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ELEMENTS
OF
PHYSICS;
OR,
NATURAL PHILOSOPHY,
GENERAL AND MEDICAL:'WRITTEN FOR UNIVERSAL USE,
IN
PLAIN OR NON-TECHNICAL LANGUAGE;
AND CONTAINING
NEW DISQUISITIONS AND PRACTICAL SUGGESTIONS.
COMPRISED IN IVE PARTS:
1. SOMATOLOGY, STATICS AND      3. PNEUMATICS, HYDF AULICS
DYNAMICS.                    AND ACOUSTICS.
2. MECHANICS.                   4. HEAT AND LIGHT.
5. ANIMAL AND MEDICAL PHYSICS.
BY NEILL ARNOTT, M. D.
OF THE ROYAL COLLEGE OF PHYSICIANS.
A NEW EDITION, REVISED AND CORRECTED FROM THE LAST ENGLISH EDITION.,WITH ADDITIONS.
BY ISAAC HAYS, M. D.
COMPLETE IN ONE VOLUME.
PHILADELPHIA:
BLANCHARD AND LEA.
1856.




Entered according to Act of Congress, in the year 1841, by
LEA AND BLANCHARD,
in the Clerk's Office of the District Court of the United States, in and for
the Eastern Distr:cli of Pennsylvania.




ADVERTISEMENT
OF THE
AMERICAN PUBLISHERS.
THE very valuable and popular work of Dr. Arnott, has passed
through several editions in this country, in the form in which it
was originally published by the author, in separate parts. A new
edition being now called for, the work has been carefully revised
and corrected, and the whole condensed into one volume. In this
form it cannot fail to be more acceptable to the public, and rendered
more convenient and useful for the purposes of instruction in the
various Colleges and Seminaries of Learning that have adopted it
as a Class Book for their pupils. This volume embraces all that
has been prepared or published by the author.








INTRODUCTION.
To appreciate the importance of PHYSICS or NATURAL PHILOSOPHY, as
an object of study not only to all persons engaged in scientific pursuits, but,
in the present day, to all who pretend to a moderately good education, we
must take a rapid glance at the nature of human knowledge generally, and
at its bearings on the existing condition of mankind.
WHILE the inferior races of animals on earth seem to have changed as
little in any respect since the beginning of human records, as the trees and
herbs of the thickets, which gave many of them shelter, the condition of man
himself has fluctuated, but, on the whole, progressed in a very remarkable
manner. The inferior animals were formed by their Creator such, that within
one life or generation they should attain all the perfection of which their
nature was susceptible. Their wants were either immediately provided foras instanced in the clothing of feathers to birds, and of furs to quadrupeds;
or were so few and simple, that the supply was easy to very limited powers
-except in a few cases where considerable art was required, as by the bee
in making its honey-cell, or by the bird in constructing its beautiful nest,
and there, a peculiar aptitude or instinct was bestowed. Thus a crocodile
which issues from its egg in the warm sand, and never sees its parent, becomes as perfect and knowing as any crocodile that has lived before or that
will appear after it.-But how different is the story when we turn to man?
He comes into the world the most helpless of living beings, long to continue
so; and if deserted by parents at an early age, so that he can learn only
what the experience of one life may teach him,-as to a few individuals has
happened who yet have attained maturity in woods and deserts,-he grows up
in some respects inferior to the nobler brutes. Now as regards many regions
of the earth, history exhibits the early human inhabitants in states of ignorance and barbarism, not far removed from this lowest possible grade, which
civilized men may shudder to contemplate. But these countries, occupied
formerly by straggling hordes of miserable savages, who could scarcely defend themselves against the wild beasts that shared the woods with them,
and the inclemencies of the weather, and the consequences of want and
fatigue, and who to each other were often more dangerous than any wild
beasts, unceasingly warring among themselves, and destroying each other
with every species of savage and even cannibal cruelty-countries so occupied
formerly, are now become the abodes of peaceful, civilized and friendly men,
where the desert and the impenetrable forest are changed into cultivated
fields, rich gardens and magnificent cities.
It is the strong intellect of man, operating with the faculty of language as
a means which has gradually worked this wonderful change. By language,
fathers communicate their gathered experience and reflections to their children, and these to succeeding children, with new accumulation; and when,
after many generations, the precious store had grown until simple memory
could retain no more, the arts of writing and then of printing, arose, making
language visible and permanent, and enlarging illimitably the repositories of




Vi                      INTRODUCTION.
knowledge. Language thus, at the present moment of the world's existence,
may be said to bind the whole human race of uncounted millions into one
gigantic rational being, whose memory reaches to the beginnings of written
records, and retains imperishably the important events that have occurred;
whose judgment analyzing the treasures of memory, has discovered many
of the sublime and unchanging laws of nature, and has built on them all
the arts of life, and through them piercing far into futurity, sees clearly
many of the events that are to come, aVd whose eyes and ears, and observant mind at this moment, in every corner of the earth, are watching and
recording new phenomena, for the purpose of still better comprehending the
magnificence and beautiful order of creation, and of more worthily adoring
its beneficent author.
It might be very interesting to show here, in minute detail, how the arts
and civilization have progressed in accordance with the gradual increase of
man's knowledge of the universe; but to do so would lead too far from the
main subject. We deem it right, however, to make evident to the student
the arousing truths, that the progress is not yet at an end; that it has been
vastly more rapid in recent times than ever; and that it seems still to proceed with increasing celerity: —and we know not where the Creator has fixed
the limits of the change! Although there are thousands of years on the
records of the world, our BACON, who first taught the true way to investigate nature, lived but the other day. NEWTON followed him, and illustrated
his precepts by the most sublime discoveries which one man has ever made.
HARVEY detected the circulation of the blood only two hundred years ago.
ADAM SMITH, DR. BLACK and JAMES WATT were friends, and the last,
whose steam-engines are now changing rapidly the condition of empires, may
be said to be scarcely cold in his grave. JOHN HUNTER died not long ago;
HERSCHEL'S accounts of newly-discovered planets, and of the sublime structure of the heavens, and DAVY'S account of chemigal discoveries not less
important to man, are in the late numbers of our scientific journals; —illustrious Britons these, and who have left worthy successors treading in their
steps. On the continent of Europe during the same period, a corresponding
constellation of genius has shone; and LAPLACE was lately the bright star
shining between the future and the past.
But there is a change going on in the world, connected closely with the
progress of science, yet distinct from it, and more important than a great part
of the scientific discoveries; —it is the diffusion of existing knowledge among
the mass of mankind.  Formerly, knowledge was shut up in convents and
universities, and in books written in the dead languages —or in books which,
if in the living languages, were so abtruse and artificial, that only a few persons had access to their meaning; and thus the human race being considered
as one great intellectual creature, a smallfraction only of its intellect was
allowed to come into contact with science, and therefore into activity. The
progress of science in those times was correspondingly slow, and the evils of
general ignorance prevailed. Now, however, the strong barriers which confined the stores of wisdom have been thrown down, and a flood is overspreading the earth; old establishments are adapting themselves to the spirit of
the age; new establishments are arising; the inferior schools are introducing
improved systems of instruction; and good books are rendering every man's
fireside a school. From all these causes there is growing up an enlightened
public opinion, which quickens and directs the progress of every art and
science, and through the medium of a free press, although overlooked by
many, is now rapidly becoming the governing influence in all the affairs of




INTRODUCTION.                             vii
man. In Great Britain, partly perhaps as a consequence of its insular situation, which lessened among its inhabitants the dread of h6stile invasion, and
sooner formed them into a united and compact people, the progress of enlightened public opinion had been more decided than in any other state. The
early consequences were more free political institutions; and these gradually
led to greater and greater improvements, until Britain became an object of
admiration among the nations.  A colony of her children, imbued with her
spirit, now occupies a magnificent territory in the new world of Columbus;
and although it has been independent as yet for only half a century, it already
counts more people than Spain, and will soon be second to no nation on
earth. The example of the Anglo-Americans has aided in rendering their
western hemisphere the cradle of many other gigantic states, all free, and
following, although at a distance, the like steps. In the still more recently
discovered continent of Australasia, which is nearly as large as Europe, and
is empty of men, colonization is spreading with a rapidity never before witnessed; and that beautiful and rich portion of the earth will soon be covered
with the descendants of free-born and enlightened Englishmen. Thence,
still onward, they or their institutions will naturally spread over the vast
archipelago of the Pacific Ocean, a tract studded with islands of paradise.
Such, then, is the extraordinary moment of revolution, or transit, in which
the world at present exists! And where, we may ask again, has the Creator
predestined that the progress shall cease? Thus far at least we know, that
he has made our hearts rejoice to see the world filling with happy human
beings, and to observe that the increase of the sciences can make the same
spot maintain thousands in comfort and godlike elevation of mind, where
with ignorance even hundreds had found but a scanty and degrading supply.
The progress of knowledge which has thus led from former barbarism to
present civilization, has gone on by certain remarkable steps, which it is easy
to point out; and which it is very useful to consider, because we thereby
discover the nature of human knowledge, with the relations and importance
of its different branches; and we obtain great facilities for studying science,
and for quickening its farther progress.
The human mind, when originally directed to the almost infinity of objects
in the universe around it, must soon have discovered that there were resemblances among them; in other words, that the infinity was only a repetition
of a certain number of kinds. Among animals, for instance, it would distinguish the sheep, the dog, the horse; among vegetables, the oak, the beech,
the pine; among minerals, lime, flint, the metals, and so forth. And becoming aware that by studying an examplar of each kind, its limited power of
memory might acquire a tolerably correct knowledge of the whole, while
this knowledge would enable the possessors more easily to obtain what was
useful to them, and to avoid what was hurtful, the desire for such knowledge
must have arisen with the first exercise of reason. Accordingly, the pursuit
of it has been unremitting, and the labour of ages has at last nearly completed an arrangement of the constituent materials of the universe, under
three great classes of MINERALS, VEGETABLES and ANIMALS; commonly
called the three kingdoms of Nature, and of which the minute description
is termed NATURAL HISTORY: and museums of natural history have been
formed, which contain a specimen of almost every object included in these'classes, so that now, a student within the limits of an ordinary garden, may
be said to be able to examine the whole of the material universe.
While men are examining the forms and other qualities of the bodies
around them, they could not avoid noticing also the motions or changes going




viii                    INTRODUCTION.
on among bodies; and here, too, they would soon make the grand discovery
that there were resemblances in the multitude. Self-interest, as in the case
of the bodies themselves, having prompted to careful classification, in the
present day, as the result of countless observations and experiments made
through the series of ages, we are enabled to say, that all the motions, or
changes, or phenomena (words synonymous here) of the universe, are merely
a repetition and mixture of a few simple manners, or kinds of motion or
change, which are as constant and regular in every case as where they produce the returns of day and night, and of the seasons. All these phenomena are referable to four distinct classes, which we call Physical, Chemical, Vital and Mental.  The simple expressions which describe them are
denominated General Truths or Laws of N3ature, and as a body of knowledge, they constitute what is called SCIENCE OF PHILOSOPHY, in contradistinction of NATURAL HISTORY, already described. Now as man cannot,
independently of a supernatural revelation, learn anything but what respects,
1st, the momentary state, past or present, of himself and the objects around
him; and 2d, the manner in which the states are changed; Natural History and Science, in the sense now explained, make up the whole sum of his
knowledge of nature.
To exemplify the process by which a general truth or law of nature is
discovered, we shall take the physical law of grarity or attraction. 1st. It
was observed that bodies, in general, if raised from the earth, and left unsupported, fell towards it; while flame, smoke, vapors, &c., if left free,
ascended away from the earth. It was held, therefore, to be a very general
law, that things had weight; but that there were exceptions in such matters, as now mentioned, which were in their nature light or ascending. 2d.
It was discovered that our globe of earth is surrounded by an ocean of air,
having nearly fifty miles of altitude or depth, and of which a cubic foot
taken near the surface of the earth, weighs about an ounce. It was then
perceived that flame, smoke, vapor, ic., rise in the air only as oil rises in
water, viz., because not so heavy as the fluid by which they are surrounded;
it followed, therefore, that nothing was known on earth naturally light, in
the ancient sense of the word.  3d. It was found that bodies floating in
water, near to each other, approached and feebly cohered; that any contiguous hanging bodies were drawn towards each other, so as not to hang quite
perpendicularly; and that a plummet suspended near a hill was drawn towards the hill with force only so much less than that with which it was
drawn towards the earth, viz, the weight of the plummet, as the hill was
smaller than the earth.  It was then proved that weight itself is only an
instance of a more general mutual attraction, operating between all the constituent elements of this globe; and which explains, moreover, the fact of
the rotundity of the globe, all the parts being drawn towards a common centre, as also the form of dew-drops, rain-drops, globules of mercury, and
of many other things; which, still further, is the reason why the distinct
particles of which any solid mass, as a stone or piece of metal, is composed,
cling together as a mass, but which, when overcome by the repulsion of heat,
allows the same particles to assume the form of a liquid or air. 4th. It
was further observed, that all the heavenly bodies are round, and must,
therefore, consist of material obeying the same law. 5th. And- lastly, that
these bodies, however distant, attract each other; for that the tides of our
ocean rise in obedience to the attraction of the moon, and become high or
spring-tides, when the moon and sun operate in the same direction.  Thus
the sublime truth was at last made evident, and by the genius of the immor



I.NTROD UCTIO N.                         iX
tal Newton, that there is a power of attraction connecting together the bodies
of this solar system at least, and probably limited only by the bounds of the
universe.
Acquaintance with the laws of nature has been very slowly obtained, owing
to that complexity of ordinary phenomena, which is produced by several
laws operating together, and under great variety of circumstance.  With
respect to many laws of Chemistry and Life, men seem to he yet little farther advanced than they were with respect to the physical law of attraction,
when they knew only that heavy things fell to the' earth.  But we have
learned enough to perceive that the great universe is as simple and harmonious as it is immense; and that the Creator, instead of interposing separately, or miraculously, in the common sense of the word, to produce every
distinct phenomenons, has willed that all should proceed according to a few
general laws. There is nothing in nature so truly miraculous and adorable
as that the endless and beneficent variety of results which we see, should
spring from such simple elements.  In times of ignoranee, men naturally
regarded every occurrence which they did not understand, that is to say,
which they could not refer to a general law, as arising from a direct interference of supreme power; and thus, for many ages, among some nations
still, eclipses and earthquakes, and many diseases, particularly those of the
mind, and the winds and weather, were or are accounted miraculous. Hence
arose, among heathens, many ceremonies, and sometimes even barbarous
sacrifices, for propitiating or appeasing their offended deities; but founded
on expectations no more reasonable than if we should now pray to have the
day of the year made shorter, or to have a coming eclipse averted. They
had not yet risen to the sublime conception of the one God, who said, " Let
there be light," and the light was; and who gave the whole of nature permanent laws, which he allows men to discover for the direction of their conduct in life-laws so unchanging, that by them we can calculate eclipses
backward or forward for thousands of years, almost without erring, by the
time of one beat of a pendulum; and as our knowledge of nature advances,
we can anticipate and explain other events with equal precision. Even the
wind and the rain, whiph, in common speech, are the types of uncertainty
and change, obey laws as fixed as those of the sun and moon; and already,
as regards many parts of the earth, man can foretell them without fear of
being deceived. He plMns his voyages to suit the coming monsoons, and he
prepares against the floods of the rainy seasons.
The general laws of Nature, divisible, as stated above, into the four classes
of, 1st, Physics, often called Natural Philosophy; 2d, of Chemistry; 3d,
of Life, commonly called Physiology; and 4th, of Mind, may be said to
form the pyramid of Science, of which Physics is the base, while the others
constitute succeeding layers in the order now mentioned; the whole having
certain mutual relations and dependencies well figured by the parts of a
pyramid. We must describe them more particularly, to show these relations.
Physics.-The laws of Physics govern every phenomenon of nature in
which there is any sensible change of place, being concerned alone in the
greater part of these phenomena, and regulating the remainder which originate from chemical action, and from the action of life. The great physical
truths, as comprehended in the present day by man, are reduced to four, and
are referred to by the words atom, attraction, repulsion and inertia.  It
gives an astonishing, but true idea of the nature and importance of methodical Science, to be told that a man, who understands these words,.viz., how
the ATOMS of matter by mutual ATTRACTION approach and cling together




X                      IN TRODU CTION.
to form masses, which are solid, liquid, or aeriform, according to the quantity or REPULSION of heat among them, and which, owing to their INERTIA
or stubbornness, gain and lose motion, in exact proportion to the force of
attraction or repulsion acting on then, —understands the greater part of the
phenomena of nature; but such is the fact! Solid bodies existing in conformity with these truths, exhibit all the phenomena of Mechanics; Liquids
exhibit those of Hydrostatics and fHydraulics; Airs, those of Pneumatics;
and so forth, as seen in the table of heads given below, at page xii. And
the whole of this work is merely a list of' the most interesting physical
phenomena, arranged in classes under these heads.
Chemistry. —Had there been only one kind of substance or matter in the
universe, the laws of Physics would have explained all the phenomena; but
there are iron, and sulphur, and charcoal, and about fifty others, which to
the present state of science, appear essentially distinct.  Now these, when
taken singly, obey the laws of Physics; but when two or more of them are
placed in contact, under certain circumstances, they exhibit a new order of
phenomena. Iron and sulphur, for instance, brought together and heated,
disappear as individuals, and unite into a yellow metallic mass, which, in
most of its properties, is unlike to either:-under other new circumstances,
the two substances will again separate, and assume their original forms. Such
changes are called chemical, (from an Arabic word signifying to burn, because so many of them are effected by means of heat,) but during the
changes, the substances are not withdrawn from the influence of the physical laws,-their weight or inertia, for instance, is not altered; and indeed
the phenomenon is merely a modification of general attraction and repulsion. Many chemical changes, besides, are only the beginnings of purely
mechanical changes, as when the new chemical arrangement produced by
heat among the intimate atoms of gunpowder, causes the mechanical or
physical motion of the sudden expansion or explosion. And all the manipulations of Chemistry, as the transferring of gases from vessel to vessel,
the weighing of bodies, pounding, grinding, &c., are directed to Physics
alone. Chemistry, then, is truly, as figured above, a superstructure on
Physics, and cannot be understood or practised by a; person who is ignorant
of Physics. The chief departments of study involving the consideration of
Chemical in conjunction with Physical laws, are enumerated in the table
below, under the head of CHEMISTRY.
Life.-The most complicated state in which matter exists, is where, under
the influence of life, it forms bodies with a curious internal structure of tubes
and cavities, in which fluids are moving, and producing incessant internal
change. These are called Organized Bodies, because of the various distinct
parts or organs which they contain; and they form two remarkable classes,
the individuals of one of which are fixed to the soil, and are called Vegetables; and of the other, are endowed with power of locomotion, and are
called Animals. The phenomena of growth, decay, death, sensation, selfmotion, and many others, belong to life, but from occurring in material
structures which subsist in obedience to the laws of Physics and Chemistry,
the life is truly a superstructure on the other two, and cannot be studied
independently of them.  Indeed the greater part of the phenomena of
organic life are merely chemical and physical phenomena, modified by an
additional principle. The science of Life is divided into animal and vegetable Physiology (see the table below).
- Mind. —The most important part of all science is the knowledge which
man has obtained of the laws governing the operations of his own MIND.
This department stands eminently distinct from the others, on several




INTR OD DUCTION.                           xi
accounts. Unlike that of organic lf/e, which could not be understood until
physics and chemistry had been previously investigated, this had made
extraordinary advances in a very early age, when the others, as methodical
sciences, had scarcely begun to exist. In proof of this assertion we need
only refer to the writings of the Greek philosophers. The most brilliant
discoveries and applications, however, were reserved for the moderns, as'will
occur to many readers, on perusing, in the table below, the several divisions
of the subject, and recollecting the honoured names which are now associated with each. It is truly admirable to see the modern analysis, deducing
from a few simple laws of mind, act the subordinate departments, just as it
deduces mechanics, hydrostatics, pneumatics, &c., from the laws of physics:
and let us hope that sound opinions on this subject, ensuring human happiness, and therefore, beyond comparison, more important than any other
knowledge, will soon be widely spread. The crowning science of Mind,
although in certain respects independent of the science of Matter, is still
closely allied to them in the following ways. The faculties of the mind are
originally awakened or called into activity solely by the impressions of matter or external nature; all the language used in speaking of mind and its
operations, is borrowed from matter; and many mental emotions are entirely
dependent on bodily conditions. The science of Mind, therefore, cannot be
studied until after knowledge acquired of an external nature; and cannot
be studied extensively until that knowledge be extensive.
Quantity.-To express most of the facts and laws of Physics, Chemistry
and Life, terms of QUANTITY are required, as when we speak (f the magnitude of a body, or say, that ihe force of attraction between two bodies
diminishes, in a certain proportion, as their distance increases.  Hence
arises the necessity of having a set of fixed measures or standards, with which
to compare all other quantities. Such measures have been adopted; and
they are, for NUMBERS, the fingers, fives and tens; for LENGTH, the human
foot, cubit, pace, &c.; and lately the second's pendulum and the French metre,
( taken from the magnitude of our globe ); for SURFACE S, the simplest forms
of circle, square, triangle, &c., compared among themselves by the lengths
of their diameters or other suitable lines; and for SOLID BULK, the corresponding simple solids, of globe, cube,pyramid, cone, &c., similarly compared
by the lengths of diameters or of other lines of dimension. The rules for
applying these standards to ail possible cases, and for comparing a  kinds
of quantities with each other, constitute a body of science, called the  cience
of Quantity, the Mlathematics.  It may be considered as a subsidiary department of human science, created by the mind itself, to facilitate the study of
the others.
Supposing description of particulars, or Natural History, to be studied
along with the different parts of the System of Science sketched in the table,
there will be included in the scheme the whole knowledge of the universe
which man can acquire by the exercise of his own powers: that is to say,
what he can acquire independently of a supernatural Revelation. And
on this knowledge all his arts are founded,-some of them on the single
part of Physics, a that of the machinist, architect, mariner, carpenter, &c.;
some on Chemistry, (which includes Physics,) as that of the miner, glassmaker, dyer, brewer, &c.; and some on Physiology, (which includes much of
Physics and Chemistry,) as that of the scientific gardener or botanist, agriculturist, zoologist, &c. The business of teachers of all kinds, and of governors,
advocates, linguists, &c.,'&c., respects chiefly the science of Mind. The art
of medicine requires in its professor a comprehensive knowledge of all the
departments.




Xii                    INTRODUCTION.
TABLE OF SCIENCE AND ART.
1. PHYSICS.                        2. CHEMISTRY.
Mechanics,                        Simple substances,
Hydrostatics,                      Mineralogy,
Hydraulics,                        Geology,
Pneumatics,                        Pharmacy,
Acoustics,                         Brewing,
Heat,                              Dyeing,
Optics,                            Tanning,
Electricity,                         &c.
Astronomy,
&c.
3. LIFE.                           4. MIND.
Vegetable Physiology.              Intellect,
Botany,                            Logic,
Horticulture,                      Mathematics,
Agriculture,                         &c.
&c.
Motives to action,
Animal Physiology,                 Emotions and Passions,
Zoology,                          Morals,
Anatomy,                           Government,
Pathology,                         Political Economy,
Medicine                           Theology,
&c.                              Education.
In the first stages of education, viz., during the years of childhood and
youth, the learning acquired is necessarily of the most mixed kind, and
much gf it is determined by what is called accident; but from the mutual
depen ence of the different departments of science, as explained in the preceding paragraphs, it follows that with a view to complete erudition, the
order exhibited in "The Table," is that in which they should afterwards be
studied, so as to prevent repetitions and anticipations, and to diminish, as
much as possible, the labor of acquirement.
Every man may be said to begin his education, or acquisition of knowledge, on the day of his birth.  Certain objects, repeatedly presented to the
infant, are, after a time, recognized and distinguished.  The number of
objects thus known gradually increases, and from the constitution of the
mind, they are soon associated in the recollection, according to their resemblances, or obvious relations. Thus, sweetmeats, toys, articles of dress, &c.,
soon form distinct classes in the memory and conceptions.: At a later age,
but still very early, the child distinguishes readily between a mineral mass,
a vegetable, and an animal; and thus his mind has already noted the three
great classes of natural bodies, and has acquired a certain degree of acquaintance with Natural History. He also soon understands the phrases " a falling
body," " the force of a moving body," and has therefore a perception of the




INTRODUCTION.                            Xiii
great physical laws of gravity and inertia. Then having seen sugar dissolved
in water, and wax melted round the wick of a burning candle he has learned
some phenomena of Chemistry. And having observed the conduct of the
domestic animals, and of the persons about him, he has begun his acquaint.
ance with Physiology, and the science of mind. Lastly, when he has learned
to count his fingers and his sugar plums, and to judge of the fairness of the
division of a cake between himself and brothers, he has advanced into Arithmetic and Geometry. Thus within a year or two, a child of common sense
has made a degree of progress in all the great departments of human science;
and in addition has learned to name objects, and to express feelings, by the
arbitrary sounds of language. Such, then, are the beginnings or foundations of knowledge, on which'future years of experience, or methodical
education, must rear the superstructure of the more considerable attainments which befit the various conditions of men in a civilized community.
In the course of the preceding disquisition, we have seen that Physics, or
Natural Philosophy, the subject of the present volume, is fundamental to
the other parts, and is therefore that of which a knowledge is indispensable.
Bacon truly calls it " the root of the sciences and arts." That its importance has not been marked by the place which it has held in common systems
of education, is owing chiefly, 1st, to the misconception that a knowledge
of technical mathematics was a necessary preliminary; and, 2d, to an
opinion, also erroneous, that the degree of acquaintance with Physics which
all persons acquire by common experience, is sufficient for common purposes; now it is true, that the toys of childhood, as the windmill, ball,
syphon, tube, and a hundred others, furnish so many exemplifications of
the laws of Physics, and may well be called a philosophical apparatus; but
they give information which is exceedingly vague, and not at all such as is
absolutely requisite in the practice of many of the arts. If, then, the study
of Physics be so easy as now appears, and so important as we shall try still
farther to show, there can be no excuse for neglecting it.
The greatest sum of knowledge acquired with the least trouble is, perhaps,
that which comes-with the study of the few simple truths of Physics. To
the man who understands these, very many phenomena, which, to the uninformed, appear prodigies, are only beautiful illustrations of his fundamental
knowledge, and this he carries about with him, not as an oppressive weight,
but as a charm supporting the weight of other knowledge, and enabling him
to add to his valuable store every new fact of importance which may offer
itself. With such a principle of arrangement, his information, instead of
resembling loose stones or rubbish thrown together in confusion, becomes
as a noble edifice, of correct proportions and firm contexture, and is acquiring
greater strength and consistency with the experience of every day. It has
been a common prejudice, that persons thus instructed in general laws, had
their attention too much divided, and could know nothing perfectly. But
the very reverse is true; for general knowledge renders all particular
knowledge more clear and precise. The ignorant man may be said to have
charged his hundred books of knowledge, to use a rude simile, with single
objects, while the informed man makes each support a long chain, to which
thousands of kindred and useful things are attached. The laws of Philosophy
may be compared to keys which give admission to the most delightful gardens that fancy can picture; or to a magic power, which unveils the face
of the universe, and discloses endless charms of which ignorance never
dreams. The informed man, in the world, may be said to be always sur



Xiv                     INTRODUCTION.
rounded by what is known and friendly to him, while the ignorant man is
as one in a land of strangers and enemies. A man reading a thousand
volumes of ordinary books as agreeable pastime, will receive only vague
impressions; but he who studies the methodized Book' of Nature, converts
the great universe into a simple and sublime history, which tells of God,
and may worthily occupy his attention to the end of his days.
We have said already, that the laws of Physics govern the great natural
phenomena of Astronomy, the tides, winds, currents, &c. We will now
mention some of the artificial purposes to which man's ingenuity has made
the same laws subservient.
Nearly all that the civil engineer accomplishes, ranges under the head of
Physics. Let us take, for instance, the admirable specimens scattered over
the British Isles:-the numerous canals for inland traffic; the dock to
receive the riches of the world, pouring towards us from every quarter; the
many harbours offering safe retreat to the storm-driven mariner; the magnificent bridges which every where facilitate intercourse; hills bored through
to open ways for commerce by canals, common roads and rail-roads, the
canals in some places being supported, like the roads, on arches across valleys
or above rivers, so that here and there the singular phenomenon is seen of one
vessel sailing directly over another; vast tracts of swamps or fen-land drained
and now serving for agriculture; the noble light-house, rearing its head
amidst the storm, while the dweller within trims his lamp in safety, and
guides his endangered fellow-creature through the perils of the night, &c., &c.
In Holland, great part of the country has been won and is now preserved
from the sea, by the same almost creating power, and now rich cities and
an extended garden smile, where, as related by Caesar, were formerly only
bogs and a dreary waste.
As a general picture, it is interesting to consider, that in many situations
on earth where formerly the rude savage beheld the cataract falling among
the rocks, and the wind bending the trees of the forest, and sweeping the
clouds along the mountain's brow, or whitening the face of the ocean, and
regarding these phenomena with awe and terror, as marking the agency of
some great but hidden power, which might destroy him; in the same situations now, his informed son, who works with the laws of nature, can lead
the waters of the cataract, by sloping channels, to convenient spots, where
they are made to turn his mill-wheel, and to do his multifarious work; the
rushing winds, also, he makes his servant, by rearing in their course the
broad-vaned wind-mill, which then performs a thousand offices for its master,
man; and the breezes which whiten the ocean are caught in his expanded
sails, and are made to waft their lord and his treasures across the deep, for
his pleasure or his profit.
In Architecture, also, Physics in supreme, and has directed the construction of the temples, pyramids; domes and palaces which adorn the earth.
In respect to machinery, generally, Physics is the guiding light. There
are, for instance, the mighty steam-engine; machines for spinning and
weaving, and for moulding other bodies into various shapes, yea, even iron
itself, as if it were plastic clay; wind-mills and water-mills, and wheel
carriages; the plough and implements of husbandry; artillery and the furniture of war; the balloon, in which man rides triumphantly above the
clouds, and the diving-bell, in which he penetrates the secret caverns of the
deep; the implements of the intellectual arts, of printing, drawing, painting,
sculpture, &c.; musical instruments, optical and mathematical instruments,
and a thousand others.




INTRODUCTION.                             XV
But Physics is also an important foundation of the healing art. The medical man, indeed, is the engineer pre-eminently; for it is in the animal body
that true perfection and the greatest variety of mechanism are found. Where,
to illustrate Mechanics, is to be found a system of levers and hinges, and
moving parts, like the limbs of an animal body; where such an hydraulic
apparatus, as in the heart and blood-vessels; such a pneumatic apparatus,
as in the breathing chest; such acoustic instruments, as in the ear and
larynx; such an optical instrument, as in the eye; in a word, such variety
and perfection, as in the whole of the visible anatomy? All these structures, then, the medical man should understand, as the watchmaker knows
the parts of a time-piece about which he is employed. The watchmaker,
unless he can discover where a pin is loose, or a wheel injured, or a particle
of dust adhering, or oil wanting, &c., would ill succeed in repairing an
injury; and so, also, of the ignorant medical man in respect to the human
body. Yet will it be believed, that there are many medical men who
neither understand mechanics, nor hydraulics, nor pneumatics, nor optics,
nor acoustics, beyond the merest routine; and that systems of medical
education are set forth at this day which do not even mention the department of Physics! That such is the case, furnishes an illustration of what is
stated in the beginning of this essay, viz., that the sciences and arts are
progressive, and that perfect methods of education must arise gradually,
like all other things of human contrivance. It is within the recollection of
persons now living, that political economy was discovered to be a grand
foundation of the art of government, indicating means of security against
many national misfortunes common in former times, yda, even against famine
and war. And the day is not distant, when the members of the medical
profession generally will understand how much the correct knowledge of
animal structure and function, and of many remedies, must depend on precise acquaintance with Physics. —Besides the more strictly professional
matters contained in the medical sections of the present work, there are
many others scattered through it which greatly interest the medical man;
such are the subjects of meteorology, climate, ventilation and warming of
dwellings, specific gravities, &c., &c.
The laws of Physics having an influence so extensive as appears from
these paragraphs, it need not excite surprise that all classes of society are at
last discovering the deep interest they have to understand them. The lawyer
finds that in many of the causes tried in his courts, an appeal must be made
to Physics,-as in cases of disputed inventions; accidents in navigation, or
among carriages, steam-engines, and machines generally; questions arising
out of the agency of winds, rains, water-currents, &c.: the statesman is constantly listening to discussions respecting bridges, roads, canals, docks, and
the mechanical industry of the nation: the clergyman finds ranged among
the beauties of nature, the most intelligible and striking proof of God's wisdom and goodness; the sailor in his ship has to deal with one of the most
admirable machines in existence: soldiers, in using their projectiles, in
marching where rivers are to be crossed, woods to be cut down, roads to be
made, towns to be besieged, &c., are dependent chiefly on their knowledge
of Physics: the land-owner, in making improvements on his estates, building,
draining, irrigating, road-making, &c.: the farmer equally in these particulars, and in all the machinery of agriculture; the manufacturer, of course;
the merchant who selects and distributes over the world the products of
manufacturing industry-all these are interested in Physics; then also the
man of letters, that he may not, in drawing his illustrations from the material




xvi                    INTRODUCTION.
world, repeat the scientific heresies and absurdities which have heretofore
prevailed, and which, by shocking the now better-informed public, exceedingly lower the estimation in which such specimens of the Belles Lettres
are held, and lessen their general utility; and lastly, parents of either sex,
whose conversation and example have such powerful effect on the character
of their children, who when grown up, are to fill all the stations in society;
all should study Physics, as one important part of their education.
And it is for such reasons that Natural Philosophy is becoming daily more
and more a part of common education. In our cities now, and even in an
ordinary dwelling-house, men are surrounded by prodigies of mechanic art,
and cannot submit to use these, regardless of how they are produced, as a
horse is regardless of how the corn falls into his manger. A general diffusion of knowledge, owing greatly to the increased commercial intercourse of
nations, and therefore to the improvements in the physical departments of
astronomy, navigation, &c.. is changing every where the condition of man,
and elevating the human character in all ranks of society. In remote times
the inhabitants of the earth were generally divided into small states or societies, which had few relations of amity among themselves, and whose thoughts
and interests were confined very much within their own little territories and
rude habits. In succeeding ages, men found themselves belonging to larger
communities, as where the English heptarchy was united; but still distant
kingdoms and quarters of the world were of no interest to them, and were
often totally unknown. Now, however, every one feels that he is a member
of one vast civilized society, which covers the face of the earth; and no part
of the earth is indifferent to him. In England, for instance, a man of small
fortune may cast his looks around him, and say with truth and exultation,
"' I am lodged in a house which affords me conveniences and comforts which
some centuries ago, even a king could not command. Ships are crossing
the seas in every direction, to bring me what is useful to me from all parts
of the earth. In China, men are gathering the tea-leaf for me; in America,
they are planting cotton for me; in the West Indies, they are preparing
my sugar and my coffee; in Italy, they are feeding silk-worms for me; in
Saxony, they. are shearing the sheep to make me clothing; at home, powerful steam-engines are spinning and weaving for me, and making cutlery for
me, and pumping.the mines that minerals useful to me may be procured.
Although my patrimony was small, I have post-coaches running day and
night on all the roads to carry my correspondence; I have roads and canals,
and bridges, to bear the coal for my winter fire; nay, I have protecting fleets
and armies around my happy country, to secure my enjoyments and repose.
Then I have editors and printers, who daily send me an account of what is
going on throughout the world, among all these people who serve me. And
in a corner of my house I have BooKS! the miracle of all my possessions,
more wonderful than the wishing-cap of the Arabian Tales: for they trans.
port me instantly, not only to all places, but to all times. By my books I
can conjure up before me, into vivid existence, all the great and good men
of antiquity; and for my individual satisfaction I can make them act over
again the most renowned of their exploits, the orators declaim for me; the
historians recite; the poets sing; and from the equator to the pole, or from
the beginning of time until now, by my books, I can be where I please."
This picture is not overcharged, and might be much extended, such being
God's goodness and providence, that each individual of the civilized millions
dwelling on the earth, may have nearly the same  enjoyment as if he were
the single lord of all.




INTRODUCTION.                           Xvi;
Reverting to the importance of Natural Philosophy as a general study, it
may be remarked that there is no occupation which so much strengthens and
quickens the judgment.  This pra;se has usually been bestowed on the Mathematics, although a knowledge of abstract Mathematics existed with all the
absurdities of the dark ages; but a familiarity with Natural Philosophy
which comprehends Mathematics, and gives tangible and pleasing illustrations of the abstract truths, seems incompatible with the admission of any
gross absurdity.  A man whose mental faculties have been sharpened by
acquaintance with these exact sciences in their combination, and who has
been engaged, therefore, in contemplating real relations, is more likely to
discover truth in other questions, and can better defend himself against
sophistry of every kind. We cannot have clearer evidence of this than in
the history of the sciences, since the Baconian method of reasoning by indication took place of the visionary hypotheses of preceding times. Until then,
even powerful minds did not recoil from the most absurd theories on all subjects. Astronomy was mixed with Astrology; Chemistry with Alchemy;
Physiology with the singular hypotheses which preceded the discovery of the
circulation of the blood; Politics with the errors of monopolies, prohibitions,
balance of trade, &c. Even Religion itself, in various ages and countries,
has felt the influence of the state of the public mind as to solid attainments.
To a man with the knowledge of nature which we now possess, the fables
and licentious abominations of the Greek and Roman theologies are shocking
indeed; as are the religions of the God of Fire in China, of Vishnoo in
India, of Mahomet's imposture and pretended miracles, &c. But the enlightened Christian minister earnestly recommends the study of nature; first
because from contemplating the beauty of creation, with the wisdom and
benevolent design manifest in all its parts, there spring up in every undepraved mind those feelings of admiration and gratitude, which constitute the
adoration of natural religion, and which form, as shown by many estimable
writers on Natural Theology, a fit foundation for the sublime doctrine of
immortality, and secondly, because a Revelation being probable only by the
miracles occurring at its establishment; to enable men to distinguish between
miracles and the usual course of nature, a perfect knowledge of that course,
or of Natural Philosophy, is essential: all the false religions of antiquity
were founded on, and upheld by pretended miracles. As regards the question of immortality, even independently of Revelation, no man who contemplates the order and beauty of the material world, and then thinks on the
hideous deformities of the moral world-where vice so often triumphs, and
modest virtue pines and dies-can for a moment believe that they are the
work of the same author, unless there be a hereafter of retribution; and
feeling thus that eternal justice requires another state for man, he embraces
with delight the cheering promises of immortality.  There have been, however, at various times, even among Christians, sincere, but weak-minded or
ill-informed men, who decried the study of the natural sciences, as inimical
to true religion; as if God's ever-visible and magnificent revelation of his
attributes in the structure of the universe could be at variance with any
other revelation. But such prejudices are now quickly passing away.
Wherever considerable knowledge of nature exists, debasing and gloomy
superstition must cease. It is not the abject terror of a slave which is inspired by contemplating the majesty and power of our God, displayed in his
works, but a sentiment akin to the tender regard which leads a favored child
to approach with confidence a wise and indulgent parent.
It remains for the author now only to say a few words with respect to the
2




Xviii                   INTRODUCTION.
present work. He was originally led to the undertaking with the view of
supplying the desideratum in medical literature, of a treatise on Medical
Physics; but soon perceiving that the preliminary investigation of General
Physics, necessary to adapt the work to medical readers, would require to
be nearly as extensive as it would for general readers, and reflecting that
every person of liberal education must now possess such a book, not to be
read once and then thrown aside as a novel is, but to be frequency consulted
as a manual, he determined to make his book as complete and as extensively
useful as possible. He has been encouraged during his labor, by the belief
that the growing light of science, which now exhibits more clearly the natural relations of the different departments of study, as attempted to be portrayed in the preceding pages, might enable him to avoid some of the defects
of former elementary treatises, and to add features of novelty and improvement to his own. The sections on Animal Physics were, of course, written
for medical men; and a great service will be rendered by the work, if it
only awakens them to a just sense of the importance of Physics as one of
the foundations of their art. But even for general readers there are few
parts of these sections which the author would exclude.  There is nothing
more admirable in nature than the structure and functions of the human
body, and there are many reasons why no liberal mind should be careless of
the study. The details here given are not more anatomical than the illustrations from the animal economy contained in the common treatises on
Natural Theology.  From the attempt in this work to compress into the
smallest possible space the greatest possible sum of scientific information,
few historical details have been adjmitted, whether relating to the distinguished men who have benefitted the world as authors or inventors, or to
the history of the progress of science: —such details form an interesting, but
distinct branch of study.
The author must not conclude without observing, that no treatise on Natural Philosophy can save, to a person desiring full information on the subject, the necessity of attendance on experimental lectures or demonstrations.
Things'that are seen, and felt, and heard, that is, which operate on the external senses, leave on the memory much stronger and more correct impressions than where the conceptions are produced merely by verbal description,
however vivid. And no man has ever been remarkable for his knowledge
of Physics, Chemistry, or Physiology, who has not had practical familiarity
with the objects. With reference to this familiarity, persons who take a
philanthropic interest in the affairs of the world, must observe, with much
pleasure the now daily increasing facilities of acquiring useful knowledge,
afforded by the scientific institutions formed and forming, not only through
this kingdom, but through most civilized nations.
Bedford Square, 1st March, 1827.




ELEMENTS
OF
NATURAL PHILOSOPHY.
SYNOPSIS, OR GENERAL REVIEW.
IF it excite our admiration that a varied edifice, or even a magnificent
city can be constructed of stone from one quarry, what must our feeling be
to learn how few and simple the elements are out of which the sublime fabric of the universe, with all its orders of phenomena, has arisen, and is now
sustained.  These elements are general facts and laws which human
sagacity is able to detect, and then to apply to endless purposes of human
advantage.
Now the'four words, atom, attraction, repulsion, inertia, point to four
general truths, which explain the greater part of the phenomena of nature.
Being so general, they are called physical truths, from the Greek word
signifying nature as also "truths of Natural Philosophy," with the same
meaning, and sometimes "mechanical truths," from their close relation to
ordinary machinery. These appellations distinguish them from the remaining general truths, namely the chemical truths, which regard particular
substances, and the vital and mental truths, which have relation only to
living beings.  And even in the cases where a chemical or vital influence
operates, it modifies, but does not destroy, the physical influence. By
fixing the attention, then, on these four fundamental truths, the student
obtains, as it were, so many keys to unlock, and lights to illuminate, the
secrets and treasures of nature.
1st. ATOM.  Every material mass in nature is divisible into very
minute indestructible and unchangeable particles,-as when a piece of any
metal is bruised, broken, cut, dissolved, or otherwise transformed, a
thousand times, but can always be exhibited again as perfect as at first.
This truth is conveniently recalled by giving to the particles the name atom,
which is a Greek term, signifying that which cannot befturther cut or divided,
or an exceeding minute resisting particle.
2d. ATTRACTION. It is found that the atoms above referred to, whether
separate or already joined into masses, tend towards all other atoms or
masses, —as when the atoms of which any mass is composed are, by an invisible influence, held together with a certain degree of force; or when a
block of stone is similarly held down to the earth on which it lies; or when
the tides on the earth rise towards the moon. These facts are conveniently




20                         SYNOPSIS.
recalled by connecting with them the word Attraction (a drawing together)
or gravitation.
3d. REPULSION. Atoms under certain circumstances, as of heat diffused
among them, have their mutual attraction countervailed or resisted, and
they tend to separate;-as when ice heated melts into water, or when water
heated bursts into steam, or when gunpowder ignited explodes. Such
facts are conveniently recalled by the term Repulsion (a thrusting asunder.)
4th. INERTIA. As a fly-wheel made to revolve, at first offers resistance to
the force moving it, but gradually acquires speed proportioned to that force,
and then resists, being again stopped, in proportion to its speed, so all bodies
or atoms in the universe have about them, in regard to motion, what may
be figuratively called a stubborness, tending to keep them, in their existing
state, whatever it may be-in other words, they neither acquire motion, nor
lose motion, nor bend their course in motion, but in exact proportion to
some force applied. Many of the motions now going on in the universe
with such regularity-as that turning of the earth which produces the phenomena of day and night-are motions which began thousands of years
ago, and continue unvarying in this way. Such facts are conveniently
recalled by the term inertia applied to them.
A person comprehending fully the import of these four words, that is to
say, having present to his mind numerous good types or exemplars of the
facts referred to them, may predict or anticipate correctly, and may control
very many of the facts and phenomena which the extended experience of a
life can display to him; and such a person is commonly said to know the
causes or reasons of things and events. Now it is important here to observe,
that when a person gives a reason or explanation of any fact, other than
that it is a fact, or than that the Creator has willed it, he is merely, although
he may not be aware of this, showing its resemblance to many other facts,
no one of which he understands better than itself-and what he calls a
general truth, or law, or principle, is merely an expression for the observed
but unaccountable resemblance of the facts. Thus, when a man says that
a stone falls because of attraction or gravitation, he only uses a word which
recalls thousands of instances which he has witnessed of one body approaching another; but by any cause of the approach, other than that God has
willed it, is to him utterly unknown. Should men, in the course of their
researches, discover that the phenomena now classed by them under the
heads of attraction and repulsion, although apparently opposite, are really
as closely allied as they already know the rising of a balloon and the falling
of a stone td be (the balloon rises like a cork in water, being pushed up by
the fluid air around it, heavier than it, and seeking to descend,) they will
not have discovered a new cause, but a new resemblance, (new to them)
among phenomena, and will only have advanced one step farther in
perceiving the simplicity of creation. In accordance with these views, it
will be found that this volume is chiefly an extensive display of the most
important phenomena of nature and art, classified so as to be explained by the'our physical truths, and mutually to illustrate one another. They will be
Distributed under the following heads or divisions:




SYNOPSIS.                            21
PART I.
CONSTITUTION OF MASSES, MOTIONS AND FORCES.
The four fundamental truths extensively examined, and used to explin
generally, in
Section
1. The nature or constitution of the material masses which compose the
universe; (a department technically called SOMATOLOGY, from Greek
words signifying a discourse on body.)
2. The motions or phenomena going on among the masses; —a department
including the common divisions of STATICS (things stationary or at
rest,) and DYNAMICS (what relates to force or power.)
PART II.
PHENOMENA OF SOLIDS.
The four truths explaining the peculiarities of state and motion among
solid bodies:-a department called, in a restricted sense, MECHANICS, (from
the Greek, and signifying machine.)
PART III.
PHENOMENA OF FLUIDS.
The truths explaining the peculiarities of state and motion among fluid
bodies: —a department called HYDRODYNAMICS (from Greek words signifying water and force.)
Section
1. HYDROSTATICS (water at rest or in equilibrium.)
2. PNEUMATICS (air phenomena.)
3. HYDRAULICS (water or fluid in motion.)
4. ACOUSTICS (phenomena of sound and hearing.)
PART IV.
PHENOMENA OF IMPONDERABLE SUBSTANCES.
The truths aiding to explain the more recondite phenomena of IMPONDERABLE SUBSTANCES, under the heads of
Section
1. HEAT or Caloric.
2. LIGHT or Optics.
PART V.
ANIMAL AND MEDICAL PHYSICS.
In this part will be ranged the most interesting illustrations afforded by
the animal economy, constituting-ANIMAL AND MEDICAL PHYSICS.
As no man can well understand a subject of which he does not carry a
distinct outline in his mind, it is recommended to the reader of this work
to study the general synopsis, and the analysis placed at the heads of the
chapters and sections, until the memory be well impressed with them.




22              CONSTITUTION OF MASSES.
PART I.
TiE FOUR FUNDAMENTAL TRUTHS MINUTELY EXAMINED, AND USED TO
EXPLAIN GENERALLY, FIRST, THE NATURE OR CONSTITUTION OF THE
MATERIAL MASSES WHICH COMPOSE THE UNIVERSE, AND SECONDLY,
THE MOTIONS OR PHENOMENA GOING ON AMONG THEM.
SECTION I.-THE CONSTITUTION OF MASSES.
ANALYSIS OF THE SECTION.
The visible universe is built up of very minute indistructible ATOMS called
matter, which by mutual ATTRACTION, cohere or cling together in masses
of various form and magnitude.  The atoms are more or less approximated, according to the quantity or REPULSION of heat among them, and
hence arise the three remarkable forms in the masses, of solid, liquid and
air, which mutuiilly change into each other with change in the quantity
of heat.  Certain modifications of attraction and repulsion produce the
subordinate peculiarities of state called crystal, dense, hard, elastic, brittle,
malleable, ductile and tenacious.
" Minute Indestructible ATOMS."*
THAT the smallest portion of any substance which the human eye can perceive, is still a mass of many ultimate atoms or particles, which may be
separated from each other, or newly arranged, but which cannot individually be hurt or destroyed, is deduced from such facts as the following:
A particle of powdered marble, hardly visible to the naked eye, still appears to the microscope a block susceptible of indefinite division; and, when
it is broken by fit instruments, until the microscope can hardly discover the
separate particles of fine powder, these may be yet further divided, by
solution in an acid; the whole becoming then absolutely invisible, as part
of a transparent liquid.
A small mass of gold may be hammered into thin leaf, or drawn into fine
wire, or cut into almost invisible parts, or liquefied in a crucible, or disolved
in an acid, or dissipated by intense heat into vapour; yet, after any and all
these changes. the atoms can be collected again to form the original mass
of gold, without the slightest diminution or change. And all the substance of
* The different heads or titles, which appear thus, throughout the work, between inverted commas, are the successive portions of the Analysis, detached for separate
consideration. The reader is particularly requested to re-peruse the analysis at the
several interruptions, that he may have constantly before him that clear view of the
general relations among the different parts of the subject, which is essentially to a perfect understanding of it.




CONSTITUTION OF MASSES.                          23
elements of which our globe is composed, may thus be cut, torn, bruised,
ground, &c., a thousand and a thousand times, but are always recoverable as
perfect as at first.
And with respect to delicate combinations of these elements, such as exist
in animal and vegetable bodies, although it be beyond human art, originally
to produce, or even closely to imitate many of them-for we cannot build up
a feather or a rose-still, in their decomposition and apparent destruction,
the accomplished chemist of the present day does not lose a single atom.
The coal which burns in his apparatus, until only a little ash remains behind,
or the wax-taper that seems to vanish altogether in flame, or the portion of
animal flesh which putrefies, and gradually dries up and disappears-present
to us phenomena which are now proved to be only changes of connection
and arrangement among the indestructible ultimate atoms; and the chemist
can offer all the elements again, mixed or separate as desired, for any of
the useful purposes to which they are severally applicable. When the funeral
piles of the ancients, with their charge of human remains, appeared to be
wholly consumed, and left the idea with survivors that no base use could be
made, in after time, of what had been the material dwelling of a noble or
beloved spirit, the flames had only, as it were, scattered the enduring blocks
of which a former edifice had been constructed, but which were soon to serve
again in new combinations.
Facts to be stated under the heads of " chemical composition" and " crystal," will prove, that the ultimate particles of any substance must be, among
themselves, perfectly similar.
"Minute." (Read the Analysis, page 22.)
The following are interesting particulars in the arts or in nature, helping the
mind to conceive how minute the ultimate atoms of matter must be.
Goldbeaters, by hammering, reduce gold to leaves so thin, that 360,000
must be laid upon one another to produce the thickness of an inch. They
are so thin, that if formed into a book, 1,800 would occupy only the space
of a single leaf of common paper; and an octavo volume an inch thick would
have as many pages as the books of a well-stocked ordinary library containing 1,800 volumes of 400 pages each; yet those leaves are perfect, or free
from holes, so that one of them laid upon any surface, as in gilding, gives
the appearance of solid gold.
Still thinner than this is the coating of gold, upon the silver wire of what
is called gold lace; and we know not that such coating is of only one atom
thick. If we place a piece of this wire in nitric acid, so as to dissolve the
silver within, the gold coating remains as a metallic tube of exquisite
tenuity.
Platinum can be drawn into wire much finer than human hair.
A grain of blue vitriol or carmine, will tinge a gallon of water, so that in
every drop the colour may be perceived.
A grain of musk will scent a room for twenty years, and will have lost
but little of its weight.
The carrion crow seems to smell its food at a distance of many miles.
The thread of the silk worm is so small, that many folds have to be
twisted together to form our finest sewing thread; but that of the spider is
smaller still, for two drachms of it by weight would reach from London to
Edinburgh, or 400 miles.
In the milk of a cod-fish, or in water in which certain vegetables have




24              CONSTITUTION  OF MASSES.
been infused, the microscope discovers animalcules, of which many thousands together do not equal in bulk a grain of sand; yet these have their
blood and other subordinate parts like larger animals; and, indeed, nature,
with a singular prodigality, has supplied many of them with organs as complex as those of the whale or elephant. Now the body of an animalcule
consists of the same elementary substances, or ultimate atoms, as the body
of man himself.  In a single pound of matter, it thus appears, that there
may be more living creatures than of human beings on the face of this globe,
What scenes has the microscope laid open to the admiration of the philosophic inquirer.
Water, mercury, sulphur, or, in general, any substance, when sufficiently
heated, rises as invisible vapour or gas; in other words, is made to assume
the aeriform state. Great heat, therefore, would cause the whole of the
material universe to disappear, the previously most solid bodies becoming as
invisible and impalpable as the air we breathe. Utter annihilation would
seem but one stage beyond this.
J laxtter."
The inconceivable minuteness of ultimate atoms, as shown above, has led
some inquirers to doubt whether there really be matter; that is to say,
whether what we call substance or matter have existence or not. In answer
to this it has been usual to adduce, besides the weights of the substances,
and the proofs of indestructibility already mentioned, which seems conclusive, the fact that every kind or portion of matter obstinately occupies some
space to the exclusion of all other matter from that particular space. This
occupancy of space is the simplest and most complete idea which we have
of material existence. The awkward word impenetrability has been used to
express it, with reference of course to the individual atoms. The following
are elucidations:
We cannot push one billiard-ball into the substance of another, and then
a second, and then a third, and so on; or the material of the universe might
be absorbed in a point.
A mass of iron on a support will resist the weight of thousands of pounds
laid upon it and pressing to descend into its place; and although a very
great weight might crush or break it into pieces, still one particle would not
be annihilated. In a forcing pump, or in Braham's water-press, millions of
pounds can not push the piston down, unless the water below it be allowed
to escape.
A weight laid upon bladders full of air, or on the piston handle of a closed
air-pump, is supported in the same manner.
A quantity of air escaping from a vessel under water ascends through the
water as a bubble displacing its bulk of water in its way.
A glass tube, left open at bottom, while the thumb closes the top, if
pressed from air into water, is not filled with water, because the air contained
in it resists; but if the air be allowed to escape by removing the thumb
from the top, the tube becomes filled immediately to the level of the water
around it. In a goblet or basin pushed into water, with the mouth downwards, the entrance of water is resisted for the like reason; and if the goblet
be inverted over a floating lighted taper, this will continue to float under it,
and to burn in the contained air, however deep in the water it may be carried-exhibiting the curious phenomenon of light below water, and being
an emblem of the living inmate of a diving bell, which is merely a larger
goblet holding a man instead of a candle.




GENERAL ATTRACTION.                            25
"Mutual attraction." (See the Analysis, page 22.)
Any visible mass of matter, then, as of metal, salt, sulphur, &c., we
know to be really a collection of dust, or minute atoms, by some cause made
to cohere or cling together; yet there are no hooks connecting them, nor
nails, nor glIb; and the connection may be broken a thousand times, by
processes of nature or art, but is always ready to take place again; the cause
being no more destroyed in any case by interruption, than the weight of a
thing is destroyed by frequent lifting from thWe ground. Now the cause we
know not, but we call it attraction. The phenomena of attraction and its
contrary, repulsion, particularly when occurring between bodies at considerable distances from each other, are as inexplicable as any subjects which
the human mind has to contemplate; but the manner or laws of the phenomena are now well understood. The general nature and extensive influence.
of attraction may be judged of from the following facts:
Logs of wood floating in a pond, or ships in calm water, approach each
other, and afterwards remain in contact. When the floating bodies are very
small, or can approach very near to each other at the water's edge-as glass
bulbs in a tea-cup-an additional force is called into play, as will be explained under the head of "capillary attraction."
The wreck of a ship, in a smooth sea after a storm, is often seen gathered
into heaps.
Two bullets or plummets suspended by strings near to each other, are
found by the delicate test of the torsion balance (which will be described
afterwards) to attract each other, and therefore not to hang quite perpendicularly.' v
A plummet suspended near the side of a mountain inclines towards it, in
a degree proportioned to its magnitude; as was ascertained by the w.ellknown trials of Dr. Maskelyne near the mountain Schehallion, in Scotland.
And the reason why the plummet in such a case tends much more strongly
towards the earth than towards the hill, is only that the earth is larger than
the hill
At New South Wales, which is situated on our globe nearly opposite to
England, plummets hang and fall towards the centre of the globe, as they
do here; so that in respect to England, they are hanging and falling upwards,
and the people there, like flies on the opposite side of a pane of glass, are
standing with their feet towards us,-hence called our antipodes. Weight,
therefore, is merely general attraction acting everywhere.
But it is owing to this general attraction that our earth itself is a globe: —
all its parts being drawn towards each other, that is, towards a common
centre, the mass assumes the spherical or rounded form.
And the moon also is round, and all the planets; nay, the glorious sun,
too, so much larger than these, is round; —suggesting the inference that all
must at one time have been a certain degree fluid, and that all are subject
to the same law.
Descending again to the earth and observing minuter masses, we have
many interesting instances of roundness from the same cause; as-the particles of a mist or fog floating in air-these, mutually attracting and coalescing
into larger drops, and so forming rain-dew-drops-water trickling on a
duck's wing-the tear dropping from the cheek-drops of laudanum-globules of mercury, like pure silver beads, coalescing when near, and forming
larger ones-melted lead allowed to rain down from an elevated sieve, and by




26              CONSTITUTION OF MASSES.
cooling as it descends so as to retain the form of its liquid drops, becoming
the spherical shot-lead of the sportsman, &c.
The cause of this extraordinary phenomenon which we call attraction, acts
at all distances.-The moon, though 240,000 miles from the earth, by her
attraction, raises the water of our ocean under her. and forms what we call
the tide.-The sun, still farther off, has a similar influence; and when the
sun and moon act in the same direction, we have the sprint tides. —The
planets, so distant that they appear to us little wandering points in the
heavens, yet, by their attraction, affect the motion of our earth in her orbit,
quickening it when she is aptproaching them, retarding it when she is receding.
The attraction is greater the nearer the bodies are to each other; as the light
of a taper is more intense near to the taper than at a distance.
A board of a foot square, represented in fig. 1 by A B at a certain distance
from a light supposed at C, just shadows a board of two feet square, as E D,
at double distance; but a board with a side of two feet has, four times as
much surface -as a board with a side of one foot, for it is not only twice as
high or long, which would make it double, but twice as broad also, which
Fig. 1.
E
-.4
makes it quadruple-as a globe of two feet in diameter requires just four
times as much paper to cover it as a globe of one foot,-and the corner, or
fourth part E F, of the larger square here shown is just equal to the whole
of the smaller square A B. Light, therefore, at double distance from its
source, being spread over four times the space, has only one-fourth of the
intensity; and for a similar reason, at thrice the distance it has only a ninth
part, at four times a sixteenth part, and so on.  Now light, heat, attraction,
sound, and indeed every influence from a central point are found to decrease
in the proportion here illustrated, viz., as the surface of squares which shadow
one another increases.  The technical expression is, "the intensity is inversely as the squares of the distance; (the distances being estimated from
the centres of attraction or radiation) or one-fourth part as strong at double,distance, four times as strong at half distance, and in a corresponding manner for all other distances.
Accordingly, what weighs 1,000 lbs. at the sea-shore, weighs five lbs. less
at the top of a mountain of a certain height, or when raised in a balloon-as
is proved experimentally by a spring balance, or other means; —and at the
distance of the moon, the weight, or force towards the earth, of 1,000 lbs.,
is diminished to five ounces, as is proved by astronomical test.
ATTRACTION has received different names as it is found acting under different circumstances.  The chief distinctions are Gravitation, Cohesion,
Capillary and Chemical attraction.
Gravitation is the name given to it when acting at sensible distance, as in
the cases of the moon lifting the tides-the sun and earth attracting each




COHESIVE ATTRACTION.                            27
other-a stone falling, &c. Most of the facts enumerated at page 25,
belong to this head.
Cohesion is the name given when it is acting at very short distances, as in
keeping the atoms of a mass together.
It might appear, at first sight, that it cannot be the same cause which
draws a piece of iron to the earth with the moderate force called its weight,
and which contains the constituent atoms of the iron in such strong cohesion;
but when we recollect that attraction is stronger as the substances are nearer
to each other, the difficulty is met.  Atoms very nearly in contact may be
a million times nearer to each other than when only a quarter of an inch
apart, aud therefore, when thfe heat among the atoms of any mass allows
them to approach very near, they should attract mutually with great force.
If, then, the surfaces of the bodies were not in general so very rough and
irregular, that, when applied to each other, they can touch only in a few
points of the million, perhaps, which each surface contains, bodies would
be invariably sticking together or cobering by any accidental contact. The
effect of artificially smoothing the touching surfaces is seen in the following
examples:-we may remark, however, that besides irregularity of surface,
there is another reason, explained a little farther on, which prevents the
cohesion.
Similar portions being cut off with a clean knife from two leaden bullets,
and the fresh surfaces being brought in contact with a slight turning pressure, the bullets cohere, almost as if they had been originally cast in one
piece.
Fresh-cut surfaces of India-rubber or caoutchouc, cohere in a similar way.
We may hence make elastic air-tight tubes, by cutting off the edges of a
strip of India-rubber and bringing the cut surfaces into contact by wirlding
the strip spirally round any small rod or cylinder, and fixing it there for a
time by tape or cord.
Two pieces of perfectly smooth plate glass or marble, laid upon each other,
adhere with great force: and so, indeed, do most well-polished fiat surfaces.
Cohesion between a solid and liquid, and between the particles of a liquid
among themselves, is seen in the following instances:
A flat piece of glass, balanced at the end of a weighing beam, and then
allowed to come into contact with water, adheres to the water, and with
much more force than the weight of water remaining upon it when again
forcibly raised! If there were not cohesion or attraction of the water particles among themselves, as well as to the glass, the latter could only be
held down by the weight of the water which directly adhered to it. In
pouring water from a mug or bottle-lip, the water does not at once fall perpendicular, but runs down along the inclined outside of the vessel; chiefly
in consequence of the attraction between this and the water; hence the difficulty of pouring from a vessel which has not a projecting lip.
The particles of water cohere among themselves in a degree which causes
small needles gently laid on the surface to float: —the weight of the needles
is not sufficient to overcome the cohesion of the water surface.
For the same reason, many light insects can walk upon the surface of
water without being wetted.
It is chiefly the different force of the attraction of cohesion in different




28              CONSTITUTION  OF MASSES.
liquids that causes their drops or gutts from the lip of a phial to be of different magnitude. Sixty drops of water fill the same measure as 100 drops
of laudanum from a lip of the same size.
In a larger mass of liquid, the attraction which, if acting alone, would
draw the particles into the form of a distinct globe, yields to that which
draws them towards the centre of the earth, and therefore the liquid assumes
more or less completely, what is called the level surface, that is to say, a
surface corresponding with the general surface of the earth.
Attraction is called capillary when it acts between a liquid and the interior
of a solid, which is tubular or porous.
When an open glass tube is partially immersed in water, the water within
it stands above the level of that on the outside; and the difference of level
is greater as the tube is less, because in small tubes, the glass all round
being nearer to the raised water, attracts it more powerfully.
Between the two plates of glass standing near to each other, with their
lower edges in water, a similar rising pf water will occur; and if they are
closer at one perpendicular edge than at the other, the surface of the suspended water will be higher there. The two plates of glass in such a case
are found to be drawn towards each other by the interposed waters with a
certain force, as happens also to glass beads, or other small bodies, floating
in water with their surfaces so near to each other at the water's edge, that
the water may rise between them,-and the nearer they approach, the
higher the water rises, and the more strongly it attracts.
Water, ink, or oil, coming in contact with the edge of a book, is rapidly
absorbed far inwards among the leaves.
A piece of sponge or a lump of sugar touching water by its lowest corner,
soonsbecomnes moistened throughout.
The wick of a lamp lifts the oil to supply the flame, from  two or three
inches below it.
A mass of cotton thread hanging over the edge of a glass from the water
within it, will empty it as a syphon would. A towel will empty a basin
of water in the same way.
Dry wedges of wood driven into a groove formed round a pillar of stone,
on being moistened, will swell so as to rive off the portion from the block.
In some portions of Germany, mill-stones are thus cut from the rock.
An immense weight or mass suspended by a dry rope may be raised a
little way, by merely wetting the rope;-the moisture imbibed by capillary
attraction in the substance of the rope causes it to swell laterally, and to
become shorter.
At one time, the small vessels of vegetables were supposed to raise the
sap from the roots, by capillary attraction; but this is known now to be
chiefly an action of vegetable life.
Attraction has received the name of chemical attraction, or affinity, when
it unites the atoms of two or more distinct substances into one perfect
compound.
There are about fifty substances in nature which appear, in the present
state of science, distinct from each other, and are therefore called kinds of
matter; such as the various metals, sulphur, phosphorus, &c.; but whether
these are in truth, originally and essentially different, or only one simple




CAPILLARY  ATTRACTION.                           29
primordial matter, modified by circumstances as yet unknown to us, we
cannot at present positively determine. Diamond and pure black carbon
are the same substance only with different arrangement of atoms; and
steel, which in the soft state the graver cuts as it would copper or silver, is
exactly the same substance as when, after being tempered by heating and
sudden cooling, it has become as hard nearly as diamond itself. Yet these
differences are more striking than appear between some substances, which
we now account essentially distinct.
It is found, however, that the atoms of what we call different substances
will not cohere and unite indifferently, to form masses, as atoms of the same
kind do,-there being singular preferences and dislikes among them, if it
may be so expressed,'or affinities, as the chemists term it: and when atoms
of two kinds do combine, the resulting compound generally loses all resemblance to either of the elements.-Thus:
Sulphuric acid will unite with copper and form a beautiful transcendent
blue salt; with iron it will form a green salt; and if a piece of iron be thrown
into a solution of the copper salt, the acid will immediately let fall the copper, and take up or dissolve the iron.-Sulphuric acid will not unite with or
dissolve gold at all.-Quicksilver and sulphur unite in certain proportions
and form the paint called vermillion; in other proportions they form the
black mass called Ethiops Mineral —Lead, with oxygen absorbed from the
atmosphere or other source, forhLs what is called red lead, used by painters.
-Sea-sand, or flint and the substance called soda, when heated together,
unite and form that most useful substance called glass. —Certain proportions
of sulphur and of iron combine and produce those beautiful cubes of pyrites
or gold-like metal which are seen in slate. Chemical attraction operating
thus, does not, in the slightest degree, interfere with general attraction or
gravity, for every chemical compound weighs just as much as its elements
taken separately.
The history and classiication of such facts connected with the combinations and analysis of different substances, constitute the science of chemistry,
so attractive and so useful.  It explains how the fifty kinds of matter above
alluded to, by variously combining, form the endless diversity of bodies
which constitute, as far as it has yet been explored, the mass of our globe.
The reasons of these various modifications of attraction are yet much hidden
from us.
It is a remarkable truth, that when different substances combine in the
wjy now described, the proportions of the ingredients are always uniform,
and such as to lead to the conclusion, that for every atom present, of one
substance, there is exactly one, or two, or three, &c., of the other; so that,
if there be ten atoms of one substance, there are exactly ten, or twenty, &c.,
of the other, but never an intermediate number, as 13 or 23 to 10, for then
a particle of the compound would consist of one atom of the first, and of
one and three-tenths, or two and three-tenths, &c., of the second substance,
an absurdity if the atom be indivisible. For instance, a certain number of
atoms of quicksilver, which weigh twenty-five grains, combine with a certain
number of atoms of sulphur, weighing two grains, and form a black compound called Ethiops Mineral, or black sulphur of mercury; and if a little
more of either ingredients be added, it lies as a foreign mixture in the sulphuret of mercury; but if just as much more sulphur be added as at first,
so that there may be two atoms of it, instead of one, in every particle of
the compound, a perfect combination of the whole will take place, and a new
substance will appear, which we call vermilion. Many elementary substances




30              CONSTITUTION  OF  MASSES.
will only unite with each other in one proportion, so that any two such substances form only one compound; but others unite in several proportions,
so that several distinct compounds arise out of the same two elements.
It thus appears, that although we do not know the exact number of atoms
in a given quantity of any substance,-whether, for instance, a grain of
sulphuret of mercury has more or less than a million of them; still, as we
know that in that grain there are just as many atoms of sulphur as of mercury, and that the weight of thewwhole sulphur to that of the whole mercury
is as two to twenty-five, we know that the single atoms must have the same
relation, or that the atom  of mercury is 12] times as heavy as that of
sulphur.
Tables have been formed exhibiting the relative weights of the atoms of
different substances; and the number standing opposite to each substance
is called its chemical equivalent,-that is to say, the weight of its atom in
relation to the weight of the atom  of some other substance chosen as a
standard.  The equivalent of a compound substance depends, of course,
both on the equivalents of the ingredients, and on the number of atoms
existing in one integrant particle of the compound.
There is no such thing as an atom of vermilion, or of any other compound,
for the ultimate molecule or particle must contain at least one atom of the
respective ingredients.
The facts of the peculiarities and constalecy of chemical unions are among
the strongest arguments for the existence of similar ultimate atoms.
Besides the simple cases of attraction now explained, there are two curious
modifications, called electrical and magnetical attractions, which from their
peculiarities are reserved for consideration in a future division of this work.
a Atoms are more or less close, according to the quantity or REPULSION of
heat among them; hence the forms of solid, ftuid, air, &c."  (Read the
Analysis, p. 22.)
Were there in the universe only atoms and attraction, as hitherto explained, the whole material of creation would rush into close contact, forming
one huge solid mass of stillness and death.  But there is also heat or caloric,
which counteracts attraction, and singularly modifies the results.  It has
been described by some as a most subtle fluid, pervading all things, somewhat as water pervades a sponge: others have accounted it merely a vibration among the atoms.  The truth is, that we know little more of heat as
a cause of repulsion than of gravity as a cause of attraction; but we can
study and classify most accurately the phenomena of both.
When a continued addition of heat is made to any body,-it gradually
increases the mutual distance of the constituent atoms, or dilates the body.
A solid thus is first enlarged and soften'ed; then melted pr fused, that is to
say, reduced to the state of liquid, as the cohesive attraction is overcome;
and lastly, the atoms are repelled to still greater distances, so that the substance is converted into elastic fluid or air.  Abstraction of heat from such
air causes return of states in the reverse order.
Thus ice, when heated becomes water, and the water when farther heated
becomes steam; the steam when cooled again becomes water as before, and
the water when cooled becomes ice. Ice, water and steam, therefore, are
three forms or states of the same substance-one of the most common in
nature, being the material of the ocean.




LIQUID  AND  AIR.                           31
Other substances are similarly affected by heat, but as all have different
relations to it, some requiring much for liquefaction, and some very little,
we have that beautiful variety of solids, liquids and air, which constitutes'
our external nature.
Dilitation.-A rod of iron, which, when cold, will pass through a certain
opening, and will lie lengthwise between two fixed points, when heated, becomes too thick and to long to do either.-'For accurate mensuration, therefore, rods or chains used as the measure, must either be at a given temperature, or due allowance must be made for the difference.
The walls of a building, under the pressure of a heavy roof, had begun
to bulge out so as to threaten its stability. No force tried was sufficient to
restore them to perpendicularity, until the idea occurred of using the contracting force of cooling iron. The opposite walls were then connected by
a number of iron bars, passing through both, and having nuts to screw close
to the wall upon their projecting ends, of which bars one-half were heated
at a time, viz., every second or alternate bar, by lamps placed under them,
and while lengthened in consequence, and projecting farther beyond the
wall, their nuts were again screwed close up; so that on cooling and contracting, they pulled the walls in a degree back to its place. The nuts of
the second set ofbars being then screwed home, the others were again heated,
and advanced the object as much as the first; and so on, until the object
was accomplished.
The iron rim of a coach wheel, when heated, goes on loosely and easily,
but when afterwards cooled, it binds the wheel most tightly, giving remarkable firmness and strength.
Iron hoops on masts and casks, are made to bind in a similar manner.
The common thermometer for measuring degrees of heat, is a glass bulb,
filled with mercury or other fluid, and having a narrow tube rising from it,
into which the fluid, on being expanded by heat, ascends, and so marks the
degree. 
A bladder not quite full of cold air, on being heated, becomes tense, and
if weak, may even be burst.
Liquid and Air.-A piece of gold, lead, pitch, ice, sulphur1 or of other
thing, if sufficiently heated, melts or becomes liquid;. each substance, however, requiring a different degree of heat-gold requires 5,000 degrees, lead
600, ice, 32, and so forth; and if the heating be afterwards continued, most
things at certain higher temperatures suddenly expand again to many time
the liquid volume, and becomes aeriform fluids.
The conversion of water into steam is familiarly known to all. One pint
of water driven off as steam from the boiler of a low-pressure steam-engine,
fills a space of nearly 2,000 pints, and raises the piston through this, with
a force of many thousands of pounds: it immediately afterwards appears
again in the cold condenser as a pint of water.
Six times as much heat is required to convert a pint of water into steam,
as to raise it from an ordinary temperature to that of boiling but the steam,
by occupying nearly 2,000 times the space of the water, proves that heat
merely produces a revulsion among the particles, and by no means fills up
the interstices. The steam rising from boiling water does not appear to the
thermometer hotter than the water itself; and hence it was that Dr. Black,
whose genius shed so much light on this part of knowledge, gave the excess
of heat the name of latent heat.
The latent heat Qf common air is made sensible in the match syringe. In
this, which is close at the bottom, the piston is driven down quickly and




32              CONSTITUTION OF MASSES.
strongly, so as to compress very much the air which is underneath it, and
the heat then condensed with the air is sufficiently intense to light a small
piece of tinder attached to the. bottom of the piston.
Not only are spirits, oethers, oils, &c., convertible, as water is into aeriform
fluid, but also sulphur, phosphorus, mercury, and, indeed, all the metals and
elementary substances;-some of them, however, requiring heats of great
intensity.
The varieties of form, then, in the bodies on the face of this earth, may
be considered accidental, as dependent on the temperature of the earth, and
do not mark the permanent nature ef the substances.
In the planet Mercury, which is near the sun, resin, tallow, wax and many
vegetable substances deemed by us naturally solid, would all be liquid, as oil
is with us; and a certain mixture of tin, zinc and lead, which with us is solid
at common temperatures, but melts in boiling water, would there be always
liquid like our quicksilver.  Our water, oils, and spirits, would there be in a
state of steam or air, and could not be known as liquids, except by cooling
processes and compression, such as we have lately learned to use for reducing
our different airs to the form of liquids.
Again, in the cold planet Herschbel, which is nineteen times farther from
the sun than our earth is, water, if it exist, can be known only as rock crystal, which fire would have to melt as it does glass with us: our oils would
be as butter or resins, and quicksilver might be hammered as. lead or silver
is with us.
On our own earth, near the equator, common sealing-wax will not retain
impressions; butter is oil in the day, and a soft solid at night; and tallow
candles cannot be used. And near our pole, in winter, the quicksilver from
a broken thermometer is solid metal; water must be melted by fire for use;
oils are solid, &c.
To judge, then, of the constitution of nature aright, we must always take
extended surveys, and not allow prejudice to.mislead us, as it did that Eastern
potentate, who put a traveller to death for saying he had visited remote
northern countries, where water was sometimes to be seen solid like crystal,
and' sometimes white and fleecy, like feathers.-The ancients believed that
there were Just four elements concerned in forming our globe, with all upon
it, viz., earth, water, air and fire. What a contrast between former and
present knowledge..
Repulsion without sensible Heat.
As we stated in a former paragraph that besides general. attraction, under
names gravitation, cohesion, capillary and chemical attraction, there are
mnodifications which have the names of electrical and magnetical attractions;
so we have now to remark, that, besides the general repulsion of heat just
dekribed, there are peculiarities which we call electrical and magnetical repiulsions.  Whether these depend altogether on different causes, or are only
modifications of effect from the same cause, we cannot yet positively decide.
And it is-a curious fact connected with the subject, that there seems to be,a film of repulsion, so to express it, covering the general surfaces of all bodies,
and preventing their meeting in absolute contact, even when they appear to
the human eye so to meet. Were it not for this, things would be constantly
approaching so closely to each other, that they would. stick or cohere, in a
way to disturb the common operations of nature. The following facts illustrate this superficial repulsien, and the means which art uses to overcome it
for particular purposes.




REPULSION OF SURFACES.                            33
Newton found that a ball of glass, or a watch-glass, laid upon a flat surface
of glass does not really touch it and cannot be made to touch it by a force
of even 1,000 pounds to the inch.
In like manner, when glass, stone, porcelain, or indeed almost any body
is broken, we cannot make the parts cohere again by simply pushing them
together in their former position. Where a union, therefore, between separate masses is desired, we are compelled to have recourse to various artifices
A few cases in which cohesion is easily affected, were enumerated at page
27: the following are other instances of a different kind.
Gold leaf laid upon clean steel, and then forciby struck by a hammer,
coheres to the steel, and gilds it permanently.
But iron can be made to cohere to iron, only by rendering both pieces red
hot before hammering: —the process is called welding. Iron and platinum
are the only metals that can be welded.
Tin and lead, in sheets, pressed together between the strong rollers of a
flatting-mill, cohere.
The other metals require to be melted before the superficial repulsion
gives way so as to allow separate quantities to cohere or run into one mass.
It is thus, for instance, that gold, silver, lead, &c., are treated.
In many cases the substances are not such as can be melted, (wood or
marble, for instance,) and then it is necessaryto use some soft glue or cement.
Cements must have strong attraction for both substances, and, when dry or
cool, must be tenacious in themselves; solder, paste, common glue, motar,
&c., are the principal substances of this kind.
Certain m.odifications of attraction produce the subordinate states, called
crystal, porous, dense, &c."  (Read the Analysis, page 22.)
It is a remarkable circumstance, that attraction, in causing the atoms to
cohere so as to form solid masses, seems not to act equally all around each
atom, but between certain sides or parts of one, and corresponding parts of
the adjoining one; so that when atoms are allowed to cohere according to
their natural tendencies, they always assume a certain regular arrangement
and form, which we call crystalline. Because in this circumstance they
seem to resemble magnets, which attract each other only by their poles, the
fact has been called the polarity of atoms. It is the cause of several of the
peculiarities above enumerated, as elasticity, &c.
"' Crystallization" is exemplified in the following particulars:
Water beginning to freeze, shoots delicate needles across the surface;
these thicken and interweave until the whole mass has become solid, but
the crystalline arrangement always remains. In most substances, this
arrangement is remarkably proved, by the forms of the surfaces left, when
the mass is broken.
Moisture, freezing on the window-pane in winter, exhibits a beautiful
variety of arborescence.
A flake of snow viewed in the microscope, is seen to be as symmetrically
formed as a fern-leaf or a swan's feather.
If a piece of copper be thrown into a solution of silver in nitric acid, it is
preferred by the acid to silver, and is dissolved accordingly: the silver in
the mean time, during its precipitation or separation, assumes the form of
a singularly beautiful shrub or tree, resting on the remaining copper as its
root. This appearance is call the arbor Diana.
3




34               CONSTITUTION  OF  MASSES.
Any metal which has been melted, when allowed to cool again, slowly
and at rest, becomes solid first on the outside of the mass.  If, before the
cooling be completed, the remaining liquid be poured from within, a curious
internal crystalline structure, like grotto work, is seen. What is called the
grain of a metal is the result of this crystallization.
Saltpetre, glaubler salt, copperas (to use popular names,) or any other of
the many neutral salts, being dissolved in water, and the water being then
allowed slowly to evaporate, reappears in beautiful regular crystals, each
salt having its peculiar forms, bounded by perfectly plane and polished
surfaces. If any such crystal be broken in any part, the broken surface
appears to the microscope as if regular layers of particles had been disturbed,
(as we see on a larger scale in a broken stack of bricks, or broken pile of
shot in a battery yard,) and the defect of the crystal will be exactly filled
up by replacing it in the evaporating solution-proving that the ultimate
particles are all of the same size.
All the precious stones are crystals, and can be well cut only parallel to
their natural surfaces.
The basaltic pillars of the Giant's Causeway in Ireland, and of the Isle
of Staffa, which appears like a garden supported on magnificent columns in
the midst of the ocean, are natural crystalline arrangements of particles,
equalling in regularity and beauty any human work, and in granduer so far
surpassing even the Egyptian pyramaids, that superstitious conjecture
naturally supposed them the work of giant architects.
It would be endless to go on enumerating crystalline masses, for nature's
forms generally, in the inanimate creation, as well as in organized bodies,
are regular and symmetrical; and what we see on earth of broken continents, and islands, and rocks, and wild Alpine scenery, are the effects of
subsequent convulsions, which have deranged a primitive and natural order.
Much ingenuity has been employed to account for the specific forms
which different crystalline bodies assume; but the subject is not yet reduced
to a state fitting it to be a part of this elementary study.  A familiarity
with the various figures which the exact science of measures treats of, is
required in the person who expects to pursue it with pleasure or advantage.
The facts are extremely curious, and the scientific investigation of them
may ultimately give important information respecting the intimate constitution of material nature.
" Porous."-The crossing of the constituent crystalline needles or plates
in bodies, causes them to be porous or full of small vacant spaces. In some
cases these are visible to the eye, in many more cases, they are visible to the
microscope, and in all, they are to be proved in some way.
Owing to the porosity arising from the new arrangement of atoms of
solidifying, water and a very few other substances become more bulky in
the change from  the liquid  to the solid state.  Water then dilates with
such force as to burst the strongest vessels which art can provide, and in
winter to split even rocks, where it has been retained in their crevices;freezing water thus curiously producing effects which surpass those of
exploding gunpowder. This agency of water contributes to the gradual
breaking down of our Alpine summits, and the falling of their destructive
fragments into the valleys.
The stone called hydrophane (agate) is opaque, until dipped into water,
when it absorbs into its pores one-sixth of its weight of the water, and afterwards gives passage to light.
Into crystallized sugar, and various stones, much water will enter without
increasing the bulk.




DENSITY.                               35
A kind of sandstone, suitably shaped, forms an excellent filter or strainer
for water.
Pressure will force water through the pores of the most solid gold:-as
was seen in the famous Florentine experiment, where a hollow, thick, golden
ball, being filled with water and squeezed, to try the compressibility of water,
was found to perspire all over.
The examples of porosity in animal and vegetable bodies, are, however, the
most remarkable.
Bone is a tissue of cells and partitions, as little solid as a heap of empty
packing-boxes.
Wood is a congeries of parallel tubes, like bundles of organ pipes.  It
has lately been proposed to prepare wood for certain purposes, as for making
the great wooden pins or nails used in ship-building, by squeezing it to half
its lateral bulk between very strong rollers, and thus making its density
approach to that of metal.
A piece of wood sunk to a great depth in the ocean, and exposed to the
pressure there, has its pores soon filled with water, and becomes nearly as
heavy as stone. Thus it was with the boat of a whale-fishing ship, which had
been dragged far under water by a whale, and which, on being afterwards
drawn up, was supposed by the crew to be bringing a piece of rock with it.
A piece of cork in a strong, close glass vessel, nearly full of water, may
be seen floating at the top; but if more water be then forcibly pumped into
the vessel, the cork will be squeezed and reduced in size, until at last it becomes heavier than water, and sinks.   On water being afterwards allowed
to escape, the cork will resume its bulk and will rise.  A cork sunk 200
feet under water will never rise again of itself.
A bottle of fresh water, corked and let down thirty or forty feet into the
sea, often comes up again with the water saltish, although the cork be still
in its place: the explanation being, that the cork, when far down, is so
squeezed as to allow the water to pass in or out by its sides, but on rising,
resumes its former size.
" Density," or the quantity of atoms which exist in a given space, is very
different in different substances.
A cubic inch of lead is forty times heavier than the same bulk of cork
Mercury is nearly fourteen times heavier than an equal bulk of water.
The density must depend on, first, the size or weight of the individual
atoms; secondly, the degree of porosity just now explained; and thirdly, the
proximity of the atoms in the more solid parts which stand between the pores.
From many circumstances it appears, that the atoms even of the most
solid bodies are nowhere in actual contact, but are retained in their places
by a balance between attraction and repulsion-thus,
A body dilates or contracts, according as heat is added or taken away
from it.
A weight placed on any upright rod or pillar, shortens it and lessens its
bulk, and if suspended from the bottom, lengthens it and increases its bulk,
-the rod in both cases returning to its former dimensions when the weight
is removed.
When a plank or rod is bent, the atoms on the concave side are, for the
time, approximated, and those on the convex side are drawn more apart.
It is remarkable in solid bodies, not only how precisely the balance between




36              CONSTITUTION OF MASSES.
attraction and repulsion determines the relative position of the particles, but
also how strongly; for any farther separation of the particles is resisted by
all the force which we call the tenacity or cohesion of the substance, and
any nearer approach by all the force which we call the hardness or incompressibility.
Tin and copper, when melted together, to form bronze, occupy less space
by one-fifteenth than when separate: proving that the atoms of the one are
partially received into what were vacant spaces in the other. A similar condensation is observed in many other mixtures.  A pound of water and a
pound of salt, when mixed, form two pounds of brine, but which has much
less bulk than the ingredients apart. So also of a pound of sugar dissolved
in a pound of water.
Water and liquids generally resist compression very powerfully, but yield
enough to show that the particles are not in contact. It is found that at
1,000 fathoms down in the sea the water is compressed by the superincumbent water so as to have bulk about a hundredth part less than it would have
at the surface.
In aeriform masses the atoms are very distant, and hence the masses are
more easily compressed. A pint of water, on assuming the aeriform state,
in which it is called steam, under ordinary pressure, acquires nearly 2,000
times its former bulk. A hundred pints of common air may be compressed
into a pint vessel, as in the chamber of an air-gun; and if the pressure be
much farther increased, the atoms will at last collapse and form a liquid.
The heat which was contained in such air, and gave it its form, is squeezed
out in this operation, and becomes sensible all around.
From these proofs of the non-contact of the atoms, even in the most solid
parts of bodies; from the very great space obviously occupied by poresthe mass often having no more solidity than a heap of empty boxes, of which
the apparently solid parts may still be as porous in a second degree, and so
on; and from the great readiness with which light passes in all directions
through dense bodies like glass, rock crystal, diamond, &c., it has been
argued that there is so exceedingly little of really solid matter, even in the
densest mass, that the whole world, if the atoms could be brought into
absolute contact, might be received into a nut-shell.  We have as yet no
means of determining exactly what relation this idea has to truth.
The comparative weights of egual bulks of different bodies are called their
specific gravities.
In thus comparing bodies, it was necessary to choose a standard; and
water, as being the substance most easily procurable at all times and in all
places, has been generally adopted.
The metal called platinum, the heaviest of known substances, is about
twenty-two times as heavy as an equal bulk of water, and is therefore said
to have specific gravity of 22 —gold is nineteen times as heavy- mercury
thirteen and a half-lead eleven-iron eight and a half-copper eight —common stones about two and a half-woods from half to one and a half-cork
one-quarter, &c.
"Hardness," is not proportioned, as might be expected, to the density of the
different bodies, but to the polarity of the atoms in them, that is, to the
force with which the atoms hold their places in some particular arrangement.
Hardness is measured generally by the circumstance of one body being




DENSITY.                               37
capable of scratching another.  It is here worthy of notice, however, that
the powder or dust of a softer body will often, through an effect of motion
to be described below, aid in wearing down or polishing one that is harder.
Gold, though soft, is four times heavier than the hard diamond; and
mercury, which is fluid, is nearly twice as dense as the hardest steel.
Diamond is the hardest of known substances. It cuts or scratches every
other body, and is generally polished by means of its own dust.
Glass-cutters use a point of diamond as a glass-knife, for dividing and
shaping their panes.
Common flint also cuts glass, as is proved by the frequent scribblings on
windows.
It is remarkable, that the preparation of iron, called steel, may either be
soft like pure iron, or from being heated and suddenly cooled, in the process
called tempering, may become nearly as hard as diamond. The discovery
of this fact is, perhaps, second in importance to few discoveries which man
has made; for it has given him all the edge tools and cutting instruments
by which he now moulds every other substance to his wishes. A savage
will work for twelve months, with fire and sharp stones, to fell a great tree,
and to give it the shape of a canoe; where a modern carpenter, with his
tools, could accomplish the object in a day or two.
The project has lately been realized, of engraving on plates of soft steel,
instead of copper, and afterwards tempering the steel to such hardness, that
it may be used as a type or die to make its impression, not on paper, but on
other plates of soft steel or of copper; each of which then is equal in value
to an original and distinct engraving. By this means the beautiful productions of art, instead of being limited to a comparatively small number of
copies and of persons, may be multiplied almost to infinity, becoming the
cheap delight of all.
" Elasticity" is present in a mass when the atoms, cohering in a particular
arrangement only, yield, however, to a certain extent, when force is applied,
but move back or regain their natural positions on the forde being withdrawn.
Elastic bodies vary much as to the extent to which they yield without
breaking, and as to the degree of perfection with which, after the bending,
or displacement of atoms, they regain their former state.  India rubber is
extensively elastic, for it yields far; but it is not perfectly elastic, for when
stretched much or often, it becomes perfectly elongated.  Glass, again, is
perfectly elastic, for it will retain no permanent bend; but, unless in very
thin plates indeed, or in fine threads, it will not bend far without breaking.
All hard bodies are elastic, as steel, glass, ivory, &c., and many soft ones,
as caoutchouc, silk, a harp string, &c.  The aeriform  bodies are all perfectly elastic, as is rudely seen in a bladder filled with air, when squeezed,
and allowed to expand again; and they will change volume to a very great
extent. Liquids also are perfectly elastic, but to a small extent.
A good steel sword may be bent until its ends meet, and yet, when
allowed, will return to perfect straightness.
A rod of bad steel, or of other metal, will be broken in bending, or will
retain a bend.
An ivory ball, let fall on a marble slab, rebounds, owing to the great
elasticity of both bodies, nearly to the height from which it fell, and no
mark is left on either.  If the slab be wet, it is seen that the ivory or mar



38              CONSTITUTION OF MASSES.
ble, or both, had yielded considerably at the point of contact, for a circular
surface of some extent on the slab is found dried by the blow. The sudden
expulsion of air from between the meeting surfaces might contribute to the
effect, but the result is very nearly the same when the experiment is made
in a vacuum. Billiard balls scarcely lose even their polish by long wear,
although the touching parts yield at every stroke.
A marble chimney-piece, long supported by its ends, is found at last to
be bent downwards in the middle; and the bend is permanent.
A steel watch-spring, although so much and so constantly bent, resumes
its original form when freed at the end of a century; but occasionally,
without evident cause, while in action, it will suddenly give way.
Elasticity is a property of bodies of great utility to man, as in his timepieces, carriage-springs, gun-locks, &c., &c.
" Br ittleness," designates that constitution of a body where, with hardness,
and elasticity perfect as far as it goes, the cohesion among the atoms exists
within such narrow limits that a very slight change of position or increase
of distance among them is sufficient to produce a rupture.  A comparatively slight force, therefore, if sudden, breaks them. It belongs to most
very hard bodies.
Glass scratches an iron hammer, proving that it is harder than iron-yet
glass is the very type of fragility; yielding to the stroke of soft wood, or,
indeed, of almost any thing which can give a blow.
Steel, when tempered so as to be very hard, becomes brittle also. The
steel chisels and tools with which artificers now shape the stones and metals
as they formerly did wood, require, of course, to be exceedingly hard; but
they thereby lose in regard to the extent of their elasticity, and hence are
frequently broken. Cast iron, which is much harder than malleable or
wrought iron, is very brittle, while soft iron and steel are the toughest things
in nature.
"Malleable," or reducible into thin plates or leaves by hammering. This
property, in opposition to elasticity and brittleness, belongs to bodies
whose atoms cohere equally in whatever relative situations they happen
to be, and therefore yield to force, and shift about among each other, without fracture or change of property, almost like the atoms of a fluid.
Gold is remarkably malleable, for it may be reduced to leaves of the thinness of 360,000 to the inch, or of 1,800 to a sheet of common paper. For
gold-beaters the metal is first formed into rods, these are afterwards rolled or
battened into ribbons; the ribbon is cut into portions, which are extended,
by hammering, to great breadth and thinness, and which being again divided into portions, are hammered and extended to the thinness described.
Silver, copper and tin may also be hammered until very thin. Most other
metals crack or are torn before the operation is carried far; and some, on
being struck, are broken at once, almost like glass.
"Ductile," or susceptible of being drawn into wire. One might expect
malleability and ductility to belong to the same substances and in the
same degrees-but they do not. In ductile substances, as in malleable,
the atoms seem to have no more fixed relation of position than in a liquid,
but yet they cohere very strongly.
One end of a rod of iron, or other ductile metal, being reduced in size so




DENSITY.                               39
as to pass through an opening in a plate of steel, is seized by strong nippers
on the other side of the plate, and the whole rod is drawn through. It is
thus reduced, of course, to the size of the opening, and is lengthened in a
like proportion. By repeating the operation through smaller holes successively, a wire may at last be obtained to the size of a hair.
Dr. Wollaston's ingenuity produced platinum  wire finer than spider's
thread. He filled a space in the axis of a silver wire with small platinum
wire. He then drew or reduced the compound piece to the smallest wire
possible, and on dissolving the silver from the outside, he exposed to view
the delicate filament of platinum.
The order in which metals may be ranged according to their ductility is,
platinum, silver, iron, copper, gold, &c.
Melted glass has great ductility. The workers draw or spin it into threads
by merely attaching a point, pulled out from the mass, to the circumference
of a turning-wheel.  A uniform thread then continues to be drawn out and
wound upon the wheel, at a rate of 1,000 yards or more per hour. This
glass thread, when lying together in quantities, resembles beautiful white
hair, and when cut in bunches, it serves as an ornament to the female head,
waving in the air like the delicate plume of a bird of paradise.
"cPliant."  In bodies distinguished by this title, the cohesion is not destroyed by considerable change of direction among the particles, but there
is little elasticity, and unlike what happens in a ductile mass, the same
atoms always remain together.
Of all pliant things, the chief are animal and vegetable fibres and membranes-as silk, bladder, lint, hemp, &c., &c.
a Tenacity" means the force of cohesion among the atoms of any mass.
It belongs more or less to all solids, and even to liquids.
This property varies much in different substances. Iron and its modification called steel possess it in the most remarkable degree.
The following table shows the comparative tenacity, or strength to resist
pulling of certain metals and woods.  Supposing similar wires or rods of
each to be used, and of such a size that the surface of a broken end or crosssection would be the one-thousandth of a square inch, the weights supported
would be nearly as follows:
METALS.
Cast Steel.            134 lbs.
Best wrought iron.   70
Cast Iron...   19
Copper...  19
Platinum...   16
Silver...   11
Gold...    9
Tin...    5
Lead..    2
WOODS.
Teak...   13
Oak...   12
Beech...   121
Ash...   14
Deal...   11




40             CONSTITUTION OF MASSES.
Iron compared in this way, is five or six times stronger than oak.
Steel wire will support about 39,000 feet, that is, 71 miles of its own
length.
Certain animal substances have great tenacity; as-the silk-worm's thread,
which is our strongest connecting or sewing material, and has such flexibility
united with its strength —the ligaments and tendons of the animal body, possessing at once such admirable strength, elasticity and pliancy: these when
dried, and otherwise prepared, constituted the tough bow-strings of our remote forefathers —the hair or wool of animals twisted into threads, and
worked into strong and beautiful textures of the loon —strips of animal intestines prepared and twisted, forming the cords of harp and violin, and in
strength and uniformity rivaling the steel wires of keyed instruments.
The gradual discovery of substances possessed of strong tenacity and which
man could yet easily mould to his purposes, has been of great importance to
his progress in the arts of life. The place of the hempen cordage of European navies is still held in China by twisted canes and strips of bamboo;
and even the hempen cable of Europe, so great an improvement on former
usage is now rapidly giving way to the more complete and commodious security of the iron chain-of which the material to our remote ancestors existed
only as a useless stone or earth. And what a magnificent spectacle is it, at
the present day, to behold chains of tough iron stretched high across a channel of the ocean, as at the Menai Strait, between Anglesea and England,
and supporting there an admirable bridge-road of safety along which crowded
processions may pour, regardless of the deep below, or of the storm; while
under it, ships with full sails spread pursue their course, unmolesting and
unmolested t




APPENDIX
TO PART I.-SECTION I.
BY THE AMERICAN EDITOR.
IF the reader has studied the preceding section with attention he is prepared
to understand the following propositions.
Prop. 1-Matter is endowed with properties.
Prop. 2.-The properties of matter are distinguishable into two classes,
first, those which are general or belong to all kinds of matter, and second,
those which are peculiar or belong only to particular kinds of matter.
Prop. 3.-The general properties of matter are, indestructibility (p. 22;)
extension or the property of occupying a portion of space (p. 24; ) divisibility ( p. 23; ) impenetrability (p. 24; ) and inertia, (p. 42. )
Prop. 4.-Every particle of matter, and also all masses, have a mutual
attraction for one another, or endeavor to get near each other; and this
attraction is inversely as the squares of the distances.
Attractions may be primarily distributed into two classes: one consisting
of those which exist between the molecules or constituent parts of bodies,
and the other between the bodies themselves. The former are called molecular or atomic attractions, the latter gravitation (p. 26:) of the former
there are several varieties, 1st, cohesion (p. 27;) when this variety of
molecular attraction is exhibited by liquids pervading the interstices of porous
bodies, ascending in crevices or in the pores of small tubes, it is called capilliary attraction (p. 28.) The other varieties of molecular attractions
are affinity or chemical attraction (p. 28, ) and electric and magnetic attraction, (p. 30. )
Prop. 5.-Attraction of gravitation, or that force by which all the masses
of matter tend towards each other, is exerted at all distances.
Prop. 6.-Attraction of cohesion acts only within certain limits, and where
its sphere of attraction ends, a repulsive force begins.
Prop. 7.-Repulsion, except when dependent on electricity or magnetism,
is owing to the presence of heat, which latter pervades all matter.
Priop. 8.-The particles of matter are more or less close, according to the
quantity of heat among them; but they are never in actual contact (p. 3031,) and hence porosity is usually considered as one of the properties of
matter.
Prop. 9. —Thepeculiar properties of matter are density (p. 35, ) hardness
(p. 36, ) elasticity (p. 37, ) brittleness (p. 38, ) malleability (p. 38, ) ductility (p. 38, ) pliability (p. 39, ) tenacity, (p. 39, ) &c.




42                 MOTIONS  AND  FORCES.
SECTION II.-THE MOTION OR PHENOMENA OF THE
UNIVERSE.*
ANALYSIS OF THE SECTION.
The bodies or masses composing the universe may be at rest or in motion,
and to change any present state, force proportioned to the quantity of
the body and to the degree of change, is equally requised, whether to yive
motion, to take it away, or to bend t: —a truth expressed by saying that
matter has INERTIA, or figuratively, a stubbornness.  Unifbrm straight
motion, then, is as naturally permanent as rest.  And the motion in any
body, measured by its velocity, quantity of matter and direction, is the
measure of the amount and direction of any single force or of any combiination of forces, which has produced it, as also of the force or momentum which the body can exhibit again when opposed or made to act itself
as a cause of some new motion.
The great forces of nature, referred to by the two words ATTRACTION and
REPULSION, acting upon INERT matter, produce the equable, accelerated,
retarded and bent motions which constitute the great phenomena of the
universe. — Tides, currents, winds, falling bodies, &c., exemplify attraction. —Explosion, steam collision, &c., exemplify repulsion.  And as in
every case of attraction or repulsion two masses at least must be concerned, there is no motion or action in the universe, without an equal and
opposite motion or re-action.;" Motion"
Is the term applied to the phenomenon of the changing of place among
bodies.
Were there no motion in the universe it would be dead. It would be
without the rising or setting sun, or river flow, or moving winds, or sound,
or light, or animal existence.
To understand the nature and laws of the motions or changes which are
going on around him, is to man of the greatest importance, as it enables him
to adapt his actions to what is coming in futurity, and often to interfere so
as to control futurity for his special purposes.
Motion, in any particular case, is described by referring to certain objects
to mark place, and to some other motion chosen as the standard of velocity.
-A man sitting on the deck of a sailing ship, has common motion with
the ship: if walking on the deck, he has relative motion to the ship: but
if he be walking towards the stern, just as fast as the ship advances, he
is at rest relatively to the bottom or shore. A ship sailing against the tide,
just as fast as the tide runs, is as much at rest relatively both to the earth
and water as if she were at anchor. Absolute motion is that which is relative to the whole universe, or rather to the space in which the universe exists. We have no means of ascertaining such: for although we know how
fast our globe whirls upon its axis and wheels round the sun, we have no
measure of the motion of the sun himself-revolving possibly round some
* The reader should here re-peruse the title and Analysis at page 22.




MOTION.                               43
more distant centre, but almost certainly having a progress in space, and
carrying all the planets along with him.
Mlotion is called rapidl, as that of lightning-slow, as that of the sun-dial
shadow; both terms have reference to the ordinary intermediate velocities
observed upon earth.  It is called straight or rectilincal, in the apparent
path of a failing body —bent, or curvilinear, in the track of a body thrown
obliquely-accelerated, in a stone falling -to the earth-retarded, in a stone
thrown upwards while rising to the point where it stops before again
descending.
"Owing to the INERTIA of bodies, force is equally required to impart motion
and to take it away." (Read again the last Analysis.)
If a man put his hand to the crank of a heavy fly-wheel or grindstone, to
turn it, he experiences a certain resistance, which, however, gradually yields
to his effort, and he leaves the wheel whirling with velocity proportioned to
the effort.  If he then puts out his hand again to stop the wheel, he experiences an opposite but similar resistance, which however, as before, gradually
yields, and he brings the wheel to rest.  In the second case the effort required of him is less than the first, by reason of the friction of the turning
axle, and the resistance of the air in which the wheel moves, —obstructions
which, when he was giving motion, opposed him, but when taking it away
assisted him. That these obstructions caused the whole difference in such a
case, and that they are the great reasons why all ordinary motions on earth
seem to tend of themselves to cease, will be shown in subsequent pages. It
is the resistance overcome in moving the wheel or in stopping it, and occasioning an expenditure of force proportioned to the mass and to the degree
of change of state, which is called the INERTIA of the mass, or the vis inertire, and sometimes, to help the conception of the student, the stubbornness,
sluggishness, or inactivity; but none of these words can originally suggest
to the mind all that is intended to be conveyed.
An exact measure of the amount of inertia is contained in the familiar fact
that a body let fall near the surface of the earth, falls rather more than 16
feet in the first second of time,-the well-known weight of the body, or
force of terrestrial attraction acting upon it for one second, being just sufficient to overcome its inertia to the extent stated. Were the inertia of matter
only half of what it is, a body near the earth would fall 32 feet in the second,
instead of 16, as it equally would, if, with present inertia, the attraction of
the earth were doubled. And were there no inertia, it would fall or pass
through any height, however great, in one instant. As the amount of inertia
thus determines the amount of other force required to give motion to a
mass, so does it determine the amount of force required to destroy motion
in a mass. A heavy cannon-ball, if wanting inertia, might be dispatched
with the speed of lightning by the slightest force, but then the stiffness of a
stalk of corn would suffice to arrest it; and while the ball, with the inertia
now existing, takes the force of pounds of gunpowder to give it its usual
motion, it may not be stopped, even by the cohesion of a block of granite,
which accordingly it shivers to pieces. The numerous examples now to
follow will prove the immense importance of inertia in the general operations of nature.
When the sails of a ship are first spread to receive the force or impulse of
the wind, the vessel does not acquire her full speed at once, but slowly, as
the continuing force gradually overcomes the inertia of her mass. When the




44                 MOTIONS AND  FORCES.
sails are afterwards taken in, she does not loose her motion at once, but
slowly again, as the continued resisting force of the water destroys it.
Horses must make a greater effort at first to put a carriage in motion than
to maintain the motion afterwards.  And a strong effort is required to stop
a moving carriage. When a carriage, of which the body hangs from springs,
is first moved, the body appears to fall back, and a person within seems to
be suddenly forced against the back cushion. When the carriage is stopped
again the body swings forward, and if the stoppage be very sudden, a careless passenger may unwittingly pop his head through a front glass. These
particulars prove the inertia, first of rest, and secondly of motion.
A man standing carelessly at the stern of a boat, when the boat begins to
move, falls into the water behind; because his feet are pulled forward while
the inertia of his body keeps it where it was, and therefore behind its support. The stopping of a boat, again, illustrates the opposite inertia of motion,
by the man's falling forward.
An awkward rider on horsback may be left behind, when his horse starts
forward suddenly: or may be thrown off on one side by the horse starting
to the other. A horse at speed, stopping suddenly, often sends his cavalier
over his ears-as was mortifyingly experienced by a coxcomb, who, on an old
cavalry horse, chose to canter along a foot-path, to the annoyance of the
company, and whose horse on hearing the word halt loudly addressed to it
by a waggish officer of the regiment, who happened to be there and to recognize it, suddenly stood and got rid of its load. The mind or will of the beau
had sinned against the law of propriety, but his body very perfectly obeyed
the laws of inertia and gravity, by shooting forward in a parabolic curve to
the earth.
A young man not yet accustomed to the whip, drove his phaeton against
a heavy coach on the road, and then to his father foolishly excused his awkwardness, in a way which led to the prosecution of the coachman for furious
driving. At the trial, the youth and the servant both deposed that the
shock of the coach was such as to throw them over their horses' heads, and
thus lost the case, by unconsciously proving, that the faulty velocity was
their own.
A man jumping from a carriage at speed is in great danger of falling forward, when his feet reach the ground; for his body has as much forward
velocity as if he bad been running with the speed of the carriage and unless
he advance his feet like a running man, to support his advancing body, he
must as certainly be dashed to the ground, as a runner whose feet are suddenly arrested. A man racing who receives a signal to stop, and a man
jumping from a flying vehicle, must check their motion nearly in the same
way.
A person wishing to leap over-a ditch or chasm, first makes a run, that
the motion thereby acquired may help him over. A standing leap falls much
short of a running one.
An African traveller saw himself pursued by a tiger, from which he could
not escape by running; but perceiving that the animal was watching an
opportunity to seize him by its usual spring or leap, he artfully led it to
where the plain terminated in a precipice hidden by brush-wood, and he had
just time to transfer his hat and cloak to a bush, and to retreat a few paces
when the tiger sprung upon the bush, and by the moral inertia of its body,
was carried over the precipice and destroyed.
From a glass of water suddenly pushed forward on the table, the water is
spilt or left behind; but if the glass be already in motion, as when carried




MOTION.                                45
by a person walking, and if it then be suddenly stopped by coming against
an impediment, the water is thrown or spilt forward.
A servant carrying a tray of glasses or china in the dark, and coming suddenly against an obstacle, hears all his freight slipping forward and crashing
at his feet: and a too hurried departure with such a load causes equal destruction, on the opposite side.
The actions of beating a coat or carpet with a cane, to expel the dust; of
shaking the snow from one's shoes, by kicking against a door-post; of cleaning a dusty book by knocking it against a table, or shutting it violently-all
illustrate the same principle.
If a guinea be laid on a card which is already balanced on the point of the
finger, a small fillip or blow to the edge of the card will cause it to dart off,
but the guinea, owing to its inertia, will remain resting on the finger, —its
inertia being greater than the friction on it of the card passing from underneath it.
When we desire a person, with suspected disease of the brain, to shake
his head and tell whether and where he feels pain, we are doing nearly as
if we touched the naked brain with the finger to find the tender part; for
the inertia of the brain, when the skull is moved, causes a momentary pressure between it and the skull, almost equivalent, for the purpose desired, to
such a touch.
This kind of pressure is sufficient to break and destroy tender wares-as
glass or eggs-in packages which are too suddenly moved or stopped.
A weight suspended by a spring on ship-board is seen vibrating up and
down as the ship pitches with the waves. It seems to fall as the ship rises,
and to rise as the ship falls: but the motion is really in the ship, and the
comparative rest is in the weight. A heavy weight so supported, and connected with a pump-rod, would work the pump.
Like the weight last mentioned, the mercury of a common barometer on
ship-board is seen rising and falling in the tube; and until the important
improvement was lately made, of narrowing one part of the tube to prevent
this, the mercurial Barometer was useless at sea. The explanation is, that
the tube rises and falls with the ship, from being connected with it; but the
mercury, which plays freely in the tube, and is supported by the atmospheric
pressure, tends, by its inertia, to remain at rest, and thus makes the motion
of the ship apparent.
What happens to the mercury in the barometer-tube on ship-board, indicates what happens to the blood in the vessels of animals under similar circumstances. In any long vein below the heart, when the body falls, the
blood, by its inertia and the supporting action of the vessels, does not fall so
fast, and therefore really rises in the vein: and as there are valves in the
veins preventing return, the circulation is thus quickened without any muscular exhaustion on the part of the individual. This helps to explain the
effect of the movement of carriages, of vessels at sea, of swings, &c., and of
passive exercise generally, on the circulation, and leaves it less a mystery
why these means are often so useful in certain states of weak health.
If a cannon ball were to break to pieces in its flight, its parts would still
advance with the previous velocity. And thus, in the deadly contrivance of
the Shrapnell-shell, which is in a case containing hundreds of musket bullets, when these are scattered at the desired distance from the devoted body
of men, they retain the forward velocity of the shell, and spread death
around like the near discharge of a whole battalion of musketry.
On the awful occasion of a ship in rapid motion being suddenly arrested




46                  MOTIONS  AND  FORCES.
by a sunken rock, all things on board, men, guns, and furniture, start from
their places and dash forwards; while the onward inertia or moral obstinacy
of the hinder parts of the ship, suffices to crush her bow against  the rock.
".Motion as naturally permanent as rest."
From the instances now given, it is seen that a body at rest would never
move if force were not applied, and that a body put in motion retains motion, at least for a time, after the force has ceased; but there is a feeling
from common experience, that motion is an unnatural or forced state of
bodies, and that all moving things, if left to themselves, would gradually
come to rest.  It is recollected that a stone projected comes to rest, or a
wheel left moving, or a bowl rolled on the green, or the waves heaving
after a storm-and in a word, that there is no perpetual motion on the earth.
On more attentive consideration, however, it may be perceived that there
are prodigious differences in the duration of motions, and that the differences
are always exactly proportioned to evident causes of retardation, and chiefly
to friction and the resistance of the air.
Friction is the resistance which bodies experience when rubbing or sliding
upon each other; and however much it may be diminished by art, it can in
no case be annihilated. Air-resistance, again, to motions going on in air, is
of the same nature as water-resistance to motions going on in water, only
less in degree: and as advancing science has shown the true nature of our
atmosphere, the amount of this resistance is perfectly ascertained.
A smooth ball rolled on the grass soon stops-if rolled on a green cloth
over a smooth plank it goes longer-on the bare plank, longer still-on a
smooth and level sheet of ice, it hardly suffers retardation from friction, and,
if the air be moving with it, will reach a distant shore.
Two little wind-mill wheels set in motion together with equal velocity, but
of which one has the flat sides of the vanes turned to their course, and the
other the edges, if moving in the air, will stop at very different times, but if
tried in a vessel from which the air has been removed, they will both go
much longer, and will then stop exactly together.
As it is to facilitate the motion of fishes in the water, that they are of
sharp form before and behind; so it is to facilitate the motion of birds in
the air that they have somewhat of a similar form.
A large spinning-top, with a fine hard point, set in motion in a vacuum,
and on a hard, smooth surface, will continue turning for hours.
A pendulum moving in a vacuum has only to overcome slight friction at
its point of suspension, and, therefore, if once put in motion, will vibrate for
a day or more.
But it is in the celestial spaces that we see motions completely freed from
the obstacles of air and friction —and there they seem eternal.
Had the human eye, unassisted, been able to descry the four beautiful
moons of Jupiter, wheeling around him for these thousands of years, with
such unabated regularity, and which now form, to the telescope of the astronomer, a perfect and magnificent time-piece in the sky, or had science long
proved that the velocity imparted to our globe, when first launched into its
present orbit, still wheels it along as swiftly as in the days of the first man,
this error or prejudice, that motion is always tending to rest, would never
have arisen.
Indeed, had these or other such truths, been long familiar to the common




MOTION UNIFORM.                             47
mind, the opposite prejudice might as well have obtained, that motion is the
natural state, and rest a forced or unknown state. We know of nothing
which is absolutely at rest. The earth is whirling round its axis and round
the sun; the sun is moving round its axis and round the centre of gravity
of the solar system, and possibly, round some more remote centre in the
great universe, carrying all its planets and comets about his path.
If there were any natural tendency in moving bodies to stop, a thing floating in a trough of water, on board a sailing ship, should always be found at
the end of the trough nearest the stern; and in all the seas and lakes of the
earth, the floating things should be accumulated on the western shores,
because the surface of the earth is always turning towards the east. We
know that neither of these suppositions is truth. A man on board a moving ship can throw any body just as far towards the bow as towards the
stern; although in the two cases the velocity, as regards the earth is so
different.
Ignorance of the law of moral inertia led a story-telling sailor to assert, as
a proof of the speed of his favourite ship, that when a man one day fell from
the mast-head, the ship had passed from under him before he reached the
deck: the fact in such a case, being, that he must have fallen on the same
part of the deck, whether the ship were in motion or at rest, because his
body had just the motion or rest which belonged to the ship.
Another equally sapient man, reflecting that the earth turned round once
in twenty-four hours, proposed rising in a balloon, and waiting aloft, until
the country which he desired to reach should be passing under him.
"Motion naturally uniform." (See the Analysis.)
It is only repeating that a body can neither acquire motion nor lose motion
without a cause, to say that free motion must be uniform.
The perfect uniformity of undisturbed motion is proved by every fact
observed in the universe.  If any continued motion, as of a planet, for instance, be found at one time to have certain relative velocity to some other
continued motion, the same relation is found always to hold: or deviations
from perfect uniformity are exactly proportioned to the disturbing causes.
Thus we can foretell the exact time of an eclipse, a thousand years before
its occurrence.
Had motion not been in its nature uniform, a man could have formed no
rational conjecture or anticipation as to future events; for it is by assuming,
for instance, that the earth will continue to turn uniformly on its axis, that
he speaks of to-morrow and of next week, &c., and that he makes all his
arrangements for future emergencies: and were the coming day, or season,
or year, to arrive sooner or later than such anticipation, it would throw such
confusion in all his affairs, that the world would soon be desolate.
To calculate faturities, then, or to speak of past events, is merely to take
some great uniform imotion as a standard with which to compare all others;
and then to say of the remote event, that it coincided or will coincide with
some described state of the standard motion.  The most obvious and best
standards are the whirling of the earth about its axis, and its great revolution
round the sun. The first is rendered very sensible to man by his alternately
seeing and not seeing the sun, and it is called a day; the second is marked
by the succession of the seasons, and it is called a year. The earth turns
upon its axes nearly 365 times while it is performing one circuit round the
sun, and thus divides the year into so many smaller parts, and the day is




48               MOTIONS AND FORCES.
divided into smaller parts, by the progress of the earth's whirling being so
distinctly marked, in the constantly varying direction of the sun, as viewed
from any given spot on the face of the earth. When advancing civilization
made it of importance for men to be able to ascertain with precision the
very instant of the earth's revolution, connected with any event, various contrivances were introduced for the purpose; as,-sun-dials, where the shadow
travels progressively round the divided circle; —the uniform flux of water
through a prepared opening —the flux of sand in the common hour-glass, &c.
But the great triumphs of modern ingenuity are those astronomical clocks
and watches, in which the counted equal vibrations of the pendulum, or
balance-wheel, have detected periodical inequalities even in the motion of
the earth itself, and have directed attention to unsuspected disturbing causes,
important to be known.
It is the natural uniformity of undisturbed motion which causes any number of bodies moving together, as the furniture of a sailing ship, to appear
among themselves as if at rest, —no one tending to pass before, or to fall
behind, or to move to one side or another. For the same reason a person
who is moving with such bodies is absolutely insensible of his uniform progression, and knows it only by reasoning from such facts as the changing
appearance of other objects around which do not share the motion, the rushing of the waves or wind, &c. When a ship is becalmed at sea, she may,
as numberless sad accidents have proved, be carried by rapid currents in any
direction, without one of the crew suspecting that she has motion at all; and
if the suspicion do arise, the truth can be come at only by such means as the
sounding line, where the bottom can be reached, or careful observation of the
heavenly bodies where it cannot. A man in the hold of a ship in a river or
tides-way cannot say whether the rushing of water, which he hears from
without, be a rapid tide passing the ship at anchor, or the effect of the ship's
advance in the river. A man in a balloon going 80 miles an hour, knows
not in what direction he is moving, nor indeed that he is moving at all, but
by observing the objects below.
This explains why men are not sensible of the motion of the earth itself,
which they know, however, to be turning around its axis once in twenty-four
hours, and therefore to have its surface near the equator moving with a speed
of more than 1,000 feet per second; and as in the case of a ship or balloon,
there will be no difference of sensation whether the speed were of one mile
per hour or of 10 or 100, so in the case of the earth, there would be none
whether it turned as now, once in twenty-four hours; or, like the planet
Jupiter, once in ten. A hunter among the hills, who during the heat of
noon, rests and contemplates around him a sublime scene of solitude and
silence, may little think that if, amidst that apparent repose of nature, he
were for a moment lifted up from the earth and held at rest above its surface, he would see its face of hill and dale sweeping past beneath him at
the prodigious rate of 1,000 miles an hour, on account solely of the whirling
of the earth.
The fact that a cannon-ball can be shot just as far upon the surface of the
earth, eastward, in the direction of the earth's motion, as westward, against
it, illustrates the truth, that whatever common motion objects may have, it
does not interfere with the effect of a force producing any new relative
motion among them. All the motions seen on earth are really only slight
differences among the common motions: as in a fleet of sailing ships, the
apparent changes of place among them are in reality only slight alterations
of speed or direction, in their individual courses.




MOTION STRAIGHT.                             49
A man continuing to throw upwards a ball or orange, or several of them
gt once, and to catch and return them alterfiately, uses no difference of art
as regards them, whether he be standing on the earth and whirling with it,
or on a sailing ship's deck, or in a moving carriage, or on a galloping horse's
back.  He and the oranges have always the same forward common motion.
And when a man, standing on a galloping horse, leaps through a hoop held
acros his course, he does not leap forward-for this would throw him over
the horse's ears-but merely jumps up and allows his moral inertia to carry
him through.
The reason why a lofty spire or obelisk stands more securely on the earth,
than even a short pillar stands on the bottom of a moving wagon, is, not that
the earth is more at rest than the wagon, but that its motion is uniform.WTere the present rotation of our globe- to be arrested but for a moment,
imperial London, with its thousand spires and turrets, would, by the moral
inertia, be swept from its valley towards the eastern ocean, just as loose
snow is swept away by a gust of wind.
" Force is required to bend motion."
If a body moving freely cannot vary its velocity without a cause, neither
can it vary its course without a cause; and free motion, therefore, is
straight as well as uniform.
A ball shot directly up or down gives men their simplest idea of straight
motion.
A bullet or arrow, projected horizontally, is gradually drawn downwards
by the attraction of the earth, but it deviates neither to the right nor to
the left.
William Tell, trusting to the natural straightness of motion, obeyed the
tyrant's order, and shot an apple placed on his child's head.
And the right eye of Philip of Macedon is said to have been destroyed
by an arrow which brought a label on it, telling its destination.
Riflemen shooting at a target, hit the very spot they choose to aim at.
A stone in a sling, the moment it is set at liberty, darts off as straightly as
an arrow from the bow-string or a bullet from a gun-barrel, and it is only
because the point of its circle, from which it should depart, cannot in practice'be accurately determined, that the same sure aim cannot be taken with it.
A body moving in a circle, then, or curve, is constrained to do what is
contrary to its inertia. A person on first approaching this subject, might
suppose that a body, which for a time has been constrained to move in a
circle, should naturally continue to do so when set at liberty. But on
reflecting that a circle is as if made up of an infinite number of little straight
lines, and that the body moving in it has its motion bent at every step of the
progress, the reason is seen why constant force becomes necessary to keep
it there, and force just equal to the inertia with which
the body tends, at every point of the circle, rather to   Fig. 2.
pursue the straight line, called a tangent, of which that
point, as seen in Fig. 2, is the commencement, than the
circle itself. The force required to keep the body in the
bent course, is called centripetal or centre-seeking force;
while the inertia of the body tending outwards, that is,
to move in a straight line rather'then in a curve, is called
the centrifugal or centre-flying force; and the term central forces is applied to both.
4




50                 MOTIONS AND FORCES.
A sling-cord is always tight while the cord is whirling: and its tension is
of course the measure both of the centripetal and centrifugal force. A means,
then, of measuring the tension of a sling-cord would experimentally demonstiate the amount of centrifugal force; and such a means we possess in the
contrivance called the "' whirling table," upon which is a leading sling, or any
mass with a string attached to it, may be placed to revolve, at any desired
distance from the centre. and with any desired velocity, while the string
passing over a pulley at the centre, is made to lift weights proportioned to
the outward dragging of the revolving mass. By this apparatus it is found,
as would be expected, that centrifugal force-in other words the force with
which the inertia of moving matter resists the bending of its course from
straight to circular, is proportioned, first, to the quantity of matter moved —
every separate particle having its own inertia; second, to the size of the
circle or orbit described in the same time-a body moving in a circle of
double diameter for instance, having to be forced inwards from the tangent,
at every departure, twice as far in a given time; third, that with a double
revolution in the same time, the centrifugal force is not double but quadruple
(a corresponding proportion existing for other velocities,) because, not only
are there twice as many bendings or angular departures from the tangent for
the two circles as for one, requiring, as may be said, twice as many tugs or
impulses of the centripetal force, but every impulse must be made with
double energy, for it has to drive the mass inwards through the required distance in half the time; and twice as many impulses, every one being twice
as strong, make a quadruple amount of force on the whole; fourthly and
lastly, it is found, agreeing with the relation between inertia and terrestrial
gravity described at page 43, that a body revolving, for instance, in a circle
of four feet diameter, that it may have centrifugal force just equal to its
weight, required to complete its revolution in one second and a half of time.
This and similar facts will be more particularly considered when we come to
treat of the motions of the planets round the sud. This analysis of central
forces will suffice to excite in the student a due interest touching the kindred
phenomena now to be described.
Bodies laid on a whirling horizontal wheel, are readily thrown off.
In a corn-mill, the grain, after being admitted between the stones through
an opening in the centre of the upper stone, is then kept turning round
between them, and is, by its centrifugal force, always tending and travelling
outwards until it escapes as flour from the circumference.
A man, if he lie down on a turning millstone with his head near the edge,
falls asleep, or dies of apoplexy, from the new pressure of blood on the
brain.
A wet mop, or bottle-brush, made to turn quickly on its handle as an axis,
throws the water off in all directions, and soon dries itself.
Sheep, in wet weather, thus discharge the water from their fleeces, by a
semi-rotatory shake of the skin. Water-dogs, on coming to land, dry themselves by the same action.
A tumbler of water placed in a sling, may be made to vibrate like a pendulum with gradually increasing oscillation, and at last to describe the whole
circle, and continue revolving about the hand, without spilling a drop:-the
water, by its inertia of straightness, or centrifugal force, tending more away
from the centre of motion towards the bottom of the tumbler, even when
that is uppermost, than towards the earth by gravity.
As solid bodies laid on a whirling table are thrown off, so water in a vessel caused to spin round in any way, as on the centre of a horizontal wheel,




CENTRIFUGAL FORCE.                             51
instead of lying at the bottom, is raised up all round, against the sides of
the vessel.
Water, poured obliquely into a funnel, runs round the interior of it, and
often leaves an open passage of air all the way down through it, as if there
were merely a lining of water to the funnel. The centrifugal force of the
turning water is a chief reason of this phenomenon: another reason will be
considered farther on, under the head of atmospheric pressure.
Great whirlpools at sea, and smaller ones, or eddies in rivers, occur whenever a current is obliged suddenly to bend, as in rounding a point of land or
a rock, or in meeting and mingling with a contrary current. The water, by
tending to continue its straight motion, falls in behind the obstruction, reluctantly as it were, and leaves there a pit surrounded by a liquid revolving
ridge. Charybdis, in the Mediterranean, and the great whirlpool off the
Norwegian coast, are noted examples.
It is owing to the centrifugal force in any bending part of a stream of
water, that is to say, the tendency away from the centre of the curvature,
that when a bend has once commenced, it increases, and is soon followed by
others, until that complete serpentine winding is produced, which characterizes most rivers in their coufs'e across extended plains. The water being
thrown by any cause to the left side, for instance, wears that into a curve or
elbow, and, by its centrifugal force, acts constantly on the outside of the bend,
until rock or higher land resists the gradual progress; from this limit being
thrown back again, it wears a similar bend to the right hand, and after that,
another to the left, and so on.
Carriages are often overturned in quickly rounding corners. The inertia
carries the body of the vehicle in the former direction, while the wheels are
suddenly pulled round by the horses into a new one. A loaded stage-coach
running south, and turning suddenly to the east or west, strews its passengers on the south side of the road. Where a sharp turning in a carriage-road
is unavoidable, the road towards the outside of the bend should always be
made higher than at the inside, to prevent such accidents.
A man or a horse turning a corner at speed, leans much inward, or towards the corner, to counteract the centrifugal force, that would throw him
away from it.
In skating with great velocity, this leaning inwards at the turnings becomes very remarkable, and gives occasion to the fine variety of attitudes
displayed by the expert; and if a skater, in running, finds his body inclined
to one side and in danger of falling, he merely makes his skate describe a
slight curve towards that side, when the tendency of his body to move
straightly, or its centrifugal force, refusing to follow in the curve, allows the
foot to push itself again under the body, and to restore the perpendicularity.
Skating becomes to the intelligent man an intellectual as well as a sensitive
or bodily treat, from its exemplifying so pleasingly the law of motion.
The last example explains, also, why a hoop rolled along the ground goes
so long without falling: if it incline to one side, threatening to fall, by that
very circumstance, the part touching the ground is made to bend its course
to that side, and as in the case of the skater who turns his foot, the supporting base is again forced directly under the mass of the body.
A coin dropped on the table or floor often exhibits the same phenomenon.
It is said to run and hide itself in the corner. Just before falling, if not
obstructed, it describes several turns of a decreasing spiral, the minute examination of which is a pleasing mathematical exercise.
The reason also why a spinning top stands, will be understood here.




52                 MOTIONS AND FORCES.
While the top is quite upright, the extremity of its peg, being directly under
its.centre, supports it steadily, and although turning so rapidly, and with
much friction, has no tendency to move from the place: but if the top incline at all, the edge or side of the peg, instead of its very point, is in contact with the floor, and the peg then becoming as a turning little roller,
advances quickly, ahd describes a curve somewhat as a skater's foot does,
until it come directly under the body of the top as before. It thus appears
that the very fact of the top inclining, causes the point to shift its place,
and to continue moving until it comes again directly under the centre of the
top. It is remarkable that even in philosophical treatises of authority the
standing of a top is still vaguely attributed to centrifugalforce.  And some
persons believe that a top spinning in a weighing scale, would be found
lighter than when at rest; and others most erroneously hold that the centrifugal force of the whirling, which of course acts directly away from the
axis, aid quite equally in all directions, yet becomes, when the top inclines,
greater upwards than downwards, so as to counteract the gravity of the
top. The way in which centrifugal force really helps to maintain the
spinning of a top is, that when the body inclines or begins to fall in one direction, its motion in that direction continues until the point describing its
curve, like the foot of a skater, has forced itself under the body again.
By reason of centrifugal force also, it is easier to do feats of horsemanship
in a small ring as at our theatres, than if the animal were running on a
straight road.  We see the man and the horse always inclining inwards to
counteract centrifugal force, and if the rider tend to fall inwards, he has
merely to quicken the pace; if to fall outwards, he has to slacken it, and all
is right again.
If a pair of common fire-tongs, suspended by a cord from the top, be
made to turn by the twisting or untwisting of the cord, the legs will separate
from  each other with force dependent on the speed of rotation, and will
again collapse when the turning ceases.  Mr. Watt adapted this fact most
ingeniously to the regulation of the speed of his steam-engine. His steamgovernor may in truth be described as a pair of tongs with heavy balls at
the ends, to make their opening more energetic, attached to some turning
part of the machine.  If the engine move with more than the assigned
speed the balls open or fly asunder beyond their middle station, and by a
simple contrivance are then made to act on a valve which contracts the steam
tube; on the contrary, with too slow a motion, they collapse and open the
valve.
A half formed vessel of soft clay, placed in the centre of the potter's
table, —which is made to whirl and is called his wheel,-opens out or
widens merely by the force of its sides and thus assists the worker in giving
its form.
A ball of soft clay, with a spindle fixed through its centre, if made to turn
quickly, soon ceases to be a perfect ball. It bulges out in the middle, where
the centrifugal force is great, and becomes flattened towards the ends, or
where the spindle issues.
This change of form is exactly what has happened to the ball of our
earth. It has bulged out seventeen miles at the equator, in consequence of
its daily rotation, and is flattened at the poles in a corresponding degree.A mass of lead that weighs one thousand pounds at our pole, weighs about
five pounds less at the equator, by reason of the centrifugal force.
In the planets Jupiter and Saturn, of which the rotation is much quicker
than of our earth, the middle or equator bulges out still more-even so as
to offend an eye which expects a perfect sphere.




QUANTITY OF MOTION.                            53
If the rotation of our earth were seventeen times faster than it is, the
bodies or matter at the equator would have centrifugal force equal to their
gravity, and a little more velocity would cause them to fly off altogether, or
to rise and form a ring around the earth like that which surrounds Saturn.
Saturn's double ring seems to have been formed in this way, and is now
supported chiefly by the centrifugal force of the parts. Were it to crumble
to pieces, the pieces rMight still revolve, as so many little satellites. His
true satellites are only more distant masses sustained in the same manner.
And our earth and the other primary planets have the same relation to the
sun that these satellites have to Saturn-all being sustained by an admirable balance between centrifugal force and gravity.
"The quantity of Motion in a body measured by the velocity and quantity
of matter.
If a single atom of matter were moving at the rate of one foot per second
it would have a definite quantity of motion expressed by these words; and
if it were moving ten feet per second, it would have ten times the quantity.
Again, in a mass consisting of many atoms, the quantity of motion would be
still as much greater as there were more atoms in it than one.
By experiment it is found, that if a ball of soft clay of one pound, suspended by a cord as a pendulum, be allowed to fall with a velocity of ten feet
per second, against a ball of nine pounds suspended in the same way, bub at
rest, the two, after contact, will start together at the rate of one foot per
second, the original quantity of motion being then diffused through ten
times the quantity of matter, and therefore exhibiting only one tenth of the
velocity.
A cannon ball of a thousand ounces, moving one foot per second, has thus
the same quantity of motion in it as a musket-ball of one ounce, leaving the
gun-barrel with a velocity of a thousand feet in the second.
"' The quantity of motion in a body is the measure of the force which produced it."
* The experiment of the balls of clay mentioned above furnishes one instance
of this truth. Again, a body falling for ten, seconds, acquires ten times as
much velocity as by falling for one second; its motion thus measuring the
force of gravity which has been exerted upon it.
When a large body or mass of many atoms falls, it of course has as much
more motion than a smaller body, as there are more atoms in it than in the
smaller; but as gravity acts equally on every atom, the force causing
either body to fall is still exactly indicated by the quantity of motion in it.
A large body or mass of many atoms falls where there is no impediment,
with the same velocity as a smaller body or a single atom; for gravity pulls
equally at each atom, and must overcome its inertia equally, whether it be
alone or with others.
This remark contradicts the popular opinion, that a large and heavy body
should fall to the earth much faster than a small and light one; an opinion
which has arisen from our constantly seeing such contrasts, as the rapid fall
of a gold coin, and the slow descent of a feather. The true cause of the
contrast is, that the atoms of the feather are much spread out, so as to be
more resisted by the air than those of the gold. If the two be let fall together in a vessel from which the air has been extracted-as in the common
air-pump experiment, they arrive at the bottom in exactly the same time;




54                MOTIONS AND FORCES.
and even in the air, if the coin be hammered out into gold leaf, it will fall
still more slowly than the feather. One brick dropped from a height, because its motion is not much affected by the air, reaches the earth very
nearly as soon as ten bricks let fall near it, whether they be connected or
separate-as a single horse may reach the goal as soon as ten horses galloping abreast.
A man's force will move a small skiff quickly, a loaded barge very slowly,
and a large ship in a degree scarcely to be perceived. In each case, however,
the quantity of motion may be the same, and a true measure of the force
which produced it.
A ball of one pound weight, impelled by a given force, moves twice as
fast as a ball of two pounds impelled with the same; yet, although the
velocities are different, the quantities of motion, as ascertained by the rule
already given, are equal, and indicate an equality of producing force.
" The quantity of motion in a body is the measure also of the force or mo~ mentum which it can exhibit again."  (See the Analysis, 42. )
Bodies, owing to their inertia, may be regarded as passive reservoirs of
force or motion, always ready to return as much as they have received. Momentum is the name given to the motion in a body; with reference to the
production by it of new motions or the overcoming of resistances, and is but
another term for the quantity of motion
A cannon ball, according to the quantity of motion in it, may have only
the force or momentum that will bruise a plank, or it may have enough to
penetrate a tree, or even to shoot its rapid way through a block of the hardest
stone.
A block of wood floating against a man's leg with moderate velocity,
would be little felt; but a loaded barge, coming at the same rate and pressing it against the quay, might break the bones; a large ship, again; although
moving no faster, would crush his body against any fixed obstacle; and an
island of ice, opposed in its approach to another, even by a first-rate man-ofwar, would destroy it, as meeting barges destroy a floating egg-shell.
A hail-stone falling, strikes rudely;- a stone rolled from a height, as of old,
by the besieged against besiegers, may carry death with it to many; an
avalanche, breaking from its hold on a mountain steep, may sweep away a
village.
To meeting bodies, the shock is the same, whether the motion be shared between them or be all in one.
If a running man come against a man who is standing, both receive a certain shock. If both be running at the ~same rate in opposite directions, the
shock is doubled. In some such cases, as where swift skaters have met, the
shock has proved fatal.
The meeting fists of boxers not unfrequently dislocate or break bones.
A man's skull is fractured as certainly by its being dashed against a tree
or beam, while he is on a galloping horse, as by a blow of a similar beam
coming upon him with the velocity of the horse.
When two ships in opposite courses meet at sea, although each may be
sailing at a moderate rate, the destruction is often as complete to both as if
with a double velocity they had struck on a rock. Many melancholy instances of this kind are on record. In the darkness of night a large ship has
met one smaller and weaker, and in the lapse of a few seconds, have followed




DIRECTION OF FORCES.                            55
the shock of the encounter, the scream of the surprised victims, and the horrible silence when the waves had again closed over them and their vessel for
ever.-In November, 1825, on the coast of Scotland, the Comet steamboat
was thus destroyed, and carried to the bottom with her about seventy passengers, into whose ears the drowning water rushed before the sound of arrested music and joy had died away.
" Direction of the force or forces producing motion."
When only one force acts on a body, the body obeys in the exact direction
of the force.
A ball floating in water, or lying on smooth ice, is driven exactly south
by a wind blowing to the south. A bullet issues from the mouth of a cannon,
in the direction of the axis of the cannon-which is, as the force impels it.
When two or more forces, not in the same direction, act upon a body at the,same time, as it cannot move two ways at once, it holds a middle course
between the directions. This course is called the resulting direction, viz.,
resulting from the composition of the forces..A ball or ship moving south by a direct wind, may, at the same time, be
carried east, just as fast, by a tide or current moving east; every instant,
therefore, it will go a little south and a little east, and really will describe
a middle line pointing south.east.
These particulars may be well represented on paper, as by fig. 3: where
b is the original place of the ball or ship, e the east, s the
south, and b a the middle line pointing to the south-east,    Fig. 3.
and showing the true course of the vessel. This figure is
called the parallelogram of forces, and is an important help
to the understanding of many facts in natural philosophy..
The minute investigation of the subject belongs to the
science of measures, or technical mathematics; but the
general truths are quite intelligble to common sense, or
the mathematics of common experience.
When two forces act upon a body, like the wind and    8          a
tide in the last example, the result is the same, whether they act together or
one after the other. For instance, if the wind drive a vessel one mile south,
as from b to s, fig. 3, and immediately afterwards the tide drive it one mile
east, as s to a, the vessel will be in the same place at last, viz., at a, as if she
had been driven at once south-east, in the line b a, by the simultaneous
action of the two. Therefore, by drawing the lines b s and b e to represent
the force and direction of the two causes of motion, and by then adding one
of them, or an equivalent, to the end of the other as s a to b s, or e a to
b e, the square or parallelogram is sketched, of which the middle line or
diagonal, as it is called, shows the resultant of the forces, and the true
course of the body obeying them.
What is thus true of the effect of continued forces like wind and tide is
Fig. 4.          Fig. 5.        Fig. 6.              Fig. 7.
<XXS      ell




56                 MOTIONS AND FORCES.
true also of momentary impulses, like the blows of clubs simultaneously
striking a ball, or of two billiard-balls striking a third.
When the forces exactly cross each other, and are equal, as in the case of
the ship above supposed, the figure becomes a square, at fig. 3; but if one
of the forces be greater than the other, the figure becomes oblong, as at fig.
4; if the forces cross obliquely, the fig. becomes as at fig. 5; and if they
cross in an opposing direction, it will be as at fig. 6. In all the cases,
however, the diagonal still shows the result. It is evident that the same line
may be the diagonal of many figures, as seen in b a at fig. 7; and therefore,
that very different degrees and directions of combined forces may produce
the same result.
Forces crossing each other so obliquely as to be represented by lines drawn
in almost opposite directions, would form a parallelogram having scarcely
any breath, that is to say, the diagonal would approach to nothing; showing
thus, that opposing forces neutralize or destroy each other. In fig. 6, by
reason of this crossing, the resultant is less than either of the constituents.
And for the same reason, when forces cross so acutely as to advance hearly
parallel to each other, the resultant is longer than either, as seen in fig. 5.
Forces directly opposed, or entirely agreeing in direction, give as their resultant their difference or their sum.
Forces crossing each other directly, or at right angles, as is true of the
exactly eastward force b e, and the exactly southward force b s, in figures 3
and 4,-do not in the slightest degree neutralize or alter each other, for the
body, when arrived at a, is just as far east as it would be at e, and as far
south as it would be at s. This explains why the progressive motion of the
planets in their orbits is not at all affected by the directly crossing centripetal force of gravity which keeps them at their due distances from the sun.
In all cases where the two crossing forces are equal, with whatever obliquity they cross, the resulting direction must be midway between them.Thus a boat impelled by oars, goes straight, although the direction in which
the oar acts is constantly changing; because the changing obliquity of the
force is always the same on both sides.-This explains also why a bird flying, or a man swimming, holds a perfectly straight course, although in both
cases the direction of the impelling forces is constantly varying. —And it
explains why a body suspended, as a plummet, or falling to the earth as an
apple does from a tree, is always in a line towards the centre of the earth:
for, while the part of the earth immediately under the body is pulling it
straight down to the centre, the action of parts on any one
Fig. 8.   side of the perpendicular is exactly counterbalanced by the,b    action of corresponding parts on the opposite side; and the
perpendicular is still the diagonal or middle line of every
pair of attracting parts. In fig. 8, b a represents the common
diagonal.  In speaking of the attraction of our earth, therefore, which really is the united attraction of all the individual.00-        atoms, we may always consider it as a single force acting towards the centre of the earth.
When a body is carried below the surface of the earth, its weight becomes
less, because the matter then above it is drawing it up, instead of down, as
before. A descent of a few hundred feet makes a sensible difference, and
at the centre of the earth, if man could reach it, he would find things to
have no weight at all; and there would be neither up nor down, because
bodies would be attracted equally in all directions.
When more than two forces act on a body, the resulting direction may be




DIRECTION OF FORCES.                             57
found, first of two, and then of the last resultant with each of the others
successively:-or the forces may be represented on paper by lines tacked
together, of which one denotes the strength and direction of each: the extremity of the last line will mark the place of the body after being acted
upon by the combined forces. A sailor, to know the true place of his ship
and the course which she has steered, considers, first, the forward progress
as found by the log, then the leeway or sideward motion produced by a
cross wind, and then the effect of any tide or current in which he may be
sailing.
Resolution of Forces is a phrase pointing to another important use of such.
parallelograms or figures as have just been described, viz., the enabling us
when force or motion is given, to find the forces or motions in any other
directions of which it may be the resultant, and those into which it may
itself be resolved.
Thus, if a line b a (in any of the preceding figures 4, 5, 6, &c.) represent
a force or motion, and the line b s represent one of two elements composing
it, we have but to complete the parallelogram b s a e to obtain the other
line, b e representing the only other force or motion which, combined with
the first element, can produce the given resultant.-If a ship pass from b to
a (fig. 5) while sailing through the water eastward, -a distance expressed by
6 e, she must at the same time have been carried by a tide current to the
distance and in the direction marked by the line b s.
Again, if a line be given representing a single
force or motion as b a, and if it be desired to know     Fig. 9.
how much there is in this capable of acting in another   D      e
direction as b d; it is only necessary to draw a line               C
in the direction, as b d, from the commencement of
b a, and to cut such line by another drawn directly
upon it-or at right angles to it, as the term is, from
the other end of b a: the length of b d, so cut off,  S 
viz., b s, shows the proportion required.
It is thus that a sailor who knows how far he has    c
sailed in an oblique direction finds out how much
he has gone north and east or south and west; in
other words, finds out the difference in latitude and longitude between his
present place and a former one. In the above figure, b a may represent the
course and distance sailed, b s the difference of latitude, and b e the difference of longitude.
Thus again, if a ball b strike a table a c,
with velocity and direction, both represented          Fig 10.
by the line b c; and if the ball be supposed                 i'
afterwards with the same velocity to approach
the table in the oblique direction e c, it will   e
then strike with as much less force than before,
as the line e a is shorter than e c.  For e a is
found according to'the rule for decomposing a
force, given above; and, to common sense, it
is obvious, that if the whole velocity of the
ball be represented by e c, the rate of approxi-    *a       c
mation towards the table, or merely downward
velocity and therefore the downward force is marked by the line e a. The
body only falls through the distance e a while moving all the way from e to c.




58                MMOTIONS AND FORCES.
Figure 10, explains the important cases of the force of wind upon ships
sails, windmill vanes, &c.; and the force of water upon float-boards, waterwheels, &c.; showing'that the moving mass exerts force upon a surface,
not in proportion to the speed with which it may be passing along or near the
surface, but to the rate of perpendicular approximation. It explains also, why
the slanting blow of a club or ball is so light, compared with the direct blow.
" The two great forces of Nature are Attraction and Repulsion." (Read
the Analysis.)
A person, on first approaching this subject, is far from supposing that the
beautiful and almost endless variety of phenomena exhibited in the universe
around, all are referrable to the two principles, attraction and repulsion,
examined in the first section:-but such is the truth.-It will first be shown
here, how the great classes of accelerated, retarded, and bent motions arise
from them.
Attraction.-Until Newton said, that what we call weight of bodies is
merely an instance of that universal attraction of matter which diminishes
with increasing distance, it was never suspected that weight was less, high
up in the air than on the ground; or on a lofty mountain than on the seashore.  But this we now know to be the case. However, in studying what
goes on in obedience to gravity near the surface of the earth, except in a few
very nice cases, gravity may be considered as a uniform power; for man has
neither approached the centre of the earth in mines, nor receded from it in
balloons, by more than about a thousandth part of his distance from it; and
weight has relation to the distance from the centre, not to the distance from
the surface.
"Accelerated Motion from Gravity."
Owing to the inertia of matter, any force continuing to act on a mass
which is free to obey it, produces in the mass a quickening or accelerated
motion; for as the motion given in the first instance, continues afterwards
without any farther force, merely on account of the inertia, it follows, that
as much more motion is added during the second instant, and as much again
during the third, and so on. A falling body, therefore, under the influence
of attraction, is, as it were, a reservoir, receiving every instant fresh velocity
and momentum.
It is said that Newton's sublime genius read the nature of attraction in
the simple incident of an apple falling before him from a lofty branch in
his garden.-The eye which perceives an apple beginning to fall, can follow
it for a time and mark the gradual acceleration of its descent, but soon sees
its path only as a shadowy line.
A boy letting a ball drop from his hand, can catch it again in the first
instant, but after a litttle delay his hand pursues it in vain.
A fragment of rock, detached from the brow of a hill by the lightning
stroke, begins its motion slowly; but once fairly launched, it gathers fresh
speed and momentum with every instant, and bounds from steep to steep
driving every obstacle before it.
Any liquid falling from a reservoir, forms a descending mass or stream, of
which the bulk diminishes from above downwards, in the same proportion as
the velocity of the particles increases. This truth is well exemplified in the
pouring out of molasses or thick syrup; if the height of the fall be considerable, the bulky sluggish mass, which first escapes, is reduced, before it
reaches the bottom, to a small thread; but the, thread is moving proportionately faster, and fills the receiving vessel with surprising rapidity. The same




MEASURE OF ATTRACTION.                             59
truth is exhibited on a vast scale in the falls of Niagara; where the broad
river is seen first bending over the precipice a deep slow moving mass, then
becoming a thinner and a thinner sheet as it descends, until at last, surrounded by its foam or mist, it flashes into the deep below, apparently with
the velocity of lightning.
When velocity becomes considerable in any case of falling; it cannot be
measured accurately by the eye, but its effects ascertain it.  A man leaps
from a chair with impunity, from a table with a shock, from a high window
with fracture of his bones, and in falling from a balloon his body is literally
dashed to pieces.
The force of gravity or general attraction is such at the surface of this
earth, that, in the first second of time, it gives to a body allowed to fall a
velocity of 32 feet nearly per second, that is, a velocity which, remaining
uniform from the end of the second, would carry it, without farther action
of gravity, through 32 feet in the next second   Yet the body falls only 16
feet in the first second; and the reason is, that the velocity of 32 feet possessed at the end of the second is gradually acquired, the body having only
half of it at the half second, and as much less than half at any distance before that time, as it has more than half at the same distance afterwards; and
the average, therefore, is only half of the 32, or 16 feet in the whole second.
In the next second, it falls, of course, through the whole 32 feet, with 16
additional, from the new action of gravity, in all three times as much as in
the first second; and in two seconds, therefore, it falls altogether four times
as far as in one second. At the end of two seconds the velocity is doubled
or is 64 feet per second, so that in the third second the body falls 64, and
other new 16, in all five times as much as in the first second; and in three
seconds, therefore, it has descended nine times as far as in one second, &c.
Knowing this progress, the velocity acquired by a falling body, and the distance through which it falls, in any given time, are easily calculated; and
the height of a precipice, or the depth of a well, may be ascertained by
marking the time required for a body to fall through the space.
The doctrines of falling bodies are of such importance in the minute examination of many of the phenomena of nature, that much attention has been
bestowed upon them.  Mr. Atwood's ingenious contrivance by which the
motion of falling bodies may be retarded in any desired degree, without the
character of the motion being otherwise altered, has enabled experimenters
to render evident to the senses all that abstract calculation had anticipated.
A pound weight, left quite free, falls towards the ground, sixteen feet in the
first second, proving that attraction of one pound is just sufficient to overcome the inertia of one pound at that rate. But if the inertia were doubled,
or tripled, or increased in any other degree, the fall of course would be just so
much slower.  Now Mr. Atwood's machine in effect increases
it, by causing falling weights to overcome not only their own    Fig. 11.
inertia, but also that of other weights; fig. 11. Thus a and b,
being weights of two pounds each, balancing each other over
the very easily turned pulley c, are moved by a weight of one
pound d, hooked to one of them; and gravity in pulling this
down, with force of one pound, has to overcome, not the inertia
of one pound, but of five, for the other two weights must move
as fast as the one pound does; and thus, the velocity being
reduced to one-fifth of what is natural to a falling body, the  c
descent can be minutely observed.  The experiments with At-.      b
wood's machine may be varipd exceedingly, and they are most
interesting.




60                 MOTIONS  AND FORCES.
" Retarded Motion," from gravity.
What has been said of the changing velocity of a falling body, from gravity, is exactly true, in a reversed way, respecting a rising body exposed to
the same influence.
A bullet shot directly upwards, every instant loses a part of its velocity,
until at last it comes to rest in the sky,-where a soaring eagle might see
the messenger of death motionless and harmless for a moment by his side:
the ball then descends again, and so that, at corresponding points of the
ascent and descent, but for the resistance of the air, the velocities would be
equal; and, on reaching the ground, it would have acquired exactly the
velocity with which it first departed.
It is explained in a preceding paragraph, that a body falls four times as
far in two seconds as in one, although the velocity at the end of two seconds
is only doubled.  For the same reason, a body shot upwards with double
velocity, rises four times as far as if shot with a single velocity; if shot with
triple velocity, it rises nine times as far, and so forth.
In aiming for amusement at bodies thrown up into the air, it is easy to
hit them near their point of turning, and more difficult always as they are
nearer to the ground, whether rising or falling.
An upward jet of water is small below, where it issues from the pipe with
great velocity, but it becomes more bulky as the water loses velocity in
ascending, and at the top it often spreads a little like a palm tree, and any
light round solid will continue supported and playing upon its summit.
The rise of a pendulum from the bottom of its arc, is an exact copy, reversed, of its previous descent to that point.
" The Pendulum"
exemplifies well both accelerated and retarded motion. The name is applicable to any body so suspended, that it may swing freely backwards and
forwards. When such a body is made of certain form and length, although
so simple, it is one of the most admirable contrivances of man's ingenuity.
Galileo having observed the hanging chandeliers of lofty ceilings to continue vibrating long and with singular uniformity, after any accidental cause
of disturbance, was led to investigate the laws of the phenomenon; and out
of what, in some shape or other, had been before men's eyes, but uselessly,
from the beginning of the world, his powerful genius extracted the most
important results. Independently of the light which the theory of the pendulum has thrown on various branches of physics, the instrument itself, with
a few wheels attached, to record its vibrations, has now become the perfect
time-keeper, regulating many of the affairs of men.
A common pendulum consists of a ball, fig. 12, as a, suspended from a rod
from a fixed point as b, and made to swing
Fig. 12.           backwards and forwards, or to vibrite unb                der this point. Being raised to c, and then
set at liberty, it falls back to a with an accelerating motion, like a ball rolling down
a slope, and when arrived there, it has just
acquired momentum enough to carry it to
i... 7'C    d, at an elevation on the other side; from
this it falls back again, again to rise; and
would so go on for ever, but for the impediments of air and friction.  The pendulum
is strictly an object of mathematical study;




PENDULUM.                                61
but we shall give a general idea of its important characteristics in common
language.
1. The times of the vibrations of a pendulum  are very neary equal,
whether it be moving much or little, that is to say, whether the arc described
by it be large or small. This remarkable property is what makes a timekeeper. The reason that a large vibration is performed in the same time as
a small one, in other words, that the pendulum always moves faster in proportion as its journey is longer —i s, that in proportion as the arc described
is more extended, the steeper are its beginning and ending, and the more
rapidly, therefore, the pendulum falls down at first, sweeps along the intermediate space, and stops at last.  It is evident, for instance, that the portion
c e of the arc (fig. 13) is much more steep than the equal portion e c.A pendulum made to vibrate in the curve called a cycloid, which, in the
central part, very nearly coincides with a circular arc, but towards the extremity rises a little more steeply, has its beats perfectly isochronous, or in
equal times, whatever their'extent.
A common clock is merely a pendulum with wbeel-work attached to it, to
record the number of the vibrations, and with a weight or spring having force
enough to counteract the retarding effects of friction and the resistance of the
air. The wheels show how many swings or beats of the pendulum have
taken place, because at every beat, a tooth of the last wheel is allowed to
pass. Now if this wheel has sixty teeth, as is common, it will just turn
round once for sixty beats of the pendulum, or seconds, and a hand fixed
on its axis projectiig through the dial-plate, will be the second hand of the
clock. The other wheels are so connected with the first, and the numbers
of teeth on them so proportioned, that one turns sixty times slower than the
first, to fit its axis to carry a minute hand, and another, by moving twelve
times slower still, is fitted to carry an hour hand.
2. The length of a pendulum influences the time of its vibration. —Long
pendulums virbrate more slowly than short ones, because, in corresponding arcs or paths, the hob or ball   Fig. 13.
of the long pendulum has a greater journey to perform, without having a steeper line of descent.  If a
pendulum b a be twice as long as another reaching
from b to e, it has twice as much to fall in its descend-,.
ing arc c a, as the other in its arc d e, while in corresponding parts of the two paths, the slope or incli-             e
nation is always equal:-the ball of the long pendulum
may be considered as having rolled twice as far down.::.:-.....
a given slope as the ball of the short pendulum. Now
as a body falls four times as far, either directly or on
any uniform slope, in two seconds, as in one, a pendulum must be four times as long, to beat once in two'
seconds, as to beat every second.  A pendulum of a little more than 39
inches beat seconds; one of four times the length is required to beat double
seconds, and one of one-fourth the length to beat half seconds.-As a pendulum to answer its purpose must be of invariable length, one which beats
seconds constitutes an easily found standard of measure.
Because the smallest change' in a length of a pendulum alters the rate of
going of the clock, it is important to be able to counteract the dilatation or
contraction of pendulums caused by the changing heat of the seasons; and
for this purpose various ingenious means have been contrived. One of the
best of these is the gridiron pendulum, as it is called, from consisting of various




62                 MOTIONS AND FORCES.
rods of metal. It renders the different dilatability by heat of two
Fig. 14.  metals composing it, the cause of unchanged length in the whole.
The adjoining sketch may show that if the central rod of brass
b    represented by the strong line from b to c, dilate alone just as
much as the two rods of steel, represented by the weaker lines on
either side of the other, dilate together (the expansion of brass by
heat is about double that of steel,) it will exactly counteract the
lengthening of these, and will keep the ball d always at the same
distance from the point of suspension a. Some astronomical
clocks in the present day are so perfect that they do not err one
beat of the pendulum in a year. Common clocks are regulated
by a screw which lifts or lets down the ball of the pendulum, and
cS     so changes the effective length, that is, the distance between the
Odc   point of suspension and what is called the centre of oscillation,
treated of in the next chapter.
3. The force of gravity, of course, is what determines how long the pendulum shall be in falling to the bottom of its arc, and how long in rising, for
the ball of the pendulum, as already stated, may be considered as a body
descending by its weight on a slope; a change in the force of gravity, therefore, would at once alter the rate of all the clocks on earth. At the equator
of our earth, where the gravity of bodies is counteracted in a small degree
by the centrifugal force arising from the earth's motion (as explained at page
53,) a pendulum vibrates more slowly than elsewhere, and must therefore be
made shorter to answer the same purpose. Corresponding results take place
when a pendulum is carried to a mountain top, and therefore farther away
from the centre of the earth, which is the centre of attraction-or when carried to the bottom of a mine, where it is attracted by the matter above it, as
well as by the matter beneath.
The popular prejudice refuted at page 53, that a large or heavy body
should fall to the earth, even in a vacuum, more quickly than a small or
light body attaches itself also to the case of a heavy and a light pendulum.
Now there isno difference for pendulums of the same length, whatever their
weight or material, but what depends on the resistance of the air. It is a
very remarkable fact thus proved, that in all substances the gravity and
inertia perfectly agree.
There is a small pendulum called a metronome, used
Fig. 15.    by musicians for marking time; which, although very
short, may still be made to beat whole seconds, or even
longer intervals.  The reason of its slow motion is, that
its rod is prolonged beyond its axis of support, at a, upwards, to b, and has a ball upon the top at b, as well as
on the bottom at c; which upper ball prevents the under
one from moving so fast as it otherwise would, just as a
small weight attached to one end of a weighing-beam,
prevents a greater weight attached to the other end from
falling so fast as it would if there were no counterpoise.
The rate of motion changes with any change in the distance of the ball b from t]e centre of motion a; and to
allow of such change, the ball is made to slide.
A pocket-watch differs from a clock in having a vibrating wheel instead
of a vibrating pendulum; and as, in a clock, gravity is always pulling the
pendulum down to the bottom of its arc, which is its natural place of rest,
but does not fix it there, because the momentum acquired during its fall




PENDULUM.                               63
from one side is just sufficient to carry it up to an equal height on the other
-so in a watch, a spring, generally spiral, surrounding the axis of the balance-wheel is always forcing this towards a middle position of rest, but does
not fix it there, because the momentum acquired during its approach from
either side to the middle position, carries it just as far past on the other side,
and the spring has to begin its work again.  The balance-wheel at each
vibration allows one tooth of the adjoining wheel to pass, as the pendulum
does in a clock, and the record of the beats is preserved by the wheels which
follow, as already explained for the clock. A main-spring is used to keep
up the motion of a watch, instead of the weight used in a clock; and as a
spring acts equally, whatever be its position, a watch keeps time although
carried in the pocket or in a moving ship.
As the rate of a clock is influenced by the length of its pendulum, so is
the rate of a watch by the size or diameter of its balance-wheel; and heat,
which retards the motion of a common clock by lengthening the pendulum,
retards the motion of a common watch by dilating the balance-wheel. Ingenuity, however, has found a remedy for the latter case as for the former,
viz., the contrivance called the expansion balance-wheel. Of this the circumference, instead of being a continuous ring, is made up of two half-rings,
each attached by one end only, to a cross bar, and which half rings being of
brass on the outside and of steel within, bend or curl inwards by heat-as
a sheet of damp paper bends when held to the fire —and thus diminish the
size of the wheel at their loose extremities, so as just to counter balance its
increase by the expansion of the cross bar.
As the motion of a pendulum has relation to the force of gravity, so has
the motion of a balance-wheel to the stiffness of the balance-spring; and the
regulator of a watch is merely a pin which bears against the balance-spring,
and by sliding backwards or forwards, so as to shorten or lengthen the part
of the spring left free to act, changes the degree of its stiffness. A change
produced by the variation of temperature is compensated for by the expansion-wheel described above.
It would be exceeding the limit marked out for this general work, to speak
more particularly here of those admirable watches which have been produced
within the last thirty years under the name of chronometers, for the pupose
of ascertaining the longitude at sea; but the author may perhaps be excused
for mentioning a moment of surprise and delight which he experienced, on
first seeing their singular perfection actually proved. After months spent
-in a passage from South.America to Asia, his pocket chronometer, with
others on board, announced one morning that a certain point of land was
then bearing east from the ship at a distance of fifty miles; and in an hour
afterwards, when a mist had cleared away, the looker-out on the mast gave
the joyous call of " Land a-head!" verifying the report of the chronometers
almost to a mile after a voyage of thousands. It is natural, at such a
moment, with the dangers and uncertainties of ancient navigation before the
mind, to exult in contemplating what man has now achieved. Had the rate
of the wonderful little instrument in all that time been changed even a little,
its announcement would have been worse than useless, —but in the night
and in the day, in storm and in calm, in heat and in cold, while the persons
around it were experiencing every vicissitude of mental and bodily condition,
its steady beat went on, keeping exact account of the rolling of the earth
and of the stars; and in the midst of the trackless waves it was always ready
to tell its magic tale of the very spot of the globe over which it had arrived.
The mode of using a chronometer for so valuable a purpose will be explained
in the section on astronomy.




61                 MOTIONS AND FORCES.
Bent or curvilinear motion from attraction.-This takes place whenever
attraction is acting across the path of an existing free motion. The flying
cannon-ball or stone, drawn down by gravity, is an example, for the projectile force ceases with the first impulse, but the bending force is acting
every instant and by every instant producing a new effect, causes a curvilinear path.
An oblique jet of water is to the eye a permanent exhibition of the curve
described by a body thus projected. The particles of the liquid move in the
line which they would describe if projected singly, and the continued succession of them marks the line of situations through which each passes in its
course to the earth.
A cannon or musket-ball, shot quite horizontally over a level plain, will
touch the ground or plain just as soon as another ball dropped at the same
instant directly froml the cannon's mouth; for the forward or projectile motion
does not, in such case at all interfere with the action of gravity. This result, which most persons, before consideration, would be disposed to doubt,
makes strikingly sensible the extraordinary speed of the cannon-ball; viz.,
that it has already moved, perhaps, six hundred feet forward, during the
half second that a ball dropped from the hand of a standing person requires
to reach the earth only four feet beneath. This fact also explains why, for
a long range, the gun must be pointed more or less upwards.
A dozen marbles swept horizontally from off a table by a stick, all reach
the floor at the same instant, how different soever the distance to which
they may respectively be driven.
The particular study of the subject projectiles is very important to military engineers; and we know how successfully they have pursued it by the
precision with which they now direct their shot and shells to objects at very
great distances.
A cannon-ball shot horizontally from the top
Fig. 16.          of a lofty mountain, would go three or four
miles.  (The mountain is here represented on
an enlarged scale, as standing on the globe b, c,
d, at a.) If there were no atmosphere to resist
/, /   ~\\\ its motion, or if the mountain top were above
the surface of the atmosphere, the same original
velocity would carry it thirty or forty. miles before it fell, as to b: with more force still, it
would reach to c, and. with still more to d. And
if it could be dispatched with about ten times
the velocity of a common cannon-shot, it would
not have approached nearer to the earth than at
first, even when it had again reached round e
or to a; and its velocity being undiminished, it would perform a second
similar tour, and then a third, and so forth: it would, in fact,'have become
a little satellite, or planetary body, revolving round the earth. In the successive ranges represented in the figure, it is seen that the centrifugal force
of the ball, or its tendency to move in a straight line becomes more and
more nearly a counterbalance to gravity, and at last is exactly equal to it.,
If the force given to the ball were more than sufficient to bring it round
again to the level of a, it would for a time fly off, or increase its distance
from the earth, acquiring somewhat the eccentric motion of a comet. There
may really be such revolving masses above our atmosphere, although invisible
to us, owing to their smallness. It has been supposed by some, that the




PROJECTILES.                              65,
meteoric stones, which fall to the earth every now and then, came from such
bodies, or are the entire masses, having become entangled in our atmosphere,
so as to lose their forward velocity. The four little planets discovered lately
beyond the orbit of Mars, are not larger than a six-thousandth part of our
earth.
REPULSION, —produces accelerated, retarded and. bent motions, like attraction, but it acts only at minute distances, while attraction draws from  the
sun, or from the very limits of the universe; repulsion acts, for instance, between the adjoining atoms of an elastic. fluid. Yet repulsion plays a part in
the economy of nature, not at all inferior to its sister attraction. We have
already seen, when considering the constitution of masses, in section first,
that repulsion prevents or modifies the contact of the atoms of all bodies;
that with increase of temperature, it causes these atoms to separate, and of
a solid forms a liquid, or even an air; that it operates around all masses as
if it were a film or covering, preventing their mutual cohesion, &c., &c.
Accelerated motion from repulsion is seen when the atoms of gunpowder
explode and propel the bullet from the bottom of a piece to the muzzle with
such rapidly increasing velocity.  The strength of this repulsion of gunpowder is so much greater than the strength of gravity or common attraction,
that its action on a bullet, during the passage along a barrel of five or six
feet in length, may not be overcome by gravity, during an ascent of a mile
or more.
A visible retarded motion from repulsion is exemplified by a moving body
coming againstoa spring or a bladder full of air, or against the piston-handle
of an air-syringe, so as to compress th'e air be-ieath it.
Any elastic body striking against another body and recoiling, exhibits in
conjunction the phenomena of retardation, acceleration, and often also of
bending, chiefly from repulsion; for instance:
An ivory ball, driven forcibly against a marble slab, does not stop at the
instant that apparent contact takes place, but still advances and compresses
that part of the substance which is against the marble,-as is proved by the
facts mentioned at page 37. While this compression of the ivory is going
on, the resistance made by the increasing repulsion of the particles gradually
retards, and ultimately destroys the forward motion of the ball; and at the
instant of its final arrest, the parts in contact, both of the ball and of the marble, being in their greatest degree of compression, act on the ball, and repel
it again with gradually accelerating motion, until it leaves the marble with
the same velocity which it had on approaching. The retardation and acceleration take place here within so small a space, and in so short a time, that
they are not apparent to sense, but the mind perceives the nature of the phenomenon as distinctly as if the ball had rolled against the end of a long steel
spring. If the ball strike the marble obliquely, as from a to c, in a path forming the angle a c d, with a perpendicular line, it does nqt rebound in the same
line by which it approached, but just as obliquely towards the other side, viz., from c to b; and it then exhibits  Fig. 17.
a bent motion from repulsion. This case illustrates also     d
the "' resolution of motions," for the oblique descent a c
being composed of a direct downward motion from a to    bz,
the table, and a horizontal or forward motion from a towards the perpendicular, the table destroys the downward motion, and converts it into an opposite directly
upward motion, but it does not affect the forward mo-        C
tion, which immediately combines again with the up



66                 MOTIONS  AND  FORCES.
ward, and carries the ball as far beyond the perpendicular at b as it was
distant from it at a.  The important law in physics, of which this case is
an example, is usually expressed —" The angles of incidence and of reflection are equal."  It applies to all reflected bodies, as balls, waves, sound,
light, &c.
If the ivory ball and marble, in the above case, were supposed to be both
perfectly hard, and without elasticity, still the repulsion which surrounds all
bodies, as a thin covering, preventing their cohesion, (see page 32,) would
act exactly as the real elasticity of the ivory, and would cause a retarded
motion until perfect rest came, and then an accelerated motion back again,
until the ball recovered its primitive velocity.
Collision between hard bodies always exhibits more or less of the truth
now described; when it occurs between soft bodies, as lumps of lead or of
moist clay, the approaching parts mutually displace each other, and there is
no recoil.
When a straight steel plate, of which the end is fixed in a block, is bent
as by a ball rolling against it, the particles on the side which becomes concave are made to approximate, and there is a resistance or repulsion gradually increasing among them; the particles on the convex side, again, are
drawn a little more from each other, and are therefore exerting attraction to
return: the recoil of the spring is thus owing to both forces trying to replace
the particles in their former relative situations.
Tides, Winds, &c., exemplify ATTRACTION."  (Read the Analysis,
page 42.)
Until we reflect attentively on this subject, we are far from perceiving that
all the phenomena of nature are only instances of attraction and repulsion,
acting under a variety of circumstances.
ATTRACTION. —Tides are raised by the attraction of the moon and sun,
and fall again by the general attraction of the earth; producing in many of
the shallower parts of the ocean very rapid horizontal currents. They do a
great deal of work for man. They carry his ships along the coasts, and up
and down the rivers; they turn water-wheels for him; they fill his docks
and canals at convenient times; they rise to receive his ships, launched from
elevated building-yards, &c.  What a busy scene is a great sea-port river,
during the rising and falling of the tide —with the thousands of people
along its bank, borrowing assistance in their various occupations!
Winds are produced chiefly by the fluid atmosphere seeking its level, in
obedience to the attraction of the earth, after the action of disturbing causes,
such as the heat of the sun, &c. They help man in the important business
of navigation; they turn his windmills, &c.
The currents of rivers are water constantly descending on slopes, that is,
regaining its level, in obedience to the earth's attraction.  Water-mills and
inland navigation are among the advantages which they afford to man.
Allfalling and pressing bodies exhibit attraction in its simplest form.
REPULSION-is instanced in explosion, steam, the action of springs, &c.
Explosion of gunpowder is repulsion among the particles when assuming
the form of air.
Steam, by the repulsion among its particles, moves the piston of the
steam-engine. In our days it performs half the labour of society.
Accidental explosion of fire-damp, or hydrogen, in mines, and the tre



PRODUCTION OF GREAT VELOCITIES.    67
mendous evolutions of elastic fluid in volcanoes and earthquakes, are other
instances of the same class.
Elasticity, as seen in springs, collision, &c., belongs chiefly to repulsion;
as seen in India-rubber, and other substances resuming their usual length
after extension, it belongs chiefly to attraction.
A spring is often, as it were, a reservior of force, kept ready charged for
a purpose; as when a gunlock is cocked, a watch wound up, &c.
It will be remarked, with respect to many of the phenomena now and hereafter to be mentioned, that it is not the original Attraction or Repulsion
which man uses as his servant, but the momentum gradually accumulated
in masses by the exertion of such attraction or repulsion; in other words,
the inertia is used as a great working power or force.
Electrical, galvanic, magnetical, and optical, phenomena, are also in great
part peculiar attractions and repulsions, as will be seen in the chapters devoted
to the explanation of them.  And even the actions of animals, so infinitely
varied, are all results of a shortening of the fleshy threads called muscular
fibres, which is produced by the mutual attraction of their component particles;-just as the varied motion of a telegraph, or of a ship's yard, are produced by the shortening of certain ropes of connection.
However closely allied the last-mentioned particular attractions and repulsions may be to the general attraction and repulsion formerly treated of, it
is found convenient to consider them apart.
In the remarkable phenomena of nature and art, all the motions being caused,
as now shown, by Attraction and Repulsion, these forces do not operate by
a single impulse, but through a repetition of impulses, or a continued action,
of which the effect is gradually accumulated in the inertia of matter. Thus
all great velocities and momenta are the terminations of an accelerated
motion.
Meteoric stones falling from great heights, bury themselves deep in the
earth by the force of their gradually acquired velocity.
When the wood-cutters among the Alps launch an enormous tree from high
on the mountain side, along the smooth wooden trough or channel prepared
for it and in fewer minutes than it traverses miles, it is seen plunging into
the lake below; it acquires its frightful velocity, not at once, but through
the action of gravity continued during the whole of its descent.
The shock or blow of the ram of a pile-engine, is not the effect of momentary attraction between it and the earth, but of that attraction accumulating
motal inertia or power, during the descent of the ram through a space of
twenty or thirty feet.
A common hammer, in its instantaneous shock, has the condensed effect
of the arm and of gravity, as accumulated through its whole previous course;
and when a powerful blow is intended, the hammer, or hatchet, or club, or
fist in boxing, is lifted high, or carried far back, that there may be time and
space for imparting greater power.
The inferior animals, by many of their actions, illustrate the same truth,
and prove their experimental or instinctive acquaintance with it.
Sea-birds carry shell-fish up into the air, and drop them on smooth stones
to break them, and to obtain the food. It is related in Grecian story, that a
bird once mistook the venerable bald head of a sage meditating on the sea



68                MOTI6NS AND FORCES.
shore for a smooth stone, and by the same act killed an oyster and the
philosopher.
There are some long-necked birds, that fight and kill their prey by a blow
of their beak. They draw back the head, bending the neck like a swan or
serpent, and then dart it forward, with a continued effort, until the strong
wedge-like beak reaches its destination, almost with the velocity of a pistol
bullet. One snake in darting its fangs at another passing swiftly across its
coil, has been known to miss its aim and inflict a mortal wound on its own
flesh.
Bulls, rams and goats, in fighting, alternately recede and run at each
other, that the shock may be great when their foreheads meet.
A horse in kicking, from the great length of his leg, and the consequent
space through which he can be adding velocity to his foot, drives at last
against the object almost like a cannon shot.
A bow-string propelling an arrow, follows it through a considerable space,
and so gives the great velocity at last produced.
- A sling gives to the hand the power of adding velocity to the stone through
a long path; for the hand moves in a small circle while the stone moves in
a larger, and the hand being kept always in somewhat advance of the stone,
pulls at it without intermission, until the moment of discharge.
The battering-rams of the ancients allowed those about them to accumulate in them the efforts of many hands, and of a considerable duration of
aption, so as to give at last one great and sudden shock.
Even the gentle action of the human breath, exerted for a time on a pea
or small hard ball of clay while passing through a long smooth tube, gives
a velocity which will inflict a sharp and painful stroke on a distant animal.
In Borneo and other of the Eastern Islands, poisoned arrows are thrown in
this way with great force and precision.
The action of gunpowder or bullets, although appearing so sudden, is still
not an instantaneous, but a gradual, and therefore accelerating action; and
accordingly we find the effect to depend on the length of the piece along
which the force pursues the ball. A small fast sailing vessel with a single
long gun, has often compelled a very superior vessel, whose guns were
shorter, to yield.
For the same reason that all great velocities require continued action or repeated impulse to produce them, so do they also destroy them; the inertia of motion and of rest being equal.
A vast mass of rock suspended like a pendulum, and allowed to sweep
down its curve to a considerable elevation, would arrive at the bottom like a
battering-ram, with force sufficient to shake a thick wall or rampart to its
foundation. The continued action of gravity would have given this force,
and if, instead of the solid resistance supposed, and which would scarcely be
sufficient to take the whole momentum away, the mass were merely allowed
to continue its course as a pendulum, and to ascend on the other side, the
continued action of gravity then opposing its motion, would bring it to powerless rest again, by the time when it had reached an elevation equal to that
from which it fell.
Soft air expanding gives gradually the death-carrying velocity to the cannon-ball; and soft air, or cotton, or wood, resisting in a close strong tube,if the bullet could be directed exactly into it-would again gradually annihilate the motion.  Were the attempt made, however, to stop the ball sud



PRODUCTION  OF GREAT  VELOCITIES.                       69
denly, by a block of the hardest granite, the block would instantly be riven
by its force.
Bales of cotton or thick masses of cork, attached round a ship, will receive
cannon-balls, and bring them to rest, without themselves suffering much,
while the nakedefirmer side of the ship would be penetrated. The cotton or
cork offers an increasing resistance through a considerable space, while the
oak opposes its hard front at once, and must instantly suffice or be destroyed.
A hard body, that it may at once destroy such a motion as we are supposing,
must be able to oppose as much force in perhaps the space of one-hundredth
of an inch, that is, in the extent to which its elasticity will let it yield without breaking, as the moving cause gave, through a much greater space (a
plate of steel will thus oppose a pistol-bullet; ) and when it cannot do this,
it must be broken or penetrated by the moving body. It is to be remarked,
however, that the continued opposition of a thick mass of wood, stone, or
earth, to an entered bullet, brings it to rest at least as an elastic unbroken
opposition would. Gunners have ascertained the exact depth in each substance to which a ball will penetrate; and they call buildings bomb-proof
or ball-proof, which have a thickness or depth exceeding that.
A hempen or silken rope, supporting the scale of a weighing beam, would
resist a greater weight falling into the scale than would be resisted by an
iron chain which were even stronger than the rope for the purpose of bearing
a quiescent weight; because the hemp or silk would yield by its elasticity,
and cntinue its resistance through a considerable space and time, and thus
would at last gradually overcome the momentum; while the iron, by scarcely
yielding at all, would require to be strong enough to stop the mass suddenly
or would break.
Yet for the same reason that iron is weakest in such a case as the last, it
is stronger than hemp or rope when used as a cable for a ship, to withstand
the sudden force of waves.
This will be understood on considering, that the chain by its weight hangs
as a curve or inverted are in the water, while the rope, being nearly of the
weight of water, is supported in it almost as a straight line from the anchor
to the ship; therefore, when a great wave dashes against the ship, the bent
chain will yield until it be drawn nearly straight, by which great extent of
yielding, and consequent length of resistance, it will withstand a great
shock; whereas, the straight rope, as it can yield only by the elasticity of
its material, and comparatively, therefore, a little way, will resist much
less.
A heavy ship moving quickly with the tide or wind, could not be stopped
instantly by a short rope or chain of any magnitude: if the attempt were
made to destroy at once so vast a momentum, something would certainly
give way; but a rope of very moderate size, kept tight between the shore
and the ship, and from time to time allowed to slip a little round a wooden
block, when the tightness threatened its breaking, would accomplish the end
very soon and easily.
The following are farther proofs that forces are to be measured as much by
the time or space through which they act, as by their difference of intensity or momentary power.
A door standing open, and which would yield readily on its hinges to the
gentle push of a finger, is not moved by a cannon-ball piercing through it.
Now the ball ieally overcomes the whole force of cohesion among the atoms




70                 MOTIONS AND FORCES.
of tough wood: but that force is allowed to act or resist for so short a time,
owing to the rapid passage of the ball, that it is not sufficient to affect the
inertia of the door, in a degree to produce a sensible motion.  The cohesion
of the circle in the door, cut out by the ball, would have borne a weight of
more than a hundred pounds laid quietly upon it, but supposing the bullet
to fly twelve hundred feet in a second, and the door to be one inch thick,
the cohesion being allowed to act for only the 14,400th part of a second, its
influence is not perceived.  The following are other examples of the same
kind.
A leaden bullet pressed slowly against a pane of glass, breaks it irregularly, where the strength happens to be least; but the same bullet shot at it
from a pistol, makes only a small round hole.  It has been amusingly said
of such a case, that the particles struck and carried away have not time to
warn their neighbours of what is happening.
A cannon.ball, having very great velocity, passes through a ship's side,
and leaves but a little mark; while one with less speed splinters and breaks
the wood to a considerable distance around. A near shot thus often injures
a ship less than one from a greater distance.
A sheet of paper standing edgeways on a table, is not driven down by a
pistol-ball fired through it.
The truth at present under consideration explains, with respect to gun-shot
wounds, why the man often remains ignorant for a time of his misfortune,
and why a rapid bullet only kills the parts which it touches, while a spent
ball may bruise and injure all around. In many cases of injury popularly
attributed to the wind of a ball, the ball itself has really reached the part.
A man lying down and receiving the blow of a great hammer on his chest
would be killed by it; but if a heavy anvil be first laid upon the chest, and
the blow then received upon the anvil, the man bears it with impunity.
Here the quantity of motion in the hammer being diffused through the great
mass of the anvil, produces but a trifling velocity, which the elasticity of the
chest, in its slow yielding, easily overcomes.
A circular plate of soft iron, made to turn with extreme rapidity, will cut
through the hardest steel file, almost as a knife cuts through a carrot.  In
cases where a soft powder suffices to polish a hard body, it acts partly like
this plate, by the motion or velocity given to the wearing particles.
C There is no motion or action in the universe, without a concomitant and
opposite action of equal amount."  (See the Analysis.)
This truth has otherwise been expressed —" action and reaction are equal
and contrary."  It is evident, that if no action or movement takes place on
earth but in consequence of either Attraction or Repulsion-and this has
now been shown-there must always be two objects or masses concerned,
and each must be attracted or repelled just as much as the other, although
one will have less velocity than the other, as it may be itself greater, or
fixed to another mass.
If a man in one boat pull at a rope attached to another, the two boats will
approach. If they be of equal size and load, they will both move at the
same rate, in whichever of the boats the man may be; and if there be a
difference in the sizes, and resistances, there will be a corresponding difference in the velocities, the smaller boat moving the fastest.
A magnet and a piece of iron attract each other equally, whatever disproportion there is between the masses. If either be balanced in a scale, and




ACTION AND REACTION EQUAL.                           71
the other be then brought within a certain distance beneath it, the very same
counterpoise will be required to preverft their approach, whichever be in the
scale.  If the two were hanging near each other as pendulums, they would
approach and meet; but the little one would perform more of the journey
in proportion to its littleness.
A man in a boat pulling a rope attached to a large ship, seems only to
move the boat: but he really moves the ship a little, for supposing the resistance of the ship to be just a thousand times greater than that of the
boat, a thousand men in a thousand boats, pulling simultaneously in the
same manner would make the ship meet them half way.
A pound of lead and the earth attract each other with equal force, but
that force makes the lead approach sixteen feet in a second towards the earth,
while the contrary motion of the earth is of course as much less.than this as
the earth is weightier than one pound,-and is therefore unnoticed. Speaking strictly, it is true, that even a feather falling lifts the earth towards it,
and that a man jumping kicks the earth away.
A spring unbending between two equal bodies, throws them off with equal
velocity; if between bodies of different magnitudes, the velocity of the smaller
body is greater in proportion to its smallness.
On firing a cannon, the gun recoils with even more motion or momentum
in it than the ball has, for it suffers the reaction of the expelled gunpowder
as well as of the ball; but the momentum in the gun being diffused through
a greater mass, the velocity is small, and easily checked.
The recoil of a light fowling-piece will hurt the shoulder, if the piece be
not held close to it.
A ship in chase, by firing her bow guns, retards her motion; by firing
from her stern she quickens it.
A ship firing a broadside, heels or inclines to the opposite side.
A vessel of water suspended by a cord hangs perpendicularly; but if a
hole be opened on one side, so as to allow the water to jet out there, the
vessel will be pushed to the other side by the reaction of the jet, and will so
remain while it flows. If the hole be oblique, the vessel will constantly turn
round.
A vessel of water placed upon a floating piece of plank, and allowed to
throw out a jet, as in the last case, moves the plank in the opposite direction.
A steamboat may be driven by making the engine pump or squirt water
from the stern, instead of making it, as usual, move paddle-wheels. There
is a loss of power, however, in this mode of applying it, as will be explained
under the head of " Hydraulics."
A man floating in a small boat, and blowing strongly with a bellows towards the stern, pushes himself onwards with the same force with which the
air issues from the bellows-pipe.
A sky-rocket ascends, because, after it is lighted, the lower part is always
producing a larger quantity of aeriform fluid, which, in expanding, presses
not only on the air below, but also on the rocket above, and thus lifts it.
The ascent is aided also by the recoil of the rocket from the part of its substance, which is constantly bursting downwards.
He was a foolish man who thought he had found the means of commanding always a fair wind for his pleasure-boat, by erecting an immense bellows
in the stern. The bellows and sails acted against each other, and there was
no motion: indeed, in a perfect calm, thene would be a little backward motion, because the sail would not catch all the wind from the bellows.




72                 MOTIONS AND FORCES.
A man supported on a floating plank, by walking towards one end of it
gives it a motion in the direction opposite.
A man using an oar, or a steam-engine turning paddle-wheels, advances
exactly with the force that drives the water astern.
A swimmer pressing the water downwards and backwads with his hands,
is sent forwards and upwards with the same force, by the reaction of the
water.
And a bird flying, is upheld with exactly the force with which it strikes
the air in the opposite direction.
A man pushing against the ground with a stick, may be considered as
compressing a spring between the earth and the end of his stick, which
spring is therefore pushing him up as much as he pushes down; and if, at
the time, he were balanced in the scale of a weighing beam, he would find
that he weighed just as much less as he was pressing with his stick.
Thus an invalid, on a spring plank or chair, who, by a trifling downward
pressure of his hand on a staff or on a table, causes his body to rise and fall
through a great range, and thus obtains the advantange of almost passive
exercise, is really lifting himself while he presses downward.
When a boy cries on knocking his head against a table or pane of glass,
be is commonly told, and truly, that he has given as hard a blow as he has
received; although his philosophy probably, looking chiefly to results,
blames the table for his head hurt, and his head for the glass broken.
The difference of momentum acquired in a fall of one foot or of several,
is well known: the corresponding intensities of reaction are unpleasantly
experienced by a man who sits down in an easy chair, or who, in sitting
down where he supposed a chair to be, unexpectedly reaches the floor.
What motion the wind has given to a ship it has itself lost, that is to say,
the ship has reacted on the moving air- as is seen when one vessel is becalmed under the lee of another.
When one billiard-ball strikes directly another ball of equal size, it stops,
and the second ball proceeds with the whole velocity which the first hadthe action which imparts the new motion being equal to the reaction which
destroys the old.  Although the transference of motion, in such a case, seems
to be instantaneous, the change is really progressive, and as follows. The
approaching ball, at a certain point of time, has just given half of its motion
to the other equal ball, and if both were of soft clay, they would then proceed
together with half the original velocity; but, as they are elastic, the touching
parts at the moment supposed are compressed like a spring between the balls,
and by then expanding, and exerting force equally both ways, they double
the velocity of the foremost ball, and destroy altogether the motion of that
behind.
If a billiard ball be propelled against the nearest one of the row of balls
equal to itself, it comes to rest as in the last case described, while the farthest
ball of the row darts off with its velocity, —he intermediate balls having
each received and transmitted the motion in a twinkling, without appearing
themselves to move
As farther illustrative of the truths, that action and reaction are equal and
contrary, and that in every case of hard bodies striking each other, they may
be regarded as compressing a very small strong spring between them, we
may mention, that when any elastic body, as a billiard-ball, strikes another
body larger than itself, and rebounds, it gives to that other, not only all the
motion which it originally possessed, this being done at the moment when
it comes to rest, but an additional quantity, equal to that with which it recoils




ACTION AND REACTION EQUAL.                           73
-owing to the equal action in both directions of the repulsion or spring
which causes the recoil. When the difference of size between the bodies is
very great, the returning velocity of the smaller is nearly as great as its
advancing motion was, and thus it gives a momentum to the body struck
nearly double of what it originally itself possessed.. This phenomena constitutes the paradoxical case of an effect being greater than its cause, and has
led persons, imperfectly acquainted with the subject, to seek from the principle, a perpetuum mobile.  A hammer on rebounding from an anvil has
given a blow nearly double the force which it had itself, for the anvil felt its
full original force while stopping it, and then, equally with itself, was affected
by the repulsion which caused its return.
Many other interesting facts might be adduced as examples of equal action
and reaction, but these will suffice.
This second section of the work has now explained the nature of INERTIA
in matter, and has shown that the infinitely varied phenomena of motion,
which the universe exhibits, are only attraction and repulsion, acting on
inertia of atoms separate or conjoined, under diversified circumstances.
And such is the sublime simplicity of the whole scheme of nature.




74                        APPENDIX.
APPENDIX
TO PART I.-SECTION II.
BY THE AMERICAN EDITOR.
THE attentive perusal of the preceding section will prepare the reader to
understand the following propositions.
Definitions.
Prop. 1.-When a body is successively changing its place, it is said to be
in motion, p. 42.
The idea of motion involves those of space, time, velocity, direction, the
quantity of matter and momentum.
Prop. 2.-The space described is the distance passed over by a body
during its motion; and is measured by the number of units of length, as a
foot, a yard, a mile, &c., contained in this distance.
Prop. 3.-The time consists of a certain number of units of time adopted
as its measure, as a second, a minute, &c., which have elapsed during the
motion of a body.
Prop. 4.-The velocity of a body is the rate at which it moves, or the
number of these assumed units of space that it passes over during the assumed unit of time.
All the above measures may be represented graphically by lines that are
proportioned to them, p. 65.
Prop. 5.-The direction of a body may be straight or curved; when
straight or rectilinear, it is the angle which its path makes with any straight
line in the same plane, adopted as an axis; when the path of a body ig a
curve, its direction at any point is the angle which the tangent to the curve
at the point makes with the fixed axis.
Prop. 6.-The momentum of a body is its quantity of motion, both the
mass and velocity being taken into consideration, and its proper measure is
the product of the mass into the velocity, pp. 53, 54.
Prop. 7.-A body is said to have a uniform motion when its velocity
remains constant, that is, when it describes equal spaces in equal successive
intervals of time, p. 47.
Prop. 8.-Every motion that is not uniform is said to be varied, and is
called accelerated or retarded as the velocity increases or decreases.
Prop. 9.-When the velocity constantly increases or decreases in the
direct ratio of the time that the body has been moved, the motion is said to
be uniformly accelerated or retarded, pp. 43, 58, 59, 60.
Prop. 10.-Whatever is capable of producing or destroying the motion
of a body is called force.




APPENDIX.                             75
Prop. 11. —A force that produces its effect instantaneously, and then
ceases to act, is called-an impulsive force.
Prop. 12.-A force that acts continually and equally is termed a constant force.
Prop. 13.-When the constant force acts in lines directed towards a
single point or centre, it is called centripetal, and the path of the body its
orbit, p. 59.
Prop. 14.-That part of the impulsive force which tends to make a body
move directly from the centre, is termed the centrifugalforce, p. 49.
Prop. 15.-A  force that is capable of destroying motion without being
able, under any circumstances, to produce motion, is termed a passive fbrce.
Prop. 16.-The state of rest produced by the action of opposite forces is
termed equilibrium.
Prop. 17.-When a body is struck, its particles yield to the impulse, and
the form of the body is changed. When the body possesses the inherent
power when thus changed, of restoring its form, it is said to be elastic;
when it has not this power, it is called non-elastic, p. 37.
Pro2p. 18.-A body oscillating below a point to which it has in any way
attached, is termed a pendulum, p. 60.
Laws of Motion.
Prop. 19. —lst. If a body be at rest it will continue at rest, and if in
motion, it will continue to advance uniformly in a right line unless compelled to change its state by some external force, pp. 47, 49.
Prop. 20.-2d. The motion of a body is in the direction of the force that
produces it, and is proportional to that force, pp. 53, 55.
Prop. 21.-3d. Action and reaction are always equal and opposed to each
other; or when a body communicates motion to another, it loses of its own
momentum as much as it gives to the other body, pp. 70, 72.
Of Impulsive force and Rectilinear motion.
Prop. 22.-The effect of an impulsive force is to produce uniform rectilinear motion, p. 49.
For during the moment of its action on any body, it must set it in motion
with a certain velocity; and by the first law of motion, the body must continue to advance in a straight line with that velocity.
Prop. 23.-In rectilinear motion the space is as the velocity multiplied
into the time.
For if a body move with the velocity of three feet per second, it is evident, that it will move oveir 6 feet in two seconds, i. e. X 32; and 9 feet in
seconds, i. e. X 33, and 12 feet in four seconds, &c. &c.
Prop. 24.-The time is as the space divided by the velocity.
For if a body passes over 12 feet for instance, when its velocity is 3 feet
per second, it is evident, that in order to find the number of seconds, which
the body has employed in passing over 12 feet of space, we need only
divide 12 by 3, (i. e., the space by the velocity) and the quotient 4 is the
time sought.
Prop. 25.-The velocity is as the space divided by the time.
For if a body move over 12 feet in 4 seconds, its velocity is evidently
3 feet per second or 12 -4.
The velocities of two bodies may be compared, in the same manner: the
velocities of two bodies A and B, for instance, of which A moves over 54




76                          APPENDIX.
feet in 9 seconds, and B, 96 feet in 6 seconds; their velocities will be as
6 (54 —. 9) to 16 (96  6.*)
Of a constant force and uniformly accelerated motion.
Prop. 26.-The effect of a constant force acting upon a body, is to produce in it a uniformly accelerated motion, p. 58.
For since the effect of force is to produce velocity, a constant force must,
in successive instants of time, afford continual and equal additions to the
velocity of the body it has set in motion; that is, the velocity will increase
in the direct ratio that the body has been moving, which is the definition of
uniformly accelerated motion.
Prop. 27.-In uniformly accelerated motion the space described is as the
square of the time, pp. 58, 59.
Thus it is found by experiment, that if a body move with a gradually and
constantly increasing velocity that would carry it through a mile in one
minute, that at the end of this time it has acquired such a velocity as would
carry it through two miles the next minute, if the force that communicated
its motion ceased to act at the end of the first minute; but if the force continues to act, it acquires a velocity that would carry it over an additional
mile, so that it will pass over three miles the second minute, or four miles
in two minutes. At the end of the second minute it has acquired a velocity
that will carry it over double the space in the third minute, that it moved
over in the first two minutes, or a velocity of 8 miles in 2 minutes, or 4
miles a minute. But the force still continuing to act, it will move a mile
farther or five miles in the third minute. - Hence, if a body acted upon by a
continued force move a mile the first minute, it will move 3 miles the second,
5 the 3d, 7 the 4th, 9 the 5th, &c.
Thus the spaces described in successive equal parts of time, by uniformly
accelerated motion, are always as the odd numbers 1, 3, 5, 7, 9, &c., and
consequently the whole spaces are as the squares of the times or ofethe lastacquired velocities. For the continued addition of the odd numbers yields
the squares of all numbers from unity upwards. Thus I is the first odd
number and the square of 1 is 1; 3 is the second odd number, and this
added to one makes 4, the square of 2; 5 is the third odd number and this
added to 4 makes 9, the square of 3; and so on for ever. Since, therefore,
the times and velocities proceed evenly and constantly as 1, 2, 3, 4, &c., but
the spaces described in equal times are as 1, 3, 5, 7, &c., it is evident that
the space described,
In 1 minute will be      -       -              1-=square of 1.In 2   "    ~              *              1+3 —4 —         2
In 3   "      "          -.           1+3+5=9 —            3
In 4   "      "          -       - 1+3+5+7=16          "   4
* For the benefit of those who are acquainted with algebra, we subjoin the following equation, which expresses all the circumstances of uniform motion.
Let t -- the time of motion,
s - the space described in the time t,
v _ the velocity;
Then, s   vt from which we obtain
V    $t
8
and t -
v




APPENDIX.                             77
Of Gravity.
Prop. 28.-The force which causes bodies to fall to the earth is of the
kind named constant, and is called gravity, p. 58.
Prop. 29.-The direction of gravity is in lines perpendicular to the earth's
surface.
Prop. 30. —The force of gravity is directly proportional to the mass of
the body.
For however small the parts into which we divide a body, we find them all
affected by gravity, since this force must act upon all the particles of a body.
Hence, in an unresisting medium, all bodies setting out from a state of
rest, fall through the same space in the same time, because the force bf
gravity acting upon them increases in proportion to the mass to be moved.
Prop. 31.-The force of gravity decreases, as the square of the distance
from the attracting body increases.
This is proved by astronomical observations.
Motion produced by joint forces.
Prop. 32.-When a body is acted upon at the same moment by a plurality
of forces, each of these forces produces its full effect: and the place of the
body at the end of any given time is the same as it would have been if the
forces had acted in succession each during that time, pp. 55, 56, 57.
Thus let A B represent the direction of a
force that would move a body, A the distance       Fig. 18.
from A to B in a certain interval of time, (a
second for example,) and A C, the direction of                 7
a force that would propel the same body from
A to C in the same interval of time. Suppose  16           H
the first force acted alone, it would move the
body from A to B in one second; if the force A C then acted at f1, by drawing
B R equal and parallel to A C, B R will represent the direction and velocity
of the force A C, and R the position in which the body would be in at the
end of the second interval of time. Unite A and R and the line A R will
represent the course of the body A if acted upon at the same moment by
the two forces A B and A C, and R the position of the body at the end of
the first interval of time.
In the same manner the action of any num-        Fig. 19.
ber of forces may be represented. Thus let
A B, A C, A D, A E, represent the separate
effects of four different forces acting in, the
same plane, capable of moving a body the
distances A B, A C, A D, A E, in a given
interval of time. Draw B c, c d d R, equal  _-D    /
and parallell to A C, A D, A E, respectively,
and join A R, A B, c d R, will represent the       c             c
pa/th of the body if these forces had acted
successively each during one interval of time,  A
and A R the path of the body if they all act
together, and R the position of the body at the end of the first interval of time.
Prop. 33.-The line A IR in figures given to illustrate the preceding
propositions represents the direction and measure of a single force equivalent
to all the others in each figure; and hence the process by which it is determined called the composition of forces, pp. 55, 56, 57.




78                         A. PPENDIX.
Prop. 34.-Any force may be decomposed into any number of other forces,
that shall be equivalent to it, by the reverse of the foregoing operation.
This process is called the Resolution of forces, p. 57.
Thus the force A R fig. 18, may be separated into two forces A B, A C,
and the force A R, fig. 19, into four forces, A B, A C, A D, and A E.
Prop. 35. —When the forces act in the same right line, we have only, in
order to ascertain the spaces described by their combined action, to add or
substract the spaces which would be described by their separate action, according as these forces act, in the same or opposite direction.
Equilibrium.
Prop. 36.-A body acted upon by a plurality of forces, in opposite directions, will remain at rest, or in equilibrio; when these forces were supposed
to act in succession each during the same interval of time, the body would
arrive at its point of departure.
The simplest and most evident case of equilibrium is that in which a
body is acted upon by two equal and opposite forces.
On the joint action of an impulsive and a constantforce.
A.  When these forces act in the same manner.
Prop. 37.-When the forces act in the same direction, the place of the
body at the end of any given time, may be determined, as in the problem
of the composition of forces, by supposing, first, that the impulsive force acts
during that time, and then that the action of the constant force commences
and acts alone during the same time: the spaces added altogether will give
the space passed over by the joint action of these forces during the assumed
time.
Prop. 38.-When the forces act in opposite directions, the place of the
body may be ascertained by a similar process; in this case, however, the
spaces are to be substracted one from the other, pp. 58, 59.
When a constant force is acting in a direction contrary to that of a moving
body set in motion by an impulsive force, the retardation that the former
produces may be determined by comparing the motion with that of a body
moved by the same force.
The degrees by which an ascending body loses its motion, are the same
as those by which it is again accelerated at the same points, when it has
acquired its greatest height and again descends, for the velocity at the corresponding parts of the ascent and descent are equal. Thus we may calculate
to what height a body will rise when projected upwards by an impulsive
force, gunpowder, for instance, and retarded by the force of gravity. Since
the force of gravitation produces or destroys a velocity of 32 feet in every
second, a velocity of 320 feet will be destroyed in 10 seconds; and according to what has been premised, a body will fall in 10 seconds through a hundred times 16 feet or 1600 feet, which is therefore the height to whichl a
velocity of 320 feet in a second will carry a ball projected, without resistance
from other cause than gravity, in a vertical direction, p. 60.
B.  When these forces act in different directions.
* When the successive directions of the constant forces are parallel.
Prop. 39.-If the constant force be that of gravity, the successive direc



APPENDIX.                              79
tions of which are assumed to be parallel, the investigation of the effects
produced constitutes the doctrine of projectiles; a projectile being a body
thrown in any direction by an impulsive force and at the same time acted
upon by the force of gravity, pp. 59, 60.
Prop. 40.-The place of a projectile at the end of any given time may
be determined as in the problem of the composition of forces, by supposing
first that the impulsive force alone has acted during that time, and then that
the action of gravity commences, and acts alone during the same time.
Thus let A H represent a horizontal plane, and A B the initial            Fig. 00.
direction and velocity of a body
projected from the point A in
the same plane.  If the impulsive force alone acted on the
body it would describe the path                   33
A B B' B" B"' &c. with uniform  i
velocity. But as the force of                B
gravity acts from the moment of
projection, the body will be
drawn downwards from the line                  " _
A B"' so as to be found after the   A
successive intervals of time, at                              3.
the points g g' g", &c., and as
the force of gravity produces a
velocity which increases as the
squares of the distances, if the distances A B, B B', B' B", B" B"' be
equal, B g, B' g', B" g"', B"' g"', &c., will be as the. squares of these distances, and the path of the projectile through the points g g' g" g"' will
be a curve, and this curve mathematicians have called a parabola.
** When the successive directions of the constant force tend to a common
centre.
Prop. 41.-This case constitutes the doctrine of central forces, see prop.
13, p. 75.
Prop. 42.-The place of the body at the end of any given time may be
determined here also by the problem of the resolution of forces.
Thus, suppose A represent a body impelled
towards H with such a force, as, by itself, would     Fig. 21.
enable it to run over the equal spaces A B, B F,   A. _l -I? G:1
F G, &c., in equal portions of time: suppose like-.
wise that it is acted upon the same time by constant force, which would enable it to pass over the  ]i. O
unequal spaces A I, I K, K L, &c:, in the same
equal portions of time.  It is evident, that the  rT  P
joint action of both these forces would compel the
body A to pass over the curvilinear path A N 0
P, &c. Through B draw the line B C, (viz., in
the centre of attraction); through I draw I N parallel to A B; and at the end of the first portion of
timA the body will be found at N, whence it would
proceed in the straight direction N R (by the first    c
law of motion), if the constant force then ceased




80                          APPENDIX.
to act. But as this force continues to act, the body at the end of the second
portion of time will be found in 0; for the like reason, at the end of the
third portion of time, it will be found in P, and so on. The course then A
N O P is not straight, but consists of the lines A N, N 0, 0 P, forming certain
angles with each other. Now it will not be difficult to conceive that, because
the attractive force acts not by intervals but constantly and unremittedly,
the real path of the body must be a polygonal course, consisting of an infinite number of sides; or more justly speaking, a continuate curved line, which
passes through the points A, N, O, P, &c., as is shown by the dotted line.
Prop. 43.-Should the action of the centripetal force cease at any instant,
the body would proceed straight forward, p. 49.
The portion of the impulsive force by which this is affected is called the
centrifugal, prop. 14.
Prop. 44.-Whilst the distance from  the centre remains unchanged, as
when the body moves in a circular orbit, the centripetal and centrifugal
forces are equal.
Laws of Central forces.
Prop. 45.-When bodies revolve in equal circles, their centrifugal forces
are proportional to the squares of their velocities.
Prop. 46.-When two bodies revolve with equal velocities at different
distances, the centrifugal forces are inversely as the distances.
Consequently (prop. 45, 46) the centrifugal forces are in all cases directly
as the squares of the velocities, and inversely as the distances.
Prop. 47.-When two bodies revolve in equal times at different distances,
their centripetal forces are simply as their distances.
In general, the centripetal forces are as the distances directly and as the
squares or the times of revolution inversely.
Prop. 48.-When the forces vary inversely as the squares of the distances,
as in the case of gravitation, the squares of the times of revolution are proportional to the cubes of the distances.
Thus, if the distance of one body be four times as great as that of another,
the cube of 4 being 64, which is the square of 8, the tinmes of its revolution
will be 8 times as great as that of the first body.
Prop. 49.-Where the orbit deviates more or less from a circular form, a
right line joining the revolving body and its centre of attraction, always describes equal areas in equal times, and the velocity of the body is therefore
always inversely as the perpendicular drawn from the centre to the tangent;
and the velocity at any point less than three-eighths, greater than that necessary to make the body describe a circle.
Prop. 50.-To propel a.body in an elliptical orbit, the force directed to
its focus must be inversely as the square of the distance.
This is proved by astronomical observations, but we have no other proof of it.
The motion of the planets round the sun in the solar system is governed
by the laws of central forces, the centripetal force in this case being that of
gravity.
On the joint effect of active and inactiveforces.
A. When they have opposite directions.
Prop. 51. —The effect of passive forces is to restrain and modify the action
of other forces so as to confine the motion of a body to a particular course or
path, and the direction of the passive force affecting a body at any moment




APPENDIX.                            81
s the line perpendicular to that part of this path at which the body is found
at this moment. If the direction of the active force be also perpendicular
to this path, the body must evidently remain at rest, since no part of this
force can be resolved into the direction of the path in which alone the body
can move.
B. When they have different directions.
General rule.
Prop. 52.-Resolve the active force into two, one perpendicular, and the
other a tangent to the path of the body, the effect of the former force will be
entirely destroyed (prop. 51,) and the body will advance by the-latter alone.
* On the motion of a body impelled obliquely against a plane.
Prop. 53.-Let M N represent the plane,           Fig. 22
and A B the direction and velocity from' the     A     *
impulsive force, resolve A B into the forces
A C perpendicular to the plane, and C B in
its direction, then by the general rule (prop. 
52) the body will move along the plane with a 
velocity of which C B is the measure.            c        3B
** On the motion of a body impelled obliquely against a curved surface.
Prop. 54.-Let M N represent the curve and         Fig. 23.
A B the direction and velocity from the impulsive force. Resolve A B into two forces, C B   W: A   C        N
perpendicular to the curve at B, and B D (equal
to A C) a tangent to the curve at.the same point.
Then B D will represent the velocity at the point
Prop. 55.-If the curve be interrupted at any:13
point, or change the direction of its concavity, the body will advance with
its last velocity in a tangent to the curve at that point.
*** On the descent of a body along an, inclined plane.
Prop. 56.-Let M N represent an in-            Fig. 24.
dined plane and A B (perpendicular to the
horizontal base H N) the force of gravity
as measured by the distance which would                        M,~
cause a body to descend in the first second
of time. Resolve A B into two, A C, per-               C
pendicular to the plane, and C B in its
direction, then the body will be urged
dbwn the plane by the constant force
measured by C B.
Laws of the descent of bodies down inclined planes.
Prop. 57. —lst. The motion of a body drawn down an inclined plane is
uniformly accelerated.
Prop. 58.-2d. The velocity acquired is proportional to the perpendicular
6




82                        APPENDIX.
descent, so that a body falling from M to H has the same velocity at H as
one descending the whole length of the plane at N.
Prop. 59.-3d. The times of descent down planes of the same heights
are as their lengths.
Prop. 60.-4th. The times of descent down all planes which are cords
drawn. to the lowest point of the same circle, are equal.
Thus, if the balls A, B, C, be placed at different
Fig. 25.      points of the circle and suffered to descend at the
same instant along as many planes which meet at
the lowest point of the circle, they will arrive there
at the same time.
Or it may be enunciated in the following terms:
the times of descent down all the cords drawn from
the same point or circumference of a circle will be
the same.
This will be made evident by supposing the above
figure inverted, ID being made the upper point and the
balls allowed to fall from that point to A, B, and C.*
**** On the descent of a body down a vertical curved line.
Prop. 61.-The times of descent down the cords of different circles are
to each other as the square roots of their diameters.
Prop. 62.-If a body fall from a state of rest down a curve, the velocity
acquired is equal to that which it would have by falling through the same
perpendicular height.
For if the curve be considered as made up of an infinite number of contiguous planes, it is evident that the angle of inclination of any two of these
adjacent planes is infinitely small, or nothing, and consequently there is no
velocity lost by a change of direction in passing from one to the other.
Therefore, as the effect of gravity is not impeded, the truth of the proposition becomes evident.
Prop. 63.-If a body be projected up a curve, the perpendicular height to
which it will rise is equal to that through which it must fall to acquire the
velocity of projection. For the body in its ascent will be retarded in the
same degree that it was accelerated in its descent.
Thus, let B A B' be a curve in which the lowest
Fig. 26.       point is A, and the parts A B, A B' are similar;
3B               B   a body in falling down B A will acquire a velocity
that will carry it to B', and since the velocities in
all equal altitudes in the ascent and descent are
equal, the times of ascent and descent are equal.
The foregoing proposition is equally true whether
the body actually move over a solid surface or be
retained in its path by a string which is in every part perpendicular to it.
Of the simple Pendulum.
Prop. 64.-The simple pendulum is conceived to be a mere material point
suspended by an imponderable and inextensible thread, p. 60.
Prop. 65.-If the simple pendulum vibrates through very small arcs,
these may, without sensible error, be conceived to coincide with their chords,
and we may derive from this consideration the following theorems:
1st. As the times of descent of the body down different chords of the same




APPENDIX.                              83
verticle circle are equal (prop. 60.) the vibrations of the same pendulum,
although performed through unequal arcs, will be very nearly equal,
p. 61.
2d. The times of vibrations of different pendulums will be to each other
as the square roots of the lengths of these pendulums, or, W*hich is the same
thing, their lengths are proportioned to the squares of the times of vibration,
p. 61.
The times of descent down the chords of different circles are the same as
would be occupied in descending vertically through their diameters, and are
consequently proportional to the square roots of these diameters.
Of the impact of bodies.
Prop. 66.-When a body in motion strikes directly another body, it always
communicates motion to the second body, and loses part of its own, and from
the.third law of motion it is evident that the momentum gained by the
second body is exactly equal to that lost by the first.
Prop. 67. —When one non-elastic body strikes against another, the two
bodies will move on together since there is no force to separate them; and as
one of the bodies gains all the momentum which the other loses, the momentum after impact will be equal to the sum of the momentum before impact.
Prop. 68.-When an elastic body strikes against another, the second is
impelled forward with double the momentum which it would have received
under the same circumstances if non-elastic.
For at the moment of impact the form of the body struck is changed by a
force equivalent to the momentum which it receives from the striking body,
and if this body be perfectly elastic, its form will be restored to it by a force
exactly equal to that by which it was changed, and this force (which we
have just seen to be equal to the original impulse,) will be exerted in driving
the body forward. The body thus receives, besides its original impulse, the
equal force of the re-bound.
Prop. 69.-The striking body, when elastic, is also acted upon by the
rebound, and loses twice as much momentum as it would have lost if
non-elastic.
In this case, as in the former, the sum of the momenta is the same after
impact as before it.; but the bodies after impact do not mole on together.
Prop. 70. —If an elastic body strike against a firm plane, the angle of
reflection will be equal to the angle of incidence, p. 66.




84                        MECHANICS.
PART II.
PHENOMENA OF SOLIDS.
THE FOUR FUNDAMENTAL TRUTHS USED TO EXPLAIN THE PECULIARITIES OF STATE AND MOTION WHICH DEPEND ON THE SOLID FORM OF
BODIES; A DEPARTMENT COMMONLY CALLED MECHANICS.
ANALYSIS OF THE CHAPTER.*
A force, which moves part of a solid body, must effect the whole or break
off the part.
If the force be directed towards a certain central point in the mass, it will
effect the whole equally, whether simply to support the mass, or to move it
or to stop it when in motion. The point, according to circumstance, is
called THE CENTRE OF GRAVITY OF INERTIA, or OF ACTION.
In solid bodies moving about an axis, as exemplified in a wheel or weighing beam the various parts describe circles or move through spaces which
are greater in proportion to their respective distances from the centre of
motion.  Bence forces differing as to speed, may still, through a solid
medium, be brought exactly to co-operate or to oppose one another; a slow
force counter-balancing or being equivalent to a quicker one, provided
that it be more intense in proportion as it is slower. The SIMPLE
MACHINES, or MECHANICAL, MEDIA called LEVER, WHEEL AND AXLE,
PULLEY, INCLINED PLANE, WEDGE, SCREW, &c., are so many arrangementsof solid parts, by which Jbrces of different velocities and intensities
maybe thus connected or opposed, or may be conveniently substituted one
for another.
By solid connecting parts, also, the direction of any existing motion or force
may be changed, as when the straight motion of running water is converted into the rotary motion of a water-wheel, &c. Hence arises an endless variety of COMPLEX MACHINES.
In all machines, an important circumstance to be considered is the resistance among moving parts which arises from FRICTION:-and in solid
structures generally, the forms and positions of parts have to be adjusted
to the STRENGTH OF THE MATERIALS) and to the strains which the parts
have to bear.
" Solid" is the term applied to a mass in which the mutual attraction of
the atoms is so strong, that the mass may be moved about as one body, without the relative positions of the component parts being thereby disturbed.
" Force moving part of a solid must effect the whole or break off the part."
This is a necessary consequence of the description or definition of a solid
just given. And it follows that in all cases of breaking, the cohesion of the
* The reader should here re-peruse the general table or synopsis at page 19.




CENTRE OF GRAVITY.                             85
atoms at the fractured part must have been less strong than the weight of
the remaining mass, or its inertia resisting the degree of change attempted, or
the force fixing it to its place, or than some combination of these particulars.
The sharp blow of a hammer given to an ivory ball, causes it to dart off
swiftly, but does not injure it, because the cohesion among the atoms struck
is stronger than the opposing inertia of the mass, even under a rapid change;
but the blow of a hammer on a large elephant's tusk indents or breaks the
part because the opposing inertia of the larger imass is- stronger than the
cohesion of the atoms which receive the blow.
A vessel of pottery-ware may be safely suspended by its handle; proving
that the cohesion which fixes the handle to it is stronger than the weight of
the vessel; but if the attempt be made to lift the vessel quickly, the handle
may rise and leave the vessel behind; because then the weight and inertia
are acting together to destroy the cohesion. Thus servants attempting to
lift too quickly the loaded stone-ware dishes at a dinner-table, often break
off the part by which they take hold.
Centre of Gravity or Inertia.
If any uniform beam or rod be supported by its middle, like a weighing
beam, the two ends will just balance each other. This is in accordance with
the general truth or law of attraction already explained; for as there is just
as much similarly situated matter on one side of the support as on the other,
there will also be just as much attraction, and therefore no reason why the
matter on one side should overpower that on the other.  If equal weights be
afterwards attached in corresponding situations on the two arms of the beam
the balance will not be thereby disturbed; and the operation of adding
weights that counterpoise, above and below, and near and far from the centre
may be continued, until a bulky mass is built up upon the beam-and instead of a beam a wheel may be used-yet the whole will remain perfectly
supported and in equilibrium about the original centre. In the pages now
to follow, it will be shown that, in every body or mass, or system of connected masses, in the universe, there is a point of this kind about which all
the parts balance or have equilibrium, and it is this point which is called
the centre of gravity or of inertia. Although in any mass, therefore, every
atom has its separate gravity and inertia, and the weight and inertia of the
whole are really diffused through the whole, still by supporting this one
point, either from above or from below, the whole mass is equally supported;
by lifting it, the whole is lifted; by stopping it, the whole is brought to
rest; and when it rises or falls, the general mass is really rising or falling.
Thus for many purposes, a body, however large, may be considered as compressed into or existing only in the single point called its centre of gravity
or of inertia.
This centre in a mass of regular shape and of uniform substance, as a ball
or cube of metal, is easily found, because it is the evident centre of the
form; but in bodies that are irregular, either as to density or form, it must
be found by rules of calculation hereafter explained.
To say that the centre of gravity will always take the lowest situation
which the support of the body will allow, is only to repeat, that bodies tend
by their gravity towards the centre of the earth. In a suspended body, therefore, as the lowest situation which the centre of gravity can find is, when it
is immediately under the point of suspension, all bodies hanging freely must
have their centre of gravity directly under that point. A plummet is an
interesting example of this; and the truth furnishes, in many cases of irregular masses, a very simple practical mode of finding the centre.




86                        MECHANICS.
Thus if an irregular piece of plank or of pasteboard, represented here by
the figure a e b d, be suspended from any point, as a, and
Fig. 27.    the cord of a plummet a g be attached at the same point, the
e~n      centre of gravity of the board must be somewhere in the
d   direction of the plummet, and a chalk line left on the board
where the cord touched it, must pass over the centre of
gravity.  If the board be then suspended by another point,
as d, and'another chalk line d e be made in the same mane    -Jo    ner, the place c, where the two lines cross or cut each other,
will indicate the centre of gravity; and the board when supported by a cord attached there, will hang evenly balanced.
go            The following cases further illustrate the truth, that the
centre of gravity always seeks the lowest place. They seem
at first to be exceptions to the law; bitt when more fully
considered, are interesting proofs of it.
A wooden cylinder or roller e d c, placed on a slope
Fig. 28.      or inclined plane a b, will naturally descend, because,-z1     its centre of gravity is thereby approaching the earth;
eA       {:-..'  but if there be a heavy mass of lead c introduced at one
side, which must rise before the roller can descend, the
< O Q'   rise of the mass being contrary to gravity, the motion
i   will be arrested.  Indeed if the roller were placed on
the plane with the lead in the position d, the lead would
fall down to the position c, and so would move the roller towards b, exhibiting the singular phenomenon of a body rolling up hill by the action of its
weight.
If a billiard-ball be placed upon the small ends of two billiard sticks or cues
Fig. 29.               a b and c d, laid on a table with
their points c and a in contact, but
with the larger ends b and d so far
e ]~~         apart that there may be just room
for the ball to touch the table between them, the ball will roll along
Ac=                               J  between the cues, sinking gradu~Jb  _ ally from its high situation near
their points, to its lower situation near b. To a careless observer, it
would then have the appearance of rolling upwards, because the cues on
which it rests are thicker towards the end d and b; but it would really be
descending in obedience to gravity. If a double cone, as represented atf,
were substituted for the ball, it would similarly roll from c to e, and with still
more of the fallacious appearance of rolling upwards, because its ends would
always be resting on the upper and rising surfaces of the cues.
The board or stick c d resting on the edge of
Fig. 30.         the table a b would naturally fall if left to itself,
c                   because more than half of it is beyond the edge
~th = J  D     / of the table; but strange to say, an additional
weight e attached to its projecting part as at b
by the cord b e, instead of pulling it down faster, shall fix or steady it on the table, provided
the weight be pushed inwards a little by a rod
d e resting against it and against a niche in the
stick at d.  It is evident that the stick c d, in
falling, must turn round the edge of the table at




CENTRE OF GRAVITY.                             87
b; but in so doing, after the arrangement now supposed, it must lift the
weight e along the path ef —which rise as the weight is heavier than the
stick (that is to say, as the common centre of gravity of the connected objects
is near e, ) gravity forbids, and therefore the stick and weight will both remain
supported by the table. An umbrella or walking cane, hanging on the edge
of a table by a crooked handle, is another instance of the same kind. And
the common toy of a little man standing on tiptoe upon the top of a pillar,
and supporting two leaden bullets by wires descending from his hands, is
another combination of parts which places the centre of gravity of the whole
the support, making the combination a kind of pendulum;
By attending to the centre of gravity of the bodies around us on earth, we
are enabled to explain why, from the influence of gravity, some of them are
stable or firmly fixed, others tottering, others falling.
If we find that a body, from its form or position, cannot be overturned.
without its centre of gravity being lifted,-knowing now that the general
mass is then lifted in the same degree, we see why a weak cause cannot effect
the change. The rise of the centre of gravity, or body, in any case of falling
over when the centre of gravity is over the middle of the sustaining base,
will be proportioned to the breadth of the base of the body, compared with
the height of the centre of gravity above the base. This is shown in the
annexed figures of which the two particulars of base and height are combined
Fig. 31.
A. "B c 
in a series of proportions. In the figures, the dot c marks the place of the centre of gravity, and the curved line beginning from the dot marks the path
of the centre of gravity, when the body is overturned. This curved line is a
portion of a circle which has the edge or extremity of the base (b, in fig. A. )
as a centre, because the body in turning must rest upon such extremity or
corner as the centre of its motion. The farther inwards, therefore, from this
extremity that the centre of gravity is, as marked by where a plumb-line as
p, hanging from it, crosses the base, the farther, of course, is the centre of
gravity from the top of the circle which it has to describe in moving, and the
steeper, consequently, will be its commencing path; and as in the case of
bodies made to roll up slopes, the steeper the ascent, the greater will be the
force necessary to give motion.-The line of a plummet hanging from the
centre of gravity is called the line of direction of the centre, or that in
which it tends naturally to descend to the earth.
In fig. A, which has a broad base and little height of the centre of gravity,
we see that the centre must rise almost perpendicularly before it can fall
over, and the resistance to overturning is therefore nearly equal to the
whole weight of the body. Hence the firmness of a pyramid.
In figures B, C, and D, progressively, the commencing path of the centre
is less steep, because the base is narrower, and hence the bodies are so much
the less stable. B, may represent an ordinary house, C, a tall narrow house,
and D. a lofty chimney.




88                         MECHANICS.
Fig. E, shows a tottering position, for the centre of gravity being directly
over a base which is a mere point, the least inclination places it on a
descending slope, and the body must fall.
Fig. 32.
In F, the position is tottering on one side, and stable on the other. This
explains how the least inclination of a standing body virtually narrows, in
$one direction, its sustaining base.
In G, which represents a ball upon a level plane; the whole mass is supported on a single point, as in E, yet the body has no tendency to move,
because, in any other possible position, the centre would still be as far from
the sustaining plane. In moving, the centre describes the straight level
line a b.
In H, the ball is on an inclined plane, and rolls down, the centre of
gravity describing the oblique line b a.
In I, which is an oval body resting on a level plane, when the body is
moved to either side, the centre of gravity must rise, as in the case of a pendulum. Hence an oval body on a level will rock or vibrate like a pendulum.
K, is a true pendulum, whose centre of gravity describes the curve here
shown, as explained in Section II., at page 60.
The importance of the subject of the centre of gravity will be farther judged
of by the facts which are now to be reviewed.
A cart loaded with metal or stone may go safely along a road of which one
side is higher than the other, as here shown, but were the same cart loaded
with wool or hay it would be overturned; because, although the sustaining
base be the same in the two cases, the line of direction falls much within it
from the low centre of gravity of the metal at c, but falls very near the wheel
at P, or altogether on the outside, from the high centre
Fig. 33.      of the wool at a, and in the latter case the centre has
offered to it a descending path.
a/ At t        This explains why lofty stage coaches or vans are so
dangerous, and particularly whein heavy luggage is
placed on the top, and why lofty gigs and curricles
have led to so many fatal accidents.  As regards any
of these, a defect of smoothness or of level in the road,
or even, in a case of quick driving, a slight lateral
d  1 < I'-  bend often suffices to produce the catastrophe.  The
aP         safety-coaches of late times are made with the wheels
far apart to give a broad base, and with the luggage
receptacles and seats for outside passengers placed low down before and
behind the body of the carriage, instead of on the top, as formerly.
The feet of tripods are generally expanded below to give a broad base.
The same is true of our common chairs; but a thoughtless child often leans
so far over the back of the chair, that he causes the line of the general centre
of gravity to fall beyond the base, and the chair with its load is overturned.




CENTRE OF GRAVITY.                             89
The small lofty chairs made to raise children to the parents' elbow at the
dinner-table, are very dangerous if the feet are not made to spread much.
Pillar-and-claw tables, candle-sticks, table-lamps, and many other articles of
household furniture, have stability given in the same manner.
The least inclination of a standing body virtually narrows the supporting base.
This truth is explained by fig. F. It shows the necessity of building the
thin walls and tall chimneys of modern houses perfectly upright. And hence
the extreme importance and utility of that simple instrument, the plummet
or plumb-line, which, when applied to a body, is a visible indication of the
line of its centre of gravity. The mason and many other workmen cannot
proceed a step without their guiding plummet.
The brick walls of ordinary houses are so thin, that, to have standing
strength, they require to rest against one anothet; and hence they occasionally exhibit the kind of stability which belongs to a child's house built of
cards. As contrasted with the masses of masonry which remain to us from
antiquity, resting on firm-spreading basements they are examples of what
is truly ephemeral, in comparison with that which has partaken of the permanency of nature's own works, covering regions with mighty ruins. What
magnificent illustrations. of strength and durability, dependent on proportions, are those ancient pyramids and temples, which still give such interest
to the banks of the Nile, and to the valleys and plains of Asia!
There are many remarkable structures on earth which lean or incline a
little; yet so long as the line of their centre of gravity remains within the
base, and the parts of the mass have tenacity among themselves sufficient to
hold together the structure will stand. The famous tower of Pisa was built
intentionally inclining, to frighten and surprise: with a height of one hundred and thirty feet, it overhangs its base sixteen feet, and assumes nearly
the air of fig. F. in page 88.
The tall monument near London Bridge inclines so much, that in high
winds, from a particular quarter, timid minds have doubted of its stability.
And many of the most lofty and beautiful of our cathedral spires or towers, as that of Salisbury, have lost something of their perpendicularity.
An oval body on a fiat level surface, as already explained by fig. I, page
88, oscillates somewhat like a pendulum, because, when disturbed from its
middle position, its centre of gravity has risen and seeks to return. The
same is true of any regular slice or portion of a solid globe, which will consequently always come to rest with its plane face turned directly upwards.
The rocking-horse of children and the common cradle are exemplifications
of the same class.
But perhaps the most curious instances are those rocks called Loggan or
Laggan stones, of which there are several among the picturesque barriers
of the British coast. An immense mass, loosened in some convulsion of
nature, is found with a slightly rounded base resting on a flatter surface of
rock below; and is so nearly balanced, that the force of a man suffices to
move it. Some of these have been objects of much superstitious veneration
to their neighbourhood.
There is an amusing Chinese toy, made in obedience to the same principle. It has the appearance of a little fat, laughing man, sitting on the
ground with his feet concealed under him; but where the feet should be,
there is only a rounded smooth surface, with heavy lead ballast placed in it,
so low, as always, when allowed, to raise the body to the erect or sitting




90                       M ECHANIC S.
attitude. A child pushes the little fellow down again and again, and would
persuade him to be still, but is surprised to see him always up the moment
after, shaking about and as lively as ever.
The vibratory motion of a pendulum, as dependent upon the circumstance
of the centre of gravity having been moved from its lowest place, which it
again constantly seeks, was so fully considered in the last chapter, that it
need not be again dwelt upon here; but we have to enumerate the following phenomena as being of the same class:
-The vibrations of a common swing.
-The rocking of a balloon when it first ascends.
-The spontaneous shutting of those gates or doors of which the upper
hinge overhangs or projects beyond the lower, causing the gate, when in the
shut position, to have its lock lower than when in any other. Such a gate
always returns of itself, from either side, to the shut position, just as a penpulum  returns to the lowest part of its arc:-the gate, in fact, is but a
sloping pendulum.
Of the same nature also is the rocking or rolling of a ship, in particular
states of wind and sea. Nhen the centre of gravity of a ship is too low,
owing to all the heavy load being placed near the keel, this pendulummotion, in rough weather, becomes excessive and dangerous.
The actions and postures of animals, and particularly of man, illustrate beautifully the observations made above with respect to the centre of gravity.
A body, we have seen, is tottering in proportion as it has great altitude
and narrow base-but it is the noble prerogative of man to be able to support his towering figure with great firmness, on a very narrrow base, and
under constant change of attitude. This faculty is acquired slowly because
of the difficulty. A child does well who walks at the end of ten or twelve
months; while the young of quadrupeds, which have a broad supporting base,
are able to stand and even to move about almost immediately after birth.
The supporting base of a man is the space occupied by and included between the feet. The advantage of turning out the toes is, that without taking
much from the length of the base, it adds considerably to the breadth.
If there be much art in walking on two perfect feet, there is still more in
walking on two slender wooden legs, with rounded extremities:-which,
however, we often see done, by mutilated soldiers and sailors.
All the ladies of the empire of China have to acquire nearly the same
talent as these victims of war; for barbarous custom has crippled them, by
confining their feet for life in such shoes as fitted them in infancy.
But surpassing in difficulty any of these instances is the practice, which is
general among the inhabitants of the sandy plains, called the Landes, in the
south-west of France, of walking on stilts.  The Landes afford tolerable
pasture for sheep; but during one portion of the year are half covered with
water, and during the remainder are still very unfit walking ground, by
reason of their deep loose sand and thick furze. The natives meet the inconveniences of all seasons by doubling the length of their natural legs, through
the addition to them of the stilts mentioned, which they call des echasses.
Mounted on these, which are wooden poles, put on and off as regularly as
the other parts of dress, they appear to strangers a new and extraordinary
race of long-legged beings, marching over the loose sand, or through the water,
with steps of eight or ten feet in length, and with the speed of a trotting
horse; their moderate journeys being of thirty or forty miles in a day. While




CENTRE OF GRAVITY.                             91
watching their flocks, they fix themselves in convenient stations by means of a
third staff which supports them behind, and then with their rough sheep-skin
cloaks and caps, like thatched roofs over them, they appear like little watchtowers, or singular lofty tripods, scattered over the face of the country.
Still beyond the art of walking on stilts is that which some persons
attain of walking and dancing on a single rope or wire; or even of keeping
the centre of gravity above the base, while standing on the moveable support
of a galloping horse.-A rope-dancer usually carries a long pole in his hand,
to balance him; it is loaded at each end, and when he inclines, he throws
it a little towards the side required, that the reaction may restore his
perpendicularity.
Much art of the same sort is shown in the attitudes and evolutions of the
skater; in the amusements of supporting a stick upright on the end of the
finger; and many other feats of a like kind.
Attitudes generally depend on the necessity of keeping the centre of
gravity of the body over the base under variety of circumstances, as in the
straight or upright part of a.man who carries a load on his head;-the
leaning forward of one who carries it on his back; the hanging backwards
of one who bears it between his arms;-the leaning to one side of him who
is carrying a weight on the other side;-the habitual carriage of very fat
people, whose head and shoulders are thrown back, giving a certain air of
self-satisfaction,-an air which belongs also to the expectant mother, and
even to the dropsical patient, although producing iii the latter so sad an
incongruity.
When a man walks or runs, he inclines forward, that the centre of gravity
may overhang the base: and he must then be constantly advancing his foot
to prevent his falling. He makes his body incline just enough to produce
the velocity which he desires.
A man, in pulling horizontally at a load, is merely causing his body to
overhang its base, so that its tendency to fall may become a force or power
applicable to the work.
When a man rises from a chair, he is seen first to bend the body forward,
or to draw the feet backward, so as to bring the feet or base under the centre
of gravity, and then he lifts the body up.  If he lifts too soon, that is,
before the body be sufficiently advanced, he falls back again.
A man standing with his heels close to a perpendicular wall, cannot without falling, bend forward sufficiently to pick up any object that lies before
him on the ground; because the wall prevents him from throwing part of
his body backward, to counterbalance the head and arms which must project forward. A' person little versed in suclh matters, might agree to give
ten guineas for permission to possess himself, if he could, of a purse of twenty,
laid on the ground before him: he of course would lose his stake.
When a man walks at a moderate rate, his centre of gravity comes alternately over the right and over the left foot. This is the reason why the
body advances in a waving line, and why persons walking arm in arm shake
each other, unless they make the movements of their feet to correspond, as
soldiers do in marching.
Sea Sickness is a subject closely related to the present.  Man requiring,
as now explained, so strictly to maintain his perpendicularity, that is, to keep
the centre of gravity always over the supporting part of his body, ascertains
the required position in various ways, but chiefly by comparing the perpendicularity, or other known position of things about him, with his own posi



92                        MECHANICS.
tion. Vertigo and sickness are the consequences of depriving him of his
standards of comparison, or of disturbing them.
Hence on shipboard, where the lines of the masts, windows, furniture, &e.
are constantly changing, sickness, vertigo and other affections of the same
class are common to persons unaccustomed to ships.  Many persons experience similar effects in carriages, and in swings; or on looking from a lofty
precipice, where known objects being distant, and viewed under a new aspect, are not so readily recognized; also in walking on a wall or roof; in
looking directly up to a roof, or to the stars in the zenith, because then all
standards disappear  on entering a round room, where there are no perpendicular lines of lightkand shade, as when the walls and roof are covered with
a paper which has no regular arrangement of spot; on turning round, as in
waltzing, or if placed on a wheel; because the eye is not then allowed to
rest long enough on any standard, &c.
People when in the dark, and therefore blind people, always use standards
belonging to the sense of touch; and it is because, on board of a ship,
the standards both of sight and touch are lost, that the effect is so very
remarkable.
But sea sickness also partly depends on the irregular pressure of the
bowels among themselves and against the containing parts, when the influence of their inertia and weight varies with the rising and falling of the ship.
From the nature of sea sickness, as discovered in these facts, it is seen
why persons unaccustomed to the motion of a ship, often find relief by keeping their eyes directed to the fixed shore, where visible; or by lying down
on their backs and shutting their eyes; or by taking such a dose of exhilarating drink as shall diminish their sensibility to all objects of external sense.
As no condition or form of matter escapes from the great laws of nature,
we find the attitudes and general condition of vegetable as well as of animal
bodies, characterized by the necessity of having the centre of gravity supported over the base. With what admiration may we contemplate the pine
and other trees in the forests or nature, springing up to heaven as perpendicularly as if the plummet had been at work to direct them; and no less
on the brows of precipitous hills than in the level plains. On a smaller scale,
we see the grasses and corn-stalks of our fields illustrating the same truth.
And whenever, in tree or shrub, accident or peculiar nature causes a deviation from perpendicularity, additional strength and support are provided.
Beauty ofform or position is often felt to exist in bodies, merely because
they possess the shape and support required, that the centre of gravity may
be stable.
In architecture, how displeasing is a wall or pillar that is not quite upright;
or a column with too small a base; or a very tall narrow house; or a long
slender chimney. On the other hand, how beautiful in a lofty edifice is the
suitable succession of columns, from the massive Doric of the basement,
supporting the whole superstructure, to the light Corinthian or kindred forms
seen above. The Chinese pagoda is a fine example of the union of certain
requisites for stability, viz., perpendicularity and expanding base, with the
other qualities of perfect symmetry, graceful proportion, and fanciful ornament. When seen crowning a rising ground in a wooded island, or springing up from the centre of a rich garden, it forms, perhaps, one of the most
beautiful objects which fancy has ever designed.
Beauty of attitude and grace of carriage in the human individual are in
great part referable to the same principle.




CENTRE  OF GRAVITY.                            93
The postures of opera dancers might pass as intentional illustrations of the
number of ways in which the centre of gravity may be kept above a narrow
base, by counteracting one disturbing motion or extension of a limb by some
opposite and corresponding motion. The common statute of the god Mercury
on tip-toe is a permanent familiar illustration of such a beautifully balanced
attitude.
Grace of carriage includes not only a perfect freedom of motion, but also
a firmness of step, or steady bearing of the centre of gravity over the base.
It is usually possessed by those who live in the country, taking much and
varied exercise, or who make gymnastics a part of their discipline. What
a contrast is there between the gait of the active mountaineer, enjoying the
consciousness of perfect nature, and that of the mechanic or shop-keeper,
whose confinement to the cell of his trade soon produces in his body a shape
and air corresponding to it-and in the softer sex what a difference is there,
between that active and graceful fair one who recalls to us the fabled Diana
of old, and that other sedentary being who, having scarcely trodden but- on
smooth pavements and carpets, under any new circumstances, carries her
person as if it were a load quite new and foreign to her.
The centre of gravity is also the centre of inertia.  When a person lifts
a uniform rod by its middle, the inertia of both ends being equal, he overcomes it equally, and raises thbm evenly together. When he lifts by a part
nearer to one end, the shorter and lighter portion having less inertia will rise
the first, and there will be a turning motion of the rod round the finger as a
centre, proportioned to the excess of inertia in the greater side.
-The centre of gravity, or inertia, however, is not necessarily in the centre
of the mass; for if a weight of -three pounds, a,
be affixed to one end of a rod, and a weight of       Fig. 34.
only one pound, b, be affixed to the other, the
two will still be balanced, if supported or lifted
by a point of the rod, c, three times nearer to the
centre of the large weight than to the centre of
the small one. This fact is explained under -the head of lever, a few pages
hence. For the sake of simplicity, in describing such experiments, the
weight of the connecting rod itself is neglected.
The centre of gravity or inertia is also the centre of centrifugalforce:for if the balls a and b of the last figure were made to spin round a common
centre, as by making the connecting rod rest and turn upon a point or pivot
at c; unless the point c were the centre of inertia of the two, the pivot
would always be drawn in the direction of that end of the rod at which there
was the greatest centrifugal force. It is on this account that in the case of
amill-stone, or great fiy-wheel, or of the balance wheel of a watch, the axis
must pass through the centre of inertia, to prevent its being more worn on
one side than on the other.
When we say in astronomy, that the earth revolves round the sun, or that
the moon revolves round the earth, we do not speak with absolute correctness, for in all such cases, both bodies are revolving round the common centre of inertia of the two. In the case of-the sun and earth, as the former is
about a million times larger than the latter, the common centre of inertia of
the two is a million times nearer to its centre than to the centre of the earth,
and is therefore within its body or circumference.
The centre of inertia in a body moving evenly is also its centre of action
orpercussion; because, if such centre come against an obstacle, the whole




94                        MECHANICS.
momentum of the body acts there and is destroyed; while if any other part
than the centre hit, the body loses only part of its momentum, and revolves
round the obstacle as a pivot or centre of motion, to pass it on the side towards which the greater inertia happened to be.
In a hammer, or a bar of iron used as a hammer, or in a pendulum, the
motion is not said to be even, because the velocity of the different parts is
different, being greatest far from the hand or centre of motion, and the centre
of all the motal inertia is nearer to the fast moving end than to the other.
Its exact place, in many cases, is easily ascertained by calculation. In a uniform rod moving as a pendulum, for instance, it is at a distance of one-third
from the lower end. In the pendulum it is called the centre of oscillation.
If a man use a bar or rod of iron as a hammer, he must take care to make
it strike the object by'its centre of action, or his own hand will receive a part
of the shock. A very heavy mass thus carelessly used will seriously strain
the wrist. In a common hammer, as the chief part of the matter is at the
end, the centre of percussion is there too, and no precaution of the kind mentioned is required.
If a rod or small log of wood be suspended horizontally by a string tied to
its middle, or be floating in water, and if a forward blow be given directly
across it near to one end, the other end will be found, in the first instant to
have moved a little backward, or in a direction contrary to the blow, as if the
rod had been fixed upon an axis. The inertia of the general mass, by resisting the motion becomes in effect a
Fig. 35.              fixed axis.  This fact is amusingly
illustrated by laying the ends of a
long stick on two wine-glasses, and
then breaking the stick by a smart
downward blow of a poker on its centre. Instead of breaking the glasses
also, as by such a blow might be expected, the ends of the stick rise at the
instant of the stroke, to turn round certain centres of resistance in the fragments, as at a and b, and then fall harmless on the table.
In this section we have seen what admirable simplicity is given to many
of our reasonings and operations, by considering bodies in reference only to
their centre of inertia, under one or other of its namas.
"In a solid body moving about an ax.Is, like a wheel or weighing-beam,
the different parts have different velocities, according to their respective
distances from the axis or centre."  (Read the Analysis.)
The truth of this proposition is perceived at once on comparing the motion
in the rim of a wheel, or near the ends of a weighing-beam, with that in
parts nearer the centres. Suppose a d to be a
Fig. 36.        line drawn across a wheel, or, along a weighingbeam, the centre of motion in either case being
at c; then the outer circular line or path, a c,
which a point in a describes when moving, is
longer than the corresponding inner line, b f,
a    c             which a point at b describes in the same time, as
a is farther from the centre than b. This admits
of easy mathematical demonstration, and is indeed merely an instance of the truth, that the
proportions existing between any parts or lines in
one circle, hold with respect to the corresponding
parts and lines in all circles.




SIMPLE MACHINES.                            95
"Hence forces with different speed may still be placed in continued connection or opposition; and they will balance or be equivalent, if the one be,as
much more intense than the other as it is slower."  (Read the Analysis.)
This is the important truth upon which the whole of mechanics may be
said to hinge. It gives to man the simple machines or mechanicalpowers,
as they have been called,-the Lever, Wedge, Pulley, &c., which enable him
to adapt any species and speed of power which he can command to almost
any work which he has to accomplish: and the discovery of it and of means
to apply it may be said to have subjected external nature to his control. His
works are of a thousand kinds, from the displacing of a rock to the spinning
of a delicate thread; while the natural powers or forces at his command are
chiefly wind, waterfalls, fire and animal effort-and of which in any particular case, he may have only one kind at his service; —still, being able to connect together his power and resistance by solid media, of which different
parts move with any desired difference of velocities, he can employ any force
for a purpose of almost any kind.
There is, however, a false and most pernicious prejudice very generally
existing with respect to the simple machines, which we must begin by
removing, viz., that they increase the quantity of power or force applied to
them.  For instance, when one pound, as a at the end of a beam or lever,
is seen balancing two pounds, as b, at half the distance on the other side of
the axis, or four pounds as c, at a quarter of the distance, many persons believe that the lever itself gives or begets
a force equal to the difference of the           Fig. 37.
weights so balanced. But we shall now
show, that levers and all the other mechanical powers (as from the erroneous
idea above mentioned they have been
called,) merely enable us to make such
substitutions, so that of a small weight                r-'-'
descending far, in place of a greater   I..... 
weight descending a little way, or of an
inferior force working long, instead of a              * I
superior force working for a shorter
time,-and thus often to accomplish
ends to which the force possessed would
be quite unsuited if applied directly. In other words the simple machines
enable us to concentrate or divide any kind or quantity of force which we
possess, so as to suit it to our various purposes, just as mill-ponds and
branching channels enable us to accumulate or divide the force of a stream
of water; but they no more increase the quantity of power than a mill-pond
increases the quantity of water. When any slender force is caused through
a machine, to produce some effect which seems proportioned to an intense
force, it has always to act longer, or through more space than the other,
just in proportion as it is more slender; as a small stream of water acting
for ten minutes, may produce the same effect as a greater gush in one
minute. Twenty feet of the action of a small horse near the circumference
of a great wheel, may be rendered, by intervening machinery, equivalent to
ten feet of the action of a heavy ox or elephant nearer the centre. And one
horse in drawing through six hundred feet, or a hundred horses in drawing
through six feet, or the piston of a great steam-engine, in raising one from the
bottom to the top of its cylinder, &c., may all be made to do the same work.




96                       MECHANICS.
To illustrate this subject farther; we shall suppose a weighing-beam xy,
with a weight of one pound hanging at the end x; then if a spring issuing
from the fixed box at E, with uniform force of one pound, be made to push
at the other end of the beam y, it will just balance the weight; and if it be
in the slightest degree stronger than
Fig. 38.             the weight, it will push the end of
the beam y down to B, and will raise
3. [    3 j      the weight to F.  If, instead of the
"*...~.. -.*......          single spring of one pound at the end
He _...   of the beam, two such springs be
applied at half way from the centre
to the end so as to press at A, where
there is just half the extent of motion, or room to act, as at B, exactly
the same effect will follow. Now because one spring at the end of the beam
is seen here doing the same work as two similar springs, or a single spring
of double strength at the middle, it might at first appear that there was a
saving of power by using the single spring and longer lever; but let it be
observed, that the two middle springs have each issued from their box only
one inch, while the single spring at the end has issued two inches: in both
cases, therefore, exactly two inches of one-pound spring have been used.
In the last.experiment, pound weights or little buckets of water might be
used instead of the springs, and with exactly the same result-one pound or
pint at the end of the arm producing the same effect as two pounds or pints
at the middle of it; but it would be observed that the single quantity fell two
inches, while the double quantity at half distance fell only one inch; and to
replace them after they had done their work, there would evidently be the
same labour, whether a person had to lift the single quantity first one inch,
and then another, or had to lift, first, one half of the double equipoise an inch,
and then the other half as much. —Each atom of matter may be considered
as held to the earth by its thread of attraction, and if one atom rise or fall
ten inches, just as much of the supposed thread of attraction will be drawn
out or returned as if ten atoms rise or fall one inch. And so where a weight
of one pound is made to do any work, instead of a weight of two pounds,
there is no more saving than in giving away two yards of single rope instead
of one yard of double rope; and in like manner for all other differences of
intensity.
If a man were to exert a force of one hundred pounds at A, in the above
figure, to lift the weight, a boy at B, with force of fifty pounds, might do
the same work; but the man would only have worked or pressed down
through one foot, while the boy would have worked through two; and therefore, although the boy, with the assistance of the lever, seemed to become as
strong as the man, the case would merely be, again, that of the one-pound
spring unbending two inches, to produce an effect equal to that of the twopound spring unbending one inch. The boy would be using two feet of his
smaller force, where the man used one foot of his greater force; and if the
work had to be long continued, the boy would have completely exhausted
himself, when the man remained yet fresh.
A case of the lever, exhibited in this diagram, serves well to explain the
nature of mechanical powers in general. Suppose A to be a weight of four
pounds at the end of a rod or lever A B, (p. 97) made to turn on c as an
axis or fulcrum, and having the arm c B four times as long as the arm c A,
(but the two arms of the lever being equipoised sd as not to conceal the action




MMPLE MACHINES.                             97
of weights subsequently attached to them;) then one pound at the end B,
would balance the four pounds at the end of A, and with the slightest additional weight would preponderate. Now let us suppose the arc B b to have
been fixed to the long arm of the lever with the four projections or shelves
here shown on which balls of one
pound might rest; then if one of the            Fig. 39.
four balls from the plane d were to roll
upon the first shelf, it would just
balance A, and, with one grain more,
would descend to the plane e, one inch    ct                  d
below; then a second ball of one pound..            _____
would occupy the second shelf, and         c.....
would descend in the same way, to be                           e
followed by a third, and afterwards by
a fourth; and when the whole four had
fallen from d to e, they would Just have lifted the four pound mass, at the
other end of the lever, one inch. So that, although one pound was seen
here lifting four pounds, it would only have lifted them one-fourth part as
far as it fell itself, and the sum of the phenomena would be, that four pounds
by falling one inch at the long end of the lever, had raised four pounds
through the same distance of one inch at the short end. No mechanical
power or machine generates force more than the lever does in this case.
It appears, then, from all this, that as the quantity of motion in a body is
measured by its velocity and the number of atoms in it conjointly, so the
quantity of force exerted in any case, is measured by the intensity of the
force conjointly with the space through which it moves. A clear mode,
therefore, of comparing forces, is to state the lengths and the intensitiesfor instance, to speak of ten feet of one-pound force, as equal to one foot of
ten-pound force, &c.
A horse pulling with the force of fifty pounds goes generally at the rate of
six miles an hour; the steam-engine piston is generally made to move at the
rate of two hundred feet per minute, bearing a pressure of steam of about
twenty pounds to each square inch of its surface; a particular mill-stream
may have a force of one hundred pounds, with a velocity of a hundred and
fifty feet per minute:-now it is easy, by simple arithmetic, and the rule of
length and intensity above explained, to compare all these and other forces
as applicable to any given work. We must warn the reader, however, that
there are many important considerations connected with the practical employment of forces, according to their respective nature and that of the resistance
to be overcome, which cannot be entered upon in this elementary work. In
very many cases there is a great waste or unavoidable loss of force, because
the resistance, in yielding, runs away or escapes from the force; as when a
ship runs away from the wind which is driving her, or the floats of a quick
moving water-wheel, from the stream which turned it. Horses drawing boats
or carriages at the rafe of five miles an hour, might exert great force, but to
have a speed exceeding twelve miles they might require their whole effort
to move their own bodies. As a general rule, although equal quantities of
force balance each other when applied to parts of a lever or wheel altogether
or nearly at rest, still when a force is made to act near its axis or fulcrum,
to produce considerable velocity in a more distant part of the machinery,
much of it. is wasted in pressure against the fixed fulcrum.
What an infinity of vain schemes-yet some of them displaying great
ingenuity-for perpetual motion, and new mechanical engines of power, &c.,
7




98                       MECHANICS.
would have been checked at once, had the great truth been generally understood, that no form or combination of machinery ever did or ever can increase,
in the slightest degree, the quantity of power applied. Ignorance of this is
the hinge on which most of the dreams of mechanical projectors have turned.
No year passes, even now, in which many patents are not taken out for such
supposed discoveries; and the deluded individuals, after selling perhaps even
their household necessaries to obtain the means of securing the expected
advantages, often sink into despair, when their attempts, instead of bringing
riches and happiness to their families, end in disappointment and ruin. The
frequency, and eagerness, and obstinacy, with which even talented individuals,
owing to their imperfect knowledge of this part of natural philosophy, have
engaged in such undertakings, is a remarkable phenomenon in human nature.
Examples of such schemes will be noticed in different parts of this work,
where they may serve to illustrate points under consideration.
" Lever, wheel and axle, &c."  (Read the Analysis, at page 84.)
These are the simplest of the contrivances which the circumstances of
solidity in masses has enabled man to adopt for the purpose of connecting
or opposing forces and resistances of different intensities. We proceed to
describe them, and to explain some of their useful applications.
" Lever."
A beam or rod of any kind, resting at one end of a prop or axis, which
becomes its centre of motion, is a lever; and it has been so called, probably,
because such a contrivance was first employed for lifting weights.
This figure represents a lever employed to move a block of stone; a, is
the end to which the power or force is applied, f, is the prop or fulcrum, and
the mass b, is the weight or resistance. According to the rule already given
and explained at page 96, the power may be
Fig. 40.          as much less intense than the resistance as
*   it is farther from the fulcrum, or moving
through a greater space. A man at a, there:
fore, twice as far from the prop as the eentre
__ -/            of gravity of the stone b, will be able to lift
a stone twice as heavy as himself; but he
will lift it only one inch for every two inches
that he descends: and two men would be required, acting at half the distance, to do the same work.
There is no limit to the difference, as to intensity, of forces which may be
made to balance each other by the lever, except the length and strength of
the material of which levers have to be formed. Archimedes said, " Give me
a lever long enough, and a prop strong enough, and with my own weight I
will lift the world." But he would have required to move with the velocity
of a cannon-ball for millions of years, to alter the position of the earth by
a small part of an inch. As stated in a former part of the volume, this feat
of Archimedes is, in mathematical truth, performed by every man who leaps
from the ground; he kicks the world away from him when he rises, and
attracts it again when he falls back.
To calculate the effect of a lever, in practice, we must always take into
account the weight of the lever itself, and the fact of its bending more or
less; but in expounding the theory of the lever, it is usual to consider, first,




SIMPLE MACHINES.                               99
what would be the result if.the lever were a rod without weight and without
flexibility.
The rule for the lever, that the opposing forces, to balance each other, must
be more or less intense, exactly as they act nearer to or farther from the
centre, holds in all cases, whether the forces be on different sides of the prop
or both on the. same side, and whether the force nearest to the prop have the
office of power or of resistance; it holds, also, whether the lever be straight
or crooked.
The following are examples of levers with the prop between the forces.
The handspike, represented in page 98, is a lever moving a block of stone.
The same form, when made of iron, with the extremity formed into claws, is
called crow-bar. Both kinds are used by gunners, in working cannon during
battle: they are also used generally for lifting and moving heavy masses
through small spaces, as the materials of the mason, the ship-builder, the
warehouse-man, &c. A short crow-bar is the instrument of house-breakers,
for wrenching open locks or bolts, tearing off hinges, &c.
The common claw-hammer, for drawing nails, is another example. A
boy who cannot exert a direct force of fifty pounds, may yet, by means of
this kind of hammer, extract a nail to which half a ton might be quietly suspended,-because his hand moves through, perhaps, eight inches, to make
the nail rise one-quarter of an inch. The claw-hammer also proves, that
it is of no consequence whether the lever be straight or crooked, provided
it produces the required difference of velocity between power and resistance.
The part of the hammer resting on the plank is the fulcrum.
A pincers or forceps consists of two levers, of which the hinge is the
common prop or fulcrum. In drawing a nail with steel forceps or nippers,
we have a good example of the advantages of using a tool: 1, the nail is
seized by the teeth of steel instead of by the soft fingers: 2, instead of the
griping force of the extreme fingers only, there is the force of the whole
hand conveyed through the handles of the nippers: 3, the force is rendered,
perhaps, six times more effective by the lever-length of the handles: and 4,
by making the nippers, in drawing the nail, rest on one shoulder as a
fulcrum it acquires all the advantages of a lever or claw-hammer for the
same purpose.
Common scissors are also double levers, and those stronger shears with
which, under the power of a steam-engine, bars and plates of iron are now
cut as readily as paper is cut by the force of the hand.
The common fire-poker is a lever. It rests on the bar of the grate as its
prop, and displaces or breaks the caked coal behind as the resistance.
The mast of a ship, with sails set upon it, may be regarded as a long
lever, having the sails as the power, turning upon the centre of buoyancy of
the vessel as the fulcrum, and lifting the ballast or centre of gravity as the
resistance.  For-this reason lofty sails make a ship heel or lean over greatly,
and if used in open boats, are dangerous. In some of the islands in the
Eastern and Pacific Oceans, for the sake of sailing swiftly, boats are used
so extremely narrow and sharp, that to counteract the overturning tendency
of their large sails, they have an outrigger or projecting plank to windward, on the extremity of which one or more of the crew may sit as a
balance.
Perhaps no instance of the lever, with the prop between the forces, is
more interesting than the weighing-beam; whether with equal arms, forming the common scale-beam; or with'unequal arms, forming the steel-yard.




100                       MECHANICS.
We have seen why quantities of matter attached at equal distances from
the prop, must be equal to each other in order to balance. A lever, therefore, which enables us to place quantities thus exactly in opposition to each
other, and which turns easily on its axis, becomes a weighing-beam. Of this
the annexed figure shows a common form.
Ftg. 41.          The axis or pivot at c is sharpened below,
CZ            wedge-like, that the beam may turn easily,
and that its centre of motion may be nicely
lli b    determined; in a delicate balance for philo5j~s~ —F' —--— ~U         sophical purposes, the axis is almost as sharp
as a knife edge, and rests on some hard smooth
surface of support, so as to turn with the
weight of a small part of a grain. The scales
also of a weighing-beam are suspended on sharp edges to facilitate motion,
and to determine nicely the points of suspension. If the two arms of a beam
be not of perfectly equal length, a smaller weight at the end of the longer will
balance a greater weight at the end of the shorter. An. excess of half an
inch in the length of a beam-arm, to which merchandise is attached, where
the arm should be eight inches long, would cheat the buyer of exactly one
ounce in every pound. This case might be detected instantly, by changing
the places of the two things balanced; for so, the lightest would be at the
short arm, and would then appear doubly too light. A beam intended for
delicate purposes, and required, therefore, to turn easily, must have its centre
of gravity very nearthe axis on which the beam turns; for if otherwise, the
beam will be in the predicament of a ship with the ballast too high or too low:
in the former case, when once inclined, it would fall over, and not to recover
itself: in the latter, it would tend to remain horizontal, and therefore would
be less free to move. The proper situation of the centre of gravity is a little
below the axis or line of support, that the beam may return with sufficient
readiness from any state of inclination,' to its horizontal position of rest.
There is a mode of arriving at very accurate results, even with a weighing-beam which is not itself accurately made, provided it has very free
motion, viz., first, very nicely to balance in one scale the substance to be
weighed, and then to remove it, and to put weights into the same scale, until
a perfect balance is produced. Such weights must be the exact equivalent
or weight of the substance, however unlike to each other the arms of the
balance may be.  A projecting rod, or plank, or branch of a tree, may thus
be made to answer the purpose of a weighing-beam, by attaching any substance to its extremity and 6bserving minutely how far such substance
bends it, and then trying what weights would bend it as much.
The steel-yard is a lever with unequal arms, and any weight as b, on the
long arm, will balance as much more
Fig. 42.            weight as a on the short arm, as the former is supported farther from the fulcrum
I_ 2  3   4.S 6  than the latter.  Thus; if the hook at the
short end be one inch from the centre of
support, c, a pound weight b, on the long
arm at four inches, will balance four
pounds a, at the short arm.  This sup.
poses, however, that the steel-yard when
bare, hangs horizontally, from having a
greater mass of matter in the short arm to counterbalance the long slender




SIMPLE  MACHINES.                         101
arm from which the shifting weight hangs. When this is not the case, a
corresponding allowance has to be made.
The Chinese, who are so remarkable for the simplicity to which they have
reduced all their common implements, weigh any small objects by a delicate
pocket steel-yard. It is a rod of wood or ivory, about six inches long, with
a silk cord passing through it at a particular part, to serve as a fulcrum, and
with a sliding weight on the long arm, and a small scale attached to the
short one.
The following are examples of levers with both forces on the same side of the
prop, and where the more distant force acts as the power.
A common wheel-barrow is.a lever, in using which a man bears as much
less than the whole weight of the load as the centre of gravity of the load is
nearer to the axle of the wheel than to his hands.
When two porters carry a load placed midway between them, on a pole,
they share it equally, that is to say, each bears a half, for the pole becomes a
lever, of which each porter is a fulcrum, as regards the other; but if the load
be nearer to one end than to the other, heto whom
it is nearest bears proportionably more of its weight.  F g 48.
A load at c is equally borne by a porter at a and       c 
by one at b; but a load at d gives three-quarters
of its weight to the man at a and only one-quarter
to him at b.
Two horses drawing a plough, act from the ends
of a cross bar, of which. the middle is usually hooked to the plough. The
horses must thus pull equally, to keep the bar directly across. When on
heavy land, three horses are yoked, and two of them are made to draw from
one end of the bar, it must be attached to the plough by a hook, not at its
middle, but half as far from one end of it as from the other.
The oar of a boat is a lever of this kind, where singularly the purpose of
fulcrum is served by the unstable water.
The common nut-crackers furnish another instance, by the lever-power of
which a person can break a shell many times stronger than he could break
with the bare fingers.
The consideration of this kind of lever explains why a finger caught near
the hinge of a shutting door is so much injured. The momentum of the
door acts by a comparatively long lever, upon a resistance placed very near
the fulcrum. Children pinching their fingers near the hinge of a door, —or of
the fire-tongs, which furnishes a similar case-wonder why the.bite is so keen.
The phenomenon of the branch of a tree gi-ving way, when in autumn
overloaded with fruit, or in winter with snow, also exhibit the action of this
kind of lever. The resistance is the cohesion of the upper side of the branch
to the tree, and the fulcrum is the part below which is last broken.
The following are examples of the lever, where the two forces are on the
same side of the pivot, but where that nearest to the pivot acts as the power.
In this kind, the power is more intense than the resistance.
The hand of a man who pushes open a gate while standing near the hinges,
moves through much less space than the end of the gate, and hence must act
with great force.
When a man uses the common fire-tongs, the ends move much further




102                       MECHANICS.
than his fingers, and therefore with less strength. No one fears a pinch
with the ends of the fire-tongs.
The most beautiful and remarkable instances of this modification of lever
are in the limbs of animals. The object in these was to give to the extremities great range and freedom of motion, without clumsiness of form; and it
has been attained most perfectly by the tendons or ropes which move them,
being attached near to the joints, which are the pivots or fulcra of the bone
levers.
In the human arm, the deltoid muscle, which forms the cushion of the
shoulder, by contracting its fibres less than an inch, raises the elbow twenty
inches, and of course, if it overcome a force of fifty pounds at the elbow, it
must itself be acting with a force at least twenty times as intense, or of one
thousand pounds.-What extraordinary strength of muscle, then, is displayed
by a man who lifts another at the end of his extended arm; yet this feat is
frequently accomplished, and even on both sides of the person at once.
How powerful again must be the wing-muscles of birds, which, by this
kind of action, sustain themselves in the sky for many hours together. The
great albatros, with wings extended fourteen feet or more, is seen in the
stormy solitudes of the Southern Wean, accompanying ships for whole days,
without ever resting on the waves.
A little contraction of the glutaei muscles of the hips gives to the human
step a length of four feet.
While the erroneous opinion prevailed, that machines increased power,
instead of, as they do, merely accommodating forces to purposes, this last
kind of lever, where a great force acting through a short distance is made to
gain great extent of motion and other benefits, was regretted by many as a
most unprofitable, contrivance and was called the losing lever.
It is almost unnecessary to say, that the same rule of comparative velocities ascertains the relations required between power and resistance, where a
combination of levers is used, as where there is only one. If a lever which
makes one balance four, be applied to work a second lever which does the
same, one pound at the long arm of the first will balance sixteen pounds at
the short arm of the second, and would balance sixty-four at the short arm
of a third such, &c.
The general rule for the lever, that a force may be less intense the farther
it is from the pivot, supposes always that the force acts at right angles, or
directly across the lever; for if there be any obliquity, there is a corresponding diminution of effect, as explained under the head of resolution offorces,
at page 57.  For instance, one pound at b on the end of the long arm of the
bent lever b d a, because its weight does not act
Fig. 44.         directly across b d, has influence only as if it were
acting directly at the end of a shorter horizontal............           arm df; and the two-pound weight at a acts only
b  as if it were on a horizontal arm at e; now e being
only half as far from the centre as f, two pounds
at a, in the position of the lever here shown,
would just balance the one pound at b.  In every
case, the exact influence of weights is known by
referring them to places directly above or below
them, on a supposed horizontal lever e f. What
is called a bent-lever balance, is made on the principle here explained.  It has
on one side a heavy weight as at a, and on the other side a scale attached at
b; and the weight of any thing put into the scale is indicated by the position




SIMPLE MACHINES.                           103
then assumed by the lever, marked by the point at which it cuts an arc of
divisions placed behind it. In any common weigh-beam, the point of suspension of the scales being a little below the axis of motion of the beam,
there is a degree of the property of the bent-lever balance, and enough to
require notice in very nice experiments.
"The Wheel and Axle"
is the next to be mentioned of the simple machines. The letter d here marks
a wheel, and e an angle affixed to it; and we see that in turning together, the
wheel would take up or throw off as much
more rope than the axle, as its circumference       Fig. 45.
or diameter were greater than that of the Ale.
If the proportions were as four to one, one                  _ __
pound at b hanging from the circumference of           e
the wheel, would balance four pounds at a, Dp
hanging from the opposite side of the axle.
The proportions are equally indicated, and are
usually expressed by comparison of the diameters of the wheel and the axle.
This figure represents the same object as the
last, viewed endways. It explains why the
wheel with its axle has been called a perpetual
lever; for the two weights hanging in opposition, on the wheel at a, and on
the axle at b, are always as if they were connected by a
horizontal lever at a c b, of which the arms are respectively  Fig. 46.
the diameters of the wheel and the axle turning on the
centre c as the prop; and while a simple lever could only
lift through a small space, it is evident that this construc-   a
tion will lift as long as there is rope to be wound up.
A common crane for raisingrweights consists of an axle
to wind up or receive the rope which carries the weight,
and of a large wheel at the circumference of which the
power is applied. The power may be animal effort exerted
on the rim or outside of the wheel, or the weight of a man
or beast walking within it, and moving it as a squirrel
moves the cylinder of his cage.
The capstan used on board of ships, is merely a large upright axle or
spindle b, which by turning, pulls the           Fig. 47.
cable or-rope a b c; and it is moved by
the men pushing at the capstan-bars d, e,,                 d
f, &c., which for the time are stuck into
holes made for them in the broader part                     e
or drum, usually appearing above the
deck at the top of the spindle. These
bars may be considered as the spokes of                        c
a large wheel, and the effect produced by    c
a man working at one of them is in proportion to his distance from the centre. The capstan is chiefly used on board
ships for lifting the anchor, and for doing any other very heavy work; but it
is also applied for certain purposes on shore.
The common winch (represented as attached to the wheel and axle at the
letter c,) with which a grindstone is turned, or a crane worked,.or a watch




104                       MECHANICS.
wound up, is really in principle a wheel: for the hand of the worker
describes a circle, and there is no difference in the result whether an entire
wheel be turning with the hand or only a single spoke of a wheel.
That part of a common watch called the fusee is as beautiful an illustration of the principle of the wheel and axle now under consideration, as it is
a useful and ingenious contriFig. 48.                 vance. The spring of a watch,
q                   _            immediately after winding up,....':qt...being more strained, is acting
more powerfully than after_________             wards when slacker, and if
there were no means of equal-.t      -,,         izing its action, it would deI        F'               stroy the wished-for uniformity En the motion of the timepiece. The fusee is this means. It may be considered as a barrel or spindle,
gradually diminishing from its large end b, to its small end a, with the
surface cut into a spiral grove to receive the chain, by pulling at which the
spring in the box c moves the watch. Now when the watch has been wound
up, by a key applied on the axis of the fusee, the fusee is covered with the
chain up to the small end a, and the newly bent and strong spring begins
to pull by this small end or short lever; and afterwards, exactly as the
spring becomes relaxed and weaker, it is pulling at a larger and larger part
of the fusee-barrel, and so keeps up an equal effect on the general movement.
A large fusee in place of a common cylindrical axle, is often used with a
winch, for drawing water by bucket or rope from very deep wells.  When
the bucket is near the bottom of the well, and the laborer has to overcome
the weight of the long rope, in addition to that of the bucket and water, he
ddes so more easily by beginning to wind the rope on a small axle, that is
to say, on the small end of the fusee; and in Rroportion as the length of rope
diminishes, he lifts by a larger axle.
By the double axle a b, very unequal intensities of force may be balanced.
We see that iu turning it, a rope unwinding
Fig. 49.          from the small end a is taken up by the large
c     end b, turn for turn, and that the rope below
7f b    go   "'  must be shortened at each turn by the difference between the circumference of the ends
a and b. If the weight rise half an inch
only, while the handle of the winch describes
a circle of fifty inches, one pound force at the
winch would balance one hundred pounds
at d.
By means of a wheel, which is very large
in proportion to its axle, forces of very different intensities may be balanced, but the machine becomes of inconvenient
proportions. It is found preferable, therefore, when such an end is desired,
to use a combination of wheels of moderate size. In the adjoining figure,
three wheels are seen thus connected. Teeth on the axle d, of the first
wheel c, acting on six times the number of teeth in the circumference of
the second wheel g, turn it only once for every six times that c turns; and
in the same manner the second wheel, by turning six times, turns the




SIMPLE MACHINES.                            105
third wheel h once; the first wheel, therefore, turns thirty-six times for one turn of       Fig. 50.
the last; and as the diameter of the wheel             i
c, to which the power is applied, is three    h       r
times as great as that of the axle f, which
has the resistance, three times thirty-six,                        e e  dc
or one hundred and eight, is the difference
of velocity, and therefore of intensity, between weights or forces that will balance
here.-An axle with teeth upon it, as d?b
or e, is called a pinion.
On the principle of combined wheels,           a
cranes are made, by which one man can
lift many tons. It is even possible to
make an engine, by means of which a little windmill, of 4 few inches in
diameter, should tear up the strongest oak by the roots; but of course it
would require a very long time for its work.
The most familiar instances of wheel-work are in our clocks and watches.
One turn of the axle on which the watch-key is fixed, is rendered equivalent, by the train of wheels, to about four hundred turns or beats of the
balance-wheel; and thus the exertion during a few seconds, of the hand
which winds up, gives motion for twenty-four or thirty hours. By increasing the number of wheels, time-pieces are made which go for a year; if the
material would last, they might easily be made to go for a hundred or a
thousand years.
Wheels may be connected by bands as               Fig. 51.
well as by teeth. This is seen in the com-       -...
mon spinning-wheel, turning-lathes, grind-...
stones, &c. &c.  A spinning-wheel, as a c,
of thirty inches in circumference, turns by    (.
its band a pirn or spindle of half an inch, b,     /.
sixty times for every turn of itself...
" The Inclined Plane"
is the third means which we shall describe, of balancing, by solid media,
forces of different intensities. A force pushing a weight from  c to d, only raises it            Fig. 52.
through the perpendicular height e d, by
acting along the whole length of the plane                 -
c d; and if the plane be twice as long as it      C
is high, one pound at b acting over the
pulley d would balance two pounds at a, or
anywhere on the plane: and so of all other c
quantities and proportions, as already explained under the head of " Resolution of forces," at page 86.
A horse drawing on a road where there is a rise of one foot in twenty,
is really lifting one-twentieth of the load, as well as overcoming the friction
and other resistances of the carriage. Hence the importance of making roads
as level as possible; and hence our forefathers often erred in carrying their
roads directly over hills, for the sake of straightness considered vertically,
where by going round the bases of the hills they would scarcely have had
greater distance, and would have avoided all rising and falling. Hence, also,




106                       MECHANICS.
a road up a very steep hill must be made to wind or zig-zag all the way; for
to reach a given height, the ease of the pull to the horses is greater exactly
as the road is made longer.  This rule of road-making is exhibited remarkably in various parts of the world, where hills with almost perpendicular
faces have very- safe and' commodious roads upon them, leading to forts or
residences near their summits. An intelligent driver, in ascending a steep
hill on which there is a broad road, winds from side to side of the road all
the way to save his horses a little.
The railways of modern times offer a beautiful illustration of this subject.
They are made generally quite level, so that the drawing horse or steamengine has only to overcome the friction of the carriage; or where heavy
loads are passing only in one direction, as from mines, they are made to slope
a very little, leaving to the horse or other power only the office of regulating
the movement.
A hogshead pf merchandise, which twenty men could not lift directly, is
often seen moved into or out of a wagon, by one or two men, who have the
assistance of an inclined plane. In some canals, or rather particular situations on canals, the loaded boats are drawn up by machinery or inclined
planes, instead of being raised by water in locks, as is the usual mode.
It is supposed that the' ancients (the Egyptians particularly) must have
used the inclined plane, to assist in elevating and placing those immense
masses of stone, which still remain from their times, specimens of their
gigantic architecture.
Our common stairs are inclined planes in principle; but being so steep,
are cut into horizontal and perpendicular surfaces, called steps, that they
may afford a firm footing.
We may here recall, that a body falling freely, in obedience to gravity,
descends about sixteen feet in the first second, and that if made to descend
on an inclined plane, it moves just as much less quickly (besides. the loss
from the friction and the turning produced) as the length of the plane is
greater than the height. On a plane sloping one foot in sixteen of its length,
a body would descend only one foot in the first second.
The descent of a pendulum in its are is investigated 1thematically by
the laws of the inclined plane. And the laws of the inclined plane itself
are mathematically explained by the principle of the resolution of forces,
explained at p. 57.
" The Wedge"
is merely an inclined plane forced in between resistances to separate or overcome them, instead of, as in thq last case, being stationary while the resistance
is moved along its surface. The same rule as to mechanical
Fig. 58.    advantage has been applied to the wedge as to other simple,   machines; the force acting on a wedge being considered as
moving through its length c d, while the resistance yields to
the extent of its breadth a b.  But this rule is far from explaining the extraordinary power of a wedge.  During the
\tremor produced by the blow of the driving-hammer, the
wedge insinuates itself, and advances much more quickly
than the above rule anticipates.
The wedge is used for many purposes; as for splitting
blocks of stone and wood: for squeezing strongly, as in the
oil-press; for lifting great weights, as when a ship of war, in
dock is raised by wedges driven under the keel, &c.




SIMPBE MACHINES.                            107
An engineer in London, who had built a very lofty and heavy chimney,
common to all his steam-engines and furnaces, found after a time that,
owing to a defect in the foundation, it was beginning to incline. However,
by driving wedges under one side of it, he succeeded in restoring it to
perfect perpendicularity.
Nails, awls, needles, &c., are examples of the wedge; as are also all our
cutting instruments, knives, razors, the axe, &c.  These latter are often
used somewhat in the manner of a saw-which is a series of small wedgesby pulling them lengthwise at the same time that they are pressed directly
forward against the object. They themselves, indeed, when viewed through
a microscope, are seen to be but finer saws. It appears that the vibration of
the particles produced by the drawing of a saw, enables its edge to insinuate
itself more easily. The sharpest razor may be pressed directly against the
hand with considerable force, and will not enter, but if then drawn along
ever so little, it starts into the flesh.
" The Screw"
is another of the simple machines. It may be called a winding wedge, for
it has the same relation to a straight wedge that        Fig. 54.
a road winding up a hill or tower has to a straight
road of the same length and acclivity. 
A screw may be described as a spindle a d, with         )
a thread wound spirally round it,-turning or work-.
ing in a nut c, which has a corresponding spiral fur — 
row fitted to receive the thread.  The nut is some-  |        
times called the female screw.  Every turn of the....
screw carries it forward in a fixed nut, or draws a
moveable nut along upon it, by exactly the distance
between two turns of its thread: this distance, there-  I —   I7
fore, is the space passed through by the resistance,
while the force moves in the circumference' of the
circle described by the handle of the screw, as atf, in the figure. The disparity between these lengths or spaces is often as a hundred or more to one;
hence the prodigious effects which a screw enables a small force to produce.
Screws are much used in presses of all kinds: as in those for squeezing
oil and juices from such vegetable bodies as linseed, rapeseed, almonds,
apples, grapes, sugar-cane, &c.: they are used also-in the cotton press,
which reduces a great spongy bale, of which a few, comparatively, would
fill a ship, to a compact package, heavy enough to sink in water; —also, in
the common printing-press, which has to force the paper strongly against the
types:-a screw is the great agent in our coining machinery,-and in letter —
copying machines:-it is a screw which draws together the iron jaws of a
smith's vice, &c. The screw, although producing so much friction as to consume a considerable part of the force used in working it, is an exceedingly
useful contrivance.
As a screw can easily be made with a hundred turns of its thread in the
space of an inch, at perfectly equal distances from each other, it enables the
mathematical instrument maker to mark divisions on his work, with a minuteness and accuracy quite extraordinary. If we suppose such a screw to be
pulling forward a plate of metal, or pulling round the edge of a circle, over
which a sharp-pointed steel marker can be let down perpendicularly, always
in the same place, the marker, if let down once for every turn of the screw,
will make just as many lines on the plate as the screw makes turns; but if




10S                       MECHANICS.
made to mark at every hundredth or a thousandth of a turn of the screw,
which it will do with equal accuracy, it may draw a hundred thousand distinct lines in one inch.
The instruments called micrometers, by which the sizes of the heavenly
bodies, and of microscopic objects, are ascertained, are worked by fine
screws.
A perpetual screw is the name given where a screw acts on the teeth of a
wheel, so as to produce a continued rotation of the wheel.
~ A common cork-screw is the thread of a screw without the spindle, and
is used, not to connect opposing forces, but merely to enter and fix itself in
the cork COmplicated cork-screws are now made, which draw the cork by
the action of a second screw, or of a toothed rod or rack and pinion.
" The Pulley"
is another simple machine, by which forces of different intensities may be
balanced. A simple pulley consists of a wheel as
Fig. 55.       a b, which rests with its grooved circumference. on the bend of a rope, c a b d, and to the axis of
which the weight or resistance is attached, as
7I i                 at e.
In such a construction, it is evident that the
i. f    weight (let it be supposed ten pounds) is equally
/~.~]    A.supported by each end of the rope, and that a man
holding up one end, only bears half of the weight,
or five pounds; but to raise the weight one foot,
he must draw up two feet of rope; therefore,
with the pulley, he is as if lifting five pounds two
feet, where, without the pulley, he would have to
lift ten pounds one foot.
Many wheels may be combined together, and
in many ways to form compound pulleys. Whereever there is but one rope running through the whole, as shown here, the
relation of power and resistance is known by the number of folds of the
rope which support the weight.  Here there are four supporting folds, and
a power of one hundred pounds would balance a resistance of four hundred.'
As persons using pulleys generally find it more convenient
Fig. 56.    to stand upon the ground than to go up and apply their
force directly to one of the supporting ropes, the last of
~5.'~'    these is commonly made to pass over a wheel above, and
to come down. apart from the others, as shown here. This
portion not being directly connected with the weight, adds
convenience to the pulley, but is not to be counted with
the others, in estimating the relation of the power and
resistance.
In fixed pulleys, like those shown at a and c, p. 109,!u Vi.     there is no mechanical advantage, for the weight just moves
as fast as the power; yet such pulleys are of great use in
changing the direction of forces.  A sailor without moving from the deck of his ship, by means of such a pulley,
Go ~  may hoist the sail or the signal-flag to the top of the
loftiest mast. And in the building of l'ofty edifices, where
heavy loads of material are to be sent up every few




SIMPLE MACHINES.                            109
minutes, a horse, trotting away with the end of the rope from d, in a level
courtyard, causes the charged basket b to ascend to the summit of the
building as effectually as if he had the power of climbing, at the same rate,
the perpendicular wall.                               Fig. 57.
There is a case, however in which a fixed
pulley may seem a balancer of different intensities of force; viz., where one end of a rope is
attached to a man's body, and the other is carried.
over a pulley above, and brought down again to
his hands;-for safety this end also sthould be
attached to his body.  By using the hands then
to pull with force equal to half his weight, he    LJ
supports himself, and may easily raise himself to
the pulley.  A man, by a pulley thus employed,
may let himself down into a deep well, or from                 c
the brow of a cliff, with assurance of being able
easily to return, although no one be near to help
him; and cases have often occurred where, by
such means, a fellow-creature's life might have been saved, or other important objects attained. How easily, for instance, might persons either
reach or escape from the elevated windows of a house on fire, by such a
pulley, which might readily be found and used where ladders could not be
obtained! This kind of pulley furnishes a convenient means of taking a
bath from a ship's stern windows, &c.
The chief use of the pulley is on ship-board. It is there called a block,
although, strictly speaking, the block is only the wooden mass which surrounds the wheel or wheels of the pulley.  It aids so powerfully in overcoming the heavy strains of placing the anchor, hoisting the masts and sails,
&c., that, by means of it, a smaller number of sailors are rendered equal to
the duties of the ship. Pulleys are also used on shore, instead of cranes
and capstans, for lifting weights, and overcoming other resistances.
Surgeons, in former days, when they trusted rather to force than to the
address which better information gives, used pulleys much to help in the
reductions of luxations,-but often hurtfully, from not understanding the
force of the pulley. A good surgeon now rarely needs a pulley, and he who
should ignorantly stretch his patient on the rack, would be well requited by
similar treatment.
The cranks of bell-wires, seen in the corners of our rooms, are bent levers
nearly equivalent to fixed pulleys.
There is no reason, but old usage, why. the appellation of mechanicalpower
should be confined to the six contrivances now explained, for those of
which the account is yet to follow equally deserve it; and, as will be seen
under hydrostatics and pneumatics, the most powerful mechanical engines
do not belong to solids at all.
Engine of oblique action, is a title which may include a considerable
variety of contrivances for connecting different Velocities.
Suppose c a and c b to represent two strong rods connected together, like
a carpenter's folding rule, by a hinge or joint at c.  If the distant ends be
made to bear against notches in two obstacles, at a and b, and by force then
applied to c, either to push or to pull, the joint c be straightened or carried
towards d, the joint c will move through a much greater space than the simul



110                       MECHANICS.
taneous increase of distance produced between a and b; and, in proportion to
the disparity, the power applied at c will overcome
Fig. 58.       a more intense resistance at the extremities.  The
mechanical power of this  contrivance increases
Cl ]rapidly, the nearer the jointed rods approach to
straightness.
If we suppose the end a to be steadied by a hinge.              dc   on frame-work, and the end b to bear upon that
part of a printing-press which carries the paper
against the types, we have imagined the simple
press called, from its contriver, the Russell-press. A
man's force at d, at the moment when the rods are
drawn nearly to a straight line, becomes equivalent
to a pressure of many tons.
For the same reason, that by urging c toward d, in the last figure, the
extremities a and b are separated with great force, so by urging c in the contrary direction, the extremities would be drawn together with corresponding
force: and if we suppose a c b to be part of a rope coming through pulleys
at a and b, to one end of which rope, beyond a, great resistance is attached,
one man, by pulling at c, may move a weight or resistance many times
greater than he could move by his direct power.
The following is another mode of connecting an oblique and a direct force
so as to balance them, although of different intenFig. 59.        sities.  If to turn a wheel (represented here by,L            the circle) a weight be suspended from d, it is acting directly, for it descends just as fast as the circumference of the wheel moves, and would, therefore, be impelling with its whole strength: but if
it were suspended from the point e, it would then
be acting obliquely to the motion of that part of the
wheel, and from not descending so fast as if at d,
it would have as much less effect on the wheel
than if there, as the line e b is shorter than the line
d c.  The reason of this will be understood by
referring to the subjects of resolution of forces
and of bent levers, in former parts of the work.
I:   *   For the same reason, if such a wheel were used
in lifting weights, a man turning it could lift as
much more attached at the point e than at the point d, as the line d c is
longer than e b. A man turning this wheel in the direction from e to a,
with a weight hanging at e, would.be lifting that weight exactly as if he
were rolling it up the inclined plane or curve e a. This figure is useful
in explaining the varying intensity of the acFig. 60.        tion of a crank or winch, in different parts of
ea                      its revolution, and of the combination of levers
used in the Stanhope printing-press, in their
different positions: it explains also the degrees
- of strength and support afforded by oblique
stays in buildings and in ships' rigging, and
many other kindred matters.
The arrangement of cross-jointed wires, represented here, connects different velocities, and
therefore is really a mechanic power.  It has




SIiMPLE  MACHINES.                         111
been applied to some curious purpses, but to none of much utility.  By
pressing the ends a and b towards each other, the wires, from being in the
position represented in the upper figure, immediately assume the position
represented in the lower; so that the end c darts outward much farther than
the ends a and b approximate.
Different intensities of force are balanced, although not simultaneously, by
the following means; which, therefore, according to the old idea, have
some claim to the name of mechanic powers.
A man may have a purpose to effect, which a forcible downward push
would accomplish: but his body being too weak to give that push directly,
he may employ a certain time in carrying a weight to such an elevation,
above his work, that when let fall its momentum may do what is required.
Here the continued effort of the man in lifting the weight, to a height of
perhaps thirty feet, may be just sufficient to produce a blow which will
cause a stake or pile to sink into the earth one inch; and the contrivance
has therefore balanced forces, of which the relation as to intensity is marked
by the spaces thirty feet and an inch.
So also hammers, clubs, battering-rams, slings, &c., are machines which
enable a continued moderate effort to overcome a great but short resistance.
The fly-wheel, which by persons ignorant of natural philosophy, has often
been accounted a positive power, in common cases merely equalizes the
effect of an irregular force.
In using a winch to turn a mill, for instance, a man does not act with
equal force all around the circle: but a heavy wheel fixed on the axis moderates acceleration, and receives or absorbs momentum, while his action is
above par, and returns it again, giving to the machine, while his action is
below par, thus equalizing the movement. And in the common instances
of circular motion produced by a crank, as when, by the pressure of the foot
on a treadle, we turn a lathe or grind-stone, or spinning-wheel, the force is
only applied during a small part of the revolution, or in the form of interrupted pushes; yet the motion goes on steadily, because the turning grindstone, or wheel, or lathe, becomes a fly and reservoir, equalizing the effect
of the force. In a steam-engine which moves machinery by a crank, the
upward and downward pushes of the piston are converted, by means of a
heavy fly-wheel into a very steady rotatory motion.
A heavy wheel, however, has sometimes been used as a concentrator of
force or a machanic power. By means of a winch, or a weight, or otherwise,
motion or momentum being gradually' accumulated in the wheel, is then
made to expend itself in producing some sudden and proportionally great
effect. Thus, a man may lift a very heavy weight by first, in any way
giving motion to a fly-wheel, and then suddenly hooking a rope from the
weight to the axle of the wheel, which rope being wound upon the axle,
lifts the weight.  
A fly-wheel moved in the same manner, and containing the result of a
man's action during perhaps one hundred seconds, if made to impel a screwpress, will, with one blow or punch, stamp a perfect medal, or from a rough
flat plate of silver will form a finished spoon, or other utensil.
A spring, in the same sense, may become a mechanical power. A person
may expend some minutes in bending it, and may then let fly its accumulated
energy in an instantaneous blow. A gun-lock shows this phenomenon on a
small scale. The slow bending of a bow, which afterwards shoots its arrow
with such velocity, is another instance.




112                        MECHAN-ICS.
These, then, are the principal means yhrich the solid state of bodies affords
us of balancing forces of different intensifies. We shall find other such means
or mechanic powers belonging to liquids and airs. All of them are of inestimable value to man, by enabling him to accommodate the forces which he
can command to any kind of work which he has to perform. Thus he makes
his millstone turn with the same velocity, whether it be moved by the slow
exertion of a horse or bullock, walking in a ring, or by the quicker motion
of a river gliding under the wheel, or by the rapid gush of a water-fall, or
by the invisible swiftness of the wind. And again, each of these forces he
can equally apply to turn the heavy millstone or to twist a cotton thread.
The wants of men seem first to have led them to use the simple machines,
for the purpose of raising great weights, or overcoming great resistances
and hence the name long used of mechanic powers,-particularly for the
Lever, Wheel and Axle, Plane, Wedge, Screw, and Pulley: but the term
conveys to the uninformed a false idea of their real nature, and has begotten
the common prejudice with respect to them, that they generate force, or
have a sort of innate power for saving labour. Now so far is this from
being true, that in using them in any case, even more labour or bodily
exertion is expended than would suffice to do the work without them. This
assertion is intentionally rendered paradoxical to arrest, attention, but its
truth will appear from the following considerations.
One man may be able, with a tackle of pulleys having ten plies of the
rope, to raise a weight which it would require ten men to raise at once without pulleys. But if the weight is to be raised a yard, the ten men will raise
it by pulling at a single rope and walking one yard, while the one man at his
tackle must walk until he has shortened all the ten plies of rope of one yard
each; that is, he must walk ten yards, or ten times as far as the ten men
did. In both cases, therefore, to accomplish the same end, we have just
the same quantity of man's work expended, in the first, performed by ten
men in one minute, in the second, by one man in ten minutes; and if the
work were of a nature to continue longer, let us say a whole day for the ten
men, it would last ten days for the single man, and there would be ten days'
wages of a man to pay in both cases. There is, therefore, no direct saving of
human effort from using pulleys; indeed, there is a loss, because of the
great friction which has to be overcome. Now exactly the same is true of
all other simple machines, or mechanic powers; none of them  save labour,
in a strict sense of the phrase; they only allow a small force to take its time
to produce any requisite magnitude of effect, at the expense of additionality
overcoming a certain amount of friction or other such risistance.
The real advantage of these machines are such as the following:
That one man's effort, or' any small power, which is always at command,
by working proportionally longer, will answer the purpose of the sudden
effort of many men, even of hundreds or thousands, whom it might be most
inconvenient and expensive, or even impossible, to bring together.
A ship's company of a few individuals easily weighs a heavy anchor by
means of the capstan.
A solitary workman, with his screw or other engine, can press a sheet of
paper against types, so as to take off a clear impression; to do which without
the press, the direct push of fifty men would scarcely be sufficient: and
these fifty men would be idle and superfluous except just at the instants of
pressing, which occur only now and then. In this way the screw may be
said to do the work of fifty men, for it is as useful.
A man with a crow-bar may move a great log of wood to a convenient




SIMPLE  MACHINES.                           113
place, where twenty men would have been required to move it without the
crow-bar; and although the single man takes twenty minutes, perhaps, to
do what the many men would have done in one minute, as the twenty might
not have been wanted again for the rest of the day, the crow-bar may really
be as useful as the twenty men.
It is so important to have correct notions on the subject of the simple
machines or mechanical powers, that more space has been here allotted to
the explanation of the general principle, than has been usual in such works.
After the examination which it has now undergone, however, the author
hopes that none of his readers will have difficulty in conceiving clearly, that
"whatever, through a machine, is gained in power, is lost in speed or in
time, and vice versa "-or will have difficulty in detecting immediately any
common fallacy connected with the subject; —as that of supposing, for
instance, that a lever, or great pendulum, or spring, or heavy fly-wheel, &c.,
can never exert more force than -has passed into it from some source of
motion.
"By solid connecting parts, also, the direction of any existing motion or
force may be changed. Hence the endless variety of COMPLEX MACHINES."
(Read the Analysis at p. 84. )
It is this power of changing the direction of motion, added to the power
of connecting and adjusting various intensities of force and resistance by the
simple machines last described, which has enabled man to make complex
machines, rivaling in their performances the nicest work of human hands.
It would be endless to attempt the enumeration of the modes in which the
directions of motions may thus be changed, for it would be to enumerate
and describe the whole apparatus of the arts and sciences; but we shall
advert to a few as specimens.
Straight motion changed into rotatory.-The straight motion of wind or
water becomes rotatory in wind or water-wheels.-The straight downward
pressure of the human foot, acting at intervals on a treadle and crank, turns
round the grindstone, and common lathe, and spinning-wheel. The alternate rising and falling of the piston of a steam-engine is made, by means of
a crank, to turn the great fly-wheel and any other wheels which a steamengine miay move.,
Rotatory motion into straight.-An axle in turning will wind up a ropie,
and lift a weight in a straight line. —A crank on a turning axle, if connected
with a pump rod, will work the piston up and down; or it will work a saw.
Pallets or teeth on a turning-wheel act on the handle of a great forge hammer, so that every one in passing lifts the hammer and produces a blow.
We need not multiply instances. By a visit to great manufacturing towns,
or, indeed, by simply directing the eyes to what is passing around, in any
part of the civilized world, we discover miracles of mechanic art:-machines
driven by wind, water or steam for grinding corn; —machines for sawing
wood and giving it various forms;-machines in which rods of metal are
seized between great rollers, and are flattened at once into thin plates, as if
they were of clay, and these plates again are slit into bars or ribbonsspinning machines, which perform their delicate office even more uniformly
than human hands, forming thousands of threads at once, in obedience to the
impulse of a single ste tm-engine;-weaving machines, which accomplish their
difficult task with the most admirable perfection;-paper-making engines,
which convert worn-out and apparently useless'remnants of our apparel, into
8




114                        MECHANICS.
the uniform and beautiful texture of paper, a texture which, with the farther
assistance of the pen, or types, or engraved plate, becomes a magic conservatory of mind, shutting up among its folds the brightest effusions of genius,
and ready, at any instant, to disclose them again to the delighted student,
nothing changed after revolving centuries;-coining machinery, which form
a bar or plate of metal cuts out and stamps thousands of beautiful medals in
an hour, and keeps an exact record of its work; —cranes,-pile engines,turning-lathes,-time-pieces,-all the implements of agriculture, of mining,
of navigation, &c., &c. If Aristotle deemed the title or definition of toolusing animal appropriate to man two thousand years ago, what title should
be given now?
In many of the complex machines, several of the simple ones are found
as elements; and in the same machine may be comprised many of the means
of changing the direction of motion.
" Friction."  (Read the Analysis, p. 84.)
In estimating the effects of mechanical contrivances, by the rule of comparative velocities of the power and resistance, there is an important correction
to be mace, on account of the mutual friction of the moving parts. In
the steam-engine, where the rubbing parts are numerous, the loss of
power, from friction often amounts to one-third of the whole.
Impediment from friction seems to be owing to two causes; 1st, a degree of
cohesive attraction between the touching substances; 2nd, the roughness of
these surfaces, even where, to the naked eye, they appear smooth.
It is supposed to be, because the roughness, or little projections and cavities, in pieces of the same or of homogeneous substances mutually fit each
other, as the teeth of similar saws would, so as to allow the bodies, in a
degree to enter into each other, that the friction is greater between such than
between pieces of different or of heterogeneous substances with dissimilar
grain.
The friction of one piece of iron, wood, brick, stone, &c., on another piece
of the same substance, has been measured by using the second piece as an
inclined plane, and then gradually lifting one end of it until the upper mass
began to slide,-the inclination of the plane, just before the sliding commences, is called the angle of repose.  This angle, different for different substances, is found to be, for metals, generally such as to mark that the force
required to overcome the friction between small pieces of them is equal to
about a fourth of the weight of the moving piece, and for woods it is about
a half. But for large pieces or great pressures, the friction is proportionably
much less.
It is this angle in the substances concerned, which determines the degrees
of acclivity which can exist in the sides of hills composed of sand, gravel,
earth, &c., in the banks of canals, rivers, &c.
If the thread of a screw winds round the spindle with an angle less than
this, the screw'can never recoil or slide back from force acting against its
point.
But for friction, men walking on the ground or pavement would always be
as if walking on ice; and our rivers, that now flow so calmly, would all be
frightful torrents.  Friction is, therefore, in these cases of great use to men.
Friction is useful, also, when it enables men, out of the comparatively
short fibres of cotton, flax, or hemp, to form their lengthened webs and
cordage, —for it is friction alone, consequent upon the interweaving and




SIMPLE MACHINES.                            115
twisting of the fibres and threads, which keeps the material of these fabrics
together.
The following means-are used to diminish friction between rubbing surfaces; and they are used singly or in combination, according to the circumstances.
1. Making the rubbing surfaces smooth;-but this must be done within
certain limits, for great smoothness allows the bodies to approach so near
that a degree of cohesion takes place.
2. Letting the substances which are to rub on each other be of different
kinds. Axles are made of steel, for instance, and the parts on which they
bear are made of brass; in small machines as time-keepers, the steel axles
often play in agate or diamond. The swiftness of a skater depends much
on the great dissimilarity between steel and ice.
3. Interposing some lubricating substance between the rubbing parts; as
oils for the metals, soap, grease, black-lead, &c., for the woods.  There is a
laughable illustration of this in the holiday sport of soaping a lively pig's
tail, and then offering him as the prize of the clever fellow who can catch
and hold him fast by his slippery appendix.
4. Diminishing the extent of the touching surfaces; as in making the
rubbing axis of a wheel very small.
5. Using wheels as in wheel-carriages, instead of dragging a rubbing load
along the ground. Casters on household furniture are miniature wheels.
6. Using what is called friction-wheels;-which
still farther diminish the friction even of a smooth   Fig. 61.
axis, by allowing it to rest on their circumferences,     a
which turn with it. Here a represent the end of
an axis, resting on the exteriors of two frictionwheels, b and c.                                   (
7. Placing the thing to be moved on rollers or
balls as when a log of wood is drawn along the
ground upon rounded pieces of wood; or when a
cannon with a fiat circular base to its carriage, turns round by rolling on
cannon-balls laid on a hard level bed. In these two cases there is hardly
any friction, and the resistance is merely from the obstacles which the rollers
or balls may have to pass over.
Of all rubbing parts the joints of animals, considering the strength, frequency and rapidity of their movements, are those which have the least
friction. The rubbing surfaces in these are covered, first, with a layer of
elastic cartilage, and then with an exceedingly smooth membrane, over which
there is constantly poured from the glands around, a fluid called synovia,
more emollient and lubricating than any oil: and which is renewed constantly
as may be required. We study and admire the perfection of animal joints,
Without being able very closely to imitate it.
Wheel-carriages merit notice here, as illustrating many of the circumstances connected with friction; and moreover as being among the most
common of machines.
Wheel-carriages have three advantages over the sledges for which they
are the substitutes:
1. The rubbing or friction, instead of being between an iron shoe and the
stones and irregularities of the road, is between the axle and its bush, of
which the surfaces are smoothed and fitted to each other, and well lubricated.




116                        M ECHAN IC S.
2. While the carriage moves forward, perhaps fifteen feet, by one revolution of its wheel, the rubbing part, viz., the axle, passes over only a few
inches of the internal surface of its smooth greased bush.
3. The wheel surmounts any abrupt obstaele on the road by the axle
describing a gently rising slope
Fig. 62.                 or curve,- as shown in this
-c..,~   - figure, where a represents an
obstacle, and where the curve. -..  from c, of which the beginning
/. X  / has the direction shown by the..... line c e, represents the path
/Nt  -   /~ \  of the axle in surmounting it.
(//1  (     The wheel is as if rising on an
inclined plane, and gives to the
_                       drawing animal the relief which
such a plane would bring. This
kind of advantage is greater in a large wheel, for evidently the smaller wheel
here represented, in having to surmount the same size of obstacles, has to
rise in the steeper curve beginning at d,-but the difference of advantage, in
this respect, is not so great as the difference of size. It is true again, that
a small wheel would sink to the bottom of a hole, where a larger one would
rest on the edges as a bridge, and would sink less. The fore wheels of carriages are usually made small, because such construction, by allowing the
wheel to go under the body of the carriage, facilitates the turning of the
carriage. It is not true, however, according to the popular prejudice, that
the large hind wheels of coaches, wagons, &c., help to push on the little
wheels before them, as if the carriage were on an inclined plane resting on
the wheels; but there is the accidental advantage, that in ascending a hill,
when the horses have to put forth their strength, the load rests chiefly on
the hind wheels, and in descending, when an increased resistance is
desirable, the load falls chiefly on the fore-wheels.
From the causes mentioned in the last paragraphs, the difference in performing the same journey of a mile, by a sledge and by a wheel carriage, is
that while the former has to rub over every roughness in the road and to be
jolted by every irregularity, the rubbing part of the latter, the axle, glides
very slowly over about thirty yards of a smooth oiled surface, in- a gently
waving line. Thus, by wheels, the resistance is reduced to about the hundredth part of what it is for a sledge.
On hilly roads, in descending, it is common to lock or fix one of the wheels
of a carriage, and the horses have then to pull nearly as much as on a level
road with the wheel free; showing the effect of a little increase of friction.
The wheel of a carriage, simple as from our extreme familiarity with it,
it now appears to us, is- a thing of very nice workmanship, and which has
exercised much ingenuity. —It acquires astonishing strength, indeed that of
the arch, from what is called its dished form, seen
Fig. 63.       here in the wheel c, as contrasted with the flat wheel
e       |a. In a wheel of this form, the extremity of a spoke
cannot be displaced inwards, or towards the carriage, unless the rim of the wheel be enlarged, or
ar -z   s A,    all the other spokes yield at the same time, and it
cannot be displaced outwards, or away from the car-:- -           riage, unless the rim  be diminished, or the other
spokes yield in the opposite direction:-now the




FRICTION.                              117
rim  being strongly bound by a rinc, or tire of iron, cannot suffer either
increase or diminution, and the strength of all the spokes is thus by it
conferred on each individually. In a fit wheel a given degree of displacement outwards or inwards of the extremities of a spoke, would less affect the
magnitude of the circumference, and therefore the rim of such a wheel,
secures much less firmly. A watch-glass and a round piece of egg-shell are
stronger than flat pieces of like substances, for the same reason that a dished
wheel is stronger than a flat wheel.-The dished form of a wheel is farther
useful by leaving more room between the wheels for the body of the carriage,
and is useful, also, in this, that when the carriage is on an inclined road,
and more bf the weight consequently falls upon the wheel of the lower side,
the inferior spokes of that wheel become nearly perpendicular, and thereby
support the increased weight more safely. The strongest form of wheel is
the doubly dished, that is, a wheel having half of the spokes passing from
within to the rim as from c to d, fig. 63, and the other half similarly from
without. This form is adopted in the wheel recently constructed entirely
of iron, in which there is a farther peculiarity that the load is supported
more by hanging by the upper spokes than by resting on the lower.  When
wheels, instead of standing upright, like b and d shown, fig. 63, are made
to incline outwards. as is common, owing to the ends of the axletrees being
bent down a little to give a security against the accident of the wheels falling
off, the pull to the horses in deep or sandy roads is much increased; for an
inclining wheel would naturally describe a curved and outward path, as is
seen when a hoop or wheelbarrow inclines; and the horses, therefore, in'drawing straight forward; have constantly to overcome the deviating tendency
in all inclining wheels.  This cause of resistance is still more remarkable
when the wheels have broad rims. Such wheels must be conical, that is, of
smaller diameter at the outer than at the inner edge, as the end of a cask is
smaller than its middle, and then, as the iron hoops or tires which cover the
different parts cannot all, by an equal number of turns, truly measure the
same length of road, there will be a constant rubbing or grinding forward of
the lesser rings, and a grinding backward of the larger, injuring the road,
rapidly wearing the iron, and exhausting the strength of the pulling animals.
Such wheels rolling free would describe a circular path, as is exemplified
when a thimble, or drinking glass, or sugar-loaf, which also are conical, is
pushed forward on any plane surface.
The application of springs to carriages, which is an improvement of comparatively recent date, not only renders them soft moving vehicles on rough
roads, but much lessens the pull to the horses. When there is no spring,
the whole load must rise with every rising of the road, and if time be given,
must sink with every depression, and the depression costs as much labour
as the rising, because the wheel must be drawn up again from the bottom of
it; but in a spring-carriage moving rapidly along, only the parts below the
springs are moved in correspondence with the road-surface, while all above,
by the inertia of the matter, have a soft and even advance. Hence arises
the superiority of those modern carriages, furnished with what are called
under-springs, which insulate from the effect of shocks, all the parts excepting the wheels and axletrees themselves.  When only the body of the carriage is on springs, the horses have still to rattle the heavy frame-work
below it over all irregularities, and' then the wheels as well as the structure
generally require to be of much greater strength and weight to bear the
consequent shocks.




118                        MECHANICS.
The subject of wheel carriages is interesting to medical men, from their
having often to direct in transporting the sick or wounded.
It is perhaps difficult to conceive anything more elegant and perfect than
the carriages of modern refinement; and, therefore, a man, who sees them
gliding swiftly along the prepared levels and slopes of our present landscapes,
and thinks of the clumsy vehicles on the bad roads of former times, may
readily imagine that absolute perfection is at last attained.  Yet we are perhaps now on the eve of a farther change which, for many purposes, will be
of greater importance than all that has yet been achieved-viz., the general
adoption of rail-roads, with new-fashioned carriages to suit them. To all
who study such subjects, it is now known, that to draw a loaded-wagon up
one inconsiderable hill, costs more force than to send it thirty or forty miles
along a level rail-way; and the conclusion is obvious, that although the original expense of forming the level line might considerably exceed that of making an ordinary road, still, in situations of great traffic, the difference would
soon be paid for by the savings, and when once paid, the savings would be
as a profit for ever. To readers conversant with political economy, it
would be superfluous to speak here of the advantages of any great facility
of intercourse, but to those who are not, the following reflections may be
interesting.
In reviewing the history of the human race, we find that every remarkable
increase in civilization has taken place very much in proportion to the facilities of intercourse offered in the particular situation. First, therefore, civilization, grew along the banks of great rivers, as the Nile, the Euphrates and
the Ganges; or along the shores of inland seas or. archipelagoes, as in the'
Mediterranean and the numerous islands of Greece; or over fertile and extended plains, as in many parts of India. When the situation thus bound a
great number of individuals into one body, the useful new thought or action
of any one unusually gifted, and which, in the insulated state, would soon
have been forgotten and lost, extended its influence immediately to the whole
body, and became the thought or action of all who could benefit'by it, besides
that it was recorded for ever, as part of the growing science of art of the community. And in a numerous society, such useful thoughts and acts would
naturally be more frequent, because persons feeling that they had the eyes of
a multitude upon them, and that the rewards of excellence would be proportionally great, would be excited to emulation in all the pursuits that could
contribute to the well-being of the society. Men soon learned to estimate
aright these and many other advantages of easy intercourse, and after having
possessed themselves with avidity of the stations naturally fitted for their
purposes, they began to imnprove the old and to make new stations. They
created rivers and shores, and plains of their own, that is, they constructed
canals and basins, and roads; and so connected, artificially, regions which
nature seemed to have separated forever.-In the British isles, whose favoured children have taken so remarkable a lead in showing what prodigies
a wise policy may effect, the advantages arising from certain lines of canal
and road first executed, soon led to numberless similar enterprises, and within
half a century the empire has been thus bound together in all directions: and
it seems as if the noble work was now to be crowned by the substitution of
level'raitways for many of the common roads and canals.* Several rail* These observations were first published (the substance had been written long
before,) soon after the Darlington rail-road, the first of any note intended for passengers, was opened.  The Manchester and Liverpool rail-road has since then
admirably verified the anticipations.




STRENGTH OF MATERIAL.                          119
roads of short extent have already been established, and although they and
the carriages upon them are far from having the perfection which philosophy
says they will admit, the results have been very satisfactory.  If we suppose the progress to continue, and the price of transporting things and persons to be thus reduced to a fourth of the present charge-and in many cases
it may be less-and if we suppose the time of journeying with safety also to
be reduced in some considerable degree,-of which there can be as little
doubt —the general adoption of such roads would operate an extraordinary
revolution and improvement in the state of society. Without in reality
changing the distances of places, it would in effect bring all places nearer to
each other, and would give to every spot in the kingdom the conveniences
of the whole,-of town and country, of sea-coast and of highland district.
A man, wherever residing, might consider himself virtually near to any other
part, when, at the expense of time and money nobv expended in travelling a
short way, he might travel very far, and he would thus find remarkably
extended, the sphere both of his business and of his pleasures.  The
over-crowded and unhealthy parts of towns would scatter their inhabitants
into the country; for the man of business could be as conveniently at his
post from a distance of several miles, as he is now from an adjoining street.
The present heavy charges for bringing distant produce t:o market being
nearly saved, the buyer everywhere would purchase cheaper, and the producer would be still better remunerated.
In a word, such a change would be affected, as if by magic the whole of
Britain had been compressed into a circle of a few miles in diameter, yet
without any part losing aught of its magnitude or beauties. All this may
appear visionary; but it is less so than seventy years ago it would have
been to anticipate much of what, in respect to travelling, has really come to
pass,-as, that the common time of passing from London to Edinburgh
would be forty-six hours.  At the recent opening (in 1825) of the railroad
near Darlington, a train of loaded carriages was dragged by one little stenimengine a distance of twenty-five miles within two hours; and in some parts
of the journey the speed was more than twenty miles an hour: the load
was equal to a regiment of soldiers, and the coal expended was not of the
value of a crown. An island with such roads would be an impregnable
fortress; for in less time than an enemy would require to disembark on
any part of the coast, the forces of the country might be concentrated to
defend it.
" Strength depends on the magnitude, form and position of bodies, as well
as on the degree of cohesion in the material."  (Read the Analysis,
page 84.)
The minute details connected with this branch of the subject belong to
the practical engineer, but there are some of the general truths which should
be familiar to every body.
Of similar bodies the largest is proportionally the weakest.
Suppose two blocks of stone left projecting from a hewn rock, of which
blocks one, as d, p. 120, is twice as long, and deep, and broad, as the other,
b. The larger one will by no means support at its end as much more weight
than the smaller, as its mass is greater, and for two reasons.  1st. In the
larger, each particle of the surface of attachment at c, in helping to bear




120                        MECHANICS.
the weight of the block itself, has to support by
Fig. 64.       its cohesion twice as many particles beyond it in
the double extent of projection, as a particle.has
to support in the shorter block at a; and 2ndly,
both the additional substance, and anything apr,-. b           pended at its outer extremity, are acting with a
C> Odouble lever advantage to break it, that is, to destroy the cohesion at c. Hence, if any such mass
anct —--------...  -rl  be made to project very far, it will be broken off
or will fall by its own weight alone.  And what
I   O  is thus true of a block supported at one end, is
equally true of a block supported at both ends, and
indeed of all masses, however supported, and of
whatever forms, if they have projecting parts. It
is to be observed, also, that masses, like an absolutely perpendicular cliff, which have no projecting
or overhanging parts, are still limited, as to size, by the degree of cohesive
force among their particles, for the upper part of such a mass tends to
crush or break down the lower. A lofty pillar cannot be formed of soft clay.
That a large body, therefore, may have proportionate strength to a
smaller, it must be still thicker and more clumsy than it is longer: and
beyond a certain limit, no proportions whatever will keep it together, in
opposition merely to the force of its own weight.
This great truth limits the size and modifies the shape of most productions
of nature and art;-of hills, trees, animals, architectural or mechanical
structures, &c.
Hills.  Very strong or cohesive material may constitute hills of sublime
elevation, with very projecting cliffs and very lofty perpendicular precipices;
and such accordingly are seen where the hard granite protrudes from  the
bowels of the earth, as in the Andes of America, the Alps of Europe, the
Himalayas of Asia, and the Mountains of the Moon in Central Africa.
But material of inferior strength exhibits more humble risings and more
rounded surfaces.  The gradation is so striking and constant, from granite
mountains, down to those of chalk, or gravel, or sand, that the geologist can
often tell the substance of which a hill is composed by observing the peculiarities of its shape.
Even in granite itself, which is the strongest of rocks, there is a limit to
height and projection; and if an instance of either, much more remarkable
than now remains on earth, were by any chance to be produced again, the
law which we are considering would prune the monstrosity. The grotesque
figures of rocks and mountains, seen in the paintings of the Chinese-or
actually formed in minature for the gardens, to express their notions of
perfect sublimity and beauty-are caricatures of nature for which originals
can never have existed.  Some of the smaller islands in the Eastern Ocean,
however, and some of the mountains of the chains seen in the voyage
towards China, along the coasts of Borneo and Palawan, exhibit, perhaps,
the very limits of possibility in singular shapes.  In the moon, where the
weight or gravity of bodies is less than on earth, on account of her smaller
size, mountains of a given material might be many times higher than on
earth- and observation proves that the lunar mountains are in fact very
high.




STRENGTH  OF MATERIAL.                          121
By the action of winds, rains, currents, and frosts, upon the mineral masses
around us, there is unceasingly going on an undermining and wasting of
supports, so that every now and then immense rocks, or almost hills, are
torn by gravity from the station which they have held since the earth received its present form, and fall in obedience to the law now explained.
The size of vegetables, of course, is obedient to the same law. We have
no trees reaching a height of three hundred feet, even when perfectly perpendicular, and sheltered in forests that have been unmolested from the
beginning of time: and oblique or horizontal branches are kept within
comparatively narrow limits by the great strength required to support them.
The truth, that to have proper strength, the breadth or diameter of bodies
must increase more quickly than the length, is well illustrated by the contrast existing between the delicate and slender proportions of a young oak or
elm, yet in the seedman's nursery, and the sturdy form of one which has
braved for centuries all the winds of heaven, and has become the monarch
of the park or forest.
Animals furnish other interesting illustrations of this law.
How massive and clumsy are the limbs of the elephant, the rhinoceros,
the heavy ox, compared with the slender forms of the stag, antelope, and
greyhound! And unless the bones were made of stronger material than now,
an animal much larger than the elephant would fall to pieces owing to its
weight alone. The whale is the largest of animals, but feels not its enormous weight because lying constantly in the liquid support of the ocean. A
cat may fall with impunity from a greater height than would suffice to dash
the bones of an elephant or an ox to pieces.
For the reason which we are now considering, the giants of the heathen
mythology could not have existed upon this earth; although, on our moon,
where, as already stated, weight is much less, such beings might be. In the
planet Jupiter, again, which is many times larger than the earth, an ordinary man from hence would be carrying in the simple weight of his body a
load sufficient to crush the limbs which supported him.  The phrase a little
compact man, points to the fact that such a person is stronger in proportion
to his size than a taller man.
The same law limits the heighth and breadth of architectural structures.
In the houses of fourteen stories, which formerly stood for protection close
under the Castle of Edinburgh, there was danger of the superincumbent
wall crushing the foundation.
Roofs. Westminster hall approaches the limit of width that is possible
without either very inconvenient proportions or central supports; and the
dome of the church of St. Peter in Rome is in the same predicament.
Arches of a bridge. A stone arch, much larger than those of the magnificent bridges in London, would be in danger of crushing or splintering its
material.
Ships. The ribs or timbers of a boat have scarcely a hundredth part of
the bulk of the timbers of a ship only ten times longer than the boat. A
ship's yard of ninety feet contains, perhaps, twenty times as much wood as a
yard of thirty feet, and even then is not so strong in proportion. If ten men
may do the work of a three-hundred-ton ship, many more than three times
that number will be required to manage a ship three times as large.  Very
large ships, such as the two built in Canada, in the year 1825, which carried
each nearly ten thousand tons of timber, are weak from their size alone; and
the loss of these first, two specimens of gigantic magnitude will not encourage
the building of others.




122                       MECHANICS.
The degree in which the strength of structure is dependent on the form and
position of their parts, will be illustrated by considering the two cases of
longitudinal and transverse compression. And the rule for giving strength
to any structure will be found to be, to cause the force tending to destroy
it, to act, as equally as possible, on the whole resisting mass at once, and
with as little mechanical advantage as possible.
In longitudinal compression, as produced by a body a, on the atoms of the
support b, the weight, while the support remains straight, can only destroy
the support, by crushing it in opposition to the repulsion and impenetrability
of all its atoms. Hence a very small pillar, if
Fig. 65.         kept perfectly straight, supports a very great
weight; but a pillar originally crooked, or
beginning to bend, resists with only part of its
strength; for, as seen in c d, the whole weight
above is supported chiefly on the atoms of the
concave side, which are therefore in greater
danger of being oppressed and crushed, while
those on the convex side, separated from their
natural helpmates, are in the opposite danger of
being torn asunder. The atoms near the centre
in such a case are almost neutral, and might be
absent without the strength of the pillar being
much lessened.
Long pillars or supports are weaker than short
b I _  pillars of the same diameter, because they are
____-_    -U           more easily bent; and they are more easily bent
i~ -~ ----'  because a very inconsiderable, and therefore
easily effected yielding between each adjoining
two of their many atoms, makes a considerable
bend in the whole; while in a very short pillar there cannot be much bending without a great change in the relation
Fig. 66.           of proximate atoms, and such as can be
effected only by great force. The weight
resting on any pillar, and bending it, may
be considered as acting (with obliquity
dependent on the degree of bending) at
the end of a long lever which reaches
from the extremity to the centre of the
pillar, against the strength resisting always
c  e   A!,,l-.-. ~. idirectly at a short lever reaching from the
side d to the centre; the strength of the
pillar, therefore, has relation to the difference between these levers and to the degree
of bending.  Shortness, then, or any stay
or projection as a e b, which, by making
the resisting lever longer, opposes bending,
b           fl _ _ t   really increases the strength of a pillar.
X` PA column with ridges projecting from
it, is on this account stronger than one
that is perfectly smooth.
A hollow tube of metal is stronger than the same quantity of metal as a
solid rod, because its substance standing farther from the centre resists bend



STRENGTH OF MATERIAL.                           123
ing with a longer lever. Hence pillars of cast-iron are generally made
hollow, that they may have strength with as little metal as possible.
In the most perfect weighing-beams for delicate purposes, that there may
be the least possible weight with the required strength, the arms, instead
of being of solid metal, are hollow cones, of which the substance is not much
thicker than writing paper.
Masts and yards for ships have been made hollow in accordance with the
same principle.
In Nature's work we have to admire numerous illustrations of the same
kind.
The stems of many vegetables, instead of being round externally, are ribbed
or angular and fluted, that they may'have strength to resist bending. Many
also are hollow, as corn-stalks, the elder, the bamboo of tropical climates,
&c., thereby combining lightness with their strength."-A person who has
visited the countries where the bamboo grows, cannot but admire the almost
endless uses to the inhabitants, which its straightness, lightness and hollowness, fit it to serve. Being found of all sizes, it has merely to be cut into
pieces of the lengths required for any purposes, and nature has already been
the turner, and the polisher, and the borer, &c. In many of the Eastern
Islands it is the chief material, both of the dwellings, and of the furniture;
there are the bamboo huts and bungallows, and then the fanciful chairs,
couches, beds, &c.; flutes and other wind instruments there, are merely
pieces of the reed with holes bored at the requisite distance: conduits for
water are pipes of bamboo; bottles and casks for preserving liquids are
single joints of larger bamboo with the natural partitions remaining; and
balnboo split into threads is twisted into rope, &c.
From the animal kingdom also we have illustrations of our present subject:-as in the hollow stiffness of the quills of birds; the hollow bones of
birds; the bones of animals generally-strong and hard, and often angular
externally, with light cellular Lexture within, &c.
Transverse Pressure.
When a horizontal beam is supported at its extremities, as at a and b,         Fig. 67.
its weight bends its middle down more  e 
or less, as here shown, the particles
on the upper side being compressed,
while the parts below are distended;                fh
and the bending and tendency to
break are greater, according as the beam is longer and its thickness and
depth is less.
The danger of breaking in a beam, so situated is judged of, by considering the destroying force as acting by a long lever reaching from an end of
the beam to the centre, and the resisting force or strength as acting only by
a short lever from the side d to the centre: while only a little of the substance of a beam on the under side is allowed to resist at all. This last circumstance is so remarkable, that the scratch of a pin on the under side of a
plank resting, as here supposed, will sometimes suffice to begin the fracture.
Because the resisting lever is small in proportion as the beam is thinner, a
plank bends and breaks more readily than a beam, and a beam resting on its
side bears less weight than if resting on its edge.  Where a single beam




124                       MECHANICS.
cannot be found deep or broad enough to have the strength required in any
particular case, as for supporting the roof of a house, several beams are
joined together, and in a great variety of ways, as is seen' in house-rafters,
&c., which although consisting of three or more pieces, may be considered
as one very broad beam, with those parts cut out which would contribute
least to the strength.
The arched form, resting against immoveable abutments, bears transverse
pressure so admirably because by means of it the force that would destroy,
is made to compress not one side only, but all the atoms or parts of both
sides nearly in the same degree. By
Fig. 68.             comparing this figure with the last,
d                  we see that the atoms on the under
side of an arch, must be compressed
about as much as those on the upper
u   side, and are therefore in no danger of
being torn, or overcome separately.
The whole substance of the arch therefore resists, nearly like that of a
straight pillar under a weight, and is nearly as strong.
An error, which has been frequently committed by bridge-builders, is the
neglecting to consider sufficiently the effect of the horizontal thrust of the
arch on its piers. Each arch is an engine of oblique force (see page 56,)
pushing the pier away from it. In some instances, one arch of a bridge
falling, has allowed the adjoining piers to be pushed down towards it, by the
thrust, no longer balanced, of the arches beyond, and the whole structure
has given way at once like a child's house or bridge built of cards.
It is not known at what time the arch was invented, but it was in comparatively modern times. The hint may have been taken from nature,
for there are instances in Alpirie countries of natural arches, where rocks
have fallen between rocks, and have there been arrested and suspended, or
where burrowing water has at last formed a wide passage under masses of rock,
and has left them balanced among themselves as an arch above the stream.
Nothing can surpass the strength and beautypf some modern stone bridges;
-those, for instance, which spans the Thames as it winds through London.
Iron bridges have been made with arches twice as large as those of stone;
the material being more tenacious and easily moulded, is calculated to form
a lighter whole. The bridge of three fine arches lately built between the
city of London and Southwark, is a noble specimen, and compared with
those erected in the preceding century, appears almost a fairy structure of
lightness and grace.
The great domes of churches, as those of St. Peter's in Rome and St.
Paul's in London, have strength on the same principle as simple arches.
They are in general strongly bound at the bottom with chains and ironbars, to aid the masonry in counteracting the horizontal thrust of the
superstructure.
The'Gothic arch is a pointed arch, and is calculated to bear the chief
weight on its summit or key-stone. Its use, therefore, is not properly to
span rivers as a bridge, but to enter into the composition of varied pieces of
architecture.  With what effect it does this, is seen in the truly sublime
Gothic structures which still adorn so many parts of Europe.
The following are instances, in smaller bodies, of strength obtained by the
arched form.-A thin watch-glass bears a very hard push; a dished or
arched wheel for a carriage is many times stronger to resist all kinds of shocks
than a perfectly flat wheel; —a full cask may fall with impunity, where a




STRENiTH  OF  MATERIAL.                         125
strong square box would be dashed to pieces; —a very thin globular flask
or glass, corked and sent down many fathoms into the sea, will resist the
pressure of water around it, where a square bottle, with sides of almost any
thickness, would be crushed to pieces.
XWe have, from the animal frame, an illustration of the arched form giving
strength, in the cranium or skull, and particularly in the skull of man,
which is the largest in proportion to its thickness:-the brain required the
most perfect security, and in the arched form of the skull has obtained it
with little weight.-The common egg-shell is another example of the same
class: what hard blows of the spoon or knife are often required to penetrate
this wonderful defence of a dormant life! The weakness of a similar substance not arched, is seen in a scale from a piece of freestone so readily
crumbling between the fingers.
To determine, for particular cases, the best forms and positions of beams
and joists, and of arches, domes, &c., is the business of strict calculation,
and belongs therefore to mathematics, or the science of measures.
It was a beautiful problem of this kind, which Mr. Smeaton, the English
engineer, solved so perfectly, in the construction of the far-famed Eddystone light-house. He had to determine the form and dimensions of a
building, which would stand firm on a sunken rock, in the channel of a
swift ocean tide, and exposed to the fury of tempests from every quarter.
Only the man who has himself been driven before the irresistible storm in
the darkness of night, and in the midst of dangers, and whose eyes have
watched the steady ray from the light-house which saved him, can appreciate fully the importance of the studies which bring such useful results; or
can feel how happy he is to have fellow-men, whose talents, although exerted usually for individual good, are yet, by God's providence, made to
accomplish the most philanthrophic ends, and to bind the whole of human
kind into one great society of helping brotherhood.
[For Animal and Medical Mechanics, see Part V. Sect. 1.]




126                     HYDROSTATICS.
]PART III.
THE PHENOMENA OF FLUIDS.*
SECTION I.-HYDROSTATICS.
ANALYSIS OF THE SECTION.
The particles of a fluid mass are freely moveable among one another, so as
to yield to the least disturbing force; and if bearing force at all, can be
at rest only when equally forced in all directions.  Hence:
1. In a mass of fluid submitted to compression, the whole is equally affected,
and equally in all directions. A given pressure, for instance, made by
a plug forced inwards upon a square inch of the surface of a fluid filling
a vessel, is suddenly communicated to every square inch of the vessel's
surface, however large, and to every inch of the surface of any body
immersed in the fluid.
2. In any fluid, the particles that are below bear the weight of those that
are above, and there is, therefore, within the mass, a pressure increasing
exactly with the perpendicular depth, and not influenced by the size, or
shape, or position of the containing vessel.
3. The open surface of a fluid is level; and if various pipes or vessels
communicate with each other, any fluid admitted to them will rise to the
same level in all.
4. A body imlmersed in a fluid displaces exactly its own bulk of it, which
quantity having been just supported by the fluid around, the body is
pressed upwards, or supported, with a force exactly equal to the weight
of the fluid displaced, and must sink or swim according as its own weight
is greater. or less than this. By compgring, therefore, the weight of a
body with the force which holds it up in a fluid, the comparative weight
or specific gravities are found.
" Fluid."
It was explained in Part I., that the same atoms may exist in the form
of a solid or of a fluid; and as a fluid, they may either constitute a dense
liquid like water, or a light elastic mass like air. A pound of ice, or a
pound of water, or a pound of steam, differs only in the particles being
more or less distant from each other, owing to the different quantities of
heat among them. In the ice, they are comparatively near, and are held
together by attraction, as if they were spitted or glued to each other; in
the water, the repulsion of heat seems nearly to balance attraction, and to
leave the particles at liberty to glide about among each other almost without
* Read again the Synopsis, page 20.




FLUIDS.                              127
friction; and in the steam, the repulsion altogether overcomes the attraction,
and the particles separate to a great distance, as if held apart by some bulky
elastic medium. The few facts not evidently reconcilable with the simple
and satisfactory explanation of so many phenomena,-as that water in freezing, and even in cooling down from forty degress to the freezing point,
increases in volume, instead of contracting, like things'in general, and like
itself in cooling at other temperatures, —and that baked clay, in proportion
as it is more heated, contracts instead of dilating,-are treated of in other
parts of our work.
Whether matter be in the solid or fluid form, the properties of the individual atoms remain unchanged, that is, the atoms always exist in accordance with the "general truths;" but as, in the chapter on Mechanics, we
found so many important modifications of effect produced by the circumstance of the attraction being in the degree which produces solid cohesion
among the particles, in this chapter on fluids we shall find as many
important results springing from the circumstances of non-cohesion or
fluidity.
In a liquid the particles, although comparatively near to one another,
seem not to be in actual contact; for the mass may be condensed indefinitely
by pressure. The force required, however, to change the volume of a liquid
in any sensible degree, is so great, that until improved means of experiment,
recently contrived, liquids were accounted absolutely incompressible. In
aeriform fluids, on the contrary each particle, under common circumstances,
has about two thousand times as much space to itself as when forming part
of a liquid or solid; and hence it is that these fluids are so extensively compressible and dilatable-or elastic, as they are called.  On account of this
elasticity, they exhibit so many important phenomena, in addition to those
of mere fluidity, that the consideration of them requires to be gone into
apart, and forms the branch of the subject called pneumatics, from a Greek
word, signifying " spirit" or " breath!"
" In a quantity of fluid submitted to compression, the whole mass is equally
affected, and similarly in all directions.  A given pressure, therefore,
made upon an inch of the surface of a fluid confined in a vessel, as by a
plug forced inwards, is suddenly borne by every inch of the surface of
the vessel, however large, and by every inch of the surface of any body
immersed in the fluid."
This truth is of great importance, both from its explaining so many remarkable phenomena of nature, and from the useful applications of it in the
construction of machinery.
When a man compresses in his hands a bladder full of air, he readily
conceives that the air immediately under his fingers is not at all more
compressed than that in every other part of the bladder; and of course
that every part of the bladders's surface must be pressing the air as much
as those parts of it on which his fingers rest, and must be bearing a reaction
or resistance of the air in an equal degree; and that every single particle
of air must be acted upon equally on every side, so that if a small opening
were made in the bladder anywhere, the air would issue from it with
equal readiness. This is in accordance with the characteristic of fluidity,
c" that the particles glide about among one another almost without friction,
so that a particle can never be at rest unless when equally urged in all directions."




128                       Y DROS T A TI CS.
In like manner, if a close vessel B be filled with water, and into the top of
it a tube a c be screwed, and if then, by means of
Fig. 69.        a cork or moveable plug in the tube at c, the surface
of the water in the vessel be pressed upon with a
a!. Ne   force of one pound, the water throughout the whole
I I.., will be squeezed or condensed in proportion to the
i:.   ii  pressure, and every other portion of the vessel B3,. of equal surface with c, will be keeping up the conIIj.,,,   densation just as much as c, and will be bearing the'resistance or elasticity of the water to the extent of
-B            one pound. And if there were another similar tube,
b, also with a plug, screwed into the top of the box B,
the force of one pound depressing the plug c would
be pushing up the plug b, with the same force.  And if there were many
other similar tubes and plugs, by acting on one, all would be equadly affected;
and a plug or piston of double size would be twice as much afected as the
smaller one; and a plug d, of ten times the size, would b2 lifted with a
force of ten pounds.  Hence it appears that, through the medium of confined fluid, a force of one pound, acting upon an inch square of the fluid surface in a vessel, may become a bursting force of ten, or a hundred, or a thousand pounds, according to the size of the vessel, or may be used as a mechanical power to overcome a force much more intense than itself.  It will be
explained below that the well-known hydrostatic press is merely a large plug
or piston as here described, forced up against the substance to be pressed by
the action of a smaller piston in another barrel.
If, in the above figure, the tube a were such as to contain just one pound
of water, on the plug c being withdrawn from it, and water being poured in
to fill it, the same pressure or condensation would take place in the box B
as when the plug was pressed with the force of one pound; and of course
exactly the same effects would follow on the sides of the vessel and on the
other pistons; and if, in the other tubes also, water were substituted for the
pistons, it is evident that, to effect a balance in all, it would require to stand
as high in every one as in the tube a c, producing the same level in all,
whatever their size.
The fact that the weight of one pound of water, or any
Fig. 70.    other force of one pound similarly applied, may be made,
through the medium of extended fluid surface to produce a
b      pressure of hundreds or of thousands of pounds, has been
called the "hydrostatic paradox," yet there is nothing in
reality more paradoxical in it than that one pound at the long
end of the lever should balance ten pounds at the short end:
indeed it is but another means, like the contrivances usually
called mechanical powers, and described in the last chapter,
of balancing different intensities of force, by applying them
to parts of an apparatus moving with different velocities.
Here the tube a being ten times smaller than the tube e, the
c      -piston a must descend ten inches to raise the greater piston
in e one inch.
This law of fluid pressure is rendered very striking in the
o   experiment of bursting a strong cask by the weight or action
of a few ounces of water. Suppose a cask a already filled
with water, and that a long small tube b c is screwed tightly
into its top, which tube will contain only a few ounces of




FLUIDS.                             129
water; by pouring these few ounces into the tube, the cask will be burst.
In explanation it is unnecessary to say more than that if the tube have an
area of a fortieth of an inch, and contain, when filled, half a pound of water,
that water would produce a pressure of half a pound upon every fortieth of
an inch all over the interior of the cask, or nearly 2,000 lbs. on the square
foot, —a pressure greater than a common cask can bear.
A similar effect is seen in what is called the hydrostatic bellows. This
consists of a long small tube a b, into which water is poured to enter the
body of the apparatus at c, which resembles the common bellows, in having
wooden boards above and below, and strong leather connecting them. If
the tube a b holds an ounce of water, and has itself only one-thousandth bf
the area of the top of the bellows, an ounce of water in it will balance
weights of a thousand ounces placed on the top of the bellows at d. If
mercury were substituted in this machine for water, the effect would be
fo]rteen times greater, because mercury is fourteen     Fig. 71.
times heavier in the same bulk. And if a man stand 
on a large bellows of the kind, he may raise himself
by blowing into the tube with his mouth.            a
The annexed cut will give an idea of Mr. Braham's
singularly powerful and useful hydrostatic or hydraulic press; which, if compared with the bellows, exhibits merely a strong forcing pump instead of the
lofty tube, and a barrel with its piston instead of the
leather and boards. The letter e points out the piston of the forcing pump worked by the handle d, and
driving water along the horizontal tube into the space
f under the large solid piston c, which last, with its   b
spreading top, is urged against the object to be compressed. If the small pump have only one-thousandth          c
of the area of the large barrel, and if a man by means
of its lever-handle d, press its piston down with a force of five hundred
pounds, the piston of the great barrel will
rise with a force of one thousand times five         Fig. 72.
hundred pounds, or more than two hundred
tons, Scarcely any resistance can withstand
the power of such a press; with it the hand
of an infant can break a strong iron bar; and                      b
it is used to condense substances, as cotton          a
or hay for sea voyages, to raise great weights,   e          S
to uproot trees, to tear things asunder, &c.
The Dilater is a surgical instrument of extensive applicability, of which
the action depends on the principle of the communication of fluid pressure.
It was proposed by-the author some years ago, and was brought to great
practical perfection by his brother Dr. James Arnott, (now superintendent
surgeon in the service of the Hon. East India Company,) in whose publication it is minutely treated of. Many professional men in this country
doubted of its power, from not being aware of the nature of fluid nature;
but it is in reality a kind of hydraulic press, allowing the operator to act
with the most gentle or most energetic force. Further remarks are made
upon it in the medical section which follows this chapter.
9




130                     HYDROSTATICS.
"In any fluid, the particles that are below, bear the weight of those that are
above, and there is therefore a pressure among them increasing in, exact
proportion to the perpendicular depth, and not influenced by the size, or
shape, or position of the containing vessels."
The atoms of matter having gravity, it is evident that the upper layer of
any mass of fluid must be supported by the second, and this with its load by
the third, and the third with its double load by the fourth, and so on. This
truth is experimentally proved by putting different heights of liquid into an
upright tube, of which the bottom is closed by a flap having a spring or lever
to support it, and to indicate the force acting on it. And what is true of the
entire column of water in the tube, may be considered true of any single line
of atoms; just as it would be true of a line of bricks piled one above another.
A tube of which the area is an inch square, holds, in two feet of its length,
nearly a pound of water; hence, the general truth, well worth recollecting,
that the pressure of water, at any depth, whether on the side of a vessel or
on its bottom, or on any body immersed, is nearly one pound on the square'
inch for every two feet of depth.
The striking effects from the increase of pressure in a fluid, at great depths,
are, of course, most commonly exhibited at sea. The following instances will
illustrate them.
If a strong square glass bottle, empty, and firmly corked, be sunk in
water, its sides are generally crushed inwards by the pressure before it
reaches a depth of ten fathoms.
A chamber of air similarly let down with a man it, would soon allow
him to be drowned by the water bursting in upon him; as really happened
to an ignorant projector.
When a ship founders in shallow water, the wreck, on breaking to pieces,
generally comes to the surface, and is cast upon the beach; but when the
accident happens in deep water, the great pressure at the bottom forces
water into the pores of the wood, and makes it so heavy that no part can
ever rise again to reveal her fate.
A bubble of air or of steam, set at liberty far below the surface of water,
is small at first, and gradually enlargens as it rises.
A man who dives deep, suffers much from the compression of his chest,
as the elastic air within him yields under the strong pressure.  This limits
the depth to which divers can safely go.
It is not known whether there is a limit to the pressure which fishes can
bear to impunity, but they are chiefly found living in the shallower waters
on coasts, or on banks in the midst of the ocean, such as the banks of
Newfoundland, the Dogger-bank, and other fishing stations out at sea. In
rounding the Cape of Good Hope, at a considerable distance from land,
ships pass over the bank of Lagullas, where a hook let down with a bit of
red rag, or almost any thing as a bait, immediately secures its codfish.
By sending a vessel prepared for the purpose, down into the deep sea,
we can readily prove the compressibility of water. Suppose the vessel to
be made with only one entrance, and that a small round opening, into which,
instead of cork, a sliding rod has been closely fitted. If, then, when filled
with water, and having the rod inserted into the opening, it be allowed to
sink into the sea, the pressure around will push the rod inwards, in a degree
proportioned to the yielding or compression of the water within; and if
there be on the road a stiff sliding-ring, or other contrivance to indicate on the
return of the vessel how far the rod has been driven inwards, the apparatus




PRESSURE  AS DEPTH.                           131
will shdw the degree of compression at the greatest depth to which it has
descended. Water a thousand fathoms below the surface is less bulky by
about one-twentieth part than when at the surface.
The following are proofs of the pressure of weight in an open fluid, operating in all directions, as any pressure does in the case of a confined fluid.
A bottle-cork carried far under water, is not flattened as if it were pressed
unequally, but is reduced in all its dimensions so as to appear a phial-cork
of the usual form.                                                  a
If a corked empty bottle be sent down into the sea, the cork is generally
forced inwards at a given depth, and equally so in whatever direction the
mouth of the bottle may happen to point.
If a vessel containing water have an opening in the side, covered by a
valve or flap so contrived as to tell the force required to keep it shut, we find
that the water tends to escape just as powerfully through such an opening
as it would through one in the bottom, with the same elevation of water
over its centre. And different equal openings in the side of a v6ssel require
to be closed with forces exactly proportioned to the heights of liquid above
their centres.
In an open square sided vessel full of water, the whole pressure on any
upright side is just half the pressure on an equal extent of horizontal bottom;
for the centre of the side being just half as deep as the bottom, the pressure on any point there is only half as great as on a point at the bottom, and
on points above the level of the centre is just as much less than half, as, at
corresponding distances below, it is more than half, and so it amnounts to an
exact half in the whole.  Considering that the pressure on every point
below the central level is greater than on every point above it, we see the
reason why, to support a sluice or flood-gate by a single stay on the outside,
the point at which the pressure has to be made is below the central level.
Calculation discovers that this point, called the centre of pressure, is at onethird from the bottom. The knowledge of such facts furnishes rules for the
construction of large vessels for liquids, canal embankments, &c.
The pressure on a given extent of the side of a narrow vessel is just as
great as on the same extent of the side of a wide vessel, having the same
depth of fluid; because, as now explained, it depends entirely on the extent
of surface acted upon and the depth of liquid.
Hence a flood-gate or sluice which shuts out the ocean, as in docks opening to the sea, bears no more pressure than if it stood only against an equal
depth of lake or river; or than if it were one of two such flood-gates become
the sides of a very narrow vessel, made to contain only a few hogsheads of
water.
Hence, again, the fear is unfounded which has been expressed with reference to the formation of a canal between the Red Sea and the MIediterranean, —that because the former, owing to the effects of easterly winds at its
mouth, &c., is twenty feet higher than the later, it might burst through the
flood-gates, and carry devastation along its course.
A deep crevice in a rock, when filled by a shower, is often the cause of
the rock being torn asunder, and of part being precipitated.
Extensive walls or faces of masonry, intended to confine banks of sand
or earth, if no openings were left for water to escape from behind them,
would be burst after a rain unless they had the strength of flood-gates of
the same size.  Ignorance of this danger has led to some extraordinary
catastrophies.




132                     HYDROSTATICS.
Other examples of. the pressure in fluids being in all directions, And proportioned to the depth, are:-the swelling and bursting of leaden pipes when
filled from a very elevated source i-the tearing up of the coverings of subterranean drains or water courses, when, during a flood, any accident chokes
them near their lower openings:-the violence with which water escapes by
an opening near the bottom df any deep vessl, or enters by an opening or
leak.near the keel of a deep-floating ship: —the great strength required in
the lower hoops and securities of those enormous vessels of porter-brewers,
called vats, some of which contain many thousand barrels of liquid.
In speaking of the pressure of a fluid in all directions, some persons have
difficulty in conceiving that there is an upward as well as a downward and a
lateral pressure. But if, in a fluid mass, the particles below had not a tendency upwards equal to the weight or downward pressure of the fluid over
them, they could not support that fluid, which entirely rests upon them.
Their tendency upward is owing to the pressure around them from which
they are trying to escape. Accordingly, if a long tube, open at both ends,
and with a sliding plug or piston in it near one end, be partially plunged into
water by the plugged end, the water is found to press the plug upwards with
force proportioned to the depth to which it is carried, and exactly equal to
the force with which water presses upon an equal extent of the bottom or
side of any other vessel having in it the same depth; or, with which, in the
same vessel, it would press other plugs in other branches of the tube projecting in various directions. On removing such a plug altogether, the upward pressure is visibly proved and measured by the column of water pushed
into the tube from below, and supported there to the level of the water around.
The pressure in a mass of fluid is proportioned to the perpendicular depth,
and is not at all influenced by the size, shape or position of the containing
vessel.
A body immersed in the water of a lake, one foot under the surface is
just as much pressed upon as if it were one foot under the surface of the sea,
and no more than if it were one foot under the surface of a small cistern.
Suppose vessels differing from each other in form and capacity, as sketched
here at a, b, and c, but all having flat bottoms,
Fig. 73.         of exactly the same area; if fluid be poured
into all to the same level or perpendicular
height, as represented here by the dotted lines,
although the quantity be very different in each,
the pressure on the bottom will be the same in
\b.        all. This truth is easily proved experimentally,
by having the bottoms moveable, and held to
their places by weights or springs capable of
measuring the pressure: or by letting the three
vessels all communicate with the same vessel of water below them, and then
observing that the water in all has still the same level.  These results are
other exemplifications of the truths, "pressure equal in all directions,"'"pressure as depth," and "pressure as the extent of surface."   For as a
column of the fluid, resting on the middle of each bottom, just presses with
its whole weight, and therefore, according to its altitude, this column could
not remain at rest if there were any greater or less pressure than its own
near it; then as the fluid really is at rest in all the cases, and in all a
central column is of the same height, the pressure must be equal on all the
bottoms.  The case of the largest vessel a, is in a degree illustrated by




FLUID LEVEL.                             133
supposing the water in it to be suddenly converted into smooth upright small
columns or rods of ice or glass; Then evidently only those pieces which
rested on the bottom, could press on it while the others would be supported
by the oblique sides of the vessel, and by the lateral resistance of the pieces
around them.
" Level surface of a Fluid."  (Read the Analysis.)
That the surface of a fluid must be level, follows from the facts of all the
particles being equally attracted towards the centre of the earth, and being
perfectly moveable among themselves. The particles forming the surface
may be regarded as the tops of so many columns of particles, supported at
any given level below, by a uniform resistance of pressure;-for no particle
of an inferior level can be at rest unless equally urged in all directions, and
therefore all the particles at such a level, and which, by equally urging one
another, keep themselves at rest, must all be bearing the weight of equal
columns: thus a higher column must sink and a lower one must rise, until
just balanced by those around; that is, until all become alike. Besides,
just as a ball rolls down a slope or inclined plane, so do the particles of
a fluid slide or move from any higher situation among themselves to any
lower'unoccupied situation near them. The account now given explains
why any accidental elevation or depression of a fluid surface, usually called
a wave, continues to rise and fall, or to oscillate, for some time with gradually diminishing forces;-for when the mass is raised above the general level
it is not quite supported, and therefore soon sinks, but in sinking, like, a
falling pendulum, it acquires momentum which carries it below the general
level, until opposed and arrested by a resistance greater'than its weight, it
then rises again, by acquiring a new momentum in its rise, it has to fall
again, again to rise, and this alteration continues, until the lateral sliding
of the particles, and the friction among them, gradually destroy it.
A perfectly level surface on earth really means one in which every particle
is equi-distant from the centre of the earth, and it is therefore truly a
spherical surface; but so large is the sphere, that if a slice of it of two miles
in diameternwere cut off, and laid on a perfect plane, the centre of the slice
would only be four inches higher than the edges. Any small portion of it,
therefore, for all common purposes, may be accounted a perfect plane.
So truly smooth does a fluid surface become, that it forms a perfect
mirror; that is, it reflects or throws back the rays of light, which fall
upon it so exactly in the order which they had on leaving the object, that
an eye which receives them may fancy the object to be placed in the
direction of the mirror. —It was over the glassy surface of the fountain or
-the lake, that the shepherdess of the young world bent themselves, to learn
the charms which nature had bestowed on them. And a child contemplates
with wonder and delight, through the window of a still pool or gliding
stream, another sky below the ground, with its cloulds, and sun or stars;
and another landscape, with inverted wood and mountains, the supposed
dwelling of fairy beings.
In the cutting of canals, the making of railways, and in many other ope.
rations of engineering, it is of essential importance
to determine the level or horizontal direction at any    Fig. 74.
place; and this is usually done by a tube or glass,
a c, filled with spirit except one bubble of air b,   C    _       C
and called a spirit level.  When this tube is horizontal, the bubble has no tendency to move to either




134                    HYDROSTATICS.
end; but if the tube inclines ever so little, the bubble rises to the end which
is highest; or to speak more correctly, the denser spirit falls down to the
lower end, and forces the light bubble away from it. Such a tube properly
fixed in a frame, with a telescope attached to it, or simply with sight-holes
to look through, becoming the engineer's guide in many of his most
important operations.
A hoop surrounding the earth would bend away from a perfectly straight
line four inches in a mile. In cutting a level canal, therefore, which may
be considered as part of a hoop, there must be everywhere a falling from
the straight line, called by geometers a tangent, in the proportion now
described. All rivers also have the curvature of hoops applied to the surface
of the earth.
Canals leading from sea-ports to the interior of countries have generally
to ascend; but as water cannot become stagnant in any channel which is not
level, the canal is divided, by gates or sluices, into portions at different
levels, like steps of a stair, the rising at the joinings being generally from
six to twelve feet. The boats are raised or lowered from one level to
another by the contrivance called a lock, which is merely a portion of the
canal, of sufficient capacity for the boat to lie in, furnished with high walls,
and with flood-gates at both ends; so that when the gates below are'shut,
and water is gradually admitted from  above, it becomes part of the high
level, ready as such to deliver a boat, or receive one; and when the upper
flood-gates are shut, and the water is gradually allowed to escape from the
lock, it becomes a part of the low level, and a boat may enter it, or leave it
by its lower gates.
The cutting of canals is one of the great items in the mass of modern
improvement, which both mark and.hasten the progress of civilization.
Adverting to the importance of easy intercouse, as explained in a former
section, we need only say here, that a horse which can draw only one ton
on our best roads, can draw thirty with the same speed in a canal-boat.
And what a glorious triumph to science and art it is, tobe able to conduct
vessels of all kinds, even those originally intendedl for the ocean surge alone,
through the quiet valleys of an interior country! In Scotland,.now, along
the Caledonian canal, a noble frigate may be seen, wandering as it were
among the inland solitudes, and displaying her grace and majesty to the
astonished gaze of the mountain shepherd; and when she has traversed the
kingdom, and visited the lonely lakes, whose waters until lately had borne
only the skiff of the hunter, she descends again by the steps of her liquid
stair, and safely resumes her place among the waves.
It was lately in contemplation to lead a ship canal across the isthmus
which joins North and South America. The elevation to which the canal
must reach, to surmount the central ridge, is considerable, and will increase
the difficulty; but such important consequences would follow the accomplishment of the object, that, with the continuance of general peace, and the
increase of political wisdom, it will probably be attained. If so, the loaded
vessel, rising from the Atlantic, would soon be descried among the mountain
heights, and, a few hours after would be safely lodged in a port of the oppo.
site sea; having performed, by a near cut, a voyage which at present costs
months of delay and hazard, in.a tedious navigation round the whole southern
continent.-And if the Red Sea and Mediterranean were joined in the same
way, as has also been proposed, the operation would, in effect, bring India
nearer to Europe, and would more and more strengthen the bonds of mutual
utility and brotherhood among the nations of the earth. Then, indeed, might




FLUID LEVEL.                            135
it be said with truth, that the world is a vast garden, given to man for
his abode, of which every spot has its peculiar sweets and treasures; but
because the cultivator of each may exchange a share of its produce for shares
in return, the same general result follows as if every field or farm contained
within itself the climates and soils and capabilities of the whole.
In a canal, the least deviation from the true level would immediately cause
any water admitted into it to flow towards the lower end. This flux to a
lower situation is what is going on in the myriads of streams, which render
the face of the earth a scene of such varied beauty and incessant change.
As in the animal body, from every the minutest point a little vein endowed
with living power, takes the blood which has just brought life and nutriment
to the part, and delivers it into a larger vein, whence it passes into a larger
still, until at last, in the great reservoir of the'heart, it meets the blood returned from every part of the body, so, in this terraqueous globe, where the
magic moving power is simply fluid seeking its level, does the rain which
falls to sustain vegetable and animal life, and to renovate nature, glide from
every point of the surface into a lower bed, and from thence into a lower still,
until the countless streams, so formed, after every variety of course combine
to form the swelling rivers, which return the accumulated waters into the
common reservoir of the ocean. In the living body, the arteries carry back
the blood with renewed vitality to every point whence the veins had withdrawn it, and so complete the circulation; and in what may be called the
living universe, the circulation is completed by the action of heat and of the
atmosphere, which, from the extended face of the ocean raise a constant
exhalation of watery vapour of invisible purity, which the winds then carry
away and deposit as rain or dew on every spot of the earth.
A very slight declivity suffices to give the running motion to water. Three
inches per mile, in a smooth straight channel, gives a velocity of about three
miles per hour.  The Ganges, which gathers the waters of the Himalaya
mountains, the loftiest in the world, is, at eighteen hundred miles from its
mouth, only eight hundred feet above the level of the sea-that is, above
twice the height of St Paul's Church in London; and to fall these eight
hundred feet, in its long course, the water takes nearly a month. The greater
river Magdalena, in South America, whose channel, for a thousand miles, is
between two ridges of the Andes, falls only five hundred feet in all that distance. Above the commencement of the thousand miles, it is seen descending
in rapids and cataracts from the mountains. The gigantic Rio de la Plata
has so gentle a descent to the ocean, that, in Paraguay, fifteen hundred miles
from its mouth, large ships arrive which have sailed against the current all
the way, by the force of the wind alone: that is to say, which on the beautifully inclined plane of the stream, have been gradually lifted by the soft
wind, and even against the current, to an elevation greater than that of our
loftiest spires.
A small lake or extensive mill-pond, with uneven bottom, if suddenly
emptied by a sluice or opening at its lowest part, would exhibit a number of
pits or pools of various size and shape left among the inequalities. But supposing rain to fall, and frequently to recur, the water seeking its level would
soon effect a very. remarkable change. In consequence of each pool discharging over its lowest part, that is, sending out a streamlet either into another
lower pool, or into a channel leading directly to the sluice or opening, there
would be a constant wearing down of the part or side of the pool over which
the water was running, that is to say, a deepening of a breach or channel
there, and the surface of water in the pool would be consequently becoming




136                    HYDROSTATICS.
lower, while, at the same time, the bottom would be rising, owing to the
deposit of sand or mud washed down by the rain from the elevations around:
and these two operations continuing, the pool would at last altogether disappear. And by this change going on in every pool through the whole of the
emptied mill-pond, the general bottom would at last exhibit only a varied or
undulated surface of dry land, with a beautiful arrangement of ramifying
water channels, all sloping with a precision unattainable by art, to the general
mouth or estuary.-The reason that, in the supposed case, and in every other,
a water course soon becomes so singularly uniform, both as to dimensions
and descent, is, that any pits or hollows in it are filled up by the sand and
mud carried along in the stream, and deposited where the current is slack;
while any elevations are worn away by the action of the more rapid current
which accompanies shallowness.
The above paragraph describes, in miniature, what has been going on over
the general face of our earth ever since that convulsion of nature which
produced its present form. In many places the phenomenon is already complete; in others it is only in progress. The whole of what is now dry land,
has at some period been under water, and much of it has evidently been a
gradual deposition from water.  By some extraordinary convulsion, therefore, our present continents and islands must have been thrown up from the
bottom of an ocean, or In ocean must have.subsided away from them; and
in either case the land must have merged as checkered and unsightly as the
bottom of the emptied lake above supposed. And it is the gradual operation of water seeking its level which has at last converted the earth into the
paradise which we now behold.
The marks of the former state of the world, and of the progressive
change, are every where most strikingly evident to the enlightened eye of
philosophy. The present kingdom of Bohemia, for instance, is the bottom
of one of the great lakes formerly existing over Europ.e. It is a basin or
amphitheatre, formed by a wall of mountains, and the only gate or opening
to it, is that remarkable one by which the water now escapes from it, and
which evidently has been gradually cut or formed by the action of the running stream. As the bottom became uncovered, owing to the sinking of the
water, and the formation of a regular sloping channel from every part, the
former lake was converted into a fine and fertile country, a fit habitation for
man; and the continued drain from it of the rains which fall over its surface,
and either pass rapidly away, or sink into the earth, and ooze again more
gradually in the form of springs, is the beautiful river which we now call
the Elbe.
In Switzerland, many of the valleys which were formerly lakes, have the
opening for the exit of water so narrow, that, as happened in one of them a
few years ago, a mass of snow or ice falling into it, converts the valley once
more into a lake. On the occasion alluded to, the accumulation of water
within was very rapid; and although, from the danger foreseen to the country below, if the impediment should suddenly give way, every means was
tried to remove the water gradually, the attempt had not succeeded when the
frightful burst took place, and involved the inferior country in common ruin.'The magnificent Danube is the drain of a chain of basins or lakes, which
must, at one time, have discharged or run over one into another; but owing
to the continued stream cutting a passage at last low enough to empty them
all, they are now regions of fertility, occupied by civilized man, instead of
the fishes which held them formerly. This operation is still going on in
all the lakes of the earth. The lake of Geneva, for instance, although con



FLUID  LEVEL.                           137
fined by hard rock, is lowering its outlet, and the surface has consequently
fallen within the period of accurate observation and records; and as, at the
same time, the wearing of the neighbouring mountains, brought down by
the winter torrents, are filling up its bed, if the town of Geneva last long
enough, its inhabitants may have to speak of the river in the neighbouring
valley, instead of the picturesque lake which now fills it. Already several
towns and villages, which were close upon the lake a century ago, have fields
and gardens spreading between them and the shore.
Illustrating this subject, it is very interesting to observe the contrast
between the pure blue water of the Rhone issuing from the lake of Geneva,
and the turbid streams which join its course a little farther down. The
torrents which fall into the lake all around, are equally charged with the
debris or wearings of the mountains; but having deposited all their load in
the still bosom of the lake, the pure water alone escapes to form the river.
The streams, however, coming to the Rhone directly from the Alps, and
bringing with 4hem their charge of broken-down earth, even after they have
joined it, are long distinguishable by their muddy waters. It is the mud
deposited as here described, which is gradually filling up all lakes, and which
has formed the vast regions of fiat country seen about the: mouths of great
rivers. The greater part of Holland is deposition of this kind, the whole
of lower Egypt, a great part of Bengal, &c., &c.
There are some lakes on the face of the earth which have no outlet towards
the sea,; —all the water which falls into them, being again carried off by
evaporation alone-and such lakes are never of fresh water, because every
substance, which, from the beginning of time, rain could dissolve in the regions around them, has necessarily been carried towards them by their feeding streams, and there has remained. The great majority of lakes, however,
being basins with the water constantly running over at one part towards the
sea, although all originally salt, have, in the course of time, become fresh,
because their only supply being directly from the clouds, or from rivers
and springs fed by the clouds, is fresh, while what runs away from them
must always be carrying with it a proportion of any substance that remains
dissolved in them. We thus see how the face of the earth has been gradually washed to a state of purity and freshness fitting it for the uses of man,
and why the great ocean necessarily contains in solution all the substances
which originally existed near the surface of the earth, soluble in water:viz., all the saline substances. The city of Mexico stands in the centre of a
vast and beautiful plain, 7,000 feet above the level of the sea, and surrounded
by sublime ridges of mountains, many of them snow-capped. One side of
the plain is a little lower than the other, and forms the bed of the lake, which
is salt for the reasons stated above; —but the lake will not long be salt, for
it now has an outlet. About 150 years ago, owing to unusual rains, an extraordinary increase of the water took place, and covered the pavements of
the city. An artificial drain was then cut from the plain, at the distance of
about sixty miles from the city, to the lower external country. This soon freed
the city from the water, and since then, becoming every year deeper by the
wearing effects of the uninterrupted stream, it is still lowering the surface
of the lake, is daily rendering the water less salt, and is converting the vast
salt marshes, which formerly surrounded the city, into fresh and fertile fields.
The vast continent of Australasia, or New Holland, (as large as Europe,)
is supposed by some to have been formed at a different time from what is
called the Old World, so different and peculiar are many of its animal and
vegetable productions; and the idea of a later formation receives counte



138                     HYDROSTATICS.
nance from the existence of immense tracts of marshy or imperfectly drained
land discovered in the interior, into which rivers flow, but seem not yet to
have worn down a sufficient outlet or discharging channel toward the ocean.
Where the soil or bed of a country through which a water-track passes
is not of a soft consistence, so as to allow readily the wearing down of higher
parts, and the filling up of hollows by deposited sand, lakes, rapids and great
irregularities of current remain. We have, for instance, the line of the lakes
in North America, the rapids of the St. Lawrence, and the stupendous falls
of Niagara, where at one leap the river gains a level lower by a hundred and
sixty feet. A softer barrier than the rock over which the river pours, would
soon be cut through, and the line of lakes would be emptied.
The contemplation of the fact, that water in seeking its level is constantly
wearing where it rubs, and carrying the abraided portions down to lower
level, and ultimately to the bed of the ocean, brings irresistibly the awful
idea, that this earthly abode of ours,.owing to natural causes already in
operation, can have but a limited existence in its present state. No shower
falls that does not send portions of mountains and plains into the depths
of the ocean, and thus cause a corresponding encroachment on the shores by
the rising water; and with revolving ages, unless new -convulsions of nature
disturb the progress, or art succeed, as in Holland and elsewhere, in shutting out the ocean from extensive low tracts by means of sea dykes or embankments, the dry land must at last disappear, and another gradual deluge
embrace the globe.
There is, perhaps, nothing which illustrates in a more striking manner
the exact resemblance among nature's phenomena, or their accordance with
the few general expressions or laws which describe them all, than the perfect
level of the ocean as a liquid surface. The sea never rises nor falls in any
place, even one inch, but in obedience to fixed laws, and therefore its changes
may generally be foreseen and allowed for. For instance, the eastern tradewinds and other causes force the water of the Indian Ocean towards the
African coast, so as to keep the Red Sea about twenty feet above the
general ocean level; and the Mediterranean is a little below'that level,
because the evaporation from it is greater than. the supply of its rivers,
causing it to receive an additional supply by the Strait of Gibralter; —but
in all such cases, the effect is as constant as the disturbing cause, and therefore can be calculated upon with confidence.
Were it not for this perfect exactness, in what a precarious state would
the inhabitants exist on the sea shores, and on the banks of low rivers! Few
of the inhabitants of London, perhaps, reflect, when standing by the side of
their noble river, and gazing on the rapid flood-tide pouring inland through
the bridges, that although sixty miles from the sea, the water there is, at the
moment, lower than the surface of the sea, which may at the time be heaving
moreover, in lofty waves, covered perhaps with wrecks and the drowning.
The horrid destruction that would follow any alteration of the level of
the ocean, may be judged of by the effects of occasional floods, produced by
rains and melting snow in the interior of countries, or by these combined
with winds and high tides on the coasts. The flood at St. Petersburg, in
1825, was dreadful, in which strong westerly winds had retarded the flow of
the Neva so much, that the water rose forty feet (the height of an ordinary
house) above its usual mark, covered all the low parts of the town, and
drowned thousands of the people.
In Holland, which is a low flat, formed chiefly by the mud and sand
brought down by the Rhine and neighboring rivers, much of the country




FLUID  LEVEL.                            139
is really below the level of the common spring-tides, and is only protected
from daily inundations by artificial dykes or ramparts, made strong enough
to resist the ocean. On one occasion the water broke into such an enclosure,
and drowned more than sixty thousand people.  What awful uncertainty
then would hang over thei existence of the Dutch, if the level of the sea
were subject to change; for while.we know that its waters, owing to the
centrifugal force or of the earth's rotation, are seventeen miles higher at the
equator than at the poles. if the level, as now established, were from any
cause to be suddenly changed but ten feet, millions of human beings would
be the victims.
Where inundation is regularly periodical, as in the Nile and many other
rivers, the hurtful effects can be guarded against, and the occurrence may
even become useful, by fertilizing the soil.
Tracts of land in contact with rivers, of which land, the surface lies be.
tween the levels of ebb and flood-tide, if surrounded with dykes, may be
kept constantly covered with water, by opening the sluices only at high water;
or may be kept constantly drained, by opening the sluices only at low water.
A vast extent of rice fields, near the mouths of rivers in India and China, is
managed in this way, the admission or exclusion of water being regulated by
the age of the rice plant. A great part also of the rich sugar plantations
of Demerara, Esequibo, &c., on the coast of South America, are in the saihe
predicament; and another advantage which these have over the plantations
on the West-India Islands, is the saving of the labour of transport effected
by the canals which intersect all the fields.
" If various tubes and vesselA communicate with one another, fluid admitted
into them will rise to the same level in all."  (Read the Analysis, p. 84.)
The following sketch may represent a variety of tubes and vessels, fixed
upon and opening into the cistern op box G.  Water poured into any one
would fill the box, and would then rise to the same level in all. The dotted
lines from a tof may represent the surfaces of the fluid in the different
vessels.  In the figure at p. 128, it
was seen why, in all upright cylin-                Fig. 75.
drical vessels, as a, b and c, the fluid   b         c c    e
rises to the same level; and the figure
at p. 132 explained why shape of the   a
vessel cannot affect the level.  Although in the oblique vessel c, represented here, there is more water
than in a,. still there is the same
pressure at the bottom of both, because e supports part of the weight
of its contained fluid on the principle of the inclined plane.
If a tube twenty miles long, and rising and descending among the inequalities of a country, were filled with water, and could have its ends
brought together for comparison, it would exhibit two liquid surfaces having
precisely the same level; and on either end being raised, the fluid would
sink in it to overflow f'm the other.
An easy mode of determining a level line at any spot is to have an open
tube, bent up at its ends a and b, and nearly          Fig. 76.
filled with liquid: b-y then looking along the
two liquid surfaces, or through floating sights  a't           J b
resting on them, an observer looks in a line
which is quite horizontal at the middle point between them.




140                     HYDROSTATICS.
If there were two lakes on adjoining hills of different heights, a pipe of
communication descending across the valley and connecting them, would
soon bring them to the same level; or if one were much higher than the
other, would empty that one into the other.
A projector thought that the vessel of his contrivance, represented here,
was to solve the renowned problem of the perpetual motion. It was gobletshaped, lessening gradually towards the bottom until it became a tube, turned
upwards at c, and putting with an open extremity into the goblet again. He
reasoned thus: A pint of water in the goblet a must more than counterbalance
an ounce which the tube b will contain, and must thereFig. 77.     fore be constantly pushing the ounce forward into the
vessel again, and keeping up a stream or circulation,
which will cease only when the water dries up.  He
b   was confounded when a trial showed him  the same
level always in a and in b.
A glass tube inserted near the bottom of a cask or
cistern of any sort, not air-tight above, -which tube is
then bent upwards, to appear on the outside like a
barometer tube, shows by the elevation of a fluid in it,
the height of the greater mass of fluid within.
In like manner a tube brought from a river into a neighbouring cellar or
pit, will indicate the height of water in the river.
A knowledge of the truth, that water in pipes will always rise again to
the height or level of its source, has enabled men in modern times to
construct those admirable systems of iron pipes, which distribute water in
great towns.  The water brought to any elevated site, in or near the town,
may be delivered from a reservoir there, by the effect of gravity alone, to
every cistern which is under the level of the reservoir, the result not being
affected by the pipes having to rise over heights and to descend into valleys
many times in their course.
On the hill north of London, on which'Pentonville stands, there is such a
reservoir to which water is brought from Hertfordshire, by a channel cut for
the purpose, upwards of thirty miles in length, and called the New River.
Another reservoir has lately been constructed by the West Middlesex Water
Company, at Primrose Hill, higher than any house in town.  It is filled by
operation of steam-engines at the Company's works, near Hammersmith,
five miles off. It will supply water to the summits of all the houses connected with it, and is exceedingly useful in cases of fire.
Many persons have believed that the ancients were ignorant of the law,
that fluid in pipes rises to the level of its source, because, in all the ruins
of their aqueducts, the channel is a regular slope. Some of the aqueducts,
as works of magnitude, are not inferior to the great wall of China, or the
Egyptian Pyramids; yet, at the - present day, a single pipe of cast-iron is
made to answer the same purpose, and even more perfectly.  It is now
ascertained, however, that it was not ignorance of the principle, but want of
fit material for making the pipes, which cost our forefathers such enormous
labour.
The supply and distribution of water in a large cid, particularly since the
steam-engine has been added to the apparatus, approach closely to the perfection of nature's own work in the circulation of blood through the animal
body. From the great pumps or a high reservoir, main pipes issue to the
chief divisions of the town; these then send suitable branches to the streets,
which branches again divide for the lanes and alleys; and at last subdivide




FLUID LEVEL.                            141
until every house has its small leaden conduit carrying its precious freight,
if required, even into the separate apartments, and yielding it anywhere to
the turning of a cock A corresponding arrangement of drains and sewers,
most carefully constructed in obedience to the law of level, receives the
water again when it has answered its purposes, and sends it to be purified
in the great laboratory of the ocean.  And so admirably complete and
perfect is this counter-system of sloping channels, that a heavy shower may
fall, and after washing and purifying every superficial spot of the city, and
sweeping out all the subterranean passages, may, within the space of an
hour, form part of the river passing by. It is the recurrence of this almost
miracle, of extensive, sudden, and perfect purification, which makes modern
London the most healthy, while it is the largest city in the world.
English citizens have now become so habituated to the blessing of a
supply of pure water, more than sufficient for all their purposes, that it no
more surprises them than the regularly returning light of day or warmth of
summer. But a retrospect into past times may still awaken them to a
sense of their obligation to advancing art.  How much of the anxiety and
labour of men in former times had relation to the supply of this precious
element! How often, formerly, has periodical pestilence arisen from the
deficiency of water; and how often has fire devoured whole cities, which a
timely supply of water might have saved! Kings have received almost
divine honours for constructing aqueducts, to lead the pure streams from
the mountains into the peopled towns. In the present day, it is he who has
travelled on the sandy plains of Asia or Africa, where a well is more prized
than mines of gold, or who has spent months on ship-board, where the fresh
water is often doled out with more caution than the most precious product
of the still, or who, in reading history, has vividly sympathized with the
victims of siege or shipwreck, spreading out their garments to catch the rain
from heaven, and then with mad eagerness, sucking the delicious moisture
— it is he who can appreciate fully the blessing of that abundant supply
which most of us now so thoughtlessly enjoy. The author of this work
will long remember the intense momentary regret with which, at once
approaching a beautiful land after months spent at sea, he saw a stream of
fresh water gliding over a rock into the salt waves-it appeared to him as
if a most precious essence, by some accident were pouring out to waste.
The subject of fluid level leads to a consideration of springs or wells, and
of the operation of boring for water.
The water which falls from the clouds, and which must all ultimately
return to the sea, may find its way to the rivers, either by running directly
along the surface of soils which refuse it admittance; or by first sinking into
porous earth, and again oozing out at lower situations in the form of springs.
If a spring be as low as the bottom of the porous earth from which it issues,
that is to say, as low as the surface of the impermeable clay or rock on
which at some depth all such earth rests, it may drain the whole; but if not,
the water will stand at a certain level among the earth as it would among
bullets in a water-tight vessel. If a hole or pit be then dug in such earth,
reaching below the level of the water lying in it, the pit will soon be filled
with water up to the level, and will be called a well. In many places this
water-level is very far below the surface of the ground; and in some places,
by reason of the water having an easy drainage towards the sea, or of the
superficial soil being altogether impermeable to it, there is none to be found
within an accessible depth.




142                     HYDROSTATICS.
A remarkable illustration of this subject occurred a few years ago, in Kent,
on the occasion of cutting between Rochester and Gravesend the canal called
the Thames and Medway canal. This canal consists of but one cut or
level, seven miles long, of which two are in a tunnel through the hill-which
level is that of high water in the connected rivers; the intention having
been to let the canal be filled always from the rivers at high water;-but as
the level of the subterranean water in the surrounding land, and therefore of
all the inhabitants' wells there, is, as might be anticipated, half-way between
the levels of high and low tides, the salt water from the rivers was no
sooner admitted to the canal than it spread into the land on either side,
where the resisting internal water-level was lower, and destroyed. all the
wells. If the canal had been dug a few feet lower, the mischief would not
have occurred, and the company would have escaped paying the heavy
damages, which rendered their undertaking a very ungainful speculation.
All the wells and springs in the world are merely the rain water which has
sunk into the earth, appearing again, and gradually escaping at lower places:
nature thus admirably making the bowels of the earth an ever-stored reservoir
of the substance most indispensable to the comfort and existci ce of man, and
of all living creatures. It is worthy of remark here, that high cultivation or
agricultural improvement of a country has a great effect on the quantity of
spring water in it. While the face of a country is rough the rain-water
remains long among its inequalities, slowly sinking into the earth to feed the
springs, or slowly running away from the surface as from bogs and marshes
towards the rivers. The rivers, hence, have a comparatively uniform and
regular supply, even when rain has not fallen for a long time:-but in a
well-drained country, the rain, by a thousand prepared channels, finds its
way to the brooks and rivers almost immediately, producing often dangerous floods or inundations of the neighboring low grounds. A friend of
the author had a waterfall and mill in Surrey, which he formerly let for a
rent of ~1,200 a year; but after agricultural improvements in the district
from which the water came, the supply of water was generally either
superabundant or deficient, and the value of the mill was reduced to one-half.
The surface of our globe is formed of different strata or layers, as of clay,
chalk, sand, gravel, &c., &c., which appear all to have been at former periods
horizontal, formed under water, and to have been afterwards thrown up, by
some convulsion or convulsions of nature, into every variety of position. In
particular situations, the upper surface is now concave or basin-shaped, the
different strata or layers, when water-tight, being like cups or basins placed
one within another, and as water poured in, to fill the space between two
basins so placed, would spring out to the height of its upper or level surface,
through any hole made in the side of either, so on boring for water, through
an innermost or superior water-tight stratum or basin of earth, the water
often springs out and rises far above the surface of the ground. L6ndon
stands in a hollow of which the first-met layer is a basin of clay, placed
over chalk, and on boring through the clay (souletimes of three hundred feet
thickness,) the water issues and in many places will form a jet considerably
above the surface of the ground; showing that there is a higher source or
level somewhere-as among the hills of Surrey, or those north of London.
When fluids of different kinds and of different weights under the same
bulk, are made to oppose or to balance each other in communicating vessels
-as water, for instance, in one leg of the bent tube b d c, and oil in the other
-the surfaces will not all rest or settle at the same height or level, but that
of the lighter fluid will be j ust as much higher than that of the other as it is




FLUID SUPPORT.                            143
lighter. Thus a column of oil must be of a
length as d o, to balance a column of water         Fig. 78.
d w: and alcohol, because lighter than oil,   b    ce
to balance the same water, would have to. 
stand higher still, as at a; while mercury,       c  
because thirteen times weightier than water,  
would stand only about m. The shape, size,                       /
or position of the vessels in which the opposing fluids might stand, would have no influence on the relative heights of the surfaces;.
for if we suppose a larger vessel, such as is
represented here by the dotted lines between
the letters e f m, to be substituted for the     in..-.. o
leg c d of the tube, the various fluids to    Li
balance the water in b d, would have to stand  dc
just as high in it as in the smaller tube.
" A body immersed in a flaid, displaces exactly its own bulk of it, which
quantity having been just supported by the fluid around the body is held,up with force exactly equal to the weight ofthe fluid displaced, and must
sink or swim according, as its own weight is greater or less than this."
A bladder full of air, and maintaining the bulk of a pound of water,
requires a force of one pound (except a few grains, the weight of the air,)
to plunge it under the water. The same bulk of gold is held up in water
with exactly the same force; so that, if previously balanced at the end of a
weighing beam, it appears on immersion to have lost one pound of its
weight.
And a piece of wood,' ivory, or any other substance, having exactly the
same bulk, is opposed on entering the fluid by the same resistance.
The reason of this is obvious, for the immersed body takes the place of
water which weighed one pound and yet was supported, and whose pressure
was necessary for the equilibrium of the rest. In a vessel of water represented here by the figure a b, let us append to any
portion of the water, a single column of particles,    Fig. 79.
for instance, represented by the line c d: we
know that each column is steadily supported in..............
its place, because the particle of the liquid 
immediately under it is tending upwards to cs
escape from the surrounding pressures, with force
exactly equal to the weight of the column; and    ]
what is true of a column of single particles, is
true of any other portion, such as the larger column represented by the
figuref h g. If such portion weighed exactly a pound, the surface under it
would be tending upward with the force of a pound; and if the portion,
without changing its bulk or form, were to become ice, it would still be
exactly supported by the surface below pressing upwards with a force of a
pound; and farther, if a similar column of wood, or stone, or metal, were there,
the surrounding pressure would still be the same. Again, if we suppose
only half the column to be solidified, the portion h g for instance, it, would
be pressed upwards with a force of one.pound at g; but its own weight of
half a pound, and the weight of the half pound of water above it, would
produce an exact balance and maintain rest.




144                     HYDROSTATICS.
It is very imlprtant to have clear notions on this subject; and as different
minds apprehend such matters with different degrees of facility, and in
different ways, we shall state the same general truth in other words.
Let us consider a mass of fluid as consisting of a vast number of extremely
minute columns of single particles standing side by side, where every
particle supports those above it by tendency upwards which it requires
through the pressure of the fluid surrounding it. Now if we suppose the
particles of a portion of a fluid mass, of any shape, to stick together, or to
become ice without change of bulk or weight, that portion when solid would
still be between the same forces as when fluid, and therefore would be equally
supported, and would remain at rest. And if god, or silver, or glass, or
wood, having the same bulk, were substituted for the supposed ice, such
new substance would still be sustained with the same force; so that a
substance of exactly the same weight as the ice or water displaced, would
have no tendency either to rise or to fall more than the water itself had;
but a substance heavier would sink, and one lighter would swim, and in
either case with force exactly proportioned to the difference between its
weight and that of an equal bulk of water.
Few persons, in now reading the statement of this truth-in appearance
so simple and obvious —would imagine that it had remained so long unknown
and that the discovery of it may be accounted one of the most important
which human sagacity ever made,-but such is the case.  We owe the discovery to one of the master-minds of antiquity-that of Archimedes. He
caught the idea one day while his limbs were resting on the liquid support
of a bath: and as his God-like intellect darted into futurity, and perceived
many of the important uses to which the knowledge was applicable, he is
said to have become so moved with admiration and delight, that he leapt
from the water, and unconscious of his nakedness, pursued his way homewards, calling out " fvCvx:, FvC7xa',"   I have found it.' He was thinking chiefly
of the ready means, thus obtained, of ascertaining in all cases what has since
been called the specific gravity of bodies, viz., the comparative weights of
equal bulks of different substances; as of gold, or silver, or copper, or iron,
compared with water; and in the case of mixtures, as of gold with silver for
instance, of declaring at once the proportion present of each-important
problems, which, until then, could not be correctly solved.
The hydrostatic law now explained, has since led to great advances in
various arts. It may be regarded as a chief foundation of chemistry, for by
it the chemist distinguishes one substance from another, distinguishes a
pure from an impure substance, and discovers the nature of many mixtures
or compounds. The merchant often judges by it of the worth of his merchandize. In any case it enables an inquirer to ascertain at once the exact size
or solid bulk of a mass, however irregular-even of a bundle of twigs. It
has become the cause of improvements in navigation, in marine architecture,
and in many other arts.
We shall now discuss more particularly the subject of comparative
weights or specific gravity.
" Theforce with which a body is held up in a fluid, being the exact weight
of its bulk of that fluid, by ascertaining this force and comparing it with
the weight of the body itself the comparative weights or SPECIFIC GRAVITIES are found."  (Read the Analysis, p. 126.)
If any body, c, a mass of gold, for instance, be suspended by a thread or




FLUID  SUPPORT.-SPECIFIC  GRAVITY.    14'5
hair from the bottom of one scale b
of a weighing-beam, and be balanced              Fig. 80.
by weights put into the other scale a,
and if a vessel of water be then lifted
under it so that the water shall surround it, the body  is pushed up or
supported by the water with force
equal to the weight of the water which
it displaces; the weights, therefore,
then required in the scale b to restore
the balance, show  truly the exact
weight of the water displaced; or of
water equal in bulk to the body; and
the weights in the two opposite scales                        c
show the comparative weights of the
body and of its bulk of water. In the
supposed case, whatever weight the
gold had in the air, it would seem to lose when the water surrounded it,
about a nineteenth part of such weight; that is, the water would support it
with this force; and gold would thus be proved to be about nineteen times
as heavy as water.
In making a table of specific gravities, it was necessary to select a common
standard with which all other substances should be compared, and this has
been done in choosing water; the reason of preference being, that water can
be so easily procured in a state of purity, and therefore of uniformity, in all
situations.  When we say, therefore, that gold is of the specific gravity 19,
and copper 9, and cork!, we mean that these substances are just so much
heavier or lighter than their bulk of pure water in its densest state, viz., at
the temperature of 40 degrees of Fahrenheit's thermometer.
As the substances in nature differ as to form and other qualities, corresponding differences have to be made in the manner of ascertaining their
specific gravities: the following cases are most important.
Solid bodies insoluble in water and heavier than it-as the metals, &e.,
are merely suspended by a thread or hair, having nearly the specific gravity
of water, to one scale of the hydrostatic balance (simply a good weighingbeam with a water-vessel below one of the scales;) and the body being first
balanced or weighed in the air, and then in water, as already described, the
weight and the loss, represented, if the operator chooses, by the weights in
the-opposite scales, are the weights of equal bulks of the two substances;
and by finding, through the arithmetical operation of division, how often
the weight of the water is contained in the weight of the solid, we find the
specific gravity of the solid, or how much it is weightier than its bulk of
water. —It is almost superfluous to remark, that putting.weights into the
scale, b or taking them out of the scale a, are equivalent operations. We
shall explain afterwards, that for very delicate purposes bodies must be
weighed first in a vacuum, instead of in air, or a suitable allowance must be
made; for air itself supports a little any body immersed in it.
Solids lighter than water, as cork, are weighed in it by attaching to
them a mass of metal or glass heavy enough to sink them, and already
balanced in water for the purpose; or by making the line which connect
them with the weighing beams pass under a small pully fixed at the bottom
of the vessel, so that the rising of the end of the beam to which they are
attached shall draw them down.
10




146                     HYDROSTATICS.
A solid soluble in water, as a chrystal of any salt, may be protected
during the operation of weighing in water, by previously dipping it in
melted wax, so as to leave a thin covering on it; or it may be weighed in
some liquid which does not dissolve it, allowance being afterwards made for
the difference between the weight of such liquid and of water.
Powders insoluble in water, such as gold dust, are weighed in a glass cup
which has previously been balanced in water for that purpose.
Powders soluble in water, must be weighed in some other liquid.
Mr. Leslie, the highly endowed professor of natural philoolphy in the
University of Edinburgh, has lately suggested a novel and ingenious mode
of acertaining the specific gravity of pulverized or porous bodies; but as it
can be understood only by persons acquainted with the doctrines of
pneumatics, the consideration of it must come under that head.
Other liquids may be compared with water in several ways. %t. If a
phial be made to hold exactly one thousand grains of distilled water, at
the temperature of 400, the weight of the same measure of any other liquid
is found, by simply filling the phial, and weighing it. Of sulphuric acid,
for instance, such a phial will contain nearly nineteen hundred grains, while
of alchohol it will receive only about eight hundred. 2d. A bulb of glass,
which loses one thousand grains when weighed in water, (which thousand
grains is therefore the weight of its bulk in water,) may be weighed in other
liquids,. and the difference of loss marks the specific gravity, as in the last
case. The bulb for this purpose may be of any size, but one which loses
in water exactly one thousand grains, is preferable, from the simplicity
thereby given to the calculations:-This remark applies also to the phial
last mentioned..3d. A contrivance which renders the beam and scales altogether unnecessary, is a hollow floating bulb of glass or metal a, with a
slender stalk rising from it to support the little scale or dish b, and with
another stalk descending to carry the weight or weights at c, which serve as
ballast to it. The whole is so adjusted that when displacing one thousand
grains, or other known quantity of pure water, it shall float with a certain
mark upon the upper stalk just at the surface of the water. By then immersing it in other liquids and finding how much weight must be added to,
Fig. 81.    or taken from it above or below, to make it float in them
at the same elevation, the comparative weights of these
other liquids and of water are found:-or the difference of
weight which makes it float at different elevations in
water, having been previously ascertained, it will only be
a         necessary, in any other case, to note exactly its elevation;
an inch of the slender stalk may be equivalent to a
difference of ten grains. This instrument is called an
hydrometer.  There are generally printed tables and
directions, accompanying all forms of it, telling the exact
import of the several indications, and the allowances to
be made for temperature, &c. It may be used for weighing solids as well as liquids, for if any mass be put into the saucer b, weights
exactly equal to the mass must be taken out of the saucer b, or from below
at c, to restore the equilibrium of the instrument. The mass may be afterwards placed at c, and weighed in water. 4th. The shortest mode of ascertaining the specific gravities of liquids, is to have a set or series of small
glass bubbles of different specific gravities, so that when they are thrown
into any liquid, those heavier than it will sink, and those lighter will swim,
while that one which marks its specific gravity will remain merely suspended.




FLUID  SUPPORT.-SPECIFIC  GRAVITY.    147
The bubbles must, of course be numbered, and the specific gravity of each
be previously known.
A common use of hydrometers is to ascertain the quality of the distilled
spirits brought to market, as of rum, brandy, gin, &c. All these consist of
alcohol more or less diluted with water; and duty or tax is levied upon them
in proportion to their strength, or the quantity of alcohol which they contain. A delicate hydrometer discovers this at once.
A shop-keeper in China sold to the purser of a ship, a quantity of
distilled spirit according to a sample shown,; but not standing in awe of
conscience, he afterwards, in the privacy of his store-house, added a certain
quantity of water to each cask.
The spirit having been delivered on board, and tried by the hydrometer,
was discovered to be wanting in strength.  When the vendor was charged
with the intended fraud, he at first denied it, for he knew of no human
means which could have made the discovery; but on the exact quantity of
water which had been mixed being specified, a superstitious dread seized
him, and having confessed his roguery, he made ample amends. On the
instrument of his detection being afterwards shown to him, he offered any
price, for what he foresaw might be turned to great account in his trade.
The specific gravity of aeriform  substances is ascertainnd by means of
a glass flask of known size, furnished with a stop-cock. It is first weighed
when emptied by the air-pump, and afterwards when filled successively
with water and with different airs or gases.  Comparison of the weights
gives the specific gravities, as already described.
The following table shows, in round numbers, the comparative weights
or specific gravities of some common substances. Water is the standard
kept in view, and any equal bulk of another substance is heavier or lighter
than water, according to the numbers severally attached to them.
Platinum....  22-  Common Salt,... 2
Gold...          19.  Brick....    2
Mercury.. 131  Alcohol.1Copper....       83  ZEther.. 
Steel and Iron..       8   Cork....
Diamond...      3   Atmospheric Air  -
Glass...            3   Hydrogen Gas..
Common stones                  2..                             -a -a
Complete tables are found in systems of Dictionaries of Chemistry.
A cubic foot of water happens to weigh very nearly one thousand ounces
avoirdupois, or 621 pounds.  Hence, in the foregoing table, the figures
denoting the specific gravities tell how many times a thousand ounces of
the different substances a cubic foot contains.  Of gold, for instance,
a cubic foot contains more than nineteen thousand ounces, being worth in
money about ~63,000 sterling. A cubic foot of common air contains only
a little more than one ounce; and of hydrogen gas, the lightest of ponderable things, a cubic foot contains less than a drachm.
The following facts are also illustrations of the truth, that a body immersed
in a fluid is held up, or has its entrance resisted, with force equal to the
weight of the quantity of fluid which it displaces.
A stone which on land requires the strength of two men to lift it, may be
lifted and carried in water by one man.  There are cases, therefore, where




148                     HYDROSTATICS.
the support of water thus rendered useful, is equivalent to the assistance of
additional hands. A boy will often wonder why he can lift a certain stone
to the surface of water, but no farther.
The invention of the diving-bell in modern times, having enabled men, in
the building of piers, bridges, &c., to work under water almost as freely as
above, many have experience of this influence of water: but workmen are
generally surprised at first, to find that below they can move much larger
and heavier stones than they can in the air.  Some had supposed the fact
accounted for by saying that the denser air of the diving-bell, wlfen received
into the lungs gave greater strength. In recovering property from a sunken
ship. by the diving-bell, everything is found to be lighter in the proportion
now stated.
This law explains also why stones, gravel, sand and mud, are so easily
moved by waves and currents.  Many people expressed astonishment, in
March, 1825, to learn that at the Plymouth Breakwater, the storm had displaced blocks of stone of many tons weight; but we now see that the moving
water had only to overcome about half the weight of the stone.
When a person lies in a bath, the limbs are so nearly supported by the
water as to require scarcely any exertion on the part of the individual. When
this softest of all beds has been indulged in for half an hour or more, the
person, on first lifting a limb out of the water, feels surprise at its great
apparent weight.  The workers about diving-bells always experience the
sensation now spoken of, on returning to the air.
The bodies of most fishes are nearly of the specific gravity of water, and,
therefore, if lying in it without making exertion, they neither sink nor rise
very quickly.  When this subject was less understood, many persons
believed that fishes had no weight in water; and it is related as a joke at
the expense of philosophers, that a king having once proposed to his men
of science to explain this extraordinary fact, many profound disquisitions
came forth, but not one of the competitors thought of trying what really
was the fact. It was beneath the dignity of science in those days to make
an experiment. At last a simple man balanced a vessel of water in scales,
and on putting a fish into the water, showed its scale preponderating just as
much as if the fish had been weighed alone.
In the sense now explained, water is said to have no weight in water.
The least force will raise a bucket of water from the bottom of a well to the
surface; but if the bucket be lifted at' all farther, its weight is felt just in
proportion to the part of it which is above the surface.., body lighter than its bulk of water will float, and with force proportioned to the difference." (Read the Analysis, p. 126.)
The reason of this is clear. If any body, the cylinder a b c d for instance,
be partially immersed in water, we know that the
Fig. 82.        upward pressure of the water on the bottom c d, is
exactly what served to support the water displaced
by the body, viz., water of the bulk, efc d. The
body, therefore, that it may remain out as far as
c'    here represented, must have exactly the weight
of the water which the immersed part of it displaces; and if it be lighter than this, it will rise
farther; if heavier, it will sink farther until the
exact balance be produced.




FLUID  SUPPORT.-SWIMMING.                         149
Hen"ce of any body which floats in water, a pound weight displaces just a
pound of water, whether the body be very light in proportion to its bulk, as
cork, or heavier, as a piece of dense wood. This is experimentally shown
by putting such bodies to float in a vessel originally full of water. The
water displaced by each must run over the sides of the vessel, and may be
caught and measured.
Hence a porcelain basin weighing four ounces will sink in water only as
far as a similar wooden basin or bowl of the same weight; and the weight of
either basin may be in the substance of which it is formed, or in anything
else put into it as a load.
Hence a boat'made of iron floats just as high out of water as a boat of similar form and sizemade of wood, provided the iron be proportionately thinner
than the wood, and therefore not heavier on the whole. An empty metallic
pot or kettle is often seen floating with a great part of it above the surface of
the water.-Prejudice for a long time prevented iron boats from being used,
although, for various purposes, they are superior to others: and there are still:
people who would fear to go on board of a ship built of the strong and singularly durable Indian teaks, because it is heavier than water, and, in the form
of a log, therefore, sinks in water. Many fine ships of thelline, however, and
East-Indiamen of fifteen hundred tons or more, are now built of teak.
Hence a ship carrying a thousand tons weight will draw just as much
water, or float to the same depth, whether her cargo be of cotton or of lead:and the exact weight of any ship and her cargo may be determined by finding how much water she displaces. In canal boats, which are generally of
a simple form, this truth affords a ready rule for ascertaining the quantity
of their load.
The human body, in an ordinary healthy state with the chest full of air, is
lighter than water.
If this truth were generally and familiarly understood, it would lead to the
saving of more lives, in cases of shipwreck and in other accidents, than all
the mechanical life-preservers which man's ingenuity will ever contrive.
The human body with the chest full of air naturally floats with a bulk of
about half the head above the water,-having then no more tendency to sink
than a log of fir. That a person in water, therefore, may live and breathe it
is only necessary to keep the face uppermost. The reason that in ordinary
accidents so many people are drowned who might easily be saved, are chiefly
the following:1st. They believe that the body is heavier than water, and therefore, that
continued exertion is necessary to keep it from sinking; and hence, instead
of lying quietly on the back, with the face upwards, and with the face only
out of the water, they generally assume the position of a swimmer, in which
the face is downwards, and the whole head has to be kept out of the water
to allow of breathing. Now, as a man cannot retain this position but by.continued exertion, he is soon exhausted, evgn if, a swimmer, and if he is not,
the unskilful attempt will scarcely secure for him even a few respirations.
The body raised for a moment by exertion above the natural level, sinks as
far below it when the exertion ceases; and the plunge, by appearing the
commencement of a permanent sinking terrifies the unpractised individual,
and renders him an easier victim to his fate.-To convince a person learning to swim of the natural buoyancy of his body, it is a good plan to throw
an egg into water about five feet deep, and then desire him to bring it up
again.. He discovers that instead of his body with the chest full'of air na



150                     HYDROSTATICS.
turally sinking towards the egg, he has to force his way downwards, and is
lifted again by the water as soon as he ceases his effort.
2d. They fear that water entering by the ears may drown, as if it entered
by the nose or mouth, and they make a wasteful exertion of strength to prevent it; the truth being, however, that it can only fill the outer ear, as far
as the membrane of the drum, where its presence is of no consequence.
Every diver and swimmer has his ears thus filled with water, and cares not.
3d. Persons unaccustomed to the water, and in danger of being drowned,
generally attempt in their struggle to keep their hands above the surface,
from feeling as if their hands were imprisoned and useless while below; but
this act is most hurtful, because any part of the body held out of the water,
in addition to the face which must be out, requires an effoqt to support it,
which the individual is supposed at the time ill able to afford.
4th. They do not reflect, that when a log of wood or a human body is
floating upright, with a small portion above the surface, in rough water, as
at sea, every wave in passing must cover it completely for a little time, but
again leave its top projecting in the interval. The practiced swimmer chooses
this interval for breathing.
5th.  They do not think of the importance of keeping the chest as full of
air as possible; the doing which has nearly the same effect as tying a bladder of air to the neck, and without other effort, will cause nearly the whole
head to remain above the water. If the chest be once emptied, while from
the face being under water the person cannot inhale again, the body remains
specifically heavier than water, and will sink.
When a man dives far, the pressure of deep water compresses, or diminishes the bulk of the air in his chest, so that, without losing any of that air,
he vet becomes really heavier than water, and would not again rise, but for
the exertion of swimming. The author of this work once saw a sailor ( a
fine-bodied West India negro ) fall into the calm sea from a yard-arm eighty
feet high. The velocity on his reaching the water was so great, that he shot
deep into it, and, of course, his chest was compressed as now explained:
probably also the shock stunned him, for although he was an excellent
swimmer, he only moved his arms feebly once or twice, and was then seen
gradually sinking for a long time afterwards, until he appeared only as a
black and distant speck, descending towards the unknown regions pf the
abyss.
Every person need not learn to swim; but every one who makes voyages
should have practiced the easy lesson of resting in the water with the face
out. The head, from the large quantity of bone in it is a heavy part of the
body, yet, owing to its proximity to the chest, which is comparatively light,
a little action of adjustment with the hands, easily keeps it uppermost; and
there is an accompanying motion of the feet, called treading the water, not
diffcult to learn, which suffices to sustain the entire head above the surface.
Many of the seventy passengers who were swallowed up on the sudden sinking of the Comet steam-boat near Qreenock, in November, 1825, might have
been saved by the boats, which so soon went to their assistance, had they
known the truth which we are now explaining.
A man having to swim far, may occasionally rest on his back for a time,
and resume his labor when he is somewhat refreshed.
So little is required to keep a swimmer's head above water, that many
individuals, although unacquainted with what regards swimming or floating,
have been saved after shipwreck, by catching hold of a few floating chips or
broken pieces of wood, An oar will suffice as a support to half a dozen




FLUID  SUPPORT.-STABILITY.                         151
people, provided no one of the number attempts by it to keep more than his
head out of the water; but often, in cases where it might be thus serviceable,
from each person wishing to have as much of the security as possible, the
number benefitted is much less than it might be.
The most common contrivances, called life-preservers, for preventing
drowning, are strings of cork put round the chest or neck, or air-tight bags
applied round the upper part of the body, and filled, when required, by those
who wear them blowing into them through valved pipes.
On the great rivers of China, where thousands of people find it more convenient to live in covered boats than in houses upon the shore, the younger
children have a hollow ball of some light material attached constantly to
their necks, so that, in their frequent falls overboard, they are not in danger.
Life-boats have a large quantity of cork mixed in their structure, or of
air-tight vessels of thin copper or tin plate: so that, even when the boats
are filled with water, a considerable part still floats above the general surface.
Swimming is much easier to quadrupeds than to man, because the ordinary motion of their legs in walking and runningis that which best supports
them in swimming.  Man is at first the most helpless of creatures in water.
A horse while swimming can carry his rider with half the body out of the
water.  Dogs commonly swim well on the first trial.-Swans, geese, and
water-fowls in general, owing to the great thickness of feathers on the under
part of their bodies, and the great volume of their lungs, and the hollowness
of their bones, are so bulky and light, that they float upon the water like
stately ships, moving themselves about by their webbed feet as oars.
A water-fowl floating on plumage half as bulky as its naked body, has
about half that body above the surface of the water; and similarly a man
reclining on a floating mattrass, as in the hydrostatic bed afterwards to be
described, has nearly as much of his body above, the level of the watersurface, as he forces of the mattrass under it.  His position, therefore,
depends on the thickness of the mattrass.
A man walking in deep water may tread upon sharp flints or broken glass
with impunity, because his weight is nearly supported by the water.
But many men have been drowned in attempting to wade across the fords
of rivers, from forgetting that the body is so supported by the water, and
does not press on the bottom sufficiently to give a sure footing against a very
trifling current. A man, therefore, carrying a weight on his head, or in his
hands held over his head, as a soldier bearing his arms and knapsack, may
safely pass a river, where, without a load, he would be carried down the
stream.
There is a mode practised in China of cat.hing wild ducks, which requires
that the catcher be well loaded or ballasted. The light grain being first
strewed upon the surface of the water to temp them, a man hides himself
in the midst of it, under what appears a gourd or basket drifting with the
stream, and when the flock approaches and surrounds him, he quickly
obtains a rich booty by snatching the creatures down one by one-adroitly
making them disappear as if they were diving, and then securing them
below. Each bird becomes as a piece of cork attached to his body.
Fishes can change their specific gravity, by diminishing or increasing the
size of a little air-bag contained to their body. It is because this bag is
situated towards the under side of the body, that a dead fish floats with the
belly uppermost.
Animal substances, in undergoing the process of putrefaction, give out
much aeriform matter. Hence the bodies of persons drowned and remaining




152                    HYDROSTATICS.
in the water, generallyrswell, after a time, and rise to the surface, again to
sink when the still increasing quantity of air shall burst the containing parts.
A floating body sinks to the same depth whether tpe mass of fluid
supporting it be great or small:-as is seen when a porcelain basin is placed
first in a pond, and then in a second basin only so much larger than itself
that a spoonful or two of water suffices to fill up the interval between them.
One ounce of water in the latter way may float a thing weighing a pound
or more, exhibiting another instance of the hydrostatic paradox:-And if
the largest ship of war were received into a dock, or case, so exactly fitting
it that there were only half an inch of interval between it and the wall or
side of the containing space, it would float as completely, when the few
hogsheads of water required to fill this little interval up to its usual watermark were poured in, as if it were on the high sea. In some canal locks,
the boats just fit the place in which they have to rise and fall, and thus the
expense of water at the lock is diminished.
The preceding examples of floating are all illustrations also of the truth
that the pressure of a fluid on any immersed body is exactly proportioned
to the depth and extent of the surface pressed upon. The lateral pressures
just balanced one another, and the upward pressure has to be balanced by
the weight of the body.
Similar reasoning to that which proves that the whole weight of a body acts
as if lodged in the point called its centre of gravity, proves that the whole
buoyancy of a body, or the upward push of the fluid in which a body is
immersed, acts as if lodged in the point which was'the centre of gravity
of the fluid displaced. This point, consequently, is called the " centre of
buoyancy."
A floating body, to be stable in its position, either must have its centre
of gravity below the centre of buoyancy-in which case it resembles a
pendulum; or it must have a very broad bearing on the water, so that any
inclination may cause the centre of gravity to ascend-in which case it
resembles a cradle or rocking-horse.
Hence arises, in the stowing of a ship's cargo, the necessity of putting
the heavy merchandise underneath, and generally of putting iron ballast
under all the merchandise. Hence, also, the danger of having a cargo or
ballast which is liable to shift its place. A ship loaded entirely with stones,
is sometimes lost by a wave making her incline for a moment so much that
the load ships to one side, which is then kept down. For a similar reason,
a cargo of salt or sugar has a peculiar danger attached to it, for if the ship
leak, the cargo may be dissolved, and then pumped out with the bilge water,
leaving her with altered trim. In a fleet coming home from India, in 1809,
four fine ships disappeared during a hurricane off the isle of France. and from
what happened to the other ships that were saved, the cause of the destruction was supposed to be, that the saltpetre of the cargoes had been dissolved
and pumped out, and that the ships in consequence became unmanageable.
Bladders used by beginners in swimming are dangerous, unless secured
so as not to shift towards the lower part of the body.
A great inventor (in his own estimation) published to the world, that he
had solved the important problem of walking safely upon the water; and he
invited a crowd to witness his first essay. He stepped boldly upon the wave,
equipped in bulky cork boots, which he had previously tried in a butt of
water at home; but it soon appeared that he had not pondered sufficiently on




FLUID SUPPORT AMONG FLUIDS.                          153
the centres of gravity and of floatation, for in the next instant all that was
to be seen of him was a pair of legs sticking out of the water, the movements
of which showed that he was by no means at his ease. He was picked up
by help at hand, and, with his genius cooled and schooled by the event, was
conducted home.-Some soldiers once finding a few cork jackets, among old
military stores, determined to try them; but mistaking the shoulder straps
for lower fastenings, they put them on as drawers, and on then plunging in,
with the hope of being able to sit pleasantly on the water, their heavy heads
went down, and they were nearly drowned -
When, on the return of summer, the ice breaks up in the polar regions,
immense islands of it are set afloat, rising high into the air and sinking deep
into the sea. The melting process, in most cases, does not go on equally in
the water and in the air, and from the mass, consequently, changing form, its
stability is often lost, and one of the grandest phenomena in nature followsthe overturning of a mountain-the sudden subversion of an island-producing a tumult in the ocean around, felt often at the distance of many leagues.
The phenomena of pressure, floating, &c., in fluids, vary in proportion to
the weight or specific gravity of the fluid.
A ship draws less water, or swims lighter, by one thirty-fifth, in the heavy
salt-water of the sea than in the fresh water of a river: and for the same reason
a man swimming supports hmiself more easily in the sea than in a river.
Many kinds of wood that float in water will sink in oil.
A man floats on mercury as the lightest cork floats on water, and with
practice he might be able to walk upon mercury.
Had the water of our ocean been but a little heavier than it is, men after
shipwreck might have died of famine and cold, but would not have been
drowned
Oil floats on water, but sinks in alcohol or rether. The term proof spirit
means spirit light enough for oil to sink in it. The strength of spirit is
proportioned to its lightness.
Cream rises in milk, and forms a covering to it.
Blood, allowed to rest after flowing from the living body, separates, into
parts or layers, which arrange themselves according to their specific gravities.
The buffy coat of inflammation (where this exists) is uppermost, forming
the surface of the general coagulum: towards the lower part of the coagulum
there is an accumulation of red globules; and the whole of the solid part
floats in the serum, which is therefore lowest of all. When the red globules
escape from the coagulum, they fall to the bottom even of the serum.
Wine, if slowly and carefully poured on water, will float upon it. In a
vessel shaped like a common sand-glass, only with a larger
opening between the chambers at c, if wine be put into the Fig. 83.
under chamber, and water into the upper, the two liquids
will gradually change places: and if the lower half of the    a
glass be covered, so as to leave the upper half with the
appearance of a simple goblet, the water will seem to have
been changed into wine.  The liquids are less mixed, and
change places sooner, when there is a tube b to carry the
water down to the bottom without touching the wine, and a
tube a to carry the wine directly to the top.
Mercury, water, oil, air, and some other fluids may all be.    b
shaken together in the same vessel, and on standing will
separate again and arrange themselves in the order of their specific gravities.




154                     HYDROSTATICS.
When, in a mass of water, part of it is heated more than the rest, that
part, by its expansion, becomes specifically lighter than the rest, and rises
to the surface. Hence, when heat is applied to the bottom of a vessel
containing water, a circulation is established, which goes on from the first
moment until the operation of heating finishes:-water is always rising
from the hotter parts of the vessel, and descending over the colder parts.
In like manner, when a tall glass containing hot water is dipped into cold
water, a downward current takes place within the glass near the sides all
round, and there is an upward current in the middle. This motion may be
rendered very obvious by small portions of amber thrown into the water, for
these being nearly of the specific gravity of water, rise and descend with it.
On account of the current established- in such cases, heat applied to the
bottom of a vessel of liquid is soon equally diffused over it; but heat
applied at the top is there confined, Jbecause the heated and lighter fluid
does not descend. Water may be made to boil at its surface, while a piece
of ice lies at the bottom. The converse is impossible.
The current in a fluid, produced by local change of temperature, is an
important part of the following process, which the author deems applicable
to various useful purposes.-Heat may be transferred from one liquid to
another, without mixing them, by making the hot liquid descend in a very
thin metallic tube, through the cold liquid rising around it in a larger tube,
Boiling water from the vessel e, for instance, may descend slowly by the
small tube ea b f, which is surFig. 84.                 rounded from a to b by cold
water ascending through the:
tube cg.  Then, as the teme           perature of two liquids, brought
q'.so nearly into contact with each
other, will not, after a very short
cLi            c[<ltime, differ, in any one place
EIL                                   more than a few degrees, it folc1.       lows that the water lately cold,
will on leaving the part of the
tube g, which is in contact with
the boiling water descending directly from e, be nearly boiling,
while the water lately hot will,
I b       on leaving the tube at b, which
is in contact with cold  water.f         just arrived from h, be itself
nearly cold; and thus equal
quantities of hotfand cold water will have exchanged temperatures. The flux
of the hot water is to be regulated by a cock b, and that of the cold water by
a cock h. The water in the part of the tube c g d rises, because it is hotter and
therefore specifically lighter than that in the part h c.-The author believes
that an apparatus made on this principle, with an arrangement of many thin
flat tubes instead of a single large tube, for the descending fluid, and a spacious
box c g to contain these and the rising fluid, would be an excellent refrigerator in a distilling apparatus, and for cooling the wort of brewers; or would
serve as a means of diminishing the expense of warm baths, by transferring
the heat from the water lately used to pure water. In distilling, the wash or
low wines, about to enter the still, might be used as the cold condensing fluid
to surround the warm or vapor tubes, and thus, without expense, would be




FLUID SUPPORT AMONG FLUIDS.                          155:
heated in its progress to the still.  Half the original expense of a great
porter brewery is in the construction of the numerous water-tight floors on
which the hot wort is thinly spread to cool. The practice of warm bathing,
so conductive to health, is less common in this country, because the present
expense is so great.
It is a general truth in nature, that substances contract in size as they
cool. There is, however, in water, a curious exception to this rule, which,
operating through the principle of specific gravities, effects most important
purposes in the economy of nature. Water contracts only down' to the
temperature of 40 deg., below which, towards 32 deg., or the freezing point,
it goes on dilating again, and as ice is much lighter than as a fluid. Ice,
therefore, floats on the surface of water, and being a very slow conductor of
heat, defends the water underneath from the cold air, and preserves it liquid,
and a fit dwelling for the finny tribe, until the return of the mild season.
And not only is the extreme of cold below thus prevented, but because very
cold water remains floating on the surface of a wintry lake, as cream floats
on milk, it preserves underneath that warmth which is agreeable to the
fishes, just as very hot water in summer remains uppermost, preserving
underneath an agreeable c7olness. By the dilation of very cold water, then,
and the formation of ice, nature has prepared a winter garb for the inhabited
lakes and rivers, as complete and effectual as for terrestrial animals, by the
periodical thickening of their wool or fuxr Had ice become heavier than
water, so tlat it must have fallen to the bottom, and have left the surface
without protection, a deep lake in European winters, would have been frozen
into a solid lifeless mass, which summer suns would no more have melted
than they now do the glaciers of Switzerland. But for this important exception, therefore, to a general law of nature, many of the now most fertile
and lovely portions of the earth's surface would have remained for ever barren
and uninhabited wastes.




156                      PNEUMATICS.
PART III.
THE PHENOMENA OF FLUIDS.
SECTION II.-PNEUMATICS:
ANALYSIS OF THE SECTI(*.
In. aeriform fluids, that is, in such as have their particles held far apart
by mutual repulsion, which yields, however, to any force applied, so that
the mass sufers great change df volume under different degrees of compression.-the phenomena are modified by the GREAT LIGHTNESS and
ELASTICITY of the fluids, but are still in strict accordance with the general
properties of fluids already explained, viz., PRESSURE EQUAL IN ALL
DIRECTIONS-PRESSURE AS THE DEPTH-LEVEL SURFACE, and FLUID
SUPPORT.  The pressure of air, in all directions, and as the depth, may
be studied in the effects of our atmosphere-on solids-on liquids: —or
when it concurs with heat, in producing thephenomena of boiling, evaporation, clouds, rain, dew, &c.; or when, by varying in degree, it allows
certain substances to exist sometimes in the liquid and sometimes in the
aeriform states.  The fluid support in air is exemplified by ballons, the
ascent of flame, and smoke, winds, &c.
WHAT'a change has taken place in the degree of man's knowledge of nature,
since philosophers thought that air was one of four primary elements,
viz., airire, water, and earth, of which all things were composed, and each
of which was for ever distinct from the others.  We now know that air or
gas is merely an accidental state, in which any body may exist, according to
the quantity of heat pervading it: the body being solid when the absence of
heat allows its atoms to obey freely their mutual attraction, and to cohereas in ice, for instance; being liquid, when so much beat is present as nearly
to balance the attraction, and to let them slide freely among each other-as
they do in water; and being aeriform when still more heat is added, causing
the atoms mutually to repel and dart asunder to a great distance-as they do
in steam. But in any one of these three states, the various substances are
as much themselves as in the others, and at the command of the chemist
will assume any of the forms which he desires. As most substances in
nature have a different relation to heat, there are some which, at the medium
temperature of our earth are solid, some which are liquid, and some aeriform.
The solids, in general, are the heaviest under a given volume, and therefore
sink down and form the great mass or centre of the earth; the liquids follow
next in order, and float upon this solid centre, filling up its inequalities with




AERIFORM FLUIDS.                            157
a level surface, so as to constitute the ocean; while the airs are the lightest
of all, and as a second ocean, rest above the sea and above the highest mountain, to an elevation of about fifty miles. Among the substances whose
relation to heat causes them, when not restrained in certain combinations, to
assume the form of air at very low temperatures, there are two in particular,
viz., oxygen and nitrogen, which are very abundant in nature in such uncombined state, and of these, therefore, the atmosphere chiefly consists; but
smaller portions of almost every other substance are found in it. Water,
among the supplementary matters, is much more abundant than any of the
others, and in various states of cloud, mist, rain, dew and snow, it answers
a thousand: useful purposes, and serves beautifully to vary the scenes of
nature. The atmosphere is about fifty miles high or deep, and therefore,
in re ltion to the bulk of the earth, is as a covering of one-tenth of an inch
in th-kness to a common library globe of a foot in diameter.
The atmospheric ocean is the great laboratory in which most of the actions
of life go on, and on the composition of which they depend. A human
being requires for breathing a gallon of fresh air every minute, dying equally
if deprived of air, or if confined to the same. All other animals also require
fresh air, but in various proportions.  And in the vegetable creation, the
beautiful green leaf and delicate flower are merely broad and tender expansions of surface for the contact of the vivifying air. Animals give out to
the atmosphere a substance which vegetables absorb, and vegetables, by the
absorption, fit the air again for the use of animals; so that, upon the whole,
in the various changes of nature, there is a perfect balancing of actions,
which preserves the atmospheric mass in a uniform state, constantly fit for
its admirable purposes.
While the ancients had that notion of air, which made them apply to it
vaguely, and almost indifferently, the names of air, ether, spirit, breath, IUfe,
&c., they never dreamed of making experiments upon it, with a view to prove
its relation to common matter: —and one of the most beautiful portions of
the modern history of man's progress in knowledge, is that which exhibits
the light gradually breaking in upon this most interesting subject. Galileo
was led to conclude that air made a definite pressure upon things at the
surface of the earth; Torricelli and Pascal proved that this was occasioned
by its weight, and hence, moreover, they deduced the height of the aerial
ocean; Priestly, Black, Lavoisier, and others, discovered that air might be
united with a metal, so as to increase its weight, and to produce a compound
of totally new qualities, for they showed that many of the ores of our mines
are merely metals concealed, by being thus united with a substance which,
when set free, ascends as one of the ingredients of the atmosphere. They at
last analyzed the atmosphere itself, and exhibited its two ingredients as
distinct substances. And within a few years the nature of air or gas has
been so thoroughly investigated, that we can now take a little of many a
light, invisible, impalpable fluid such as we breathe, and squeezing the heat
out of it by strong pressure, can make its particles collapse from their
aeriform distances to assume the state of a tranquil fluid; which may
then be retained as such for ever, or may be decomposed and made solid in
combination with other bodies, or may be again set at liberty.
The suspicion once excited, that air was as much a material fluid as water,
only much less dense, by reason of a greater separation and repulsion of the
particles, it was easy to follow out the parallel, and to confirm the supposition
by reference to the commonest facts. Thus, a leathern sack or pouch, opened
and dipped into water so far as to become full, if its mouth be then carefully




158                       PNEUMATICS.
closed, retains the water, and its sides cannot afterwards be pressed together:
a similar sack or bladder, opened out, and then closed in air, is found to
remain, in a corresponding way, bulky and resisting, and forms what is called
an air-pillow. The motion of a flat board is resisted in water: the motion
of a fan is resisted in the air. Masses of wood, sand, and pebbles, are rolled
along or floated by currents of water: chaff, feathers, and even rooted trees
are swept away by currents of air.  There are mills driven by water; and
there are mills driven by the wind. Oil set free under the surface of water,
or placed there in a bladder, rises to the -surface: hot air or hydrogen gas
placed in a balloon, rises in the air. A fish moves itself by its fins in water:
a bird moves itself by its wings in the air; and as on taking the water from
a vessel in which a fish swims, the creature falls to the bottom, gasps a few
moments, and dies, so, on exhausting the air from a vessel in which birds
or butterflies are enclosed, their useless wings may flap; but they sink to
the bottom, and if the cruel experiment be continued, they soon become
motionless and forever.
We proceed now to prove that air or gas, as a fluid, differs from the
other fluids, which we call liquids, only in the two circumstances of great
lightness or rarity, and of being very extensively elastic, that is to say, the
particles being so related, that pressure brings'them much more nearly into
contact, and on ceasing, allows them to regain their former distance.
Lightness of Air.
The lightness or rarity of atmospheric air, as it is found on the general
surface of the earth, is such, that if, by the action of a pump, a bag of it
holding a cubic foot be emptied into the copper ball of an air-gun, the ball
weighs about an ounce and a quarter more than before.  The same volume
of water weighs nearly a thousand ounces; so that common air is about eight
hundred times lighter than water. Other gases, or substances in the aeriform state, have their various specific gravities, just as the same substances
have when liquid or solid. T'hus water in the form of air, that is to say,
when existing as steam, and of the common density, is little more than half
as heavy as the same bulk of common air; hydrogen is only one-fourteenth
part as heavy: and carbonic acid gas, which is the air that rises out of sodawater, brisk ale, champagne wine, &c., is so much heavier, that even in the
atmosphere it may be poured out of one open vessel into another, as a liquid
might, or, more exactly, as water might be poured out under oil.
Elasticity of Air.
A small bladder full of air may be pressed or squeezed between the hands
so as to be much reduced in size, but on being relieved from the
Fig. 85. pressure, it will immediately resume its former bulk.
~cl       If a metalic tube or barrel of perfectly uniform  bore a b, be
fitted with a moveable plug or piston c, which is covered with
leather and oiled, so as to slide up and down without allowing
the air to pass by its sides, the air between the piston and the
close bottom  b may be compressed to a hundredth or less of its
usual bulk; but when allowed, will push the piston back again
with the same force as it opposed to the condensation, and will
C   recover the volume which it had before the experiment.
Again, if the plug, at the commencement of the experiment,
were only an inch from the bottom, enclosing air of the usual
b   density, on drawing it up to the top, the inch of air beneath it




ELASTICITY OF AIR.                           159
would expand so as to occupy the whole tube, having become, of course,
proportionally less dense.
To the question why the air, which admits of such various density, is
found to have that certain degree of it met with at the surface of the earth,
we answer, that as the water, in any place near the bottom of the ocean, is
pressed with force exactly proportioned to the quantity of water above it, so
the air at the surface of the earth bears the pressure of the superincumbent
mass of air, and on account of its extensive elasticity, suffers, like the lowermost bags of cotton or.wool in a great heap, that degree of compression
which the superincumbent mass is calculated to produce. We shall see
below that the density of the air near the earth is changing with every
circumstance which affects the weight of the atmosphere above, as winds,
cloudS, rain, &c., and that it bears relation to the altitude of the place of
observation above the level of the sea.
The tube with its piston, described in the last page, becomes, according to
the position of its valves, either a syringe for injecting and condensing air,
or a pump for exhausting or removing it from any vessel; both operations depending on the elasticity of air.
A barrel and piston is a condensing syringe, when in a passage of communication between the bottom of the syringe and a receiving vessel, there
is a flap or valve allowing air to pass towards the receiver but not to return.
The piston, therefore, at each stroke, forces what the barrel contains of air
into the. receiver. When the piston is lifted again after the stroke, air
re-enters the barrel from the atmosphere, either through a valve in the
piston, or through a small hole near the top of the barrel. That useful
contrivance, a valve, for whatever purpose used, and in whatever way formed,
is in principle merely a moveable flap, placed on an opening, against which
it is held by its weight, or by some other gentle and yielding force. Such
a flap, it is evident, will allow fluid to pass only in one direction, viz., outwards from the opening, for any fluid tending inwards must shut the flap,
and press it the closer, the greater the tendency.
To convert a forcing syringe or pump into an exhausting syringe or pump,
commonly called an air-pump, it is only necessary to reverse the position of
the'valves; then, on the descent of the piston, all the air between it and the
bottom, instead of entering the vessel or receiver, as in the last case, escapes
by a valve in the piston itself towards
Fig. 86.              the atmosphere, and on the rising of
r c7                        the piston, a perfect vacuum would be
q   as e C   5           left under it, but that the valve below,
then opened by the elasticity of the air
in.the receiver, allows a part of that
air to follow it.  Thus, at each stroke,
A    _ It   a quantity of the air, proportioned to
the size of the pump, is removed from
ac   b                      the receiver.  In a good air-pump,:.....:-:..: ~ B there are two similar pumping barrels,
as a and b, to quicken the operation
of exhausting; and both are worked
at the same time by the reciprocating
winch or handle f, with its pinion e,
acting on the teeth of the piston rods d




160                       P NEUMAT-ICS. 
and c. This double construction has the farther advantage, that the atmospheric pressure, if fifteen pounds per square inch on the upper surface of
either piston, and which for a single piston would have to be overcome by
the worker in lifting it, as here balanced always by the corresponding
pressure on the other piston. Both pumps communicate with the tube g h,
which at h rises tightly through the round plate of the machine to i. This
flat plate is so smooth, that a glass bell or receiver k, with a smooth
ground lip, when placed upon it, forms an air-tight joining. On working
the pump, such a bell is exhausted of its air, aqd fitted for showing the
many interesting phenomena which the air-pump can display,-and which
will pass under review as we proceed. The supporting frame-work of the
pump is not shown here.
The law of the elasticity of air is, that its spring, or resistance to compression, increases exactly with its density or the quantity of it collected in a
given space. Hence, by finding in any case either the density of the air,
or the spring, or the compressing force, we know all the three.
It has been ascertained by experiments described a few pages farther on,
that in the atmospheric ocean surrounding the earth, there are nearly fifteen
pounds of air above every square inch of the surface of the earth; and that
the air nearest the earth, and bearing this superincumbent weight or pressure,
has the density of an ounce and a quarter of weight to a cubic foot of volume.
We further find that such air is reduced to half its bulk, or becomes of what
is called double atmospheric density, by an additional pressure of fifteen
pounds on the inch, and of triple density, by tripple pressure, and so forth;
and on the other hand, that it dilates to double bulk, if the pressure be
diminished to half, and to any greater bulk, even beyond a thousand-fold, if
the pressure be diminished in a corresponding degree; and any air bearing
a given force or pressure, is always acting as a spring of that force on
whatever it touches.
It is very important to be familiar with this truth or law, for it holds very
nearly with respect to all aeriform fluids as well as common air, and throws
light, therefore, on the action of steam-engines, air-guns, pneumatic machines
generally. It also explains the condition of our atmosphere as to density at
various elevations; telling us, for instance, that when a balloon has risen
through half of the atmospherical mass, the air around it will be of only
half the density which exists at the surface of the earth.
We know not exactly to what extent the rarefaction of air may go on the
removal of pressure; in other words, at what distance the gravity of the
particles becomes just a balance to their mutual repulsion; and therefore we
know not exactly what the degree of rarity is at the top of our atmosphere;
but we know that it must be exceedingly great, from the fact that the air
left in the receiver of an air-pump has still spring or elasticity enough to lift
the valve of the pump, when less remains than the thousandth part of the
original quantity. In the most perfect air-pumps, that the exhaustion may
be as complete as possible, the machine itself is made to raise the valve.
The expansion of air is well illustrated by a bladder, having a very little
air in it, placed under the receiver of an air-pump. On exhausting the
receiver, the bladder gradually swells, with force sufficient to lift a moderate
weight laid upon it, and at last appears quite full, and may even be burst.
A shriveled apple treated in the same way becomes plump. The explanation of such phenomena is, that at first the air in the bladder or apple is in a




ELASTICITY  OF AIR.                          161
state of condensation, like all air, at the surface of the earth under the pres -
sure of the superincumbent atmosphere; but that its volume increases as that
pressure is diminished by the air-pusp: —it is rarefied in the same proportion as the air which remains in the receiver surrounding it.
The curious instrument called She air-gun has a strong globular vessel of
copper attached under the lock, into which air is usually forced to be thirty
or forty times as dense as the air in the atmosphere around: hence the
pressure or elasticity tending outwards is thirty or forty times fifteen pounds
on the inch, and when the valve is opened for an instant by the action of
the lock, a portion of the air issues and propels the charge with this force.
The effect of air thus condensed nearly equals that of gunpowder, and one
charge of the ball suffices for many shots, the force, however, becoming less
for every successive discharge.
If a bottle or vessel a b, partly filled with water, have a
tube c d passed tightly through the cork to near the bottom  Fig. 87.
of the water; and if more air be then forced through this
tube in any way, so as to accumulate in the upper part of the     I
vessel above the water surface a b; on turning the cock c,,
which opens the tube, the elasticity of the condensed air!: li i
will press the water out as a beautiful jet, to a height proportioned to the condensation, and gradually diminishing as
the condensation diminishes.  Or if such a vessel, with air
of common density, be placed under a tall air-pump receiver,  r
on working the pump so as to diminish the density of the
air in the receiver, the jet of water will equally rise.-A
table-lamp, by the force of condensed air, may be supplied
with oil from a reservoir far below the wick: and lately an
enema syringe and a shower-bath have been constructed on
the same principle.
The elasticity of air is rendered very serviceable in con-,'
nection with great water-pumps, such as those used for the
supply of cities.  A pump throws its water by a distinct,     i
gush at each stroke, while the current through the pipe
towards the city should be uniform.  Now uniformity is       (
attained by causing the gushes from the pump a to enter by
the passage b at one side of a large vessel c, of which the
upper part is full of the condensed air, and from the other side of which at d the
water issues on its way. The air in this vessel
(called the air-vessel) is condensed, as a spring, by   Fig. 88.
the entering water, and its resisting elasticity, both
immediately, and afterwards during the interval of
the strokes, forces the water along the pipe d. Each
entering gush has only the effect of compressing
the air a little more for the time, while the flow in
the great pipe continues nearly uniform.  The a
pump itself is made to take in a little air at each     b
stroke, so that not only is the vessel always supplied, but some air is constantly passing on with
the water, and effecting the highly useful purpose
of giving an elasticity to the whole contents of the
pipe and its ramifications.
The same object is attained by the same means in the fire-engine used to
check conflagration. In it there are generally several water-pumps working
11




162                      PNEUMATICS.
tdgether, which throw their interrupted supply into an air-vessel whence it
passes in a nearly uniform jet to the point desired.
The compressibility and corresponding spring of air are remarkably exhibited in that singular contrivance of modern times, the diving-bell, in which
men now descend with safety to considerable depths in the ocean, there to
reside and labor, attaining many objects ot high importance to them:-they
recover sunken treasures,-they are enabled to pursue works of submarine
architecture, as in constructing light-houses and noble harbours, where
formerly no foundations could have been laid, &c. The diving-bell, in
point of utility, has proved a remarkable contrast to its sister invention, the
balloon, which, although so wondrously bearing man aloft to the regions of
the clouds, has brought him as yet little advantage to compensate for the
many fatal accidents which its use has occasioned.
The diving-bell is a large heavy open-mouthed vessel, with accommodation in it for one or more persons. It is let down into the water with its
mouth undermost, from a crane to which it is suspended, and which rests on
a suitable carriage either on the shore, or on the deck of a ship, or barge
fitted for its service. On first entering the water it appears full of air; but
air being compressible, according to the law now explained, and the pressure
of the water around the descending bell increasing with the depth, the volume
of the air gradually diminishes, and at thirty-four feet is reduced to half.
The bell, then, unless more air be supplied, will of course be half full of
water, and a person breathing in it, at each inspiration will receive twice as
much air into the lungs as when breathing at the surface. A constant supply
of fresh air is sent down to the bell by a forcing-pump above: and the heated
and contaminated air, which has served for respiration, and which rises to
the top of the bell, is allowed to escape by a cock placed there for the purpose. The men who work at a distance from the bell have tubes of communication with it, by which they inhale the air required; and they allow
the used air to rise through the water above them. A man cannot breathe
comfortably by such a tube if he be either much above or much below the
level of the water in the bell; for if above, the air in the bell is more compressed than his chest, and is forced towards him so as to require an effort
to control its admission; and if below, his chest is bearing greater pressure than the air in the bell, and he must therefore act strongly with the
muscles of the ribs to draw the air down to him. A phenomenon similar
to this takes place when two bladders of air are connected by a long tube,
and immersed in water to unequal depths. the air being always strongly
forced from the lower into the upper one, because the lower one is more
pressed. The difficulty of pumping air down to the diving-bell increases,
of course, with the depth to which it has ascended: for if the bell be so low
that the water is pressing on the air in it with a force of fifteen pounds per
inch, (which would happen at thirty-four feet, ) it is evident that a syringe
or pump can not inject more air unless it act with a force greater than this.
Divers might often,'if not always, more conveniently receive their supply
of air through tubes from an air vessel kept charged to the necessary density
in a boat over them, or on the shore, than from a bell below.  If they
would, moreover, dress in India rubber cloth, and use a hood of metal with
windows for the head, they might work under water without wetting any
part but their hands.
It is remarkable, when the use of the diving-bell has become so familiar,
that a kindred and still more simple contrivance of the same class has not
been introduced for certain purposes, particularly of sudden emergency, such




ELASTICITY  OF AIR.                          163
as to aid in the recovery of the bodies of drowning persons. A ten-gallon
cask, or vessel of any kind, filled with air, and made heavy enough just to
sink in water, with a breathing tube from it like that of a diving-bell, would
be a provision of.air for a man below water for ten minutes; and a man with
it under his arm, might instantly descend from a boat or walk from the shore,
into water of any depth, to recover the body of a fellow-creature lately sunk,
and in time probably to save the life, which a few minutes wasted in waiting
or in unsuccessful dragging would suffer to be lost. The author would propose this as an addition to the apparatus of the Humane Society for the
recovery of persons apparently drowned.-It shows the remoteness from
common trains of thinking of the truths connected with the constitution of
our atmosphere and sea, when a means so simple and easily procured should
never have been thought of or tried in any way by pearl-fishers, or by persons who gain their bread by diving to recover things dropped overboard in
harbours or anchoring stations; all of whom have hitherto been limited to
the single gulp of air taken on descending. In the case of a man working
under water, cask after cask of air might be sent down, to enable him to
remain as long as necessary.
There is an exceedingly beautiful philosophical toy, of which the action
depends chiefly on the elasticity of air; and as it moreover
illustrates most of the laws of fluidity, it is deemed worthy of    Fig. 89.
description here. It is a small balloon or thin globe of glass
c, having an opening at the bottom, and its little car or basket  a.
hanging to it. If put to float in water while the globe contains air only, it is so light that half the globe remains above:.
the surface; but water may be introduced to adjust the specific
gravity of the whole, until it becomes only a little less than 
that of water.  If the balloon be then placed in a tall jar of
water a b, the mouth of which is closely covered by bladderskin or India-rubber tied upon it, on pressing such covering
with the hand, the balloon will immediately descend in the
water, to rise again when the pressure ceases, and will float
about, rising, or falling, or standing still, according to the
pressure made.  The reason of this is, that pressure on the    -b
top of the jar first condenses the air between the cover and the
water surface; this condensation then presses upon the water below, and by
influencing it through its whole extent, compresses also the air in the balloon
globe, forcing as much more water into this as to render the balloon heavier
than water, and therefore heavy enough to sink. As soon as the pressure
ceases, the elasticity of the air in the balloon repels the lately entered water,
and the machine, becoming as before, lighter than water, ascends to the top.
If the balloon be adjusted to have a specific gravity too nearly that of water,
it will not rise of itself after once reaching the bottom, because the pressure
of the water then above it will perpetuate the condensation of the air which
caused it to descend. It may even then, however, be made to rise again by
inclining the water-jar to one side, so that the perpendicular height of water
over it shall be diminished.
This toy proves many things-the materiality of air, by the pressure of
the hand on the top being communicated to the water below through the air
in the upper part of the jar-the compressibility of air, by what happens in
the globe just before it descends-the elastic force of air shown in expansion,
when, on the pressure ceasing, the water is again expelled from the globethe lightness of air, in the buoyancy of the globe;: —it shows, also, that in a




164                      PNEUMATICS.
fluid the pressure is in all directions, because the effects happen in whatever
position the jar be held-it shows that pressure is as the depth, because less
pressure of the hand is required the farther that the globe has descended in
the water-and it exemplifies many circumstances of fluid support.  A
young person, therefore, familiar with this toy, has learned the leading
truths of hydrostatics and pneumatics, and has had much amusement as
well as instruction.
On the same principle as the balloon now described,
Fig. 90.      three or four little figures of men may be formed of
i, glass, hollow within, and having each a minute openI   ing at the heel, by which water may pass in or out.
If these be placed in a jar as the balloon was, and be
adjusted by the quantity of water admitted into them,
so that in specific gravity, they shall differ a little from
each other, and if then, a gradually increased pressure
C    be made on the cover of the jar, the heaviest figure...l..    --— ld    will descend first, and the others will follow in succession; and they will stop or return to the surface
in reverse order when the pressure ceases. A person
while exhibiting these figures to spectators who do not
understand them, may appear only carelessly to rest
his hand on the cover of the jar while he is making the
required pressure, and he will seem to have the power
of ordering their movements by his will. If the jar
containing the figures be inverted, and the cover be
1, cI       placed over a hole in the table, through which, unoblLi;  11     served, the exhibitor can act by a rod rising through
Fig. 91.      the hole and obeying his foot, he may produce the most
amusing and surprising evolutions among the little
men, in perfect obedience to his word of command.
The beautiful fountain, called the fountain of Hiero,
by which water is made to spout far above its source,
depends for its action upon the resisting elasticity
of compressed air. The vessel d is first filled with
water, while b and a contain air only.  On then
pouring water into a, the water of d darts upwards
through the jet-pipe e, to an elevation nearly equal to
the length of the tube from a to b. The reason is,
that the water from a descends by the tube to b, and
compresses the air at c; which compression conveyed
along the other tube from c to b, acts on the water in
the vessel d, and causes it to jet.  As the pressure
is produced by the column of water a b, the jet is
proportioned to the length of that column.-This
kind of fountain may have its parts concealed under
a variety of forma as here exemplified, and may thus
become a beautiful ornament among flowers in a summer drawing-room. It may be made of size to play
for an hour or more, and it will always recommence
on the water being shifted from the low to the high
reservoir.-The useful table-lamp, consisting of a
simple column or pillar with the oil rising to the
flame from far below, is a Hiero's fountain, only the




PRESSURE AS THE DEPTH.                           165
oil, instead of being allowed to jet out, rises in a tube to the flame. The
contrivance for maintaining the two columns always of the same. length, not.
withstanding the expenditure of oil has to be explained some pages hence.
Having now explained the two peculiarities which distinguish aeriform
froum other fluids, viz., their lightness and extensive elasticity, we proceed to
show that they have the four other properties already described under
hydrostatics, as belonging to fluids generally: and first,
" Pressure in all directions." (Read the Analysip, at pages 140 and 172.)
A quantity of air or gas shut up in any vessel and compressed, is equally
affected throughout, and its tendency to escape from the pressure is equal in
all directions, as is proved by the force necessary to keep similar valves close
wherever placed. Hence the hydrostatic press and hydrostatic bellows
described in the last section, which depend for their action on this law, may
be worked by air or gas as by a liquid.
Owing to this law, air, when allowed, will always rush from where there
is more pressure to where there is less. The actions of the common firebellows, and of the animal chest in breathing, blowing, sucking, &c., are so
many instances.
The suddenness with which any compression made on part of a confined
aeriform fluid is communicated through the whole, is strikingly seen in the
simultaneous increase or burst of all the gas-lights over an extensive building or even in a long street, at any instant when the force supplying the gas
is augmented.
Many very interesting illustrations of the fluid pressure of air being in
all directions, will occur under the next head, joined with proofs of the
atmospheric pressure being as the depth.
"Pressure as the depth."
On first approaching this subject, a person is naturally surprised to hear
the depth or height of the atmosphere spoken of as something perfectly
ascertained, although nobody can ever have approached the surface to measure it; but science often furnishes means of reaching precise truth, in
cases where ignorance would not even dream of the possibility of making
an approximation. It may facilitate the apprehension of this point as regards
air, to describe first some parallel cases in which water is concerned.
The bottom of a lake evidently supports all the water in the lake, and
each portion bears just the weight of the water directly over it: a means
then of ascertaining the weight or pressure of water on any portion of the
bottom would tell how much water stood over that portion, and by the
known relation of the weight and bulk of water would tell also the depth
at that part.  In like manner the ocean of air which surrounds the globe
rests with its whole weight upon the surface of the globe, and each portion
of the surface bears its share: if we ascertain, then, the pressure of the
atmosphere on a given extent of the surface, we find how much air is standing directly over it; in other words, the weight of a column of air resting
on such surface as its base, and reaching to the top of the atmosphere.
Having then the weight of the whole column, and finding the weight of a
given bulk of it at the botton (ascertained as described at page 158,) and
knowing the law of aerial elasticity (explained at page 158, ) we determine
the depth or height of the column by a simple calculation. Now accurate




166                      PNEUMATICS.
experiments show that there are nearly fifteen pounds of air over'every
square inch of the earth's surface; producing the same pressure as would
be made by a depth of water of thirty-four feet, or by a depth of quicksilver
of thirty inches; and from this fact and the ascertained lightness and elasticity of air, we know that its depth on earth must be nearly fifty miles,
which, as already stated, is about as much in relation to the size of the earth
as the tenth of an inch is to a globe of one foot in diameter. The remaining
part of this section has chiefly to trace the effects of this mass of matter
resting upon the earth's surface, and as a fluid embracing and compressing
every object placed there.
Water is a substance much more obvious to the human senses than air,
and which is constantly under observation; yet many of its most important
agencies escape the notice of common observers. Few persons, for instance,
of themselves discover the law explained in the last section, of the pressure
in water being proportioned to the depth: but when made to observe that a
piece of cork plunged deep into it is compressed to much smaller bulk, and
that strong empty vessel of glass, or even of metal under the same circumstances, are crushed or broken inwards, and that pieces of sunken wood are,
at great depths, filled with water through all their pores, so as to become
nearly as heavy as stone, &c., their minds are roused to a sense of the important fact that a fluid presses, and in proportion to its depth: If the truths
of hydrostatics thus escape notice, we need not wonder that those of pneumatics escape still longer.
If a piece of bladder-skin or a pane of glass be laid at the bottom of a
vessel, holding water, the bladder or glass exhibits no sign of being pressed
upon, although it bears on its upper side the whole weight of the water
directly above it: the reason being that water beneath the bladder resists
just as strongly as the water above presses, in the same way that one stone
in a pillar resists those above it: but if the bladder be tied closely over the
mouth of a common drinking glass or tumbler filled with water, and placed
at the bottom of the vessel, and if then, by means of a syringe or pump,
the water be extracted from within the glass, the bladder itself has to bear
the whole pressure of the water above it, (independently of a pressure of
air, to be explained afterwards, ) and will probably be torn or burst. The
degree of pressure, and consequently the depth of the water, in such a case,
might be ascertained by placing some support, of which the action could
be measured, under the bladder to sustain it after the removal of the
interior water.-Now this case may be closely copied in our atmosphere or
sea of air. A glass held in the hand is immersed in the fluid air, and is full
of it as the other glass was supposed full of water: its mouth may be covered
over with bladder, and no external pressure will be apparent, because there
is a resistance of the air within, just equal to the pressure of the air on the
outside: —but if the air be extracted from under the covering by means of
an air pump, the bladder is first seen sinking down and becoming hollow
from the weight of the air overdit, and at last bursting inwards with a great
noise or crack. By placing a circular piece of wood under the bladder-skin,
for it to rest on, and a spring of known force to support the wood, we may
ascertain very nearly the weight and pressure of the air over it. This
mode, however, of ascertaining the weight of the atmosphere, is not that
commonly used, but is described here as a good illustration of the present
subject; the problem being solved much more elegantly and accurately by
means of the barometer described farther on. The phenomenon of atmospheric pressure is often exhibited by placing the hand on the mouth of a




ATMOSPHERIC PRESSURE ON SOLIDS.   167
glass so as to cover it closely. and then extracting the air from underneath
the hand: the weight of the atmosphere holds the hand down on the mouth
of the glass with the forc of fifteen pounds to the inch.
As should follow, from the pressure of fifteen pounds per inch thus detected
at the surface of the earth, being the weight of our superincubent atmosphere, we find that exactly as we rise from the earth, and leave part of the
atmosphere beneath us, the pressure diminishes. This fact now furnishes
the readiest means of ascertaining the'heights of mountains and of balloon
ascents, as will be explained in considering the barometer.
After the many explanations here given of fluid pressure being equal in
all directions, it is almost superfluous to remark, that the downward weight
of the atmosphere becomes a pressure in all directions. This is seen in the
fact of the bladder seen above, being as readily burst if turned sideways
as if turned directly upwards. Every body or substance, therefore, on the
surface of. the earth, dead or living, solid or fluid, is compressed with this
force. In general the pressure on one side of a body. is just balanced by the
equal pressure on the other, so that no sensible effect follows; and it is on
this account that philosophers were so long in discovering it at all, and that
half-informed persons are still disposed to doubt its existence; but the proofs
offered on all sides to the now awakened attention are irresistible. We shall
speak first of
C' Atmospheric pressure on solids."
The atmosphere, then, presses on the two sides of a plate of glass or metal,
with force of fifteen pounds on the inch. Under ordinary circumstances, no
sensible effect follows, because the opposite pressures counterbalance; but if
two plates of smooth glass or metal be laid against each other, and the air
be prevented from entering between them, they cannot be separated by less
force than fifteen pounds per inch of their surface.
In like manner, to draw down the piston of a syringe from the bottom of
its barrel, while no air is allowed to enter between them, requires force of
fifteen pounds to the square inch of surface of the piston. But if the
experiment be made in the exhausted receiver of an air-pump, the piston
falls by its own weight. It is pushed back immediately on re-admitting
the air.  Wherever a vacuqm is produced at the surface of the earth, there
is an external pressure, of the force statesd, seeking admittance all round.
An air-pump receiver of five inches diameter has nearly twenty square
inches of surface in its upper part or roof, and bears a weight or pressure of
atmosphere, of twenty times fifteen, or three hundred pounds. While it has
air within it, this pressure is exactly balanced, and is not sensible; but when
exhausted on the plate of the air-pump, it is pressed against the plate with
this force. As the atmospheric pressure is in all directions, the pump-plate,
of course, is equally pressed upwards again.st the receiver, and the sides of
the receiver are pressed towards each other. This explains why air-pump
receivers must be made arched or of dome-shape to withstand the great
pressure. A flat piece of glass of great thickness, laid upon the upper
mouth of a receiver, so as to form an air-tight cover to it, is broken
instantly by exhausting the air beneath; and a bottle or receiver with flat
side, when exhausted suffers in the same manner.
Illustrative of this pressure on solids there is the experiment of the Magdeburgh hemispheres, as it is called. Two hollow half globes of metal a and




168                      PNEUMATICS.
b, are fitted to each other, so that their lips when touching
Fig. 92.  may be air-tight. While there is air between them or within,
resisting the pressure of the outward air, they can be separated
ajrcc)2    from  each other without difficulty; but when the air is
exhausted from within by the air-pump, a force is required to
separate them of as many times fifteen pounds as there are
square inches in the area of the mouth.  The air is extracted
by unscrewing one of the handles at b, and then connecting
the remaining stalk (which is hollow and has a stop-cock)
with the air-pump. —This experiment merits recollection,
because it was one of the first which drew attention to the
material nature and properties of the air; and it astonished
the world. Otto Guericke, Burgomaster of Magdeburgh, the inventor, had
hemispheres made of three feet in diameter, and once when he exhausted
them, on the occasion of a public exhibition, twenty coach-horses of the
emperor were unable to pull them asunder. There being no air-pump when
Guericke began his experiments, although he himself invented it afterwards,
he originally emptied the balls of their air by first filling them with water,
and then extracting the water by a common pump or syringe applied to the
bottom.
It is a phenomenon of the same kind as the last described, when a boy
with his foot presses a circular piece of wet leather as a, against a fiat-faced
stone as b, and then lifts the stone by pulling at a cord c, rising from the
centre of the leather.  If the leather be so close in its texture that air cannot
pass through it, and stiff enough not to be puckered or drawn together, he
must exert a force before detaching it, of as many times fifteen pounds as
there are square inches of surface covered by it for
Fig. 93.    such is the weight or pressure of the air over it, while
there is no counterbalancing pressure underneath nearer
than on the other side of the stone. The weight of the
stone that may be lifted is thus determined by the
size of the leather.  The contrivance has been called a
sucker, or pneumatic tractor.  A  very large sucker
applied, upon a rock or wall, would resist the pull of
horses like the Magdeburgh hemispheres.
This contrivance seems suited to some purposes of
surgery.  It might assist, for instance, in raising de.
pressed portions of a fractured skull, and might thus
sometimes save the operation of trepanning:-for such
a purpose it would be preferable to the small cuppingglass sometimes used, from its being perfectly inactive,
except during the instants when pulled at; whereas the
cupping-glass, by keeping up a continual flow of blood
to the part, might do injury. There is another surgical application spoken
of in the last section of the pregntpart, which the professional reader may
consult immediately.
It is from having feet that act on the principle of the tractor, that the
common fly and other insects can move along ceilings, and even polished
surfaces of glass or metal with their bodies hanging downwards; and there
are many marine animals which attach themselves to rocks, or other objects
by a similar action.
If two pneumatic tractors be applied to each other, men pulling opposite
ways, to separate them, must act with a force of fifteen pounds to the square




ATMOSPHERIC PRESSURE ON LIQUIDS.  169
inch of the surface of contact, as if they were separating the Madgeburgh
hemisphere.
The case of the pneumatic tractor may be well illustrated by an experiment made in a vessel containing a liquid. If a body with a flat surface be
applied to the bottom of the vessel so as perfectly to exclude the liquid, the
body bears the whole weight of liquid directly over it, and cannot be detached
without force equal to this. The case is striking when a flat piece of cork is
pushed against the smooth bottom or side of a vessel containing mercury,
and is found not to rise again when the hand is withdrawn from it, but to
be firmly held down by the weight of the mercury.  We have to remark
that in such experiments made in vessels open to the air, the weight of the
atmosphere on the liquids adds a pressure of fifteen pounds on every inch of
the surface of a body immersed in it.
" Atmospheric pressure on liquids."
The pressure of the atmosphere on liquids produces many important
effects, and now that we comprehend them, we wonder that they should have
been so long misunderstood. We have familiar examples of it in the working of pumps and syphons.  All such phenomena, in former times, were
referred to what was called nature's horror of a vacuum, or to an obscurely
imagined principle of suction.  It was. not until the time of Galileo that
their true nature began to be detected. The discovery has led to many very
important results in the arts.
Persons may at first have a difficulty in conceiving that a fluid so rare and
subtle as air should affect or resist a dense liquid like water: but the action
or resistance of air in contact with water, is familiarly shown in the facts
that a glass does not become full of water when plunged, with its open mouth
downwards, from the air into water; and that when a tube, open at both
ends, has been partially immersed in water, and, therefore, partially filled,
the water can be forced out of it by blowing air in at the upper end, to
return only when the blowing ceases.  Then it may be recollected that a
hundred pounds of feathers are as great a load as a hundred pounds ot lead.
That there are fifteen pounds of air above every square inch of the earth's
surface, is confirmed by the effects above described of the atmospheric pressure on solids; and we now proceed to show that many of the phenomena
among liquids, which long appeared so mysterious, are merely the necessary
consequences of the same -pressure upon them.  It will facilitate the comprehension of these effects, if we first view them as they may be produced
by more visible agents, viz., by one liquid pressing upon another; and for
this purpose the author has contrived the apparatus represented in the next
page, in which a layer of oil rests upon a layer of water, or upon a layer of
mercury.
It has already been shown, that an ocean of oil, spread over the earth, to
have the same weight as our atmosphere, requires to be about thirty-seven
feet deep   A vessel, then, a b c, with water in it up to the level W, and
with thirty-seven feet of oil above this, up to the level 0, is fitted to illustrate many of the phenomena of atmospheric pressure on liquids.  The
following are the seven principal cases.
1st. The weight of the oil pressing with a force of fifteen lbs. per inch on
the water at W, would not at all disturb the level surface of the water.
Neither does the weight of the atmosphere of fifteen lbs. per inch disturb
any liquid surface.




170                      PNEUNATICS.
2. If the oil were gradually poured into
Fig. 94.          the vessel a b c, over the water, the water
would rise in the tube i w, as already excL            C     plained by the figure at page 143; so that
when there were thirty-seven feet in height,
or fifteen pounds in weight of oil on the
o   inch, the water i w would stand thirty-four
717 I El _X_ feet above its level in the large vessel.  If
these thirty-four feet of water were then
lifted out of the tube by a plug or piston
1_   1 _ _   drawn up from the bottom of it at i, a second
1  -       ~ _._equal quantity would be pressed up by the
way as the first, and the tube and piston
would constitute a pump.  Now when the
to press upon a water surface in such a
-CizI!       vessel, but is excluded from the tube, the
water rises in the tube thirty-four feet, as
W   in the last case; and if this quantity be liftILs ~1~ —-M-l    ed out of the tube by a piston, a second equal
quantity is pressed up, and the tube and
b.      piston become a complete example of the
common lifting or sucking pump. We have
to describe it more particularly hereafter.
3d. If there were a quantity of mercury or of quicksilver at the bottom
of the vessel a b c, filling it up to the level M, and if a tube i m issued from
under this level, the mercury pressed upon by thirty-seven feet of oil would
rise in this short tube as the water did in the larger; but by reason of its
greater specific gravity, it would only reach a height of thirty inches above
its level, the water having stood at thirty-four feet.  Now thirty inches of
mercury is the height of column which the atmospheric pressure, acting
in the same way really produces, as is seen in a similar apparatus made
expressly for measuring that pressure, and called a barometer or measure
of weight.
4th. If a tube d, of an inch square and open at both ends, were pluDged
into the oil, it would of course always be full up to the level of the oil on
the outside of it; and if it were pushed low enough to touch the water at
W, it would just contain fifteen pounds of oil resting on an inch square of
the water-surface at its mouth; which surface would therefore be bearing a
weight of fifteen pounds like every inch of the surface around, but would
not yield, owing to the force with which it tended upwards to escape from
the pressure corresponding to its depth in the oil.  Then if the tube were
pushed a little farther down, and if, by a piston or plug in it, the fifteen
pounds of oil were lifted out of it, water would rise into it until enough had
entered to reproduce the pressure of fifteen pounds on the surface below
as before; that is to say, the water would rise thirty-four feet, as in the
external tube w i. This int'ernal tube and piston again would form apump.
In like manner, when a tube open at both ends is plunged from the air into
water, the air presses on the surface of the water within the tube, as on the
surface around it, with a force of fifteen pounds to the inch, and the two
surfaces are not affected by the equal pressures; but if, by a piston, we lift
the air out of the tube, as we suppose the oil lifted in the last experiment,




ATMOSPHERI-C  PRESSURE  ON  LIQUIDS.    171
the water will then rise, following the piston to the altitude of thirty-four
feet. This arrangement of parts is the most usual for the Ulfting or household pump.
5th. If a common bottle or vessel of any shape, as the bent tube e, were
filled with water, and placed under the oil with its mouth or mouths reaching below the water surface at the level W, it would remain full of water,
owing to the pressure of the oil surrounding it.-For a similar reason, any
such vessel or tube, surrounded only by air, when filled with water, and
placed with its mouth or mouths under the surface of water, remains full;
and if such a bent tube has one of its ends in another vessel lower than the
first, a current is established in it; —the contrivance being then called a
syphon.
6th. A fish in the water below the level W, would be bearing the pressure of the oil from O to W, as well as the pressure of the water.-So a
fish in water open to the air, is bearing the atmospheric pressure of fifteen
pounds per inch, in addition to that of the water itself. This is proved by
extracting the air from over water in which a fish is swimming: for then
the air-bag of the fish, situated near its under side, as already described,
immediately dilates and turns the fish upon its back.
7th. To separate the Magdeburgh hemispheres, or to produce a vacuum
in any way, under the water level W, would require force proportionate to
the weight of oil above, it addition to that required on account of the'
water;-and to separate the Magdeburgh hemispheres under any watersurface pressed upon by the atmosphere, a force is required of fifteen pounds
per inch beyond what would balance the effect of the water itself.
The following remarks illustrate more minutely some of the objects which
we have just been explaining.
The common lifting-pump (or sucking-pump as it used to be called,) is
then merely a barrel a b, With a close-fitting moveable plug or piston in it c.
When the lower end b, is plunged into water, and the piston is drawn up
from the bottom, the atmosphere being prevented from pressing on the surface of the water within the tube, the pressure  Fig. 9.5.
on the surface external to the tube, drives the water up
after the piston. That the water which thus rises may not
fall again, there is a valve or flap at the lower part of the
pump-barrel b, which opens only to water passing upwards;  a.   dand that the piston may be allowed to pass downwards
through the water in the barrel, to repeat its stroke, there
is in it a similar valve.  The piston, in rising during a
second or succeeding stroke, causesall the water above it to
run over at the spout d.-Formerly a lifting-pump was                1
said to act by sucking the water up from the well beneath
it; the true meaning of which phrase we now perceive to
be, that the piston merely lifts or holds off' the air which
was pressing on the water within the barrel, and allows the
water to rise in their obedience to the external pressure of the  b
air around.  The reason is apparent, then, why, in the
lifting-pump, the water will only follow the piston to a
certain elevation, viz., until its weight balances the external
pressure of the atmosphere.




172                      PNEUMATICS.'When the piston of a pump is solid, or without a
Fig. 96.      valve, as at c, the machine is called a forcing-pump.
The water rises beneath the piston, as already explained for the lifting-pump, but then as it cannot
pass through the descending piston, as in the liftingpump, it is forced into any other desired direction,
as to d.  A forcing-pump can bring water from only
thirty-four feet below the piston, but can send it to
a,  any elevation. -In forcing-pumps, it is usual to make
the water enter an air-vessel d a (already explained
at page 161,) from which it is again urged by the
r           i     elastic air, through the pipe b, in a nearly uniform
stream.
The animal action of sucking is an approximation
to what we have described in the lifting-pump.  The
difference is that the chest or mouth can make only
a partial vacuum, and therefore cannot raise a liquid
very far.
A syphon remains full of liquid, although partially
raised above the general surface of the liquid, as explained above. For common purposes, a syphon is made of the form here represented, viz., a bent
tube c b a, with one end longer than the other.  To use it, the end c is first
immersed in liquid, and the end a being then stopped for the time by the
finger or a cock, the air is extracted by the mouth or otherwise, through the
small tube a d, and the atmosphere immediately fills the whole tube with
liquid from c. If the instrument be then left to act, the liquid will run from
the longer leg, because a long column of liquid overbalances a short one,
until the shorter has drunk up all within its reach. Whether the external
extremity be in the air only, or immersed
Fig. 97.           in liquid, makes no difference, except that
/'             the immersion shortens so much the descending column.  If both extremities be
/  immersed in liquid, and in different vessels,
by alternately lifting one vessel or the
other, the liquid will be made to pass and
repass, and will come to rest in the syphon
only when the surfaces in the two vessels
are at the same level.  Thus the same leg
whha abecomes alternately the long and the short
leg, according to the height of the liquid
in which it is immersed.  A syphon is
sometimes made with both legs equal and
turned up, as here represented, so that it
remains full of liquid although lifted away
from the vessel, and therefore is always
ready for action.  As it is the same cause
which lifts the water in a pump and in a syphon, the top of a syphon must
evidently be within thirty-two feet of the water-surface below. In the syphon
as the cases of balancing liquids, described at page 131 (which see,) the
comparative diameters of the legs are of no importance, nor their oblique
length, provided the perpendicular heights of the two columns have the necessary relation:-even an inverted tea-pot may be used as a syphon.  This
truth is well exemplified in what may be called the syphon-paradox an exact




PRESSURE  OF LIQUIDS. —-SYPHON.                       173
counterpart of the paradox of the "Hydrostatic Bellows,"
already explained.  If the apparatus of the bellows be filled    Fig. 98.
with water in the ordinary way (see page 130,) and be then
reversed or turned so that the tube becomes like the long leg of l
a syphon, the little stream of water issuing from it at a will lift
as great a weight. suspended from the board d, as the same
slender column in the standing position can lift upon the board.
As farther illustrative of the atmospherical pressure exerted in
producing this effect, and in rendering a syphon active, we may
advert to the striking fact, that a long small tube of water
screwed into the side or bottom  of a close cask of water so
as to communicate with it, and then allowed to discharge like
the long leg of a syphon, will cause the cask to be crushed
inwards, just as the same tube screwed into the top of the cask, as represented at page 131, causes the cask to burst outwards.
The syphon is very useful for drawing off liquids, where there is a sediment that should not be disturbed, or where it is desirable not to make an
opening in the lower part of the vessel. A large syphon would empty a
lake or mill-pond over its bank without injuring the bank. To fill a large
syphon that i~ may act the most convenient way is, instead of pumping out
the air from it, to close the two ends for the time, and to pour in water
through a cock at the top.
There is a pretty syphon-toy,"called a Tantalus-cup,'having in it a standing human figure which conceals a syphon. The short branch of the syphhon
rises in one leg of the figure to reach the level of the chin, and the long
branch descends in the other leg to pierce the bottom of the cup towards a
reservoir below. On pouring water into the cup, the syphon begins to act
as soon as the water reaches.the chin of the figure, and the cup is.then
emptied as if by magic.
Among the infinitely varied water-drains or courses in the bowels of the
earth, some are syphons, and produce what are called intermitting wells or
fountains. These may alternately run and cease for longer or shorter
periods, according to the comparative magnitudes of the collecting reservoir
and the' drain. The reservoir may be an internal cave of a mountain,
receiving a regular supply of water by a slow filtering of moisture from
above, and the drain is a syphon-formed channel, which, like that of the
Tantalus-cup, begins to act only when the water in the reservoir has reached
the level of the top of the syphon, and then carries off the water faster
than it is supplied. There are some fountains that flow constantly, but at
regular intervals have a remarkable increase. In them a common spring is
joined with a syphon-spring.
The author has suggested an application of the syphon, which obviates a
strong objection to the high operation for stone, as explained in the next
medical section.
The following facts have close relation to those now explained, as farther
illustrative of atmospheric pressure on liquids.
A long glass of jelly, if inverted and placed with its mouth just under
the surface of warm water, will soon be found to have lost the jelly, but to
be full of water in its stead.  The jelly is heavier than water, and whei
melted by the heat sinks down, and. is replaced by water from below, sent
up by the atmospheric pressure.




174                       PNEUMATICS.
The slaves in the West Indies steal rum, by inserting the long neck of a
bottle full of water through the top aperture of the rum-cask. The water
falls out of the bottle into the cask, while the lighter rum ascends in its stead.
The common water-glass for bird-cages has its only opening near the bottom through the neck b; yet no water can escape
Fig. 99       from it but when the level of the water at c, in the
open part, becomes low enough for some air to pass
into the body of the glass by the channel b. When....   Cl  a bubble of air does pass in, an equal bulk of water
comes out, and by raising the water level in c, prevents the passage of more.-An ink-glass made on
this principle preserves the ink well, because there
is so small a surface exposed to the air; if made too
I/          large, however, the accidental expansion of the air
kb - c       in in it by heat may cause it to overflow.
In the common Aryand or fountain-lamp, the
provision of oil is in a vessel like an inverted bottle, higher than the flame,
and with its mouth immersed in a small reservoir of oil, nearly on a level
with the flame, then no oil can escape from above, but as the flame consumes
the free oil from the, small reservoir, which supply is thus maintained
always at the same elevation.-In the Hiero's fountain lamp, mentioned at
page 164, that the two balancing columns of oil may be always of the same
height, the oil is suppplied to them from high reservoirs, with the mouths
dipping in them as above described, and keeping their tops, therefore,
always at the same level; and that the descending column may not be
shortened by the rising of the oil in the low reservoir c. the tube containing
it is turned up at the bottom like an end of the "ever ready syphon," and
discharges near the top of c.
We have hitherto been contemplating only the direct weight or downward
pressure of the atmosphere on liquids: in the following instances we have
proof of the same pressure acting upon them in all directions.
If a bottle or cask be filled with liquid, and closely corked, and if a small
-hole be then drilled in the bottom or side, the liquid will not escape by it,
because of the existing pressure of the atmosphere, and of there not being
room in the opening for a current of air to enter while the current of water
escapes: but if a second hole be drilled in the top, a jet from the lower
opening will follow immediately, because then theair will press on the upper
surface of the liquid as well as on the lower, and the weight of the liquid
will be free to act:-thus, a cask, of beer or wine cannot be emptied by a
cock near the bottom, unless what is called a vent-hole be made at the top.
If the lower opening, however, in any case be so large, that the air may
enter by one side of it, as is seen in decanting a bottle of wine. In such a
case, it is the interrupted entrance of the air which causes that gurgling
sound so delightful to the ear of the drunkard, instead of allowing the
smooth stream which fall from a funnel.
Even a large opening at the bottom of a vessel which is close above, may
be prevented by the pressure of the air, from discharging liquid if any
mutual passing of the two currents of air and liquid be rendered difficult.
An inverted bottle of water will not discharge, if a piece of paper simply be
applied against its mouth. Even a wineglass filled with water may be




ATMOSPHERIC  PRESSURE.-  BAROMETER.  175
inverted, and yet will spill none, if the piece of paper, laid loosely upon its
mouth, be held to it during the turning, —the pressure of the atmosphere
against the paper keeping it in its place, and supporting the water above it.
Any vessel or tube of water of less height than thirty-four feet may be kept
closed at the bottom in the same way.
The animal body is made up of solids and fluids, and is affected by the
atmospheric pressure accordingly.
There is a difficulty at first in believing that a man's body should be
bearing a pressure of fifteen pounds on every square inch of its surface, while
he remains altogether insensible of it: but such is the fact, and the reason of
his not feeling the fluid pressure is its being perfectly uniform all around.
If a pressure of the same kind be even many times greater, such, for instance,
as fishes bear in deep water, or as a man supports in the diving-bell, it equally
passes unnoticed. Fishes are at their ease in a depth of water where the
pressure ar6und will instantly break or burst inwards almost the strongest
empty vessel that can be sent down; and men walk on earth without discovering a heavy atmosphere about them, which, however, instantly crushes
together the sides of a square glass bottle emptied by the air-pump, or even
of a thick iron boiler, left for a moment by any accident, without the counteracting internal support of steam or air.
The fluid pressure on animal bodies, thus unperceived under ordinary circumstances, may be rendered instantly sensible by a little artificial arrangement. In water, an open tube partialy immersed becomes full to the level
of the water around it, and the water contained in it is supported, as already
explained, by that which is immediately below its mouth: now a flat fish
resting closely against the mouth of the tube, would evidently be bearing on
its back the whole of this weight, perhaps one hundred jounds; but the fish
would not thereby be pushed away, nor would it even feel its burden, because
the upward pressure of the water immediately under it.would just counterbalance the weight, while the lateral pressure around would prevent any
crushing effect of the upward and downward forces. But if, while the fish
continued in the situation supposed, the hundred pounds of water were suddenly lifted from off its back by a piston in the tube, the opposite upward
pressure of one hundred pounds would at once crush its body into the tube.'At a less depth, or with a smaller tube, the effect might not be fatal, but
there would be a bulging or swelling of the substance of the fish into the
mouth of the tube.-In air and onf the human body a perfectly analogous
case is exhibited.  A man without pain or any peculiar sensation, applies
his hand closely to the mouth or opening of a tube, or of any vessel containing air, but the instant that the air is withdrawn from within the tube or
vessel, the then unresisted pressure of the external air fixes the hand upon
the opening, causes the flesh to swell or bulge into it, and makes the blood
ooze from any crack or puncture in the skin.
These last lines describe closely the surgical operation of cupping; the
essential circumstances of which are, the application of a cup or glass, with a
smooth blunt lip, to the skin of any part of the body, and the extraction by a
syrifige, or other means, of a portion of the air from within the cup. To
some minds the exact comprehension of this phenomenon may be facilitated,
by considering the case of a small bladder or bag of India-rubber full of any
fluid and pressed between the hands on every part of its surface except one:at that one part it would swell, and even burst if the pressure were strong
enough. So in cupping, the whole body, except the surface under the cup,




176                       PNEUMATICS.
is squeezed by the atmosphere, with a force of fifteen pounds to the square
inch, while in that one situation the pressure is diminished according to the
degree of exhaustion in the cup, and the blood consequently accumulates
there. - The application of a cup with exhaustion only, constitutes the operation called dry-cupping. To obtain blood, the cup is removed and the tumid
part is cut into by the simultaneous stroke of a number of united lancets:
and the cup is then applied again as before and exhausted, so that the blood
may rush forth under the diminished pressure.
The partial vacuum in the cup may be produced either by the action of a
syringe, or by burning a little spirit in the cup and applying it while the
momentary dilation effected by the heat has driven out the greater part of
the air. The human mouth applied upon any part becomes a small cupping
apparatus, and formerly, in cases of poisoned wounds, was used as such.
Our present perfect cupping glasses, of stronger and more permanent operation, are not yet always used, as they might be, to assist in removing the
poison after the bites of rabid or venomous animals.
The author has suggested an extension and modification of the operation
of the dry cupping, which he believes will prove an important remedy in the
hands of the medical practitioner. It is intended as a substitute for bleeding
in certain cases where blood can ill be spared, and as a more sudden and
effectual check than even bleeding itself, in certain cases of inflammatory
disease. It is explained in the next medical section of this work.
The atmospheric pressure on living bodies produces an effect which is
rarely thought of, although of much importance, viz., keeping all the parts
about the joints, firmly together, by an action similar to that exerted on the
Magdeburgh hemispheres.  The broad surfaces of bone forming the kneejoint, for instance, even -if not held together by ligaments, could not, while
the capsule surrounding the joint remained air-tight, be separated by a force
of less than about' a hundred pounds; but on air being admitted to the
articular cavity, the bones at once fall to a certain distance apart. In the
loose joint of the shoulder, this support is of great consequence. When the
shoulder or other joint is dislocated, there is no empty space left, as might
be supposed, but the soft parts around are pressed in to fill up the natural
place of the bone. XVhen a thigh bone is dislocated, the deep socket called
thee actabulum instantly becomes like a cupping-glass, and is filled partly with
fluid and partly with the soft solids. In all joints, it is the atmospheric
pressure which keeps the bones in, such steady contact, that they work
smoothly and without noise.
The baroneter we have seen at page 170, is a column of fluid supported
in a tube by the pressure of the atmosphere, and therefore indicating most
exactly the degree of that pressure. It is an instrument now of such importance, both in a scientific point of view and in the business of common life,
that for the sake of minds which conceive such subjects with difficulty we
shall add here the two following farther illustrations of its nature.
If mercury be poured into a bent tube open at both ends, it will stand at
the same level in the two legs, as at a and b, and the air will be pressing on
the two surfaces at a and b with equal force of 15 lbs. per square inch. If the
air be then removed from one leg a, by a piston or otherwise, while it continues
to press in the other leg b, the mercury will be pushed down in b, until the
growing height of a column in a produces a weight so much greater than
that in b, as just to counteract the pressure: now this balance takes place, in
fact, when the mercury in a stands about thirty inches higher than in b; that
being the height of a column of mercury weighing 15 lbs on the square inch.




ATMOSPHERIC  PRESSURE.-BAROMETERS.   177
If the top of the tube a were then closed permanently, the mercury would for ever remain elevated in it, marking most perfectly  Fig. 100.
the atmospheric pressure; now this construction, only with the 
empty and useless part of the tube above d cut off or wanting,  c l e
forms a common barometer. The exact altitude of the mercury
in it is known by observing how much the surface near c is
higher than that near d. Often, in such a barometer, a little mass
of metal is placed to float on the mercurial surface at d, and as  " b
it rises and falls, is caused, by a thread passing from it over a
wheel or pulley, to move an index like the hand of a clock connected with the wheel, and this index tells the degree of eleva-    c
tion.  This modification is called the wheel barometer.
Again, as water at a, in the bottom of a closed pump-barrel, if
pressed upon by the piston b c, of which the rod d were hollow
or tubular, would rise in the rod to a height proportioned to the pressure made
by the piston, so, in a straight exhausted barometer-tube, which is as this
hollow piston-rod, the mercury or water rises, because the atmospheric pressure around it is as the piston forcing thelfluid up.  To make a barometer
of this kind it is only necessary to procure a glass tube more than thirty inches
long, and close at one end, and then having filled it with
mercury, to plunge its mouth (stopped by the finger       Fig. 101.
while turning) into a small cup or basin of mercury;the fluid falls away a little from the top of the tube,
leaving a vacuum  there, and stands at the elevation          ti.
which the atmospheric pressure is fitted to maintain.
We know from the law of hydrostatics already explained
that it is of no importance, in such a case, what the shape,
or inclination, or size of the tube may be, as only the
perpendicular height can measure or be measured by the
pressure. This fact enables us to construct barometers  "
with the upper part of the tube bent obliquely, so that
for one inch rise of mercury in a perpendicular tube,
there shall be an advance of several inches in the oblique top, rendering
any change of elevation so much more apparent.
Galileo had found that water would rise under the piston of a pump to a
height only of about thirty-four feet.  His pupil Torricelli, conceiving the
happy thought, that the weight of the atmosphere might be the cause of the
ascent, concluded that mercury, which is about thirteen times heavier than
water, should only rise under the same influence to a thirteenth of the elevation; —he tried and found that this was so, and the mercurial barometer was
invented.  Pascal then, to afford farther evidence that the weight of the
atmosphere was the cause of the phenomena, carried the tube of mercury to
the tops of buildings and of mountains, and found that it fell always in
exact proportion to the portion of the atmosphere left below it; —and he
found that water-pumps in different situations varied as to sucking power,
according to the same law.
It was goon afterwards discovered, by careful observation of the mercurial
baromneter, that even when remaining in the same place, it did not always
stand at the same elevation; in other words, that the weight of the atmosphere
over any particular part of the earth was constantly fluctuating; a truth which
without the barometer could never have been suspected.  The observation
of the instrument being carried still farther, it was found, that in serene dry
weather the mercury generally stood high, and that before and during storms
12




178                      PNEUMATICS.
and rain it fell:-The instrument, therefore, might serve as a prophet of the
weather, becoming a precious'monitor to the husbandman or the sailor.
The reasons why the barometer falls before wind and rain will be better
understood a few pages hence; but we may remark here, that when water
which has been suspended in the atmosphere, and has formed a part of it,
separates as rain, the weight and bulk of the mass are diminished: and that
wind must occur when a sudden condensation of aeriform  matter, in any
situation, disturbs the equilibrium of the air; for the air around will rush
towards the situation of diminished pressure.
To the husbandman the barometer is of considerable use, by aiding and correcting the prognostics of the weather which he draws from local signs familiar
to him; bat its great use as a weather-glass seems to be to the mariner who
roams over the whole ocean, and is often under skies and climates altogether
new to him. The watchful captain of the present day, trusting to this extraordinary monitor, is frequently enabled to take in sail and to make ready for
the storm, where, in former times, the dreadful visitation would have fallen
upon him unprepared.  The marine barometer has not yet been in general
use for many years, and the author of this work was one of a numerous crew
who probably owed their preservation to its almost miraculous warning. It
was in a southern la:titude; the sun had just set with placid appearance, closing
a beautiful afternoon, and the usual mirth of the evening watch was proceeding, when the captain's order came to prepare with all haste for a storm. The
barometer had began to fall with appalling rapidity. As yet, the oldest sailors
had not perceived even a threatening in the sky, and were surprised at the
extent and hurry of the preparations: but the required measures were not
completed, when a more awful hurricane burst upon them than the most
experienced had ever braved. Nothing could withstand it; the sails already
furled and closely bound to the yards, were riven away in tatters: even the
yards and masts themselves were in great part disabled; and at one time the
whole had nearly fallen by the board.  Such, for a few hours, was the min
gled roar of the hurricane among the rigging, of the waves around, and of
the incessant peals of thunder, that no human voice could be heard, and
amidst the general consternation, even the trumpet sounded in vain. In
that awful night, but for the little tube of mercury which had given the
warning, neither the strength of the noble ship, nor the skill and energies
of the commander, could have availed anything, and not a man would have
escaped to tell the tale. On the following morning the wind was again at
rest, but the ship lay upon the yet heaving waves, an unsightly wreck.
The marine barometer differs from that used on shore, in having its tube
contracted in one place to a very narrow bore, so as to prevent that sudden
rising and falling of the mercury which every motion of the ship would'else
oecasion.
Civilized Europe is now familiar with the barometer and its uses, and
therefore, that Europeans may conceive the first feelings connected with it,
they almost require to witness the astonishment or incredulity with which
people of other countries still regard it. A Chinese once conversing on the
subject with the author, could only imagine of the barometer, tha;t. it was a
gift of miraculous nature, which the God of Christians gave them in pity,
to direct them in the long and perilous voyages which they undertook to
unknown seas.
A barometer is of great use to persons employed about those mines in
which hydrogen gas, or fire-damp, is generated and exists in the crevices.
When the atmosphere becomes unusually light, the hydrogen being relieved




ATMOSPHERIC PRESSURE.-BAROMETERS.   179
from a part of the pressure which ordinarily confines it to its holes and
lurking-places, expands or issues forth to where it may meet the lamp of
the miner, and explode- to his destruction. In heavy states of the atmosphere, on the contrary, it is pressed back to its hiding-places, and the miner
advances with safety.
We see from this, that any reservoir or vessel containing air would itself
answer as a barometer if the only opening to it were through a long tubular
neck, containing a close-sliding plug; for then, according to the weight and
pressure of the external air, the density of that in the vessel would vary, and
all changes would be marked by the position of the movable plug. A barometer has really been made on this principle, by using a vessel of glass, with
a long slender neck, in which a globule of mercury is the movable plug.
The state of the atmosphere, as to weight, differs at different times in the
same situation, so as to produce a range of about three inches in the height
of the mercurial barometer; that is to say, from twenty-eight to thirty-one
inches. On. the occasion of the great Lisbon earthquake, however, the mercury fell so far in the barometers, even in Britain, as to disappear from that
portion at the top usually left uncovered for observation. The uncovered
part of a barometer is commonly of five or six inches in length, with a
divided scale attached to it, on which the figures 28, 29, &c., indicate the
number of inches from the surface of the mercury at the bottom to the
respective divisions: —on the lower part of the scale the words wind and
rain are generally written, meaning that when the mercury sinks to them,
wind and rain are to be expected; and on the upper part, dry and fitne
appear, for a corresponding reason: but we have to recollect, that it is not
the absolute height of the mercury which indicates the existing or coming
weather, but the recent change in its height; a falling barometer usually
telling of wind and rain; a rising one of serene and dry weather.
The barometer answers another important purpose, besides that of a weatherglass-in enabling us to ascertain readily the height of mountains, or of
any situation to which it can be carried.
As the mercurial column in the barometer is always an exact indication of
the weight or pressure of air above its level, being, indeed, as explained in
the foregoing paragraphs, of the same weight as a column of the air of equal
base with itself, and reaching from it to the top of the atmosphere-the
mercury must fall when the instrument is carried from any lower to any
higher situation, and the degree of falling must always tell exactly how much
air has been left below. For instance, if thirty inches barometrical height
mark the whole atmospheric pressure at the surface of the ocean, and if the
instrument be found, when carried to some other situation, to stand at only
twenty inches, it proves that one-third of the atmosphere exists below the
level of the new situation. If our atmospheric ocean were of as uniform
density all the way up as our watery oceans, a certain weight of air thus left
behind in ascending would mark everywhere a change of level nearly equal,
and the ascertaining any height by the barometer would become one of the
most simple of calculations: the air at the surface of the earth, being between
eleven and twelve thousand times lighter than its bulk of mercury, an inch
rise or fall of the barometer would mark everywhere a rise or fall in the
atmosphere of nearly twelve thousand inches or one thousand feet. But
owing to the elasticity of air, which causes it to increase in volume as it
escapes from pressure, the atmosphere is rarer in proportion as we ascend,




180.                      PNEUMATICS.
so that to leave a given weight of it behind, the ascent must be greater, the
higher the situation where the experiment is made: the rule, therefore, of
one inch of mercury for a thousand feet, holds only for rough estimates near
the surface of the earth. The precise calculation, however, for any case, is
still very easy; and a good barometer, with a thermometer attached, and
with tables, or an algebraical formula, expressing all the influencing circumstances, enables us to ascertain elevations much more easily, and in many
cases more correctly, than by trigonometrical survey.
The weight of the whole atmospherical ocean surrounding the earth being
equal to that of a watery ocean of thirty-four feet deep, or of a covering of
mercury of thirty inches, and the air found at the surface of the earth being
828 times lighter than water, if the same density existed all the way up, the
atmosphere would be 34 times 828 feet high, equal to about five miles and
a half. On account of the greater rarity, however, in the superior regions,
it really extends to a height of nearly fifty miles. From the known laws of
airial elasticity, explained at page 158, we can deduce what is fQund to hold
in fact, that one-half of all the air constituting our atmosphere exists within
three miles and a half from the earth's surface; that is to say, under the
level of the summit of Mont Blanc. A person unaccustomed to calculation,
would suppose the air to be more equally distributed through the fifty miles
than this rule indicates, as he might at first also suppose a tube of two feet
diameter to be only twice as capacious as a tube of one foot, although in
reality it is four times as capacious.
In carrying a barometer from the level of the Thames to the top of St.
Paul's Church, in London, or of Hampstead Hill, the mercury falls about
half an inch, making an ascent of about five hundred feet. On Mont
Blanc it falls to half of the entire barometric height, making an elevation
of fifteen thousand feet; and in Dl)u Lucts' famous balloon ascent it fell to
below twelve inches, indicating an elevation of twenty-one thousand feet,
the greatest to which man has ever ascended from the surface of his earthly
habitation.
The extreme rarity of the air on high mountains must, of course, affect
animals.  A person breathing on the summit of Mont Blanc, although
expanding his chest as much as usual, really takes in at each inspiration
only half as much air as he does below-exhibiting a contrast to a man in
a diving-bell, who, at thirty-four feet under water, is breathing air of double
density, at sixty-eight feet of triple, and so on. It is known that travellers,
and even their practised guides, often fall down suddenly as if struck by
lightning, when approaching lofty summits, on account, chiefly, of the
thinness of the air which they are breathing, and some minutes elapse before
they recover. In the elevated, plains of South America, the inhabitants
have larger chests than the inhabitants of lower regions-furnishing another
admirable instance of the animal frame adapting itself to the circumstances
in which it is placed. It appears from all this, that although our atmosphere be fifty miles high, it is so thin beyond three miles and a half, that
mountain ridges of greater elevation are nearly as effectual barriers between
nations of men, as islands or rocky ridges in a sea are between the finny
tribes inhabiting the opposite coasts. The intense cold which appertains to
high situations, and forms another obstacle to human approach, remains to
be considered in our next division.
A barometer connected with an air-pump, indicates exactly the progress
and degree of exhaustion in the receiver. When the mercury falls to half its
height, it shows that half of the air is extracted; and so for all other propor



ATMOSPHE R I C PRESSURE. -BAROMETERS.   181
tions.  A barometer then is a necessary appendage to a complete air-pump;
but as its chief purpose is to mark when the exhaustion is carried nearly to
completion, a very short tube, corresponding to the bottom of a common
barometer, is all that is generally provided, and it is usually made of syphon
form.
The ingenious method, mentioned at page 145 of ascertaining the specific
gravity of the solid material forming any porous mass or powder, includes
the agency of a.barometer.  It proceeds upon this reasoning.  The interstices of a porous or pulverized mass are filled with air of the density of the
surrounding atmosphere, and if the atmospheric pressure on which that density depends be diminished upon the mass in any given degree, an exactly
corresponding proportion of the air will issue from the pores, and if measured
will declare the whole quantity, and, therefore, the amount of interstices
or pores in the solid mass. Now if the substance were inclosed at the end
or bottom of a syringe, the pressure of the atmosphere might be held off
from it in any degree by drawing at the piston, and the air would issue
from  the pores as described, and would follow the pistom; but as, owing to
the friction or a solid piston, it would be difficult to measure the precise
action, the liquid piston of a mercurial column has been substituted, of which
the force is always proportioned to the length. The operator
takes an open glass tube, a e, of known dimensions, and pre-   Fig. 102.
pare a part of its top, a b, as a receptacle for the substance,L
under trial, by affixing a partition at b, which shall support the
substance, but allow passage to air.  Having then filled a b      b
with the substance, he gradually immerses the tube in a vessel   c
of mercury df, until the mercury stand both inside and outside
of the tube at the level of b, the air from the tube having
passed out through the substance in a b. It is evident that
on then closing the tube at a in the air-tight manner, and
lifting the tube, a column of mercury will remain standing in
it, above the level of the external mercury at d, and will be
acting as a piston pulling down from b with force proportioned
to its height. If the tube be lifted until such mercurial
column c d be just of half the length in a column in a common
barometer, the air in the pores of the substance will be relieved
from half of the atmospheric pressure, and will dilate to double
bulk: so that while half of the air will remain in the pores,
the other half will have issued forth to occupy a space, as b c,
between the surface of the mercury at the partition at b. This  e
space b c, therefore will be' exactly equal to the amount of
the pores or interstices; and as it may be measured and compared with the whole.space a b, its ascertained magnitude will
solve the problem.  It has been found in this way that charcoal, which is usually said to be only half as heavy as its bulk
of water, is really formed of matter nearly four times as heavy;
proving, in a new way, the identity of charcoal and diamond,
and that light pumice stone consists of matter heavier than granite or
marble. This very ingenious application of the barometer may lead
ultimately to many useful results: and the contrivance merits consideration
here, as exhibiting, under a new and interesting aspect, the rationale of
barometric action and the elasticity of air.




182                       PNEUMATICS.
Atmospheric pressure determining the liquid or aeriform state of certain
substances. (See the Analysis, page 156.)
It has already been stated that the permanent gases-or substances usually
in the aeriform state-may be reduced to the liquid, or even solid form, by
simple pressure, and abstraction of the heat which is combined with them
in the aeriform state. Carbonic acid, the common coal gas, &c., have been
treated in this way.  Now it becomes an interesting question whether many
of the substances known as liquids on the face of the earth, where they are
bearing the pressure of the atmosphere, would not appear as airs if that
pressure did not exist.
On investigating this subject by experiment, we accordingly find, that
tether, alcohol or ardent spirits, volatile oils, &c., and even water itself, are
known to us here as liquids, only because their particles are kept together
by the weight and pressure of a superincumbent atmosphere   Any of these
substances, relieved by art from such pressure, quickly becomes an air or
gas, just as a cornnon gas, which has been kept in the state of liquid by
any great pressure, becomes air again on being relieved.
In our first chapter we explained the dependence of the three forms which
any body may assume, viz., of solid, liquid, or air, on the quantity of heat
diffused among the particles; we now see, however, that to understand the
subject completely, we must consider also the effect of accidental pressure;
for, while heat is the power separating the atoms in the changes mentioned,
it has to overcome both the mutual attraction of the atoms and the additional
force of the atmosphere pressing them together. The combined influence
of these forces is fully displayed in the two phenomena called boiling and
evaporation, which exhibit the progress or the change of a liquid into an
aeriform fluid.  We now proceed to examine these phenomena.
Boiling.-If water be placed in a suitable vessel over a common fire, or
over the flame of a lamp, it is gradually heated to a certain degree; and
then small bubbles of aeriform matter, viz., water, in the state called steam,
are seen forming at the bottom of the vessel; and successively rising to the
surface, where they disappear by mixing with the atmosphere; and the
operation being continued, the quantity of water diminishes with every
bubble, until the whole vanishes under the new form of air.
This change takes place in water, under common circumstances at the
degree of heat mark 2120 on Fahrenheit's thermometer, and called, on that
account, the boilingpoint of water; at which therefore, the repulsive power
among the particles is just sufficient to overcome both their natural attraction, and the compressing force of the atmosphere of fifteen pounds on the inch.
But a less degree of heat suffices if the pressure of the atmosphere be lessened
or removed: and a greater degree is required if pressure be increased. Water
on the top of Mount Blanc boils at 1800, because relieved from the pressure
of the air that is below the level of the mountain's summit; and at all intermediate heights in descending to the level of the sea, and beyond that'into
mines, there is a corresponding increase of the boiling temperature.  So
exactly is -this the case, that we now find it to be a good method of ascertaining the heights of places, merely to observe the heat of boiling water at
them.  To many persons the information here given that boiling water is not
equally hot in all places,will appear extraordinary; and they will not understand that even in the same place, at different tinles, when the barometer is
high or low, there will be corresponding differences. Again, near the bottom
of a boiler, the water is hotter than above, because it is bearing an additional




ATMOSPHERIC PRESSURE.-BOILING.                           183
pressure proportioned to the depth and does not, therefore, give out the
steam which it would part with if a.little higher up. In very large and deep
boilers, therefore, such as are used in great porter breweries, the liquor is
much more heated that it can be in smaller vessels; a circumstance which
probably has an influence on its ultimate quality.
While water, under common atmospheric pressure, or when the barometer
stands at thirty inches, boils at 212~, other substances, with other relations
to heat, have their boiling points higher' or lower: mether, for instance, at
98~; spirit or alcohol at 1740; fish-oil and tallow at about 6000; mercury
at 650~.
It is in consequence of the different temperatures at which the paticles of
different substances requires repulsion enough to rise against the atmospheric
resistance that we are enabled to perform the operation called distilling. If
a mixture of spirits and water, for instance, be heated up to 180 deg., the
spirit will pass off in the aeriform state, leaving the water behind, and may
be caught apart and cooled to condensation in any fit receiver. Distillation
is the best means we possess of separating many substances from each other,
as spirit from wine and other fermental liquor; various acids from water;
water itself from its common impurities;-and even the separation of
mercury from silver or gold which it has been used to dissolve from among
the rubbish of a mine or river-bottom, is merely a distillation.
We must call to mind here what was mentioned in a former part of the
work, that a large quantity of heat combines with every substance during the
-change of form from solid to liquid, or from liquid to air; a quantity which,
from not remaining sensible to the thermometor, has received the name of
latent or concealed heat. The whole of this is given out again in the contrary
change. In the conversion of water into steam, the heat which thus disappears is about 1,000 degrees, or six times as much as is required to raise the
cold water to the boiling point; this is proved by the time and fuel expended
in boiling any quantity to dryness, and by the fact that a pint of water in
the form of steam will combine instantly with six pints of cold water, raising the whole to boiling heat.
But for the fact of latent heat, the conversion of a liquid into air would
not be the gradual process of boiling which we now see, but a sudden and
terrible explosion: for when any quantity of water were raised to the boiling
heat one degree more would be sufficient to convert the whole into steam.
And but for the same reason, the thawing of winter snow would always be
a sudden and frightful inundation; the whole load of a mountain or plain
becoming at once as a lake bursting from its enclosing barriers. On the
other hand, if water in freezing had not to give out again its latent heat, after
any quantity were at once cooled down to the freezing point, the abstraction
of one degree more would instantly convert the whole into a solid mass.
Thus, then, by an arrangement effecting most important purposes in nature
and art, all changes from solid to liquid and from liquid to air, and the
converse changes, are very gradual.
If a little heat be abstracted from steam, a part of the steam proportioned
to the abstraction is immediately condensed into water. What is called steam,
in common language-as the vapour which becomes visible at a little distance
from the spout of a boiling kettle or the chimney of a tea-urn-is not truly
steam, but small globules of water already condensed by the cold air and
mixed with it. Steam is as dry and invisible as air itself; but the instant
that it comes in contact with air or other bodies colder than itself, it becomes
water.




,184           rPN E U MAT IC S.
By means of the exhausting air-pump on one hand, and of the condensing
syringe on the other, all the above-mnntioned phenomena, depending on
the atmospheric pressure, and its increase or diminution, may be strikingly
shown.
Thus, to exhibit the effect of diminished pressure, water not heated by
several degrees to the boiling point of ordinary low situations, but which
would be boiling at the top of Mont Blance, is caused to boil instantly by
placing it under the receiver of an'air-pump, and making a few strokes of the
piston; if the exhaustion be rendered nearly complete, the water will boil,
even when colder by 20 degrees than the blood of animals; and at degrees of
temperature still much lower, it will rapidly assume the form of air, although
not with force sufficient to produce the violent agitation of boiling. Other
liquids, as spirits, sether, &c., from requiring inferior degrees of heat to
separate their particles to aeriform distances, boil under the receiver of an
air-pump at very low temperatures; aether, for instance, when as cold as
freezing water.
On the other hand, to exhibit the effect of increasing pressure, if we con.
fine the particles of a liquid still more than by a common atmospheric or
equivalent pressure, degrees of heat higher than the common boiling point
will be required to separate them. In a diving-bell, the boiling point of water
is higher than 212 deg. in proportion to the depth which the bell has reached:
and if at the surface of the earth, we heat water in a close vessel into which
air is forced so as to press thirty pounds on the inch instead of fifteen, as the
atmosphere does; or from which we prevent the steam's escaping until it
has acquired the force of a double atmosphere,-before making the liquid
boil, we shall have to raise the heat, in a corresponding proportion beyond
212 deg. Unddr a very strong pressure, water may be rendered almost redhot, but the force with which its particles are then tending to separate is
almost that of inflamed gunpowder. Even then, however, if a gradual issue
were allowed, only a certain quantity of the water would absorb and render
latent the existing excess of heat above 212 deg., and would become common
steam, leaving behind a considerable portion as boiling water of the ordinary
temperature.
The fact that liquids are driven off, or made to boil at lower degrees of heat
when the atmospheric pressure is lessened or removed, has recently been
applied to some very useful purposes.
The process of refining sugar is to dissolve impure sugar in water, and
after clarifying the solution, to boil off or evaporate th.e water again, that
the dry crystallized mass may remain.
Formerly this evaporation was performed under the atmospheric pressure,
and a heat of 218~ or 220~ was required to make the syrup boil; by which
degree of heat, however, a portion of the sugar was discoloured and spoiled,
and the whole product was deteriorated.  The valuable thought occurred to
Mr. Howard, that the water might be dissipated in boiling the syrup in a
vacuum, or at least a place from which air was nearly excluded, and therefore,
at a low temperature.  This was done accordingly.; and the saving of sugar
and the improvement of quality were such, as to make the patent-right,
which secured the' emoluments of the process to him and other parties, worth
many thousand pounds a year. The syrup, during this process, is not more
heated than if in a vessel merely exposed to a summer sun.
In the preparation of many medicinal substances, the process of boiling in




ATMOSPHERIC  PRESSU R E. -'B OILIN G.                 185
vacuo is equally important. Many extracts from vegetables, have their
virtues impaired, or even destroyed, by a heat of 212~; but when the water
used in making the extract is driven off in vacuo, the temperature need
never be higher than blood-heat, and all the activity of the fresh plant
remains in the extract.
In the same manner, in the process of distillation,-which is merely the
receiving and condensing again in appropriate vessels the aeriform matter
raised by heat from any mass, —substances which are changed and injured
by an elevated temperature may be obtained of admirable quality by carrying on the operation in a vacuum. The essential oils of lavender, peppermint, &c., never had the natural flavour and virtues of the plants until
within the last few years, since this plan has been adopted.
* The influence on the human system of vegetable medicines obtained in
the old or in the new way, is so different, that the prescriber should carefully
advert to the circumstance.
The apparatus for evaporating and distilling in vacuo consists of vessels
strong enough to bear, when quite empty, the external atmospheric pressure
and which are therefore generally of arched form. The vacuum is produced
and maintained by air-pumps driven by a steam engine or otherwise; or by
first admitting steam to expel the air, and then condensing the steam into
water.
The author has suggested a very simple contrivance to answer, in certain
cases, the purpose of such air-pumps and steam-engines or
apparatus.  It is merely to establish a communication    Fig. 103.
between a close boiler, as a, and the vacuum at the top of
a water barometer, as b, To produce that vacuum, the        e
strong vessel b forming the top of the barometer, and   (
thirty-six feet of tube below, reaching to d are first filled b a
with water through a cock c at the top; this cock being
then shut, and another cock d at the bottom, which was
shut, being opened, the water will sink down out of the
vessel b, until the column in the tube be only thirty-four
feet high. as at f, that being the height which the atmosphere will support. On then opening a communication
between the boiler a and the vacuum in b, the operation
will go on as desired, and the steam rising from a may be
condensed in b by a little stream of cold water allowed constantly to run through from above. This water, it is evident, would always pass downwards to form part of the
column below, without filling up or impairing the vacuum.
If air should find admittance in any way, the original degree
of vacuum could easily be reproduced as at first; and to
prevent interruptions, it might be convenient to have two
vessels like b, of which one could always be in action while'......
the other was being emptied of air. The author planned this as a simple
apparatus for the preparation of medical extracts; and it appears well suited
also for the manufacture of sugar in the colonies, where air-pumps and nice
machinery can with difficulty be either obtained or managed. On many
sugar estates there is a fall of water, which would supply the barometer without the trouble of pumping. The tube d c need not be perpendicular, provided it be longer in proportion to its obliquity; and it may be very small;
some yards of common lead-pipe would answer.
When it was understood that, at common temperatures, water and many




186                      PNEUMATICS.
other liquids would be existing in the form of air, but for an atmospheric
pressure opposing the separation of the particles, it became of great importance in many of the arts, and for comprehending certain phenomena of
nature, to ascertain, very exactly, with respect to some of these liquids, the
degrees of expansive force belonging to them at different degrees of temperature. The subject, as far as water is concerned, has been investigated with great
care, and the following table shows part of the results. The left-hand column
marks temperature from 32 deg. of Fahrenheit's thermometer or the freezing point of water, to 290 deg.; and the right-hand column marks the corresponding degrees of force with which the water tends to expand into the
state of steam, and therefore also the force and density of the steam existing
in any vessel above the water which it contains. One ounce and a half
per square inch, is the force exerted on the sides of any containing vessel by
steam rising from freezing water, that is to say, the force with which freezing
water seeks to dilate into steam or air; and sixty pounds per inch is the force
of water heated to 290 deg. To many readers the idea will be quite new
and surprising, that if some freezing water, or even ice, be placed in a bladder
containing nothing else, and the bladder be then placed in the exhausted
receiver of an air-pump or other vacuum, the bladder will quickly be distended with steam strong enough to support one ounce and a half on every
square inch of its upper surface.
At 32~ force of steam is 1  oz. per inch.
50  23 oz.
100  - - - - -  13
150  - - - - -   4  lbs.
180  71 lbs.
212     - - - -  15  lbs.
250 -                30 lbs.
275  45  lbs.
290 -                60 lbs.
In this table we have to remark how much more rapidly the tendency
to become steam increases than the temperature of the water: for a rise of
eighteen degrees, viz., from 320 to 500, at the beginning of the scale, only
increases the dilating force one ounce and a quarter on the inch, while an
equal rise at the top of the scale, viz., from 272 deg. to 290 deg., increases
it Sfifteen pounds. It is most important to distinguish, however, between the
tendency to form  steam  at any temperature, and the bulk or quantity of
steam formed by a given quantity of heat; for the matter imperfectly understood has led to many vain schemes for improving the steam-enyine. The
truth is, that high-pressure steam is merely condensed steam, as high-pressure air is condensed air; in other words, the density of steam is greater,
or there must be more of it, exactly as its force is greater according to the
rule- explained at page 160: and the heat absorbed in its formation-being
proportioned to the quantity of steam in a given space, or the density, the
force and the cost in fuel have always nearly the same relation to each other.
In one pint of steam, at 290 deg., having an elastic force of sixty pounds on
the inch, there are very nearly four times as much water and four times as
much latent heat as in one pint of steam at 212 deg., which has a force of
fifteen pounds- on the inch; indeed, the one pint, at 290 deg., may be
changed into four pints at 212 deg., or the contrary, by merely changing
the degrees of pressure. It does not accord with the- plan of the present




STEAM  ENGINE.                          187
work to enter farther into the details of this subject, but they may be found
in various modern treatises.
Seeing the rapid increase of the expansive force in the preceding table,
we have the explanation of the terrible effects occasionally produced by confined water when overheated. A boiler of any kind completely closed, and
having no safety valve, if heated to a certain degree, will explode as if
charged with gunpowder. Unhappily the instances are too numerous where
the incautious or ignorant use of steam has produced explosions, which have
shattered buildings and destroyed whole neighborhoods.
The prodigious force generated by heating water would at first only surprise and terrify men, but in the course of time would lead inventive
minds to enquire whether it might not be turned to use; in other words,
whether some mechanism, to be called a steam-engine, might not be contrived to enable men to make it aid them in their various labours. To this
inquiry, after numerous less successful attempts, a glorious answer has been
given in our own day by the illustrious WATT;-and to this part of our
work it belongs to consider what he has accomplished, viz., to describe
The Steam-Engine,
which, in the few years since the genius of WATT carried it to its present
state of perfection, has changed the direction of human industry, and may
almost be said to have elevated man in the scale of existence.
The name of steam enyine, to most persons, brings the
idea of a machine of the most complex nature, and hence to   Fig. 104.
be understood only by those who will devote much time to
the study of it; but he who can understand a common pump,
may understand a steam-engine. It is in fact only a pump
in which the fluid passing through it is made to impel the
piston instead of being impelled by it, that is to say, in which
the fluid acts as the power instead of being the resistance.
It may be described simply as a strong barrel or cylinder c d,
with a closely fitting piston in it, here shown at b, which is  j
driven up and down by steam admitted alternately above and   a
below it from a suitable boiler; while to the end of the piston rod a, at which the whole force may be considered as
concentrated, there is attached in any convenient way the
work which is to be performed. The power of the engine is of course proportioned to the size or area of the piston, on which the steam acts with a
force, according to its density, of from 15 to 100 or more pounds to each
square inch. In some of the Cornish mines there are cylinders and pistons
of more than ninety inches in diameter, on which the pressure of the steam
equals the efforts of six hundred horses.
In one place this wonderful piston-rod may be seen acting upon the end
of a great vibrating beam, to the other end of which capacious water-pumps
are attached, whose motion causes almost a river to gush up from thie bowels
of the earth. In another place, it is seen working a crank, and urging complicated machinery.  One steam-engine four miles from London is at the
same instant filling all the water reservoirs, and baths, and fountains of the
finest quarter of the town. One engine stretching long arms over a great
barrack or manufactory, keeps in one quarter, thousands of spinning-wheels
in motion, while in another it is carding the material of the thread, and in
another weaving the cloth.  In like manner, one steam engine, in a great




188                       P N EUMATICS.
metropolitan brewery, may be seen at the same time grinding the malt, pulling up supplies of all kinds from wagons around the building, pumping cold
water into some of the coppers, sending the boiling wort from others up to
lofty cooling-pans, over which it is turning the fans, perhaps also working
the mash-tub, drawing water from the deep wells under ground, and loading
the drays-in a word, performing the offices of a hundred hands.  Again,
there are manufactories where this resistless power is seen with its mechanic
claws seizing masses of iron, and in a few minutes delivering them out again
pressed into thin sheets, or cut into bars and ribbons, as if the iron had become to it like soft clay in the hands of the potter. And now for some
years1 over nearly the whole world, has this wonderful piston-rod, working at
its crank, been turning the paddle-wheels of innumerable steam-boats, thereby sitting at defiance the violence of the winds and waves, and the currents
of the fleetest rivers, while it carries men and civilization into the remote
recesses of all the great continents. To wherever a river leads, the region,
although concealed perhaps since the beginning of the world, is now by the
steam-engine called as it were from its solitude, to form a part of the great
garden which civilized man is beautifying.-Such are a few of the prodigies
which this machine is already performing, and every day is witnessing new
applications of its utility.
The following account of the parts of the steam-engine is intended, without entering into minute practical details, still fully to explain the principle
or general nature of the machine.  It should serve to render very interesting
to an attentive reader, a visit to any place where a steam-engine is in use:
and it should make evident the folly of many of the modern'schemes for
improving the engine. To avoid complexity in the figure, the parts which
the reader can easily conceive are not here sketched.
Fig. 105.
ii                  9, C
I   Io      t
A.                t/
1st. The part which first claims attention is the great barrel c d, already
spoken of as the centre or main portion of the machine, in which the piston
P is moved up and down by the action of steam entering alternately above
and below it, through the pipes e c and e d. The barrel or cylinder is
bored with extreme accuracy, and the piston is padded round its edge with
hemp or other soft material so as to be perfeetly air or steam-tight. Lately




STEAM  ENGINE.                            189
pistons have been made altogether of metal, and, in some cases, from working with less friction, these answer even better than the others.-2d. The
next part to be mentioned is the boiler B, which is made of suitable size and
strength.-3d. The steam passes from the boiler along the pipe to e, and there
by any suitable cock or valves, workell by the engine itself, is directed alternately to the upper and under part of the barrel; and while it is entering to
press on one side of the piston, the waste steam is allowed to escape from
the other side, either to the atmosphere, for high-pressure engines, or into4th, the condenser at C, for those of low-pressure; the condenser being
always kept' at a low temperature by cold water running into it and pumped
out again by the piston k.-5th. The supply of steam from the boiler to the
cylinder is regulated by a valve placed somewhere in the pipe B e, and made
obedient to what is called-6th, the governor, a contrivance not represented
here, but already described at page 52, to illustrate centrifugal force.  We
may recall it by saying, that it consists of two balls hanging by jointed rods
like the legs of a tongs, from opposite sides of an upright spindle, which is
made to revolve by connection with some turning part of the machinery:when the spindle turns at all faster than with the desired speed, the balls fly
more apart and are made to affect the steam valve so as to narrow the passage;
and, on the contrary, when it turns more slowly than is desired, they collapse,
and by so doing open the valve wider.-7th. The supply of water to the
boiler is regulated by a float on the surface of the water in the boiler; which
float, on descending to a certain point, by reason of the consumption of
water, opens the valve to admit more. —8th. There is a safety valve in the
boiler, viz., a well-fitted flap or stopper, held against an'opening by a weight,
but loaded so as to open before danger can arise from the over-heating of the
water.-9th. The rapidity of the combustion, or force of the fire, is exactly
regulated by the state of the boiler and the wants of the machine, thus:there is a large open tube (not represented here) rising from near the bottom
of the boiler, through its top, to the height of several feet, and when the
water in the boiler is too hot, and the steam therefore too strong, part of the
water is pressed up into this tube, and by the agency of a float which rests
on its surface, it shuts the chimney valve or damy2er: the draught is then
diminished and the fuel saved, until a brisker fire is again required.-10th.
In this figure a i g marks the place of the great beam, turning on an axis
at i, and transmitting the force of the piston to the remote machinery.
When the object is to raise water, the pump rods are simply connected with
the end g of the beam; but when any rotatory motion is wanted, the end g
is made to turn.-11th. A crank 1 n by the rod g 1; and uniformity of
motion is obtained by the influence of-12th, the great fly wheel m, fixed
to the axis of the crank.
The smallest and simplest steam engine, and therefore the cheapest, is
that called the high-pressure engine.  In it steam is used of great density,
and consequently of great force, as of 50 lbs. or more to the inch; and while
the fresh steam is admitted to press on one side of the piston, the steam
which has already worked, is allowed to escape, or is driven out to the air,
from the other side.  The atmospheric resistance to the issue of the steam
diminishes the working force of the piston just 15 lbs. per inch. The simplicity of this form of engine recommends it, but the danger of a large
boiler of overheated water, always, like inflamed gunpowder, seeking to
escape, has, by numberless fatal accidents, been proved to be so great, that
the use of such an engine is lirhited to certain situations. Notwithstanding
all the ingenious securities recently contrived against the danger, and which




190                            PNEU'MATIC S
will suffice for small engines, such as are required for steam carriages, the
high-pressure engine is not employed in a single English passage-vessel.*
In the low-pressure engine, the steam is used generally of force not
exceeding 20 lbs. on the inch, which force is only 5 lbs. more than the atmospheric pressure, and is insufficient to birst a common boiler, or to do serious
mischief:J but as the interior of the low-pressure engine is kept in a state of
* In this country, also, what is called the low pressure engine, is at present employed in
all the steamboats on our eastern waters, but we have reason to believe that few, if any, of
them are worked with a less pressure than 25 lbs. on the square inch, and we have seen
many of them worked with a much higher peressure. It is stated, that the steam guage on
board the Pulaski, just before the fatal explosion, indicated a pressure of 26 lbs., which was
considered by the engineera safe working force. The.British Ocean Steamers work with a
pressure of only 31 to 4~ lbs. to the square inch. On our western waters, the low pressure
engine has been discarded in favour of the high pressure.                 Am. ED.
t Our author must be understood as saying, only, that explosions do not take place in low
pressure engines whilst working under a pressure of only 20 lbs. to the squnare inch, But
from various circumstances, the elastic force of the steam in a low pressure engine may be
greatly increased, and instead of its ordinary power of 20 lbs., it may acquire one of 100 or
200 lbs., and if the boiler, as is often the case, is unable to bear this pressure, it will burst.
in this way explosions have repeatedly taken place in low pressure engines. Several of
these will be found related in an interesting memoir by M. Arago, originally published in
the " Annuaire du Bureau des Longitudes," and which has been translated and published in
that useful periodical, " The Journal of the Franklin institute of Pennsylvania." M. Arago,
in this memoir, remarks, " I ought not to conclude so long a paper on the subject of the
explosion of steam boilers, without explaining why I have not separated the examples of
the explosion of high pressure boilers from those of the low pressure; it is because I think
there is no reason to make such distinction. Every one must, in fact, admit, that at the
time of an explosion, all boilers contain high pressure steam."
The belief that low pressure boilers are not liable to burst, or do mischief, has led, as
has been already observed, to their exclusive use in passage vessels; this belief, however,
is founded in error. M. Arago, in the memoir already quoted, observes, "' it does not
appear established by any means, that explosions take place more frequently in high than
in low pressure boilers; the contrary has been maintained by different engineers, among
whom may be classed Messrs. Perkins, Oliver Evans, &c." Indeed, a little reflection will
show that high pressure boilers ought not to be more liable to explosion than the low.
Boilers may be made of either iron or copper of sufficient strength to resist a much greater
force than that of the steam ever employed in high pressure engines. Now boilers are
always constructed of a strength proportionate to the pressure they are to sustain. Thus,
in a low pressure engine working with a force of 20 lbs., the boiler is made of strength
calculated to support from 3 to 5 times that pressure; in a high pressure engine destined
to work with a pressure of 150 lbs., the boiler is constructed so as to resist from 3 to 5 times
that pressure. It will immediately be asked, why cannot the boiler of a low pressure
engine be made of the same strength as if they were for a high pressure engine? In answer
to this it may be remarked, that independent of many difficulties to be overcome before we
can exceed certain limits in the thickness of boilers, the weight and cost of the low presure
engine are already so great that it would be impossible to persuade owners of steamboats
to incur any addition to these particulars-and even would they do so, perfect safety would
still not thus be obtained. We have already observed, that at the time of an explosion, all
boilers contain high pressure steam, and as we know no limits to the force of this steam,
however strong the boiler may be, it may burst, unless this be prevented by other means.
It was long ago known,. that if a vessel, however strong it might be, containing water,
be placed over a fire, it will burst, unless an opening is provided for the escape of -the
steam as fast as produced. The temperature which will cause the rending of a vessel,
must depend upon its form and dimensions, and upon the tenacity and thickness of the
material of which it is made. If we could keep the heat of our furnace below a certain
limit, no other precaution would be required to prevent explosions. But it is evident that
this cannot be done, we must, therefore, resort to some other expedient, and the safety
valve, invented by Papin, would seem to answer this purpose. We must be allowed to
anticipate the subject a little, in order to explain the nature of this valve.
The safety valve consists of a hole, say of an inch square, made in the upper part of the
boiler, upon which is placed a metal plate loaded with a certain weight. It is evident
that the hole will remain closed as long as the pressure of the steam within the boiler is
Less that the weight of the valve, together with that of the atmosphere, upon the square
inch, and that as soon as the pressure within shall exceed this, the valve will be raised
and give a free vent to the steam.
It would lead us too far to explain how it has happened that so simple and apparently




STEAM ENGINE.                                  191
vacuum, except where the steam is acting, the whole pressure of 20 lbs. is
made available, and the engine has the -same power, if of equal size, as a,
high pressure engine working with steam of 35 lbs. on the inch    The required vacuum is preserved by means of a separate vessel or box, represented
at C. called the condenser, into which cold water is constantly running to
condense the steam, and is afterwards pumped out with the condensed
steam, and with any little air that may have entered: the pump is represented at k in the figure.  Steam, on coming into contact with a cold body,
is condensed almost with the rapidity of an explosion; and therefore the
instant that the opened valves make a communication between the cold condenser and any part of'the engine containing steam, this rushes to the condenser, and becomes water, leaving a vacuum behind.   The great merit of
Mr. Watt was in the contrivance of this separate condenser, for, until his
time, cold water had always been thrown directly into the working cylinder,
cooling it so much, that twice or thrice its fill of steam was destroyed at
each stroke to warm it again before it could work. This single change
saved three-fourths of the quantity of fuel formerly expended.
Before Watt's day, the only steam engine in use was a rude single-stroke
engine as it was called, in which steam, admitted under the piston, allowed
the weight of the pump-rods at the far end of the beam to lift the piston,
and the steam being then condensed so as to leave a vacuum in the cylinder,
the pressure of the atmosphere pushed the piston down to do its work: on
this last account the engine was also called an.atmospheric engine.  It was
used almost solely for pumping water; but it wasted so much fuel, from
causes.of which the chief is mentioned in the last paragraph, that the
expense was not much less than that of employing horses.
In the atmospheric engine, the steam which lifted the piston against the
atmospheric pressure, required to be at least as strong as that pressure, to the
very end of the stroke.   Another of Watt's great improvements was, his
efficient means has not always proved efficacious; since these causes are various and many
of them as yet not perfectly understood.
Those who wish to investigate the subject, we refer to the memoir of M. Arago, and
to the report of a committee of the Franklin Institute, who have collected an account of
all the explosions in this country, and who have instituted a very interesting series of experiments, in order to examine into the causes of the explosion of steam boilers, and devise
means for its prevention. This report has been published in the Journal of the Institute.
We must not omit, however, to mention that when a low pressure boiler does explode,
it has been found to produce greater destruction than a high pressure one, in consequence
of the great size, and, therefore, larger quantity of water contained in the former. It
may, perhaps, be supposed that the steam from a high pressure engine would scald more
severely than that from a low pressure one. This, however, is not the fact: on the contrary,
whilst the steam issuing from a low pressure engine scalds at all moderate distances from
the boiler, that from a high pressure one scalds only at certain distances. Thus the hand
may be placed an inch from an aperture in a high pressure engine without any inconvenience being felt; at a greater distance, however, it will scald most severely. A friend has
informed us that he has placed his hand within an inch of the aperture in a boiler from
which the steam was issuing at a time when the force of the steam within the boiler was
equal to 300 lbs. on the square inch, without feeling any inconvenience. Some interesting
experiments on this subject have been instituted by Peter Ewort, Esq., and an account of
which will be found in the fifth volume of the Journal of the Franklsin Institute.
It must not be supposed from any thing that we have said in this note, that explosions
of steam boilers cannot be prevented. But we may be allowed to quote on this subject
the following remarks of M. Arago. " No cause of explosion exists which cannot be avoided,
by means at once simple and within the reach of every one. As we should not trust firearms in the hands of children, so, I think, we should not trust the direction of a steam
engine to a man either unskilful, without experience, or wanting in intelligence. It is a
mistaken idea, that because steam engines usually move without attention to them, such
attention is not required; Watt contended strongly against this error."  AM. ED.




192                       PNEUMATICS.
excluding altogether the air from his machine, by doing which he not only
avoided the cooling effect of the air, but was at liberty to shut off the steam,
as it is expressed, or to stop the supply for each stroke before the cylinder
was full, and then to make the farther expansion of the quantity admitted
impel the piston to the end of the stroke. This principle of causing the mere
expansion of steam to do work was afterwards carried to a great extent by
Messrs. Hornblower, Woolfe, and others, who constructed engines with two
barrels, in the first and smaller of wliich, the steam was made to act in its
dense or strong state, as it issued from the boiler, and when it had finished
a stroke there, instead of being at once sent useless to the condenser, it was
admitted to a larger piston, which it moved by its continued expansion alone:
-the same steam thus doing double work or more. All the advantages of
the two cylinders, however, are obtainable from the single cylinder with its
condenser, as now used in most of the Cornish mines. Steam of about 60 lbs.
pressure on the inch is admitted to the cylinder, until the piston is driven
nearly one-third of its way, and the valve being then shut, the same steam is
left to finish the stroke by its expansion.  The pressure of the expanding
steam gradually diminishes, it is true, in proportion as the volume increases:
but in pumping water there is a great saving of time, from having the power
more intense at the beginiiing of the stroke, when the vast mass of water and
machinery has first to be put into motion. Steam, while doubling its volume
by mere expansion, will do about two-thirds as much work as while originally rising from the boiler, and by every subsequent doubling it might do
as much as by the first: the increasing size of the cylinder, however, and
increased friction, confine this mode of using it to.narrow limits.
It might be supposed that high pressure engines without condensers would
be comparatively wasteful, because in them the steam which has acted must
be driven out of the cylinder against the powerful resistance of the atmosphere, while in the low pressure engine it has instant access to the condenser, and leaves effective the whole pressure of the fresh steam on the
opposite side of the piston. But as in the low pressure engine, nearly half
the power of the steam is expended in overcoming the friction and other
impediments of the numerous parts, while in that of high pressure, the parts
are so much fewer, and the piston is so much smaller in proportion to the
force acting upon it, that the loss from friction is often less than a fourth or
even a sixth of the steam power, although the resistance of the air is to be
overcome by the high pressure engine, still there is often a saving on the
whole. The saving becomes very considerable if the steam be allowed to
act by its expansion also, as described in the last paragraph.
From misapprehension of the law of increase of force by increase of heat
in water, explained by the table at page 186, some exceedingly false conclusions have been drawn and acted upon at great expense (as lately by Mr.
Perkins) in attempts to make engines work with an excessively high pressure.  Besides making the error now alluded to, and others, Mr. Perkins
overlooked the fact, that we possess no material for cylinders and pistons
strong enough to bear the contemplated pressure and friction even for a
moderate time.  Perhaps more striking examples could not be adduced of
the absurdities into which even highly ingenious men may fall, when not
sufficiently acquainted with the general truths of nature on which the arts
which occupy them are founded, than in the history of supposed inventions
and improvements connected with the steam-engine.
The fertile genius of James Watt did not stop at the accomplishment of the
two or three important particulars described above, butbthroughout the whole




EXPLOSION.                             193
detail of the component parts, and of the various applications of the engine,
he contrived miracles of simplicity and usefulness.  We should exceed the
prescribed bounds of this work by entering more minutely into the subject;
but we may remark that, in the present perfect state of the engine, it appears
a thing almost endowed with intelligence. It regulates with perfect accuracy
and uniformity the number of its strokes in a given time, counting or recording them, moreover, to tell how much work it has done, as a clock records
the beats of its pendulum;-it regulates the quantity of steam admitted to
work;-the briskness of the fire;-the supply of water to the boiler;-the
supply of.coals to the fire;-it opens and shuts its valves with absolute precision as to time and manner; —it oils its joints; —it takes out any air which
may accidentally enter into parts which should be vacuous;-and when any
thing goes wrong which it cannot of itself rectify, it warns its attendants by
ringing a bell:-yet with all these talents and qualities, and even when exerting the force of hundreds of horses, it is obedient to the hand of a child;its aliment is coal, wood, charcoal, or other combustible; —it'consumes none
while idle; —it never tires, and wants no sleep; —it is not subject to malady
when originally well made; and only refuses to work when worn out with
age; —it is equally active in all climates, and will do work of any kind; —it
is a water-pumper, a miner, a sailor, a cotton-spinner, a weaver, a blacksmith,
a miller, &c., &c.; and a small engine in the character of a steam pony, may
be seen'draging after it on a rail-road a hundred tons of'merchandize, or a
regiment of soldiers, with thrice the speed of our fleetest horse coaches.  It
is the king of machines, and a permanent realization of the Genii of Eastern
fable, submitting supernatural powers to-the command of man.
We need not wonder that the inventor of an engine having such qualities,
should be deemed deserving of the highest honours from his fellow-men. In
November, 1825, a public meeting was called, to vote a monument to WATT,
then not long deceased; and the most distinguished men of the empire, of
all parties, philosophers and statesmen, met to vie with each other in speaking his praise. Perhaps a series of such eloquent discourses has rarely been
pronounced at one time; but perhaps in the progress of the arts of civilization there can rarely be offered such motive and occasion.  The common
voice of that assembly scarcely exaggerated, when attributing to WATT'S
genius and perseverance that increase of our national commerce and riches,
which had enabled free Britian, single-handed, at an extraordinary crisis of
human affairs, to contend with Europe combined against her, and at last to
triumph, so as to secure her own happy destinies, and probably much to
influence those of the human race.
As science and the twin sister art are making constant advances, who
shall say that even the steam-engine, perfect as we have described it, forms
the limit to human discovery of mighty yet obedient force? It is true that
the nature of steam, and the laws of its formation and action, are now so
well understood, that the intelligent engineer no more hopes for great
improvement in steam-engines, than he hopes for it in the mode of using
a waterfall to turn a mill; but still there are kindred regions of nature left
almost unexplored.  We shall have occasion to make a remark on this
subject in our chapter on the nature of heat.
The explosion of gunpowder and of all fulminating mixtures bears so strong
an analogy to the phenomenon of the formation of steam, that the mind
may advantageously contemplate the subject in this place.
The ingredients of. which gunpowder is formed are chiefly substances
13




194                       PNEUMATICS.
which, when separate, exist, at any common temperature, in the form of
air; and the combustion sets them loose, with a production of intense heat,
causing an increase of volume which is instantaneous, and almost irresistible.
By experiment and mathematical deduction, it appears that the exploding
particles begin to separate from each other with a velocity as if ten thousand volumes of air had been condensed into one: and this explains the
corresponding force and swiftness with which a bullet is propelled.
All the fulminating metals are chiefly combinations of the like substances with the metals; and the ingredients are held together by so slight
a tie, that a little friction or elevation of,temperature disunites them so as
to produce the explosion.
The escape of condensed air from the chamber of an air-gun, is a species
of explosion; but is very gentle compared with the shock of discharged
gunpowder.
It has lately been shown that a gun-barrel may be connected with a highpressure steam-boiler, in the same manner as with a chamber of condensed
air; and as the steam may be supplied as long as water remains.-in the
boiler, if bullets be allowed to fall into the barrel fast enough, a hundred or
more may be thrown out every minute, with the same force and precision
as if each issued from a common fire-arm. The rapid succession resembles
the issue of water from a jet pipe; and if such an engine were used in a
field of battle, its barrel of death, made to point gradually along a line of
men, would mow them down like corn-stalks before the scythe-none could
escape.  The horrible idea and proposal have been excused by saying, that
to prove the possibility of such carnage must have the effect of putting an
end to war altogether.
The invention of gunpowder, with the consequent change of military
tactics, because it gave to a handful of men possessing it the mastery over
thousands who had it not, was hailed by the philosophers of the day as a
certain security against the relapse of civilized mankind into such a state of
barbarism as followed the irruption into Europe of the Goths and Vandals:none but well-instructed and disciplined armies could thenjenter a European
kingdom.  This consideration, however, has lost its interest, since the invention of printing, and other changes in society, have afforded still better
and more humane securities.
Besides the interesting instances above cited of the pressure of the atmosphere determining whether certain substances shall or shall not have
the form of air, there are others that deserve mention, where the effect is
modified by the mutual attraction of substances.
The pressure of the atmosphere at the surface of the earth keeps a certain quantity of air in combination with water, so as to form part of the
liquid mass. This air re-appears at once on taking off the pressure. If we
place a glass of water under the receiver of an air-pump and then exhaust
this, the water is soon crowded with bubbles of air, seen adhering to the
glass all round, or rising through the water. This admixture of air in
water is necessary to the life of fishes. It is driven off by boiling, and hence
the vapid taste of water that has recently been boiled.
In the making of beer, wine, and other fermented liquors, there is formed,
during the fermentation, a large quantity of the substance called carbonic
acid.  Much of it flies off in its usual form of gas, but, because of the pressure of the atmosphere, much still remains in union.with the liquid. On




PRESSURE AFFECTING TEMPERATURE.   195
removing this pressure suddenly, the liquid appears almost to boil, as' when
a glass of warm beer is placed in the air-pump vacuum.
A degree of pressure still greater than that of the atmosphere keeps a proportionably larger quantity of this carbonic acid in liquid combination; as in
bottled porter or sparkling champagne before the cork is drawn; but as soon
as the compression maintained by the cork is removed, the gas escapes, causing the thin champagne to sparkle, and the more viscid beer, which retains
the little bubbles as they rise, to be covered with froth. After the sparkling or frothing has ceased under the atmospheric pressure, the phenomenon
may be renewed by placing the glass in the air-pump receiver.
Carbonic acid so readily becomes liquid when its attraction for water assists
the compression, that enough of it may be united with water to make a pint
become a pint and a half. The soda water, or aerated water, now so generally used as a drink in warm weather, is water with several times its bulk
of carbonic acid forced into it by pressure; and a part of this is seen escaping
always at the instant of the confining cork being drawn.
Carbonic acid forms nearly half of the substance of marble or lime-stone.
When an acid with stronger attraction, as vinegar or sulphuric acid, is
poured upon marble, it dispossesses the carbonic acid, and unites itself with
the pure lime. The carbonic acid in rising, constitutes the effervescence which
then appears. Carbonic acid, for the manufacture of the common soda water
and other aerated drinks, is obtained in this way.
Many mineral waters contain carbonic acid, which remains in tranquil
combination, while the water is bearing a certain pressure under ground, but
which in part escapes as soon as the water issues to the air and only the
atmos1pheric pressure remains: such waters are called sparkling waters.
The reason that champagne and aerated waters are so cool when first
decanted is, that the carbonic acid, in assuming its gaseous form, absorbs
as latent heat, a large proportion of the heat which was previously existing
in the liquid.
The atmospheric pressure, by making the density of the air in any place
dependant upon the height of the place above the level of the sea, causes
corresponding differences of temperature.
The explanation of this is simple. If a gallon of air, at the surface of the
earth, contain a certain quantity of heat, this must be diffused equally through
the space of the gallon; but if the air be then compressed into one-tenth of
the bulk, there will be ten times as much heat in that tenth as there was before; the increase affecting the thermometer to an extent modified by circumstances explained in a future part of this work. In like manner, if by taking
off pressure, the gallon be made to dilate to ten gallons, the heat will be in
the same degree diffused, and any one part will be colder than before. It is
known that air may be so much compressed under the piston of a syringe,
that the heat in it, similarly concentrated, becomes intense enough to inflame
tinder attached to the bottom of the piston;-this means, under the name of
the match-syringe, being in common use for obtaining an instantaneous light.
Now, for the reason here explained, the air near the surface of the earth,
forming the bottom of the atmosphere, because condensed by the weight of
the air above it, is much warmer than if it were suddenly carried higher up,
to where, from the pressure being less, it would be more expanded or thin.
In many cases the height.of mountains may be estimated by the difference of
temperature observed at the bottom and at the top.  While a thermometer




196                       PNEUMATICS.
stands at 60~ at the bottom of St. Paul's Cathedral, in London, another
marks only 580 at the top of the dome; and in the lofty ascent of a balloon,
the thermometer soon falls to the freezing point and below it, the cold to
the aeronaut becoming almost insupportable.
In every part of the earth,.at a certain elevation in the atmosphere, different according to the latitude or proximity to the equator, the thermometer
never rises above the freezing point,-and this limit in tlte atmosphere is
called the line or level of perpetual congelation. In Norway it is at five
thousand feet above the level of the sea; in Switzerland at six thousand five
hundred; in Spain and Italy at seven thousand; farther south, at Teneriffe,
at nine thousand; directly under the sun, as in central Africa, and among the
Andes in America, it is about fourteen thousand. We see therefore why the
snow-capt mountains are not the tenants only of high northern and southern
latitudes. In this effect of elevation which renders many of the tropical
regions of the earth not only tolerable abodes for man, but as suitable as any
others, contrary to the opinion of the ancient philosophers of Europe, who
accounted them by reason of the great heat, an everlasting barrier, as regarded man, between the northern and southern hemispheres.  Much of the
tropical land of America is so raised, that it rivals, as to agreeable temperature, even a European climate; while the lightness and purity of the air, and
the brightness of the sun, add delightfully to its charms. The vast expanse
of table land forming the empire of Mexico is of this kind, enjoying the
immediate proximity of the sun, and yet, by its elevation of seven thousand
feet above the level of the ocean. possessing the most healthful freshness.
The land in many parts has the fertility of a cultivated garden, and can
produce naturally most of the riches which vegetation offers over the diversified face of the globe. The plains of Columbia in South America, and
indeed all along the ridge of the Andes, are similarly circumstanced. The
contrast is very striking, after sailing a thousand miles up the level river
Magdelena, in a heat scarcely equalled on the plains of India, at once to
climb to the table-land above, where Savte Fe de Bogota, the capital of the
republic, is seen smiling over interminable plains, that bear the livery of the
fairest fields of Europe.
Persons not understanding the law which we are now illustrating, will
express surprise that wind or air blowing down upon them from a snowclad mountain, should still be warm and temperate. The truth is, that there
is just as much heat combined with an ounce of the air on the mountain top
as in the valley; but above, the heat is diffused through a space perhaps
twice as great as when below, and, therefore, is less sensible. KIt may be the
same air which sweeps along as a warm gale on a plain at the foot of a mountain,-which then rises and freezes water on the summit-and which in an
hour after, or less, is playing among the flowers of another valley, as warm
and genial as before,
As the temperature in different parts of the atmosphere depends thus
upon the rarity of the air, and therefore upon the height, the vegetable productions of each distinct region or elevation are of a distinct character; and
many other peculiarities of place and climate acknowledge the same cause.
Because the atmospheric pressure determines the temperature of the air in
different situations, as now explained, it has also a corresponding influence
upon the'state of aerial humidity, which is modified by the temperature.
It was explained at page 184, that water and other liquids under a vacuum,




PRESSURE AFFECTING TEMPERATURE.  197
rise in the form of air or vapour with force, and in quantity having a strict
relation to the temperature-heat being in fact the cause of their rising; and
the table at page 186 exhibits the force, and therefore the density of watery
vapour corresponding to some certain temperatures.  Now it is a remarkable
circumstance, that vapour in the same quantity and of equal tension rises
from any liquid, whether placed under the pressure of air, or under a vacuum;
only through a space containing air it diffuses itself more slowly than if the
air were not present. As regards the former case, it was for a long time
supposed that the air dissolved a liquid as a liquid dissolves a salt: but it
now appears that there is merely a mechanical mixture of the two. If the
vapour, while rising from a liquid, has not a tension or elastic force equal
to the pressure of the atmosphere the process is tranquil, and is called
evaporation, and it goes on only as the vapour can diffuse itself among the
particles of the air, and therefore slowly in air perfectly quiescent, but quicker
as the air is moving more, or as the density of the air is less. But when
the vapour, owing to a greater heat, is strong enough to overcome the atmospheric pressure of fifteen pounds per inch, and the weight of a certain quantity
of liquid over it, the phenomena of boiling arises as already described.
For the reason now explained, the air of our atmosphere contains diffused
through it a large quantity of invisible aeriform water; and if there were no
intestine motion, and no changes of temperature in the atmosphere, the
quantity of water would soon everywhere reach a maximum, or would be
the greatest that the temperature of the place could support; instead of this,
however, from a variety of causes to be explained below, the air is moving
about constantly as winds, and the local temperatures are ever fluctuating,
and when the temperature is lowered, in situations where a maximum  of
watery vapour is present, part of this is instantly reduced to the state of
water again, and appears, according to circumstance, in the form of mist,
rain, snow or hail; while to supply material for these phenomena, evaporation is going on wherever, over water, there is not a maximum of vapour
in the air. These opposing operations of evaporation and condensation keep
up that constant circulation of moisture which is the life of nature.
When a given quantity of water assumes the aeriform state, it contains the
same quantity of latent heat in all cases, whether rising, for instance, from
a boiling caldron, or from the surface of the lake.  Hence we see why evaporation is so cooling a process to any liquid or moistened solid from which it
is arising: and as we have already shown that a rapid passing of dry air, or
the substance being placed in a vacuum, quickens evaporation, we now see
why both of these conditions accelerate the cooling. Wet linen placed in a
strong wind, which does not contain a maximum of moisture, becomes dry
almost immediately; a bottle of wine covered with a wet cloth and suspended
in a current of air, as is practised in warm climates to prepare wine for the
table is quickly cooled; mats hung around the walls of houses in India, and
frequently wetted through the day, preserve a delightful freshness in the
apartments. Sprinkling water or vinegar over a hot sick room cools and
refreshes it; and watering the streets of a city moderates in them the intensity of summer heat. In warm climates water is cooled for drinking by
being put into vessels so porous that the external surface is always moist,
the vessel being then suspended in a current of air, or during a calm being
made to vibrate in the manner of a pendulum. Again, the rapidity of evaporation from water under the exhausted receiver of an air-pump, and particularly when some other substance which powerfully absorbs watery vapour is
included in the receiver, is so rapid, and carries off the heat so quickly, that




198                       PNEUMATICS.
the mass of water freezes before much of it has been carried away. This
process is used for making ice in India.
It is partly because air saturated with moisture, -that is to say, having
as much water diffused in it as can be supported in the invisible. or aeriform
state of the existing temperature,-lets fall a part on any reduction of the
temperature, that air which as a portion of the atmosphere, has been heated
by the sun during the day, and has received such moisture, lets it fall again
during the night, and exhibits the night fogs of certain seasons, which float
upon the surface of the earth, until again acted upon by the beams of the
next morning's sun.  Fog, when farther condensed, by groups of the minute
particles uniting, forms rain; and rain when cooled becomes snow or hail.
The quantity of dew which falls at night is influenced by the quantity of
moisture taken up by the atmosphere during the heat of the day; and the
immediate cause of the dew is, as was ingeniously proved by Dr. Wells,
some years ago, that the temperature of the objects on which it settles has
become lower during the night than that of the air around, and than is
required to maintain in the invisible state, the moisture in the surrounding
atmosphere. There is a tendency in heat to diffuse itself uniformly among
bodies, by a constant radiation from one to another, rapid in proportion to
the differences of temperature, and which, if continued, would reduce all to
the same degree.  The earth, therefore, during the day, receives radiated
heat from the sun, and becomes comparatively hot, and during the night it
gives out heat again by radiation towards the sky, from which there is little
or no return.  When there are clouds in the atmosphere at night, they
receive the heat darted upwards from the bodies on the earth's surface, and
they radiate heat back, becoming thus, as it were, a clothing to maintain the
warmth of the earth beneath thern,-and on cloudy nights there is no dew,but with a clear sky, the heat radiated upwards, darts into boundless space,
and is lost altogether to the objects which emitted it.  These objects, therefore, which during the day had the same, or even a higher temperature than
the atmosphere around, now become colder, and the aeriform water which
comes in contact with them is condensed, and forms what we call dew.  This
beautiful provision of nature supplies the necessary moisture to vegetables
during seasons when rain is deficient. Dew on very cold objects freezes as
it settles, and is then called hoar frost.  A phenomenon which may be
classed with dew, is the perspiration, as it is vulgarly called, of massive
walls and furniture, occurring on the sudden setting in of warm weather, or
on the occasion of a warm moist air of higher temperature than the walls
being suddenly introduced, as when a crowd assembles in a cold church:the wall or other object then, from not having yet acquired the temperature
of the surrounding air, condenses upon itself a copious deposition of the
atmospheric moisture.  For a similar reason a bottle of wine brought from a
cold cellar or from an ice-pail, into a room with company, is soon covered
with thick moisture or dew; as are the glasses also into which the wine is
poured. It is another phenomenon of the same kind when we see the
moisture of warm breath condensed on any cold polished surface, as on a
mirror's face, or on the glasses of a carriage shut up, or on the windows of
a room in winter, when the surface is very cold, the moisture being frozen
with the appearance of beautiful aboresence.
Many instruments have been contrived, with the name of hygrometers,
for indicating the quantity of water in the atmosphere. A prepared human
hair is the essential part of one of the best of those formerly hsed; the
lengthening or shortening of the hair, according to the quantity of moisture




PRESSURE AFFECTING MOISTU4E.                          199
around it, being caused to move an index like that of a wheel-barometer; to
mark the degrees. This, however, and other common hygrometers, are only
philosophical toys; but Mr. Daniel (see his excellent work, entitled Meteorological Essays) has lately given to the philosophical world a correct and
simple instrument for the purpose, depending on the principle explained
above,-that whenever the temperature of a body in the atmosphere is
reduced below that at which the quantity of watery vapour in the air around
it can be maintained in the aeriform or invisible state, dew forms on the
body. His apparatus consists of a bulb of glass, which can be cooled to any
desired degree from being connected with another bulb enveloped in an
evaporating liquid; and when moisture begins visibly to settle upon the first,
its temperature is exhibited on a thermometer enclosed within it; and the
proportion of water mixed with the air around is then, as indicated by the
table, partially copied here, at page 186.
A great fall of the barometer marks a diminished pressure in the atmosphere around, with a consequent dilatation of the air and fall of temperature,
as explained a few pages back; and if the air at such a time hold a inaximum of moisture, a part of this must become visible as fog or rain.  Thus
a fall of the barometer, a fall of temperature, and a fall of rain, often occur
as associated phenomena.
Illustrating this by experiment, we find, that on the extraction of air from
the receiver of an air-pump, a cloud of mist generally appears in it with the
first strokes of the piston:-the reason being that the still remaining air,
because cooled by the refraction, absorbs heat from  the vapor in combination with it, and renders the water visible.  The mist is then removed by
the subsequent action of the machine, or is re-dissolved when the usual
quantity of air is re-admitted.
We understand from this why rain happens so much more frequently
among mountains than on extended plains. When air saturated with moisture approaches the mountain ridge to rise over it, for every foot that it rises
it escapes from a degree of the pressure which it bore while lower down,
and in then dilating, it becomes colder, and lets fall part of its moisture.
It is the rain copiously thus produced in mountainous regions which constitutes the chief supply of their many rivers, and which, with periodical
changes of wind bringing more moisture, causes the extraordinary annual
-verfiowing of such rivers as the Nile, the Gauges, &c.
Those who have visited the Cape of Good Hope, will recollect a striking
phenomenon illustrative of our present subject, observed there when the wind
blows from the south-east. Beyond the city, as viewed from the bay, there
is a mountain of great elevation, called from its extended flat summit, the
Table Mountain. In general its rugged steeps are seen rising in a clear sky;
but when the south-east wind blows, the whole summit becomes enveloped
in a cloud of singular density and beauty. The inhabitants call the phenomenon the spreading of the table-cloth. The cloud does not appear to be at
rest on the hill, but to be constantly rolling onward; yet to the surprise of
the beholder, it never descends, for the snowy wreaths seem falling over the
precipice towards the town below, vanish completely before they reach it,
while others are formed on the other side to replace them. The reason of
the phenomena is this: The air constituting the wind from the south-east
having passed over the vast southern ocean, comes charged with as much
invisible moisture as its temperature can sustain.  In rising up the side of
the mountain it is rising in the atmosphere, and is therefore gradually escaping from a part of the pressure lately borne; and on attaining the summit it




200                       PNEU MATICS.
has- dilated so much, and has consequently become so much colder, that it
lets go part of its moisture. This then appears as the cloud just described;
but it no sooner falls over the edge of the mountain, and again descends in
the atmosphere to where it is pressed, and condensed, and heated as before,
than it is re-dissolved and disappears: —the magnificent apparition dwelling
only on the mountain top.
When the elevation to which moisture is suddenly carried is very great,
the fall of temperature is proportioned, and the separating water becomes
snow instead of rain. This phenomenon is remarkably illustrated by a great
Hero's fountain, used in one of the mines of Hungary; during the play of
which, the air in one place is so compressed, that on being suddenly released
it expands and cools enough to cause the moisture driven out with it to
appear, even in summer, as a shower of snow.
The foregoing reasoning explains why, along the sides of mountain ridges,
clouds are generally seen floating at a certain height only, and therefore in
horizontal strata. The water is separated from the air at a certain temperature, which is dependent on the height, and above that height the air is at
the time too dry and rare to have clouds.  Very lofty summits are always
seen much above the clouds, and the admirer of nature who climbs towards
them, may often contemplate the grand phenomena of the thunder-storm
far beneath his feet.  Teneriffe soars so sublimely, that the distant sailor
not unfrequently mistakes the line of clouds hanging around its sides for
the white streak which elsewhere indicates the cliffs and waves of the
sea-shore.
Fluid support orfloating in air.  (Read the Analysis, page 156.)
When it was explained under " Hydrostatics," that any body immersed in
a fluid has its downward tendency or weight resisted with exactly the force
which supported the quantity of the fluid previously occupying the same
space, and therefore that the body will sink or swim, according as it is
heavier or lighter than its bulk of the fluid, the reasoning was as applicable
to the case of a body immersed in an air or gas as in a liquid.
We hence see why a body weighed in an air appears lighter, by the exact
weight of its bulk of the air, than when weighed in an empty space or
vacuum;-and why, for the same reason, the jocular question, whether S
pound of lead or a pound of cork be the heavier, it is not truly answered by
saying that they are of equal weight; the cork being really the heavier, for
when balanced in air, bulky cork is more supported than dense lead. A
small weighing beam having attached to its opposite ends pieces of cork and
lead which equipoise in the air, if placed under the exhausted receiver of an
air-pump, quickly exhibits the cork preponderating.
As any liquid lighter than water, such as oil or spirits, on being set at
liberty under the surface of water, will rise, while any heavier liquid, such
as brine, syrup, or sulphuric acid, will sink; and in both cases with force
proportioned to the difference of specific gravities: so we find that in common air, a mass of hydrogen, or hotter air, ascends, because specifically
lighter; while oxygen, carbonic acid gas, or colder air, descends, because
specifically heavier. This truth is well exemplified in
The Balloon,
which is a thin light bag of varnished silk, generally shaped like a globe or
egg, and filled with a fluid lighter than common air. It is made sufficiently




FLOATING. —BALLOONS.                         201
large that the difference between its weight when filled and that of an equal
bulk of common air, may enable it to carry aloft the material of which it is
constructed, with the aeronauts, and their apparatus. It is in principle like
a bladder of oil immersed in water.  A globe of thirty-five feet diameter
has a capacity of nearly twenty-two thousand cubic feet. This quantity of
common air weighs about sixteen hundred pounds, and the same quantity
of hydrogen gas, of easily obtained purity, weighs only one-eighth as much
as two hundred pounds. Such a globe, therefore, being buoyed up, or
supported in common air, with a force of sixteen hundred pounds, while, if
filled with hydrogen, it only weighs two hundred, will carry up into the sky
fourteen hundred pounds of material and load.
The first balloon was exhibited by a man ignorant of what he was really
effecting. Seeing the clouds float high in the atmosphere, he thought that
if he could make a cloud and enclose it in a bag, it might rise and carry
him with it. Then, erroneously deeming smoke and a cloud the same, he
made a fire of green wood, wool, &c., and placed a great bagover it with the
mouth downwards to receive the smoke. He soon had the joy to see the
bag full, and, when set free, ascending; but he understood not that the
cause was the hot and dilated air within, which, being lighter than the surrounding air was buoyed up; while the visible parts of the smoke, which
chiefly engaged his attention, was really heavier than the air, and was an
impediment to his wishes.
This modification called the hot air or fire balloon, was afterwards better
understood, and was used by aeronauts, until the more commodious and less
dangerous modification, called the inflammable air balloon, or balloon of
hydrogen gas, was substituted.
Since the modern introduction of gas lights, the carburretted ihydrogen
prepared for them is generally employed for filling balloons.  It is considerably heavier than pure hydrogen, but is so much more readily obtained,
that aeronauts like better to make a larger balloon to suit it, than a smaller
one which obliges them to prepare the other.-A thin paper bag, filled with
the hot.air rising from a large lamp, is a miniature hot air or fire balloon;
and a common soap bubble, filled with hydrogen is a little inflammable air
balloon, which mounts with great rapidity.
There are, perhaps, few occasions on which a youth is more surprised and
delighted than when he first beholds a balloon sailing high in the bosom of
the air and bearing a human being to regions far beyond what the soaring
eagle has ever reached; while to the intrepid aeronaut himself, the scene of
a world displayed beneath him is unquestionably the grandest, except that
of the starry heavens, which mortal eye has ever compassed. To him even
wide spread London, the queen of the cities of the earth, and a little world
within itself, when viewed from a great elevation in the sky, appears but
as a dusky patch upon a map, with the far-famed Thames winding there
as a silvery line, and the magnificent temples and palaces scattered around
appearing but as darker points rising out of the general mist of buildings,
in which a million and a half of human beings reside.
The frst aeronautic expeditions astonished the world, and endless reveries
passed through mens' minds of important uses to which the new discovery
might be applied; but more mature reflection, and now frequent trials have
shown that the balloon, while furnishing philosophers with the opportunity
of making some observations in elevated regions of the atmosphere, is still
interesting chiefly as a philosophical toy. The French, under the Directory




202                      PNEUMATICS.
in 1796, attempted to use it as a military station, from which the position
and motions of an enemy might be descried: but the plan was eventually
abandoned. It has since been thought of as a means by which travellers
might obtain information while penetrating into unknown countries, like the
almost interlineable plain of Australasia. Although aeronauts, while aloft,
have the power of making the balloon rise farther by throwing out part of
the sand-ballast which they carry with them, or of making it descend by
opening a valve at the top, through which the hydrogen may escape, still
they have no power of producing a lateral motion.  The idea which yet
strongly excites the minds of some projectors, that by wings or other means,
a balloon may be directed in the sky nearly as a ship is directed on the sea,
is not much more reasonable than to suppose that an insect, suspended to a
huge block of wood, driven along at the rate of eight or ten miles an hour
by river torrent, should have power to stop or sail against the stream.  A
man in a balloom would generally have to resist or change a motion exceeding fifty miles in an hour.
A balloon wh'ich is only half full at the surface of the earth, becomes
quite full when it has risen three miles and a half, because, at that altitude,
air from below doubles its volume on account of the diminished pressure.
A balloon, therefore, if quite distended on first rising, must let air escape
as it ascends, or it will burst: this is true also of the drum of the human ear
under the same circumstances, and in a contrary way under the opposite
circumstances of descending in a diving-bell.
The downy seeds of plants seen floating about upon the winds of autumn
are not lighter than air, but have so much bulk and surface in proportion to
their weight, that the friction upon them of the moving air, is greater than
their weight, and carries them along.
A sheet of paper made in some degree to resemble a balloon, by its having
a little weight, representing the hanging car, attached by threads from its
angles, is often seen rising at a street corner, to the delight of the boy who
watches it. Its rise depends upon eddy winds or currents which the corner
produces.
The ascent of flame and smoke
in the atmosphere, affords other examples of a lighter fluid rising in a
heavier; for both these are merely hotter air rising in the midst of colder.
The phenomenon of flame is produced when a burning substance contains
some ingredient capable, on being heated, of assuming the form of air or gas,
which ingredient, or ascending, burns or combines with the oxygen of the
atmosphere, with intensity of action sufficient to produce a white heat.  It
is because charcoal and coke have nothing in them thus volatile, that they
burn without flame, appearing like red-hot stones. The flame of a lamp or
candle is merely the oil, wax, or tallow converted into gas, and allowed to
burn as it is disengaged and rises. The same gas obtained by heating the
oil, &c., in vessels which exclude the atmosphere, so as to prevent immediate
combustion, and from which tubes lead to suitable receptacles, is the common
oil-gas used for illumination.
Smoke consists of all the dust and visible particles which are separated
from the fuel without being burned, and are, moreover, light or minute
enough to be carried aloft by the rising current of heated air; but all that is
visible of smoke is really heavier than air, and soon falls again as powdered
chalk falls in water. In the receiver of an air-pump, where a candle has
be-n extinguished by.exhausting the air, the steam of smoke that continues




FLOATING.-FLAME AND SMOKE.                           203
to pour from the wick after the exhaustion, is seen to fall on the pump-plate,
because there is no air to support it.
Chimneys quicken the ascent of hot air by keeping a long column of it
together. A column of two feet high rises, or is pressed up with twice as
much force as a column of one foot, and so in proportion for all other lengths;
just as two or more corks strung together and immersed in water, tend upwards with proportionally more force than a single cork; or as a long spear of
light wood, allowed to ascend perpendicularly from a great depth in water,
acquires a velocity which makes it dart above the surface, while a short piece
under the same circumstances rises very slowly.  In a chimney where one
foot in height of the.columrn of hot air is one ounce lighter than the same
bulk of the external cold air, if the chimney be one hundred feet high, the
air or smoke in it is propelled upwards with a force of one hundred ounces.
In all cases, therefore, the draught, as it is called of a chimney, is proportioned to its length. The following facts are consequences of this truth.
In low cottages, and in the upper floors of houses, the annoyance of
smoky rooms is much more frequent than were chimneys are longer.
If there are two fires in the same room, or in any rooms open to each
other, which have chimneys of different lengths, and of which the doors and
windows are very close, so that the air to supply the draught cannot enter
by them, the taller chimney will overpower the shorter, and cause it to
smoke into the room; just as the long leg of a syphon overcomes the short
one, or as a long log of wood, held down in water by a cord passing from it
round a pulley at the bottom to a shorter log also floating, will rise, and
pull down the shorter log.]
A long chimney, for the reasons above explained, causes a current of
air to pass through the fire very rapidly, and it has the advantage also of
acting more uniformly than any bellows or blowing machine.  On these
accounts, of fires of steam engines, and many others, it is the means of
blowing generally preferred.  The importance of length in a chimney
explains the remarkable appearance of some mining districts and modern
English towns, where steam-engines abound.
When we heap dying embers together, so that the hot air rising among
them may become a mass or column of considerable altitude, this column
has the effect of blowing them gently, and helps to light them up again.
A piece of burning paper thrown upon the top of a half-extinguished fire,
often makes it blaze afresh, by causing a more rapid current of air to pass
through it from below.
The action or draught of a chimney, influenced as we have seen, by its
length, depends also on the degree in which the air in it is heated, because
this determines the dilitation, or comparative lightness, which makes the
air ascend.
In what are called open fire-places, such as those in the sitting-rooms of
Britain, a large quantity of air directly from the apartment enters the chimney
above the fire, and mixes with the hot air from the fire itself. This mixture
ascends. more slowly than if hot air alone entered, and in a proportion
dependent on the degree of mixture. The effect of excluding a part of this
colder air, is seen when a board or plate of metal is suspended across the
opening of the chimney, so as to narrow the entrance: —almost instantly a
quicker action is produced, and the fire begins to roar as if blown by a bellows. This means is often used to blow the fire instead of bellows, or to
cure a smoky chimney by increasing the draught.  What is call a register




204                       PNEUMATICS.
stove is a kindred contrivance. It has a flap placed'in the throat of the chimney, which serves to widen or contract the passage at pleasure. Because the
flap is generally opened only enough to allow that air to pass. which rises
directly from the fire, the chimney receives only very hot air, and therefore
acts well. The register stove often cures smoky chimneys: and by prevening the too ready escape of the moderately warmed air of the room, of which
so much is wasted by a common fire-place, it also saves fuel. In what are
called closefire-places, as those of steam-engines, or brewers' coppers, when
the furnace door is shut, no air can enter the chimney but directly through
the fire; hence the action of such chimneys is very-powerful.
In a room with two fires, or in drawing rooms communicating with each
other, although the chimneys be of equal length, that one over the best fire
will act the most strongly; and if the doors and windows of the apartment be
so close as to prevent a sufficiency of air from entering by them to supply
both fires, cold air will enter by that chimney which has the weakest fire, and
the smoke from it will spread into the room.  How often is an assembling
dinner party annoyed by the smoke of a second drawing-room fire just lighted
before their arrival, and which had therefore to contend with the antagonist
fire already in powerful action all the day. While only one fire was lighted, the
cold chimney was admitting the air to feed it, just as an open pane in the
window would have done. A room may be so close that no air can find entrance, and in such a case the smoke of its fire must all spread into the room.
When all the windows and doors of a house fit so closely as not to admit
air for the acting chimneys, the supply comes down the chimneys that are not
in use.  Inattention to this fact causes many a good chimney to incur the
imputation of being smoky, because on the attempt being made to light a
fire at it, the smoke at first is always thrown back. The truth is, that at
the time when the servant begins to light the fire, there is a downward current in the chimney, repelling of course, any heated air and smoke that
approaches it, and spreading them over the whole house; but were the
room door to be shut for a few minutes, so as to cut off communication with
the other drawing chimneys in the house, while at the same time the windows
were opened, the chimney would act at once; and when sufficiently heated,
would continue to act in spite of the others, as well as they.
There are some cases of smoky rooms not to be so easily corrected as what
we have now mentioned. When a low house adjoins a lofty house, the wind
blowing towards the latter, is obstructed and becomes a gathering or condensation of air against the wall; and if the top of a low chimney be there, the
compressed air enters it and pours downwards. The same happens occasionally
from the proximity of trees or rocks.  In such cases, to avoid the influence,
the chimneys of the low houses are often made very lofty. A gain, whenever,
from the nature of buildings, eddies of wind occur, or unequal pressures, as
at street corners, &c., the chimneys around do not act regularly.  It is proverbial, that corner houses, or those at the end of a row, are smoky houses;
and we see the uniformity of architecture in a street often destroyed by the
necessity of lengthening the chimneys of the houses at the extremities.
When smoke is found descending into a room where there is no fire, the
empty chimney is serving as an inlet for air to the house, while the smoke
of a neighboring chimney is passing closely over the top of it.
In summer, when fires are not in use, there is often a strong smell of soot
perceived in the apartments during the whole of the day, but.which ceases
at night. The reason is, that during the day the chimney is colder than the
external air, and by condensing the air which enters it, causes a downward




FLOATING.-FLAME AND SMOKE.                            205
current through the soot.  During the night, again, when the external air
becomes colder, owing to the absence of the sun, the chimney, by retaining
the heat absorbed during the day, is hot enough to warm the air in it, and
to cause an upward current. These currents, in chimneys left open during
the days and nights of summer, are almost as regular as the land and sea
breezes of tropical countries.
All these remarks prove how important it is to be able to conceive clearly
of the motions going on, according to the simple laws of matter, in the
invisible air around us.  Were such subjects better and more generally
understood, many prevalent errors in the arts of life, influencing much the
comforts and health of the community, would soon be corrected.
If we are filled with admiration on discovering how perfectly the simple
law of a lighter fluid rising in a heavier, provides a constantly renewed supply of fresh air to our fires, which supply we should else have to furnish by
the unremitted action of some expensive blowing apparatus, still more must
we admire that the operation of this law should effect the more important
purpose of furnishing the ever renewed supply of the same vital fluid to
breathing creatures. The air which a man has once respired becomes poison
to him; but because the temperature of his body is generally higher than
that of the atmosphere around him, as soon as he has discharged any air
from the lungs, it ascends completely away from him into the great purifying laboratory of the atmosphere, and new air takes its place.  No art or
labor of his, as by the use of fans or punkas, could have done half so well
what this simple law unceasingly and invisibly accomplishes, and accomplishes without effort or even attention on his part, and in his sleeping as in
his waking hours.  Truly in this, may he be said to be watched over by a
kind Providence.
The warming and ventilating of houses,
is an important art, founded chiefly on the foregoing considerations, and at
present too little understood, not only by the public at large, but even by
medical practitioners, whose management of disease, though judicious in
other respects, is often rendered vain by error or omission in this.
Excellent fuel is so cheap in Britain, owing to the profusion which
beds of rich coal are scattered in it, that a careless domestic expenditure has
arisen; which, however, instead of securing the comfort and health that
might be expected, has led to plans of warming which often prove destructive to both. The mischief lies chiefly in the unsteadiness or fluctuations of
our domestic temperature; for in still colder countries, and where fuel is
more expensive, as in the north of continental Europe, the necessity for
economy has led to contrivances which give steady temperature and impunity.
In cold countries, to retain and preserve the heat once obtained, the houses
are made with thick walls, double windows, and nice fittings; and, moreover, with close stoves or fire-places, which draw their supply of air, not
from  the apartments where they are placed, wasting the temperate air of
these, but directly from without. Thus fuel is saved to a great extent, and
a uniformity of temperature is produced, both as regards the different parts
of the room, so that the occupiers may sit with comfort where they please,
and as regards the different times of the day, for the stove being once heated
in the morning, often suffices to maintain a steady warmth until night.
The temperature can be carried tq any required degree, and sufficient
ventilation is easily effected.
In England, again, the apartments, with their open chimneys, may be




206                       PNEUMATICS.
compared to great air funnels, constantly pouring out their warm contents
through a large opening, and constantly requiring to be replenished. They
thus waste fuel exceedingly, because the chimney being large enough to
allow a whole room-full of air to pass away in two or three minutes, the air
of the room has to be warmed, not once in the course of the day, but very
many times. The temperature in them is made to fluctuate by the slightest
causes, as the opening a door, the omitting to stir the fire, &c.  The heat is
very unequal in different parts of the room, rendering it necessary in general
for the company to sit near the fire; where they must often submit to be
almost scorched on one side, while they are chilled on the other. There is
generally a warm stratum of air above the level of the chimney-piece, surrounding, therefore, the upper part of the bodies of persons in the room, while
a cold stratum below envelopes the sensitive feet and legs. As a very rapid
current is constantly ascending in the chimney, a corresponding supply must
be entering somewhere; and it can only enter by the crevices and defects
in the doors, windows, floors, &c.:-now there is nothing more dangerous
to health than to sit near such inlets, as is proved by the rheumatisms, stiff
necks and catarrhs, not to mention more serious diseases, which so frequently
follow the exposure.  There is an old Spanish proverb, thus translated,
"If cold wind reach you through a hole,
Go make your will and mind your soul,"
which is scarcely an exaggeration.
Consumption is the disease which carries off a fifth or more of the persons
born in Britain; owing in part, no doubt, to the changeableness of the external climate, but much more to the faulty modes of warming and ventilating
the houses. To judge of the influence of temperature in producing this disease
we may consider,-that miners who live under ground, and are always,
therefore, in the same temperature, are strangers to it, while their brothers,
and relatives, exposed to the vicissitudes above, fall victims,-that butchers
and others, who live almost constantly in the open air, so as to be hardened
by the exposure, enjoy nearly equal immunity,-that consumption is scarcely
known in Russia, where close stoves and houses preserve a uniform temperature within doors, while fit clothing gives safety on going out,-and that in
all countries and situations, whether tropical, temperate or polar, the frequency of the disease bears relation to the degree and manner of change.
We may here remark, also, that it is not consumption alone which springs
from changes of temperature, but a great proportion of acute diseases, and
particularly of the common winter diseases of England.   There are a few
cases of these in which the invalid has not to remark, that if he had avoided
cold or wet on some certain occasion, he might yet have been well.
WVhile temperature is thus so frequently an original cause of disease, it is
also a circumstance of the very highest importance in the treatment, —as is
proved by every fact bearing upon the question. We may, therefore, at first
wonder that it should be so negligently and unskilfully controlled as we often
see it; disease and death being thence allowed to lurk, as it were, undisturbed in the sanctuaries of our homes: but when we reflect on the subtile
and invisible nature of air and heat, and that the science which detects their
agencies has been hitherto so little an object of general study, and is, indeed,
of modern discovery, the fact is accounted for.
In England, the open fire-place is so generally in use for common dwellings, and the cheerful blaze is accounted so essential to the comforts of the
winter days and long evenings, that it would be difficult to persuade persons




FLOATING.-WARMING AND VENTILATING. 207
to abandon it: let us hope, then, that when the subject which we are now
discussing comes to be better and more generally understood, the open fire,
with close flooring, better for double windows, doors that fit well, register
stoves, and good general management, may be rendered almost as efficient
for warming, and as safe to health, as any other contrivance.
The following considerations present themselves in this place: —Small
rooms in winter are more dangerous to health than large ones, because the
cold air, entering towards the fire by the doors and windows, reaches the persons in the room before it can be tempered by mixing with the warmer air
already around them.  Stoves in halls and stai'-cases are useful, because
they warm the air before it enters the rooms; and they prevent the hurtful
chills often felt on passing through a cold stair-case from one warm room to
another;  It is important to admit no more cold air into the house than is
just required for the fires and for ventilation; hence there is a great error
in the common practice of leaving all the chimneys that are not in use quite
open, each admitting air as much as a hole in the wall, or an open pane in
the window would do. Perhaps the best mode of admitting air to feed the
fires is through tubes, leading directly from the outer air to the fire-place, and
provided with what are called throttle-valves, for the regulation of the quantity; the fresh air admitted by them being made to spread in the room
either at once, or after having been warmed during its passage inwards, by
coming near the fire.  In a very close apartment, ventilation must be expressly provided for by an opening near the ceiling, through which the impure
air, rising from the respiration of the company, may pass away. With an
open fire the purpose is effected, although less perfectly, by the frequent
change of the whole air of the room which that construction occasions.
With a view to have, in rooms intended for invalids, the most perfect
security against cold blasts and fluctuations of temperature, and still to retain
the so much valued appearance of the open fire, a glazed frame or window
may be placed at the entrance to the chimney or stove, so as completely to
prevent the passage of air from the room to the fire. The room will then be
warmed by the fire through the glass, nearly as a green-house is warmed by
the rays of the sun.  It is true that the heat of combustion does"not pass
through glass so readily as the heat of the sun; but the difference for the
case supposed is not important.  The glpss of such a window must, of
course, be divided into small panes, and supported by a metallic frame-work
to resist the heat; and there must be a flap or door in the frame-work, for
the purpose of admitting the fuel and stirring the fire. Air must be supplied
to the fire, as described above, by a tube leading directly from the external
atmosphere to the ash-pit. The ventilation of the room may be effected by
an opening into the chimney near the ceiling; and the temperature may be
regulated with great precision by a valve placed in this opening, and made
to obey the dilatation and contraction of a piece of wire affixed to it, the
length of which will always depend on the temperature of the room. The
author contrived the arrangements here described, for the winter residence of
a person threatened with consumption, and the happy issue of that particular case, and of others treated on similar principles, has led him to doubt
whether many of the patients with incipient consumption who are usually
sent to warmer climates, and who die there after suffering hardships on the
journey, and distress from the banishment sufficient to shake even strong
health, might not be saved by judicious treatment in properly warmed and
ventilated apartments, under their own roofs, and in the midst of affectionate
kindred. And if a boy be almost certainly secured from consumption by




208                       PNEUMATICS.
being' made a miner or a butcher, may we not hope that, when all the influencing circumstances come -to be better understood, something of the same
immunity may be obtained for persons in all the professions and conditions
of civilized society?
It must not be supposed that the remarks made in this section exhaust
even nearly the very important subject of temperature as affecting health.
The questions of clothing, of hot and cold bathing, of exercise, and others,
equally belong to it, but the consideration of them falls under other departments of study.
Winds or currents in the atmosphere
are also phenomena, in a great measure dependent on the law, that, lighter
fluids rise in heavier. As oil let loose under water is pressed up to the
surface and swims, so air near the surface of the earth, when heated by the
sun, rises to the top of the atmosphere, and spreads there, forced up by the
heavier air around; this heavier air rushing inwards, constitutes the wind
felt at the surface of the earth. The cross currents in the atmo.sphere arising
as now described, are often rendered evident by the motion of clouds or
balloons.
If our globe were at rest, and the sun were always beaming over the same
part, the earth and air directly under the sun would become exceedingly
heated, and the air there would be constantly rising like oil in water, or like
the smoke from a great fire; while currents or winds below would be pouring towards the central spot, from all directions. But the earth is constantly
turning round under the sun, so that the whole middle region or equatorial
belt may be called the sun's place: and therefore, according to the principle
just laid down, there should be over it a constant rising of air, and constant
currents from the two sides of it, or the north and south, to supply the
ascent. Now this phenomenon is really going on, and has been going on
ever since the beginning of the world, producing the steady winds of the
northern and southern hemispheres, called trade winds, on which in most
places within thirty degrees of the equator, mariners reckon almost as confidently as on the rising and setting of the sun himself.
The trade winds, however, although thus moving from the poles to the
equator, do not appear on the earth to be directly north and south, for the
eastward whirling, or diurnal rotation of the earth, causes a wind from the
north to appear as if coming from the north-east, and a wind from the south
as if coming from the south-east.  This fact is illustrated by the case of a
man on a galloping hbrse, to whom a calm appears to be a strong wind in his
face;. and if he be riding eastward, while the wind is directly north or south,
such wind will appear to him to come from the north-east, or south-east:
or again, is illustrated by the case of a small globe made to turn upon a perpendicular axis, while a ball or some water is allowed to run from the top of
it downwards; the ball or water will not immediately acquire the whirling
motion of the globe, but'will fall almost directly downwards, in a track
which, if marked upon the globe, will appear not as a direct line from the
axis to the equator, that is, from north to south, but as a line falling obliquely.
Thus, then, the whirling of the earth is the cause of the oblique and westward direction of the trade winds, and not, as has often been said, the sun
drawing them after him.
The reason why the trade winds at their external confines, which are about
30 degrees from the sun's place, appear almost directly east, and become




FLOATING.-WINDS.                             209
more nearly north and south as they approach the central line, is, that at the
confine they are like fluid coming from the axis of a turning wheel, and which
has approached the circumference, but has not yet acquired the velocity of
the circumference; while, nearer the line, they are like the fluid after it has
for a considerable time been turning on the circumference, and has acquired
the rotary motion there, consequently appearing at rest as regards that
motion, but still leaving sensible any motion in a cross direction.
While, in the lower regions of the atmosphere, air is thus constantly
flowing towards the equator and forming the steady trade winds between
the tropics in the upper regions, there must, of course, be a counter-current
distributing the heated air again over the globe:according]y, since reasoning
led men to expect this, many striking proofs have been detected.  At the
summit of the Peak of Teneriffe, observations now show that there is always
a strong wind blowing in a direction contrary to that of the trade wind on
the face of the ocean below.  Again, the trade winds among the West India
Islands are constant, yet volcanic dust thrown aloft from the Island of St.
Vincent, in the year 1812, was found, to the astonishment of the inhabitants
of Barbadoes, hovering over them in thick clouds, and falling, after coming
more tlian 100 miles directly against the strong trade wind, which ships
must take a circuitous course to avoid.  Persons sailing from the Cape of
Good Hope to St. Helena, have often to remark that the sun is hidden for
days together, by a stratum of dense clouds passing southward high in the
atmosphere; which clouds consist of the moisture raised near the equator
with the heated air, and becoming condensed again as it approaches the
colder regions of the south.
Beyond the tropics, where the heating iufluence of the sun is less, the
winds occasionally obey other causes than those we have now been considering, which causes have not yet been fully investigated. The winds of
temperate climates are in consequence much less regular, and are called
variable; but still, as a general rule, whenever air is moving towards the
equator, from the north or south poles where it was at rest, it must have the
appearance of an east wind, or a wind moving in the contrary direction of the
earth itself, until it has gradually acquired the whirling motion of that part of
the surface of the earth on which it is found; and again, when air is moving
from the equator, where it had at last acquired nearly the same motion as
that part of the earth, on reaching parts nearer the poles, and which have less
eastward motion, it continues to run faster than they, and becomes a westerly
wind. In many situations beyond the tropics, the westerly winds, whichl;are
merely the upper equatorial currents of air falling down, are almost as regular
as the easterly winds within the tropics, and might also be called trade winds:
-witness the usual shortness of the voyages from New York to Liverpool,
and the length of those made in the contrary direction. North of the equator,
then, on earth, true north winds appear to be north-east, and true south winds
appear to be south-west:-which are the two winds that blow in England for
three hundred days of every year. In southern climates the converse is true.
While the sun is beaming directly over a tropical island, he warms very
much the surface of the soil, and, therefore, also, the air over it; but the rays
which fall upon the ocean around penetrate deep into the mass, and produce
little increase of superficial temperature.  As a consequence of this, there is
a rapid ascent of hot air over the island during the day, and a cooler wind
blowing towards its centre from all directions.  This wind constitutes the
refreshing sea-breeze of tropical islands and coasts. A person must have
been among these, to conceive the delight which the sea-breeze brings after
14




210                       PNEUMATICS.
the sultry stagnation which precedes it. The welcome ripple shorewards is
first perceived on the surface of the lately smooth or glassy sea; and soon
the whole face of the sea is white with little curling waves, among which
the graceful canoe, lately asleep on the water, now shoots swiftly along.
During the night a phenomenon of opposite nature takes place. The
surface of the earth then no longer receiving the sun's rays, is soon cooled
by radiation, while the sea, which absorbed heat during the day, not on the
surface only, but through its mass, continues to give out heat all night.
The consequence is, that the air over the earth becoming colder than that
over the sea, sinks down, and spreads out on all sides, producing the landbreeze of tropical climates. This wind is often charged with unhealthy
exhalations from the marshes and forests, while the sea-breeze is all purity
and freshness. Many islands and coasts would be absolutely uninhabitable
but for the sea-breeze.
The peculiar distribution of land in the Asiatic part of the globe, produces
the curious effect there of a sea-breeze of six months, and a land-breeze of
six months.  The great continent of Asia lies chiefly north of the line, and
during its summer, the air over it is so much heated, that there is a constant
steady influx from the south-appearing south-west, for the reason given in
a preceding page; and during its winter months, while the sun is over the
southern ocean, there is a constant land breeze from the north-appearing,
for a like reason, north-east. These winds are called monsoons; and if their
utility to commerce were to be a reason for a name, they also deserve the
name of trade winds. In early periods of navigation, they served to the
mariner the purpose of compass, as well as of moving power; and one
voyage outward, and another homeward with the changing monsoons, filled
up his year.-On the western shores of Africa and America, also, the trade
winds are interfered with by the heating of the land; but much less so than
in Asia, and always in accordance with the laws now explained.
The frightful tornadoes, or whirlwinds, which occasionally devastate
certain tropical regions, making victims of every ship or bark caught on
the waters, and the shore gusts or squalls met with every where, are
owing to some sudden chemical changes in the atmosphere, not yet fully
understood.
The Pneumatic Trough and Gasometer
of the chemist are contrivances constantly displaying the truth now under
consideration, "that a lighter fluid is pushed up and floats on a heavier."
They are important parts of the apparatus for operating on substances while
in the form of air.
The trough a may be made of metallic plate, or of wood lined with metal,
and of any convenient size.  It is nearly filled
Fig. 106.         with water, and has at one end about an inch
under the surface of the water, a shelf on which
jars or vessels, as b and e, may rest.  Any
C.!   b             particular air or gas is preserved separate from
_l          a    the atmosphere, by being placed in one of these
jars with the mouth downwards.  The gas is
passed into the jar by the operator first immersI a          ing the jar in the trough, so as to fill it with
water and to expel the common air from it; and
then holding its mouth over the gas while rising
under the water from another vessel or pipe:



FLOATING.-GAS APPARATUS.                           211
d represents a long-necked vessel, used to contain the ingredients for the production of gases by chemical action.  The gas of course rises to the top of
the jar b, and gradually displaces the water. During the operation of filling,
the jar may be supported by the hand or by resting on the shelf;-in the
latter case the gas is allowed to rise into it through a hole in the shelf, provided with a small funnel gaping downwards to catch the air more readily.
The shelf may have room on it for many jars, and it may have more holes
than one; and if the gas under operation be such that water absorbs or
changes it, some other liquid, as mercury, may be used instead of water.
A gasometer or gas-holder, is merely a larger jar or vessel as a, dipping
into water, with its mouth downwards, in a
trough of its own shape, b c, and so sup-            Fig. 107.
ported or counterpoised by a weight at d,
over pullies, that very little force suffices to            - 
move it up or down.  Air forced into it
through a pipe f opening under it, causes it
to rise or float higher in proportion to the
quantity.  The air is made to pass from it           a
again when wanted, either through the same
tube or through another as e.
The huge gasometers, exceeding in size    -                  c
an ordinary house, and containing the supply,           T
of gas for the lamps of a town, are vessels
suspended as above represented, in great pits                    LJ
or troughs, filled with water. The gas issues
with force proportioned to the downward
pressure of the containing vessel, which may   ---
be nicely regulated in a variety of ways, and
is generally made to equal the action of a column of water of two inches in
height; that is to say, such, that a pipe issuing from the gas holder, and
dipping into water at its other end, shall allow gas to escape, if immersed
less than two inches perpendicularly.
It would be encroaching on the province of the chemist to treat here particularly of the substances which most generally exist in the aeriform state;
but to give an increased interest to the description of the gas apparatus, a
few leading facts may be mentioned.
Of about fifty distinct substances known as the materials of our globe, five,
when uncombined, and under common circumstances of heat and pressure,
exist as air or gases   The water used to fill the apparatus above described
is a compound of two of the substances, viz., oxygen and hydrogen.  By
directing an electrical current through water, it is. gradually decomposed,
and from one side, a stream of aeriform oxygen may be received, and from
the other a stream  of hydrogen   The two gases may be again united to
form water, by mixing them in a proper vessel, and passing an electric spark
through them. They combine with explosion.
This oxygen, so called from its relation to acids, (the name consisting of
two Greek words, signifying acid and to form,) has been accounted, for
many reasons, the most important substance in nature. It forms eightninths, by weight of the ocean; one-fourth of the atmosphere; and perhaps
one-fourth of the solid matter of the globe: possibly, therefore, although
most persons think of it only as an air or gas, there is not a millionth part
of the quantity of oxygen in the world existing as air. It unites readily
with most other substances, and generally with such intense action as to




212                        PNEUMATICS.
produce the phenomena of fire or combustion; the word combustible chiefly
apply to substances that quickly combine with oxygen.
Oxygen assumes a singular variety of character in its different combinations;  Thus with hydrogen, it forms water; with lead, it forms the substance called red-lead; with nitrogen, in one proportion, it forms atmospheric
air, in another proportion, the nitrous oxide, or what is called the laughing
gas, in a third, the acid called aqua fortis; with sulphur it forms the
sdlphuric acid or oil of vitriol; with iron, and all metals it forms their ores
called oxides: and so forth. But the most important character in which we
know it, is as that ingredient of our atmosphere, without which animals and
vegetables cannot live, and fire cannot burn. Oxygen, from this part of its
history, was long named vital or pure air.
Pure oxygen, in the state of air is a little heavier than common air; but
when holding a quantity of charcoal in solution, it forms aeriform carbonic
acid, which is nearly twice as heavy as common air, and may be poured out
of one vessel into another like water.  Carbonic acid is what issues from
soda-water, brisk ale, champagne, &c., while they sparkle. If drawn into
the lungs in breathing, it is fatal to life. A charcoal fire left in a close room
with sleeping persons, has often been fatal to them, because carbonic acid
gas is the product of the combustion.  So likewise, houseless wretches in
winter lying down in a brickmaker's field to leeward of a burning-heap of
bricks, often fall asleep for ever. The famous Grotto del Cane, in Italy, is
a cavern always full 6f carbonic acid, which springs into it from below, as
water springs into a well, and runs over like water from a well:-it received
its name from the circumstance of dogs dying instantly when thrown into it.
Carbonic acid rising in fermentation has often proved fatal to persons leaning
over the edge of fermenting vats.  It is common to see a rat die instantly,
in the attempt to run a plank laid across the mouth of a fermenting tub.
Hydrogen, the other ingredient of water, so called from its relation to
water (the name consists of the Greek words for water and to form,) when
in the state of air, is sixteen times as light as oxygen. With it balloons are
filled. When it holds in solutioi' a certain quantity of carbon or charcoal it
becomes the common gas used for illumination, and is the fire-damp of
mines, of which the burning and explosion are so terrible. It forms oneninth of the ocean, and much of the animal and vegetable bodies.
Nitrogen, so called from its relation to nitric acid, is the third and last
substance which we shall mention. ~ It is what remains of the atmosphere
when the oxygen is removed. It forms about four-fifths of the atmosphere,
one-fourth of the animal flesh, and is found in small quantities in the other
combinations. It will not support life by itself, and therefore formerly was
called azote: with a larger portion of oxygen, it forms nitric acid or the
aqua fortis of old.
The last few paragraphs may serve to show how many of the manipulations of chemistry are directed by thee principles of physics or mechanical
philosophy; and therefore, how essential to the chemist the preliminary
study of physics becomes.




DISCHARGE FROM APERTURES.                          213
PART III.
OR
THE PHENOMENA OF FLUIDS.
( CONTINUED. )
SECTION III.- HYDRAULICS-PHENOMENA OF FLUIDS IN
MOTION.
ANALYSIS OF THE SECTION.
Whether the particles of matter exist in the form of solid or fluid, the circumstance does not afect their properties of INERTIA and GRAVITY.Hence liquids and airs, in proportion to their quantity, resist; receive,
and impart motion, and have weight and friction, as is true of solids.
This is seen in the phenomena of
1. Fluids issuing from vessels, or moving in pipes and channels.
2. Waves.
3. Fluids resisting the motion of bodies immersed in them; or themselves
moving against other bodies.
4. Fluids lifted, or moved in opposition to gravity.
" Fluids issuing from vessels, or moving in channels."
WATER admitted to a tube ascending from near'the bottom of a reservoir
will rise in it, as already explained, to the level of the liquid-surface in the
reservoir. If such a tube be afterwards cut off, except a small part at the
bottom, then prepared as a jet-pipe, the water will spout from this still to
the same height, with a certain deduction for the resistance of the air and
friction. Now as a body shot upwards to any height has that velocity in
departing, which it again acquires by falling back to the same place or level,
(with a certain deduction for the resistance of the air,) as explained at page
60, it follows that fluid issues from any orifice in a reservoir with velocity
equal to what a body acquires in falling as far as from the level of the fluid
surface in the reservoir to the orifice. By referring then to the law of falling bodies, as explained at page 50, we may learn the velocity of the issue
of water in any case, and therefore the quantity delivered by an opening of
a given magnitude.




214                       HYDRAULICS.
Thus, a body by gravity falls sixteen feet in the first second, with speed
gradually increasing, and at the end of the second has a velocity of thirty-two
feet per secdnd;. therefore a reservoir with an opening of an inch square at
sixteen feet below the water's surface, will deliver, in one second of time, with
a certain deduction for resistance of air, friction, &c., thirty-two feet of a jet
of water of an inch square; and according to the same rule, an opening at
four times the depth should deliver a d.ouble quantity; at nine times the
depth, a triple quantity; and so on, as really happens.  An inquirer is at
first surprised that the quantity should not be quadruple, where the height of
column or pressure forcing it out is quadruple, ninefold when tie pressure
is ninefold, &c., but on reflection, he may perceive that the real effects, as
stated above, are still exactly proportioned to the causes; for when only
twice as much water is forced out in the same time, there is still an effect
four times as powerful, because each particle of the double quantity issues
with twice the force or velocity, and increase of velocity costs just as much
force as increase of quantity.  Similar reasoning holds with respect to the
triple or other quantities. Because a body shot upward with a double velocity gains a quadruple height, ( see page 60, ) the jet issuing with only double
velocity from four times the depth, still reaches the level of the surface of
the reservoir.
The knowledge of this rule for discharging orifices is of the greatest
importance in the construction of water-works, because when joined with
other rules assigning the effects of friction, bending, unequal width, &c., in
pipes, it ascertains the quantity of water which a conduit of any magnitude,
length and slope, will deliver.
It is a curious fact, that more water issues from a vessel through a short
pipe, than through a simple aperture of the same diameter as the pipe; and
still more if the pipe be funnel-shaped, or wider towards its inner extremity.
The explanation is, that the issuing particles coming from all sides to
escape, cross and impede each other in rushing through a simple opening,
as is proved by the narrow neck which the jet exhibits a little beyond the
opening; but in a tube, this narrowing of the jet cannot happen without
leaving a vacuum around the part, and the pressure of the atmosphere, resisting the vacuum, causes a quicker flow. The funnel-shape again leads
the Water by a more gradual inclination to the point of exit, and thus considerably prevents the crossing among the particles; besides that, because
its mouth surrounds the narrow neck of the jet, it allows that part to be
deemed the commencement of the jet.
Another remarkable effect of atmospheric pressure on running liquids is,
that in a tube of considerable length, descending from the reservoir, it much
quickens the discharge. Water naturally falls like any other body with
accelerating velocity, but if it so fall in a tube which it fills like a piston,
either portions of it below must outstrip portions above, leaving vacuous
spaces between, or water from above must be pressed into the tube by some
other force than its weight. Now the atmospheric pressure becomes this
force, and it prevents a vacuum, partly by impelling water more rapidly into
the top of the tube, and partly by resisting the discharge from below. The
forcing in of the water at the top of the tube causes that depression of the
water surface in the reservoir over it, which becomes more conspicuous as
the depth in the reservoir diminishes, and at last is a deep hole in the water
extending far into the tube, and sometimes even as in a common funnel
extending quite through.
The friction or resistance which fluids suffer in passing along pipes is




DISCHARGE FROM APERTURES.                          215
much higher than might be expected. It depends on the cohesion of the
particles to the surface of the pipe and among one aiother, and on the particles near the outside being constantly driven from their straight course by
the irregularities in the surface of the pipe. An inch tube of two hundred
feet in length, placed horizontally, is found to discharge only a fourth part
of the water which escapes by a simple aperture of the same diameter. All
passing along tubes is still more retarded. A person who erected a great
bellows at a water-fall, to blow a furnace two miles off, found that his
apparatus was totally useless.  When gas lights were first proposed,
some engineers feared that the resistance by friction to the passing airwould be fatal to the enterprise.
Higher temperature in a liquid increases remarkably the quantity discharged by an orfice or pipe,-apparently by diminishing that cohesion of
the particles which exists in certain degrees in all liquids and affects so
much their internal movement. The additition of 100 degrees of heat will,
in certain cases, nearly double the discharge.
The flux of water through orifices under uniform circumstances is so
steady, that before the invention of clocks and watches, it was employed as
a means of measuring time.  The vessels were called clepsydrae.  That of
Ctesibius is famous, in which the issuing water took the form of tears from
the eyes of a figure, deploring the rapid passing of precious time; and these
tears being received into a fit vessel, gradually filled it up and raised
another floating figure, who pointed to the hours marked on an upright
scale. This vessel was daily emptied by a syphon, when charged to a
certain height, and its discharge worked machinery which told the month
and the day. The common hour-glass of running sand is another modification of the same principle, with this remarkable difference, however, that
depth of the sand does not quicken the flux.
The progress of water in an open conduit, such as the channel of a river
or acqueduct, is influenced by friction, &c., in the same manner as in close
pipes. But for this, a river like the Rhone, drawing its waters from the
elevation of 1,000 feet above the level of its mouth, would pour them out,
with the velocity of water issuing from the bottom of a reservoir, 1,000 feet
deep; that is to say, at the rate of about 170 miles per hour. The ordinary
flow of rivers is about three miles per hour, and their channels slope three
or four inches per mile.
The velocity of a water current is easily ascertained by immersing in it
an upright tube, of which the bottom bent at right angles becomes an open
mouth turned towards the stream. The water in the tube
will stand above the surface of the stream, as much as would    Fig. 108.
be necessary in a reservoir, according to the explanation
given above, to cause a velocity of jet equal to the velocity   c.of the stream.  A  modification of this contrivance may be
made to measure the velocity of the wind. A common mode
of telling the velocity of an open stream, is to observe with
a stop-watch the progress of a body floating in some part of
it from which its medintm speed may be known; and knowing that speed and the depth and width of the channel the
quantity delivered in a given time becomes a matter of simple calculation. The speed of the wind may be ascertained
by observing how long the shadow of a cloud takes to
pass across a field of known dimensions.
The friction of water moving in water is such, that a small stream directed




216                       HYDRAULICS.
through a pool, with speed enough to rise over the opposite bank, will soon
empty the pool. Extensive fens have been drained on this principle. The
friction between air and water is also singularly strong, and is proved on a
great scale by the magnitude of the ocean-waves, which is a consequence of
it; and on a small scale, by the amusing experiment of making a light
round body dance or play upon the summit of a water-jet-a chief cause of its
remaining there being, that the current of air which rises around the jet by
reason of the friction, presses it inward again, whenever it inclines to fall
over.  Oil thrown upon the surface of water, soon spreads as a film over it.
and defends it from farther contact and friction of the air.  If oil be thus
spread at the windward side of a pond where the waves begin, the whole
surface of a pond becomes as smooth as glass; and even out at sea, where
the commencement of the waves cannot be reached, oil thrown upon them
smooths their surface to leeward of the place, and prevents their curling
over or breaking.  It is said that boats having to reach the shore through
a raging surf, have been preserved by the crews first spilling a cask of oil
in the offing.
The most magnificent examples that ever existed, or probably ever will
exist of artificial water-courses, were the acqueducts of ancient Rome, about
twenty in number. Several of them exceeded forty miles in length, passing
through hills in their way, and resting on tiers of splendid arches across the
valleys. They were constructed of such durable materials, and so skilfully,
that the principal of them remain perfect to this day.  Considered as one
object, they rank, in point of magnitude, with anytother work of human
labour, not excepting the pyramids of Egypt.
While the acqueducts are cited as specimens of grandeur, we may mention
the fountains in the gardens of France and Italy as specimens of beauty.
Those at Versailles are well known. In them the most magical effects are
produced by varying the ways in which water is made to spout from orifices.
In one place it is seen darting into the air as a single upright pillar: in others
many such pillars rise together, like giant stalks of corn; sometimes an
inclination given to the jets makes them bend so as to form beautiful arches,
of which a portion appear as the roofs of apartments built of water while
others mingle together with endless variety: here and there water-throwing
wheels throw out spiral streams, and hollow spheres with a thousand opening
are the centres of immense bushes or trees of silvery boughs.  Such effects
amidst cascades, smooth lakes, and scenes of lovely landscapes, constitute a
whole as enchanting, perhaps, as art by moulding nature has ever produced.'" Taves."
The form, magnitude and a velocity of waves, are subjects admitting of
deep mathematical research; and are rendered the more interesting, because
certain phenomena of sound and light are of kindred nature.  Here, however, they must be treated with great brevity.
A stone thrown into a smooth pond, causes a succession of circular waves
to spread from the spot where it falls as a common centre. They become of
less elevation as they expand, and each new one is less raised than the preceding, until gradually the liquid mirror becomes again perfect as before.
Several stones falling at the same time in different places, cause crossing
circles which, however, do not disturb the progress of one another-a phenomenon seen in beautiful miniature at each leap of the little insects which
cover the surface of our pools in the calm hours of summer. The rationale




WAVES.                              217
of the formation of waves in such cases is as follows: When the stone falls
into the water, because the liquid is incompressible, a part of it is displaced
laterally, and becomes an elevation or circular wave around the stone. This
wave then spreads outward in obedience to the laws of fluidity, already
explained, and the circle is seen to widen. In the meantime, where the
stone descended, a hollow is left for a moment in the water, but owing to
the'surrounding pressure, is soon filled up, chiefly by a sudden rush from
below.  The rising water does not stop, however, at the exact level of that
around, but like a pendulum sweeping past the centre of its arc, it rises
almost as far above the level as the depression was deep. The central elevation now acts as the stone did originally, and causes a second wave, which
pursues the first; and when the centre subsides, like the pendulum still, it
sinks again almost as much below the level as it had mounted above: hence
it has to rise again, again to fall, and so on for many times, sending forth
a new wave at each alternation. Owing to the friction among the particles
of the water, each new wave is less raised than the preceding, and at last
the appearance dies away.
A wave passing through any gap or opening, spreads from it as a new
centre; and a wave coming against a perpendicular surface of wall or rock,
is completely reflected from this, and acquires the appearance of coming
from a point as far beyond the reflecting surface, as its real origin or centre
is distant on the side where it is moving.
So absolutely level is a liquid surface, and so sensitive or mobile, that the
effect of any disturbing cause is perceived at great distances. A boat rowed
across a still lake, ruffles its surface to a great extent; and although the
widening waves become at last such gentle risings as not to be perceptible
to the eye, they still produce a rippling noise where they fall among the
pebbles on shore. In seas liable to sudden but partial hurricanes, the roar
of breakers on distant coasts often tells of the storm which does not otherwise reach them. The author once, in the eastern ocean, had an opportunity
of contemplating waves of extraordinary magnitude rolling along during a
gloomy calm, and therefore with unbroken surface, appearing like billows of
molten lead. At that very time, about a hundred and fifty miles to the
north-east, four of the finest ships of the India Company were perishing in a
storm.-in the polar seas which are comparatively tranquil, because partially
defended from the wind by the floating islands of ice, a few sudden waves
are occasionally observed, and quickly all is calm again. Such a phenomenon
announces, that the occurrence described at page 153 has happened somewhere, of an island of ice turning over, when the place of its centre of gravity
is changed by partial melting.
The common cause of. waves is the friction of the wind upon the surface
of the water. Little ridges or elevations first appear, which, by continuance
of the force, gradually become loftier and broader, until they are the rolling
mountains seen where the winds sweep over a great extent of water. The
heaving of the Bay of Biscay, or still more remarkably, of the open ocean
beyond the southern capes' of America and Africa, exhibits one extreme, and
the stillness of the tropical seas, which are sheltered by near encircling lands
exhibits the other. In the vast archipelago of the east,'where Borneo and
Java and Sumatra lie, and the Molucca Islands and the Philippines, the sea
is often fanned only by the land and sea breezes, and is like a smooth bed in
which these islands seem to repose in bliss-islands in which the spice and
pefume gardens of the world are embowered, and where the bird of paradise
has its home, and the golden pheasant, and a hundred other birds of brilliant




218                       HYDRAULICS.
plumage, among thickets so luxuriant, and scenery so picturesque, that
European strangers find there the fairy land of their youthful dreams.-One
who has visited these islands in his early days, may perhaps be pardoned for
thus adverting to their beauties.
In rounding the Cape of Good Hope, waves are met with, or rather a
swell, so vast, that a few ridges and a few depressions occupy the extent of a
mile, But these are not so dangerous to ships, as what is termed a shorter
sea, with more perpendicular waves. The slope in the former is comparatively gentle, and the rising and falling are much less felt; while among the
latter, the sudden tossing of the vessel is often destructive.  When a ship
is sailing directly before the wind, over the long swell now described, she
advances as if by leaps; for as each wave passes, she is first descending headlong on its front, acquiring a velocity so wild that she can scarcely be steered;
and soon after, when it has glided under her, she appears climbing on its
back, and her motion is slackened almost to rest, before the following wave
arrives. To a passenger perched at such a time on the -extremity of the
bowsprit, and looking back on the enormous body of the ship, with perhaps
its thousands of a crew, a hundred feet behind him, heaved by those billows
as a cork is on a ruffled lake, the scene is truly sublime. When a coming
wave lifts the stern and in the same degree depresses the bow, he is deep in
the hollow or valley between the waves, and sees only the ship rushing headlong down towards him as if to be engulphed; but soon after, when the stern
is down, and the bow is raised, he looks from his station in the sky upon an
awful scene beneath him and around.
The velo ity of waves has relation to their magnitude. The large waves
just spoken of, proceed at the rate of from thirty to forty miles an hour.It is a vulgar belief that the water itself advances with the speed of the wave,
but in fact the form only advances, while the substance, except a little spray
above, remains rising and falling in the same place, with the regularity of a
pendulum. A wave of water, in this respect, is exactly imitated by the wave
running along a stretched rope when one end is shaken; or by the mimic
waves of our theatres, which are generally undulations of long pieces of carpet moved by attendants. But when a wave reaches a shallow bank or
beach, the water becomes really progressive, for then, as it cannot sink
directly downwards, it falls over and forwards, seeking the level.
So awful is the spectacle of a storm at sea, that it generally biases the
judgment; and, lofty as waves really are, imagination pictures them loftier
still. Now no wave rises much more than ten feet above the ordinary sealevel, which, with the ten feet that the surface afterwards descends below
this, give twenty feet for the whole height, from the bottom of any watervalley to an adjoining summit. This is easily verified by a person who tries
at what height on a ship's mast the horizon remains always in sight over the
top of the waves-allowance being made for accidental inclinations of the vessel, and for her sinking in the water to considerably below her water line, at
the time when she reaches the bottom of the hollow between the two waves.
The spray of the sea, driven along by the violence of the wind, is of course
much higher than the summit of the liquid wave; and a wave coming against
an obstacle, or entering a narrow inlet, may dash to an elevation much greater
still. At Eddystone light house, which is about ninety feet high, placed
on a solitary rock ten miles from land, when a surge breaks which has been
growing under a storm all the way across the Atlantic, it often dashes to 100
feet above the lantern at the summit.'the magnitude of waves is well judged of when they are seen breaking




WAVES.                              219
on an extended shore or beach. In the deep sea the wive is only an elevation of the water, sloping on either side; but as it rolls towards the shore,
~its front becomes more and more perpendicular, until at last it curls over
and falls with its whole weight, and when several miles of it break at the
same instant, its force and noise may shake the country abroad.
Along the east, or Coromandel Coast of India, at certain season, vast
waves are constantly breaking; and as there are nogood harbours there, communication between the sea and land is rendered impossible to ordinary boats.
The natives of the coast, at Madras, for instance, have hence become almost
amphibious. They reach ships beyond the breakers by the help of what are
called catamarans, consisting of three small logs of wood tied together. On
these they secure themselves, and boldly advance up to the coming wall of
water; which they shoot into, and rise to the smooth surface beyond it, like
water-fowls after diving. Boats unsuited to the breakers often perish in them.
The author of this work had gone on shore with a watering party on the
coast of Sumatra, and during the hours spent there, a swell had risen in the
sea, which on their return was already bursting along the beach and across
the river's mouth in lofty breakers. The boat in which he happened to be,
regained the high sea in safety, but a larger boat which followed at a short
distance was overwhelmed, and an officer and part of the crew perished.
There is a phenomenon observed at the mouths of many great rivers, called
the Boar, which has resemblance to a wave.  When the tide returning from
sea meets the outward current from the river, and both have the force which
in certain situations belongs to them, the stronger mass from the ocean assumes
the form of an almost perpendicular wall, moving inland with resistless sweep.
This is called the boar. It is in fact the great sea-wave of the tide, produced
twice a-day by the attraction of the moon, rolling in upon the land and inlets,
where contracting channels concentrate its mass. In the different branches
of the Ganges the boar is seen in a remarkable degree. Its roaring is heard
long before it arrives. Smaller boats and skiffs cannot live where it comes;
and as it passes the city of Calcutta, even the large ships at anchor there
are thrown into such commotion, as sometimes to be torn away from their
moorings.-The nature and effects of this boar are strikingly illustrated
upon certain coasts where extensive tracts of sand are left uncovered at low
water. In such situations, of which there are many on the western shores
of Britain, the returning tide is seen advancing with steep front, and with
such rapidity, that the speed of a galloping horse can scarcely save a person
who has incautiously approached too near. Many, every year, are the
victims of temerity or ignorance on these treacherous plains.
In the end of the year 1831, on the low flat coast of the Indian peninsula,
north of Madras, one great wave of the kind now described was produced
during a very high-spring tide of midnight, by an extraordinary wind, and
spread ten miles in upon the inhabited land. It had retired with the ebbing
tide before morning, but the next day's sun disclosed a scene of devastation
rarely matched. Amidst the total wreck of the villages and fields, there lay
the drowned carcases of more than ten thousand human beings, mixed with
those of elephants, horses, bullocks, wild tigers and the other inhabitants
of the land.
It has been proposed lately to construct sub-marine boats, or vessels calculated to swim so deep in the water as to be below the superficial motion of
the waves, and therefore beyond the influence of storms at the surface. Such
a boat has been tried with considerable success; and man's increasing familiarity with sub-marine matters since the invention of the diving-bell, may




220                       HYDRAULICS.
ultimately lead to improvements rendering the sub-marine vessel, for certain
purposes, commodious and safe.
" Fluids resisting the motion of bodies immersed in them, or themselves
moving forcibly against other bodies." (See the Analysis.)
The same force is required to give or to take away, or to bend motion, in
a fluid, as in an equal quantity of solid matter. A pound of water enclosed
in a bladder is not more easily thrown to a given height than a pound of ice
or of lead; nor, if falling into the scale of a weighing beam, does it require
less as a counterpoise; nor, if made to revolve at the end of a sling, does it
render the cord less tight.
A convenient measure of the force of moving water on an obstacle, or of
the resistance of still water to a moving body, exists in the facts already
explained, that the pressure of a known height of fluid column produces from
an orifice a certain velocity of jet, while conversely, that jet, or a current of
equal speed, directed against the orifice supports the column. The impulse
given or received, therefore, by a flat surface in water, such as the vane of
a water-wheel, whether that of a steam-boat pressing against the water, or
that of a corn-mill pressed by it, is measured by the weight of the column
alluded to, the height of which is, according to the velocity and the breadth
or diameter, according to the breadth or extent of the solid surface concerned.
This estimate supposes that the pressure of air upon the surface is direct; if
it be oblique, there is a diminution according to the rule given under the
head of " resolution of forces."
Many persons looking carelessly at the subject of fluid resistance, would
expect that if a body, as a boat, moving through a fluid at a given rate, meets
a given resistance, it should just meet double resistance when moving twice
as fast. Now the resistance is four times greater with a double rate.
This fact is but another example of a principle already explained, and
when more closely examined, is easily understood. A boat which moves
one mile per hour, displaces or throws aside a certain quantity of water, and
with a certain velocity;-if it move twice as fast, it of course displaces twice
as many particles at the same time, and requires to be moved by twice the
force on that account; but it also displaces every particle with a double
velocity, and requires another doubling of the power on this account; the
power then being doubled on two accounts becomes a power of four. In the
same manner with a speed of three, three times as many particles are moved
and each particle with three times the velocity; therefore, to overcome the
resistence, a force of nine is wanted; for a speed of four, a power of sixteen;
for a speed of five, a power of twenty-five, and so forth: the relations being
that which mathematicians indicate by saying that the resistance increases as
the square of the speed.  The corresponding numbers, up to a speed of ten
are as here shown.
Speed..   1   2    3    4    5    6    7    8    9    10
Corresponding   1   4    9   16   25   36   49   64   81   100
resistance
Thus, if even the resistance at the bow of a vessel was all that had to be
considered, the force of one hundred horses would only drag the vessel ten
times as fast as the force of one horse.  But there is another important
element in the calculation, viz, the lessening, as the vessel's speed quick



WAVES.                              221
ens, of the usual water pressure onothe stern,-which pressure while she is
at rest is equal to the pressure on the bow, and the force therefore required
to produce an increased velocity is still considerably greater than as noted
in the table.
There is not a more important truth in physics than the law of fluid
resistance to moving bodies here treated of; it explains so many phenomena
of nature, and becomes a guide in so many matters of art. We will now
set forth some interesting examples.
It explains at what a heavy expense of coal high velocities are obtained in
steam-boats. If an engine of about 50 horse power would drive a boat 7
miles an hour, two engines of 50, or one of 100 would be required to drive
it 10 miles, and three such to drive it 12 miles, even supposing the increased
resistance to the bow, as already stated, to be the measure of the whole work
done, which it is not, and that engines worked to the same advantage with a
high velocity as with a low, which they do not.-For the same reasons, if all
the coal which a ship could conveniently carry were just sufficient to drive
her 1,000 miles, at a rate of 12 miles per hour, it would drive her more
than 3,000 at a rate of 7 miles per hour; and more than 6,000 at a rate of
5 miles per hour. This is a very important consideration for persons
concerned in steam navigation to distant parts.
The same law shows the folly of putting very large sails on a ship; the
trifling advantage in point of speed by no means compensating for the additional expense of making and working the sails, and the risk of accidents in
bad weather. The ships of the prudent Chinese have not, for the same
tonnage, one-third so much sail as those of Europeans, and yet they move
but little slower on that account. A European ship under jury-masts does
not lose so much of her usual speed as most people would expect.
This law explains also why a ship glides through the water one or two
miles an hour when there is very little wind, although with a strong breeze
she would only sail at the rate of eight or ten miles. Less than the 100th
part of that force of wind which drives her ten miles an hour, will drive her
one mile per hour, and less than the 400th part will drive her half a mile.
Thus, also, during a calm, a few men pulling in a boat can move a large
ship at a sensible rate.
These considerations show strikingly of what importance to navigation it
might be to have, as a part of a ship's ordinary equipment, one or two waterwheels, ( or ready means-of forming them,) to be affixed upon the ship's side
when required, like the paddle-wheels of a steam-boat, and by turning which;
the crew might easily deliver themselves from the tedium, or even disastrous
consequences of a long calm at sea. —This idea occurred to the author while
in a ship completely becalmed for weeks on the Line: during which wearisome periods, the breezes were often seen roughening the water a mile or
two farther on; and any means that could have enabled the ship's company
to advance her that little distance might have saved the delay.  The wheels
might be driven by connection with the capstan, at which, under such
circumstances, the crew would most willingly work. Delay in a large vessel
often costs hundreds of pounds per day, and may retard the execution of
important projects. —But the propelling of a ship in a calm seems, by no
means, the most important purpose which such wheels might serve. If
from disease, fatigue, or other cause, the crew were inadequate to existing
necessities, two wheels affixed to the extremities of an axis crossing the ship
might be equivalent in many cases to additional hands, or to a steam-engine
of great power; for when acted upon by the water as the ship sailed, they




222                       HYDRAU LI CS.
would turn with the force of water-wheels on shore, and might be made to
move the pumps, to hoist the sails, and to do any work which a steam-engine
could perform. Many a gallant vessel has perished because the exhausted
crew could not longer labour at the pumps, where such water-wheels as now
contemplated, or a wind-mill wheel in the rigging would have performed the
duty most perfectly.
The law that resistance to a body moving in a fluid increases in a greater
proportion than in speed of the body, applies where the fluid is aeriform,
as well as where it is liquid.
A bullet shot through the air with a double velocity, for the reason
assigned above, experiences four times as much resistance in front, as with
a single velocity: the motion is retarded also by the diminution of the usual
atmospheric pressure of 15 lbs. per inch on the posterior surface, which
diminution is proportioned to the speed.  It is farther true, that when the
velocities of bodies moving in the air are very great, the resistance increases
in a still quicker ratio than in liquids, —probably because the compressibility
of air allows it to be much condensed or heaped up before the quick moving
body. It is useless to discharge a cannon-ball with a velocity exceeding
1,200 feet in a second, because the powerful resistance of the air to any
velocity beyond that, soon reduces it to that at least.
The rule of reciprocal action between a solid and fluid, now explained, holds
equally irhen the fluid is in motion against the solid, as when the solid
moves through the fluid.
If a ship be anchored in a tide's way, where the current is four miles an
hour, the strain on her cable is not one fourth part so great as if the current
were eight miles.
A wind moving three miles an hour is scarcely felt: if moving six miles,
it is a pleasant breeze; if twenty or thirty miles, it is a brisk gale; if sixty,
it is a storm; and beyond eighty, it is a frightful hurricane, tearing up
trees and destroying every thing.
Supposing the wind to move one hundred miles per hour, there are one
hundred times as many particles of matter striking any body exposed to it,
as when it moves only one mile per hour, and each particle strikes, moreover, with one hundred times the velocity or force, so that the whole increase
of force is a hundred times a hundred, or ten thousand. This explains how
the soft invisible air may by motion acquire force sufficient to unroof houses,
to level oaks which have been stretching their roots around for a century,
and in some West India hurricanes, absolutely to brush every projecting
thing from the surface of the earth.
The law of rapidly increasing resistance assigns a limit to many velocities,
both natural and artificial.!
It limits the velocity of bodies falling through the air. By the law of
gravity, a body would fall with a constantly accelerating speed, but as the
resistance of the air increases still more quickly than the speed at a certain
point, this resistance and the gravity balance each other, and the motion
becomes uniform,




ACTION BETWEEN FLUIDS AND SOLIDS.  223
The parachute, by means of which a person may safely descend to the
earth from a balloon at any elevation, furnishes a good example. The contrivance resembles a large flat umbrella. The aeronaut attaches himself
underneath it, and when it is let loose from the balloon, he is partly supported by the resistance which its broad expanse experiences in falling
through the air, and falls, therefore, in a corresponding degree more slowly.
After the first second or two, for the reason stated above, it descends with
a uniform motion; and its breadth is generally made such, as to allow a
velocity of about eleven feet in a second, or that which a man acquires in
jumping from a chair two feet high.
No ship sails faster than fifteen miles in an hour.-And it is because the
resistance to be overcome in steam-carriages on rail-ways, viz., their friction,
does not increase with their velocity like the fluid resistance to steamboats,
that the speed of the former may so much exceed that of the latter.
No fish swims with a velocity exceeding twenty miles an hour; not the
dolphin, when shooting ahead of our swiftest frigates, nor the salmon, when
darting forward with a speed which lifts him over the water-fall.
And the flight of birds through the thin air has a limited celerity. The
crow, when flying homewards against the storm, cannot face the wind in the
open sky, but skims along the surface of the earth in the deep valleys, or
wherever the swiftness of the wind is retarded by terrestrial obstructions.
The great albatross, stemming. upon the wing the current of a gale so as to
keep company with a driving ship where the air is passing at the rate of a
hundred miles an hour, often takes shelter momentarily under the lee-side
of the lofty billows. The bird called the stormy petrel abides chiefly in the
midst of the Atlantic Ocean, but the irresistible violence of the wind occasionally sweeps it from the waves, and causesits appearance on the western
shores of Europe. Vessels from the high sea, approaching a coast from
which the wind blows, generally become resting places to exhausted land
birds driven off the shore by wind which they have not had strength of wing
to stem;-sad evidences of the myriads which are constantly perishing where
no resting place is found, and where no human eye notes their fate.
The action or resistance between A meeting fluid and solid, is influenced by
the shape of the solid.
This follows from what has already been said of direct and oblique impulse.
If a flat surface directly opposed to the fluid experience a certain resistance,
a projecting surface like that of a sphere or short wedge is resisted in a less
degree, and a concave surface in a greater. The explanation is, that a flat
or plane surface throws the particles of fluid almost directly outwards from
its centre to its circumference, and therefore with greater velocity, while the
convex or wedge-like surface, although displacing them just as far, still does
so more slowly, and therefore with less expenditure of force, in proportion
to the obliquity of surface, or as its point is in advance of its shoulder or
broadest part; and a concave surface must give to some of the particles a
forward as well as lateral motion. The shape of the hinder part of a solid
moving through a fluid is of importance for corresponding reasons.
The following are instances of projecting or wedge-like surfaces, intended
to diminish the resistance. Fishes are wedge-like, both before and, behind,
their form being modified, however, in relation to other objects than mere
speed of motion. Birds are so, also; and they stretch out their necks while
flying, so as to make their form perfect for dividing the air. In the form




224                       HYDRAULICS.
of the under part of boats and ships, men have, in a degree, imitated the
shape of fishes. The light wherries which shoot about upon the surface of
the Thames, appear the very essence of what imagination can picture of form
combining utility and grace. There are boats used in China called snakeboats, which are only a foot or two broad, but perhaps a hundred feet in
length, and when moved, as they often are, by nearly a hundred rowers,
their swiftness is extreme.  The problem of which it is the object to assign
for a ship's hull or bottom the best possible form that she may have speed
of sailing, is not yet completely solved; so that a kind of empiricism prevails
in the matter, and very unexpected results often arise.  Yet the subject
merits much attention, for when vessels have to chase and to flee, speed
becomes of the greatest importance; and.at all times the sailor's heart swells
with delight to find his well-beloved vessel performing well.
The following instances exhibit the mutual influence of meeting solids and
fluids, where the surface of the solid is plane or concave. —In a water-wheel,
whether the water be moving against the wheel, as is the case where a stream
acts to drive machinery, or the wheel be moving against the still water, as
in the case of the paddle-wheels of a steam-boat, the extended faces of the
vanes or float-boards give or receive a powerful impulse. When a wheel
with float-hoards has its lower part merely dipping into a stream of water,
to be driven by the momentum, it is called an undershot-wheel; when the
water reaches the wheel near the middle of its height, and turns it by falling
on the float-boards of one side as they sweep downwards in a curved trough
fitting them, the modification is called a breast-wheel; and when the floatboards are shut in by flat sides, so as to become the bottoms of a circle of
cavities or buckets surrounding the wheel, into which the water is allowed
to fall at the top of the wheel; and to act by its weight instead of its momentum, the modification is called the overshot-wheel. To have a maximum of
effect from wheels moved by the momentum of water, they are generally
made to turn with a velocity about one-third as great as that of the water;
and wheels moved by the simple weight of water usually have their circumference turning with a velocity of about three feet per second. The subject
of water-wheels is one of the most important in practical mechanics; for
moving water performs a great deal of labor for man.
Oars for boats are made flat, and often a little concave, that the mutual
action between them and water may be as great as possible. The webbed
feet of water-fowl are oars; in advancing, they collapse like a shutting umbrella, but open outwards in the thrust backwards, so as to offer a broad
concave surface to the water.  The expanded wings of birds are in like
manner a little concave towards the air which they strike.  The sails of
ships, when they are receiving a fair wind, are left slack so as to swell and
become hollow.
The resistance between a meeting solid and fluid being nearly proportioned
to the breadth of the solid, it follows that large bodies, because containing
more matter in proportion to their breadth or surface than smaller bodies
of similar form, are less resisted in proportion to their weights, than
smaller bodies.
The science of measures tells us that a bullet or other solid of two inches
diameter, has eight times as much matter in it as a similar solid of one inch
diameter, while it has only four time the breadth or surface. Thus, by
putting eight dice or little cubes together, as here represented, we have a




ACTION BETWEEN FLUIDS AND SOLIDS.  225
larger cube, of which compared with a single dice, the edge is evidently
twice as long, the surface four times as great; and the quantity of matter
eight times as great; —again, twenty-seven dice similarly put together form a cube with sides three times    Fig.'109.
as long, and the surface nine times as great; and sixtyfour dice form a cube with sides four times as long,
and a surface sixteen times as great. All solids similar
have to each other this kind of relation, which, in the 
language of the science of quantity, is called the relation of cubes: they are said to be to each other as the
cubes of any of their corresponding lines. Hence if a
bullet of eight pounds, and a bullet of one pound be shot off with equal
velocity, because that of eight pounds has only half as much surface in proportion to its weight, and therefore to its moral inertia or force, as the
other, it will go much farther than the other.
This important rule explains why shells and large shot may be thrown
four or five miles, while smaller cannon-balls, musket-bullets, pistol and
swan shot, and the common small shot of the sportsman, all of which are
generally discharged from their respective pieces with the same commencing
velocity, have a shorter range, as the size of the projectile is less.  Even
water is sometimes thrown from a gun or powerful syringe to stun birds,
that they may be obtained with uninjured plumage; but it soon divides in
the air so minutely that it reaches only a short distance.
Water falling through the air from a great height, goes on suffering a
gradual division into smaller and smaller portions, which at last may be said
to be nearly all surface: and then the resistance of the air lets them fall
very slowly indeed. The relation of the size and resistance is well shown
by the difference of celerity in the descent of a minute fog, a drizzling mist,
and common rain.  The toy called the water-hammer, is merely a little
water enclosed in a tube exhausted or empty of air; and when, by turning
the tube, the water is made to fall from one end to the other, as there is no
air to impede or divide it in its descent, it falls as one mass, and makes a
sharp noise like the blow of a hammer.
The same law explains why a spider's thread or a single filament of silk
floats so long in the air before it falls;-why there is almost constantly suspended in the air, wherever active man resides, that immense quantity of
very minute solid particles, which when rendered visible by the sun's light
passing directly through them, are called motes in the sunbeam —particles of
which are constantly settling on household furniture, and rendering necessary
the daily operation of dusting or cleaning; — why the fine dust sent aloft
during the eruption of volcanoes is often carried by the wind to a distance
of hundreds of miles;-why in the deserts of Africa the strong winds often
transport fine sand from place to place, overwhelming caravans, and forming
new mountains, which succeeding blasts are'again to lift;-why in the
bottom of a river, or in a tides-way, fine mud is found where the current is
slow; sand where it is quicker; pebbles, or large stones, where it is quicker
still; while in rapids and water-falls, only massy rocks can resist the fluid
force. Now rocks, pebble, sand and mud, may all be the same material in
portions of different magnitude.
This law explains the operation of leviyating, by which substances insoluble in water are obtained in the state of a very fine powder. Any such
substance is first ground or powdered in the ordinary way, and mixed with
water. The grosser parts then soon fall to the bottom, while the fine dust
15




226                        H YD RAULICS.
remains longer suspended. This is afterwards obtained separately by pouring the fluid which bears it into another vessel, and allowing more time for
the slow subsidence. The fine powder of flint used in the manufacture of
porcelain'is obtained by levigation; as is also that of calamine stone, and
other powders used in medicine and various arts.
This law farther explains how, by means of air or water, bodies of different specific gravities, although mixed ever so intimately, may be easily
separated.  If pieces of cork and lead be let fall together through the air,
the lead will reach the ground first, and may be swept away before the cork
arrives; but in a vacuum the whole would reach the ground at the same
time as is proved by the common experiment of the guinea and feather
falling in the exhausted receiver of an air-pump. Again, when a mixture of
corn and chaff, as it comes from any threshing machine, is showered down
from a sieve in a current of air, the chaff being longer in falling, is carried
farther by the wind, while the heavier corn falls almost perpendicularly.
The farmer, therefore, by winnowing in either a natural or artificial current
of air, readily separates the grain from the chaff; and, if he desire it, may
even divide the grain itself into portions of different quality. Similar to the
operation of separating chaff from corn by wind, is that of separating sand
or mud from  gold-dust by water:-the soil containing' gold-dust is first
spread on a flat surface, over which a current of water is then made to pass;
which current carries away the lighter rubbish, and leaves the gold. If a
mass of metal be affixed on the end of a rod of wood, the rod then, whether
simply falling through the air, or advancing as an arrow, will follow the
heavier metal as its points. The cork of a shuttlecock is always foremost
for the same reason.
The instances enumerated under this head serve to show how many and
varied the results may be which flow from a single principle.
When a fluid and a solid meet each other obliquely, the impulse or effect is
still perpendicular to the surface of the solid, as if they met directly, but
is less forcible as the obliquity of the approach is greater.
Suppose a b to represent the upper edge of a smooth board or of any flat
polished surface, standing in a current, the fluid approaching this surface
in whatever direction, must act upon it as if approaching perpendicularly,
because on account of its smoothness, the fluid can take
Fig. 110.      no hold of it to push it endways, either towards a or b., But the impulse of a stream acting on the surface will
be less forcible if the surface be oblique to the stream,.'   both because less fluid will touch, and because the velocity of the effective approach will be less. The line c d
marks the breadth, and therefore force, of the part of
M/'"X"is   a stream reaching the board directly; and the shorter,-XC   linefe marks the smaller breadth that can touch it, of
i   a stream coming obliquely in the direction c b: in the
X/          oblique stream, moreover, if the line c b mark the whole
b  /v        Id   velocity, the shorter line c a marks the slower rate of
the direct approach of any one particle to the board.
(This subject was treated of at page 57, under the head
of Resolution of Forces.)
Hence the wind blowing upon the sail of a ship, however obliquely




OBLIQUE  FLUID  ACTION.                           227
always presses it directly forward or perpendicularly      Fig. 111.
to its surface, but acts less forcibly as the obliquity is
greater.  If the wind be represented, as to direction
and strength, by the line e d approaching the sail a b,
it will act on the sail as if it came fromf, but with   c
the smaller forcefd, instead of the whole force e d.
The effect, therefore, is the same as if the sail were
pulled by the rope d c.'We see in this how a ship
can be made to sail in a certain degree against the
wind:-for all the sails being adjusted so as to receive
the wind in the direction here shown, they all act to
produce the same result as if ropes were pulling from
each in the direction of d c; and a force likefd, or a rope like d c, urging sideways as well as forwards-as instanced in the tow-rope of a canal boatmakes the vessel advance rapidly forward, but scarcely at all sideways, because the form of vessels causes them to pass forward at least twenty times
more easily with their sharp bow than sideways with the long keel; and
therefore a force, urging equally sideways and forwards makes a ship advance
twenty miles in the direction of her keel, that is forwards, for one mile which
she deviates sideways. The deviation sideways, which, in sailing vessels,
must take place to a certain extent whenever the wind is
at all oblique, is called the lee-way.                       Fig. 112.
A vessel having to sail from b to a, while the wind            a
blows directly against her course, or from  a to b, is
obliged to sail close to the wind, as represented in fig. 112,         c
first perhaps to e, as represented by this figure, with the
left or larboard side to the wind, then to tack as it is  c<- 
called, or turn around, at e, and to sail to d, with the
right or starboard side in the wind; then to go on the larboard tack again to c, and thence to the port at a.                    e
In making way against a contrary wind, the sails of a
ship are pointed so nearly edgewise to the wind, that             b
unless very fiat, a great portion of their surface becomes useless.  The
Chinese manner of rigging is, in this respect at least, superior to the European; for in it bamboo reeds attached across the sails, render them as flat
as boards.  When a Chinese ship has her sails pointed edgeways to a spectator, he only sees the masts which support them.
The reason why a ship of several masts generally sails faster when the
wind is more or less from a side, than when directly astern, is, that in the
former case all the sails are acting, although individually not to the best
advantage, while in the latter the sails in front are becalmed by those behind
them. A ship with a side-wind may move faster than the wind itself, as is
often true of the outer extremities of the wind-mill's vanes. A corresponding relation of motions is observed when a slippery wedge is forced out two
or three inches laterally from its place, by a weight which descends only one
inch perpendicularly.
The law now under consideration explains the action of the rudder of
ships,-that contrivance, by which a single steersman can direct the course
of an enormous vessel through rocks and shoals more steadily and safely
than an adroit charioteer can guide his tiny vehicle on a common road. The
helm or rudder is a fiat projection from the stern-post of the ship, turning on
strong hinges, in the manner of a door or gate, and moved by a beam or lever




228                       HYDRAULICS
Fig. 113.   called the tiller, which proceeds from it forwards to where the
steersman stands. In small vessels the tiller is above the deck
and the steersman applies his hand directly to it; but in large
ships it is below, and is moved by ropes, rising from it to the
wheel on the deck, where the steersman stands, with the compass before him.  While the rudder points directly astern, as
to a, like a continuation of the keel and stern post, it does not
C\I     affect the vessel's course; but if it be inclined ever so little to
one side, as to b on the left or. larboard side, the water imme~b?~i m  diately acts on it in the direction of c b, perpendicular to its
surface, and pushps the stern to the right or starboard side,-.
an action equivalent to pulling to the left or larboard.
It is possible to make a ship or boat steer itself, by placing a powerful
vane on the mast-head, and connecting it with the tiller-ropes by two projecting arms from its axis. If. it were desired to make the ship sail directly
before the wind, the tiller-ropes would be fixed to the arms of the vane so
that the helm should be in the middle position, when the vane was pointing
directly forward: should the vessel then from any cause deviate from her
course, the vane, by its changed position with respect to her, would have
produced a corresponding change in the position of her helm, just such as
to bring her back to her course. Again, it is evident that, by adjusting such
a vane and rudder to each other in different ways, any other desired course
might be obtained, and which would alter only with the wind.  The vane,
to have the necessary power, would require to be of large size; it would be
a wide hoop, for instance, with canvas stretched upon it; and the rudder, to
turn with little force, might be hung on an axis passed nearly through its
middle, instead of, as usual, by hinges at one edge.  Cases have occurred
where shipwrecked persons might have sent intelligence of their disaster to
a distant coast, by a small vessel, or even a block of wood fitted up in this
way. The method admits also of other applications, particularly in war.
As fluids act on surfaces, in a direction perpendicular to them, the water
on the right side of a ship's bow is always pressing it towards the left side,
but owing to the equivalent and contrary pressure there, the ship holds her
course evenly between the two, or straight forward. When a ship, however,
owing to the side wind, lies over, or heels, as it is called, that side of the bow
which sinks most is more pressed than the other; and were there not a
counteracting inclination of the rudder then made, constituting what is
called weather-helm, the ship's head would come round to the wind.  Now
ships so rarely have the wind exactly astern, that to diminish the almost
constant necessity for weather-helm, the mast or masts, and consequently
the mass of the sails are placed more towards the bow than the stern.
Again, because the bow of a ship is oblique downwards as well as sideways, the water, when she moves, is constantly tending to lift the bow;
hence when the vessel is dragged by a low horizontal rope, as in the case of
a boat attached to a sailing ship's stern, or is moved by paddle-wheels like
steam-boats, the bow rises much out of the water, and the stern sinks in the
hollow or furrow of the track: but when she is driven by sails, as these are
high on the mast, and are acting therefore on a long lever to depress the
bow, the two opposite tendencies just balance each other, and the vessel sails
evenly along.
The form of the fore part of a ship has less influence upon her speed of
sailing than the form of the hind part, called the run, from the middle to the
stern. When a ship is at rest, there is of course as much forward pressure




OBLIQUE FLUID ACTION.                          229
of water about the stern as of backward pressure on the bow; but when she
sails, she is running away from the propelling pressure, and is increasing
the resisting pressure. A gradual tapering of the hind part, therefore, or a
fine run, as it is called, which allows the water to apply itself readily to it,
as it passes along, must influence much the rate of sailing. The fore part
of any mass drawn through the water, however blunt or square, becomes in
effect sharp or rounded by a quantity of water which it pushes on before it.
A tree, or the tapering mast of a ship, can be drawn through the water
more easily with the large end foremost than in a contrary way.
The common windmill furnishes another illustration of the action of fluids
on oblique surfaces. The face of the windmill
is turned directly to the wind, but the four flat      Fig. 114.
vanes or sails, of which the great wheel consists,
are individually oblique.  Thus the edge a of the
vane a e, is more forward as regards the coming
wind or a spectator in front, than the edge e, and  d\
the action of the wind, therefore, being perpen-                   b
dicular to the oblique surface a e, pushes it in a
degree towards a. The same remark applies to
each of the other vanes where the edges b c and          c
d are in front and those marked by the fainter
lines are behind; so that each vane produces an
equal effect in turning the wheel. The law of the " decomposition of forces,)'
explained in page.57, tells in what proportion the force of the wind is exerted
to push the wheel backwards against its supports, and to turn it round.
Windmills were first used in Europe in the fourteenth century, and they
are still of great importance in countries where there are no waterfalls, and
little fuel for steam-engines. In some of the richest European landscapes,
every height is crowned by its busy windmill, grinding corn, or sawing
wood, or pressing oil-seeds; and over the plains, similar wheels are pumping water for domestic use, or incessantly draining the land.
The smoke-jack of our chimneys is a small windmill, driven by the
ascending current of air in the chimney.
The feathering of an arrow acts in part on the principle of the windmill.
The feathery projection from the shaft is not quite straight, but winds round
it a little, like the thread of a screw; and the arrow, therefore, constantly
turns as it flies, and goes straight to its object although the shaft itself be
bent, because any deviation is constantly correcting itself.
The rifle-barrel in fire-arms has spiral furrows or threads along its interior
surface, so that the bullet in passing out receives a turning motion corresponding to that of an arrow, and producing similar results. A bullet which
receives any other turning motion than round the line of its course —and
most bullets from a common barrel do acquire such, owing to the irregularity
of their form, or unequal friction at the mouth of the piece-is sure to
deviate from its course, because unequally pressed or resisted by the atmosphere. A good rifle fixed to its place will send a succession of shots through
the hole made in the target by the first shot, at the distance of 200 yards.
Duels have been fought with rifles, and the parties having fired at the same
moment, have been corpses the moment after.
It might.be supposed that a wheel which the wind turned by direct action
on the rim, as water turns common water-wheels, would be preferable to the
windmill-wheel above described, which is turned by oblique action on the
face: accordingly, a wheel like a water-wheel, only with broader vanes, has




230                       HYDRAULICS.
been placed in a house or cover, so that only one side at a time was exposed
to the wind; —jut it is a powerless machine. The oblique vane wheel may
apply to use only half or less of the force of the air which' reaches it, but
its wide expanse receives a stream of air of thirty feet in diameter, while an
ordinary window would admit that required for a wheel of equal size of the
other construction.
There are some situations where it would be an advantage to have waterwheels like the common windmill-wheel, viz, where the stream is sluggish,
and is deep enough to allow a large wheel to be wholly immersed.
A small wheel of this sort, with broad oblique vanes, has been used as a
means of ascertaining the rate of a ship's sailing. It is allowed to drag
astern in the water; and the number of revolutions made in a given time
marks the ship's speed.
A windmill-wheel made to turn during a calm by force applied to its axle,
would be pressed endways, or in the direction of its axle, just as if wind
were blowing upon it, owing to the reaction of the still air, through which
its oblique vanes were made to sweep. Such a form of wheel fitted to work
in water, and called a water-screw, has been applied at the bow or stern of
steamboats to propel them in canals where there was no room for side
wheels. But as from the obliquity of the surfaces only a part of the applied
power becomes propulsive-the remainder being wasted in the lateral strain
or twisting of the water-the method is not applicable to general purposes.
Two small windmill-wheels placed horizontally one above the ohber, on
the same axis, and made to turn in opposite ways by springs or otherwise,
would rise in the air, carrying a certain load with them, and would constitute, therefore, a flying-machine.
The effect of a single oar projecting from the stern, used to propel a boat
or vessel, in the manner called sculling, is referable to the law now under
consideration. The oar or scull rests on a round-headed prop or nail at the
stern, and is made to vibrate from side to side. In all positions it has the
surface which pressess the water turned obliquely backwards; hence the
reaction of the water drives the boat forward. In China, large vessels are
moved by a single sculling oar, which half the ship's company may be urging
at the same time. A sculling oar may be regarded as a single vane of such
a propelling-wheel or water-screw as above described, made to sweep across,
behind the vessel, alternately to the right and to the left.
The action of a fish's tale and of the bending of an eel or snake in water,
partly resembles that of the sculling oar. Many people believe that the tail
of the fish is only the rudder of the body, and that the fins give it forward
motion-as is true of a bird's tail and wings-but the fish's tail is.in fact
the great instrument of motion, while the fins serve
Fig. 115.       chiefly to steady and direct the motion.
A paper kite rising in the air is another example
belonging to this place.  Its cord d is attached to it. above the middle of its loop, and therefore so as to
make it present always an oblique surface to the wind;
c    and by the action of the wind, perpendicular to its
surface, it rises as if pushed up in the direction c a,
or as if drawn up in the direction of a b.  A kite
might be made large enough to lift a man.  Cats
have been sent up at kites' tails, and have fallen
down safely under parachutes from the greatest elevations. It might be safer for a man to rise at a kite's




LIFTING  OF WATER.                          231
tail to reconnoitre an enemy's position, or to survey an unknown country,
than under a balloon, as was practiced by the French during the revolutionary
wars. He might have the security of a parachute, and the power of regulating the obliquity of attachment of the rope, so as to command his ascent or
descent at pleasure. An exhibition was made in October, 1827, between
Bath and London, of a car drawn along the highway by kites. That they
might ascend to a great elevation, where the wind is generally stronger than
below, they were attached to each other in a row, so that the second kite
mounted as if its cord were held by a hand at the first, the third as if rising
from the second, and so forth. The projector of this novelty hoped that he
had pointed out a most valuable means of travelling across extensive plains,
sandy deserts, tracks of snow, &c., and in all cages, nearly with the speed of
the wind.
" Fluids lifted in opposition to gravity."  (See the Analysis.)' Water, as we have seen in former parts of this work is to the living universe, in some degree what the blood is to the animal body, and a constant
supply and circulation are required. This supply has been provided for to
an extraordinary extent, by the operation of natuial causes; but for many
purposes of human society, water is still required where none natuPally
exists. A great variety of means have been employed in raising it, some
of which, sufficient to illustrate the whole, are now to be considered.
Water may be raised in a bucket which is attached to a rope to be pulled
up by the hand.-The rope carrying the bucket may be drawn up more
easily by being vound round a barrel or axle turned by a winch.-There
may be a succession of buckets on a rope, rising one after the other, and
when emptied, descending again on the opposite side of the wheel or axle
which lifts them: the rope to which they are attached being a circle or endless rope, and constituting with them what is called the bucket-machille.Instead of buckets on such an endless rope or chain there may be a succession of fiat pieces of wood, which, on being drawn up through a large tube
or barrel, like loose-fitting pistons, will raise a copious stream of water; this
is the contrivance called the chain-pump.-Or simply an endless rope of hair,
very rough, passing round one wheel above, another below, may be whirled
quickly by turning the upper wheel, so that a mass of water adhering by
friction to its rising half, shall be thrown into a reservoir at the top where it
passes over the upper wheel: several such ropes may be joined side by side
to increase the effect. —But the most important of all water-raising engines
are the lifting and forcing pumps, already described at pages 171 and 172.
They are used to draw from wells, to drain mines, to send a supply over
cities, to pump ships, to throw water for extinguishing fires and for many other purposes.           Fig. 116..
A stream of water passing through a garden,
or in the midst of fields, may have beauty with
little utility, unless it can be employed to irrigate
the vegetable creation around.  In the fields and
gardens of Persia, where the heat of the sun is
very intense, the streams are caused by their own
action, to lift a part of'their water into elevated
reservoirs, from which it again flows in sloping
channels to wherever it is required. A large
water-wheel is placedso that the stream may turn




232                      HYDRAULICS.
it, and around its circumference buckets are attached to be filled as they
sweep along below,.and to be emptied into a reservoir as they pass aboveor instead of buckets, the spokes of the wheels themselves are made hollow,
and curved as here represented, so that their extremities dip into the water
at each evolution, they receive a quantity of it, which runs along them as
they rise, and is discharged into a reservoir at the centre. These are usually
called Persian wheels, but they are as commonly employed on the banks of
the Nile and elsewhere as in Persia.
A pipe wound like a screw upon a sloping barrel, and made to dip its lower
mouth into water at each revolution of the barrel, will also raise water; the
lower portions of the turning pipe will always be full of it, and it will be
rising in them to the top, as if on
*    Fig. 117.              an inclined plane.  Archimedes
was the inventor of this beautiful
water-screw, and his name has remained to it. It may be turned by
the hand or by a passing stream
which acts on the vanes of a waterwheel fixed upon it.
Water may be raised by producing Centrifugal force at the upper end of a bent pipe which dips into a reservoir. Supposing the pipe to be bent as here represented,
Fig. 118.  and the horizontal arm a to turn like the spoke of a wheel,
round the upright portion as the axis, if the pipe be once
CZ,      >   filled with water, and be turned with sufficient speed, it will
continue to throw out a constant stream frorg the end a. To
increase the discharge there may be several horizontal arms
from one large upright pipe, and emptying themselves into a
circular trough or reservoir; and to prevent the necessity of
refilling the apparatus after every interruption of its motion,
a valve opening upwards must be placed at the bottom. This
contrivance has been called the centrifugalpump, because the
water is raised at b as in a pump, by the pressure of the atmosphere, to supply the place of that which is thrown out from a by the centrifugal force.
The velocity of rotation must bear proportion to the height of the discharging
aperture a, above the surface of the water in the reservoir.
It had long been observed in household experience and elsewhere, that
while water is running through a pipe, if a cock at the extremity be suddenly
shut, a shock and noise are produced there. The reason is, that the forward
motion of the whole water contained in the pipe being instantly arrested, and
the momentum of a liquid being as great as if a solid, the water strikes the
cock with as much force as if it were a long bar of metal or a rod of wood
having the same weight and velocity as the water. Then as a fluid presses
equally in all directions, a leaden pipe of great length may be widened, or
even burst in this experiment.-Lately this forward pressure of an arrested
stream has been used as a force for raising water, and the arrangement of
parts contrived to render it available has been called, on account of the shocks
produced, the water-ram. The ram may be described as a sloping pipe in
which the stream runs, having a valve at its lower end, to be shut at intervals to arrest the stream, and having a smal tube rising fromn near that end
towards a reservoir above, to receive a portion of the water at each interruption. Now water allowed to run for one second, in a pipe ten yards long,




LIFTING   OF WATER.                          233
two inches wide and sloping six feet, acquires
momentum enough to drive about half a pint,        Fig. 119.
on the shutting of the cock, into a tube leading to a reservoir forty feet high.  Such an
apparatus, therefbre, with the valve shutting
every second, raises about sixty half-pints or
four gallons in a minute.  The valve is so
contrived that the steam works it as desired  -    -
In this figure, which represents the lower end   e  B
of the water-ram, a is the opening by which
the stream escapes from it, and the valvedor
flap seen below the opening is that which by suddenly shutting arrests the
stream. The valve is made so heavy, that the. stream must. run for a certain
time to acquire force enough to shut it: and in the instant of its shutting,
a little of the advancing water passes upwards through the valve b towards
the reservoir. The water in the main pipe then becoming stagnant again,
no longer has power, by its weight alone, to keep the valve a shut: this,
therefore, falls open, and the stream begins again, again to be arrested as
before; and as long as the supply of water lasts, the action of the apparatus
continues. The action of a water-ram has been compared to the beating of
an animal's pulse. The upright tube has usually a reservoir at the bottom,
where it first receives the water, constituting there an air-vessel b, (described
at page 161) which, by the air's elasticity, converts the interrupted jets first
received, into a nearly uniform current towards the reservoir. The supply
of air to this vessel is manifested by the contrivance called a shi'ftiny-valve.
In the preceding examination of the dgctrines of fluidity, we have had to
touch on many of those phenomena of nature and art which are the most
important to man; yet we have seen how beautifully simple and intelligible
they are all rendered when referred, by a methodical arrangement, to a few
heads dependent on the "fundamental truths."  Each one of the many
particulars belonging to this department, and which, when now explained,
appears so simple and obvious, has yet been a distinct step in the slow
progress of discovery or invention, and probably, when first made, has filled
some ingenious mind with intense and purest delight.




234                        ACO U S T I C S.
PART III.
(CONTINUED.)
SECTION IV.-ACOUSTICS,
OR PHENOMENA OF SOUND AND HEARING.
ANALYSIS OF THE SECTION.
1. SOUND is heard when any sudden shock or impulse is given to the air,
or to any other body which is in contact directly or indirectly with the ear.
2. If such impulses be repeated at very short intervals, the ear cannot attend
to them individually, but hears them as a CONTINUED SOUND, which is
UNIFORM, or what is called a TONE, if the impulse be similar and at equal
intervals, and is GRAVE or SHARP, according as they are few or many in
a given time; and all continued sound is but a repetition of impulses.
3. When the number of impulses in a given time producing some uniform
continned sound has a simple relation, as of half, third, fourth, &c., to
the number producing some other such sound, which is heard either sihnultaneously with it, or a little before or after, the ear is generally much and
pleasingly affected by the circumstance; and the sounds are said to have
MUSICAL RELATION to each other, or to be ACCORDANT, while all others
are termed DISCORDANT.
4. The shock which causes the sensation of sound SPREADS or is propagated
in all bodies, somewhat as a wave spreads in water, wtith decreasing
strength as the distance increases, but with a velocity nearly uniform, and
which in air is about 1,142 feet per second.
5. Sound is REFLECTEDfrom smooth surfaces, and hence arise many curious
and pleasing effects called ECHOES, &c.
6. Th/e structure of the ear illustrates the law of sound.
EARLY inquiries into nature had remarked that in most instances of noise
or sound there was present a shock or trembling of the sounding body, often
visible, but sometimes only sensible to the touch, or discoverable by other
means; it was noted for instance, in the string of a harp, in the reed of a
hautboy, in -the prongs of a tuning-fork, in the lip of a bell; but it was
reserved for the moderns to understand fully, that the animal organ called
the ear, is merely a structure of parts admirably adapted to be affected by
the concussions or tremblings of things around; and that sounds in all their
varieties are merely such motions, affecting the ear through the medium of




CONTINUEb SOUND.                           235
the air which surrounds it, or of some other body, or series of bodies reaching from the trembling thing to the ear.
The delicacy and complexity of an organ destined to feel and to distinguish such slight and varying influences, and the vast importance of it to
man, as that which makes him capable of using language, besides being his
ever-watchful monitor of surrounding occurrences, and the chaninel by which
the fascination of music enters, render this subject, to all who love to read
in nature the attributes of its author, -a most favourite study.
Because all the bodies around us are immersed, in common with ourselves,
in the ocean of air which covers the earth, we are much more frequently
warned of the shocks and tremblings of which we have been speaking, by
their effect on the air, than in any other way; hence the early prejudice that
air was necessary to sound, and hence, in part, the reason why the doctrines
of sound have generally been accounted a part of pneumatics. We shall
now find, however, that all bodies are more or less fitted to convey these
tremblings, and that air in many cases is neither the quickest nor the best
conductor.  Although our notions on the subject are thus corrected, it is
still convenient to study the theory of sound as a part of Pneumatics.
1. " Sound is heard when any sudden shock or impulse occurs in a body
having communication through the air or otherwise, with the ear."
(Read the Analysis.)
Common instances of a single impulse are- the blow of a hammer-the
clap of hands-the crack of a whip-a pistol-shot-any explosion —the
thunder-clap.
The loudness of sound conveyed by the air depends on the air's density. A
bell enclosed in the receiver of an air-pump is heard less and less distinctly
as the air is exhausted, and in a vacuum is not heard at all.-Even the blow
of a hammer, in a vacuum, is not heard if care is taken to prevent the shock
from being communicated through neighbouring solid bodies.-In the thin
air surrounding a lofty mountain-top the report of a pistol is much less loud,
and human voices are weaker.-In the condensed atmosphere of i divingbell a whisper is loud.-When volcanoes and various other Iesemblances
to the constitution of our earth were first discovered in the moon, some
persons fancied that during the stillness of night we should hear the thunder
there: —but supposing the thunder to happen, and to be ever so loud, it
could not be heard on earth, because there is no medium to bear thither the
pulses of sound-there is a vacuum between.
2. Impulses quickly repeated cannot be individually attended to by the ear,
and hence they appear as one continued sound, of'which the pitch or tone
depends upon the number occurring in, a given time; and all continued
sound is but a repetition of impulses.  (Read the Analysis.)
If a wheel with teeth be made to turn and to strike any elastic plate, as
a piece of quill, with every tooth, it will, when moved slowly, allow every
tooth to be seen and every blow to be separately heard; but with increasing
velocity the eye will lose sight of the individual teeth, and the ear ceasing
to perceive the separate blows, will at last hear only a smooth continued
sound called a tone, of which the character will change with the velocity of
the wheel.
In like manner the vibrations of a long harp-string, while it is very slack,
are separately visible, and the pulses produced by it in the air are separately




236                         ACOUS TICS.
audible; but as it is gradually tightened, its vibrations quicken, so that,
where it is moving, the eye soon sees only a broad shadowy bellying line;
and the distinct sound which the ear lately perceived, seeming now to run
together on account of the shortness of the intervals, are felt as one uniform
continued tone, which constitues the note or sound then belonging to the
string.
Again, if a current of air passing through a tube or opening, be in any
way interrupted at regular and very short intervals, as by a little stop-cock
placed in the opening, of which cock the plug, instead of being only partially
turned by a cross handle, as in a common beer.cock, has a wheel fixed upon
it, so that any desired rapidity of rotation may be given to it, —then at every
time when the passage for air becomes open, there will be a certain shock
given to the air around, and the repetition of such shocks will constitute a
musical tone. This apparatus can produce all tones, and it enables us with
great precision to ascertain the number of pulses required to constitute any
given tone.
It is the elasticity of any string used to produce a tone which causes the
repetition of the percussion, and therefore the continuance of the sound,
thus:-the string having been pulled at its middle to one side, and then let
go, is, owing to its elasticity, carried back quickly to the straight position;
but by the time that it has reached this, it has acquired a momentum which,
like the momentum of a vibrating pendulum, carries it nearly as far beyond
the middle. station as the distance whence it came:-it has to return,
therefore, by its elasticity, from this second deviation, in the same way; but
still passing the middle as before, it has again to return; and thus continues
vibrating uniformly as a pendulum does, until the resistance of the air and
friction gradually bring it to rest.  A large vibration of any string, like a
large oscillation of a pendulum, occupies very nearly the same time as a
smaller, because the farther that the string is displaced or bent, the more
forcibly, and therefore quickly, is it pulled back again by its elasticity;
hence the uniformity of the tone produced by a musical string is not injured
by the different force with which the finger of the player may touch the
string. According, however, as the vibrations of a string are more extensive
or quicker, the impulses given to the air are more sharp or forcible, and
hence the sound becomes louder. And this explains why sharp sounds are
generally also loud. Vibrations which are comparatively few and slow,
strike the ear very gently, as in the flapping tf a pigeon's wing, or in the
play of a switch.
The most familiar instance of sounding vibration is that of an-elastic cord
extended between two fixed points, as in stringed instruments of music; but
because elastic bodies generally, when by any force their natural form is for
a time altered, recover it when allowed, not by a first effort, but like the
string of a pendulum, after a series of oscillations, almost all such bodies
repeat many times an impulse once given to them, and thus may become the
means of producing a continued sound.-If a solid rod of steel, glass, or any
other elastic substance, be firmly fixed at one end and left free at the other,
and if that other be then pulled a little to one side of its station or rest, and
suddenly let go, it will immediately seek its station again, but by the momentum acquired in the approach, will go beyond it: it will then return as before, but again to pass, and so will continue to vibrate with diminishing force
for considerable time.-A boy at school, thus, sticks the point of his penknife into the bench, and by one touch makes it produce a continued uniform
sound of considerable duration.-The prongs of a tuning-fork, or of the cornm-.




CONTINUED SOUND.                            237
mon sugar tongs, vibrate and sound in the same way. In the musical snuffboxes and chimney-clocks, the sounds are produced by the vibration of little
rods of steel, fixed by one end, in a row, like the teeth of a comb, and
touched by small pins or points projecting from a turning barrel.  Any
elastic flap, as of metal or of tough wood, placed over an opening, so as to
stand away from it a little when not pressed by passing air, but to close the
opening if so pressed, becomes a sounding reed when air is gently forced
through the opening: thus, the air pressing on the flap to close them causes
a momentary interruption of the current, but the flap immediately recoiling
from the blow, as well as by reason of its own elasticity, again opens the
passage, and the continued rapid alteration of the shutting and opening produces the tone. -,The reed of a clarionet is a thin plate-of elastic wood, made
to vibrate in this way. The drone of the bag-pipe and the common strawpipe, are reeds of nearly the same kind. The Chinese organ, and the sweet
instrument lately introduced under the name of ]Eolian, have reeds which
differ from these, by beating through the opening instead of merely on its
face. Elastic rods simply resting on supports at both ends, or suspended by
their middle, will also vibrate; a musical instrument is thus made of pieces
of glass laid upon two strings, and struck by a cork hammer: in the island
of Java, a rude instrument of the same kind is made of blocks of hard
elastic wood. A portion of a hollow sphere of elastic metal very readily
takes on a vibration, during which its form is constantly changing from the
perfect round to the oval, and conversely; there are consequently repeated
percussiofi of the air, and a continued sound, and the thing is called a bell.
A bell admits a great variety of shapes, and may be made of any elastic substance, as metal, glass, earthenware, (buyers sing earthenware to ascertain
its soundness,) and even of hard wood. The Chinese gong is a metallic vessel
shaped like a common sieve, having a manner of vibration very peculiar,
and producing sounds that are rousing and sublime. The drum has a tense
elastic membrane on which the blows of the drum-stick are received: its
tone ceases quickly, because the motion of so broad a surface is much
resisted by the air.  In the flute, flageolet, common organ-pipes, &c., the
air is forced through narrow passages, and is divided by sharp edges, in such
a way as to suffer repeated but perfectly regular condensations or interruptions sufficient to affect the ear; and hence the endless variety of sweet
continued sounds which these contrivances are known to produce.
To the production of a tone, it is of no consequence in what way the
pulses of the air are caused,,provided they follow with sufficient regularity;
witness, in addition to some of the instances given above, the pure sound
produced by the motion of a fly's wing-supposed by many to be the voice
of an insect.  The clacking of a corn-mill, and the noise of a stick pulled
along a grating, are not tones, because the pulses follow too slowly.
Where a continued sound is produced by impulses which do n4ot, like
those of an elastic body, follow in regular succession, the effect ceases to be
a clear uniform sound or tone, and is called a noise.  Such is the sound of
a saw or grind-stone —the roar of the waves breaking on a rTocky shore, or
of a violent wind in a forest-the roar and crackling of houses or of a wood
in flames —the mixed voices of a talking multitude-the diversified sounds
of a great city, including the rattling of wheels, the clanking of hammers,
the voices of street-criers, the noises of manufactories, &c.; which rough
elements, however, at last mingle so completely that the combined result
has often teen called "the hum of men," from analogy to the smooth
mingling miniature sounds which constitute the hum of a bee-hive.




238                        ACOUSTICS.
" Grave and sharp sounds."  (Read the Analysis.)
The difference of sounds, which depends on the different number of vibrations of the sounding body in a given time, divides them into those called
bass, low, or grave notes, for comparatively few and slow vibrations; and
those called high, shrill, or sharp, for vibrations more numerous and quick.
The frequency of vibrations in strings increases with their shortness, lightness and tension —for if a string be long or heavy, there is a greater mass of
matter to be moved, and hence a slower motion; and if a string be slack,
the force of elasticity which pulls it from any deviation back to the straight
line is so much the less.  It is found that a string taken of half the length
or of one-fourth the weight, or of quadruple the tension of. another string,
vibrates twice as fast on any one of these accounts.
These truths are familiarly illustrated in the violin.  The low or bass
string is thick and very heavy from being covered with metallic wire, and the
others gradually diminish in magnitude and weight, up to the smallest or
treble.  The strings are tuned to each other by being attached by one end
to moveable pins, which, when tuned, increase or diminish their tension; and
the sound produced by each may be afterwards varied to a certain extent,
by the performer pressing different parts of it with the finger against the
board, so as to shorten the vibrating portion.
An analogous law, as to the influence upon tone, of weight and dimensions, holds with respect to bells, glasses, reeds, &c., and enables us to use
these also in the construction of musical instruments.
3. " When the number of impulses producing some continued sound has a
simple rotation, as of half, third, fourth, &c., to the number producing
some other sound, which is heard either simultaneously, or a little before
or after it, the ear is much and pleasingly affected; and the sounds are
said to have musical relation to each other, or to be accordant, while all
others are termed discordant."  (Read the Analysis.)
Understanding now that all continued uniform sounds are produced by a
repetition of similar beats or vibrations, we perceive that.in the series from
grave to sharp, there must be such as, with respect to the number of beats
in a given time, are related to each other, as the numbers 1, 2, 3, 4, &c., or,
which is the same thing, as 10, 20, 30, &c.  Now as between two sounds,
one of which has 20 beats while another has 10, there will be a coincidence
by every second beat of the quicker, and between sounds whose beats are to
each other as 30 to 20, there must be a coincidence at every third beat of
the quicker, and so forth, we should naturally expect the ear to be differently
affected by such correspondence than when the coincidence is either less
frequent, or is irregular.  Accordingly we find that all sounds which have
such simple relations to each other, are remarkably agreeable to the ear,
either when heard together, or in close succession; while those in which the
coincident beats are farther apart, are heard with indifference, or are felt to
be positively harsh or disagreeable. It is in fact offering itself to be noticed
here, that the coincident or double pulses of any two concordant sounds become the cause of elements of a third sound, perfectly distinct from them,
but which is always heard with them, and is called their grave harmonic or
resultant: it is the same as a simple sound having as many vibrations in a
given time as there are coinciding beats between the two other sounds.
If a long musical string be made to sound, and the number of its vibrations
in a given time be ascertained, we find that if only half of it be allowed to




MIUSIC.                             239
vibrate at a time, as when a finger presses its middle against a board, that
half will vibrate twice as fast; and similarly, a third part three times as
fast; a fourth part four times as fast; and so on, producing the sounds or
tones most nearly related to each other. A fine illustration of this is afforded
by the string of a violoncello, when made to vibrate by a bow moved very
gently across it, near the bridge; for it often divides itself spontaneously
into two, three or four, &c., equally vibrating parts or bellies, with points
of rest between them called knots: when this happens, there are heard not
only the sound or note belonging to the whole length of the string, but,
also, more feebly, the subordinate notes belonging to its half, third, or
fourth, &c., according to circumstances, beautifully mingling with the first
sound, and forming with it a rich harmony. Often in such a case the subordinate sounds swell with such force as to overpower for a time the fundamental note: but any one such sound is rarely of long duration. The same
harmonic sounds. may be produced still more certainly, while drawing the
the bow across the string, by touching the string lightly with the finger, at
one of the points where we wish it to divide. Even a tune may be so
played.
The sounds thus' belonging to a single cord or string, and produced by its
spontaneous division into different numbers of equal parts, constitute, when
heard together or in succession, what may be called the simple music of
nature herself. It is produced pleasingly, as just described, by the single
string of a violoncello; but in the most perfect manner by the instrument
called the ZEolian harp.
The ZEolian harp is a long box or case of light wood, with harp or violin
strings extended on its face. These are generally tuned in perfect unison
with each other, or to the samepitch, as it is expressed, except one serving
as a bass, which is thicker than the others, and vibrates only half as fast;
but when the harp is suspended among trees, or in any situation where the
fluctuating breeze may reach it, each string, according to the manner in
which it receives the blast, sounds either entire, or breaks into some of the
simple divisions above described; the result of which is the production of
the most pleasing combination and succession of sounds that ear has ever
listened to, or fancy perhaps conceived. After a pause this fairy harp may
be heard beginning with a low and solemn note, like the bass of distant
music in the sky: the sound then swells as if approaching, and other tones
break forth, mingling with the first, and with each other; in the combined
and varying strain, sometimes one clear note predominates and sometimes
another, as if single musicians alternately led the band: and the concert
often seems to approach and again to recede, until with the unequal breeze
it dies away, and all is hushed again.-It is no wonder that the ancients,
who understood not the nature of air, nor consequently even of simple
sound, should have deemed the music of the IEolian harp supernatural, and,
in their warm imaginations, should have supposed that it was the strain of
invisible beings from above, come down in the stillness of evening or night
to commune with men in a heavenly language of soul intelligible to both.
But even now that we understand it well, there are few persons so insensible
to what is delicate and beautiful in nature, as to listen to this wild music
without emotion; while the informed ear finds it additionally delightful, as
affording an admirable illustration of those laws of sound which human
ingenuity at last has traced.
And the simple scale of sound, called a chord, which nature thus gives
by the spontaneous dividing of a single string, has considerable vacancies in




240                         A CO U STIC S.
it, human taste or feeling, long before there was any theory of music, had
joined to it the notes of two additional strings, one sharper or more acute
than it, and the other more grave; of which additional notes, while part
agreed, or were in unison with certain notes of the principal chord, the
remainqder just served to fill up its larger intervals, and to complete a scale
of nearly uniform interval-as three ladders having unequal intervals
between their steps, might still, if placed together, complete a stair of easy
ascent.  The relation between these strings or chords is such, that the principal beats thrice or twice of the low chord, and the high chord beats thrice
for twice of the principal: and in the complete scale of notes, the principal
is five notes above the lower and five notes below the higher. So truly
natural is the scale thus formed, that it has arisen in all nations, however
remote and unconnected; and On untutored individual in attempting to raise
his voice by regular steps, falls into it almost as readily as the learned professor. The scale has eight steps or notes between any tone, and the tone
above it vibrating twice as fast, or the tone below it vibrating half as fast;
these two tones or notes being hence called the octaves above and below the.note with which they are compared, and the intermediate notes which fill up
either octave from the fundamental note are distinguished by the names of
second, third, fourth, &c., in ascending or descending. The numbers which
express the relations of beats among the notes of an octave are easily found,
from  our knowing the relative number of beats in the notes of any one
simple chord, and the relation as above described of the three chords forming the compound scale. The following table exhibits these numbers or
the arithmetical expression for the notes of an octave, as well as the
corresponding lengths of a given string required to produce them, and the
English designation of the notes by letters, and the continental designation
by names, these names being the first syllables of certain verses sung by
learners.
Number of vibra-      1    9    5    4    3    5    1     2
tions.. 
Length of string      I    8    4             _            5
English characters    C    1}   E  i j       1   G A         C
Continental names  |ut   re  mni | fa  sol   l la   si    ut
The musical scale, however far extended, is a repetition of similar octaves,
so that any note in it vibrates just twice as often as the corresponding note
in the octave below, and half as often as that in the octave above. The
lowest note which is perceptible to the human ear has about thirty beats in
a second, and the highest about thirty thousand: and there is included between these two, a range of nearly ten octaves.  To certain ears the extremes
of this range are totally inaudible, as if their power did not reach so far.
Some persons do not hear at all the sharp note of the grasshopper, while
some are equally insensible to the lowest tones of an organ or piano; and
yet to all the perception of intermediate sounds may be equally perfect.
Few musical instruments comprehend more than six octaves, and the human
voice in general has only from one to three, the female voice being in pitch
an octave higher than the male.
If the intervals in the musical scale were all equal, a performer might
choose indifferently any note as a fundamental or key-note, and would only
have to attend to the number of intervals above and below it; but, in fact,
the relation of the three constituent chords is such that the third and seventh
intervals, in ascending from a key-note, are only about half as large as the
others.  It is owing to this circumstance that in changing the key on any




MUSIC.                             241
instrument, certain notes belonging to other keys are half a note too low or
too high, that is, tooflat or too sharp, and must be changed accordingly.
And hence, when an instrument is to be used to play on all keys, its larger
intervals must be divided into two parts. The fact of these unequal intervals, ill understood, is what gives an appearance of great complexity and
difficulty to musical science.
Melody, in music, is when notes, having the simple numerical relations
of beat which we have been describing, are played in succession; harmony
is when two or more such notes are sounded together. The effect of both
is delightfully increased by what is called measure, viz., making the duration of the notes or strains correspond with certain regular divisions of time.
This gives to the ear a prescience, to a certain degree, of what is coming,
with the pleasure of having expectation rearized, as happens similarly from
the metre and rhyme of poetry; it moreover enables the memory to retain
musical combinations of sound-for the airs of the iEolian harp, which
observe no time, cannot be learned or repeated. The music of a single
drum is that of time only.
Melody, harmony, time and varying intensity of sound, are the four constituents of music, and it seems that almost every state of mind has, in some
combination of these, an appropriate expression, intelligible to the general
feelings of the human race.  The exact relation between the movements of
the animal spirits, as it has been expressed, or the fluctuating stream of
feeling, and the varying flow of sound in a musical composition is not clearly
understood, but the fact of their correspondence and its consequences are
most remarkable. Under many circumstances, the association between the
feeling and expression is so strong, that the latter is often spontaneously betraying itself;-witness the almost constant humming or low song of some
contented beings —the singing and whistling of careless childhood, or of the
light-hearted rustic living among the beauties of nature-the heart-rousing
strain of the hunter or warrior-and the tender expression of many of the
modifications of anxiety and sorrow. The musical sensibilities are by no
means limited to the human race, for there is no expression more exquisite
than in the song of the nightingale during the evenings of spring, or of the
thrush and blackbird, in the same season, amid the quiet retreats of our
woodlands, —the music of which untutored songsters is made up of the same
elements as our own.
The accompaniment of an air afforded to a singer by one or more instruments, and which is so pleasing, is chiefly the sounding, simultaneously, in
a subdued manner, some other notes of the chords to which the several vocal
notes belong. Duetts and more complicated concert-pieces have their origin
from the same source: and highly cultivated musical sense can even follow
and enjoy several melodies played together.
Musical notes, by whatever instrument produced, have to each other the
same numerical relations, in the beats or vibrations which constitute them.
The different qualities of tone, therefore, from different instruments, can
only depend on the peculiarities of the single beat, as to whether they are
sharp or soft, strong or weak, &c. Such is the extraordinary nicety of per.
ception which the human ear possesses in this respect, that it cannot only
distinguish different kinds of instruments, as a flute and clarionet, playing the
same note, but different instruments of the same kind, even to the extent
for instance, of recognizing each one of a hundred voices singing the same
air. One of the greatest charms of concert music is, that the voice and the
different instruments may take up separately, parts of the strain suited to
16




242                       ACOUSTICS.
their individual expression-the flute and clarionet, for instance, breathe softness; the trumpet and drum arouse; the harp rolls forth its brilliant chord;
the violin leads the flowing sounds through rapid and endless variety; and
so of the rest.
That there might be correspondence in instruments when played together
and a known pitch when played apart, it became necessary to fix on some
tond or number of vibrations as a point of comparison.  Hence, tuning-forks
have been made of steel, with length of prongs calculated to produce a certain
note.  The note is usually the fourth, A or la from the bass of the pianoforte, and vibrates about 430 times in a second;-and when the note of the
same name on any instrument is tuned in unison with this, the other notes
can be easily adjusted according to the harmonic relations above explained.
Almost every substance or contrivance that can produce a uniform continued sound may enter into the composition of a musical instrument; hence
the almost endless variety which the world has seen, The chief classes of
instruments are stringed instruments, bwind instruments and bells or rods.
Of the stringed instruments we may mention the harp, the lyre or lute,
the guitar, the viol of all sizes, and piano-bforte. The harp, lyre and lute
were the inventions of antiquity, and have brought down with them to the
present times a thousand delightful associations. They awakened to inspiration the bards and poets of the young world, and they were the beloved
companions of many of the noblest spirits of succeeding times.  Their
great charm appears to have been in their power to heighten the emotions
produced by music's twin sister, poetry; and the combined effects seem to
have been magical.-The other instruments mentioned are of comparatively
modern invention, particularly the piano-forte; and their perfection has
assisted in carrying the combination of musical sound to degrees of complexity and difficulty of which antiquity dreamt not. It is a question, however, whether the style of much of the music now in vogue does not prove
rather a degeneracy, than a desirable refinement of musical taste. Music is
a language of nature, intelligible at once to all susceptible minds, and, in a
degree even to inferior animals; but modern art is attempting to make of it
an artificial and conventional language, in which there may be fashion and
change.  The ornaments and accompaniments are now often so overwhelming, that the melody, in which the idea and sentiment really reside, is
masked and almost lost; and an unpractised ear, particularly if listening to
an organ, often discovers only an unmeaning succession of chords. And
when a singer, abandoning the natural simplicity of melody, strains to execute with the voice the complicated movements which belong properly to
instrumental accompaniments, the attempt destroys the poetry, by either
rendering the words inaudible, or by sacrificing their natural expression to
some supposed appropriate expression of the ornamental music.  These
considerations may account in part for the insensibility of so many highlyendowed persons to what is now called excellent music.  Some of the
tricks on the voice and on instruments, at present so common, are, to
natural or graceful music, what tumbling or rope dancing are to natural
or graceful gesture. And when we hear noted professors avow their inability to sing a simple ballad, or to play an unadorned melody, must we
not conclude that the natural sense of music has left them, as the relish
for simple but the most invigorating fare has left the morbid epicure?
The guitar, as affording an accompaniment to vocal music, has many
advantages.  It is not too loud, yet the strains are very distinct; it admits of
most touching expression; it is very easily learned by any one who should




MUS I C.                           243
attempt to learn music; it is portable and cheap. The great facility of accompaniment on it depends on this, that the player is able by one position
of the hand to touch the strings so that the sound of all the six shall belong
to the same chord: —three positions of the hand, therefore, for one key, produce all the notes and chords which a simple accompaniment requires; and
the hand soon falls into these so readily, that the player is hardly sensible
of exerting volition.
Among wind instruments are the flute, the flageolet, the organ, the clarionet, the hautboly, the horn, the trumpet, &c. The pitch or tone of a tubular
wind instrument, just as of a musical string, has relation to its length; and
the vibrations causing the sound seem to be waves or condensations of air
passing from the mouth to the extremity of the tube; being more frequent,
therefore, as the tube is shorter; —when the bottom of the tube is closed,
the wave has to come back again, and thus renders the note twice as grave.
It appears, also, that on blowing more strongly, the air in the tube divides
into separate vibrating portions, as a string may divide to produce its harmonic sounds, and produces thus all the harmonic sounds belonging to the
fundamental note at the tube. By blowing into a common German flute,
for instance, it is possible to produce five ascending harmonics without moving the fingers at all. The music of a trumpet is limited to these five notes
of the same chord; but in the flute and other instruments with hbles, the
effective length of the tube is calculated from the upper end to the nearest
hole left open; and each length has its harmonics.-If a tuning fork, Jew'sharp, or any such sounding body, be held at the open end of a tube or other
empty space of dimensions calculated to produce a frequency of undulation,
in its contained air, according with the pulses of the sounding body, then
the tube or space will immediately give out its own beautiful tone; and if
the space be enlarged or diminished in a double, treble or any other simple
proportion-as a tone may be, by a piston moved up or down in it-then
will its note become the fifth, octave, twelfth, &c., above or below the original tone, although that tone continues unchanged.  The tones of the Jew'sharp are well known to depend altogether on the varying dimensions of the
player's mouth; but to obtain perfect music from it, three harps at least, to
be substituted one for the other during the performance, are required to
produce the notes of the three constituent chords of the common musical
scale.-In. wind-instruments with reeds, the tone depends on the stiffness,
weight, length, &c., of the vibrating plate or tongue of the reed, as well
as on the dimensions of the tube or space with which it may be connected.
This truth is well illustrated in that instrument, the 2Eolian, already mentioned, which, in improved and varied forms, promises to become common,
and one of the most expressive wind-instruments.-The sounds of the human
voice are the sweetest of all, and are produced by the vibrations of two delicate membranes situated at the top of the windpipe, with a slit or opening,
called the glottis, left between them, for the passage of the air. The tones
of the voice are grave or acute, according to the varying tension of these
membranes, and to the size of the opening.-In the organ there is a pipe
for each note, and wind is admitted from the bellows to the pipes, by the
action of the keys, like the keys of a piano-forte. The organ may be
played also very perfectly by a barrel, made to turn slowly under the keys,
and to lift them in passing, by pins projecting from it at the required situations. Very complicated pieces of music are thus set on barrels, but by a great
cost of study and labour, and, therefore, of money; now a plain barrel, made
to turn near the keys of an organ during performance on it by the hands,




244                        ACOUSTICS.
might be made to record, with mathematical accuracy, every touch of the
most finished player, by receiving marks of some kind from the keys as they
were lifted; and to repeat with absolute accuracy, therefore any performance,
however delicate and exquisite, it would only be farther necessary to drive
pins into the barrel where the marks remained, and afterwards make these
pins lift the keys.
Bells are often conjoined in sets, having the musical relation, and to some
persons their music is very agreeable. There are, in the tolling of a single
bell, a loudness and a solemnity rendering it a fit accompaniment of funeral
rites.
The Chinese gong partakes of the nature both of the bell and of a great
drum, and has something in its sound which is singularly affecting. In its
own country it bears a part in one of the most imposing ceremonies which
man has ever imagined. On certain festivals as the sun is sinking in the
west, the whole population of China, a host of more than a hundred millions,
issues forth under the single canopy of heaven, to testify, amid the thunder
of gongs and the continued discharge of fire works, that adoration and gratitude towards the Deity which human nature, in all ages and climes, has felt
to be due, and has eagerly sought to express, however blindas to the sublime
simplicity of religious truth.
Bells or goblets of glass sound still more perfectly than those of metal,
and by gentle friction on their edges with a bow or the wetted finger, their
tones may be continued for any length of time, and may be made to swell
and diminish like a human voice or the
Fig. 120.           notes of a violin.  A set of glasses, therefore, attuned to each other, according to
~c Q    Q a1,  the harmonic scale, becomes, for certain
o0      species of music, the most perfect of all
O instruments.  It is in fact an ZEolian harp
o    H     at command.  Dr. Franklin, who  first
constructed a set, doubled the long line of........ Oglasses upon itself, and placed the halfnotes as outside rows.  The author of this
j0jE( 57     X work, however, during some experiments
--   on sound, found the zig-zag arrangement
Qhere represented to possess certain advantages.  The small open circles represent
b   the mouths of the glasses standing in a box
a b c, and the relation of the glasses to the
written musical notes is shown by the
common music lines and spaces which connect them. The learner discovers
immediately that one row of the glasses produces the notes written upon the
lines, and the other row the notes written between the lines; and he is mentally master of the instrument by simple inspection. This arrangement also
renders the performance easy, for the notes most commonly sounded in succession are contiguous: and the relations of the notes forming a tune are so
obvious to the eye, that the theory of musical combination and accompaniment is learned at the same time. The set of glasses here represented has
two octaves, and with the additional flat seventh and fourteenth, seen at a and
c, which, when required, may be substituted for the corresponding glasses in
the rows, it is capable of playing the greater part of our simple melodies.
All the half-notes, if desired, may be placed in outside rows. The player
stands at the side of the box between a and b, and has the notes ascending
towards the right hand, as in a piano-forte.




MUSICAL EAR.                            245
Musical ear.
Philosophers have not yet been able to account for a remarkable difference
among individuals, as regards their. perception of the musical relations of
sounds. Many persons, without understanding any thing of acoustics, or
having studied music as a science can tell instantly whether various notes
heard together or in succession, have the mutual relations which we call
musical-and which we now know to depend on the comparative numbers
of beats in a given time; and they quickly recognize and learn to repeat
tunes, and to sing a fit second or bass to the performance of another; —while
there are persons, again, with an equally perfect sense of hearing, who can
neither know if an air be played in tune, nor what air it is, nor can they ever
sing alone or accompany.  The former class of persons are said to have a
musical ear, and the latter to want it; and although cultivation will raise
mediocrity to considerable expertness, it cannot bestow the faculty where
originally deficient. On this subject there is a very common misconception,
which becomes a source of great mortification on one side, and of arrogance
on the other, viz., that the possession of a musical ear, or the power of distinguishing notes, is the indication of all the finer qualities of the mind
while the want of it proves an opposite deficiency; and Shakspeare's opinion
of him " that hath no music in himself," is often triumphantly cited as applicable to all who want the distinguishing ear. The truth, however, is, that
many who possess this characteristic in a remarkable degree, are deficient in
almost all else that humanity reveres,-witness the weak minds and disorderly lives of so many professed musicians, —while many, again, who have
it not, are otherwise examples of excellence, and exquisitely sensible to other
beauties and harmonies of nature. They may not be deaf, for instance, to
the general music of spring, when all nature bursts forth in voice of rejoicing,
nor to the awful music of the storm-they may feel as touching music the
silence of a lone wood, contrasted with the unceasing din of multitudes-or
even the stillness of night in a great city, where the astronomer, contemplating the wondrous spheres above, hears only the tongues of passing time in
the church towers, or the call of watchmen, faintly sounding in the distance.
In fine, many distinguished poets and philosophers have had no musical ear.
-That the charm of music is often as much from early associations as from
peculiar aptitude in the individuals, is proved by the effects so well known
of the Swiss airs, when heard by native Swiss in foreign lands; and, indeed,
of the national melodies of all countries, whose people are happy, and mix
song with their usual occupations,-it not being in nature, that at any period
of life, or in any clime, a man should cease to deem those modulations lovely,
which recall the ecstatic emotions of his infancy and childhood; modulations learned in general from a parent's voice, perhaps an excellant mother's,
whose affection was so long around him as a shield; whose tears fell to
chide his errors, and to reward where there was promise of virtue; whose
steady judgment was his guide, whose faultless life was his example,
and who in all things to him was a personification of God's goodness on
earth.
It is the prejudice of which we are now speaking with respect to musical
ear and musical taste, that in the present day, condemns many young women,
possessed of every species of loveliness and talent, except that of note-distinguishing, to waste years of precious time in an attempt to aquire this
talent in spite of nature; but when they have succeeded as far as they can,




246                       ACOUSTICS.
they have only the merit of being machines, upon which tunes are set as
upon a barrel-organ, and of which the performance is often far from being
pleasing to good judges. Such persons when liberty comes to them with age
or marriage, generally abandon the offensive occupation; but tyrant fashion
will force their daughters to run the same course. The waste of time now
spoken of, is only one of many evil consequences which arise from the prevailing false notions with respect to music: a subject which, however interesting, cannot be farther pursued in this place.
"The trembling which causes the sensation of sounds spreads in all bodies.
solid or fluid."  (Read the Analysis.)
As air consists of material particles held far apart from each other by the
repulsion of heat among them, we can conceive how an impulse given to a
certain portion of the particles is transmitted to those beyond, by the increase
of repulsion as they approximate; and from the second layer in the same
manner to the third, and so on. And as in fluids the particles all mutually
rest against, or repel each other, we can conceive why a motion produced in
any part of a mass should be felt in every direction. The explosion of gunpowder, in which there is a sudden formation of a quantity of air, gives a
shock all round which spreads a spherical wave to a great distance.
Although material particles in the form of liquid or solid are much nearer
to each other than in the form of air, we still have many proofs, as stated at
page 30, that they are not in absolute contact, and we therefore see the reason why the impulses producing sound should be transmitted through a
liquid or solid in the same manner as through air, and even, by reason of
the greater proximity'of the particles, more quickly and forcibly than in air.
Instances of air carrying sound were given at page 235. —As farther examples we may cite the cases of what are called sympathetic sounds. Every
elastic body being sonorous, that is to say, being fitted to tremble when struck,
with a certain frequency of oscillation, depending on its weight and shape,
&c., if the air around it be made to tremble by any cause, with the velocity
which it is fitted to take on or produce, it immediately begins to tremble in
unison with the air; and its motion or sound may continue after the original
cause has ceased. —Thus almost any sound produced near a piano-forte whose
dampers are raised, finds a responsive string, and if bits of paper are strewed
upon the strings generally, those falling on the strings which return unisons
or octaves to the sounding body are soon shaken off, while the others remain.
A harp or guitar in a room with talking company, is often mingling a note
with their conversation. —A wine-glass or goblet may be made to tremble,
and if on a table at all inclined, even to fall, by.a person sounding on a violoncello near it, the note accordant to its own
Sounding bodies vibrate much more quickly, or have sharper tones, if
placed in light hydrogen, than in common air, and more quickly in common
air than in any of the heavier gases:-because the lighter the surrounding
fluid, the less is the resistance to a body moving in it. Thus also a bell will
ring under water, but with a much graver sound than in the air.
That water is a vehicle of sound, is proved by the fact last mentioned,by the distinctness with which the blows of workers around a diving-bell are
heard above,-by the fact that fishes hear very acutely, &c.
And the following are instances of sound conveyed by solids.-A scratch
of a pin at one end of a wooden log is distinctly heard by a person applying
his ear at the other end, although through the air it is not at all audible even
to the person who makes it. —Savages often discover the proximity of ene



SPREADING OF SOUND.                            247
mies or of prey, by applying an ear to the ground and hearing the tread.The approach of horsemen at night is easily discovered in the same way.The report of a cannon placed on the ice is carried much farther by the ice,
than by the air around. —In the military operation of mining, or cutting away
under ground for the purpose of entering a citadel, or blowing up fortifications,
the approach of the enemy is often discovered by the subterranean sound of
the pioneer's tools.-The awful muttering of earthquakes is merely the sound
of subterranean explosions, conveyed from amazing distances, by the solid
earth
A superstitious man sleeping in the upper story of a lofty house had for
some time heard, during the stillness of the night, a singular beating noise
near the head of his bed. There was no adjoining house beyond the wall,
nor was there any thing going on near him in his own house to account for
it, and he at last deemed it supernatural.  Accident at last discovered that
in a hovel built at the bottom and outside of the wall against which his bed
stood, there was a wooden clock hanging, of which the sound, while all else
was still, became audible aloft.
It is easy to ascertain whether a kettle boils, by putting one end of a stick
or poker on the lid, and the other end to the ear; the bubbling of the water
then appears as loud as the rattling of a carriage in the street.-A, slight
blow given to a steel poker or common triangle, of which an end is held to
the ear, produces a sound which is even painfully strong.
The readiness with which solids receive and transmit sound is farther perceived in the fact, that a small musical box, while held in the hand, is
scarcely audible, but when pressed against a table, or a door, will rival a
little harp. The vibration communicated from the box pervades the whole
of the wood, and the extended surface then acting on the air increases the
effect.  The construction of violins, harps, guitars, &c., and of sounding
boards generally, is governed by the same law. In the dancing-master's
kit or small fiddle, which he carries in his pocket, there may be the same
strings and the same bow as for a violin, but it has very little sound, because
the extent of its surface is so small.  A heavy piece of metal called a
sourdine, when fixed upon the bridge of a violin, damps the sound, because
it is a dead mass resisting the motion of the elastic wood.
The fact of solids conveying sounds so much more perfectly than air has
lately been applied to useful purposes in medicine. Dr. Laennec, of Paris,
proposed some years ago to listen to what'was going on in the interior of the
body, and of the chest particularly, by applying one end of a wooden
cylinder, which he called a stethoscope or chest inspector, to the surface, and
resting the ear against the other end. The results of this happy thought
have been important.
The actions going on in the chest are, the entrance and exit of the air in
respiration, the voice, and the motion of the blood in the heart and bloodvessels; —and so perfectly do all these declare themselves to a person listening through the stethoscope, that an ear once familiar with the natural and
healthy sounds, instantly detects certain deviations from them. Hence this
instrument becomes a means of ascertaining certain diseases in the chest
almost as effectually as if there were convenient windows for visual inspection; and when it is considered that a fourth or fifth part of the inhabitants
of Europe die of diseases'of the chest, such as inflammations, abscesses,
consumption, dropsical collections, aneurism, and various affections of the
heart and blood-vessels, each of which requires an appropriate treatment,
the importance of such a means may be judged of.  By many medical men




248                        ACOUSTICS.
this instrument was at first ridiculed as quackery and nonsense, and many
have yet to learn the use of it. May not both of these facts be attributed
to the error which has existed in medical education, of leaving so many
practitioners without that knowledge of the general laws of nature, which
should enable them to appreciate at once any means likely to be useful in
their art, from whatever quarter offered?' Trelocity of sound." (See the Analysis.)
The velocity of light is such, that for any distance on earth its passage
may be regarded as instantaneous. The velocity of sound is very much less.
— If a woodman be observed at his occupation on the hill, his axe is seen
to fall a considerable time before the sound of his blow reaches the spectator's ear.-The flash of a gun fired at a distance is seen long before the
report is heard.
Most accurate experiments have been made to ascertain the velocity with
which sound travels in the atmosphere; and it is found to be 1,142 feet per
second, or a mile in about four seconds and-a-half; varying little either with
the density or temperature of the air.
By noting then how long the flash of a gun is seen before the report
reaches the ear, we learn the distance of the ship or battery from which the
gun is fired. A chasing ship may thus often discover whether she be nearing
or not the object of her pursuit.  In the same manner the distance of
thunder may be ascertained: and the reason of the long-continued roll of
thunder is, that although the lightning darts instantly through the chains of
clouds, perhaps of miles in length, the claps or explosion at each interruption
of the chain are only heard successively, as the sound arrives at the ear.
The pulse at the wrist of a healthy man is a convenient measure of time
for ascertaining distances by the motion of sound,-each beat making
nearly a second, and therefore indicating a distance of nearly a quarter of
a mile.
A line of muskets fired at the same instant cannot appear a single report
to any person who is not in the centre of a circle, of which the line forms
a part.
An extended orchestra of musicians cannot be heard equally well from all
situations near them.
Windd affects the velocity of sound just as a current in water affects the
motion of a sailing ship.
Sound decreases in intensity from the centre where it originates, according
to the same law as gravitation or light; that is to say, at double distance it
is only one-fourth part as strong, at triple, a ninth, and so on.
By confining it, however, in tubes, which prevent it spreading, its force
diminishes much less rapidly, and it will, therefore, extend to much greater
distances.-In many manufactories, and even private dwellings now, there
are pipes for the conveyance of sound leading to all parts; so that on ringing a bell to attract attention, verbal orders may be given through them to
great distances.
Sound travels in water four times quicker, and in solids from ten to
twenty times quicker, than in air.  The blow of a hammer given to a wall
by a person at one end, may be heard twice by a person at the other, viz.,
almost immediately by an ear applied to the wall, and a little after through
the air.
"Reflection of sound."  (Read the Analysis.)
As a wave of water turns back at a smooth wall or obstacle, so that at any




REFLECTION OF SOUND.                           249
distance after the reflection, it appears what it would have been at the same
distance beyond the wall, only moving in an opposite direction; so the
pulses or waves of sound are regularly reflected from flat surfaces, and
produce what is called an echo. Such flat surfaces of nature's works are
found only among the rocks and hills; and hence arose the beautiful fiction
of the ancient poets, that echo was a' nymph who dwelt concealed among
the rocks. Science has now disclosed the secret of the viewless echo; but
who does not vividly recollect the wonder and delight with which he has
listened in the morning of his days, to his shrill call returned to him from
some bold precipice, across the plain or river, or, perhaps, sent down to him
again from the vaulted roof of ocean's caves!
The quickness with which an echo is returned to the spot where the
sound originates, depends, of course, upon the distance of the reflecting surface; and, as sound travels 1,142 feet in a second, a rock at half that distance
returns a sound exactly in one second. The number of syllables that can
be pronounced in a second, will, in such a case, be repeated distinctly, while
the end of a longer phrase would mix with the commencement of the echo.
The breadth of a river may easily be ascertained where there is an echoing
rock on the farther shore. A perpendicular mountain's side, or sublime
cliffs, such as in many parts skirt the British coasts, return an audible echo
of artillery, or of thunder, to a distance of many miles.
If two bold faces of rock or wall be parallel to each other, a sound produced between them is repeated often, playing like a shuttlecock between
them, but becoming more faint each time until it is heard no more. In
some situations, particularly when the sound plays thus above the smooth
surface of water, a pistol-shot may be counted forty times.
The resonance of enclosed spaces depends on this continued reverberation.
It often increases the effect of music by converting a simple melody, which
is a succession of notes, into a harmonized piece, where each note is accompanied by some accordant tones; and a young flute-player is often first
charmed with his own music when he finds himself performing a duet with
echo in a cave or under a spacious arch:-but resonance injures the distinctness of speech, so as even in some ill-contrived halls of assembly or theatres,
to render the articulation unintelligible.  Small rooms or near surfaces give
no perceptible echo, because the interval of time between the original sound
and its repetition is too short for the ear to appreciate.
It is worthy of remark, that every apartment or confined space, has a
certain musical note proper to it, the pitch of which depends upon the
number of pulses or repetitions of a sound produced there in a given time
by the returns from its walls. The velocity of sound being uniform, this
number must depend on the size of the apartment.
There is a curious effect of echo which both illustrates the nature of the
phenomenon, and proves that a tone or musical sound is merely a repetition
of pulses following each other very quickly. Iron railings are generally
formed of square bars, of which any side is a plane surface, and may produce
an echo. Now a sound such as the sharp blow of a hammer, occurring near
the end of such a railing, is echoed to a corresponding place on the other side
by every bar in it; and as the echoes do not return all at once, but in regular
succession, according to the increasing distances of the bars, the consequent
regular succession of slight pulses, with uniform and small intervals, affects
the ear, not as the echo of a single blow, but as a continued musical tone,
the pitch of which depends on the distance of the bars from each other.
The writer of this had observed, in passing on horseback along a particular




250                        ACOUSTICS.
portion of road, where there was first a piece of wall and then two pieces of
paling with rails or bars of different width,-that there was from the wall a
clear echo of the horse's cantering feet, and afterwards opposite the palings
a ringing sound for every step of the horse.  He had first concluded that
the road there was singularly hard, although it did not appear so, and he
slackened the horse's pace to save his teet, until, observing one day that the
ringing sound was of different pitch opposite the two pieces of pailing, and
so as to correspond with the different width of the bars, the true explanation
occurred to him that the sound was an echo of the nature above described.
That an echo may be perfect, the surface producing it must be smooth,
and of some regular form; for the wave of sound rebounds according to the
same law as a wave of water, or a ray of light, or an elastic ball, &c., as
explained at page 65, viz., perpendicularly to the surface, if it fall perpendicular, but if it fall obliquely on one side, departing with an equal degree
of obliquity on the other. To express this very important law shortly, we
say that " the angle of reflection is equal to
Fig. 121.          the angle of incidence."-According to this
law, any irregular surface must break an echo;
mp~~-:i~N%.-.~  and if the regularity be very considerable,
7i' "       - i / i        there can be no distinct or audible reflection
e                     g    at all.  A regular concave surface, on the
contrary, as e g, may concentrate soun'd, and
bring all which falls upon it, as from a b c d,
to the same centre or fbcus, as atf, so as to
a   Z   c,  d          produce there a very powerful effect.
We thus see the reason why echo is much
less perfect from  the front of a house which has windows and doors, than
from the plane end, or any plane wall of the same magnitude, —and why
the resonance of a room is so irregular and indistinct'when the room contains
curtains, carpets and other furniture, or a crowded assembly.  Halls for
music have generally plane bare walls.  Theatres for the drama, again, have
boundaries broken in all ways by rows of boxes, and various ornaments.
The concentration of sound by concave surfaces produces many curious
effects both in nature and in art.
There are remarkable situations where the sound from a cascade is concentrated by the surface of a neighbouring cave so completely, that a person
accidentally bringing his ear into the focus, is suddenly astounded, as if the
universe were crushing around him.  A chair placed in the cave, so that a
person sitting down in it must bring his ear into the focus, insures the success
of the sometimes amusing experiment.
The centre of a circle is the focus in which sound issuing
Fig  192.   from it is again collected after reflection; hence the powerful
echo near the centre of a round apartment. An oval has two
centres orfoci-one towards each end, as a and b-and the
nature of the curve is such, that sound, or light, or heat,
" issuing around from either of the foci, as a, by obeying the
law  of reflection above stated, is all directed from the various
b I.       points, as at c d e, &c., to the other focus, as at b.  Hence
a person uttering a whisper in one focus of an oval room is
very audible to the other, although he may not be heard by
persons placed between.  Such a room may be called a whispering gallery.
Concave surfaces facing each other, as two alcoves in a garden, or covered
recesses on opposite sides of a street or bridge, will enable persons seated in




REFLECTION.                             251
their foci to converse by whispers across louder noises in the space between,
and without themselves being overheard in that space.
The reason why a tube conveys sound so far, is, that its sides confine or
repress by a continued reflection, the advancing sound which, in the open
air, would quickly spread laterally and be dissipated. And the reason that
the plane surface of a smooth wall, or of water, &c., also conveys sound so
far, is, thatit similarly prevents the lateral spreading and lissipation, although
only on one side.-Persons far apart may converse along a smooth wall.The barking of dogs and the clear voice of a street-crier, in a town situated
on the border of a lake, may be heard across the water in a calm evening, at
a distance of more than five miles-the sound of bells, of course, is audible
much farther.-And in the stillness of night, even the splashing oars of a
boat will announce its approach to persons waiting at a great distance.
If a sound-reflecting surface be curved inwards, that is, be concave, it not
only prevents the spreading of any sound which passes along it, but is constantly condensing the sound by driving the external part inwards. Hence,
in a circular space, such as a gallery under a dome, persons close to the
wall;~may whisper to each other at all distances.
An ear-trumpet is a tube wide at one end where the sound enters, and
narrow at the other where the ear is applied: its sides are so curved that,
according to the law of reflection, all the sound which enters is brought to a
focus in the narrow end. It thus increases many fold the intensity of a
sound which reaches the ear through it, and enables a person who has become deaf to common conversation, to mix again with pleasure in society,
The concave hand held behind the ear answers in some degree the purpose
of an ear-trumpet, and in a very large theatre is sometimes useful even to
persons of quick hearing. A notorious instance of a sound-collecting surface
was the ear of Dionysius, in the dungeons of Syracuse: the roof of the prison
was so formed as to collect the words and even whispers of the unhappy
prisoners, and to direct them along a hidden conduit to where the tyrant sat
listening. The wide-spread sail of a ship, rendered concave by a gentle
breeze, is also a good collector of sound. It happened one day on board a ship
sailing along the coast of Brazil, far out of sight of land, that the persons
walking on deck, when passing a particular spot, heard very distinctly, during
an hour or two, the sound of bells, varying as in human rejoicings. All on
board came to listen, and were convinced, but the phenomenon was most
mysterious. Months afterwards it was ascertained, that at the time of observation the bells of the city of St. Salvador, on the Brazilian coast, had been
ringing on the occasion of a festival: their sound, therefore, favoured by a
gentle wind, had travelled over perhaps 100 miles of smooth water, and had
been brought to a focus by the concave sail in the particular situation on
the deck where it was listened to. It appears from this that a machine
might be constructed having the same relation to sound that a telescope
has to light.-A friend of the author, on the 18th of June, 1814, while
sitting near the wall of his garden, situated near Dover, heard distinctly the
firing of the cannon at the battle of Waterloo.
The speaking-trumpet is made according to the same law of reflected
sound, with the view of directing.the strength of the voice to a particular
point.  The sea captain uses' it to kail ships at a distance, or to send his
orders aloft, where the unaided voice would be lost in the noise of the wind
and waves.. A similar form of mouth is used for the bugle horn and
common trumpet, and fits them to sound the note of command amid the
uproar of contending armies.




252                        ACOUSTICS.
Some amusing effects have been produced by operating on sounds with
tubes and concave surfaces. What was terined the invisible girl, was a
contrivance where the questions of vistors were caught by a concealed concave, and carried to the director who sat at a distance; and his replies, as
in the whispering gallery, become audible to the inquirers alone.
The concave, undulating, and perfectly polished surface of many seashells, fits them to catch, mix, and return the pulses of sounds that happen
to be trembling about them. so as to produce that curious resonance from
within which closely resembles the sound of the distant ocean-so closely,
that the spirited boy, after studying the interesting stories of voyagers which
paint dangers to be nobly braved, and charm of nature to be seen in distant
lands, often feeds his imagination with this voice of a shell, and fancies
himself already riding among the billows.
The animal ear,
so admirably adapted to perceive the evanescent tremblings of the air, has
of course a structure in nice relation to their nature as now explained. The
parts of the ear, and the progress of the sound to the sentient nerve, may
be simply described as follows:
Ist. There is external to the head, a wide-mouthed tube or ear-trumpet
a, for catching and concentrating the waves of
Fig. 123.          sound.  It is moveable in many animals, so
that they can direct it to the place from which
the sound comes.
2d. The sound concentrated at the bottom of
the ear-tube, falls upon a membrane stretched
across the channel, like the parchment of an
\J - \\.         d ordinary drum, over the space called the tyzm\panum or drum of the ear b, and causes the
membrane to vibrate. That its motion may
be free, the air contained within the drum has
free communication with the external air, by the open passage f, called the
Eustachian tube, leading to the back of. the mouth.  A degree of deafness
ensues when this tube is obstructed, as by wax; and a crack or sudden
noise, with immediate return of natural hearing, is generally experienced
when, in the effort of sneezing or otherwise, the obstruction is removed.
3d. The vibrations of the membrane of the drum are conveyed farther in.
wards, through the cavity of the drum, by chain of four bones (not here
represented on account of their minuteness,) reaching from the centre of the
membrane to the oval door or window leading into the labyrinth e.
4th. The labyrinth, or complex inner compartment of the ear, over which
the nerve of hearing is spread as a lining, is full of water; and therefore by
the law of fluid pressure (see page 128,) when the force of the moving membrane of the drum, acting through the chain of bones, is made to compress
the water, the pressure is felt instantly over the whole cavity, as in a hydrostatic press. —The labyrinth consists of the vestibule e, the three semi-circular
canals c, imbedded in the hard bone, and a winding cavity, called the
cochlea d, like that of a snail-shell, in which fibres, stretched across like
harp-strings, constitute the lyra.-The separate uses of these various parts
are not yet perfectly known. The membrane of the tympanum may be
pierced, and the chain of bones may be broken without entire loss of hearing.
Considerable diversity of form and dimension is found in different animals.




THE  EAR.                           253
The bone containing the cavities of the ear is the hardest in the body,
and is the first formed.
The ear has the power of judging of the direction in which sound conies.
A person in a thicket, listening to the song of various birds, although they
be concealed from his eye by the luxuriance of the vernal foliage, still judges
correctly by the ear in what tree every little songster is concealed.-The
same truth is strikingly exemplified in the fact that, when horses or mules
march in company at night, those in front direct their ears forward; those
in the rear, backwards; and those in the centre, laterally or across; —the
whole troop seeming to be actuated by one feeling, which watches the
common safety.
The intensity of sound is to the ear a measure of distance. In a windy
night, the sound of a distant bell may be brought so quickly, that it has not
yet had time to spread and be weakened; and a person is often roused from
a reverie by its unusual loudness and apparent nearness.-When a stormy
wind blows directly upon a coast, and rolls the great waves in upon the
sandy beach or among the rocks, the countryman living far inland hears the
uproar, as if the ocean had burst its barriers, and were pouring in upon the
land.  The scene-contrivers at our theatres heighten the illusion of an
approaching procession, by letting the accompanying music be first heard
from a closed chamber or in a feeble tone, and afterwards with gradually
increasing loudness.  To the imagination, already excited perhaps to the
highest pitch by the drama of some divine mind, the advancing host is thus
most vividly portrayed; and when at last, with the thunder of drums and
trumpets from the front of the stage, the troop also appears, the effect is complete.  It is the varying loudness of the IEolian harp which produces the
feeling that the heavenly choir is sometimes approaching and sometimes
receding.
[For an account of the Doctrines of Fluidity in relation to animals, see
Part V. Sec. II.]




254            IMPONDERABLE SUBSTANCE.
PART IV.
DOCTRINES OF IMPONDERABLE SUBSTANCE.
To minds beginning this study, it may facilitate the conception of a substance which is without weight, or at least is imponderable by human art,
to consider the nature of air.  Until lately men were so imperfectly
acquainted with the constitution of the universe around them, that a person
placed in an apartment which offered to view nothing but the naked walls,
would have said that it was empty, meaning literally what he said; and
even when advertised that there was air in the room, he would still have been
far from possessing a clear notion that it was full of aerial fluid just as an
open vessel immersed in the sea is full of water, and that if air were not
allowed to escape from it, even so small a body as an apple could not be
pressed into it additionally by less force than fifty or sixty pounds. This truth,
however, is now clearly understood, and daily exemplified in easy pneumatic
experiments, and in no way more strikingly than by the recent adoption of
the substance of air in place of feathers, as stuffing.for beds and pillows.
An air-tight bag or sack suspended by its lip in the air, and held quite open
by a hoop near its mouth, would appear empty, but if then firmly closed above
the hoop, it would have imprisoned its fill of air, just as a bag similarly managed underwater would imprison its fill of water; and while in some respects
the air would be softer and locally more yielding than feathers, its entire mass
would be much less compressible. Now this air, when weighed by means which
modern science has furnished, is found in a cubic foot to contain somewhat
more than an ounce, and by strongly pressing it, or by causing it to combine
chemically with some other substance, we can reduce it to a very small bulk,
either with the form of a liquid or of a solid; proving how small a quantity
of ponderable matter, under certain circumstances, will occupy great space.
And common air is by no means the lightest known substance, which as
powerfully resists the intrusion of other bodies where it exists.  Hydrogen
gas, fcr instance, of the same space-occupying force, weighs only a fourteenth
part as much, and therefore a few drachms of it confined in a bag or bhed as
broad as the foundation of a house, would support a house or cask as large
as a house filled with water to a height of thirty feet, the gas itself being
then eighty thousand times lighter than its bulk of gold;-and if the pressure
on it were diminished, it would readily expand to a volume a thousand times
as great, and would still be exerting a considerable outward elasticity. Again,
a mixture of oxygen and hydrogen gases, while uniting with explosive force
to form water, dilates for the time, even under the great pressure of the
atmosphere, to a bulk about twenty times greater than the gases have while
separate.
The mind, pursuing the idea of such expansion or occupancy of space by
a small quantity of matter, and reflecting on the wonderful divisibility of
matter or minuteness of the ultimate atoms, as explained in Part I. of this
work, might almost admit as a possible reality Newton's hypothetical illustra



IMPONDERABLE  SUBSTANCE.                          255
tion of that divisibility, viz., that even one ounce of substance uniformly distributed over the vast space in which our solar system exists, might leave
no quarter of an inch without its particle. Now a fluid in any degree
approaching in rarity to this, although it might press, resist, communicate
motion, and have other influences in common with more ponderable matter,
would have neither weight nor inertia discoverable by means at present
known to man. While we are contemplating, then, or modifying the agencies of what causes the phenomena of heat and cold, of light and darkness,
of electricity in its forms of thunder and lightning, of galvanism, or of magnetism, in a word, the most striking phenomena of nature, we may be dealing
with matter of the subtle constitution now spoken of.  And as in the terrestrial atmosphere there are at least two fluids present, viz., oxygen and nitrogen,
of distinct nature, so in a more subtle ether, filling all space, there may be
various ingredients.
A majority of philosophers'now incline to the opinion here sketched, that
there is at least one such subtle fluid or ether occupying completely the
space of the universe, and tending to uniform diffusion by reason of a strong
mutual repulsion of its particles, which fluid pervades denser material substances somewhat as water pervades a sponge or a mass of sand, being
attracted in a peculiar way by each substance, and which fluid may or may
not have weight and inertia.  They believe farther that the phenomena
above alluded to, and which human art can exhibit with highest beauty, or
with awful intensity, are produced by the motion of other affections of that
fluid, as the sensation of sound in all its varieties is produced in the delicate
structure of the ear by a certain motion in the air, or in any other body
having communication with the ear; or as the sensation of jar is perceived
by a hand held to one end of a log of wood when a blow is given to the other
end. Some philosophers again suppose that the causes of the phenomena
are material particles projected through space, somewhat as sand might be
scattered by an explosion, and which particles are present only when the
effects are apparent. Some combine these two hypotheses. And some hold
all the phenomena of heat to be mere motions in the common matter of the
bodies in which the heat exists.
We mention these hypotheses, not with the view of entering upon a minute
examination of their respective merits, or even of asserting that any one of
them is true, but merely to make the reader aware of the directions which
inquirers' minds have taken in pursuing the investigation. To understand
the subjects as far as men yet usefully understand them, and sufficiently for
a vast number of most useful purposes, it is only necessary, as in other.departments of science, to classify important phenomena, so that their nature
and resemblances may be clearly perceived. When, in treating of the human
mind, we speak of its retainivg an idea, or being depressed, or being heated
with passion, &c., we speak of subjects sufficiently definite, although we may
have no hypothesis as to the intimate nature of the phenomena:-and in the
same manner may we speak Of the accumulation, radiation, or other affections of heat and light.  We know nothing of the cause -even of gravity, the
grandest influence in nature, but we can calculate its effects with admirable
precision.




256                            HEAT.
PART IV.
(CONTINUED.)
SECTION I.-ON HEAT.
ANALYSIS OF THE SECTION.
Beat ( by some called Caloric ) may be strikingly referred to as that which
causes the difference between winter and summer, between tropical gardens
and polar wastes.  Its inferior degrees are denoted by the term COLD. It
cannot be exhibited apart, nor prove to have weight or inertia, and the
change of its quantity in bodies is most conveniently estimated by the concomitant change of their bulk; any substance so circumstanced as to allow
this to be accurately measured constituting a THERMOMETER.
Heat diffuses itself among neighbouring bodies until all have the same temperature, that is, until all similarly affect a thermometer.  It spreads
partly through their structure, or by conduction, as it is called, with a
slow progress, different for each substance, and in fluids modified by the
motion of their particles; and it spreads partly also by being shot or
radiated like lightfrom one body to another, through transparent media
or space, with readiness affected by the material and state of the giving
and receiving surfaces.
Heat, by entering bodies, expands them, and through a range which includes, as three successive stages, the forms of SOLID, LIQUID and AIR or
GAS; becoming thus in nature the grand antagonist and modifier of that
attraction which holds corporeal particles together, and which, if acting
Hlone, would reduce the whole material universe to one solid lifeless mass.
Each particular substance, according to the nature, proximity, &c., of
its ultimate particles, takes a certain quantity of heat (said to mark its
capacity ), to produce in it a given change of temperature or calorific tension; undergoing expansion then in, a degree proper to itself, and
changing its form to liquid and air at points of temperature proper to
itself; the expansion in bodies generally increasing more rapidly than
the temperature, because the cohesion of their particles lessens with increase of distance; being remarkably greiter therefore in liquids than in
solids; and in a'ir thqa in liquids; and the rate of expansion, moreover,
being much quickened as the bodies approach their points of changing
foi m to liquid or air, to produce which changes, a large quantity of heat
enters them, but in the new arrangement of particles and increased
volume of the mass, it becomes hidden from the thermometer, and is
therefore called LATENT HEAT. For any given substance the changes of
form happens so constantly at the same temperature, that they mark fixed
points in the general scale of temperature, and enable us to regulate and




CAUSE OF SEASONS AND CLIMATES.    25
compare thermometers.   ieat, by expanding diffzrent substances unequally, influences much their chemical comrn bination.
eleat influences also the functions of vegetable and animal life.  The great
source of heat is the sun; but electricity, combustion and other chemical
action, condensation, friction, and the actions oj' ligb  are also excitants.*
"Heat may be strikingly referred to as that which causes the d'fference
between winter and summer, between the gardens of the equator and
polar wastes."  (See the Analysis,'page 256.)
In the winter of climates, where the temperature is for a time below the
freezing point of water, the earth with its waters is bound up in snow and
ice, the trees and shrubs are leafless, appearing everywhere like withered
skeletons, countless multitudes of living creatures, owing either to the bitter
cold or deficiency of food, are perishing in the snows-nature seems dyinlD
or dead; but what a change when spring returns, that is, when heat returns!
The earth is again uncovered and soft, the rivers flow, the lakes are again
liquid mirrors, the warm  showers come to foster vegetation, which soon
covers the ground with beauty and plenty.  Man, lately inactive, is recalled
to many duties; his water-wheels are everywhere at work, his boats are
again on the canals and streams, his bisy fleets of industry are along the
shores; winged life in new multitudes fill the sky, finny life similarly fills
the waters, and every spot of earth teems with vitality and joy.   MIany
persons regard these changes of season as if they came like the successive
position of a turning-wheel, of which one necessarily brings the next; not
adverting that it is the single circumstance of change of temperature which
does all.  But if the colds of winter arrive too early, they unfailingly produce the wintery scene, and if warmth come before its time ih spring, it
expands the bud and the blossom, which a return of frost will surely destroy.
A seed sown in an ice-house never awakens to life.
Again, as regards climates, the earthly matters forming the exterior of
our globe, and, therefore, entering into the composition of soils, are not
different for different latitudes,-at the equator, for instance, and near the
poles. That the aspect of nature then, in the two situations, exhibits a contrast more striking still than between summer and winter, is merely to an
inequality of temperature, which is permanent. Were it not for this, in both
situations the same vegetables might grow, and the same animals might find
their befitting support. But now, in the one, namely, where the heat abounds,
we see the magnificent scene of tropical fertility; the earth covered with
luxuriant vegetation in endless, lovely variety, and even the hard rocks festooned with green, perhaps with the vine, rich in its purple clusters. In the
midst of this scene, animal existence is equally abundant, and many of the
species are of surpassing beauty-the plumage of the birds is as brilliant as
the gayest flowers. The warm air is perfume from the spice-beds, the sky
and clouds are often dyed in tints as bright as freshest rainbow, and happy
-* It is to be remarked here, that many phenomena in which heat plays an important
part, have been already described in preceding chapters of this work; —for instance, the
action of the steam-engine, the phenomena of winds, many facts in [meteorology, &e.,
under the head of Pneumatics. In a separate treatise on heat, these could not with propriety have been omitted; but in a comprehensive system of science like the present, they
find their fit place, where, being surrounded by subjects resembling them in more intricate particulars, they can be more concisely and clearly explained.
17




258                            H E AT.
human inhabitants call the scene a paradise. Again, where heat is absent,
we have the dreary spectacle of polar barrenness, namely, bare rock or
mountain, instead of fertile field; -water everywhere hardened to solidity;
no rain, nor cloud, nor dew; few motions but drifting snow; vegetable life
scarcely existing, and then only in sheltered places turned to the sun-and
instead of the palms and other trees of India, whose single leaf is almost
broad enough to cover a hut, there are bushes and trees, as the furze and fir,
having what may be called hairs or bristles, in the room of leaves.  In the
winter time, during which the sun is not seen for nearly six months, new
horrors are added, viz., the darkness and dreadful silence, the cold benumbing all life, and even freezing mercury-a scene into which man may penetrate from happier climes, but where he can only leave his protecting ship
and fires for short periods, as he might issue from a diving-bell at the bottom
of the ocean. That in these now desolate regions, heat only is wanted to
make them like the most favoured countries of the earth, is proved by the
recent discoveries under ground of the remnant of animals and vegetables
formerly inhabiting them, which now can live only near the equator. WNhile
winter, then, or the temporary absence of heat, may be called the sleep of
nature, the more permanent torpor about the poles appears like its death;
and when we farther reflect, that heat is the great agent in numberless important processes of chemistry and domestic economy, and is the actuating
principle of the mighty steam-enzgne which now performs half the work of
society, how truly may heat, the subject of our present chapter, be considered as the life or soul of the universe.
" Heat cannot be exhibited in a separate state, nor proved to have weight or
inertia."  (Read the Analysis, page 256.)
Although heat is known to be abundant in the sunbeam, and to radiate
around from a blazing fire, we cannot otherwise arrest or detect it in its progress than by allowing it to enter, and remain in some ponderable substance.
We know hot iron, or hot water, or hot air, but nature no where presents to
us, nor has art succeeded in showing us, heat alone.
If we balance a quantity of ice in a delicate weigh-beam, and then leave
it to melt, the equilibrium will not be in the slightest degree disturbed. Or
if we substitute for the ice, boiling water or red-hot iron, and leave this to
cool, there will be no difference in the result.  If we place a pound of mercury in one scale of the weigh-beam and a pound of water in the other, and
then either heat or cool both through the same number of thermometric
degrees, although about thirty times more heat (as will be explained below)
enters or leaves the bulky water than the dense mercury, they will still
remain equivalent weights.
Again, a sunbeam, with its intense light and heat, after being concentrated by a powerful lens or mirror, may be made to fall upon the scale of
a most delicate balance, but will produce no depressing effect on the scale,
as would follow if what constitutes the beam had the least forward motal
inertia or momentum.
Such are the facts which have led certain inquirers to deny the material
or separate existence of heat, and to hold that it is merely motion of one
kind among the material particles of bodies generally, as sound is motion
of another kind among the same particles. The following facts they consider
to have the same bearing in the argument: Heat can be produced without
limit by friction, as-when savages light their fires by rubbing together two




CAUSE  OF SEASONS AND  CLIMATES.                       259
pieces of wood —when Count Rumford made great quantities of water boil,
by causing a blunt borer to rub against a mass of metal immersed in waterwhen Sir Humphrey Davy quickly melted pieces of ice by rubbing them
against each other in a room cooled below the freezing point, &c. Intense
heat is produced by the explosion of gunpowder or other fulminating mixture, yet it cannot be conceived to have existed in the small bulk of the
powder before the explosion. Other inquirers, on the contrary, have deemed
to be proofs of the separate materiality of heat such facts as now follow;that it is radiated through the most perfect vacuum which we can produce,
and even more readily than through air; that it radiates in the same place
in all directions, without impediment from the crossing rays; —that it becomes instantly sensible on the condensation of any material mass, as if then
squeezed out from the mass; as when, by compressing air suddenly, we inflame a match immersed in it; or when, on reducing the bulk of iron by
hammering, we render it very hot, the warming being greater at the first
blow (which most changes the bulk) than afterwards,-that when, on mixing bodies which combine so intimately as to occupy less space than when
separate, there is a disengagement of heat proportioned to the diminution of
the volume:-that the laws of the spreading of heat in bodies do not
resemble those of the spreading of sound, or of any other motion known
to us;-and that, as to the great and sudden extrication of heat by friction
or explosion, it may be as truly a rush of the fluid to the part, as in the
case of an electrical accumulation or discharge   These facts, moreover,
they think, square well with their assumption that the phenomena of heat
are produced by an exceedingly subtle fluid, or ether, pervading the whole
universe, and softening, or melting, or gasifying bodies, according to the
quantity present in each; its own parts being strongly repulsive of each
other, and seeking, therefore, widest and most equable diffusion.
The change of its quantity in bodies is most conveniently estimated by the
concomitant change of their bulk, any substance so circumstanced, as to
allow this to be accurately measured, constituting a thermometer."  (Read
the Analysis, page 256.)
If we heat a wire, it is lengthened; if we heat water in a full vessel, a
part runs over; if we heat air in a bladder, the bladder is distended; in a
word, if we heat any substance, its volume increases in some proportion to
the increase of temperature,-and we may measure the increase of volume.
The reasons why, in such investigations, a contrivance in which the expansion of mercury may be observed, viz., the mercurial thermometer, is commonly preferred to others, can only be fully understood by the mind which
has considered the wh-ole subject of heat; and we touch upon the matter
here, only for the purpose of stating that a mercurial thermometer is a
small bulb, or bottle of glass filled with mercury, and having a long very
narrow stalk or neck, in which the mercury rises when expanded by heat,
or falls when heat is withdrawn; the stalk between the points at which
mercury standings in freezing and boiling water, being divided into an
arbitrary number of degrees, which division appearing on a scale applied to
the stalk, is continued similarly aboe and below these points.
"4 Heat diffuses itself among neighboring bodies until all have acquired the
same temperature; that is to say, until all willt similarly affect a thermometer." (See the Analysis.)
An iron bolt thrust in among burning coals soon becomes red hot like




260                            HEAT.
them.  If it be the heater of a tea-urn, it will, when afterwards placed
amidst the water, part with its lately acquired heat to the water, until both
are of the same temperature. Boiling water, again soon imparts heat to an egg
placed in it, and a feverish head yields its heat to a bladder of cold water
or ice. A hundred objects enclosed in the same apartment, if tested, after
a time, by the thermometer, will all indicate the same temperature.
" The inferior degrees of heat are denoted by the term COLD."
When the hand touches a body of a higher temperature than itself, it receives heat according to the law now explained, anal it experiences a peculiar sensation; when it touches a body of lower temperature than itself, it
gives out heat for a like reason, and experiences another and very different
sensation. The two are called the sensations of heat and of cold. Now
heat and cold, considered as existing in the bodies themselves, although thus
appearing opposities, are really degrees of the same object, temperature,
contrasted by name, for convenience sake, in reference to the particular
temperature of the individuals speaking of them-just as any two nearest
mile-stones on a road, although merely marking degrees of the same object,
distance, might receive from persons living between them the opposite names
of east and west, or of north and south.  It is to be remarked, moreover,
that the sensation of heat is producible also by a body colder than the
hand, provided it be less cold than a body touched immediately before, or
than the usual temperature; and the sensation of cold is produced under
the opposite circumstances of touching a comparatively warm body, but
which is less warm than something touched just before. This explains the
remarkable fact that the same body may appear at the same time, and to
the same person. both hot and cold. If a person transfer one hand to common spring-trater from touching ice, that hand will deem the water very
warm: while the other hand, transferred to it from a warm bath, would
deem it very cold. For a like reason; a person from India, arriving in
England in the spring, deems the air cold, while the inhabitants of the
country are diminishing their clothing, because the heat to them is becoming oppressive. Such facts show how necessary it was for men to discover
more correct thermometers than their bodily sensations.
Spreading partly through their structure, or by conduction, as it is called,
with a progress proper to each substance."  (Read the Analysis, page
256.)
If one end of a rod of iron be held in the fire, a hand grasping the other
end soon feels the heat coming through it. Through a similar rod of glass
the transmission is much slower, and through one of wood it is slower still.
The hand would be burned by the iron, before it felt warmth in the wood,
although the inner end were blazing.
On the fact that different substances are permeable to heat, or have the
property of conducting it, in different degrees, depend many interesting
phenomena in nature and the arts; hence it was important to ascertain the
degrees exactly, and to classify the substances. Various methods for this
purpose have been adopted.  For solids-similar rods of the different substances, after being thinly coated with wax, have been placed with their inferior extremities in hot oil, and then the comparative distances to which, in
a given time, the wax was melted, furnished one set of indications of the
comparative conducting powers:-or, equal lengths of the different bare rods




SPREADING BY CONDUCTION.                          261
being left above the oil and a small quantity of explosive powder being placed
on the top of each, the comparative intervals of time elapsing before the
explosions gave another kind of measure:-or equal balls of different substances, with a central cavity in each to receive a thermometer, being heated
to the same degree and then suspended in the air to cool, until the thermometer fell to a given point, gave still another list. A modification of the last
method was adopted by Count Rumford to ascertain the relative degrees in
which furs, feathers, and other materials used for clothing, conduct heat, or,
which is the same thing, resist its passage.  He covered the ball or stem of
a thermometer with a certain thickness of the substance to be tried, by placing
the thermometer in a larger bulb and stem of glass, and then filling the
interval between them with the substance; and after heating this apparatus
to a certain degree, by dipping it in liquid of the desired temperature, he
surrounded it by ice, and marked the comparative time required to cool the
thermometer a certain number of degrees. The figures following the names
of some of the substances in the subjoined list, mark the number of seconds
required respectively for cooling it 60~.
These experiments have shown, as a general rule, that density in a body
favours the passage of heat through it. The best conductors are the metals,
and then follow in succession diamond, glass, stones, earths, woods, &c., as
here noted:
Metals-silver, copper, gold, iron, lead.
IDiamond.
Glass.
Hard stones.
Porous earths.
Woods.
Fats or thick oils.
Snow.
Air  -                         576
Sewing silk     -    -    -    917
Wood ashes    -    -           927
Charcoal  -    -    -    -    937
Fine lint  -    -             1,032
Cotton    -    -    -         1,046
Lamp black    -    -          1,117
Wool           -               1,118
Raw silk'-    -    -        1,284
Beavers' fur    -    -    -  1,296
Eider down     -    -    -  1,305
Hares' fur -    -    -        1,315
Air appears near the middle of the preceding list, but if its particles are
not allowed to move about among themselves, so as to carry heat from one
part to another, it conducts (in the manner of solids) so slowly that Count
Rumford doubted whether it conducted at all. It is Probably the worst conductor known, that is, the substance which when at rest impedes the passage
of heat the most. To this fact seem to be owing, in a considerable degree,
the remarkable non-conducting quality of porous or spongy substances, as
feathers, loose filamentuous matter, powders, &c., which have much air in
their structure, often adherent with the force of attraction which immersion
in water, or even being placed in the vacuum of an air-pump, is insufficient
to overcome.
While contemplating the facts recorded in the above table, one cannot but




262                            IIE A.
reflect how admirably adapted to their purposes the substances are which
nature has provided as clothing for the inferior animals;-and -which man
afterwards accommodates with such curious art to his peculiar wants.  Animals required to be protected against the chills of night and the biting blasts
of winter, and some of them which dwell among eternal ice, could not have
lived at all, but for a garment which might shut up within it nearly all the
heat which their vital functions produced. Now anycovering of a metallic,
or earthy, or woody nature, would have been far from sufficing; but out of a
wondrous chemical union of carbon with the soft ingredients of the atmosphere, those beautiful textures are produced called fur and feather, so greatly
adorning while they completely protect the wearers;-textures, moreover,
which grow from the bodies of the animals, in the exact quantity that suits
the climate and season, and which are reproduced when by any accident they
are partially destroyed.  In warm climates the hairy coat of quadrupeds is
comparatively short and thin; as in the elephant, the monkey, the tropical
sheep, &c. It is seen to thicken with increasing latitude, furnishing the soft
and abundant fleeces of the temperate zones; and towards the poles it is
externally shaggy and coarse, as in the arctic bear. In amphibious animals,
which have to resist the cold of water as well as of air, the furs grow particularly defensive, as in the otter and beaver. Birds, from having very warm
blood, require plenteous clothing, but require also to have a smooth surface, that they may pass easily through the air;-both objects are secured
by the beautiful structure of feathers, so beautiful and.wonderful that writers
on natural theology have often particularized it as one of the most striking
exemplifications of creative wisdom.  Feathers, like fur, appear in kind and
quantity suited to particular climates and seasons. The birds of cold regions
have covering almost as bulky as their bodies, and if it be warm in those of
them  which live only in air, in the water-fowl it is warmer still.  These
last have the interstices of the ordinary plumage filled up by the still more
delicate structure called down, particularly on the breast, which in swimming first meets and divides the cold wave. There are animals with warm
blood which yet live very constantly immersed in water, as the whale, Seal,
walrus, &c.  Now neither hair nor feathers, however oiled, would have
been a fit covering for them; but kind nature has prepared an equal protection in the vast mass of fat or thick oil which surrounds their bodies-substances which are scarcely less useful to man than the furs and feathers of
land animals.
While speaking of clothing we may remark, -that the bark of trees is also
a structure very slowly premeable to heat, and securing, therefore, the temperature to vegetable life.
And while we admire what nature has thus done for animals and vegetables,
let us not overlook her scarcely less remarkable provision of ice and snow,
as winter clothing for the lakes and rivers, for our fields and gardens. Ice,
as a protection to.water and its inhabitants, was considered in Sec. I., in the
explanation of why, although solid, it swims on water. We have now to
remark that snow, whfth becomes as a pure white fleece to the earth, is a
structure which resists the passage of heat nearly as much as feathers.  It,
of course, can defend only from clouds below 320 or the freezing point; but
it does so most effectually, preserving the roots and seeds and tender plants
during the severity of winter. When the green blade of wheat and the beautiful snow-drop flower appear in spring rising through the melting snow, they
have recently owed an important shelter to their wintry mantle. Under deep
snow while the thermometer in the air may be far below zero, the tempera



SPREADIN G BY  CONDUCTION.                        263
ture of the ground is rarely below the freezing point. Now this temperature,
to persons some time accustomed to it, is mild and even agreeable.  It is
much higher than what often prevails for long periods in the atmosphere of
the centre and north of Europe.  The Laplander, who during his long winter
lives under ground is glad to have additionally over head a thick covering of
snow. Among the hills of the west and north of Britain, during the storms
of winter, a house or covering of snow frequently preserves the lives of travellers, and even of whole flocks of sheep, when the keen north wind catching them unprotected, would soon stretch them lifeless along the earth.
It is because earth conducts heat slowly, that the most intense frost penetrates but a few inches into it, and that the temperature of the ground a few
feet below its surface is nearly the same all the world over. In many mines,
even although open to the air, the thermometer does not vary one degree in
a twelvemonth. Thus also water in pipes two or three feet under ground
does not freeze, although it may be frozen in all the smaller branches
exposed above.  Hence, again, springs never freeze, and therefore become
remarkable features in a snow covered country. The living water is seen
issuing from the bowels of the earth, and running often a considerable way
through fringes of green, before the gripe of the frosts arrest it; while
around it, as is well known to the sportsman, the snipes and wild duck and
other birds are wont to congregate. A spring in a frozen pond or lake may
cause the ice to be so thin over the part where it issues, that a skater arriving
there will break through and be destroyed. The same spring water which
appears warm in winter, is deemed cold in summer, because although always
of the same heat, it is in summer surrounded by warm atmosphere and
objects.  In proportion as buildings are massive, they acquire more of those
qualities which have now been noticed of our mother earth. Mlany of the
Gothic halls and cathedrals are cool in summer and warm in winter-as are
also old-fashioned houses or castles with thick walls and deep cellars.
Natural caves in the mountains or sea-shores furnish other examples of a
similar kind.
When in the arts it is desired to prevent the passage of heat out of, or into
any body or situation, a screen or covering of a slow conducting substance
is employed.  Thus to prevent the heat of a smelting or other furnace from
being wasted, it is lined with fire bricks, or is covered with clay and sand, or
sometimes with powdered charcoal.  A furnace so guarded may be touched
by the hand, even while containing within the melted gold. To prevent the
freezing of water in pipes during the winter, by which occurrence the pipes
would be burst, it is common to cover them with straw ropes, or coarse flannel, or enclose them in a larger outer pipe with dry charcoal, or saw dust,
or chaff, filling up the interval between.  If a pipe on the contrary, be for
the conveyance of steam or other warm fluid, the heat is retained, and, therefore, saved by the very same means.  Ice-houses are generally made with
double walls, between which, dry straw placed, or saw dust, or air, prevents
the passage of heat.  Pails for carrying ice in summer, or intended to serve
as wine coolers, are made on the same principle-viz., double vessels, with
air or charcoal, filling the interval between them. A flannel covering keeps
a man warm in winter-it is also the best means of keeping ice from melting
in summer. Urns for hot water, tea pots, coffee pots, &c., are made with
wooden or ivory handles, because if metal were used, it would conduct the
heat so readily that the hand could not bear to touch them.
It is because glass and earthenware are brittle, and do not allow ready passage to heat, that vessels made of them are so frequently broken by sudden




261                            HEAT.
change of temperature. On pouring boiling water into such a vessel, the
internal part is much heated and expanded (as will be explained more fully
in a subsequent page) before the external part has felt the influence, and
this is hence riven or cracked by its connection with the internal. A chimney
mirror is often broken by a lamp or candle placed on the marble shelf too
near it.  The glass cylinder of an electrical machine will sometimes be
broken by placing it near the fire, so that one side is heated while the other
side receives a cold current of air approaching the fire from a door or window.
A red hot rod of iron drawn along the pane of glass will divide it almost
like a diamond knife. Even cast iron, or the backs of grates, iron pots, &c.,
although conducting readily, is often, owing to its brittleness, cracked by
unequal heating or cooling, as from pouring water on it when hot. Pouring
cold water into a heated glass will produce a similar effect.  Hence glass
vessels intended to be exposed to strong heats and sudden changes, as retorts
for distillation, flasks for boiling liquids, &c., are made very thin, that the
heat may pervade them almost instantly and with impunity.
There is a toy called a Prince Rupert's Drop, which well illustrates our
present subject.  It is a lump of glass let fall while fused into water, and
thereby suddenly cooled and solidified on the outside before the internal part
is changed; then as this at last hardens and would contract, it is kept
extended by the arch of external crust, to which it coheres. Now if a portion of the neck of the lump be broken off, or if other violence be done,
which jars its substance, the cohesion is destroyed, and the whole crumbles
to dust with a kind of explosion. Any glass cooled suddenly when first
made, remains very brittle, for the reason now stated.  What is called the
Bologna jar is a very thick small bottle, thus prepared, which bursts by a
grain of sand falling into it. The process of annealing, to render glass ware
more tough and durable, is merely the allowing it to cool very slowly by
placing it in an oven, where the temperature is caused to fall gradually.
The tempering of metals by sudden cooling seems to be a process having
some relation to that of rendering glass hard and brittle.
It is the difference of conducting power in bodies which is the cause of a
very common error made by persons in estimating the temperature of bodies
by the touch.  In a room without a fire all the articles of furniture soon
acquired the same temperature; but if in winter, a person with bare feet were
to step from the carpet to the wooden floor, from this to the hearth-stone, and
from the stone to the steel fender, his sensation would deem each of these
in succession colder than the preceding. Now the truth being that all had
the same temperature, only a temperature inferior to that of the living body,
the best conductor, when in contact with a body, would carry off heat the
fastest, and would, therefore, be deemed the coldest. Were a similar experiment made in a hot-house, or in India, while the temperature of every thing
around were 980, viz., that of the living body, then not the slightest difference
would be left in any of the substances: or lastly, were the experiment made
in a room where by any means the general temperature were raised considerably above blood heat, then the carpet would be deemed considerably the
coolest instead of the warmest, and the other things would appear hotter in
the same order in which they appeared colder in the winter room.  Were a
bunch of wool and a piece of iron exposed to the severest cold of Siberia, or
of an artificial frigorific mixture, a man might touch the first with impunity,
(it would merely be felt as rather cold;) but if he grasped the second, his
hand would be frost-bitten and possibly destroyed; were the two substances,
on the contrary, transferred to an oven, and heated as far as the wool would




SPREADING BY CONDUCTION.                           265
bear, he might again touch the wool with impunity (it would then be felt as
a little hot,) but the iron would burn his flesh.  The author has entered a
room  where there was no fire, but where the temperature from  hot air
admitted was sufficiently high to boil the fish, &c., of which he afterwards
partook at dinner; and he breathed the air with very little uneasiness.
lle could bear to touch woolen cloth in this room, but no body more solid.
The foregoing considerations make manifest the error of supposing that
there is a positive warmth in the materials of clothing.  The thick cloak
which guards a Spaniard against the cold of winter, is also in summer used
by him as a protection against the direct rays of the sun:-and while in
England, flannel is our warmest article of dress, yet we cannot more
effectually preserve ice than by wrapping the vessel containing it in many
folds of softest flannel.
In every case where a substance of different temperature from the living
body touches it, a thin surface of the substance immediately shares the heat
of the bodily part touched-the hand generally; and while in a good conductor, the heat so received quickly passes inwards, or away from the surface,
leaving this in a state to absorb more, in the tardy conductor the heat first
received tarries at the surface, which consequently soon acquires nearly the
same temperature as the hand, and therefore, however cold the interior of the
substance may be, it does not cause the sensation of cold.'The hand on a
good conductor has to warm it deeply, a slow conductor it warms only superficially. The following cases further illustrate the same principle. If the
ends of an iron poker, and of a piece of wood of the same size, be wrapped in
paper and then thrust into a fire, the paper on the wood will begin to burn
immediately, while that on the metal will long resist:-or if pieces of paper
be laid on a wooden plank and on a plate of steel, and then a burning coal
be placed on each, the paper on the wood will begin to burn long before that
on the plate. The explanation is, that the paper in contact with the good
conductor loses to this so rapidly the heat received from the coal, that it
remains at too low a temperature to inflame, and will even cool to blackness
the touching part of the coal; while on the tardy conductor the paper becomes
almost immediately as hot as the coal.  It is because water exposed to air
cannot be heated beyond 2120, that it may be made to boil in an egg-shell or
a vessel made of paper, held over a lamp, without the containing substance
being destroyed; but as soon as it is dried up, the paper will burn and the
shell will be calcined, as the solder of a common tin kettle melts under the
same circumstances. The reason why the hand judges a cold liquid to be so
much colder than a solid of the same temperature is, that from the mobility
of the liquid particles among themselves, those in contact with the hand are
constantly changing.'The impression produced on the hand by very cold
mercury is almost insufferable, because mercury is both a ready conductor and
a liquid. Again, if a finger held motionless in water feel cold, it will feel
colder still when moved about; and a man in the air of a calm frosty morning
does not experience a sensation nearly so sharp as if with the same temperature there be wind. A finger held up in the wind discovers the direction in
which the wind blows by the greater cold felt on one side, the effect being
still more remarkable if the finger is wetted. If a person in a room with a
thermometer, were with a fan or bellows to blow the air against it, he would not
thereby lower it, because it had already the same temperature as the air, yet
the air blown against his own body would appear colder than when at rest,
because being colder than his body, the motion would supply heat-absorbing
particles more quickly. In like manner, if a fan or bellows were used against




266                            H E A T.
a thermometer hanging in a furnace or hot-house, the thermometer would
suffer no change, but the air moved by them against a person, would be
distressingly hot, like the blasting sirocco of the sandy deserts of Africa. If
two similar pieces of ice be placed in a room somewhat warmer than ice, one
of them may be made to melt much sooner than the other, by blowing on it
with a bellows. The reason may here be readily comprehended why a person
suffering what is called a cold in the head, or catarrh from the eyes and nose,
experiences so much more relief on applying to the face a handkerchief of
linen or cambric than one of cotton:-it is, that the former, by conducting,
readily absorbs the heat and diminishes the inflammation, while the latter,
by refusing to give passage to the heat, increases the temperature and the
distress. Popular prejudice has held that there was a poison in cotton.'C Heats spreading in fluids chiefly by the motion of their particles."
(Read the Analysis, page 256.)
Owing to the mobility among themselves of fluid particles, heat entering
a fluid anywhere below the surface, by dilating and rendering specially lighter
the portion heated, allows the denser fluid around to sink down and force up
the rarer; and the continued currents so established, diffuse the heat through
the mass much more quickly than heat spreads by conduction in any solid.
Count Rumford's experiments led him at first to conclude that liquids,
but for this carrying process, by the particles changing their place, were
absolutely impassable to heat.  A piece of ice will lie very long at the bottom of water which is made to boil at the top by the contact of any hot
body; and when it at last melts, Count R. believed that it did so entirely
from the heat which passed downwards through the sides of the vessels
containing the water. But an ingenious experiment by Dr. Murray decided
the question differently. He made a vessel of ice, which, of course, could
not carry downwards any heat greater than 320, as ice melts at that degree;
and having put into the vessel a quantity of oil at 320, with the bulb of a
thermometer being a quarter of an inch under the surface of the oil, he
placed a cup of boiling water in contact with the surface of the oil:-in a
minute and a half the thermometer rose nearly a degree, and in seven minutes
it rose five degrees, beyond which it did not go. The heat then must have
passed downwards through the liquid, proving a conducting power;-unless,
indeed, it passed by radiation, as explained in a subsequent page.
The internal currents or circulation produced by heat in fluid masses, and
of which there are so many important instances in nature, were more fitly
explained in the chapter on Hydrostatics and Pneumatics; we shall here,
therefore, allude to them very shortly.
Perhaps the best experimental illustration of the subject is obtained by
placing a tall glass jar, filled with water, in which small pieces of amber are
diffused to show its movements, first in a warm bath, and then in a cold
bath. In the first case, the water and amber near the outside of the jar
where they are heated, will exhibit a rapid upward current, while in the
centre of the jar they will form an opposite and downward current. In the
second case, or when the jar is placed in a cold bath, the direction of the
currents will be reversed.
Consideration of these currents led the author of this work, some years
ago, to propose what he deemed a great improvement on the construction and
management of boilers and evaporating pans generally; namely, to convert
the upward and downward current in the mass of boiling liquid into a lateral




SPREADING BY CONDUCTION.                         267
current below, constantly and rapidly sweeping the bottom of the vessel.
In ordinary boilers, when a portion of liquid is converted into steam in contact with the horizontal bottom, it does not separate from the bottom immediately, but remains until a steam-bubble of considerable size be formed,
and in the mean time the part of the boiler defended by it from the contact of the liquid, becomes overheated, and the following evil consequences
ensue:-1. Rapid destruction of the boiler, and on that account a rapid
expense; 2. Necessity for having originally much thicker, and therefore
dearer boilers; 3. The thickness being an impediment to the passage of
heat, there is a proportionate waste of fuel; 4. and last, When the liquid is
a vegetable juice or extract, as sugar-cane juice, of a nature to be carbonized
and blackened when overheated, the quality of the product is often exceedingly deteriorated. The patentees of an apparatus, described at 184, for
boiling sugar in vacuo, and therefore at a low and steady temperature,
gained, it was said, more than $40,000 a year by preventing the injury now
spoken of. And when the liquid is a saline solution, like the sea-water
used in steam ships, the salt soon encrusts the bottom of the boiler; and
powerfully both prevents the passage of heat and destroys the boiler. Now
a current sweeping the bottom prevents all these consequences, and may be.easily obtained.  The most obvious method is, to place in the liquid some
upright tubes with open tops at a certain distance under the surface of the
liquid, and with the bottoms also open, but latterly, and all in one direction; the consequence will be, that as soon as the liquid begins to boil, the
general mass, consisting of liquid mixed with bubbles of steam, becomes of
considerably less specific gravity than the liquid in the tubes, remaining
unmixed, because steam will not enter the lateral mouths of the tubes, and
the columns of heavier liquid will therefore descend rapidly and issuing by
the lateral openings of the tubes all in one direction along the bottom of the
boiler, will powerfully and uninterruptedly sweep it. In a long wagonshaped boiler the tubes, instead of being round, should be made flat and
broad enough to reach from side to side; and if a very rapid current be desired, they must be made larger than the spaces between them in which the
steam has to rise, for thus the steam bubbles, being driven closer together,
will make the rising column so much the lighter, and its ascent consequently
the more rapid; and there will be a corresponding rapidity of issue of the
sweep;ng current. In a moderate sized pan or boiler of the usual basin or
half-globe shape, the simplest method of producing the current is to have a
smaller vessel of similar shape- made of thin metal, and placed within the
other so as to have about an inch space all round between them, and having
one large opening at its bottom, —then all the steam mixed with fluid will
rise between the outer and inner vessel, while the unmixed liquid will
descend through the open bottom of the inner vessel, and spread in every
direction, sweeping the bottom of the outer. The sweeping of the bottom
of a boiler might also be effected by a wheel kept turning, to cause the
liquid to resolve horizontally (as was done with another view in the large
Scotch whiskey stills,) or by a frame made somewhat like a rake or gridiron,
kept moving backwards and forwards upon the bottom.
As stated in a previous section, it is the heating and dilatation of the fluid
air over a tropical island while acted upon during the middle of the day by
the powerful rays of the sun, which allow the colder and heavier air from
the faoe of the ocean around to press inwards upon it and force it upwards
in the' atmosphere-the cold current forming the delightful sea-breeze of
the climate.' And it is the general heating of the air over the whole equa



268                           H E AT.
torial belt of the earth, which, rendering it specifically lighter than the air
nearer the poles, allows this to assume the form of cool trade-winds, constantly blowing towards the sun's path, and pressing upwards the hot air,
which then spreads away on the top of the atmosphere towards the poles, to
mitigate the severity of the northern and southern cold. In the watery
ocean also there is a circulhtory motion of the same kind, although less in
degree, tending to distribute heat and equalize temperature, and contributing
to produce some of the great sea currents known to mariners.
The vertical currents produced by heat, in the ocean and in great masses
of water generally, preserve in and over them a comparatively uniform temperate freshness, while the rocks and soil on the shores around may be
either parched under a burning sun, or bound up in frost.  A keen frost
chills, and soon hardens in its icy grasp the surface of the ground; but of
water similarly exposed, the part first cooled descends to the bottom by its
increased density, and forces up a warmer water to take its place; this in its
turn is cooled and descends, and a continued circulation is established, so
that the surface cannot become ice until the whole mass, of whatever depth,
has been cooled down to its greatest density. Hence the very deep sea is
not frozen even in the coldest climates, and in temperate climates, the
severest winter does not freeze even the ordinary lakes.  During this intestine movement in the water, that which ascends to the surface to be cooled,
by losing one degree of its heat, warms more than 500 times its bulk of air
one degree, and thus tempers remarkably the air passing over it.  Hence
places in the vicinity of the. sea and of lakes are warmer in winter than
places further inland, although nearer to the equator. England is much
warmer in winter than central Germany, which lies south of England; and
the coast of Scotland and of the north of Ireland are warmer than London:-sn'ow never lies long upon these coasts. As continental or inland
countries ha-ve thus in winter an extreme of cold, so they have in summer
an extreme of heat.  Water admits the rays of the sun, and absorbs the
heat into the whole thickness of its mass, and therefore is warmed very
slowly; but the dry earth retains all the heat near its surface, and is therefore soon heated to excess.
The ventila tion of our dwellings and halls of assembly (as explained previously) is owing to the motion produced by the changed specific gravity of
air when heated.  The air which is within the house becomes warmer than
the external air, and the latter then presses in at every opening or crevice
to displace the other. The ventilation of the person by the slow passage of
air through the texture of our clothing is a phenomenon of the same kind;
and thicker clothing acts chiefly by diminishing the rapidity of this passage.
Hence an oiled-silk or other air-tight covering laid on a bed has greater influence in preserving warmth than'one or two additional blankets, and is
not generally used, only because it prevents ventilation, and by shutting in
the insensible perspiration, soon produces dampness.  From the part of bedclothes immediately over the person there is a constant outward oozing of
warm air, and there is an oozing inward of cold air in lower situations
around. In many persons the circulation of the blood is so feeble that in
winter, they have great difficulty in keeping their feet warm, even in bed,
unless with the assistance of a bottle of hot water or some such means, and
in consequence they often pass sleepless nights, and suffer in their general
health.  In such cases, at the suggestion of the author, a long flexible tube
has been used,-as of spiral wire, covered with leather or varnished cloth,
by which a person can send down to his feet his hot breath, and thus apply




SPREADING BY COND'UCTION.                           269
them effectually a natural animal warmth, as in a cold day he does to his
hands by blowing upon them through his gloves.
The power of fluids to diffuse heat being due to their power of carrying,
and not of conducting it, the consequence should follow, that any circumlstance which impedes the internal motion of the fluid particles, should
diminish the suffusing power. Accordingly we find, that fluids in general
transfer heat less readily in proportion as they are more viscid. Water, for
instance, transfers less quickly than spirits; oil than water; molasses or syrup
than oil; and water thickened by starch dissolved in it, or which has its
internal motion impeded by feathers or thread immersed in it, less quickly
than where it is pure and at liberty. Cooling being merely a motion the
reverse of heating, it is influenced by the same law. Hence the reason why
thick soups, pies, puddings, and all semifluid masses, retain their heat so
long —so much longer than equal bulks of mere fluid.  The same law affords
explanation of the facts, that very porus masses and powders, as charcoal,
metal filings, saw dust, sand, &c., conduct heat more slowly than denser
masses, —their interstices being filled with air, which scarcely conducts heat,
and which, by the structure of the substance, has no freedom of motion or
circulation by which it might carry the heat.
lec eat spreads, also, partly by being radiated or shot light from one body
to another, through transpvarent media or space with readiness affected by
the material and. the state of the giving and receiving surfaces."  (Read
the Analysis, page 256.)
If a heated ball of metal be suspended in the air, a hand brought in any
direction near to it, will experience the sensation of heat; and beneath it the
sensation will be as strong as on the sides, although the heat has to shoot
down through an opposing current of air approaching the heated ball, to rise
from it, as explained in a previous section.  A delicate thermomneter substituted for the hand, will equally detect the spreading heat, and if held at
different distances, will prove it to diminish in the same ratio as light diminishes in spreading from any luminious centre, viz., to be only a fourth
part as intense at a double distance, in a corresponding proportion for other
distances.  If the heated body be enclosed in a vacuum, a thermometer
placed near it will still be affected in the same manner.  If a screen be interposed between the body and the thermometer, the latter will not be affected
at all, proving the heat to spread in straight lines.  Heat, when diffusing
itself in this way, to distinguish it from heat passing by contact and communication, as described in the last section, is called radiatt heat; that is
to say, spreading in radii or rays all around its source as light spreads.
Radiant heat resembles light yet in other respects.  It as rapidly permeates certain transparent substances, and its course suffers in them  a degree
of the bending, termed refraction by opticians.  It is reflected from many
kinds of polished surfaces, just as light is reflected from a common mirror;
and many such surfaces directed to one point or centre (as when Archimedes
made the sun his assistant to burn the Roman ships) or a single concave
surface, having its own centre or focus, will concentrate heat just as light.
Its motion in the sun-beam is so rapid, as for any distance at which men can
try the experiment, to appear instantaneous; and the rays of heat from hot
iron or burning charcoal concentrated at great distances by suitable mirrors,
affect a thermometer as quickly as the heat of the sun similarly reflected.
Although light and heat are united in the sun's ray, they are still separable




270                             HEAT.
by our glass prisims or lenses; and the focus of heat behind a burning glass
is not precisely the focus of light. Heat, in radiating through air, does not
warm the air, and its passage is not sensibly affected by winds or any other
motion of the air.-These resemblances in the phenomena of light and heat
have by some inquirers been held to prove that the two classes of appearances are only different modifications of action in the same subtile substance
or either,
The diffusion of heat by radiation, as it takes place in an instant to any
distance, and begins whenever there is any inequality of temperature between
bodies exposed to each other, would produce instant balance of temperature
throughout nature, but that heat leaves and enters bodies with readiness,
depending on the condition of their surfaces, and on their internal conducting powers. A black stone-ware ter-pot, for instance, will radiate away 100
degrees of its heat in the same time that a pot of polished metal will radiate
only 12 degrees.
Professor Leslie was the first to see the importance of investigating this
subject, and he had the merit of contriving well-adapted means, and of detecting many of the important facts.  As common thermometers are not sufficiently delicate to determine very sudden changes of temperature, where the
influence is so light as in many cases of radiant heat, he used the beautiful
differential thermometer contrived by himself, in conjunction with concave
mirrors, (as represented on next page,) to concentrate the heat, and accumulate its energy.   Then taking as his heated body a cubical tin vessel filled
with boiling water, and covering it successively with plates or layers of different substances and with different colorsi and exposing the thermometer
to it for a given time under ill the changes, he noted the number of degrees
which the thermometer rose, (as seen in the table which here follows) and
thus ascertained the radiating power of each sort of covering.
~Lamp black.......    100~
Writing paper........   98
Crown glass.......        90
Ice.........        87
Isinglass........       75
Tarnished lead.......  45
Clean lead........        19
Iron polished........   15
Tin plate....                            12
Gold, silver and copper.. - 12
He next reversed the experiments by using his hot-water vessel always in
the same state, and covering the thermometer bulb with the different substances and colours, and thus he ascertained that the comparative absorbing
powers of the substances and colours were very nearly proportioned to their
radiating powers: laamp-black, for instance, absorbed or was heated 1000,
while the polished metals absorbed or were heated only 120, and so for the
others.  And, lastly, the absorbing powers being an indication of the opposite or reflecting powers (for a body absorbing only a given proportion of
the heat which falls on it, must reflect the remainder,) he, by the same
experiments, ascertained the radiating, absorbing, and reflective or mirror
powers of the bodies, and therefore all the important points respecting radiant
heat in its relation to them.
It seems paradoxical that the putting a clothing of a thin cotton or woolen
fabric upon the polished tin vessel, should cause the heat to be received by




SPREADING BY RADIATION.                            271
it or dissipated from it much sooner than if the vessel were naked, but such
is the fact.  And metal with a scratched or roughened surface radiates or
receives much more rapidly than polished metal.
The property of absorbing heat depends much upon the colour of the
substance, and, as a general rule, the dark colours, viz., those which absorb
most light, absorb also most heat.  Dr. Franklin proved this by laying
pieces of cloth of different colours on snow, and exposing them  during a
given period to the sun's rays: while he noted the different depths to which
by the melting of the snow under them, the pieces sank. Hence comes the
importance of having a white dress in summer, that by it, with the sun's
light, the heat also may be repelled. And a white dress in winter is good
because it radiates little.  Polar animals have generally white furs.  White
horses ars both less heated in the sun, and less chilled in winter, than those
of darker hues.
The rate of cooling in heated bodies must be influenced by all the particulars noted above viz., substance, surface, colour, and by the excess of heat
in the cooling body as compared with those around it.
The concentrating apparatus used for experiments on the radiation of
heat consists of two concave tin mirrors, here represented at a and b, so
formed and placed in relation to each other that all the rays of light or heat
issuing from the focus of one, as at c, shall, after a double reflection, be
collected in the focus of the other, d. A stand under one focus c is intended
Fig. 124.....,:    I
to support the body giving out or receiving heat, and a stand under the
other d is meahnt to support the thermometer.  For farther explanation of
the action of such mirrors, we may refer to what was said of the concentration of sound in the section on Acoustics, or to what follows in the section on Optics, on the concentration of light.  The general rationale of such
facts is, that heat, light, sound, and elastic ball, &c., when reflected from any
point of a surface, returns, if it fall perpendicularly to that point, in the
same line by which it approached; but if it fall obliquely, or from one side
of the perpenpicular, it returns in a line deviating as much on the other side.
Now the surfaces of concave mirrors are so formed, that every ray issuing
from the focus shall, when reflected, become parallel to every other ray-as
represented by the dotted lines in the figure; and it is the property of a
similar mirror receiving parrallel rays to make them all meet in its focus:thus, any influence radiating from c towards the mirror d, will again, after
two reflections, be collected at d. The purpose and effect of such mirrors
in experiments on heat, are merely to concentrate feeble influences, so that
they may be more accurately estimated. To show the effect and mode of
action of such mirrors, they may be placed exactly facing each other at any
convenient distance, and then a hot body of any kind, as a metallic ball or
a canister of boiling water, being placed in one focus while a thermometer




272                            HEAT.
stands in the other, the thermometer will instantly rise; although if left in
any intermediate situation nearer to the hot body, and therefore not in the
focus, it will not be affected. If burning charcoal be placed in one focus,
and a readily combustible substances in the other, the latter may be set fire
to, at the distance of thirty feet or more.
If, in one focus of the mirror apparatus described above, there be placed,
instead of the canister of hot water, a piece of ice, the thermometer in the
other focus immediately falls. This has been called the radiation of cold,
and persons were at one time disposed to think that it proved cold to have
a positive existence distinct from heat.  The case, however, is merely that
the thermometer happens then to be the hotter body in one focus of the
mirrors, placed in close relation with a colder body, the ice, in the other,
and consequently.by the law of equal diffusion, it muot share its heat with
the ice, and will fall. The mirrors in any case have merely the effect, by
preventing the spreading and dissipation of the radiant heat from either
focus except towards the other of making two distant bodies act upon each
other as if they were very near.  All the heat that seeks to radiate from
the thermometer d in the direction of the surface of the mirror b, if not
met by an equal tension or force of temperature in the other mirror or
focus, to which they are directed at a and c, will radiate away to c, and
become deficient at d. Some inquirers have believed that heat was constantly
radiating in exchange from  substance  to substance (as light radiates
between opposed bodies) only more copiously from the side where the
temperature was highest: others have held that motion took place only
where there was excess of heat; that is, when the balances of temperature
was destroyed; and this is the simplest view.
There is a remarkable difference in one respect between the heat of the
sun and that radiated from any other source, namely that the first passes
through air, glass, water, and transparent bodies generally, very readily,
while the latter, although not obstructed by air, is almost totally intercepted
or absorbed, in passing through any of the other substances namied., In our
drawing-rooms it is common to have plate.glass fire-screens, which, while
they allow the light to pass, defend the face from the heat; but all persons
know that the heat of the sunbeams, as well as their light, enters our greenhouses through the glass which covers them.  A glass screen interposed between the concave mirrors in the apparatus above described, destroys almost
entirely the effect of the heated body placed in one focus, on the thermometer in the other, and the trifling effect really produced has appeared to
some to be owing to the heat that is absorved by the screen on one of its
sides, and then after passng through it by conduction, is radiated from the
other. This conclusion seemed to be supported by the fact that screens of
metal or of glass, covered with lamp black, paper, &c., allow transmission
nearly in proportion to their several absorbent and radiant powers.  MIore
careful experiments, however, have seemed to prove that, even at a low
temperature, a certain portion of the heat is suddenly radiated through the
glass, and at a high temperature, a much larger portion. A glass mirror
reflects the light of a fire, but at first retains nearly all the heat, and only
radiates it afterwards as a hot body.
The doctrines of radiant heat make us aware of the importance of having
vessels of polished metal for containing liquids or other things which we desire to keep warm; hence, tea and coffee-pots, dishes for soup, &c., should
be polished.  As a black earthen tea-pot loses heat by radiation nearly in
proportion to the number 100, while one of silver or other polished metal




SPREADING BY RADIATION.                         273
loses only as 12, there will be a corresponding difference in their aptitude
for extracting the virtues of any substance infused in them. Pipes for the
conveyance of steam or hot air, if left naked, should be of polished metal;
but after arriving at a place where they have to give out their heat, their
surface should be blackened and rough.. A coat of polished mail is not a
cold covering.  A mirror intended to reflect heat should be of highly
polished metal, and such, for an obvious reason, the interior of a screen behind
roasting meat is attempted to be made. A fireman's mark is usually covered
externally with smooth tin foil. It is of advantage that the bottom of a teakettle or other cooking vessel be externally black, because the bottom has
to absorb heat, but the top should be polished because it has to confine.
The interesting phenomenon of dew was not at all understood until lately,
since the laws of radiant heat have been investigated. At sun-rise, in particular states of the sky, every blade of grass and leaflet is found not wetted, as if by a shower, but studded with a row of distinct globules most
transparent and beautiful, bending it down by their weight, and falling like
pearls when the blade is shaken.  These are formed in the course of the
night by a gradual deposition on bodies rendered by radiation colder than
the air around them, of part of the moisture which rises invisibly from
water surfaces into the air during the heat of the day. In a clear night the
objects on the surface of the earth radiate heat to the sky through the air
which impedes not, while there is nothing nearer than the stars to return
the radiation.; they consequently soon become colder, and if the air around
has its usual moisture, part of this will be deposited on them, in the form
of dew, exactly as the invisible moisture in the air of a room is deposited
on a cold bottle of wine when first brought from the cellar.  Clouds, by
obstructing the radiation spoken of, obstruct the formation of dew. Air
itself seems not to lose heat by radiation. A thermometer placed upon the
earth any time between sunset and sunrise, generally stands considerably
lower than another suspended in the air a few feet above it; owing to the
radiation of heat upwards from it and from the earth, while the surrounding
air remains nearly in the same state. During the day, while the sun shines,
the earth is warmer than the air.  The reason why the dew falls, or is
formed so much more copiously upon the soft spongy surface of leaves and
flowers, where it is wanted, than on the hard surface of stones and sand
where it would be of no use, is the difference of their radiating powers,
There is no state of the atmosphere in which artificial dew may not be made
to form on a body, by sufficiently cooling it and the degree of heat at which
the dew  begins to appear is called the dew point being an important
particular in the meteorological report of the day.  In cloudy nights heat
is radiated back from the clouds, and the earth below not being so much
cooled, the dew is scanty or deficient. And it is, when uninformed persons
would least expect the dew, viz.: in very warm clear nights, and perhaps
when the beautiful moon invites to walking, as in some of the evenings of
autumn with the harvest moon and harvest occupations-that the dew is
more abundant, and the danger greater to delicate persons of taking harm
by walking among the grass.
18




274                            HEAT.
" Heat by entering bodies expands them, and through a range which includes
as three successive stages the forms of solid, liquid, and air or gas; becominy thus, in nature, the grandI antagonist and modifier of the effects
of that attraction which holds corporeal atoms together, and which, if acting alone, would reduce the w4ole material universe to one solid, lifeless
mass. (Read Analysis, page 256.)
If an experimenter take a body which is as free from heat as human art
can obtain it-a bar of solid mercury, for instance, as it exists in a polar
winter-and if he then gradually heat such body, it will acquire an increase of bulk with every increase of temperature; first, for a time, there
will be simple enlargement or expansion in every direction; then the mass
will in addition be softened; then it will be melted or fused; that is to say,
in the case supposed, the solid bar will be reduced to the state of liquid mercury, with the cohesive attraction of the atoms nearly overcome: if the
mass be still farther heated, it will gain bulk until at a certain point, the
atoms will be repelled from one another to much greater distances, constituting then a very elastic fluid called an air or gas, many hundred times more
bulky than the same matter in the solid or liquid state, andforcibly distending an appropriate vessel as common air distends a bladder; susceptible,
moreover, of dilating indefinitely farther, by farther additions of heat, or by
diminution of the atmospheric, or other pressure, against which it had to
rise during its formation. A subsequent removal of the heat. from the gaseous mercury, will cause a progress of contraction corresponding to the, previous progress of expansion, and the various conditions or forms of the substance above enumerated, will be reproduced in a reversed order, until the
solid mass re-appear, as at first.
What is thus true of mercury is proved, by modern chemical art, to be
true also of all the ponderable elements of our globe, and of many of the
combinations of these elements, —as water, for instance, familiarly known
in its three forms of ice, water and steam; although compound substances
generally, by great changes of temperature, are decomposed into their
elements.
A student might at first have difficulty in believing that the beautiful
variety of soil, liquid, and air, found among natural bodies could depend
upon the quantitiesof heat in them, because these forms are all seen existing at the same common temperature; but he afterwards learns that each
substance has its peculiar relation or affinity to heat, and that hence, while
at the medium temperature of the earth, some bodies contain so little as to
be solids-like the metals, stones, earths, &e.; others have enough to be
liquids-as mercury, water, oils, &c., and others have enough to be airsas oxygen, nitrogen, hydrogen, &c. Men, until better informed, are prone
to deem the tastes in which bodies are most frequently observed by them,
the natural or essential states of such bodies; and the Indian king reasoned
but in the usual way when he held the Dutch navigators, newly arrived on
his shores, to be gross imposters, because they said that in their country, at
one time of the year, water became so hard that they could walk upon it,
and drive their carriages upon it, and shape it into solid blocks.  All persons err like this king, who in thinking of the different substances familiarly
known to them, regard their accidental state of solid, liquid or gas, which
state is really dependent on the temperature of the bodies, and therefore
on the particular climate or situation on earth where they are found, to he in
them an essential natural character. As well might a person who had never




CAPACITY  FOR  HEAT.                           275
seen silk, but as a delicate gauze or satin enveloping some lovely human form,
refuse to recognize it in the unsightly coil of the worm which produces it.
The degree in a general state of temperature at which the substances most
important to man change their states from solid to liquid, or from liquid to
air, will be noticed in a future page. Here we have only to remark, that the
differences are very great.  Mercury melts at about.80 degrees below the
melting point of ice, and porcelain at about 30,000 degrees above. There
are some substances which require so high a temperature for their fusion or
for their conversion into gas. that human art has difficulty, or even finds it
impossible, to produce the changes by simple concentration of heat; but all
such substances are quickly reducible to the liquid form when placed in contact with others for which they have a chemical affinity, and which possess
already the form of liquid or air; as when gold or platinum are dissloved in
nitro-muriatic acid-flint in the fluoric acid-carbon in hydrogen gas. Now
many persons may not have reflected that the dissolving a solid in any fluid
menstruum is merely another mode of melting it by heat; yet this is the
truth, for the menstruum is itself fluid, only because of the much heat which
it contains, and in dissolving the more obdurate substances, it does so merely
because its atraction for the substance brings the particles into union with
the heat which already exists in itself.  Heat, then, is the one universal
solvent, or cause of fluidity.  Its influence in this view is interestingly seen
in the fact, that a fluid when heated can dissolve much more of solid than
when cold.  Water, while hot, keeps dissolved twice as much of many salts
as it can when its temperature has fallen, as is proved by the crystals of salt
formed in any saturated solution as it cools.-There are, again, in nature
many substances having such an affinity for heat, that, until lately, they
have been known only as airs; and even in the present advanced state of
art, they cannot, by any degree of mere cooling, be reduced to the liquid or
solid form; yet all such, when pressure is added to the cooling, or when the
chemical attraction for them of some other substances which already exist
in the liquid or solid state is made to co-operate, may be reduced. An instance
is afforded when oxygen is made part of a liquid acid, or of a solid ore.
Of solids, some on receiving heat become very soft before they are liquefied, as pitch. glue, iron, &c.; others change completely at once, as ice in
becoming water: and some pass at once to the state of air, without, therefore,
having assumed at all the intermediate state of liquid-they are subligned,
as it is called, and on cooling again may be caught in a powdery state, as
seen in that form of sulphur or of benzoin, termed the flower of the substance.  Of the latter class, also, are camphor, arsenic, corrosive sublimate,
and the substance called iodi e, which last, from the state of rich ruby
crystals, becomes at once, on being heated, a dense transparent gas of the
same hue, and in cooling resumes its crystalline form.
The reader having arrived at this place, may peruse again w.ith advantage
the five pages near the beginning of this work, which treat of the influence
of heat on the constitution of masses.
"Each particular substance, according to the nature, proximity, &c., of its
ultimate particles, takes a certain quantit#y of heat, (said to mark its CAPACITY) to produce in it a given change of temperature or caloric tension."
(Read the Analysis, page 256.)
A pound of water, for instance, that its temperature may be raised one
degree, takes thirty times as much heat as a pound of mercury. This may




276                            HEAT.
be proved in various ways. First, if the heat be derived from any uniform
source, the water must remain exposed to it thirty times as long as the mercury. Second, if both substances, after being equally heated, be placed in
ice until cooled to the freezing point, the heat, which escapes from the water
will melt thirty times as much ice as that which escapes from the mercury,
instead of the two becoming of a middle temperature, as in the case when
equal quantities of hot and cold water are mixed, and every degree of heat
lost by the one quantity becomes just a degree gained by the other-the pound
of hot water, by giving up one degree to the pound of cold mercury, raises
the temperature of the latter thirty degrees; and in the same proportion for
other differences:-or on reversing the experiment, a pound of hot mercury
will be cooled thirty degress by warming a pound of water one degree.
Now each particular substance in nature, just as water or mercury, has
its peculiar capacity for heat; and experiments made by the modes of mixture and of melting ice above described have led to the construction of tables
which exhibit the relations. The following short table is an abstract, showing the comparative capacities of equal weights of some common substances.
Water, for reasons of convenience, has been chosen as the standard of comparison. It appears that a pound of hydrogen gas takes about twenty times
more heat to.produce it in a given change of temperature, than a pound of
water, while a pound of gold takes about twenty times less, and, therefore,
four hundred times less than the hydrogen. The figures in the table, by
marking the comparative capacities for heat of various substances necessarily
indicate, also, the comparative quantities of ice melted by equal weights of
the substances in cooling through an equal number of degrees.-A pound
of water, the standard, must cool 140 degrees, that is, must give up 140
degrees of its heat, to melt one pound of ice.
Gases.
Hydrogen         -                              21 
Atmospheric air  -       -      -       -
Carbonic acid gas -      -      -
Common steam    -        -       -               I
Litzquds.
Solution of carbonate of ammonia  -     -        2
Alcohol   -          -       -      -               1
Water         -          -      -       -        1
Milk      -       -      -       -      -        1
Olive oil -.       -      -       -
Linseed oil -    -
Sulphuric acid    -      -          -
Quicksilver      -.                     -
Solids.
Ice       -                                      9- 
Wheat   -        -       -      -       -
Charcoal -.. 
Chalk    -       -       -      -. 
Glass    -       -      -           - 
Iron     -       -        
Zin t    -                
Gold     -.




CAPACITY FOR HEAT.                          277
We may remark here that some late researches, by another mode of trial,
make the capacity of air to be only a quarter that of water, although in the
preceding table it appears to be one and three-quarters. Now as the other
aeriform fluids have been compared with water through the medium of atmospheric air, if there be an error with respect to this, it must run through all
the figures noting the capacity of other aeriform substances.
If we seek a reason or reasons why there should be among bodies the differences of capacity here stated, the circumstances chiefly attracting attention
are the following. 1st. Equal weights. of the various substances have very
different bulks or volumes, and therefore have different room in which the
heat may lodge;-as a pound of mercury, for instance, is only one-fourteenth
part as bulky as a pound of water. That the bulk, however, is not the only
influencing circumstance appears in the fact, that mercury only has one
thirtieth of the capacity of water. 2d. In equal bulks of different substances, the space may be more completely occupied by the particles of one
than of another-as is probably true of the particles of mercury compared
with those of water. 3d. But as the facts are not fully accounted for by
these two circumstances, we must infer that there is some difference in the
ultimate particles of bodies affecting their relations to heat. We shall now
review more particularly the various circumstances mentioned.
First. The influence of bulk or volume, in determining the capacity for
heat, is proved by the facts stated in the preceding table, and by many others.
In the table, for instance, it is seen that hydrogen and the gases generally,
with their great comparative bulk, have also great capacity; that liquids
have less capacity than gases; that solids have less than liquids-but the
capacity, as already stated, is not in strict proportion to bulk; for hydrogen
which is many thousand times more bulky than an equal weight of water,
has only twenty-one times the capacity. Again, if any body whatever be
suddenly compressed into less bulk, heat will issue from it as if squeezed
out. Thus iron or other metal, suddenly condensed by the heavy blow of
a hammer, is thereby rendered hotter, that is expelled heat gradually spreads
from it. Because water and spirit, on being mixed, occupy less space than
when separate, there is from the mixture a corresponding discharge of heat.
But the truth is most remarkably exemplified in airs or gases, owing to their
great range of elasticity. They may be condensed or dilated a hundred-fold
or more, and there will be a simultaneous concentration or diffusion of their
heat, that is to say, the production, in the space occupied by them of intense
heat or cold. The heat of air just condensed, or the cold of that which has
just expanded, is much greater than even the most delicate thermometer can
indicate, for there is so little heat altogether even in a considerable volume of
air, that the mass of a mercurial thermometer, although absorbing a great
part of it, would be little affected. The extent, however, of the change of
temperature is seen in the facts, that, by the sudden condensation of air we
may inflame tinder immersed in it, and by allowing air suddenly to expand,
we may convert any watery vapour diffused through it into ice or snow.
Nay, air, containing carbon in perfect solution, as is true of the common coal
gas, if first condensed to expel heat, and then allowed suddenly to expand,
will be so cooled that the carbon will be separated like a black cloud, as snow
is separated in the case before described. The cold which separates or
freezes carbon from a gas holding it in solution, must be very intense. It
might be expected that air suddenly compressed into half its previous volume,
should become just twice as hot as before, or if suddenly dilated to double
volume, should be only half as hot, thus enabling us to ascertain the whole




278                            HEAT.
quantity of heat contained in it; but the facts are not so; the temperature
changes, near the middle degrees of the scale at least, much less than the
density. Air in doubling its volume from, a common density, becomes colder
only by about 500 of Farenheit's thermometer.
The different capacity for heat of air in different states of dilation, produces effects of great importance in nature as well as in the arts. Thus,
On the surface of the earth near the sea-shore, the air of the atmosphere
has a certain density (a cubic foot weighs about one ounce and a quarter) dependent on the weight and pressure of the superincumbent mass; but on a
mountain top 15,000 feet high, as half the mass of the atmosphere is below
that level (see " Pneumatics,") the air is bearing but half the pressure, and
consequently any quantity of it has twice the volume of an equal quantity
at the sea-side, with a temperature consequently many degrees inferior. The
air which is at any time on a mountain-top, may, a little while before, have
been on an adjoining plane or shore, and in gradually climbing the mountain
side as a wind, it must have been gradually expanding and becoming cooler
in proportion to the diminishing pressure. It is found that air, on rising
from the sea-shore, becomes one degree colder nearly, for the first 200 feet
of perpendicular ascent, and that air becomes altogether about 50~ colder in
rising 15,000 feet; so that at this latter elevation, water exposed to the air
is frozen even near the equator, where the temperature of low plains is at
least 800. It thus appears that if a man could travel with the wind so as to
remain always surrounded by the same air, he might begin his jouiney
with it from the summer vineyard of the Rhine, might soon after find it
the piercing blast of the Alpine summits; and again, a little after, without
any change having occurred.in the absolute quantity of its heat, might feel
it as the warm breath of the flowers on the plains of Italy.
The explanation is thus given of why very elevated mountains in all parts
of the earth are hooded in perpetual snows. We have just said that even at
the equator, where the average temperature near the sea is 840, water will
be frozen when carried to an elevation of 15,000 feet.  A line, therefore,
traced round a mountain at this level would divide the portion of it' destined
to sleep under lasting ice and snow from the portion below covered with
green herbage.  This line, wherever found, is called the snow line, or line
of perpetual congelation.  At the equator it is high in the atmosphere, because there is a difference of about 400 between the average temperature of
the country and the freezing point of water, viz., the difference between 840
and 320, and an elevation of 15,000 feet corresponds to this difference; but in
a progress towards the poles, the line is m'et with gradually nearer'to the earth,
as the difference between the average temperature and the freezing point is
less. In Switzerland, the snow line is at 6,500 feet above the sea; in Norway,
it is below 5,000.  With respect to the line of congelation, it is farther to be
remarked, that in tropical countries, because the temperature of the air is nearly
uniform during the whole year, the line or limit of frost and snow is distinct
and unvarying, that is to say, is narrow, particularly where the acclivity is
considerable; but in countries to the north and south, which have strong contrast of summer and winter, the line rises in summer and falls in winter, and
thus becomes broad and less evident; in the hot season much snow is melted
or half melted above the middle of the line or belt, while in winter much snow
and ice is accumulated below that, to be melted again when summer returns.
In the breadth of the line of congelation for changeable climates, we have
the reason of the formation of what are called glaciers around snow-capped
mountains situated in such climates, and around such only. The snow near




CAPACITY FOR HEAT.                           279
the upper part of ths broad line having been only softened or half thawed in
the preceding summer, becomes in winter almost as solid as ice, and in the
succeeding summer vast masses of it, detached by the action of the sun and
of the internal heat of the earth; and loaded with more recently deposited snow,
are constantly falling down into the neighbouring valleys within the broad
line of congelation; where,beingaccumulated, and the crevices filled up with
snow or with water which hardens to ice, they form at last the huge glaciers
or seas of ice, mers de glace, which renders certain regions so remarkable.
The falling of the masses above described (called in Switzerland avalanches,)
is what renders the ascent to snow-clad mountains so terrific and dangerous.
Around Mount Blanc, Pn the awful solitudes of the elevated valleys, the
avalanches are thundering down almost without interruption during the whole
summer,-in which season only the attempt to ascend the mountain can be
made: and a pistol shot, or any considerable agitation of the air, may suffice
to set loose masses that would sweep away a whole convoy. Beneath glaciers4there is always going on a melting of that part of the ice which is in
contact with the earth, and hence a stream of water constantly issues from
the bed of every glacier. These streams in Switzerland are the beginnings
of the magnificent rivers the Rhine and Rhone -Like the avalanches breaking loose in summer among the mountains, there are in polar seas vast masses
of ice detached from the shores, and which afterwards drift into warmer seas
to be melted. These often become as rafts to the arctic bear, and to his surprise, carry him to new latitudes, and leave him at last to perish in the midst
of the wide ocean, when his support has vanished from beneath him.
Although the proofs are not so immediately apparent, the line of congelation exsits as truly every where in the open sky, over sea and plane, as
where there are mountain heights to wear its livery; and considerably below
the line, the cold, aided by electrical agency, is sufficient to produce in the
form of mist or clouds, a deposition from the air of the watery vapour contained in it. There is thus in nature an admirable provision to shade the
earth at proper times from the too powerful rays of the sun, or to supply rain
as wanted, without the transparency of the inferior regions of the atmosphere
being much affected. As the watery vapour rising from sea or lake, and invisibly diffused in the atmosphere, can only reach to the height where the cold
is great enough to condense it, the clouds may in general be regarded as the
top of that atmosphere of watery vapour or aeriform water, which is always
mixed more or less with the atmosphere of mere air; and as the quantityof
watery vapour which can exist invisibly in a given space, depends altogether
on the intensity of heat present, the clouds in a cold or humid atmosphere
will be low, and in a warm or a dry atmosphere will be high, or there may
be none. An aeronaut mounting in his balloon through a clear sky, often
enters a dense cloudy stratum, and for a time is surrounded by the gloom
almost of night, the face of eaxth being hidden from him below, while the
heavenly bodies are equally veiled from him above; but rising still higher, he
again emerges to brightness, and looks down upon the fleecy ocean rolling
beneath him, as the climber to a lofty peak looks down from the ever pure
atmosphere around on the inferior region of clouds and storms.
The diminished temperature of air in the higher regions of the atmosphere,
often enables the natives of temperate climates, when forced to reside in hot'
tropical countries, inimical to their health, to find near' at hand, on some
mountain height, the congenial temperature of their early homes.  The
author of this work, during a visit to the then not long inhabited Island of
Penang in the strait of Malacca, examined this fact with pleasure not readily




280                           IEAT.
forgotten. The centre of the island is occupied by a lofty mountain ridge
thickly wooded, on the northern summit of which a few residences visible
from the sea-shore like eagles' nests on a cliff, had just been constructed.
Towards these one morning at sunrise, on an active little horse of the
country, and along a tolerable road, he began to climb from the hot plain
below. At first there were around him purely tropical objects, inspiring
tropical feelings, —the latter modified indeed by the reflection that his track
lay through a forest, in which until lately the foot of man never penetrated,
and where the trees nursed through ages to their greatest growth, and the
stupendous precipices.and the sublime waterfall had so recently been exposed
to human observation; but as he gradually ascended, he perceived the
character of the vegetation to be changing and the air to be becoming so light
and cool as strongly to awaken in him thoughts of distant England —nay,
almost the illusion that he was there. When he had reached the summit,
however, and a clear space opened to view the whole country around, his
attention was soon recalled to the fervid land of the sun. At first, from the
elevation being so great, the eye took account only of the grander features of
the scene, and which were such nearly as might be met with on a Grecian
or Italian shore: the expanse of sunny water in that beautiful. strait, the
opposite continent with its river winding seaward across the plain, the town
and the roadstead near it crowded with ships, which appeared only as specks
on a wide-spread map; but, on closer inspection, and particularly with the
aid of the telescope, were described the rich groves or cocoa-nut and banana,
the plantations of spice, and cotton, and sugar-cane, the tawny labourers, the
bamboo dwellings, the fanciful canoes, or prows, and other objects of the
like character. And such was the scene which even under the equator, a
person could place under his eye, while the thermometer near him stood as
in an English month of May.
The interiors of the islands of Jamacia and HIayti have many situations
of great extent, which combine, as above described, the advantages of tropical situation and temperate climate, and which might well be inhabited by
English labouring colonists. The vast plain of Mexico, and much of the
central land of South America, are similarly circumstanced: and it is not
uncommon, where the ascent to the gigantic Andes is gradual, to find at the
bottom of the ridge a towin, whose markets are stored only with the productions of the equator, while in a town higher up will be seen only what
belongs to the temperate skies of Europe:-climates of the earth naturally
distant, thus meeting, as it were, in amicable vicinity, on the same rising
plain.
The facts detailed in the preceding paragraphs are intended to illustrate
the subject, of the relation of volume in a body to the capacity for heat.  We
now proceed to speak of density in the same respect.
Second. It might be anticipated that a dense body, or one in which the
constituent particles may be supposed to fill more completely the space occupied by it than the particles do in a rarer body, would have smaller capacity
for heat, in proportion to the smaller space left vacant in its mass: and in a
general comparison of the capacities of equal bulks of different substances,
such anticipation is partly verified,-as when a pint of dense mercury is found
to have only about half the capacity which a pint of lighter water has. The
relation, however, is by no means universal, nor at all in proportion to the
differences of density. Water, which is denser than oil, and according to
the hypothesis should have less capacity, yet has nearly double the capacity;
and mercury, which being nearly fourteen times denser than water, might




EXPANSION OF BODIES.                           281
be expected to have only a fourteenth of the capacity, has really for equal
volumes a half, or, as formerly stated, for equal weights, a thirtieth.
Third. We are at last, therefore compelled to admit that the relation between various substances and heat, which we call capacity for heat, depends
much more on the nature of the ultimate particles of the substances than
either on the absolute bulk or comparative density of the masses. Throwing
much light on this subject, it has been ascertained in late times, that all
material substances are composed of extremely minute unchangeable atoms,
of which, in different substances, the comparative weights have been determined, although not the absolute weights; that is to say, for example, the
atom of gold is known to weigh four times as much as the atom of iron,
although we do not know how many thousands or millions of atoms are
required to form a grain of either. Now very recent researches seem to
prove that for each ultimate atom, no matter of what substance, nearly the
same quantity of heat is required to produce in a mass of the atoms a given
change of temperature. Thus an ounce of iron, which has four times as
many atoms as an ounce of gold, has four times the capacity for heat.  This
law seems to hold for all simple substances; but for compounds there seems
to be another law not yet ascertained.
Instead of the term capacity for heat used in the preceding pages, with
respect to particular substances, that of specific heat has by some authors
been preferred; but as the latter gives to a commencing student the idea
rather of kinds of heat than of quantities, the term capacity has been
retained.
" Each substance in nature, for a given change of temperature, undergoes
expansion in a degree proper to itself, the expansion generally increasing
more rapidly than the temperature, as the cohesion of the particles becomes weaker from increased distance; being remarkably greater, therefore, in liquids than in solids, and in airs than in liquids; the ratebeingq
quickened, moreover, near the points of change."  (See the Analysis,
page 256.)
The following table, containing the names of some common substances,
solid, liquid, and aeriform, shows, by the figures following each name, how
much the substance increases in bulk, by having its temperature raised from
that of freezing to that of boiling water. A lump of glass, for instance,
would gain one cubic inch for every 416 cubic inches contained in it; while
a mass of water would gain one inch for twenty-three, dilating thus for the
same range of temperature eighteen times more than glass.
Solids.
Glass gains one part in    -    -    - 416
Deal      -               -            416
Steel     -    -    -    -    -    - 283
Iron      -    -    -    -    -    - 271
Brass     -    -                       177
Silver    -    -    -    -    -    -  175
Lead      -    -    -    -    -    - 117
Liquids.
Mercury gains one part in  -    -    -   55
Water    -    -    -    -    -    -   23
Fixed oils     -    -    -    -         12
Alcohol   -    -..                   9




282                           HEAT.
Airs.
Common air, 
all gases and  gain one part in -   -  3
vapours    J
We have to warn readers here not to confound the increase by heat of
the general bulk of a solid body with the increase of its length. The latter
is only one-third as great as the former. This will be understood by considering that the increase of bulk is divided between the length, breadth, and
depth (or thickness.  If the substance of a metalic square rod or wire, be
dilated bv heat a one-hundreth part of its bulk, it does not gain all that
hundreth at its end, becoming 101 inches (or other measure) long, instead
of 100; but every part becomes deeper and broader in the same proportion
as it becomes longer (we may suppose it divided into a row of equal little
cubes,) and the rod gains in length only the third part of an inch. A fluid
enclosed in a tube unchangeable by heat (if such tube there were) would
show its whole dilatation in an increase of length, because there could be no
swelling laterally, and its extremity, therefore, from any variation of temperature, would have a triple extent of motion. A degree of this consequence
is obtained in our common thermometers, because the containing glass, although dilatable by heat, is so much less dilatable than the fluid within.
As regards solids, we have to inspire so much more frequently respecting the
dilatation in length, breadth, &c., that is:to say, the linear dilatation, in one
direction, than the increase of general bulk, that tables are frequently made
stating only the linear dilatation. It may be found at once from the above
table, by recollecting that it is one-third of the increase of bulk: —thus, as
glass, in passing from the freezing to boiling heat of water, dilates one part
in 416 of its bulk, it will dilate only one-third of a part in length, or a whole
part in an extent of three times 416 or 1,248.
The expansion of solids by heat has been ascertained by bringing microscope instruments to bear on rods of the different substances heated to various
degrees, in troughs of oil or water. The expansion of fluids, again, is found
by filling a glass vessel with a known weight of any fluid, and then ascertaining how much is made to run over or escape by a given increase of heat;
or how much the fluid rises into a long tubular neck like the stock of a thermuometer. This quantity, added to vhat is required to fill the increased dimensions of the heated glass vessel, (which from the ascertained expansion
of glass is known,) forms the whole of the increase
The general and comparative expansion of solids by heat is exemplified in
the following cases:
An iron bullet, when heated, cannot be made to enter an opening, through
which when cold it passes readily.
A glass stopper sticking in the neck of the bottle, often may be released
by surrounding the neck with a cloth taken out of warm water; or by immersing the bottle in the water up to the neck; or by making strong friction
on the neck by a tape or any soft rope put around it, and then pulled backwards and forwards. By any one of these means the binding ring of the
neck is heated and expanded sooner than the stopper, and so becomes for a
short time slack or loose upon it.
Pipes of cast-iron for conveying hot water, steam, &c., if of considerable
length, must have joinings which allow  a degree of shortening and




EXPANSION OF BODIES.                           283
lengthening, otherwise a change of temperature may destroy them. An incompetent person undertook to warm a large manufactory by steam from one
boiler.  lie laid a rigid main pipe along a passage, with lateral branches
passing through holes into the several apartments; but on his first admitting
steam,"the expansion of the main pipe tore it away from all its branches.
In an iron railing, a gate which during a cold day may be loose and easily
shut or opened, in a warm day may stick, owing to their being greater expansion of it and of the neighbouring railing than of the earth on which they
are placed. Thus also the centre of the arch of an iron bridge is higher in
warm than in cold weather; while on the contrary, in a suspension or chain
bridge, the centre is lowered.
The iron pillars now so commonly used to support the front walls of those
houses of which the ground stories meant to serve as shops have spacious
windows, in warm weather really lift up the wall which rests upon them,
and in cold weather allow it again to sink or subside considerably more than
if the wall were brick from top to bottom.
In some situations (as lately was seen in the beautiful steeple of Bowchurch, in London) where the stones of a building are held together by
clpmps or bars of iron driven into the stones, the expansion in summer of
these clamps will force the stones apart sufficiently for dust or sandy particles to lodge between them: and then, on the return of winter, the stones
not being at liberty to close as before, will cause the ends of the shortened
clamps to be drawn out, and the effect increasing with each revolving year,
the structure will at last be loosened and may fall.
The pitch of a piano-forte or harp is lowered in a warm day or in a warm
room, owing to the expansion of the strings being greater than of the wooden
frame-work; and in cold the reverse will happen. * A harp or piano which
is well tuned in a morning drawing-room cannot be perfectly in tune when
the crowded evening party has heated the room.
Bell-wires too slack in summer, may be of the proper length in winter.
One admirable contrivance for keeping the pendulum of a clock always
of the same length, by making the greater expansion by heat of a middle
bar of brass counteract the smaller expansion of two side-rods of steel, was
explained under the head of " Pendulum," as was also the construction of a
balance-wheel having a corresponding property. A difference of a 100th of
an inch in the length of a common pendulum, causes a clock to err ten
seconds in twenty-four hours, and a rise or fall of 250 of Farenheit's thermometer produces this difference. Another kind of compensation pendulum,
not less admirable, distinguished by the name of its inventor Graham, is
obtained by substituting for the solid bob or ball at the bottom, a glass vessel containing mercury. The mercury on expanding by heat, has its centre
of gravity raised just enough to compensate for the lengthening of the rod
of the pendulum.
Crystals when heated, do not expand quite equally in breadth and in
length. The same is true of fibrous substances, as wood which expands
and contracts more in breadth than in length. This is instanced in the
leaking, during cold weather, of a ship's deck which, in warm weather, is
tight; —an occurrence which the author, in rounding the Cape of Good
Hope, had to regret as the cause of destruction to some valuable specimens
of natural history which he had collected among the Eastern Islands.
Bodies expanded by heat, unless when their intimate composition is
changed by it, regain exactly their former dimensions on being cooled.




284                            HEAT.
As is seen in the preceding table, the expansion of liquids by heat is much
greater than of solids.
A cask quite filled with liquid in the winter, must, in summer, force its
plug or burst; and a vessel which has been filled to the lip with warm Liquid,
will not be full when the liquid has cooled.  Hence a cunning dealer in
liquids has tried to make his chief purchases in very cold weather, and his
chief sales in warm weather.
There exists in the case of water, an extraordinary exception, already mentioned to the law of expansion by heat and contraction by cold, producing
unspeakable benefits in nature.  Water contracts only down to the temperature of 40~, while, from that to 320, which is its freezing point, it again
dilates. One curious consequence of this peculiarity is exhibited when a
pool or well happens to be formed on the upper surface of a mass of ice, as
on one of the glaciers of Switzerland and elsewhere, namely, that the well
goes on quickly deepening itself, until it penetrates to the earth beneath.
Supposing the surface of the water originally to have nearly the temperature
of the melting ice, or 320, but to be afterwards heated by the air and sun,
instead of the water being thereby dilated or rendered specifically lighter,
and detained at the surface, it becomes heavier the more nearly it is heated
to 400, and therefore sinks down to the bottom of the pit or well; but there,
by dissolving some of the ice, and being consequently cooled, it is again
rendered lighter, and rises to be heated as before, again to descend; and this
circulation and digging cease only when the water has bored its way quite
through.
Airs are expanded by heat still more than liquids.
The expansion of aeriform bodies by heat produces many important effects
in nature. Some of these have already been considered in the preceding
parts of this work, as the rising of heated air in-the atmosphere, causing the
winds all over the earth; the same in our fires an-d chimneys supporting
combustion and ventilating and purifying our houses; the same again from
around animal bodies, removing the poisonous or contaminated air which
issues from the lungs, and insuring a constant supply of fresh air for the
support of life, &c.
It is remarkable with respect to aeriform bodies, that, unlike solids and
liquids, they are all equally dilated by the same change of temperature, receiving an increase of about a third part of their bulk (3 7  parts in 100) on
being heated from the freezing to the boiling point of water, viz., 1800,their bulk being therefore doubled from the same standard point by about
500~.  This general truth holds, not only with respect to the more permanent airs or gases, but also with respect to all steams or vapours in the dry
state, that is, when not in contact with the liquid producing them. The
probable reason of this uniformity is, that cohesive attraction which varies
so much in different solids and liquids, modifying the effects of heat upon
them in aeriform fluids does not exist at all.
The extent of this dilatation for airs is so much greater than for liquids
or solids, that it forces itself much more strikingly upon the common attention. Thus, a bladder containing considerably less than its fill of air, becomes tense immediately on being held to the fire. The air in a balloon just
escaping from a cloud, has been so suddenly expanded by the direct rays of
the sun, as to injure the texture of the balloon; and probably some of the
fatal accidents among aeronauts have been owing to this occurrence.  Burning fuel conveyed into a vessel or case which can be suddenly and strongly




EXPANSION OF AIR.                           285
closed, will produce an expansion of the air confined with it, capable of bursting any vessel of ordinary strength —in short, will produce an explosion.
Now, if not before, at any rate soon after steam-engines began to be used,
and had so strikingly shown to what important purposes the force of an
expanding acriform fluid might be applied the thought would naturally
occurr that the force of common air dilating by heat might also be rendered
useful. Accordingly a variety of air-expansion engines have been proposed,
but as yet no one has been reduced to profitable practice. Had the truth been
generally known, which very recent investigations have proved, that a given
quantity of heat, when used to dilate air, produces several times as much
expansive power as when used to form steam, the attempts to bring such an
application of heat under control would probably have been more numerous,
and possibly, by this time, more successful. The subject is so interesting
that we shall subjoin a few remarks upon it.
To produce a cubic foot of common steam from water originally cold,
about 1,160 degrees of heat are required, as will be explained a few pages
hence. The same quantity of heat would double the volume of about five
cubic feet of atmospheric air,-as is known from the comparative capacities
for heat of the two substances, and the rate of dilatation of air when heated.
Now the value for work of the foot of steam passing from the boiler into a
working cylinder would be, to press up the piston of the steam-engine
through a foot, as from c d to a b, with a force all the way of 15 lbs. per
inch of the piston surface; while the working valve of the five feet of air ia
dilating to double bulk, would be to lift up the piston five times as far as
the steam, viz., from g h to e f, but with a force
gradually diminishing (represented here by the         Fig. 125.
shaded part of the figure) as the expansion went            e      f
on, from 15 lbs per inch at the begining until the
air had dilated to its destined volume, when the
force would altogether cease; its whole effect,
therefore, would be five feet impulsion of the piston with a pressure averaging between 15 lbs. and
nothing, viz., 7~ lbs. per inch; and the friction in
the two cases and the varying intensity of the latter pressure being neglected, the force of the air
would be 21 times as great as that of the steam.   az_   a
But it is farther to be considered, that only about 
half the heat of a fire is applied to use in a steam-   cL.     g  /
engine, viz., that part which enters the boiler,
while the remainder passed up the chimney; and
in an air-engine probably the whole might be applied.  In an air-engine,
moreover, there might be a great increase of power from the combustion, or
semi-explosion of the inflammable gas evolved from the fuel. We see from
this of what importance the discovery would be of a means enabling us
effectually to apply the force of expanding air.
If we suppose a fire a to be placed on a grate near the bottom of a close
cylinder, d a, and the cylinder to be full of fresh air recently admitted, and
if we farther suppose the loose piston g d to be pulled upwards, it is evident that all the air in the cylinder above d will be made to'pass by the
tube e through the fire, and will receive an increased elasticity tending to the
expansion or increase of volume, which the fire is capable of giving it. If
there were only the single close vessel d a, the expansion might be so strong
as to burst it; but if another vessel b c of equal size were provided, com



286                             HE A T.
municating with the first through the passage b,
Fig. 126.        and containing a close-fitting piston e f, like that of
a steam-engine, the expansion of the air in the
first cylinder would act to lift the said piston, and
so might work water-pumps, or do any other service which a steam-engine can perform,  At the
end of the lifting stroke of the piston f c, it might
rc.be made to open an escape-valve for the hot air,
placed in any convenient part of the apparatus,................and to cause the descent of the blowing piston d
to expel that air, while a new supply of fresh air
would enter by another valve into the cylinder
above d.  The engine would then be ready to
repeat its stroke as before, and the working
would be continued as in a steam-engire.
The preceding simple conception of an air-engine occurred to the author's
thoughts while considering the application of a condensed air-furnace to some
chemical purposes. It appeared to him that, in applying any such engine
to use, the chief difficulties to be surmounted would be, to prevent the very
heated air and dust from injuring the valves and other working parts of the
engine, and to obviate the inconvenience of the inequality of power at different parts of the stroke. Various expedients occurred to him. The overheating might be prevented by surrounding the cylinder,
Fig. 127      &c., with water; and both cylinder and piston would
suffer less from dust, if, instead of the common piston c.
represented above, a great hollow plunger a were used,
(such as is here represented, and, is now  common in
water-pumps for mines) embraced by an air-tight neck or
collar at b c, which neck would be the only part of the
e    cylinder requiring to be made with nicety.  But a more
complete security would be obtained by interposing water
between the hot air and the piston, as represented in this
other sketch, where the working cylinder d has a w:ter-,
vessel b connected with it, and the heated air is admitted
to b to press upon a float on the water-surface, to lift the
e Z-'?-I. working piston d e.  This construction, too, if desired,
would allow the fire chamber a to be made larger than
the cylinder, and to be kept constantly filled with highly
expansive air, each discharge of which into the spaice b would be replaced by
cold air either from the space
Fig. 128.                 above the piston d, driven in
through a tube as the piston,//).'. C.......]. e;    ascended, or from  a distinct
blowing cylinder worked by! _r lthe beam.  And if it were
wished  to  apply the  same
principle to an engine work-'ffi Em- cb~              ing with double strokes, that
is, forcing the piston alternately up and down, as in the
___t=E  double stroke  steam-engine,
the object might be attained,
by having a second water-vessel f communicating with the




EXPANSION OF AIR.                            287
part of the working cylinder above the piston d; and the air would pass
alternately to the one or the other vessel b or f, by the operation of the cock
c, as steam passes in a steam-engine; the supply of fresh air to the chamber
a would be given by a blowing cylinder worked through a connection with
the engine, as the air-pump of a steam-engine is worked.
The sketch of an air-engine, as here given, was included in the specification of a patent for another object engaged some years ago by a friend of
the author's; but that friend being almost immediately called to other business, and the author's professional engagements forbidding his attention to the
subject, it was not prosecuted.  In the specification, drawn up by an engineer in the town, some minor adaptations were described. One experiment
has lately been made by a Swedish engineer with the simple form of dry
apparatus described at page 285, for the purpose of ascertaining its power,
and the effect was found to be several fold greater than of steam from the
same quantity of fuel; but the apparatus was rude, and only calculated to
prove in a short trial, the existence of the power, but not the fitness of the
machine to endure uninjured, or to be rendered easily obedient to control; a
complete experiment, therefore, remains still to be made. Could an obedient and durable engine be contrived, at all approaching in simplicity to the
plan given above, its advantages over the steam-engine would be very considerable. First, its original cost would be much less, by reason of its small
comparative size, its simplicity, and the little nicety of workmanship required. Secondly, it would require much less room, and would be very
light; hence its peculiar fitness for purposes of propelling ships and wheelcarriages. Thirdly, the quantity of fuel required being
so much less, would not load the ship or carriage leav-  Fig. 129.
ing little room for anything else.  Fourthly, the expense of fuel and repairing would be little. Fifthly,
the engine could be set to work in a few minutes,
where a steam-engine might require hours.  Sixthly,
little or no water would be required for it.
Another modification of air-engine, called a gas va.
cuum enlgine, has lately been proposed, and many
expensive trials have been made of it; but it is in its
nature a most wasteful machine, evidently throwing
away at least nine-tenths of the power which the    b
combustion generates, It was of this nature in an experiment which the author witnessed.  A little of the
common coal-gas was admitted by the cock b at the
bottom of the cylinder a, and was there inflamed, the
lid e being at the time raised.  The combustion rarified the lower stratum of air, so that the air above was  ~
expelled, and about one-fifth of the original contents
of the cylinder was caused to occupy the whole. The
lid was shut down, as nearly as could be judged, at
the moment of greatest expansion, so that when the
small portion of air and vapour remaining within was
again cooled, the interior of the cylinder approached
nearly to the state of vacuum.  It, in fact, retained
only a fifth of the air.  A communication being then        I
opened from the vacuous cylinder by the tube e to a. ---
water reservoir ten feet below, the water was driven
up by the atmospheric pressure, until it filled more




288                           H EAT.
than half of the cylinder. The water so raised was then made to turn a
common water-wheel, and to do work. A much larger quantity of water,
however, could be raised to the same height at less expense by a steamengine. The proposer also hoped that he would be able to make the atmosphere pressing into its imperfect vacuum, act directly upon a piston as steam
does, and with power cheaper than that of steam; but in this anticipation
too he was completely in error. To produce his imperfect vacuum cost him
very nearly at the same rate as it costs to produce the perfect vacuum in
a steam-engine, and his vacuum for equal bulks was worth
Fig. 130.   as a working power, only about one-fourth as much as the
el/          steam. vacuum.  This may be understood by considering,
that in a perfect vacuum a piston rises all the way with the
same force, which if common steam be used, is 15 lbs.
per inch, the piston may be supposed to rise from c d to
a b, but if the vacuum were only three-fourths towards
being complete, the pressure on the piston would be only
d   three-fourths of 15 lbs. at the commencement of the stroke,
and then rapidly diminishing, would have ceased altogether
when the piston had made three-quarters of its journey, or
to f. The force in the first case would be represented by
the whole line c d and the whole space c d b a, and in the second by the
shortening lines and the triangular space c ef.
On considering the foregoing diagrams, we may perceive that in the vacuum
engine, by far the greater part of the force produced by the combustion of the
gas is absolutely wasted, or put to no use, namely, the whole expansive force
during the sudden combustion or explosion. It is evident that if a tenth
part of the aeriform contents of a cylinder acquire elasticity enough, (a fourteenth part of a nice experiment does so) to be able afterwards to occupy the
whole cylinder, that tenth must begin its expansion with the force of a tenfold atmospheric condensation, or pressure of 150 lbs. on the square inch of
a piston withstanding it, which pressure will then gradually diminish as the
piston rises, but will amount to an average of five times the atmospheric
pressure, or 75 lbs. per inch all the way; being therefore quadruple, or more,
that of steam against a perfect vacuum; and, therefore, again, by our former
calculation, more than twelve times greater than the force obtained from the
imperfect vacuum of the engine under consideration.
It is a question which the author thinks will one day be answered in the
affirmative, whether nearly the whole force of exploding gas may not be converted into a calmly working power, producing from a
Fig. 131.    given expenditure, ten times or more the effect obtained
in the vacuum-engine described above, and, therefore, an
effect more than equal to that of a steam-engine incur-I.... --- ring the same expense. There are probably various ways
I~-'"] —-  in which the object may be obtained. The following
b1         sketch is offered merely to give the reader an idea of a
machine for such a purpose.
Suppose b to be a very heavy close-fitting piston sliding
in the cylinder containing it; and suppose the space d
open to the cylinder, to be filled with atmospheric air of
double or greater density; then if a mixture of explosive
gases admitted by a cock to the chamber a (formed be- le      tween the piston and end of the cylinder) be inflamed,
the heavy piston will be shot forward like a cannon-ball,




EXPANSION OF AIR.                            289
against the condensed air in d; and owing to the momentum acquired in the
first instance, it will advance much beyond the point where the exploded gas
and air in d would balance each other at rest. The quantity of gases admitted
would be just such as to carry it to the end of the cylinder. The piston rod
e would then by a catch or racket, be connected with the work to be done,
and after the condensation of the exploded gases in a cylinder, would be
pressed back again, with the greater than atmospheric force in d, as if urged
by high pressure steam. Figure 127 at page 286 represents a form of cylinder which might also answer for this purpose, the heavy plunger being
thrown up, to work by its weight in descending.
It is to be remarked that the first modification of air-engine described at
page 285, is partly an explosive engine such as contemplated above, for the
gas separated from the coal during the moment of slackened combustion while
the lately used air is passing out, becomes an explosive accumulation for the
fresh air about to enter, The trial alluded to above proved this to be the fact.
The expansion of bodies by heat increases more rapidlf/ than the temperature, and particularly near the melting and boiling points, that is, their
points of changing into liquid or air being, however, exactly proportioned
to the temperature after the change into air.  (See Analysis, page 256.)
If a given quantity of heat, that, for instance, contained in some measure of
boiling water or of common steam, be added to a mass of cool water, it will
produce in this a certain increment of bulk; and if other equal quantities of
heat be afterwards successively added under the nice management which
such an experiment requires, each new addition will produce a greater increment of bulk than the preceding, particularly when the water approaches to
boiling; but after the water is converted into steam, any farther increase of
bulk will be exactly proportioned to the increase of temperature. The same
truths may be proved by the converse experiment of abstracting successively
equal quantities of heat from steam or water (as by making it melt equal
quantities of ice,) and noting the rate of contraction. What is thus true of
water in relation to heat, is true also of bodies generally, each, however,
having a rate of expansion and temperature for melting and boiling proper
to itself. The quickened rate of expansion in solids and liquids might have
been anticipated from reflecting, that each successive quantity of heat added
to a mass, meets with less resistance to its expanding power than the preceding quantity, owing to the diminishing force of cohesion of the particles
as the mass enlarges; while in an air or gas, again, as cohesion has altogether ceased, each addition of heat is at liberty to produce its fulland equal
effect. —If the capacity of substances for heat did not increase with their
bulk, the terms "increase of heat" and "increase of temperature" would
have the same meaning, and the subject would be more simple.
The reflection will naturally occur here, that as in the common thermometer, the mercury must rise or expand more for a given quantity of heat
added at a high than at a low temperature, the scale should be divided to
correspond with the inequality. Now this reasoning is good, but the difficulty of complying with it in practice is such, that the inconvenience of the
slight error arising from an equal division is commonly submitted to. An
air thermometer with equal divisions is very correct, but from wanting many
of the advantages of the mercurial thermometer, is little employed; and
fortunately in the mecurial thermometer there is such a counterbalancing
relation between the expansion of the mercury and of the containing glass,
19




290                            HEAT.
as to render the error alluded to, at least for any middle range of temperature,
very trifling.  The subject of unequal thermometric dilatation in the same
liquid, and of the differences in that respect in different liquids, depending
on the proximity to their boiling points, &c., is well illustrated by Du Luc's
experiment of filling various thermometer-glasses with different liquids, and
while they are being heated through the same range of temperature, noting
their comparative indications.  He marked on each tube the points at which
the liquid in it stood when the bulb was placed, first in freezing and afterwards in boiling water, and he then divided the intervening space into eighty
-parts.or degrees.  The discordance of the dilatations in the different tubes
when the instruments were afterwards placed together, and heated from the
freezing to the boiling degrees of water, was as here detailed.
Mercury.         Olive Oil.         Alcohol.            Water.
0    -    -      0      -         -    0   -   -       0
10    -    -      9.5   -    -    7.9       -    -      0.2
20     -    -    19.3    -    -   16.5      -    -      4.1
30    -    -    29.3    -    -   25.6       -    -    11.2
40    -    -    39.2    -    -   35.1       -    -    20.5
50    -    -    49.2    -    -   45.3       -    -    32
60    -    -    59.3   -    -  56.2         -    -    45.8
0     -    -    69.4   -          678       - -        62
80    -    -    80          -     80        -    -    80
The singular discrepancy in the case of water is owing to the peculiarity
described in former pages, of its contracting by cold only down to 400 of
Fahrenheit, and then dilating again until it freezes.
Laborious investigations have been made by the French.chemists to discover a comprehensive law determining the rate of expansion in all bodies,
but the object is not yet satisfactorily accomplished.
" To melt a solid body, or to vaporize a liquid, a large quantity of heat
enters it, but irn the new arrangement of the particles and generally increased volume of the mass, the heat becomes hidden from the thermometer
and is called LATENT HEAT.  It reappears during the contrary changes,
after whatever interval."  (See the Analysis, page 256.)
The expansion of bodies by heat, instead of proceeding throughout in
some nearly uniform or gradual manner, exhibits in its course two singular
transformations of the body: the first, when the solid breaks down into a
liquid; the second, when the liquid swells out into an air or gas; so that
there are, in all, three very distinct modifications or states of existence for
the body dependent on the agency of heat. The substance of water, for
instance when at a low temperature, exists in the solid form called ice; but
at 32~ of Fahrenheit, on receiving more heat it gradually becomes liquid or
water, and on receiving more at 2150, even under the resisting pressure of
the atmosphere, it acquires a bulk nearly 2,000 times greater than it had as
a liquid, (gradually as regards the whole, but suddenly as regards each separate portion,) being then called steam or aeriform water. And other bodies
under analogous circumstances, undergo similar changes. It is farther
remarkable, that although during the changes a large quantity of heat enters
the mass, producing in one case liquidity, in the other the form of air, the
temperature is the very same, immediately after, as immediately before the




LATENT HEAT.                            291
change, the last received heat becoming hidden or latent in the mass: —thus
water running from melting ice affects the thermometer but as the ice does,
and steam over boiling water appears no hotter than the water. The glory of
originally discovering the facts, to recall which the terms latent heat
or caloric of fluidity, have since been used, belongs to the illustrious Dr.
Black. The construction of the modern steam-engine was an early result of
kindred investigations made by his friend, James Watt.
We select the following instances as serving to display the subject of latent
heat in its various bearings.
A mass of ice brought into a warm room, and there receiving heat from
every object around it, will soon reach the temperature of melting or 320
but afterwards both the ice and the water formed from it-will continue at
that temperature until all be melted;-the heat which continues to enter,
effecting a change only in the form of the mass. And in the case supposed,
whatever time was required for heating the mass of ice one degree, just one
hundred and forty times as much will be required for melting it; proving
that 1400 is the latent heat of water.
If two similar flasks, one filled with ice at 320, and the other with water
at 32~ be placed in the same oven or over the same flame, the water will
gain 140 degrees of heat while the ice is nearly being melted into water at
32: and in the course of the experiment, a correspondence will always exist
between the phenomena; for instance, when the water has gained 140 of
heat, it will be found that just a tenth part of the ice is mated.
If equal quantities of hot and cold water be mixed together, the whole
acquires a middle temperature, each degree lost by the hot water becoming
a degree gained by the cold; but if a pound of ice at 320, and a pound of
water 1400 hotter be mixed together, the 1400 of heat will go merely to melt
the ice, for there will result two pounds of water at 32~.
If a flask of water at 320, or its freezing point, and a-similar flask of strong
brine (which does not freeze until cooled to near zero) also at 32~, be
exposed together in the same cold place, it will be found that when the brine
has lost 100 of its heat the water flask will still exhibit an undiminished
temperature, but a fourteenth part of its contents will be converted into ice.
Now as in such a case the water flask must continue to radiate away heat just
as much as the other, it can maintain its temperature by absorbing into its
general mass the heat which was latent in the portion of water frozen.
It is possible, by slowly cooling water which is kept in perfect repose, to
lower its temperature, while yet liquid, ten degrees below its ordinary freezing point; but then on the slightest agitation, ice will be formed. It might
be expected, in such a case, that the whole water will instantly freeze,
because all is colder than common ice; but no, only a fourteenth part freezes;
and singularly, both that fourteenth and the remaining liquid are rendered
in the moment ten degrees warmer-rising to 32~. Here the 140~ of latent
heat escaping from the fourteenth part of the water which freezes, become
100 of sensible heat for the whole mass, so that the remaining water has the
temperature at which it only begins to freeze.
Strong solutions in hot water of various neutral salts, if allowed to cool
while exposed to atmospheric pressure, soon deposit crystals of the salts; but
in a close vessel, which protects them from such pressure, they will remain
liquid even when cold. Now at the moment of opening such a vessel to
admit the pressure, the salt immediately crystallizes, and the latent heat




292                            HE A T.
given out by the solidifying particles warms very sensibly the remaining
liquid and the vessel.
From the preceding facts it may be perceived, that the quantity of ice
formed or melted in any case, becomes a correct measure of the quantity of
heat transferred.  From this consideration, the illustrious Lavoisier constructed his calorimeter, or heat measure.  It is a case or vessel lined with
ice, and the quantity of heat given out by any body placed in it is indicated
by the quantity of water collected from the melted ice.
Had the latent heat of water been only 1~ or 20 instead of 1400, the earth,
except in its tropical regions, would have been scarcely habitable. The cold
of a single night might have frozen an ocean, and the heat of a single day
might have converted the accumulated snows of a winter into one sudden
and frightful inundation. As the fact is, however, both changes are beautifully gradual, and easily controlled or prepared for.
The fact'of latent heat in other liquids than water is familiarly exhibited
in the slow melting of various substances as-of the metals; lead or pig-iron
for instance-of butter or oils-of glass, &c.; and on the other hand, in the
slow solidification of any melted masses when heat is again abstracted.
The substances below enumerated, while passing from  the solid to the
liquid state, absorb and render latent the quantities of heat -here noted;
which quantities are therefore called the latent heats of the liquids.
Ice       -      -       -      -    1400
Mercury      -       -       -       142
BTes' wax        -         -     -    170
Tin    -    -        -       -       442
Zinc     -       -       -      -    492
If a piece of frozen mercury (the temperature of which is at least 40~
below zero) be thrown into a little water at 32~, the latent heat of the water
immediately passes into the mercury and melts it; but, singularly, the water
in the act of melting the mercury, is itself frozen.
" Latent Heat of aeriform fluids."
Water in a vessel placed over a fire gradually attains the boiling temperature or 212~, but afterwards its temperature rises no more, for the farther
addition of heat becomes latent in the steam escaping during the ebulition.
One way of determining the quantity of heat which becomes latent in steam
is to note how much more time is required for boiling a quantity of water to
dryness, than for merely heating it to the boiling point, or through any certain number of degrees.  The experiment indicates about 1,0000; that is to
say, 1,000 times as much heat is latent in any quantity of water formed
into steam, as would raise the temperatnre of the liquid water one degree.
Watt had found that water in a vessel placed over a lamp was about six times
as long in being completely evaporated, as in being originally heated from
an ordinary temperature to that of boiling.
If we place in the same oven, or over similar flames, two like vessels containing water, one of which is open at the end and the other is strongly closed,
the two will gain heat equally up to the boiling point, but afterwards the
open vessel from giving out steam will remain at the same temperature, while
the other, by confining the heat which enters, will show the temperature
continuing to rise as before, until the increasing tendency of the water to




LATENT HEAT.                            293
dilate forces the vessel open. Supposing the water in the latter Vessel, before vent is given, to have become 1000 hotter than common boiling water,
instead of the whole, when at liberty, being immediately converted into
steam, as might be expected, only a tenth part will be so changed (the same
quantity as will be found to have already escaped from the other vessel,) for
the tenth part requiring in the form of steam 1000~ of latent heat, will take
the excess of 1000 from the other nine parts, and will leave them as common
boiling water. If, however, water heated considerably beyond the boiling
point be allowed to expand very suddenly, the whole is blown out of the
vessel as a mist, by the steam formed at the same instant through every part
of the mass; but the whole mass in such a case is no more converted into
true steam than the whole of very brisk soda-water is converted into gas
when similarly thrown out by the sudden extrication of the carbonic acid
gas, on uncorking the bottle. Misconception of this matter has led to most
wasteful experiments on steam-engines of very high pressure. Mr. Perkins,
for instance, thought he truly described what was accomplished by saying of
the water that it had " flashed into steam."
The same indication of the latent heat of steam is obtained by the converse
experiment of first converting a quantity of water into steamy and then admitting it to cold water or to ice. A pound of steam will raise the temperature
of ten pounds of cold water 100 degrees, or will melt about 8~ pounds of ice.
In the great quantity of heat which becomes latent in steam, we perceive
the reason why water projected upon a raging fire so powerfully represses it,
and hence, again, why fire and water are so often adduced proverbially as
exemplifying a fierce antagonism.
It was when Watt had discovered how much heat was lost when steam
was lost, that he contrived the separate condenser for his steam-engine, by
which he at once saved three-fourths of the fuel formerly used
Substances differ among themselves in regard to the latent heat of their
vapours as much as in their other relations to heat. Thus the latent heat
of the vapor or steam ofWater.....              is 1,0000
Vinegar......  900
Alcohol.......    442
Ether......         300
Oil of turpentine.....    177
From the less latent heat in these last-mentioned vapours than in that of
water, we might at first suppose that there would be great advantage from
using them in steam-engines. Accordingly numerous experiments have been
made, and patents secured under this idea; but the fact is, that in the same
proportion as the heat is less, the volume of the vapour is less, and therefore
-no mechanical advantage is obtainable.
The influence of external pressure in keeping the particles of liquids together,
in opposition to the repulsion of heat seeking to render their mass aeriform,
was considered in the chapter on " Pneumatics;" but to make the present
section complete, the subject must be shortly resumed.
Because water or any liquid, under the pressure of the atmosphere, while
receiving heat, remains tranquil, and apparently unchanged, until it reaches
what is called its boiling point, at which a bubbling or conversion into vapour
takes place, we might suppose its ordinary boiling temperature necessary to




291                            HEAT.
enable it under any circumstances, to assume or to maintain the form of air.
But this is no more true than that a common spring compressed against any
obstacle or force, has no tendency to expand or recover itself till the moment
when at last it overcomes the obstacle.  Liquid water with its heat is really
a spring compressed by the powerful weight of the atmosphere, and seeking
to expand itself into steam with force proportionate to its temperature. Even
at 320, or its freezing point, as is found by placing it in a vacuum, it seeks
to assume the form of air, with a force of pressure 1  ounce on each square
inch of its surface, and can be restrained only by a counter-pressure of that
amount; and at any higher temperature, to correspond with the greater
dilating tendency, the restraining force must also be greater: at 1000, for
instance, it must be 13 ounces; at 1500, 14 lbs.; at 212~, 15 lbs.; at 2500, 30
lbs., and so on as stated in the preceding part of this work:-and whenever
the restraining force is much weaker than the expansive tendency the formation of steam takes place rapidly and far below the surface of the liquid, so
as to produce the bubbling and agitation called boiling. Now it is because
the atmosphere or ocean of air which surrounds the earth happens to have in
it 15 lbs. weight of air over every square inch of the earth's surface, and
presses on all things thare accordingly, that 2120 happens to be called the
boiling point of water. An atmosphere less heavy
Fig. 132.       would have allowed liquid to burst into vapour at
lower temperatures, and one more heavy would
b    have had a contrary effect.-The exact degree of
expansive force for every degree of temperature in
water and other liquids, has been ascertained by
heating them in vessels furnished either with properly loaded valves, as at f in this figure, or with a
tall upright tube, as d b, into which the liquid c may
force a column of mercury to an elevation marking
the expansive tendency; the valve and mercury
being, of course, protected from the external atmospheric pressure, or the necessary allowance being.
made for that pressure. Boiling at the bottom of a
_'          deep vessel is resisted by the weight of the liquid
C)    H i!- in addition to that of the atmosphere, as already explained, and consequently the temperature at which
_     it occurs there, is higher than near the surface of the
vessel.  Boiling heat is greater, also,-in a deep
mine, where of course there is additional depth and
weight of atmosphere over any exposed liquid,at times when the barometer is unusually high, that is to say, when the
atmosphere is unusually heavy-in cases where air or steam is confined over
the boiling surface so as to press more upon it, as when brewers for a time
shut the lid or valve of their great boilers, &c. Water placed on the fire in
a strong vessel, from which steam cannot at all escape, may be rendered even
red-hot, without a bubble forming or one particle being dissipated; but the
tendency.to expand into steam is then great enough to burst any known
material of moderate thickness. The Marquis of Worcester exploded a
cannon by shutting up water in it and then surrounding it with fire.-Boiling
temperature is lower again when the experiment is made on mountains or in
other situations above the level of the sea, where there is less height of air
resting over the boiler. In the city of Mexico, which is 7,000 feet above the
sea, water boils before it reaches the heat of 200~, instead of, as in places




BOILING.                               295
near the sea-level, at 2120.  Wollaston's thermometer, beautifully adapted
for determining the height of mountains, balloon ascents, &c., by merely
indicating the heat of boiling water in any situation, is a fine illustration of
this truth. If in any place we take off the atmospheric pressure from a liquid,
as by placing it in the receiver of an air-pump, it will boil at very low temperatures indeed.  W~ater thus treated boils at a temperature many degrees
below the heat of English summer days; and ether boils when colder than
commnon ice.-Generally, in a vacuum, substances boil at a temperature
124~ lower than while restrained by the atmospheric pressure.
Consequences of these truths respecting the boiling temperature are the
following.
As water at any temperature is tending to dilate itself into steam, with
force proportioned to the temperature, the steam rising from any mass of
water presses on the surface of the vessel containing it with that force; and
in a steam-engine, therefore, the temperature of the water in the boiler tells
the degree of force with which the steam is acting on the piston.
Because in the case of steam  the same law holds as for aeriform fluids
generally, viz., that the outward elasticity or spring increases in proportion
as the fluid is more condensed-high pressure steam is merely condensed
steam, just as high-pressure air is condensed air; and to obtain a double
or triple pressure, we must have twice or thrice the quantity of steam under
the same volume.
The reason that high pressure steam issuing from a boiler heated to 3000
or more is not hotter than low-pressure steam from a boiler at 212~, is, that
in the instant when the high-pressure or condensed steam escapes into the
air, it expands until balanced by the pressure of the atmosphere, that is, until
it becomes low-pressure steam, and it is cooled by the expansion, as air is
cooled on escaping from any condensation.
The vessel called Papin's Digester, is merely a metallic pot or boiler,
which can be kept closed in spite of the force of the steam formed within
it; and in such a vessel, water can be heated far beyond the ordinary boiling
point,-sufficiently for instance, to dissolve and extract all the gluten or
jelly of bones, and to form from them a rich soup where common boiling
would procure nothing;-or even to melt lead lying in water.
The person who increases the fire under a boiling pot with the hope of
making the water hotter, is foolishly wasting the fuel, for the water can only
boil, and it does boil at 2120 of the thermometer.
As different substances under any given pressure, become aeriform at different temperatures, mixtures of such may be decomposed by heat.  If a
mixture of spirit and water, for instance, be placed over a fire, the spirit will
boil off long before the water.  If the spirituous vapour be caught apart and
condensed, the operation is called distillation.  All distillations are of the
same nature.
The instrument here represented consists of a glass tube blown into bulbs
at the two ends a and b, and hermetically sealed
after receiving into it some water, but no air..    Fig.133.
There will always be in the apparently empty
part a stream  or aeriform water of density pro-  
portioned to the temperature. If one of the bulbs
be heated more than the other, the steam or va-                     -
pour in that one will, for the reasons stated above
be denser and stronger than in the other, and will




296                            HEAT.
therefore be forcing its way into the other; where, owing to the lower temperature, a part of it will be relapsing into the state of water, making room
for more. Hence, if the difference of temperature between the bulbs be long
maintained, the whole water will, by a sort of distillation, gradually pass
into the colder bulb.  If the difference of temperature become at any time
considerable, the liquid will boil in the warmer bulb, even although the
source of heat be only the living hand grasping it.
To the author of this work it appears that by a larger apparatus made on
this principle, fresh water might be conveniently obtained from salt witer on
board ship, or on islands deficient in fresh springs.  Suppose any two
air-tight vessels like a and b, of large size, communicating by a tube furnished
with a stop-cock near b: then if the vessel a were filled with salt water, and
were heated by being exposed to the sun, (its surface being blackened to absorb heat, and protected by glass from the cooling effect of the air,) and if
the other vessel b were made a vacuum by pumping out from its bottom the
water with which it had been previously filled, and were then kept as cold
as possible by wetted coverings and a current of air,-on opening the cock
at b, vapour would pass ov~er from the warmer vessel to be condensed in the
colder, and there would be a distillation from sea-water by the natural action
of the sun alone, of a water naturally fresh and pure.  Cases have occurred
where a knowledge of this fact would have saved shipwrecked crews from
perishing by thirst; and there are rocky islands in the ocean where there is
no supply of fresh water but from precarious rains or importation from abroad,
but which might be rendered pleasantly habitable by the adoption of such a
means.
When a substance has reached the temperature at which it boils, that is to
say, at which its power of emitting vapour becomes rather more than a balance
to the atmospheric pressure, its dilating force is strong indeed.  Persons may
not reflect that 15 lbs. on a square inch is about a ton on a square foot.and such is the power with which the vapour of all boiling substances rises
from them-sufficient in a single Cornish steam-engine to urge the piston
with the force of 600 horses! But even at temperature much below boiling, the tendency to expand, as already stated, is still very great, and although
not attracting common attention, is silently working many beautiful and important ends in the economy of nature.-As in a perfect vacuum, freezing
water gives out a steam or vapour that would lift an opposing weight with
force of 1I ounce per inch, or 16 lbs. on a square foot: and even solid ice
gives out its vapour of nearly equal strength,-so also do many other liquids
and solids give out their vapours. Thus in the apparently empty space called
the Terricellian vacuum, over the mercury in a barometer tube, there is
always an aeriform mercury, dense in proportion to the temperature; and
around camphor, and the essential or volatile oils, &c., there is similarly an
atmosphere of the substance in the form of air.
It had for a considerable time been known that into a perfect vacuum
many bodies emitted almost instantly in the form of air, a quantity of their
substances proportioned to their temperature; but it was reserved for Mr.
Dalton to make the admirable discovery, that even into any space filled with
air, these vapours arise in quantity and density the same as if air were not present-the two fluids seeming to be independent of each other, with the exception that in a vacuum the equal diffusion of vapour takes place at once,
while in a situation already occupied by air, it proceeds more slowly as the
vapour can forceeits way through the particles of the air, and in general takes
place by a tranquil evaporation from the surf.ce instead of the agitation of




EVAPORATION.                             297
ebullition. In an apartment with an open vessel of water in it, there is soon,
although invisible, a steam of watery vapour mingled with the air, as dense
as if the room were a vacuum at the same temperature.
Consequences of this important truth are the following:
That it is only an atmosphere of the substance of each body, which, by
pressing on the body, can prevent its farther dissipation by heat. Thus we
can save camphor, musk, smelling oils, spirits, water, &c., only by placing
them in closed bottles or vessels, in which, additionally to the air present, an
atmosphere of their own substance is formed, involving the remaining masses
with pressure proportioned to their temperature and its density.
The important process of drying things is merely the placing them under
an elevated temperature if attainable, and in an atmosphere not containing so
much of the liquid as to be saturated at the temperature. The effect of
wind or motion of the air in quickening evaporation, is owing to its removing
air saturated with the moisture, and substituting air which is not-thus
producing nearly the case of the substance placed in a vacuum.
If air at a certain temperature, contain mixed with it as much water as can
be sustained in the form of invisible vapour at that temperature, and if then,
by any cause, as by rising in the atmosphere, the air be cooled, it will abstract
heat from the vapour, and cause a portion to be precipitated or visibly condensed into a fog or rain.  Water rising as invisible vapour from. the surface
of a lake or river, often, when it has reached a certain height, is condensed
into the stratum of clouds which there appears, and which for a time may
remain usefully protecting the fields from the intense meridian sun, or may
fall again as refreshing showers over the country.
It is the tranquil and invisible evaporation of which we are now speaking,
which lifts from the surface of the wide ocean all the water which, after
condensation, returns to the ocean in the form of the myriads of river streams
which give life and beauty to the face of nature.
In warm climates there are inlets of the sea, occasionally shut off from the
parent ocean, and where, after the sun's rays have drunk up all the water,
the deposited salt remains to be carried away in loads for the uses of man, as
sand is carried away from any ordinary shore. There are in the bowels of the
earth prodigious accumulations of salt, some of which may have been formed
in this way, during the revolutions of the world in remote past time, and
which are now turned to man's account as salt-mines. When the Nile overflows its banks with water holding in solution, although in almost imperceptible proportion, mineral substances brought from the interior of Africa,
some of that water admitted into reservoirs, and afterwards dried up by the
sun's heat, leaves a rich store chiefly of chrystallized natron or soda.
The following are other instances of vapour which is invisible while at a
higher temperature, but is thickly precipitated when air, with which it is
mixed, is cooled, or when it touches a colder solid body:-the steam observed
at night and. morning hovering over brooks and marshes heated by the sun
during the day:-the frost-smoke, as it is called, which lies on the whole face
of the greenland seas in the beginning of winter, where the water, warmed
by the long day of the polar summer, continues to emit its vapour for a considerable time after summer is past, into an atmosphere, become too cold to
preserve it invisible:-the breath or perspiration of animals, of horses in
particular, after strong exertion, becoming so strikingly visible in cold and
damp weather, or even in warm weather, when the air is already charged with




298                             HEAT.
moisture:-in cities where there are deep drains communicating with
kitchens, manufactories, &c., and constantly filled with moist warm air, the
vapour-loaded air, although clear or transparent in the drain, immediately
on escaping into a frosty atmosphere lets go its moisture, with the appearance
of steam issuing from a great subterranean caldron.  Steam  over water in
any boiler, is transparent or perfectly aeriform —as may be seen when water
is made to boil in a vessel of glass, but as soon as it is cooled by contact or
admixture of colder air, it ceases to be true steam, and is condensed into
small particles of water suspended in the air. Many persons while thinking
of steam, figure it only in this latter state, as particles of water mixed with
air nearly as a subtile powder might be mixed, and its substance occupying
really no more space than the original water did.  Now until steam is cool
and condensed, it is of a nature to fill alone any appropriate vessel and
powerfully distend it, just as air fills and distends a bladder. Steam issuing
from the spout of a kettle is hardly seen near the mouth, but as its distance
from the spout increases, it is cooled into a thick cloud or vapour.
In a vessel from which air and atmospheric pressure are excluded, even
the temperature of freezing air being sufficient to maintain permanently in
the state of gas or air, many substances which exist as liquid under the
atmospheric pressure-and the whole mass of such a substance, when placed
in a vacuum, not being instantly converted into gas, because the portion
which first rises becomes an atmosphere weighing upon the remaining mass,
and because, moreover, that portion, by absorbing from the mass much heat
into the latent state, cools the mass much below the freezing point; we see
why the liquids now spoken of are so rapidly cooled to at least the freezing
point if placed where a vacuum can be maintained, that is to say, where,
after common air has been removed, the aeriform matter arising from them,
and absorbing their heat, is also promptly, and in a continued manner,
-abstracted.  It is thus that water placed in an exhausted receiver of an airpump is so rapidly cooled, and that when there is beside it a vessel of concentrated sulphuric acid, or other substance capable of absorbing the watery
vapour as formed, it is soon reduced to the state of ice'; or again, that
water, or even mercury, surrounded by ether evaporating in a vacuum, is
so quickly frozen.  It is thus, also, that if one bulb of the instrument
described at page 296, be immersed in a freezing mixture, the water in the
other and distant-bulb will soon become ice; for the vapor arising from
that water in the vacuum maintained throughout the apparatus by the
freezing mixture, is immediately condensed again in the immersed bulb,
and leaves the vacuum still free for the ascent of more vapour, to carry
away more heat from the water as latent heat, and to make it freeze.
As we have explained,'also, that in a liquid there is the same tendency
to evaporation, whether it be or be not exposed to the air, we see the reason
why all evaporation is a very cooling process.  The effect, however, in air
is neither so rapid nor so great as in a vacuum: first, because the presence
of the air impedes the spreading of the newly-fo'rmed vapour from the liquid
surface, and keeps it where its pressure resists the formation of more vapour;
and, secondly, because the air in contact with the liquid, shares its higher
temperature with the liquid.  Still, in India, fiat dishes of water, placed
during the night on beds of twigs and straw kept wet and in a current of
air, soon exhibit thin cakes of ice-and thus, ice is procured in India for
purposes of luxury.
The absorption of latent heat i-n the evaporation which goes on from the
sea and earth in all warm climates, greatly tempers the heat of these climates,




EVAPORATION.                             299
and the vapour afterwards spreading to-the poles, as explained in "Pneumatics," under the head of Winds, carries warmth thither to be given out
when it is recondensed into the form of rain, or is solidified as snow. The
formation any where of mist or rain warms the air most sensibly, by the
liberation of the latent heat from the precipitated vapour. Again, the liquid
water, which, during winter, is converted into snow or ice, had been a reservoir of latent heat stored to temper the frosty air of the commencing cold
season; and in the following spring, such ice and snow serve as empty receptacles, in which the first violence of the returning sun hides or expends
itself; allowing the temperature to change more gradually, and for many
living beings, therefore, more safely. The vast stores of ice and snow
among high mountains, as among the Alps and Pyrenees, are often, during
the summer, stores of mild temperature to regions around: for besides cooling the air near them, they are the never-failing sources of the rivers which
run from them during the whole of summer, carrying freshness through distant lands:-from the Alps, for instance, proceed the Rhine and the Rhone
-most romantic and beautiful of European streams; and from the Pyrenees,
the rapid Gave, &c., which, while channels around from lower regions are
almost dried up by the summer heat, flows only the more freshly as the heat
is greater, and the feeding snows are more abundantly dissolved.
MIen in artificially raising temperature, are generally causing the liberation
of heat which had been previously latent; and in lowering temperature
or producing cold, they affect their purpose almost solely by rendering a
quantity of heat latent.
Lavoisier thought that the heat of all combustion was merely the latent
heat of the oxygen gas concerned in the combustion, given out during its
combination with the burning body. It is so in part, but we now know that
it depends more on the intensity of the chemical action between the combiaing substances.  The water thrown upon quick-lime to slake it, becomes
solid in combination with the lime, and gives out its latent heat so remarkably as often to set fire to a wooden vessel or ship containing it.
When dwelling-houses, green-houses, manufactories, &c., are warmed, as
is now common, by the admission of steam into systems of pipes which
branch over them, the heat is chiefly that previously latent in the steam,
and which spreads around as soon as the steam, by touching pipes.of a lower
temperature, is condensed to a state of water. The modes of most profitably
effecting these purposes have to be considered in a future chapter.
For producing artificial cold, our processes generally involve the circumstance either of a solid changing into a liquid, during which't absorbs, and
hides in its new constitution much of the heat previously sensible in it and
in the liquid dissolving it; or of a liquid changing into vapour, during which
heat equally becomes latent. Thus by dissolving a salt, nitre, for instance,
in water, we obtain a solution which is very cold.
In India, the common mode of cooling wine for table is to surround the
bottles with nitre thus melting; and the water of the solution being evaporated again before next day, the salt is left ready for use as before. Such is
the mutual attraction of water and many salts, that they readily combine,
assuming the liquid form, even when water is used in the solid state of
ice; and as, in that case, both the water and the salt render heat latent, the
fall of temperature is very great. Thus common salt and snow (or ice)
when mixed, dissolve into liquid brine 370 colder than freezing water, or 50
below the zero of Fahrenheit.-The last mentioned fact explains the com



300                           n1 EAT.
mon practice of sprinkling salt on an ice-covered pavement before a street
door to clear away the ice. The salt and ice quickly combine and form
liquid brine, which either of itself runs off into the gutter and disappears, or
is easily swept of, or its water evaporates, leaving only the salt behind. It
is true that the brine is at first a refrigerating mixture, which cools still more
the pavement and the neighboring ice, but all which is touched by the salt
is melted. Servants usually err in using a pickaxe or spade immediately
after the sprinkling, instead of waiting, and with a broom spreading the
melting salt compleatly over the place.
Frigoriffic Mixtures.
Substances mixed.                 Thermometer sinks.
Common salt..           1 part) From  any temperature to 50 below
Snow or pounded ice.    2 -              zero.
Common salt...    5 
Snow or ice. 1                  From any temperature to 250 below
Nitrat6 of ammonia.     5               zero.
Snow.......3 -    From 32~ above to 23~ below zero.
Diluted sulphuric acid.2 -     F
Fused Potass              4 -    From 32 above to 510 below zero.
Snow.    3    ov
Nitrate of ammonia.1        )
Water.                           I -- _ From 50~ to 4~ above zero.
Sulphate of soda          8 -    From 500 to 00 or zero.
Mluriatic acid..       5 -_
We have already described under other heads the frigorific effect of
evaporating in a vacuum or in the air, and of the operation of condensing
a gas to squeeze the heat out of it before letting it expand again to a great
volume.
For any given substance, the changes of state from solid to liquid, and firom
liquid. to air, happen under similar circumstances, so precisely at the
same temperature, that they mark fixed points in a general scale of temperature, and enable us to regulate and compare our various thermometers. (See Analysis, page 256.)
As we can neither weigh heat, nor measure its bulk, nor see it, and as,
even if our sense of touch were a correct judge in the matter, which it is
not, we dare not touch things which are very hot or cold, some other means
were wanted for estimating the presence in bodies of this very subtile principle;-and a mean has been found in the measurement of its most obvious
and constant effect, namely that dilatation or expansion of bodies, which
again ceases when the heat is withdrawn.  Any substance so circumstanced
as to allow this expansion to be accurately measured, becomes to us a thermometer or measure of heat.
In solid substances the direct expansion by heat is so small as to be seen
or measured with diffculty. In airs, again, the expansion is very extensive;
but there is the objection that in any apparatus yet contrived which will
allow their expansion completely to appear, they cannot be protected from the
varying pressure of the atmosphere —an influence which affects their volume
even more than common changes of temperature. But liquids are free from
both disadvantages, and when placed in a glass bulb, as a, having a long




TABLE OF TEMPERATURE.                          301
neck or stalk a b, into which the liquid may rise when expanded by heat, to
be measured, they form the most generally convenient
of thermometers.  Then, among liquids, mercury is,     Fig. 134.
on several accounts, singularly pre-emijuent.  In mercury, the range of temperature between freezing and
boiling reaches a higher point than in any other liquid,   3 —....
and a lower point than in all others except alcohol; its
little capacity for heat and ready conducting power,
causes it to be very quickly affected by change of temperature; its expansion is singularly equable for equal
increase of heat through the important middle part of
the scale, which includes the common temperature on
earth, namely, from freezing to the boiling heat of   F -- _
water; and it is easy to proportion the bulb and the
stalk to each other, so that a small difference of tem-  )
perature shall cause the mercurial column in the stalk
to rise or fall very conspicuously.
Now, when the important fact was ascertained that solid water or ice melts
in every case at precisely the same temperature, and that pure liquid water
in a metallic vessel, and under a given atmospheric pressure, boils always at
the same temperature, it followed that by placing such a thermometer as
above described, first in melting ice and then in boiling water, and marking
upon the stalk the two points at which the mercury stood, as represented
here by F and B, two fixed or variable points would be obtained, and the
interval between them might be divided on the glass, or on a suitable scale
to be attached to the glass, into any convenient number of parts to be called
degrees; it followed farther, that by continuing the division to any extent
both above and below the fixed points, a general scale of temperature would
be obtained, with respect to which all thermometers made on the same principle would perfectly agree, although the size of the divisions on the stalks
would vary according to the comparative capacities of the bulk and stalk in
the different instruments.  Our Newton had the honor first to propose the
regulating points of freezing and boiling, and they are now universally
adopted, but the interval between them has been variously subdivided;-that
is to say, there has not been agreement among philosophers as to what they
would call a degree of heat. In the Centigrade thermometer, which is the
most simple, the division is into 100 equal parts; in Reaumur's, which is
commonly used in France, it is into 80 parts; and in Fahrenheit's, which is
used in England, it is into 180~. In Fahrenheit's, moreover, the freezing
point, instead of being called zero, as in the others, is called 320, because
the maker choose to begin counting from the lowest heat which he met in
Iceland, and which was 32~ below freezing of his scale.-To turn the
degrees of any one of these thermometers into degrees of any other, we
have only to recollect that 90 of Fahrenheit are equal to 50 of the Centigrade, and to 40 of Reaumer. Therefore, multiplying by 9 and dividing by
5 or 4, or the reverse, and adding or subtracting the 320 of Fahrenheit,
gives, as the result, the degree desired.
The bulb of a mercurial thermometer is formed by heating to fusion in
the flame of a lamp, the end of a glass tube, which has a very small and
equable bore, and then blowing into the tube until the softened end swells
like a soap-bubble, to the size desired. The mercury is forced into such a
bulb through its lon'g stalk by the pressure of the atmosphere,-thus. First,
a portion of the air originally in the bulb being expelled by warming the
bulb, the open end of the stalk is immersed in mercury, and when the air




302                            HEAT.
remaining in the bulb cools and contracts, a little mercury enters. Secondly
this admitted mercury having been made to boil, so as to fill with its vapour
the whole capacity of the bulb and tube, and to expel the air, on the open
end being again immersed in mercury,and the mercurial vapour within being
condensed, the atmosphere presses in fresh mercury to fill the whole vacuum. To complete the making of the thermometer, the bulb is agin heated
to expel so much of the mercury as that when cold, the tube shall be about
one-third full of it, and. then, before the heated mercury begins to recede,
the end or opening is hermetically closed by directing upon it the point of
a blow-pipe flame which fuses the glass.
Although the direct expansion of any solid body by a moderate change
of temperature is so inconsiderable as to be with difficulty measured, IM.
]Breguet, of Paris, lately with much ingenuity contrived a thermometer
which makes it very evident. Having soldered side by side two very small
flattened wires of silver and platinum, or of any other metals having different
expansibility by heat, he found that all changes of temperature made such
compound wires bend to a great extent, as a sheet of damp paper curls on
being held before the fire. The metal most shortened or least lengthened
acting like a bow-string to pull the other into the arched form; and he then
found, on giving to the compound wire a spiral or cork-screw form, and
securing the upper end of it to a fixed stand, while the lower was left free to
move, that an index like the hand of a watch attached to the lower end was
turned completely round by a certain change of temperature, and afforded
on a circle of degrees placed like a watch face below it, indications which
perfectly agreed with those of good mercurial thermometers.  Other modifications of the same principle have since been successfully tried, so simplified
and reduced in bulk as to be introduced into the structure of a pocket watch.
Air is a substance on several accounts admirably adapted to the formation
of a thermometer; for it has great extent of dilatation from small increase of
heat; it quickly receives impressions, and its dilatation is equal for equal
increments of heat at all temperatures; —but, as already stated, there is the
strong objection that the pressure of the atmosphere cannot be excluded,
without at the same time confining the air, and effecting its expansion. Mr.
Leslie, however, has used for particular purposes an air thermometer, which
he calls the differential thermometer.  It consists of two bulbs a and b, filled
with air and connected by a bent tube d c, containing
Fig. 135.    liquid, the instrument being hermetically sealed, so that
the atmosphere cannot affect the air within.  The greater
Cb   heat in the bulb b than in the other, as when that bulb is
touched by the warm hand or is exposed to the sun's ray,
is marked by the descending of the liquid in the tube d,
which has a scale attached to it. —We may observe that
c      d       equal divisions or degrees marked on the scale of this
thermometer, do not mark equal changes of temperature,
for the increasing condensation and resistance of the air in
the cold bulb require the force overcoming it progressively
to increase.  If the resistance, on the contrary, were unvarying, as in an air thermometer open to a steady atmosphere, equal extent of motion in the fluids would mark
equal increments of heat.  An air-thermometer made of a
simple bulb and long stalk of semi-transparent porcelain,
with the mouth downwards, and containing in its neck
melted lead or other fusible metal instead of mercury, is
well adapted for measuring very high temperatures.




TABLE OF TEMPERATURE.                             303
Temperatures, below that of freezing mercury, are usually measured by
alcohol which substance has not yet been frozen; and temperatures higher
than of boiling mercury, are measured by the expansion of air orof metals,
as above described, or by the contraction of pieces of baked clay, which,
when highly heated, lose water and become semi-vitrified, The use of baked
clay was proposed by Wedgewood, and the apparatus has been called
Wedgewood's PPyrometer, or fire measure.  All contrivances for measuring
heat may be graduated so as to correspond with the scale adopted for the
mercurial thermometer.
It is most interesting, while considering the vast number and importance
of the phenomena produced by heat, to observe the degrees in the general
scale of temperature at which these severally take place. In the following
table, a selection of the facts is classified, the temperatures being all referred
to the scale of Fahrenheit's thermometer.
Table of facts connected with the influence of heat corresponding to certain
temperatures.
-Wedgewood.     Fahrenheit.
Highest temperature measured... 240~           32,27170
Chinese porcelain softened... 156            21,357
Cast-iron thoroughly melted...  150           20,577
Greatest heat of a commen smith's forge.         125           17,327
Flint glass surface.... 114            15,897
Stone ware baked in...          102           14,337
Welding heat of iron...     92 to 95           13,427
Delft ware baked in....   41            6,407
Fine gold melts.....   32            5,237
Settling heat of flint glass...   29            4,847
Fine silver melts....   28            4,717
Brass melts.....   21            3,807
Full red heat (the beginning of Wedgewood's
Pyurometer).....    0             1,077
Heat of a common fire......'790
Iron red in the dark......   750
Quicksilver boils......    660
Linseed oil boils......   600
Lead melts.......   594
Sulphur melts.....     26
Water boils........ 212
A compound of three parts tin, five of lead, and eight of bismuth
melts.      -.....    210
Alcohol boils... v....    174
Bees'-wax melts......    142
Ether boils.......     98
The present medium temperature of the globe...      50
Water freezes..                                            32
Milk freezes...                                            30
Vinegar freezes......      28
Strong wine freezes......      20
Weak brine freezes....      zero            0
Quicksilver freezes...     below zero      40
The air sometimes at Hudson's Bay.                               50
Greatest artificial cold yet measured....      91




304                           HEAT.
There is reason for thinking that the higher temperature noted in this
table are considerably too high, owing to the insufficiency of the thermometer
or Pyrometer (Wedgewood's) by which they were estimated.
It is a curious inquiry, suggested by contemplating the preceeding table,
how much heat may yet remain in bodies at the lowest temperature which
we know? No conjecture was hazarded on the subject until Dr. Irving thought
it might be elucidated by comparing the quantity of heat which becomes
latent in a body on changing form, with the capacity of the body before and
after the change. For instance, with respect to water, he said: as it requries
one-tenth more heat to make a certain change in the temperature of water
than of an equal quantity of ice, probably ice-cold water contains altogether
just one-tenth more heat than of an equal quantity of ice at the melting point:
then as we know the water to contain exactly 1400 more heat than the ice,
viz., its latent heat, the whole or absolute quantity of heat in it will be ten
times 1400, or 1,4000. By applying this reasoning, however, to other substances than water, it evidently is fallacious; and the conclusion follows that
we have as yet no means of solving the question;-the thermometer no more
telling us the absolute quantity of heat in any body than the rising and falling
of the tide between any two rocks tells us the total depth of the rocky chasm.
From what is said in the last and in preceding paragraphs, it is evident
that the thermometer gives very limited information with respect to heat: it
merely indicates, in fact, what may be called the tension of heat in bodies,
or the tendency of the heat to spread from them.  Thus it does not discover
that a pound of water takes thirty times as much heat to raise its temperature
one degree as a pound of mercury; nor does it discover the caloric of fluidity
absorbed when bodies change their form, and which is called "latent heat"
only because hidden from the thermometer; nor does it tell that there is more
heat in a gallon of water than in a pint; and if an observer did not make
allowance for the increasing rate of expansion with increasing temperature,
in the substance used as a thermometer, he would believe the increase of heat
to be greater than it is; and lastly, when a fluid is used as a thermometer,
the expansion observed is only the excess of the expansion in a fluid over
that iii the containing solid, and subject to the irregularities of expansion in
both substances;-all proving that the indications of the thermometer, unless
interpreted by other circumstances and our knowledge of the general laws
of heat, no more disclose the true relations of heat to bodies, than the money
accidentally in a man's pocket tells his rank and riches.
" Heat,.by its different relations to different substances, has a powerful influence on their chemical combinations." (See Anaylsis, page 266.)
By observations made and recorded through past ages, man has now come
to know that the substances constituting the world around him although
appearing to differ in their nature, almost to infinity, are yet all made up of
a few simple elements variously combined; and he has discovered that the
peculiar relations of these elements to heat, particularly their being unequally
expanded by it, and their undergoing fusion and vaporization at different
temperatures, furnish him with ready means of separating, combining, and
new-modifying them to serve to him most useful purposes. Where the primitive savage, looking around on rocks and soils, saw in their diversified aspect
almost as little meaning as did the inferior animals which participated with
him the shelter of the wood or cave, his descendant, with penetration sharpened by science, descries at once the treasures of the mine, and aided by




ABSOLUTE QUANTITY OF HEAT.                          305
heat, whose wonderful energies he has learned to control, pursues through
all the Protean disguises of ores, and salts, and solutions, each of the wishedfor substance, until he secures it apart. For instance, in what to his forefathers for thousands of years appeared but a red dross, he knows that there
lies concealed the precious iron-king of metals! and soon forcing this in
his ardent furnace to assume the metallic form, he afterwards, with implements made of it, moulds all other bodies to his will: the trees from the
forest, and the rocks from the quarry, in obedience to these, are fashioned
by him as if they were of soft clay, and at his command rise into magnificent
structures of palaces and ships, with which the earth and the ocean are now
so thickly covered.-The minute detail of the relations to heat of particular
substances forms a great portion of the department of science called chemistry
(a name taken from an Arabic word signifying fire;) but a general review
of the subject belongs to this work.
The most common ores of metals are combinations of the metals with
oxygen, carbonic acid, or sulphur, substances all of which are volatilized at
much lower temperatures than the metals. Now simply roasting, as it is
called, or strongly heating the ores, suffices often to drive away great part
of these adjuncts; and where additional assistance is required, it is obtained
by mixing with the ore something which when heated attracts the substance
to be expelled more strongly than the metal does.  Charcoal, for instance,
heated with oxide-ore, takes the oxygen, and flying off with it as carbonic
acid, leaves at the bottom of the furnace or crucible the vivified or pure metal.
Mercury mixed with the dross of a mine, dissolves any particles of gold
or of silver existing in the dross, and the ingredients of the solution may
afterwards be obtained separate by mere heating-the mercury passing away
as vapour to where it is cooled and again condensed for subsequent use, and
the more fixed gold or silver remaining in the vessel —and just as in all other
distillations, like that of spirit from wine, or of essential oils from water, &c.,
there is the separation by heat of a more volatile from a less volatile substance. The only difference between what is called drying by heat and distilling, is that in the one case the substance vaporized, being of no use, is
allowed to escape or become dissipated in the atmosphere; while in the other,
being the precious part, it is caught and condensed into the liquid form.
The vapour from drying linen, if caught would be distilled water. The
abundant vapour from wheaten bread while being baken, is chiefly a spirit
like what is obtained from malt, and by an ingenious apparatus lately contrived, may be caught and preserved.
A piece of cold charcoal lies in the air for any length of time without
change: but if heated to a certain degree, the mutual cohesion of its particles
is so weakened, in other words, the particles are so repelled and separated
from each other, that their attraction for the oxygen in the air around is
allowed to operate, and they combine with that oxygen, so as to produce the
phenomenon of combustion. The same is true, under similar circumstances,
of almost any dry vegetable or animal substances, and of several of the
metals.
Nitre, sulphur and charcoal, while cold, may be mixed together most
intimately without any change taking place; but if the mixture or any part
of it, be heated to a certain degree, the whole explodes with extreme violence,
for it is gunpowder. By the change of temperature, and the consequently
altered relative attractions of the different substances, a new chemical
arrangement of them then takes place with the intense combustion and
expansion, which constitute the explosion.
20




306                            HE A T.
Sea sand and soda very intimately mixed, and even ground together, if
remaining cold, remain also merely an opaque and useless powder; but if
the mixture be heated, to diminish the cohesion of the particles of each substance to those of its own kind, so that the mutual attraction of the two substances may come into play, the two substances melt together, and unite
chemically into the beautiful compound called glass; a product than which
art has formed none more admirable-which, in domestic use alone, is fashioned into the brilliant chandelier and lustre, into the sparkling furniture of
the side-board, into the magnificent mirror plate, and when extended across
the window opening, admits the light while it repels the storm.
Perhaps the influence of temperature on chemical union is nowhere more
remarkably exhibited, than in retarding or hastening the decomposition of
dead vegetable and animal substances.  The functions of life bring into'
combination to form the various textures of these organic or living bodies,
chiefly four substances, viz., carbon, or coal; the ingredients of water, or
oxygenz and hydrogen; and lastly, nitrogen-which substances, when in the
proportions found in such bodies, have but slight attractions for each other,
and all of which, except the carbon, usually exist as airs. Their connection,
therefore, is easily subverted; and particularly by a slight change of temperature, which either so weakens their mutual hold as to allow new arrangements to be formed, or altogether disengages the more volatile of them.-At a
certain temperature, a solution of sugar (which consists of the three substances
first mentioned, carbon, oxygen, and hydrogen) undergoes a change into a
spirituous wash, from which spirit or alcohol may then be obtained by distillation: but if the heat be continued under certain circumstances the liquid
undergoes a second change, or new arrangement of constituent particles, and
becomes vinegar: under still other circumstances, it undergoes a third change,
which is a destructive decomposition, or rotting, as we call it, and the oxygen
and hydrogen ascends away as airs.  But sugar and many similar vegetable
compounds, preserved at a low temperature, remain unchanged for ages.
Again, as regards dead animal substances, we find that although at a certain,
not very elevated temperature, they undergo that change in the relations of
their elements which we call putrefaction, during which nearly their whole
substances rise again to form part of the atmosphere, still at or below the
temperature of freezing water, they remain unaltered for any length of time.
In the middle of summer, recently caught salmon, or other fish, packed in
boxes with ice, may be conveyed fresh from the most remote parts of Britain
to the capital. In our warmest weather, any meat or game may be long
preserved in an ice-house. In Russia, Canada, and other northern countries,
on the setting in of the hard frosts, when food for the cattle and poultry is
with difficulty procured, the inhabitants kill their winter supply, and store up
their provender of frozen flesh or fowl, as in other countries men store that
which is salted or pickled.-The most striking illustration which we can
adduce of this kind is the fact that on the shore of Siberia, in 1801, in a vast
block or island of ice, then accidentally broken and partially melted, the carcass of what has been called the antediluvian elephant was found, perfectly
preserved-an elephant differing materially from those now existing on earth,
and having a skelton exactly similar to the fossil specimens found deep
buried in various countries. The carcass was soon discovered by the hungry
bears of the district, which were seen eagerly feeding on its flesh, as if it had
died but yesterday, although it must have been of an era long anterior to that
of any existing monument on earth, of human art, or even of human being.
After it had fallen from the ice to the sandy beach, and its tusks had been




INFLUENCE OF HEAT ON ANIMATED BEINGS. 307
carried away for sale by a Tunguisian Fisherman, and much of its flesh had
been devoured, a naturalist from St. Petersburg who visited it foumd an ear
still perfect, and its long mane, and a part of its upper lip, and an eye with
the pupil, which had opened on the glories of a former or younger world!
About 30 lbs. weight of its hair, which had been trodden into the sand by
the bears while eating the carcass, and part of the skin, were preserved, and
are now distributed in different museums of natural curiosities. A piece of
the skin with the hair upon it is to be seen in the museum of the London
College of Surgeons.
H" eat has powerful influence also on animated nature, both vegetable and
animal." (Read the Analysis, page 256.)
As the detail of the relations of heat to particular inanimate substances
belongs to the province of chemistry, so does the detail of its relations to
particular living vegetables and animals belong to the department of
Physiology; but a general review of the subject is required in a treatise on
Natural Philosophy.
The influence which heat exerts on inanimate nature, is, by the common
mind, more immediately and completely perceived than its influence on beings
which have life. Thus, to all it is obvious, that the contrast between a winter
and summer landscape, is owing chiefly to the effect of heat on the water of
the landscape;-that during the absence of heat, there is the dry barren deformity of accumulated ice and snow, covering every thinif, the roads impassable,
the rivers bound up, perhaps hidden, the air deprived of moisture, and loaded
often with powdery drift:-but that on heat returning, the gliding streams
again appear, the cascades pour, the rills murmur, the canals once more offer
their bosom to the boats of commerce, the lake and pool again show their
level face, reflecting the glories of the heavens, and the genial shower falls
upon the bosom of the softened earth, become ready to receive the spade or
the ploughshare. But this change is not at all greater than what happens
to a winter tree acted upon by the warmth of spring.-Again, it may be said
with truth, that heat applied to the cold boiler of a steam-engine, is the cause
of all its succeeding motions; of the heaving of its beam and pumps, the
opening and shutting of its valves, the turning of its wheels, and its ultimate
performance of any work, as of spinning, or weaving, or grinding, or propeling vehicles by land and water; but as truly may it be said, that heat coming
to a seed which has lain cold for ages, is the cause of its immediate germination and growth; or coming to a lately frozen tree is the cause of the rising
cf its sap, the new budding and unfolding of its leaves and blossoms, the
ripening of its fruits. And what is true of one seed or tree, is true of the
whole of the vegetable creation. When the warm gales of spring have once
breathed on the earth, it soon becomes covered, in field and in forest, with
its thick garb of green, and soon opening flowers or blossoms are every where
breathing back again a fragrance to heaven,-among these the heliotrope is
seen always turning its beautiful disc to the sun, and many delicate flowers
which open their leaves only to catch the direct solar ray, closing them often
even when a cloud intervenes, and certainly when the chills of night approach.
On the sunny side of a hill, or in the sheltered crevice or a rock, or on a
garden wall, with warm exposure, there may be produced grapes, peaches,
and other delicious fruits, which will not grow in situations of an opposite
character-all acknowledging heat as the immediate cause, or indispensable
condition, of vegetable life.




308                            HEAT.
And among animals, too, the effects of heat are equally remarkable. The
dread silence of winter, for instance, is succeeded in spring by one general
cry of joy. Aloft in the air the lark is every where caroling; and in the
shrubberies and woods, a thousand little throats are similarly pouring forth
their songs of gladness-during the day, the thrush and blackbird are heard
above the rest, and in the evening the sweet nightingale: for all birds it
is the season'of love and of exquisite enjoyment.  And it is equally so for
animals of other kinds: in favoured England, for instance, in April and May,
the whole face of the country resounds with lowings, and bleatings, and barkings of joy.  And even man, the master of the whole, whose mind embraces
all times and places, is far from being insensible to the change of season.
His far-seeing reason of course draws delight from the anticipation of autumn,
with its fruits; and his benevolence rejoices in the happiness observed among
all inferior creatures; but independently of these considerations, on his own
frame the returning warmth exerts a direct influence. In his early life, when
the natural sensibilities are yet fresh and unaltered by the habits of artificial
society, spring to man is always a season of delight. The eyes brighten, the
whole countenance is animated, and the heart feels as if new life were come,
and has longings for fresh objects of endearment. Of those who have passed
their early years in the country, there are few who, in their morning walks
in spring, have not experienced, without very definite cause, a kind of tumultuous joy of which the natural expression would have been, how good the
God of nature is to us. Spring, thus, is a time when sleeping sensibility is
roused to feel that there lies in nature more than the grosser sense perceives.
The heart is then thrilled with sudden ecstacy, and wakes to aspirations of
sweet acknowledgment.
Besides the effects of heat now mentioned, and which are comparatively
transient as being connected with the seasons, there are other effects on animated nature of a more prominent character.  Certain species of vegetables
and animals, by their relations to heat, are confined to certain latitudes or
climates; as the orange tree and bird of paradise, to warm regions; the fir tree,
and arctic' bear, to those that are colder:-and when individuals of either
class can support diversity of climate, they acquire a certain character according to the climate —as seen' in the sheep and dogs of the various regions of
the earth.  In this latter respect there is no instance more interesting than
that furnished by the varieties of the human race.  If we assume that the
whole sprung from one stock, what a contrast is there between the native of
equatorial Africa, of temperate Europe, and of the Polar zone: between the
Negro, the Greek, and the Esquimaux: or, again, between the dark slender
children of Hindostan, the strongly-knit fairer Roman or Spaniard, and the
taller, ruddy, powerful Briton. And in the female sex of the last-named
countries, we may remark the gentleness and singular devotedness of the
Indian woman, the more commanding dark eye and gesture of the graceful
nymph of Italy or Spain, and the happily attempered mixture of qualities in
the fair and much-favoured daughters of Britain.
The very important influence of heat upon the temporary bodily state of
animals, becomes an object of much study to the physician. It explains
among many other facts, the connection of temperature with the rise of fevers
and other pestilences, the powerful remedial efficacy of hot and cold bathing,
of changes of climates, of regulating the temperature of air breathed by invalids; the protection from clothes, houses, &c.




THE  dUN  THE  GREAT  SOURCE.                       309
T" he great natural source of heat is the sun."  (See Analysis, page 256.)
To admit this, it is only necessary to think of the comparative temperatures of night and day, of seasons and of climates, and to reflect that the sun
is the sole cause of the differences. We need not wonder, then, that to
many savage nations, seeking the course of their life and happiness, the sun
has been the object, not only of admiration, but of worship.
The heat comes from the sun with all his liaht. If a sunbeam enter by a
small opening an apartment otherwise closed and dark, it illuminates intensely
the spot or object on which it first falls, and its light being then scattered
around, all the objects in the room become feebly visible. Again, a cold
thermometer, held to receive the direct ray, ri'es much, while, in any other
situation, it is less affected; proving the heat to be, like the light, at first
concentrated, and then widely diffused, losing proportionally of its intensity.
Light passes from the sun to the earth in about eight minutes of time, as
will be fully explained in a future chapter; and there is every reason to
conclude that heat travels at the same rate.
Human art can gather the sunbeams together, and the intense heat
existing in the focus of their meeting, is another proof that the sun is the
great source of heat. A pane of glass in a window, or a small mirror will
reflect the sun's rays so as strongly to affect the eye at a distance of milesand the heat accompanies'the ray, for by many such mirrors directed towards
one point, a combustible object placed there may be inflamed   Archimedes
set fire to the Roman ships by sunbeams, returning from many points to
one, his God-like genius thus rivaling by natural means, the supposed feats
of fabled Jupiter with his thunderbolts. Again, when the light of a broad
sunbeam is made by a convex glass or lens to converge to one point or focus,
the concentrated heat is also there-for a piece of metal held in the focus
will drop like melting wax: and if the glass be purposely advanced, its
focus will similarly advance, and will pierce through the most obdurate
substances, as red-hot wire pierces through. paper or wood. A hunter on the
hill, and travelling hordes on the plains, often conveniently light their fires
at the sun himself, by directing his energies through a burning glass.
The direct ray of the sun, simply received into a box which is covered
with glass to exclude the cold air, and is lined with charcoal or burned cork
to absorb heat, and to'prevent the escape of heat once received, will raise a
thermometer in the box to the temperature of 230~ of Fahrenheit, a temperature considerably above that of boiling water. And the experiment
succeeds in any part of the earth where there is a clear atmosphere, and
where the sun attains considerable apparent altitude.  We see, therefore,
that a solar oven might in some cases be used.  In operating with the
apparatus suggested by the author, and described at page 295, for distilling
water by the heat of the sun, the vessel intended to absorb the heat, and to act
as the still, should be enclosed in a case covered and lined as above described.
Reflecting on such facts as now recorded, and on the globular form and the
motions of our earth, we have a reason and the measure of the differences of
climate and of season found upon the earth. It is evident that the part of the
globe turned directly to the sun, receives his rays as abundantly as if it were
a perfect plane, directly facing him, while on parts, which, as viewed from
the sun, would be called the sides of the globe, with the increasing obliquity
of aspect, an equal breadth or quantity of rays is spread over a larger and
a larger surface; and at the very edge the light passes level with the surface 
and altogether without touching. The sunny side of many a steep hill in




310                            HEAT.
England receives the sun's rays in summer as perpendicularly as the plains
about the equator; and such hill-sides are not heated like these plains, only
because the air over them is colder-just as mountain tops, even at the
equator, owing to the rarified, and, therefore, cold air around them, remain
for ever hooded in snow. In England, at the time of equinoxes, a level
plain receives only about half as much of the sun's light and heat as an equal
extent of level surface near the equator: and in the short days of winter it
receives considerably less than a third of its summer allowance.
There are few contrasts in nature more striking than between the consequences of different intensity of the sun's influence:, for instance, the inhabitants of India, at midday, with the thermometer at 1200, are running to
the shade of their bangralows, darkening their windows, hanging wetted mats
upon the walls and roofs, sprinkling water on the floors, fanning themselves
with ever moving punkas, and feeling the slightest covering or exertion too
much-while, on the other hand, the dwellers in Greenland, with the thermometer below zero, are loaded with furs, and are seeking the direct sunshine or heat from a fire, as their life and comfort. Again, there is the
contrast observed on passing, as the author once did, very rapidly, from
such a paradise as Rio de Janeiro, with all its vegetable riches, to Tristan
de Cunha, and the Isle of Desolation in the Southern Ocean, which exhibit
only cold and naked rocks; but yet, where the scene swarms with its appropriate inhabitants-the sea with seals, and the air with clouds of sea-fowl,
playing over the never-resting waves like flakes of eddying snow. Were a
person for a moment to doubt whether the sun be the real cause of such
differences, and of the fact that certain creatures are found only in certain
zones of the earth, let him reflect on the extraordinary migration of animals,
which have their home, not in any fixed region, but wherever the sun has
for a time particular degree of influence, and which acccordingly follow the
sun in the changes of season. We have the swallow in vast numbers coming
to visit the British isles in the spring, to play over our woods and waters,
in pursuit of the insects which the heat breeds to fill the air,-welcome
harbingers of the coming summer and its riches; and in autumn, the same
creatures are seen congregating orn our shores, to wing their flight back in
united multitudes to more southern countries, where, in turn, there is a
temperate influence of the sun. The same reason brings to England the
nightingale, and makes our woodlands resound with the notes of the cuckoo.
In the waters of our bays and coasts, again, there appear with the seasons
the vast shoals of fish, as of the herring and mackeral, which prove such
abundant food for millions of human beings; and we have salmon, at stated
times, penetrating from the ocean far up the mountain streams, to deposit
its spawn for further supply; all by their movements contributing to the
harmonious and beneficent system of the universe.
With respect to the sun as a source of heat, there have been two opinions
among philosophers, one class believing that the sun is an intensely heated
mass, which radiates its heat and light around, like a mass of intensely
heated iron; and another class holding that heat is merely an affection or
state of an ethereal fluid, which occupies all space, as sound is an affection
or motion of air, and that the sun may produce the phenomena of light and
heat without waste of its temperature or substance, as a bell may produce
the phenomenon of sound; holding, farther, that the sun, below its luminous
atmosphere may be habitable, even by such animals as live on this earth.
Those who take the first view, are awakened to the dread contemplation of
a universe carrying in itself the seeds of its own decay, or at least of great




COMBUSTION.                             311
periodical revolutions: the others may view the universe as destined to last
nearly unchanged, until a neW act or will of its Creator shall again alter or
destroy it.
Of one fact there can be no doubt, namely, that the present temperature
of the surface of the earth is much lower than the temperature in remote
past time. The rocks called primitive, as granite and gneiss, constituting
the interiors of our great mountain masses and the substrata of our plains,
bear evident marks of there having been at one period a molten state, from
which they have been solidified by a very gradual cooling: and even the
whole mass of the earth at some time must have been so fluid or soft, as, in
obedience to gravity, to have assumed its rounded form, and in obedience to
the'centrifugal force of its whirling, to have bulged out, at its great circumference or equator, the thirty-four miles which its equatorial diameter exceeds
the polar; the same, by the by, in degrees corresponding to the various
speed of rotation, being true of all the other planets belonging to the solar
system. Again, while in excavating below the surface of the globe, or in
examining its structure as exposed to view by volcanic or other convulsions,
men encounter, in very many situations, a thickness of more than a mile, of
the wreck and remains of former states of the world-as, on digging eighty
feet under vineyards near Mount Vesuvius, they encounter the more recently
buried cities of Herculaneum and Pompeii-they farther discover that the
animal and vegetable remains buried, without number, in the present cold
climates of the earth, and evidently near where the creatures lived, are all
of kinds now inhabiting only the warmer or tropical regions. Lastly, in
the operations of mining, the deeper men go, the higher they find the temperature to be, at the rate of a degree for about 200 feet of descent; which
fact as heat tends to equable diffusion, proves both that the central heat of
our earth must have had another source than a radiation from the sun of the
present intensity; and that the surface of the earth is now radiating away
more heat than it receives from the sun. The conclusion then follows, that
the temperature of the world is still falling although perhaps so slowly that
a change may not be detected even within centuries. Possibly, in very
remote antiquity, that may have been'" true which the early Greeks erroneously thought true in their day, viz., that the equator of the earth, by reason
of its great heat, the sun's influence there being joined to the heat from
within, was a barrier, impassable by man, between the northern and
southern hemispheres.
" Electricity a source of heat." (See the Analysis.)
This subject can only be satisfactorily entered upon in the chapter devoted
exclusively to electricity, and is, therefore deferred.  Suffice it here to say,
that while an electrical discharge of current passes from one situation to
another, the substance serving as a conductor is often heated, melted, or dissipated, in such a manner as to make it doubtful whether human art has any
more powerful means of producing these effects.  We may remark, too, that
in certain cases of the electrical current, the heat is accompanied by as
intense a light as art can exhibit.
a Combustion and other chemical actions as sources of heat. "(See Analysis,
page 256.)
Of the phenomena of nature there is perhaps none which, to the uninstructed, appears so inexplicable and so wonderful as that of fire or combus



312                           HEAT.
tion-whether contemplated in its beauty or in its terrors. Fire is seen in
its beauty when used by man for his domestic purposes, as when it blazes
cheerfully over his parlour hearth, or beams a steady light around from his
lamps and chandeliers. It is seen again in its terrors, when spreading by
accident from some focus, it envelopes in sudden flame and quickly consumes
the surrounding objects, perhaps the draperies and other furniture of a single
apartment; or wider spread, the valuable contents of a spacious mansion; or
still wider spread, and deafening uproar, a whole town or a forest:-and it
is fire which, labouring within the bowels of the earth, first prepares and then
urges up to heaven the volcanic eruptions of flames and red-hot rocks, during
which the region around often quakes and is uptorn, so that the cities are demolished into the sudden tombs of the inhabitants, the course of the riversis
changed, the plains are converted into lakes, or the lakes'-beds into dry land.
Fire is awfully seen also in some meteors, and when, intentionally lighted by
human hands, it bursts from the cannon to produce the carnage of battle. Fire
among many nations of antiquity was regarded with awe and holy reverence,
the sun himself being honoured chiefly as its concentration or supposed abode.
There were sacred fires in many of the temples, and fire was used to complete
the splendour of the most august ceremonies. Nay, even Moses, a worshipper
of the one true God, has given records of the Burning Bush and of burntofferings made to that God: and at the present days in many Christian churches
there are ever-burning lamps and frequent magnificent illuminations. Now
this principle offire, which, when the savage man first saw it spreading after
the thunder-clap or the rubbing of forest branches in a storm, so as to threaten
universal destruction, he so natuarally accounted the demon, if not the God of
nature; this principle man's art has now tamed to be a most obedient and by
far the most useful of his servants. Fire being in truth but a concentration of
the element of heat, which in its tranquil and invisible diffusion we have already contemplated as the beneficent life or soul of the universe-the cause of
seasons and climates and of all the changes or activity which distinguish a
living world from adead and frozen mass; man, by acquiring command over
it, commands heat when and where he wills, and thus truly becomes in a
second degree the ruler of nature. Fire in man's service may be figured
as a legion of serving spirits to whom no labour is difficult, who, in any
particular case have power or magnitude exactly proportioned to the quantity.of food or fuel afforded; of whom, moreover, man can, at any moment,
conjure up one or many by the magic stroke of his flint and steel. In every
private dwelling he has of these fiery spirits as domestic slaves-in the
kitchen and in the parlour. In his manufactories they are melting glass for
him, and reducing ores, and boiling and evaporating for a hundred purposes.
But it is chiefly when chained to the steam-engine, that they show their
prodigious powers:-as when, putting forth a giant's strength, they heave a
river from the bottoin of a mine, or urge up a vast ship through the winter
storm; or when equaling, if not surpassing, in nice dexterity, the work
of human hands, they twist the silken or cotton threads, and weave them
into most delicate fabrics.  Men, grown familiar with such prodigies, have
almost ceased to be moved by them; but even now few persons can resist a
feeling of wonder and admiration when chemistry is calling forth the hidden
spirit of combustion in some new and less familiar guise:-as, for instance,
when a piece of iron wire, lighted as a taper in oxygen gas, burns with such
resplendent brilliancy; or when phosphorus similarly placed, throws around
its overpowering flood of flame; or when small portions of the metal called
potassium, cast upon the surface of water, become as beads of most intense




COMBUSTIONO.                          313
light running about there, and crossing as in a merry dance;-or, lastly,
when flames produced from particular substances are seen rising deep-tinged
with most vived and beautiful colours.
Singularly interesting, then, to philosophers, as in such particulars the
phenomena of combustion must always have appeared, one may wonder
that its true nature could remain to them so long a mystery; but until the
admirable researches of Davy, made only a few years ago, their conjectures
had scarcely approached the truth. An opinion long prevailed, that in every
combustible substance there was present a certain quantity of a something
denominated phlogiston, which, on being disengaged or separated, became
obvious to human sense as light and heat. The white oxide of zinc, for
instance, named the flowers of zinc, and into which the metal is changed by
burning, was supposed to be the metal deprived of its phlogiston; and when
on this oxide being heated with charcoal, the metal again appeared, it was
supposed simply to have recovered phlogiston from the charcoal. The illustrious Lavoisier had the merit of most clearly disproving this hypothesis, by
showing that the flowers or powders obtained from a metal by burning it,
were heavier than the piece of metal from which they were produced, by the
exact weight of the oxygen gas, which disappeared in the combustion, &c.;
and he showed farther, that in this and many other cases, combustion was
merely the act of two substances combining chemically; but he fell into an
error almost as great as that which he overthrew, by supposing that for combustion, oxygen had always to be one of the combining substances, and that
the heat and light given out in every case had been previously latent in the
oxygen.
When Sir Humphrey Davy began his labours on the subject, than which
labours there is not, perhaps on record a more perfect specimen of scientific
research, it was already known, first,-that bodies when compressed or by
any means reduced in bulk, generally give out a part of their heat, as-when
air condensed in the match-syringe lights tinder,-or when water and sulphuric acid, uniting into a compound of smaller volume than the separate
ingredients, becoming very hot,-or when water, poured upon quick-lime to
slake it, and becoming solid with it, produces heat sufficient to inflame wood,
as has been fatally proved by the burning of many lime-loaded ships:-and
that in such cases, the heat produced during the chemical union depended
more upon the energy of the action which united the substances than upon
the change of volume produced.
Farther, it was known that any substance having its temperature raised,
by whatever means, to 8000 or more of Fahrenheit's thermometer, became
incandescent or luminous-as when iron, or stone, or any substance not dissipate,d by heat, is placed in a common fire; in the first degree the substance
being said to be red-hot, and at higher temperatures to be white-hot.
Out of these two truths Davy constructed his explanation. He assertedthat in any case, combustion is merely the appearance produced when substances, having a still stronger attraction for each other than quick-lime and
water, are, with intense energy,'combining chemically, so as to become
heated at least to the degree of incandescence, and that during the phenomenon there is not, as was formerly supposed, something altogether consumed or destroyed, or something called phlogiston escaping, but that the
substances concerned are only assuming a new form or arrangement. Thus,
if a piece of charcoal be enclosed in a glass vessel filled with air, of which
vessel the mouth dips into a liquid to confine the air, and if the charcoal be
then heated to a certain degree, by means of a burning-glass or otherwise,




314                           HEAT.
the cohesion of its particles yields to their attraction for the oxygen of the
air around them, and they immediately begin to combine with the oxygen so
energetically as to produce a heat still much greater, accompanied by the
light or inbandescence of combustion. The charcoal, under these circumstances, soon entirely disappears, or is dissolved in the air, as sugar may be
dissolved in water; but if the air be afterwards weighed, it is found to have
gained in weight the exact weight of the charcoal which has disappeared;
and a chemist can again separate the charcoal from the air, and use either
for any purpose as before. In like manner, if a piece of iron wire be heated
at one end, which is then plunged into a jar of oxygen gas, it will burn as a
most brilliant taper, and will gradually fall in the form of oxidized drops or
scales of iron, to the bottom of the vessel. Now during this process, the
quantity of oxygen will be diminished, but if the scales mentioned be collected, they will be found to weigh just as much more than the original wire
expended, as there is of oxygen lost or combined with them. A chemist
can separate this iron and oxygen, and exhibit'them apart as before, without
loss. Again, if iron and sulphur, in certain proportions, be heated together,
they unite with vivid combustion, but the product weighs exactly as much
as the original ingredients.
While every instance of combustion is thus only a case of chemical union
going on with such intensity of action as to produce incandescence, still,
according to the nature of the substances combining, the appearance varies
much. It may be, for instance, with flame or without flame.  The great
combining substance in nature, that is to say, the most universally distributed,
is oxygen, of which the name is now become familiar even to the ears of the
unlearned.  It forms four-fifths of the substance of water and one-fifth of our
atmosphere, being on the latter account present wherever man can be, and
ready to unite itself with any matter exposed to it at the necessary temperature. Now, of substances burning in air, those which are originally aeriform,
as coal-gas, or which on being heated, are rendered aeriform, or vapourized
before the union takes place as oil or wax, assume the appearance of flamewhich means that the aeriform particles usually invisible are raised to the
incandescent temperature;' but when the substance combining with the
oxygen remains solid, while its particles are gradually lifted away by the
oxygen acting only at the surface of the mass, it appears during the whole
time, only as a red-hot stone. The latter is the case of charcoal, coke, Welch
stone-coal, &c., while in the case of wood, common coal, &c., a greater or
less portion of the inflammable matter is by the heat of the combustion converted into vapour, and produces the beautiful appearance of flame.
Of the substances called combustible, and thus called because they combine
with oxygen so energetically as to become incandescent, there are only a
few, as the metals called potassium, sodium, &c., which will begin to unite
with oxygen, or to burn at the common temperature of our globe, the others
requiring to be at some higher temperature. Thus phosphorous begins to
burn at 1500, sulphur at 5500, charcoal at 750~, hydrogen at 8000, &c., it
appearing that up to these temperatures the attraction of the atoms of the
substances among themselves is sufficient to resist the other attraction or that
of oxygen. But when the combustion once begins, the temperature, from
the effect of the combustion itself, rises instantly much beyond the degree
necessary for the commencement of the process. Oxygen and hydrogen,
which begin to burn or combine at 8000, produce a flame of as intense heat
as human art can excite.
On the circumstance that bodies require to have certain preparatory tem



COMBUSTION.                            315
perature before beginning thus to combine with oxygen, depend many important facts of nature and art.  Hence the safety with which most combustibles may be exposed at ordinary temperatures to the contact of atmospheric air: otherwise, coal, wood, &c., in the moment of being exposed to
the air, would catch fire, as really happens to phosphorated hydrogen gas;
or to the'metal called potassium, even when thrown into cold water, the
metal attracting the oxygen from the water instantly, and with intense combustion.  If a fire or flame be so small that, from the rapid absorption or
heat by bodies around, it does not produce heat enough to maintain the
inflaming temperature of the substance, the combustion will soon be extinguished.  Thus a common coal fire, if not watched, and the remaining fuel
occasionally gathered together to reduce the surface of wasteful radiation,
will be extinguished long before the whole fuel is expended:-but not so
with a fire of wood or of paper, which substances burn more readily than
coal. The Welch stone-coal can only be made to burn in very large masses,
or when mixed with a more inflammable coal or other fuel, or when fed by
air already heated. Some of our manufactures have lately been improved
by causing the air which feeds the working fire to pass through metal tubes
heated by lying in another fire. In common fires much coal is burned at
temperatures so low as to be nearly useless. A substance placed in pure
oxygen gas burns with much greater intensity, and will begin burning at a
lower temperature than if placed in atmospheric air, which contains only
one-fifth of oxygen and four-fifths of another substance, nitrogen, which
does not aid the combustion-because, in the latter case, the nitrogen by
absorbing much of the heat of the combustion, lowers the temperature. Iron
wire will burn as a taper in oxygen, but not in common air; and a commbn
taper or flaming piece of wood just extinguished by blowing on it, will immediately be rekindled if placed in oxygen. Again, a lamp with a very small
wick, as of one thread, and producing, therefore, very little heat, will not
burn in cold weather, or in an ice-house, and at any time will be extinguished
by a foreign body brought near it so as to cool it-a piece of ice, for instance,
or a small metallic nob, presented to it on the end of a wire, or a metallic
ring let down over it; but if the ball or ring be hot, the effect will not follow. By more powerful refrigerating processes, even a considerable lamp or
candle may be put out. These discoveries led Davy to the construction of
his miner's safety-lamp, which is merely a lamp surrounded by a wire gauze,
of which the meshes are of such size that a flame of hydrogen gas attempting
to pass through is so cooled by the heated absorbing and heat-conducting
power of the metal as to be extinguished. A wire gauze gradually let down
upon any common flame, annihilates the part of it which should appear above
the gauze; but the combustible vapour passing invisibly through the gauze,
may be lighted afresh on its upper side. Oxygen and hydrogen, which are
the constituents of water, when uniting, produce such intense heat that the
momentary expansion of the newly formed water, then in the state of steam,
is such as to constitute a violent explosion; and when a jet of the two gases
mixed gives a continued flame, the most refractory substances melt in it like
wax in a common taper-yet these gases may be kept mixed together in the
cold reservoir of a blow pipe without combining, and when they are set on
fire, while issuing as a jet, the flame does not travel inwards through the
opening, as might be feared, because it is cooled by the metal of the orifice.
A mixture of oxygen and hydrogen passing through the small channels left
in a tube filled with fire, may be lighted at the end of the tube without
danger of explosion.




316                           NIEAT.
While solid bodies become very visible or incandescent at about 1000 of
Fahrenheit, airs, owing to their tenuity of condition, require to be heated
much farther before they take o.nthe vivid appearance of flame; and airs of
light atoms, like hydrogen, require to be heated still more than heavier airs.
Thus the flame of pure hydrogen is pale and blue, but a wire held in it becomes much more luminous than the flame itself; and the flame of mixed
oxygen and hydrogen escaping from a very minute orifice in a glass tube,
may itself be scarcely visible, while the extremity of the tube heated by it
becomes like a brilliant star. Hence the light of many flames may be increasea by placing a wire gauze or other solid body in the flame; as is seen
when a piece of lime is placed in a flame of mixed oxygen and hydrogen.
Consideration of this subject enables us to explain why common coal gas,
which consists of hydrogen holding a quantity of carbon in solution, gives a
stronger light than pure hydrogen; and why oil gas, which contains about
twice as much carbon as the coal gas, gives also about twice as much light;
-or it appears that the atmospheric air, which first mixes with these gases
as they issue to burn, is sufficient to combine with all their hydrogen (which
it most strongly attracts) but not at the same time with all their carbon:
the particles of the carbon, therefore, are first separated or precipitated in
the flame, and become so many solid particles most intensely heated and
luminous; and afterwards, when they have ascended a little higher, they
meet with new oxygen and burn in their turn, giving a second quantity of
light. That this decomposition of the gas really occurs is proved by placing
a wire gauze in the flame, for then we find that if it be held near the middle
of the flame, it becomes immrediately loaded with particles of charcoal separasted there, and cooled by it so as to cohere; while if held at the bottom of
the flame where the carbon is not yet separated, it retains none, and if held
at the top of the flame, where they are already burned, it retains none. A
candle or lamp is said to smoke when the heat produced by it, or the quantity of oxygen allowed to approach the flame, is not sufficient to effect the
total combustion of the. carbon which rises in the flame. The hollow or circular wick of the common Argand lamp, and the similar form given to gas
flames is useful, by admitting air to the inside as well as to the outside of
the flame, and the lofty glass chimney is to quicken the currents of air.
When oxygen mixed with certain of the inflammable gases or vapours is
heated, although only to a temperature considerably below that of common
burning or explosion, a union still takes place, but very slowly, so that the
temperature never rises to what is necessary to exhibit flame. This phenomenon has been called invisible combustion. It is remarkably exemplified
on plunging plantinum or gold wire moderately heated into such a mixture:
the combination then goes on in the immediate vicinity of the'hot wire;
and although without flame, still with sufficient disengagement of heat to
maintain the wire in an incandescent or luminous state, so long as there are
gases left to combine. Thus the vapour always arising at a common temperature from the mouth of a phial of ether, (ether consists chiefly of hydrogen
and carbon), if made to pass through a coil of heated plantinum wire, while
combining, by this slow combustion, with the oxygen of the air around it,
gives out heat enough to keep the wire so luminous as to serve as a little lamp
by which to read from the dial-plate of a watch through the night. A beautiful modification of this principle has been adopted in the miner's safetylamp; and when the air of the mine is too impure to maintain the flame, it
still suffices thus to produce a continued light from the incandescent metal.




FUEI.                             317
" Fuel."
Heat being, in the sense already explained, the life of the universe, and
man having command over nature chiefly by his power of controlling heat,
which power again comes to him with the ability to produce combustion, it
is of great interest to inquire what substances he can most easily procure as
food for combustion, or fuel, and how these may be most advantageously employed. To speak on this subject at all fully in reference to the various arts
of life would be to compose an extensive work, but an interesting sketch
may be comprised within narrow limits.
Although there are a great number of substances, which, in the act of
their chemical union, occasion the heat and light which constitute the combustion, still by far the greater part of these, in an uncombined state, are so
sparingly distributed in nature, and are therefore procurable with such difficulty, that heat obtained by sacrificing them would be much too expensive
to be within common means.  Providence, however, has willed that the elementary substance in nature which has the most energetic attraction for
almost all other substances, and which therefore produces in uniting with
them the most intense heat, is also of all the most universally distributed.
This substance is oxygen.  It forms part of our atmosphere, and therefore
penetrates, and is present wherever man can breathe, offering itself at once
at his service. Then for the purpose of combining with the oxygen, there
are chiefly two other substances also very widely scattered, and therefore
easily procurable and cheap. These are carbon and hydrogen, the great materials of all vegetable bodies, and therefore of our forest trees, and of coal
beds, many of which are evidently the remains of antediluvian forests. Carbon is found nearly alone in hard coal, but it is united with a large proportion
of hydrogen in caking coal, and in wood,. wax, resins, tallow and oils. The
gases obtained from these last-mentioned substances and now used for illumination are merely hydrogen, holding in solution certain proportions of
carbon; and all bodies which burn with flame give out such gases in the act
of combustion. In the mass of the earth, as' far' as known to man, the
stones, earths and water, forming its surface, are already combinations of
oxygen with other substances, and are therefore not in a state to produce'fresh combustion; but carbon and hydrogen, by various processes of vegetable and animal life, are in numberless situations becoming accumulated,
so as to be fit for fuel; —as'by other processes the atmosphere is always
preserved with its due proportion of oxygen.
The name fuel is given only to the substances which combine with oxygen,
and not to the oxygen itself, probably because the former being solid or
liquid, and therefore more obvious to sense, had attracted human notice as
producers of combustion long before the existence of the aeriform agent,
oxygen, was even suspected.
Oils, fat, wax, &c., from becoming aeriform in their combustion, exhibit
the appearance of flame, as already explained, and hence these substances are
chiefly used for the purpose of giving light; while wood and coal are more
frequently used for mere heating. But the chemist's lamp, by which he
distills and evaporates, and his common blow-pipe for directing the point of
any flame upon a substance to melt it, and his blow-pipe fed with mixed
oxygen and hydrogen, whose flame is capable of melting the most refractory
substances-prove that it is chiefly the expense of the former kinds of fuel
which has limited them so much to the office'of light-giving. Lately an




318                            HEAT.
important application of coal-gas and of oil or fat as heat-giving fuel has
been made in a general cooking apparatus, which promises to effect a considerable diminution of house-keeping expense.
WTood was the common fuel of the early world when coal-mines were not
yet known, and still in many countries it is so abundant as to be the cheapest fuel. Charcoal is the name given to what remains of wood after it has
been heated in a close place; during which operation the hydrogen and other
minor ingredients are driven away in the form of vapour. Charcoal is nearly
pure carbon. Coke, again, is the charcoal obtained by a similar process from
coal. The wood and coal, if much heated in the air, would burn or combine
with the oxygen of the air; but heated in a vessel or place which excludes
air, they merely give out their more volatile parts.
Good coal, where it abounds, is now for ordinary purposes by much the
cheapest kind of fuel; and since within a few years men have learned to
separate from it, and to use, instead of oil and wax, its illuminating gas,
namely, its hydrogen holding in solution a little carbon, it has become doubly precious to them. A person reflecting that beat is the magic power
which vivifies nature, and that coal is what best gives heat for the endless
purposes of human society, cannot without admiration think of the rich stores
of coal which exists treasured up in the bowels of the earth for man's use.
And Britain, in this respect, is singularly favored. Her extensive coal mines
are in effect mines of latent labour or power vastly more precious than the
mines of gold and silver in Peru, —for a hundred men in England, of whom
a part dig coal, and the remainder apply it in manufactures, can get in exchange for the merchandize they produce more of either gold or silver than
a hundred men working in the American mines are able to extract. These
coal-mines, therefore, may be said to produce abundantly every thing which
labour and ingenuity can produce, or which our money can buy, and they
have essentially contributed to render Britain the mistress of the industry
and commerce of the earth. Britain has become to the civilized world around,
nearly what an ordinary town is to the rural district in which it stands, and
of this vast and glorious city the mines in question are the coal-cellars,
stored at the present rate of consumption for above 1,000 years; a supply
which, as coming improvements in the arts of life will naturally bring
economy of fuel, or substitution of other means to effect similar purposes or
changes in the channels of industry,-may be regarded as exhaustless.
The coal in many mines is evidently the remains of forests, which have
existed in very remote time, swept together either during convulsions of
nature or by the more gradual operation of rivers, (as may be seen at present
where the great Mississippi is carrying an uninterrupted stream of floating
timber to the sea, and depositing it at the bottom of the Mexican gulf) the
accumulated wood being afterwards compressed and solidified by superincumbe,'t deposit sof earthy matters, aided probably by the action of heat. In
many coal-beds the trees yet retain their form, so that their species can be
easily distinguished, and there are buried among them other vegetable and
animal remains of cotemporaneous inhabitants of the earth.  Coal is found
of different qualities.  In some places it is almost unmixed carbon, and
exceeding solid, as if it had been coked by subterranean heat.  Such is
the stone-coal of Wales, which in 100 parts contains 97 of pure carbon, with
only three of hydrogen and earthy matter.  In other places the coal contains hydrogen in nearly as large proportion as wood does, so combined with
part of the carbon as to form the oily or pitchy substances existing in the




FUEL.                             319
coal which, when burning, produce flame, and when rising unburned, constitute smoke.
The comparative values, as fuel, of different kinds of carbonaceous matter,
have been found to be as in the following tables.
One pound of                                 Melts of ice
Good coal                                      90 lbs.
Coke  -      -       -      -              -   84
Charcoal of Wood  -      -                     95
Wood -        -                                32
Peat.                     19
Lavoisier, in making experiments on combustibles generally, to ascertain
the quantities of oxygen expended, and of heat given out during the combustion of a given quantity of each, obtained the following results:
One pound of                   Melts of ice Takes of oxygen
Hydogen gas      -       -      370 lbs.       71 lbs.
Carburetted hydrogen -      -    85            4
Olive oil  -     -       -      120            3
Wax.                          110            3
Tallow    -      -       -      105            3
Charcoal     -.         -    95            2*
Phosphorus-      -       -      100            1k
Sulphur      -       -      -   25             1
The following remarks with respect to fuel, and the modes of using it, seem
to demand a place here.
A pound of coke produces nearly as much heat as a pound of coal; but
a pound of coke is the produce of a pound and a third of coal, although the
coke is more bulky than the coal.
It is wasteful to wet fuel, because the moisture in being evaporated carries
off with it as latent, and therefore useless heat, a considerable proportion of
what the combustion produces. It is a very common prejudice, that the
wetting of coal, by making it last longer, is effecting a great saving; but it
does so merely by restraining the combustion or producing a smaller fire, and
with the bad fire, there is also much waste of heat. To illustrate the influence of watery vapour upon combustion, we may mention the fact, that a
manufacturer who tried to blow his fire by forcing steam into the furnace
with the air, extinguished his fire; and the analogous fact, that ordinary fires
burn better in winter than in summer, although the temperature be lower,
because cold air is generally much drier than that which is warm.
Coal which contains much hydrogen, as all flaming coal does, is used
wastefully when any of the hydrogen is allowed to escape unburned; for,
first, the great heat which the combustion of such hydrogen would produce
is not obtained; and, secondly, the hydrogen while becoming gas, absorbs
into the latent state still more heat than an equal weight of water would.
Now the smoke of a fire is the hydrogen of the coal rising in combination
with a portion of carbon. We see, therefore, that by causing a fire to
destroy or burn its smoke, we not only prevent a nuisance, but effect a great
saving. The reason that common fires give out so much smoke is, either
that the smoke or vaporized pitch is not sufficiently heated to burn, or that




820-                           HEAT.
the air mixed with it as it ascends in the chimney, has already, by passing
through the fire, been deprived of its free oxygen.  If the pitch be very
much heated, its ingredients assume a new arrangement, becoming transparent, and constituting the common coal-gas of our lamps; but at lower temperatures, it is seen jetting, from cracks or openings in the coal, as a dense
smoke-a smoke, however, which immediately becomes a brilliant flame if
lighted by a piece of burning paper or the approximation of the combustion.
The alternate bursting out and extinction of these burning jets of pitchy
vapour, contribute to render a common fire the lively and agreeable object
which it is in the winter evenings.  When coal is first thrown upon a fire,
a great quantity of vapourized pitch escapes as a dense smoke, too cold to
burn, and for a time the flame is smothered, or there is none; but as the
fresh coal becomes heated, its vapour reproduces the flame as before.  In
close fire-places, as those of steam-engines, brewing and dyeing apparatus,
&c., all the air which enters after the furnace-door is shut, must pass through
the grate and the burning fuel, so that its oxygen is consumed by the redhot coal before ascending to where the smoke is.  The smoke, therefore,
however hot, passes away unburnt; unless, sometimes, as over foundry furnaces, where the heat is very great indeed; and it burns as a flame or great
gas lamp at the chimney-top on reaching the oxygen of the open atmosphere.
There have been many modes proposed of destroying smoke: one has
been to admit to the space above the fire, by a suitable opening, a certain
quantity of fresh air, the oxygen of which may inflame the smoke.  This
plan is efficient at a certain point of time after the addition of fresh fuel,
and then for a time effects the saving of fuel; but the difficultyof admitting
just the quantity of air required to suit the varying demand for it, has not
been overcome, and hence, from there being no saving on the whole, the
plan has been abandoned.  When just enough air entered, the flame produced gave so intense a heat as in several cases to have burned or destroyed
the parts of valuable boilers exposed to it; and when,. on the contrary, too
much air entered, it injuriously cooled the boiler.  A contrivance at present
commonly adopted for burning smoke, is that of Mr. Brunton, namely, a
circular fire-grate, kept turning like a horizontal wheel, on which, by machinery, coal is made to fall in a.gradual manner, so as to be uniformly
spread over it.  The coal falls so gradually, that although there is generally
a little smoke from it, there is never much,-the oxygen which finds entrance through and'around the grate, being always in quantity the same,
and nearly sufficient.  A smoke-consuming fire would be perfect, in which
the fuel were made to burn only at the upper surface of its mass, so that
the pitch and gas disengaged from it, as the heat spread downwards, might
have to pass through the burning coals where fresh air were mixing with
them; and thus the gas and smoke, being the most inflammable parts,
would burn first and be all consumed.  This was the principle proposed
in a fire-place suggested by the author for the great brewery of Mr. MIeux
in his neighbourhood, and tried at the time when attempts were extensively
made to abate the nuisance of smoke in towns. The experiment proved
the theoretical perfection of the method, and that it would produce a saving
of 15 or 20 per cent: on the expenditure of coal; but before a durable grate
of the kind was completed, the Welch stone-coal was introduced, which
has 97 per cent. of pure carbon, and therefore has no pitch to evaporate,
and no smoke,-and it was at once adopted there and in many other places.
A  little common coal is added to it to make it more easily combustible.




FUEL.                              821
Coal in a deep narrow trough, as a b c d, if
lighted at its surface a b, burns with a lofty       Fig. 136.
flame as if it were the wick of a large oillamp; for all the gas given out from the coal
below, as that is gradually heated, passes
through the burning fuel and becomes a flame.
Now, if we suppose several such troughs::::::
placed together with intervals between them,   C
like the fire bars of acommon grate or furnace,  e          U        -
there would be a perfect non-smoking fireplace.  Such was that made on the occasion
mentioned; and although, as a mere experimental apparatus, it was flimsy
and imperfect, it put beyond a doubt the possibility of accomplishing its
object. The reason of the vast saving of fuel by such a grate is, that the
smoke instead of stealing away latent heat-being itself also the most combustible and precious part of the fuel, here gives all its powers and worth to
the purpose of the combustion. The coal rested on moveable bottoms in
the troughs (here shown by dotted lines) and was moved up like the wick
of a lamp by its screw. The bottoms may be lifted in many ways. Or,
without bottoms the coal may be raised in each trough successively by an
iron lever or bar introduced by the fire-man under the coal, fresh coal being
at the same time put into the trough to fill the space under the lever.  The
author believes that this principle of construction will still be extensively
adopted for the Newcastle or flaming coal, —the consequences would be so
important. The principle has been already introduced for common parlour
fires by Mr. Cuttler in his stove, which is merely a common grate, having,
instead of bottom-bars, a deep box to hold the coal for a whole day, with a
moveaible bottom, which lifts the coalup as wanted. From such a fire there
is always ascending a long beautiful flame; and much more heat is given
out than from the same quantity of coal burned in the common way; the
chimney never requires sweeping, for there is absolutely no smoke and therefore no soot.
Smoke is effectually consumed also in a fire-place in which the air feeding
the combustion is caused to pass downwards through the burning fuel, so as
to carry the smoke with it, instead of upwards as usual. This result is attained by having the chimney to communicate with the ash-pit. The chief
objection is, that the fire-bars are quickly destroyed by the intense heat to
which they are exposed; and to obviate this instead of solid bars, tubes
have been used with water passing through them, and admitting the feeding
air only above.
It is evident that if a house or apartment with the air in it, were once
warmed to a certain degree, it would for ever retain its temperature, but for
the escape of heat through the walls and windows, or with the air from
within, whether passing away as necessary ventilation or as waste. A perfect system of heating, therefore, would consist in diminishing as much as
possible these causes of loss, with reference, however, both to the expense
of the means and the salubrity of the dwelling, and in producing and distributing the heat judiciously. It may be asserted that a fourth part of the fuel
generally expanded in English houses, if more skilfully used, would better
secure comfort and health than all that is now expanded. But it does not
accord with the character of this general work to enter into minute detail on
the subject. Remarks were made upon it in the chapter on " Pneumatics,"
21




322                           HEAT.
under the head of "warming and ventilating," and more minute information may be obtained from Mr. Tredgold's work, expressly devoted to it.
The detailed consideration of furnaces, blow-pipes, &c., may appear to
some so closely connected with our present subject as to demand a place
here, but by so treating of them we should be encroaching on the province
of the chemist, &c. We may state generally, that furnaces are merely structures by which coal or other fuel heated to the degree at which it combines
rapidly with the oxygen of the atmospheric air, is placed in circumstances
favourable to the rapid renewal of the air,-and a common blow-pipe is
merely a jet of air thrown from a minute opening into any flame, so as with
great decision to direct the point of the flame upon the body to be heated.
The sand-bath and water-bath of the chemist are merely means of insuring
a more uniform or steady temperature:-a vessel imbedded in sand, so that
heat can reach it only through the sand, cannot be very suddenly heated or
cooled, because sand is a slow conductor; and a vessel immersed in boiling
water, can never have greater heat than 212~, or the boiling heat of water.
For certain purposes, hotter baths, as of high-pressure steam, or of solutions
of certain salts, or of vapour of oil of turpentine, or of boiling whale oil, have
been used. On such subjects, readers may consult works on " chemistry
applied to the arts."
ac Condensation and friction as causes of heat." (Read Analysis, page 256.)
A soft iron-nail laid upon an anvil, and receiving in rapid succession three
or four powerful blows of a hammer becomes hot enough to light a match,
and if longer hammered, will become incandescent or red hot,-partly from
the diminished volume or condensation of the iron, on the principle already
explained, and partly from the percussion or friction, in a way not yet well
understood, but probably electrical
In the familiar case of the mutual percussion of flint and steel, small port
tions of one or both are struck off by the violence of the collision, in a state
of white heat, and the particles of the iron burn in passing through the air;
in a vacuum, the heated particles are equally produced, but are scarcely
visible from this combustion not occurring.  In both cases, they suffice to
inflame gunpowder, or to light tinder.  When the materials are good the
shower or burst of sparks from the sudden blow is most brilliant.
The heat produced by friction alone without perceivable condensation of
the bodies concerned, is exemplified in many facts. Two dry branches of a
tree kept strongly rubbing against each other by the wind, have sometimes
set a wood on fire. Savages light their fires by analogous means. Men warm
their cold hands in winter, by rubbing them against each other, or against
their coat sleeves.  Again, the axle-tree of a heavily laden wagon or other
carriage, if left without oil, is often heated so as to inflame wood near it.
The line attached to a whale-harpoon as it runs over the side of the boat
when the whale dives after the harpoon has entered his flesh, requires that
water be constantly thrown upon it to prevent its setting fire to the boat.
The cable of a ship drawn very rapidly through the hawse-hole by the falling
anchor produces there intense heat and smoke.  When a great ship is
launched from the builder's yard into the deep, and glides along the
sloping beams, a dense smoke usually rises from the points of rubbing
contact.




ANIMAL FUNCTIONS A SOURCE OF HEAT.  323
"The functions of animal life a source of heat."  (Read the Analysis,
p. 256.)
It is one of the remarkable facts in nature, that living animal bodies, and
to a certain degree living vegetables also, have the property of maintaining
in themselves a peculiar temperature, whether surrounded by bodies that are
hotter or colder than they. VCaptain Parry's sailors, during the polar winter,
where they were breathing air that froze mercury, still had in them their
natural warmth of 980 of Fahrenheit: and the inhabitants of India, where
the same thermometer stands sometimes at 115~ in the shade have their
blood at no higher temperature.
It was at one time the favourite explanation of this, that animal heat was
produced in the lungs, during respiration, from the oxygen then admitted.
Thi's oxygen combines with carbon from the blood, and becomes carbonic
acid as in combustion, and it was supposed to give out a portion of its latent
heat, as in actual combustion; which heat, being then spread over the body
by the circulating blood, maintained the temperature.  We now know that
if such a process assist, which it probably does,-for the animal heat has
generally a relation to the quantity of oxygen required in any particular
case, and when an animal being already much heated needs no increase,
very little oxygen disappears, — still much of the effect is dependent on
the influence of the nerves either directly or indirectly, through the vital
functions governed by them. Mr. Brodie, in his important experiments
upon the subject found that, although in animals apparently dead from injury
done to the nervous system, he could artificially continue the action of respiration, with the usual formation of carbonic acid, still the temperature fell
very quickly. Again, the maintenance of low temperature in an animal
immersed in air hotter than itself, is partly attributable to the copious perspiration and evaporation which then take place, and which absorb into the
latent form the excess of heat then existing.  Perspiration, both from the
skin and internal surface of the lungs, occurs generaly in proportion to the
excess of heat.  Dogs and other animals when much heated, as they cannot
throw off or diminish their natural covering, increase the evaporating surface
by protruding a long, humid tongue.
The power in animals of preserving their peculiar temperature has its
limits.  Intense cold coming suddenly upon a man who has not sufficient
protection, first causes a sensation of pain, and then brings on an almost
irresistible sleepiness, which if indulged in proves fatal. Sir Joseph Banks
having gone on shore one day near the cold Cape Horn, and being fatigued,
was so overcome by the feeling mentioned, that he entreated his companions
to let him sleep but for a little while. His prayer, if granted, might have
allowed to come upon him that sleep which ends not-as, under similar
circumstances, it came upon so many thousands of the army which Buonaparte led into Russia, and lost there during the disastrous retreat through
the snows. The celebrated bulletin which allowed that, in one night, when
the thermometer of Reaumur stood at 190 below zero, 30,000 horses died,
declared not the number of human victims-tenderly loved husbands and
brothers, and children of thousands anxiously waiting their return, but doomed
never to see them more. Cold in inferior degrees, and longer continued.
acting on persons imperfectly protected by clothing, &c., induces a variety
of diseases which destroy more slowly; as many of the winter diseases of
England. A great excess of heat, again, may at once excite a fatal appo



324                            HEA T.
plexy, and heat in inferior degrees, but long continued, may cause those
fevers, &c., which prevail in warm climates, and which are so destructive to
strangers.
Each species of animal has a temperature natural to it, and in the diversity
are found creatures fitted to live in all parts of the earth, what is wanting in
internal bodily constitution being found in the admirable covering which has
been provided to protect them —covering which grows from their bodies,
with form of fur or feather, in the exact degree required, and even so as in
the same aiimal to vary with climate and season.  Such covering, however,
has been denied to man; but the denial is not one of unkindness,-it is an
indication of his superior nature and destinies.  God-like reason was bestowed on him, by which he subjects all nature to his use, and he was left
to clothe himself.
The human race is naturally inhabitant of a warm climate, and the para-dise described as Adam's first abode, may be said still to exist over'ast
regions about the equator. There the sun's influence is strong and uniform,
producing a rich and warm garden, in which human beings, however ignorant of the world which they had come to inhabit, would have their necessities at once supplied. The ripe fruit is there always hanging from the
branches; of clothing there is required only what moral feelings may dictate,
or what may be supposed to add grace to the form; and as shelter from the
weather, a few broad leaves spread on connected reeds, complete the Indian
hut. The human family, in multiplying and spreading in all directions from
such a centre, would find to the east and west, only the lengthened paradise,
with slightly varying features of beauty; but to the north and south, the
changes of season, which make the bee of high lattitudes lay up its winter
store of honey, and send migrating birds from country to country in search
of warmth and food, would also rouse man's energies to protect himself.
His faculties of foresight and contrivance would come into play, awakening
industry: and as their fruits, he would soon possess the knowledge and the
arts which secure to him a happy existence in all climates, from the equator
almost to the pole. And it is chiefly because man has learned to produce at
will, and to control, the wonder-working principle of heat, that in the rude
winter, which seems the death of nature, he, and other tro pical animals and
plants which he protects, do not in reality perish-as exemplified when a
Canary bird escapes from its cage, or an infant is exposed among the snowhills. By producing heat from his fire, he then obtains a novel but most
pleasurable sort of existence: and in the nights while the dark and freezing
winds are howling over his roof, he basks in the presence of his mimic sun,
surrounded by his friends and all the delights of society; while in his storerooms, or in those of merchants at his command, he has the treasured delicacies of every season and clime. I-Ie soon becomes aware, too, that the
dreary winter, instead of being a curse, is really in many respects a bleseing,
-by arousing from the apathy to which the eternal serenity of a tropical sky
so much disposes. He sees that in climates where labour and ingenuity
must precede enjoyment, every faculty of mind and body is invigorated; and
that hence the sterner climates produce the perfect man;-that in them the
arts and sciences have reached their present advancement, and the brightest
examples have arisen of intellectual and moral excellence: while from them,
as centres, knowledge and example are spreading over all the earth and
promising soon to render the whole of human kind but one large and happy
family.




LIGHT.                              325
PART IV.
(CONTINUED.)
SECTION II.-ON LIGHT, OR OPTICS.
ANALYSIS OF THE SECTION.
Light is an emanation from the sun and other self-luminous bodies, becoming less intense as it spreads, and which, by falling on. other bodies, and'
being reflected from them to the eye, renders them visible.  Its absence is
called darkness.  It moves with great velocity, and in straight lines
where there is no obstacle —leaving shadows where it cannot fall.  It
passes readily through some bodies-which are therefore called transparent-but when it enters or leaves their surfaces obliquely, it suffers at
them a degree of bending or refraction proportioned to the obliquity.
And a beam of white light thus refracted or bent, does n6t all bend equally,
but is divided or resolved into beams of what are called the elementary
colours, which colours, on being again blended, become the white light as
before.
Transparent bodies, as glass, may be made of such form as, by the powers.
of refraction thence received, to cause all the rays which pass through
them from any given point to bend and meet again in another point
beyond them;-the body, then, because usually in form somewhat resembling a flat bean or lentil, being called a LENS. And when the light thus
proceeding from every point of an object placed before a lens is collected
at corresponding points behind it, a perfect image of the object is there
produced, which may be seen from any situation on a white screen placed
to receive it, or in the air, if,viewed from behind. Now the most important
optical instruments, and even the living eye, are merely various arrangements af parts for producing and examining such images as now described.
When this image is received upol a suitable white surface or screen in a
dark room, the arrangement is called, according to minor circumstances,
a CAMERA OBSCURA, a MAGIC LANTERN, or a SOLAR MICROSCOPE. Andl
the EYE itself is, in fact, but a small camera obscura, enabling the mind
to judge of external objects, by the size, brightness, colour, &c., of the very
minute but smost perfect images or pictures formed at its back part, on the
-smooth screen of nerve called the retina.  The art of painting aims at
producing on a larger scale such a pictui e as is formed on the retina, and
which, when afterwards held before. the eye, and reproducing itself in miniature upon the retina, may excite the same impression at the original
objects. — When the image beyond a lens, formed as above described, is
viewed in the air, by looking at it from behind, that is, from a situation
where the light continuecdfrom it passes, then there is exhibited the arrange.
ment of parts constituting the TELESCOPE or COMMON MICROSCOPE.




826                           LIGHT.
Ligh failing on very smooth or polished surfaces, is reflected so nearly in
the order inz which it falls, as to appear to the eye receiving it as if coming
directly from  the objects originally enmitting it-and such surfaces are
calledl mirrors.  Mirrors may be plane, convex, or concave; and certain
forms will concentrate light, to produce  images by reflection, just as
lenses produce them by refraction; so that there are reflrcting telescopes,
&c., as there are refracting instruments of the same name.  Light, again,
falling on bodies of rougher or irregular surface, or which have other
peculiarities, is so modified as to produce all those phenomena of colour
and varied brightness seen among natural bodies, and giving them their
distinctive characters and beauty.
"Light."  (See the Analysis.)
The phenomena of light and vision have always been held to constitute a
most interesting branch of natural science; whether in regard to the beauty
of light, or its utility. The beauty is seen spread over a varied landscapeamong the beds of the flower-gardens, on the spangled meads, in the plumage
of birds, in the clouds around the rising and setting sun, in the circles of the
rainbow. And the utility may be judged of by the reflection, that if man
had been compelled to supply his wants by groping in utter and unchangeable
darkness, even if originally created with all the knowledge now existing in
the world, he could scarcely have secured his existence for one day. Indeed,
without light, the earth would have been an unfit abode, even for grubs,
generated atnd living always amidst their food. Eternal night would have
been universal death. Light, then, while the beauteous garb of nature,
clothing the garden and the meadow-glowing in the ruby-sparkling in the
diamond, is also the absolutely necessary medium of communication between
living creatures and the universe around them. The rising sun is what converts the wilderness of darkness which night covered, and which, to the
young mind not yet aware of the regularity of nature's changes, is so full of
horror, into a visible and lovely paradise; —no wonder. then, if in early ages
of the world, man has often been seen bending the knee before the glorious
luminary, and worshipping it as the God of Nature.  When a mariner, who
has been toiling in midnight gloom and tempest, at last perceives the dawn
of day, or even the rising of the moon, the waves seem to him less lofty,
the wind is only half as fierce, and hope and gladness beam on him with the
light of heaven. A man, wherever placed in light, receives by the eye from
every object around, nay, from every point in every object and at every
moment of time, a messenger of light to tell him what is there, and in what
condition. Were he omnipresent, or had he the power of flitting from place
to place with the speed of the wind, he could scarcely be more promptly
informed. Then, in many. eases where distance intervenes not, light can
impart at once knowledge, which, by any other conceivable means, could
come only tediously, or not at all. For example, when the illuminated coun.
tenance is revealing the secret workings of the heart, the tongue would in
vain try to speak, even in long phrases, what one smile of friendship or
affection can in an instant convey; —and had there been no light, man never
could have suspected the existence of the miniature worlds of life and activity
which, even in a drop of water, the microscope discovers to him; nor could
he have formed any idea of the admirable structure of many minute objects.




E M A NATION  FROM  TIHE SUN, ETC.                    327
It is light, again, which gives the telegraph, by which men readily converse
from hill to hill, or across an extent of raging sea-and it is light which,
pouring upon the eye through the optic tube, brings intelligence of events
passing in the remotest regions of space.
"Emanation from  the sun," &c.  (See the Analysis, page 325.)
The relation of the sun to light is most strikingly marked in the contrast
between night and day.  In tropical countries where the sun rises and sets
almost perpendicularly, and allows not the long dawn and twilight of temperate latitudes, the change from perfect darkness to the overpowering effulgence of day, and the contrary change, are so sudden as to be most impressive. An eye turned in the morning to the east has scarcely noted a commencing brightness there, when that brightness has already become a glow;
and the clouds floating near so as to meet the upward rays, appear like
masses of gold fleece suspended in the sky: a little after the whole atmosphere is bright, and as the stream of light reaches the lofty mountain-tops,
it makes them shine like burnished pinnacles; then as that stream descends
to lower and lower levels, the inhabitants in succession see the radiant orb
first rising above the horizon like a tip of flame, and soon displaying all its
breadth and glory, too bright for the eye to dwell upon. With evening the
same appearances recur in a reversed order, ending, as in the morning they
began by complete darkness.
Light emanates also from the stars, but they are so distant as in that respect to be of little importance to this earth. And all bodies in combustion
are self-luminious, as exemplified in our common fires and lamps. And
there are still other transient sources in animal and vegetable nature, and
among solar phosphori.
There have been two opinions respecting the nature of light; one, that it
consists of extremely minute particles darting all around from the luminious
body; the other, that the phenomenon is altogether dependent on an undulation among the particles of a very subtile elastic fluid diffused through space
-as sound is dependent on an undulation among air-particles. To admit the
first opinion, the particles of light must be held to be most wonderfully
minute, for a common taper can fill with them during hours a space of four
miles in diameter; and with the extreme velocity of light, if its particle possessed at all the property of matter called inertia, their momentum should be
very remarkable;-yet, even a large sunbeam collected by a burning-glass,
and, with the precautions necessary in the case, thrown upon the scale of a
most delicate balance, has not the slightest effect upon the equilibrium.
Such, and many other facts to be treated of in subsequent parts of this work,
lead to the opinion that there is an undulation of an elastic fluid concerned
in producing the phenomenon of light,-although the fact of light spreading
so nearly in straight lines, as if only the crown of the wave had existence,
instead of being diffused like sound, is an important difference.
" Becoming less intense as it spreads." (See the Analysis, page 325.)
Any emanation from a central point in spreading through water space,
becomes proportionally thinner or less intense. Thus, if a taper be placed
in the centre of a box, each side of which is a foot square, the light falling
on the sides of the box will have a certain intensity there:-if the taper be
then placed in a box with sides of two feet square, there will be only the




328                           LIGHT.
same quantity of light, but it will be spread over four times the surface, (a
square of two feet is made up of four squares of one foot;) and will, therefore,
on any part of that surface, be.only one-fourth part as strong or intense as
in the first box:-and so for any other size of box or space, the intensity
will diminish as the square of the distance increases.
Hence four times as much light and heat fall upon a foot of this earth's
surface as upon a foot of the surface of the planet Mars, which is twice as
distant from the sun:-as four times as much light and heat fall on a man
who is at one yard from the fire, as on another who is distant two yards.
"Falling on other bodies makes them visible."  (Read the Analysis,
page 325.)
If the window-shutter of an apartment be perfectly closed, an eye there
turns upon an absolute blank: it perceives nothing.
If a ray of the sun be then admitted, and made to fall upon any object, that
object becomes bright, and affects the eye as if it were itself luminious.  It
returns a part of the light which falls upon it, and it is visible in all directions,
proving that it scatters the received light all around. This scattered light,
again falling on other objects, and reflected from and among them until absorbed, like echo repeated many times and lost between perpendicular rocks,
makes all of them visible, although in a less degree, and the whole apartment
is said to be lighted. If the sun's ray be made to fall upon a thing which,
from its nature, reflects much of the light, as a sheet of white paper, the
apartment will be well lighted: —if, on the contrary, it be received on black
velvet, which returns hardly any light, the apartment will remain dark;and, again, if received on a polished surface or mirror, which returns nearly
the whole light, but in one direction only, and therefore throws it upon some
other single object, the effect will be according to the nature of that object,
and nearly as if the ray had fallen directly upon it.
Now all bodies on earth, and among these the constituent particles of the
mass of atmosphere surrounding the earth, retain and diffuse among themselves, for a time, the light received directly from the sun, and by so doing,
maintain every where that milder radiance so agreeable to the sight, which
renders objects visible when the sun's direct ray does not fall upon them.
But for this fact indeed, all bodies shadowed from the sun, whether by intervening clouds or by any other more opaque masses on earth, would be
perfectly black or dark; that is, totally invisible. And without an atmosphere, the sun would appear a round luminious mass in a perfectly black sky.
On lofty mountain summits, where half the atmosphere is below the level, the
direct rays of the sun are painfully intense, and the sky is of darkest blue.
A shadow is the name given to the comparative darkness of places or
objects which are prevented by intervening things from receiving the direct
rays of some luminious body shining on the things around. The apparent
darkness of a shadow, however, is not proportioned to its real darkness,
but the intensity of the surrounding lights.  A  landscape may be very
bright, even when the sun is veiled by a cloud, and then little or no shadow
is perceived; but as soon as the cloud passes away, deep shadows are cast
behind or beyond every projecting object. Yet the objects and places then
appearing so dark, are in reality more illuminated than before the shadow
existed, for they are receiving, apd again scattering new light from all the
more intensely illuminated objects around them.  A finger held between a
candle and the wall casts a shadow of a certain intensity; if another candle




LIGHT  MOVES  WtTII GREAT  VELOCITY.    329
be then placed in the same line from the shadow, the shadow will appear doubly
dark, although in fact more light will be reaching it and reaching the eye from
it than before; it will be more dark only by comparison. If the candles be
separated laterally, so as to produce two shadows of the finger, but which
coincide or overlap in one part, that part will be of double darkness, as
compared with the remainder, The most accurate mode of comparing lights
is to place them at such distances from a screen or wall, as to make them at
the same time throw equally dark shadows of the same object; and then
according to the law of decreasing intensity explained above, to calculate the
intensities of the sources of light by the difference of their distances from
the wall. The eye judges very easily of the equal intensity of compared
shadows of the same object.
The real darkness of a shadow, then, depends on the number and nature
of the light reflecting objects around it. Thus shadows are less remarkable
opposite to any white surface, as that of a recently painted wall, than in other
situations. The reason why the moon, when eclipsed, that is, as will be
afterwards explained, when passing behind the earth, or through the shadow
cast by the earth in a direction away from the sun, becomes almost, if not
quite invisible, is that there are no other moons or bodies bearing laterally on
the moon to share their light with it. And the reason why our nights on
earth are darker than the shadows behind a house or rock in the sunshine of
day, is merely that there are not other earths near us to reflect light into the
great night-shadow of the earth, as there are other houses and rocks to illuminate the day-shadow of these. The moon is the only light-reflecting body
which the earth has near it; and we perceive how much less dark the earth's
night-shadow is when the moon is so placed as to bear upon it. The eclipsed
moon, again, is invisible to men on earth, because it receives neither sunshine
nor reflected light from this earth, for the side turned towards us faces the
shadowed part of the earth; but when the moon is near the situation in
which it is called new moon, or between us and the sun, the shaded side of
the moon is thenj in a degree, visible to us because facing the enlightened
side of the earth; the bright crescent, or part of the moon illuminated by
the sun, appearing to embrace the non-illuminated part, and giving occasion
to the popular saying, that the new moon holds the old moon in its arms.
Many persons have doubted whether the light of the moon could be
altogether reflected light of the sun; the mton appearing to them more
luminous than any body on earth merely exposed to the sunshine. Their
error has arisen from contrasting the moon while returning direct sunshine
with the shadows of night on the earth around them. But could they at
night see on a hill near them, a white tower or other object scattering light
as when it receives the rays of the sun, that object being nearer than the
moon, would appear to them almost to be on fire, and much brighter than
the moon. The moon when above the horizon in the day time, is perfectly
visible on earth, and is then throwing towards the earth just as much light
as during the night; but the day moon does not appear more luminous than
any small white cloud: and although visible every day, except near the
change, many persons have passed their lives without ever observing it. The
full moon gives to the earth only about a one-hundred-thousandth part as
much light as the sun.
" Light moves with' great velocity" (See Analysis, page 325.)
The extraordinary precision with which the astronomical skill of modern
days enailes men to foretell the times of remarkable appearances or changes




330                          LIGHT.
among the heavenly bodies has served for the detection of the fact, that
light is not an instantaneous communication between distant objects and the
eye, as was formerly believed, but is a messenger which requires time to
travel: and the rate of travelling has been ascertained.
The eclipses of the satellites or moons of the planet Jupiter had been
carefully observed for some time, and a rule was obtained which foretold the
instants in all future time when the satellites were to glide into the shadow
of the planet and disappear, or when again to immerge into view. Now it
was found, that these appearances took place 161 minutes sooner when Jupiter was near the earth, or on the same side of the sun with the earth than
when it was on the other side, that is to say more distant from the earth
by one diameter on the earth's orbit; and at all intermediate stations, the
difference diminished from the 166 minutes, in exact proportion to the less
distance from the earth. This proves, then, that light takes 16~ minutes
to travel across the earth's orbit, and 8~ minutes for half that distance, or
to come to us from the sun.
The velocity of light, ascertained in this way, is such, that in one second
of time, viz: during a single vibration of a common clock pendulum, it
would go and come from London to Edinburgh 200 times, the distance
between these being 400 miles. This velocity is so surprising that the
philosophic Dr. Hooke, when it was first asserted that light was thus progressive, said he could more easily believe the passage to be absolutely instantaneous, even for any distance, than that there should be a progressive
moment so prodigiously rapid. The truth, however, is now put quite beyond
a doubt by any collateral facts bearing upon it.
As regards all phenomena upon earth, they may be considered as happening at the very instant when the eye perceives them; the difference of time
being too small to be appreciated; for, as shown in the preceding paragraph,
if our sight could reach from London to Edinburgh, we should perceive a
phenomenon there in the four-hundredth part of a second after its occurrence.
It is hence usual and not sensibly incorrect, when we are measuring the
velocity of sound, as when a cannon is fired, by observing the time between
the flash and the report, to suppose that the event takes places at the very
moment when it is perceived by the eye.
In using a telegraph, no sensible time is lost on account of light requiring
time to travel.  A message can be sent from London to Portsmouth in a
minute and a half; and at the same rate a communication might pass to Rome
in about half an hour, to Constantinople in forty minutes, to Calcutta in a
few hours, and so on. A telegraph is any object that can be made to assume different forms or appearances at the will of an attendant, and so that
the changes may be distinguished at a distance. A pole with movable arms
is the common construction, each position of the arms standing for a letter,
or cypher, or word, or sentence, as may be agreed upon. Telegraphic signals
between ships at sea are generally made by a few flags, the meanings of each
being varied by the mast on which it is hoisted and by its combination with
others.
" Light proceeds in straight lines," &c. (Read the Analysis, page 325.)
We have scarcely a clearer notion of a straight line than that received from
the direction in which light moves:-but we can verify a line so obtained by
other means, as by stretching a cord between the two extremes, or by suspending a weight by a cord, and making a moveable solid measure to correspond with the cord, which standard may be used in any other case.




LEAVING SHADOWS WHERE IT CANNOT FALL. 331
We can see through a straight tube, but not through a crooked one. The
vista through a long straight tunnel is striking as an illustration of this fact
and of the diminution of the apparent size of objects as they are more distant.
If a person enter one end of the canal tunnel two miles long, cut through the
chalk-hills near Rochester as part of the canal which joins the Thames and
MIedway rivers, the opening at the distant end is seen as a minute luminous
speck, having the form of a general arch; and a person who has advanced
half way through the tunnel may see the luminous speck at each end, then
appearing a little larger than in the former case.
In taking aim with gun or arrow, we are merely trying to make the projectile go to the desired object nearly by the path along which the light comes
from the object to the eye.
A carpenter looks along the edge of a plank, &c., to see whether it be
straight.
Because light moves in straight lines, if a number of similar objects be
placed in a row from the eye, the nearest one hides the others.  In a wood
or city, a person sees only the trees or houses that are next him.
He who believes that a squinting person can see round a corner, may also
believe that a crooked gun can shoot round a corner.
All astronomical and trigonometrical observations are made on the faith of
this property of light, the observer holding that any object is situated from
him in the direction in which the light comes to him from it. When the
mariner, after watching for hours in cloudy weather has caught a glimpse of
the sun or star through his sextant-glass, he has ascertained his place among
the trackless waves, and boldly advances through the mist of hidden dangers. And the beam darting from the, light-house across the stormy sea,
would be useless if the light moved not in a straight line.
"Leaving shadows where it cannot fall." (See the Analysis, page 325.)
The form of shadows proves that light moves in straight lines, for the outline of the shadow is always correctly that of the object as seen from the
luminous body. If the light bent round the body, this could not be.
The shadow of a face on the wall is a correct profile.
As a wheel presented edgeways. to the eye appears only like a broad line,
becomes oval or round as it is more turned, so a wheel presented edgeways
to the sun or other light casts a linear shadow on the wall behind it, the
shadow becoming oval or round as the position is changed.
A globe, a cylinder, a cone, and a flat circle, will all throw the same round
shadow if held with their axes pointing to the luminous body, and, therefore, by the shadow only, these objects could not be distinguished.
The figure of a rabbit cut in paste-board, will throw the same shadow on
the wall as the animal itself; and, again, that shadow may be well imitated
by a certain position of the two hands joined, as is known to those who find
pleasure in witnessing the surprise and delight of a child who beholds such
a shadow made to mimic the actions of life.
A man under the vertical sun stands upon his little round shadow; but
as the sun declines in the afternoon, the shadow juts out on the opposite side,
and at last may extend across a whole field.
A distant cloud which appears tothe eye of an observer only as a streak
along the sky, may yet be broad enough to shadow a whole region; for
clouds generally form in level strata, and when viewed by a spectator on
earth at a distance are seen nearly edgeways.




332                           LIG HT.
The velocity of the wind may be ascertained by marking the time which
the shadow of a cloud takes to pass over a plain or other space of known
dimension.
A body held between a candle and the wall darkens a portion of the wall,
or casts its shadow there; and the whole space between it and the wall
is a shadowed space, for anything introduced there is as much shadowed as
the portion of the wall. Thus, also, all the heavenly bodies which revolve
about the sun cast a shadow beyond them or away from the sun, as is seen
when one of them, before brightly visible, passes where the shadow of another is. The satellites or moons of Jupiter, when they suddenly disappear
to our glasses, or are eclipsed as we term it, have generally only plunged
into the shadow of the planet, and are not hidden by being then on the
other side of the planet, as many suppose. When our own moon is eclipsed,
that phenomenon so awful in the early ages of the world, she is only passing
through the long shadow which the earth casts beyond it.
When in the case of a luminous centre and a body casting a shadow, the
centre is larger than the body, then the cross section of the shadowed space,
or the shadow as thrown on a plane surface, will be less than the body, and
less, moreover, the farther the surface is from the body, for the shadowed
space terminates in a point. This is true of the shadows of all the planets
and of the earth, because they are less than the sun. On the contrary, if the
light-giving surface is smaller than the opaque body, the shadow will be larger
than the body. The shadow of a hand held between a candle and the wall
is gigantic; and a small pasteboard figure of a man placed near a narrow
centre of light, throws a shadow as big as a real man. The latter fact has
been amusingly illustrated by the art of making phantasmagoric shadows.
When the surface which receives a shadow is not directly exposed to the
light, the shadow may be much larger than the object, even although the
sun himself be throwing the light;-as is seen when a slightly projecting
roof, or a veranda, shadows from the high sun of summer noon the whole
front of a house; or, as is proved by the long evening shadows of all countries, a low wall will shadow from the setting sun a whole field.
"'ight passes readily through some bodies —which are, therefore, called
transparent; but when it enters, or leaves their surfaces obliquely, its
-course is bent." (Read the Analysis, page 325.)
It may well excite the surprise of inquirers that light, of which the constitution is so fine or flimsy, should still be able to.dart readily and in every
direction through great masses of solid matter, but such is the truth. Thick
plates of solid glass, blocks of rock crystal, mountains of ice, &c., are instantly pervaded by the beam of the sun.
What it is in the constitution of one, mass as compared with another, which
fits the one to transmit light, and the other to obstruct it, we cannot clearly
explain, but we perceive that the arrangement2 of the particles has more influence than their peculiar nature. Nothing is more opaque than thck masses
of the metals, but nothing is more transparent than equally thick masses of
the same metals in solution, nor than the glasses of which a metal forms a
large proportion. The thousand salts formed by the union of the metals or
earths with the diluted acids, are all transparent, when in cooling from the
fluid to the solid state, their particles have been allowed to arrange' themselves
according to the laws of their mutual attraction, that is to say, to' form crystals; but the same substances in other states, as when reduced to powder,




REFRACTION'.                              333
are opaque.  Even the pure metals themselves, when reduced to leaves of
great thinness, are transparent, as may be perceived by looking at a lamp
through a fine gold leaf. It is to be remarked, however, that even the most
transparent bodies intercept a considerable part of the light which enters
them: a depth of seven feet of pure water intercepts about one-half, so that
the bottom of the sea is very dark. And of the sun's light, when passing
obliquely through the atmosphere towards the earth, as when the sun has
lately risen or is about to set, only a small part arrives.
Light having once entered a transparent mass of uniform nature passes
forward in it as straightly as in a vacuum; but at the surface, whether on
entering or leaving it, if the pavage be oblique, and if the mass be of a
different density from the transparent medium around it, a very curious and
most important phenomenon occurs, namely, the light suffers a degree of
bending from its antecedent direction, or a refraction, proportioned to the
obliquity.
But for this fact, which to many persons might at first appear a subjectof
regret, as preventing the distinct vision of objects through all transparent
media; light could have been of little utility to man. There could have
been neither converging lenses as now, nor any optical instruments, of which
lenses form a part, as telescopes and microscopes; nor even the eye itself,
which has its crystalline lens.
Light falling from the air directly or perpendicularly upon a surface of water, glass,           Fig. 137.
or any such transparent body, passes through
without suffering the least bending; —a ray
for instance, shot from a to the point c, in the
surface of a piece of glass g h, would reach
directly across to o and b; but if the ray fell'     7'" e  7,
obliquely, as from  d to c, then, instead of
continuing in its first direction to i and, k, it
would at the moment of its entrance be bent       z.
downwards in the path c e, nearer to a line c.
o, called the perpendicular to the surface at  Az.the point of entrance,-and the  moving' i
straightly while in the substance of the glass
it would, when it passed out again at e, in the
opposite surface, he bent just as much as at filst, but in the contrary direction, or away from a similar perpendicular at that surface, viz., into the line
e f, instead of e n. A ray, therefore, passing obliquely through a transparent
body with parallel surfaces, has its course shifted a little to one side of the
original course, but still proceeds in the same direction, or in a line parallel
to the first-as here shown in the line ef, parallel and near to the line i k; if
the surfaces of the body are not parallel, the ray is ultimately bent as will be
explained some pages hence.
The degree of bending or refraction of light in traversing a single transparent surface is measured by comparing the obliquity of its approach to the
surface with the obliquity of its departure after passing; and for this purpose
a line is supposed to be drawn perpendicularly through the surface of the
point where the ray passes (as a b in the above figure drawn through c, where
the ray d c passes) and the relative positions of the ray to this line on both
sides of the surface, are easily ascertained. Thus the line a d, drawn from
any point of the ray before passing to such perpendicular, is a measure of the
original obliquity or angular distance of the ray, and is called the sine of the




334                           LIG HT.
angle of incidence, and the other line o c drawn from a corresponding point
of the ray after passing to the perpendicular, is a measure of the obliquity
after refraction, and is called the sine of the angle of the refraction:-by
comparing these two lines in any case, the problem is solved.
When light passes obliquely from air into water, the refraction or bending
produced is such, that the line a d measuring the obliquity before refraction,
is always longer than the line o e measuring it after refraction, by nearly onethird of the latter, and the refractive power of water is, therefore, signified by
the index 14. or 1,33; in like manner the greater refractive power of common
glass has the index a, of diamond the index 1-, and so on. And it is important to remark, that for the same substance the same relation holds,
whatever the obliquity of the incidence ray may be. If, for instance, where
the obliquity, as measured by its sine, is 40, and the refraction, is half or 20,
then in the same substance an obliquity of 10 will occasion a refraction
of 5, and obliquity of 4 will occasion a refraction of 2, and so on.
As a general rule, the refractive power of transparent substances of media
is proportioned to their densities.  It increases, for instance, through the
list of air, water, salt, glass, &c. But Newton, while engaged in his experiments upon this subject, observed that inflammable bodies had greater refractive powers than others, and he then hazarded the Conjecture, almost of
inspired sagacity, which chemistry has since so remarkably verified, that
diamond and water contained inflammable ingredients.  We now know that
diamond is merely crystallized carbon, and that water consists of hydrogen
or inflammable air and oxygen. Diamond has nearly the greatest lightbending power of any known substances, and hence comes in part its
brilliancy as a jewel.
No good explanation has been given of the singular fact of refraction; but
to facilitate the conception and remembrance of it, we say that it happens
as if it were owing to an attraction between the light and the refracting body
or medium. The light approaching from d to c, for instance (in the last
figure,) may be supposed to be attracted by the solid body below it, so as at
the surface to be bent into the direction c e; and, again, on leaving the body
to be still equally attracted and bent back, so as to take the direction ef,
instead of e  sn; and we see why the attraction and bendingishould be greater,
the greater the obliquity.
Tle following are familiar examples of this bending of light in passing from
one medium to another,
If an empty basin or other vessel b c f e, be in the sun's light, so that the
rays falling within it may reach low on the side as to d, but not to the bottom, then, on filling the vessel with water, the sun will be found to be shining on the bottom or down to e, as well as on
Fig. 138.          the side.  The reason of this phenomenon is,
that water being a denser medium than air,
the light, on entering it at c, is bent towards
the perpendicular (c f,) at the point of inci/~ c /cd   dence, and so reaches the bottom.  Again, if
a coin or metal, were laid on the bottom  of
such a vessel at e, it would not, while the vessel were empty, be seen by an eye at a, but
e, would be visible there immediately on the
vessel being filled with water; —because then,
the light leaving the coin in the direction e c




REFRACTION.                             335
towards the edge of the vessel, would at c, on passing from the water into
air, be bent away from the perpendicular, and instead of going to g, would
reach the eye at a. The coin, moreover, would appear to the eye to be in
the direction of c d, instead of the true direction c e: for the eye not being
able to discover that the light had been bent in its course, would judge the
object to be in the line by which the light came from it.
It is thus because objects at the bottom of water, when viewed obliquely,
do not appear so low as they really are, that a person examining a river or
pond, or any clear water, from its bank, naturally judges its depth to be less
than it is. Many a young life has been sacrificed to this error. A person
looking from a boat directly down upon the objects at the bottom of water,
sees them in their true directions, but even then not in their true distances,
as will be afterwards explained; and if he view them more and more obliquely, the appearance becomes more and more deceiving, until at last it
represents them as at much less than half of their true depth.
The ship in which the author sailed, once in the middle of the China Sea,
where no danger was apprehended, entered by a narrow passage a large
hore-shoe enclosure of coral rocks.  When the looker-out gave the alarm,
the predicament had become truly terrific. On every side, in water most
singularly transparent, the rocks appeared to be almost at the surface of the
water, and the anchor, which in the first moment had been let go to arrest
the ship, appeared to have been dragged to a shallow place. It seemed that
if the ship, then.drawing 24 feet, or the depth of a two-storied house, moved
but a little way in almost any direction, she must inevitably meet her
destruction   On sending boats around to sound and to search, the place of
entrance was again discovered, and was safely traversed a second time as an
outlet from that terrible prison.
On account of this bending of light from objects under water, there is
more difficulty in hitting them with a bullet or spear. The aim by a person
not directly over a fish, must be made at a point apparently below it, otherwise the weapon will miss by flying too high. The spear, sometimes used
in this country for killing salmon, is a common weapon among the islanders
of the Atlantic and Pacific Oceans for killing the albacore; the use of it, like
that of the fly-hook in England, affording to the fishermen, sport as well as
profit.  The author once witnessed at St. Helena, this employment of the
spear.  A small fish previously stunned, that it might not try to escape, was
every minute or two thrown upon the water as a bait, in the sight of perhaps
a hundred great albacores, greedily waiting for it at one side below, and
knowing the danger to which they exposed themselves by darting across to
seize it. Some albacore bold enough, soon made at the mouthful, apparently
with the speed of lightning, but yet with speed which did not save him, for
every now and then the thrown spear met him, and held him writhing there
in a cloud of his death-blood   After a victim so destroyed, the scene of
action was changed.
The bending of light when passing obliquely from water, is also the reason
of the following facts. A straight rod or stick, of which a portion is immersed
in water, appears crooked or broken at the surface of the water, the portion
immersed seeming to be bent upwards. That part of a ship or boat visible
under water, appears much flatter and shallower than it really is. A deepbodied fish seen near the surface of water, appears almost a flat fish. A
round body there appears oval. A gold fish in a vase may appear as two
fishes, being seen as well by light bent through the upper surface of the
water, as by straight rays passing through the side of the glass. To see




336                           L I GHT.
bodies under water, in their true directions and nearly of their true proportions, the eye must view them through a tube, of which the lower end closed
with a plate glass, is held in the water.
As light is bent on entering from air into water, glass, or other substance
denser than air, so it is also bent on coming from void space into the ocean
of our atmosphere. Hence none of the heavenly bodies, except when directly
over our heads, are seen by us in their true situations. They all appear a
little higher than they really are, and more so the nearer they are to the horizon; as when to a spectator at d,
Fig. 139.              suppose on the surface of the earth,
a star really at A appears to be at.....-  ~  a, because its ray, on reaching the.A.    atmosphere at c, is bent downwards.
In astronomical books there is al-.    ways introduced a table of refraction
-------------------''"'  as it-is called, showing what correc/    tion must be made on this account
for different apparent altitudes.
This effect of our atmosphere so
bends the rays of the sun, that we
see him in the morning before he
is really above the horizon, and we
see him in the evening after he is really below it-for the ray coming horizontally from e to d, appears to come from b, although in truth it really
comes from the lower situation B, and is bent into the level line only at e.
Our atmosphere thus, by the bending of light as well as by itself becoming
luminous, lengthens at dawn and twilight the durations of the precious day.
As the atmosphere is denser near the surface of the earth than higher up,
the light is more and more bent as it descends, and hence describes a course
which is sensibly curved, and therefore unlike the course of light in water.
Certain states of the atmosphere, depending chiefly on its humidity and
warmth, change very considerably its ordinary refractive power; hence, in
one state, a certain hill or island may appear low and scarcely rising above
the intervening heights or ocean, while in another state, the same object will
be seen towering above; and from a certain station, a city in a neighbouring
valley may be either entirely visible, or it may show only the tops of its
steeples, as if the bed on which it rested had sunk deeper into the earth.
In days of ignorance and superstition, such appearances occasionally excited
a strange interest.
Owing to the bending of light in passing through the media of different
densities, a beautiful phenomena is often observable in a day of warm sunshine. Black or dark-colored substances, by absorbing much light and
heat from the sun's rays, and warming the air in contact with them, until
it dilates and rises in the surrounding air, as oil rises in water, cause the
light from more distant objects, reaching the eye through the rarefied medium, to be bent a little;-and owing to the heated air rising irregularly
under the influence of the wind and other'causes, these objects acquire the
appearance of having a tremulous or a dancing motion. In a warm, clear
day, the whole landscape at last appears to be thus dancing.. The same phenomenon is to be observed at any time, by looking at an
object beyond the top of a chimney from which hot air is rising. An illicit
distillery has been discovered by the exciseman happening thus to look across




REFRACTION.                               337
a hole uged as the chimney, although charcoal was the fuel, and there was
no vestige of smoke.
This bending of light by the, varying states of the atmosphere renders
precaution necessary in making very nice geometrical observations:-as in
measuring base lines for the construction of maps or charts.
As it is the obliquity with which a ray traverses the surface, which, in any
case of refraction, determines the degree of bending, a body seen through a
medium of irregular surface appears distorted according to the nature of that
surface. It is because the two surfaces of common window-glass are not as
in the case of plate-glass perfect planes, and perfectly parallel to each other,
that objects seen through a common window appear generally more or less
out of a shape; and hence come the elegance and beauty of plate-glass window: and hence the singular distortion of things viewed through that swelling or lump of glass, which appears at the centre of certain very coarse
panes and which remains where the glass-blower's instrument was attached.
The refraction or bending of light is interestingly. exemplified in the effect
of the glass called a prism, viz., a
wedge or three-sided rod of glass                 Fig. 140.
such as that of which the end is   d7 
here represented at b c.  A  ray
from a falling on the surface at b is
bent towards the internal perpendicular, and therefore reaches c, but
on escaping again at c, it is bent
away from the external perpendicular and thus with its original deviation doubled, goes on to cd.
The.law of lights bending, according to the obliquity
with which it traverses the surfaces of a transparent body,  Fig. 141.
is well elucidated by the effect of what is called a multiplying glass; that is to say, a piece of glass like a b c e,
having many distinct faces cut upon it at angles with each
other.  If a small object, a coloured bead for. instance, be
placed at d, an eye at e will see as many beads as there
are distinct surfaces or faces at the glass; for first, the
ray d a, passing perpendicularly, and therefore straight
through, will form an image as if no glass intervened;
then, the rays from d to the surface b, will be bent by the
oblique surface, and will show the object as if it were in
the direction e b; and the light falling on the still more
oblique surface, c, will be still more bent, and will reach
the eye in the direction c e, exhibiting a similar object
also in that direction-and so of all the other surfaces.
If the eye were at d, and the object at e, the result would
still be the same. A plate of glass roughened, or cut into
cross furrows, becomes a very good screen or windowblind, by disturbing the passage of light through it so that 
objects beyond it are not distinguishable.
" And a beam of white light thus made to bend, is resolved into beams of the
various primor/  colours; which beams, however, on being again blended,
become white light as before." (Read the Analysis, page 325.)
The most extraordinary fact connected with the bending of light is that a
pure ray of white light from the sun admitted into a darkened room by a hole
22




338                           LIGHT.
in the window-shutter, and- made to
Fig. 142.              bend by passing through transparent
surfaces which it meets very obliquely (as the ray a, admitted and made
d                              to bend by passing through the prism
a    of glass b c, to fall upon the wall at
d,) instead of bending altogether and
appearing still as the same white
ray, is divided into several rays,
which falling on the white wall, ate seen to be of different most vivid
colours. The original white ray is said thus to be analyzed, or divided into
its elements.
This solar spectrum, as it is called, formed upon the wall, consists, when
the light is admitted by a narrow horizontal slit, of four coloured patches
corresponding to the slit, and appearing in the order, from the bottom of red,
green, blue, and violet.  If the slit be then made a little wider, the patches
at their edges overlap each other, and produce by the mixture of their elementary colours, certain new tints. Then the spectrum consists of the seven
colours commonly enumerated and seen in the rainbow, viz., red, orange,
yellow, green, blue, indigo, and violet. —Had red, yellow, blue, and violet
been the four colours obtained in the first experiment, the occurrence of the
others, viz., of the orange, from the mixing edges of the red and yellow-of
the green, from the mixture of the yellow and blue,-and of the indigo,
from the mixture of blue and violet, would have been anticipated. But the
facts of the case not being such, we see that they are not yet well understood. When Newton first made known the phenomenon of the manycoloured spectrum, and the extraordinary conclusions to which it led, he excited universal astonishment; for the common idea of purity, the most unmixed was that of white light. In farther corroboration of the notion of
the compound nature of light, he mentioned, that if the colours which appear on the spectrum be painted separately around the rim of a wheel, and
the wheel be then turned rapidly, the individual colours cease to be distinguished and a white band only appears where they are whirling; also, that
if the rays of the spectrum, produced by a prism, be again gathered together
by a lens, they reproduce white light. The red is the kind of light which
is least bent in refraction, and the violet that which is most bent. It was at
one time said, as an explanation, that the differently coloured particles in
light had different degrees of gravity or inertia, and were therefore, not all
equally bent. It is farther remarkable, with respect to the solar spectrum,
that much of the heat in the ray is still less refracted than even the red light,
for a thermometer held below the red light rises higher than in any part of
the visible spectrum;-and there is an influence or something in the beam
more refrangible than even the violet rays, and capable of producing powerful chemical and magnetical effects. The different spots of colour in the
spectrum are not all the same size, and there is a difference in this respect
according to the refracting substance.
All transparent substances in bending light produce more or less of the
separation of colour; but it is an important fact, that the quality of merely
bending a beam, or of refraction, and that of dividing it into coloured beams,
or of dispersion, are distinct qualities, and not having the same proportions
to each other in different substances.  Newton, from not discovering this,
concluded that a perfect telescope of refraction could neverbe made; he supposed that the bent light would always become coloured, and so render the




REFRACTION BY LENSES.                          339
objects indistinct. We now know, however, that by combining two or more
media, we may obtain bending of light without dispersion,-thus, by opposing a glass which bends five degrees and disperses one degree, to another
glass which bends three degrees and disperses one, the opposing dispersions
will just counterbalance or neutralize each other, while the two degrees of
excess of bending will remain to be applied to use.
The diversified colours of the substances around us depend merely upon
their fitness, from texture or other cause, to reflect or transmit other modifications of common light, and the colour is not a part or property of the
body itself. We shall soon find that the vivid -colours of the iainbow are
merely the white light of the sun, reflected to us after being bent and
modified by the colourless drops of falling rain; and that the sparkling with
appearance of rubies and emeralds, which we see in cut-glass lustre, is a
phenomenon of the same kind:-and that by scratching the surface of a
piece of metal so as to have a given number of lines in a given space, we
can cause the same substance to appear of any colour we please.
"Transparent bodies, as glass, may be made of such form as to cause all
the rays of light which pass through them from any one point, to bend so
as to 7neet again in another corresponding.point beyond them,-the body
itself, from the required form generally resembling that of a bean or
lentil, being then called a LENS." (Read the Analysis, page 325.)
The innumerable rays of light (of which five only are here represented,)
issuing from any point as c, towards any surface in the situation a b, are said
to form a cone or pencil of diverging light. Now it is evident that to make
Fig. 143.
—.-".....  -
all such rays converge or meet again in one place as f. beyond a transparent
body place at a b, it would be necessary, while the middle ray or axis of
the pencil c d did not bend at all, for the others to be bent more and more, in
proportion as they fell upon. the body farther and farther from the centre d.
Recollecting, then the law of refraction, that light entering from air through
the surface of any denser medium, as glass, is bent there towards the perpendicular at the internal surface, in proportion to the obliquity of incidence,
and on leaving the opposite surface, is correspondingly bent away from its
external perpendicular, (see the case of the prism at p. 337,) we see that if
a piece of glass were placed at a b, of such form that the rays falling upon
it from c should meet and leave its surfaces with greater and greater obliquity
in some regular proportion, as the points of incidence were more distant from




340                           LIGHT.
the centre d, the purpose would be obtained. And we have the satisfaction
of knowing that a glass, of which the surface is ground-which it easily may
be-to have a regular convexity or bulging, as if it were a portion cut off
from the surface of the globe, can be shown to answer very correctly the required condition. Such a glass similarly ground on both sides, is here represented edgeways between a and b, where the ray c d falling on its middle, or
perpendicularly, and similarly leaving it, is seen going straight through to f,
but the ray c e meeting the surface with a certain degree of obliquity is bent
down a little, first on entering the surface at e, and then as much more on
leaving the bpposite surface with equal obliquity, and so arrives atf; then
the ray c a, for corresponding reasons is still more bent, and equally arrives
at f;-and the case would be similar of any other rays that might be examined.  The pointf is usually called a focus (meaning a fire-place,) because
when the light of the sun is thus gathered, the heat concentrated with it is
powerful enough to make combustibles inflame.-W- e have here to remark
farther, that in accordance both with calculation and experiment, the direction
in which a pencil of rays falls upon a lens does n3t affect the result of the
covergence to a focus, only the focus is always in the direction of the central ray of the pencil or beam; it will be at p, for instance, for light issuing
from o, and at z for light issuing from x.
The lens represented at a b above or in the annexed diagram, at fig. 1,
having both sides convex, is called a double convex lens. A glass convex
only on one side, and plane or fiat on the other, as shown at fig 2, would
Fig. 144.
Fig. 1    2       3      4       5    6
as effectually gather the rays, but with half the power, and the point of
meeting or focus would be therefore proportionably more distant.  Such a
glass is called a placno-convex lens.  Then the gathering or converging
power of any glass, whether doubly or singly. convex, is in proportion to
the degree of its convexity or bulging of surfaces, for the less it bulges,
the more nearly does it approach to a plane glass, and the more it bulges,
the more obliquely will the rays, at any distance from the centre, fall upon
its surface, and the sooner, therefore, in consequence of their being more
bent, will they all meet the axis ray; hence, fig. 1 would converge much
more quickly than fig. 3, which represents nearly a common spectacle glass;
and a very minute globe is the form most powerfully converging of all.
The surfaces of fig. 1 are proportions of a smali globe; those of fig. 3, are
smaller portions, but of a globe much larger. Concave lens as-fig. 4,
a double concave, and at fig. 5, a plano-concave lens, in obedience to the
same law of refraction, spread rays, or bend them away from the axis of
the pencil, in the same degree that similarly convex lenses gather them.
A concave lens, therefore, receiving the converging pencil of rays from a
convex lens, might restore them to their former direction. Very useful
purposes, as will be afterwards explained, are served in optics, by certain




CAMERA OBSCURA.                            341
combinations of differently formed lenses. A lens may be convex on one
side and concave on the other, as at fig. 6, called a meniscus lens, because
it resembles the crescent moon, and its effect; will be according to the curve
which predominates.
A person collecting the case of the " multiplying glass," described a few
pages back, might say,-but is not a convex lens merely a multiplying glass
of a much greater number of faces, and if so, why instead of one image,
does it not make thousands?  The answer is, that the multiplying glass, by
every face, bends a set of rays, capable of forming a distinct and complete
image; but the lens has no surface large enough to bend more than a
single ray and it concentrates all the single rays into one place, to form
there one image of great vividness and beauty.
" And when the light proceeding from every point of an object placed before
a lens is collected in corresponding points behind it, a perfict image of
the object is there produced.  When, the image is received upon a suitable
white surface in a dark place, the arrangement is called, according to
minor circumstances, the CAMERA OBSCURA, SOLAR MICROSCOPE or MAGIC
LANTERN."  (Read the Analysis, page 326.)
Words are wanting to express the admirable consequences to man of the
curious property of a lens that it can bring together to focal points behind
all the rays of light which traverse it from any points-of an object placed
before it.  The following instance will lead to the understanding of others.
If a lens as a, be placed so as to fill
up an opening made in the window-                Fig. 145.
shutter of a darkened room, then, from
anydbject before that opening —as the,
cross here presented, all the light t...,a./
which each point emits towards the..
lens will be concentrated or gathered- 
together in a corresponding focal point
behind the lens or within the room
and if a sheet of paper be held there at the distance of the focal points, a
beautiful image of the object will be seen upon the paper.
In these few words, we have described the interesting contrivance called
the camera obscura or dark chamber; and when a glass is chosen of
proper size and focal distance, and a screen or the wall of the chamber (if at
the required distance,) is properly prepared to receive the light, the most enchanting portraiture is instantly produced of the whole scene which the window commands. With what rapture does the school-boy first view this
lovely picture drawn by nature's own pencil, and with colors taken directly
from the sun's bright ray —with what rapture, as his eyes search over it,
does he recognize, perhaps, his playmates there, and the river in which he
bathes, and where he sails his boat, and the wood in whose solitudes he
loves to wander, and the mountain heights which he climbs to meet the fresh
breeze, and at a distance from the world, to allow his young fancy to work,
beginning to shoot far into time and space. The great peculiarity of such
a picture is, that it does not, like others, portray still-nature, but every thing
with appropriate motion or changes: the playmates are all in action: the
leafy trees wave in the wind, the clouds sail along, the sun may rise or may
set, and even the lightning's gleam may dart across: or, again commenced
enterprize may be brought to a close; the traveller may climb the distant




342                            LIG HT.
hill and disappear; the fisherman may draw his net and secure his prize;
the contested race may be won or lost. A Maylan chief in the island of
Sumatra, was so surprised and pleased by a small portable camera obscura
which the author happened to have among his apparatus, that he seemed
disposed to give for it almost any thing he possessed.
It appears in the last diagram that the image formed beyond a lens by the
gathered light, is in a contrary position to the object itself,-that is, inverted,-because the light from the top of the object darts through the opening
or glass in a descending direction, and that from the bottom rises to the
opening, and in the same direction passes beyond it. It is usual, therefore
in a camera obscura, to place a small mirror immediately behind the lens,
so as to throw all the light which enters downwards, to a whitened table,
upon which the picture may be conveniently contemplated.
The camera obscura often gives very useful assistance to young painters,
by enabling them to trace correctly the outlines of the objects placed before
it, and also to study effects of light, shade and colour, more profitably than
they at first can, by looking at the objects themselves. The laws of perspective are most intelligibly illustrated in this most true picture
An effect, approaching in a degree to that of the complete camera obscura
now described, is produced by merely making a small hole in the shutter of
a dark room, and letting the light which enters by it fall on any white
surface beyond. The whole landscape is then dimly portrayed upon the
surface. Barry, the painter, while lying on a sick bed, mistook such a
scene appearing on the ceiling of his room for a supernatural vision.  If a
cross be held before the opening as in the last figure, it is evident that from
every point of the cross light will enter by the opening, and will fall on
corresponding parts of a sheet of paper held behind, —but as the light from
each point is not a single ray, but a spreading pencil or cone of light, it will
fall on the paper, not on one point, but o- a surface at least as large as the
opening, and thus the light from adjoining points will mix-at the edges, and
will render the images misty and indistinct, somewhat like those on the
back of tapestry. If the opening be very small, the picture will be well
defined, but very feebly illuminated; and if the opening be of considerable
size, the mixing of the pencils will be so great as to leave no particular
object distinguishable. But, in the latter case, and however large the
opening be, if a lens be introduced, it will converge. every pencil of light to
an exact point, and the picture will instantly be rendered perfectly clear.
A lens is never held up in the light without forming beyond it pictures such
as now described, of every visible object about it, and the pictures are not
seen, only because there are no screens placed to receive them, and because
they are so numerous as to confuse one another,-in other words, because
they are not admitted singly into a dark chamber.
The distance from a lens at which an image is formed or the rays of the
light meet, depends first upon the refractive or bending power of the lens,
and therefore, on its form and on the nature of its substance; and, secondly,
upon the direction of the rays of light when they reach the lens, as to whether
they are divergent, parallel or convergent. We have already explained that
glass refracts about twice as much as water, and that diamond refracts about
twice as much as glass: and we have considered the effect of different degrees
of convexity in lenses-arising equally whether the lens be of water
enclosed between glasses like watch-glasses, or of solid glass, or of rockcrystal or of diamond itself. We now proceed to consider the joint effect
of the refractive power, and of the direction of the incident rays.




CAMERA  OBSCURA.                           343
Rays falling from a on a comparatively flat or weak lens at L, might meet
Fig. 146.
only at d, or even farther off: while, with a stronger or more convex lens,
thcy might meet at c or at b; a lens weaker still might only destroy the divergence of the rays, without being able to give them any convergence or to bend
them enough to bring them to a point at all-and then they would proceed all
parallel to each other, as seen at e and f;-and if the lens were yet weaker,
it might only destroy a part of the divergence, causing the rays from a to go
to g and h, after passing through, instead of to iand k, in their original direction.
In an analogous manner, light coming to the lens in contrary directions
from b c d, &c., might, according to the strength of the lens, be all made
to come to a focus at a or at 1, or in some more distant point; or the rays
might become parallel, as m and n, and, therefore, never come to a focus,
or they might remain divergent.
It may be observed in the figure above, that the farther an object is from
the lens, the less divergent are the rays which fall from it upon the lens;
or the more nearly do they approach to being parallel. From b there is
much divergence in the exterior rays, from c less, from d less still, and rays
from a great distance, as those represented by e andf, appear quite parallel.
If the distance of the radiant point be very great, they really are so nearly
parallel, that a very nice test is required to detect thq non-accordance. Rays
for instance, coming from the earth to the sun, do not diverge the millionth
of an inch in a thousand miles. Hence, where we wish to make experiments
with parallel rays, we take those of the sun.
Any two points so situated on the opposite sides of a lens, as that when
either becomes the radiant point of light, the other is the focus of such light,
are called conjugate foci. An object and the image of it formed by a lens,
must always be in conjugate foci, and as the one is nearer the lens, the other
will be in a certain proportion more distant.
What is called the principal focus of a lens, and by the distance of which
from the glass we compare or classify lenses among themselves, is the point
at which the sun's rays, or any parallel rays, are made by it to meet; and
thus, by holding the glass in the sun, and noting at what distance behind it
the little luminous spot or image of the sun is formed, we can at once ascertain the focus of a glass-as at a for the rays e and f
It is a remarkable coincidence that the bending power of the common glass
used for lenses should be such, that the focus of a double lens is just where
the centre of the sphere would be, of which the surface of the lens is a portion. This gives us another fact with whith to associate the recollection
that the focus is nearer as the convexity of the lens is greater, that is to say,
as the surface is a portion of a smaller sphere,  And such being the law, it
may be proved by calculation as well as by the fact, that if a candle be held
in relation to a lens at twice the principal focal distance, suppose at c for a




344                           LIGHT.
lens with the focus at a, the image of the candle will be formed at 1 just as
far on the other side. Thus, then, by trying with the lens until the image
of a candle is formed at the same distance from it as the object is, we have
a second mode of ascertaining the focal distance of a lens. Other kinds of
glass and other substances refract with different power; but the facts now
stated should be retained in the memory as standards of comparison.
Because the focal point of light passing through a lens is at the same distance from the centre of the lens, in whatever direction the light passes
through, a surface placed to receive the image of any broad object should
really be concave, that is to say, all parts of it should be at the same distance
from the centre of the lens, otherwise the image will be more perfect either
at its middle than towards its edges, or vice versa-but it is not found necessary to attend to this in common practice, where the object and its image
are not of great extent.
The size of an image formed behind a lens is always proportioned to the
distance of the image from the lens, and the image is much larger or smaller
than the object as it is farther from or nearer to the lens than the object. This
will be evident from considering the anFig. 147.            nexed figure c represents the place of a
lens, and the lens, according to its power,
will form an image of the cross a b, in some
_~h            _-             situation, as at d, e, g, &c.  Now whenever the image is formed and by whatever
e3d                        lens, one end of it must be in contact with
the line a g, and the other end with the
line b A; and as these lines cross each
other at c, and widen regularly afterwards,
a line adjoining them (and the image is such a line,) must always be shorter
the nearer it is to c, that is to say, shorter in proportion to the converging
power of the lens.
Many persons may not have reflected, that the luminous circle called the
focus of a burning glass, is really but the image or picture of the sun formed
by the glass or lens. The intensity of the heat and of the light is of course
in proportion as the image is smaller than the glass which forms it, and the
nearer that the image is formed to the lens, or the more powerfully convergent
that the lens is, the smaller will the image be. Mr. Parker's famous burning
lens, which cost ~700, and is now the property of the Emperor of China,
was three feet in diameter, and the diameter of the sun's image formed by it
was one inch: it concentrated the light and heat, therefore, about 1,300 times.
To render the effect still more powerful, a smaller lens was placed behind
the larger, farther reducing the size of the image to one-sixth. Very surprising effects were produced by this lens, in the melting of metals, inflaming
of combustibles, &c. The size of burning lenses, until lately, was limited
by the difficulty of obtaining the great pieces of glass required to form them:
but they are now built up of many pieces suitably united together.  Some
large lenses have been made of water, that is, of water enclosed between
menicus glasses, like watch-glasses. A common goblet of water, or a vase
holding gold-fishes, has in some cases acted as a burning glass, setting fire to
the curtains, near which it had been left in the sunshine.
And the nearer that an object is brought to a lens, the more distant, and
therefore the larger will its image be; for, as the rays falling upon a lens are
divergent in proportion to the nearness of the object, and therefore with the
same power of lens, must meet farther behind (as seen in the figure at page




MAGI.C  LANTERN.                           345
343,) then the axis of the rays, as the lines c a and c b in the last figure,
will have separated far before the rays meet, and will have made the image
proportionally larger. If we suppose little d in the same diagram to be the
object, its image would be a b. Thle sun is exactly as much larger than his
image formed by a burning-glass, as he is more dstant.from it than the
image; and if we had a screen of sufficient size hung up in a distant space,
a very bright object of a quarter of an inch in diameter might be made by a
lens to form an image as broad as the sun.
From all these considerations, we see that, in a camera obscura, the screen
should be from the lens, at the distance of its principal focus for distant objects, and a little farther than this for near objects Accordingly the lens is
generally fixed in a sliding piece, which allows the distance from the screen
to be adjusted to circumstances.  If the representation be desired large, the
lens must be of a long focus; if small the lens must be of a short focus.
Again, when the reversed use of the lens, a small object as d is to be magnified to such a size as a b, then the object must be placed a little beyond
the focus of the glass; for if placed nearer, the pencils of rays from it would
never be gathered to focal points at all, and no image would be formed at
any distance.
When, as alluded to in the last sentence, a small object is placed very near
a lens, and the image of it is thrown upon the wall of a dark room, perhaps
a hundred times farther from the lens than the object is, the image is a greatly
magnified representation of the object, viz., it is a hundred times longer and
a hundred times broader, and therefore has ten thousand times as niuch surface as the object; but if in this experiment the object be illuminated only
in an ordinary degree, the light from it is so scattered as not to suffice for
distinct division.  Hence, to attain fully in this manner the purpose of a
microscope, avery strong light, concentrated by a suitable mirror or glass,
must be directed upon the object.  When the light of the sun is used in
-such a case, the complete apparatus is called a solar microscope, and serves
beautifully to display the structure of any minute objects.  When artificial
light is used, as of a lamp, the apparatus is called the lucernal microscope
or magic lantern.
A good solar microscope becomes one of the most interesting presents
which science has made to man, for aiding him in his researches into the secrets
of nature. With the late improvements in the construction of lenses by which
the dispersion of light or the rainbow-fringe, is prevented, (as will be explained under the head of Telescopes,) objects may be magnified two or
three hundred thousand times, and still be so luminous as to be beautifully.distinct; thus a cheese-mite will appear of the dimensions of a hog, and creatures altogether invisible to the naked eye. or perceived by it only as minute
white points are discovered to be animated beings, having the perfect proportions, and often the beauty of larger animals, and endowed with similar
appetites, passions, and apparent ingenuity, but with an activity far surpassing that met with in the more bulky creation. A judicious selection of objects
for the solar microscope is calculated exceedingly to surprise the mind on its
first attending to them, and to fill it with high conceptions of the infinity of
creation. With the common microscope only one person at a time can feast
his wonder; but with a solar, a whole roonfull of company may at once contemplate the same objects and witness the same actions, and thus have their
admiration increased by the consciousness of sympathy.
The magic lantern, we have said, consists of a powerful lens, with objects,
highly illuminated by lamp-light, placed so near it that their images are




346                           LIGHT.
formed far off, and are therefore proportionally larger. For the magic lantern the objects are generally paintings made on thin plates of glass with
transparent colours; and each plate is formed to slide through a slit or passage behind the lens. The lens itself, or what may be called half of it, (for
there are often two letes joined to give greater power,) is moveable with
the tube which is seen projecting from the lantern, so that its distance from
the object may be varied, and thus a corresponding approach to or receding
from the screen may be allowed, which will produce an increase or lessening
of the magnitude of the visible picture on the wall.
Some public lectures on astronomy and other branches of natural history,
have had the drawings and paintings required for the elucidation of their
subjects made in miniature upon glass, to be magnified afterwards to the
degree desired, and shown upon any part of the lecture-room by the magic
lantern.
A thick fog or smoke at night will sometimes reflect the images of a
magic lantern so as to make them distinctly visible; and there are several
cases on record, where persons, wickedly ingenious in this way, have terrified ignorant individuals almost to death, by throwing spectres from a concealed lantern. Some years ago a sentinel in St. James' Park was thus
persuaded that he had seen supernatural beings near him among the trees.
A very charming illusion is produced by a magic lantern manceuvred on
one side of a thin screen, while the spectators not aware of the existence of
a screen, are sitting on the other side.  The image-let us suppose it that
of a genius flying in the air —may be first thrown upon the screen from the
lantern while very near, and then it will be small, and, if desired, exceedingly bright because the light is much concentrated.  If the exhibitor then
gradually recede from the screen, adjusting at the same time the distance of
the lens fromn the picture, the image will become progressively larger, and
to the spectators will appear to be soaring and approaching, until at last the
expanded wings and limbs seem hovering almost over their heads. An endless variety of most ingenious and beautiful exhibitions of this kind have
been made under the name of phantasmagoria or raising of spectres.
" The EYE itself is, in fact, but a small camera obscura."  (Read the
Analysis, page 325.)
Who could at first believe that in describing the camera obscura, as we
have now done, we had in reality been describing only a large model of that
most interesting of the objects of creation, the living eye itself, the inlet of
man's knowledge, —or what may be called the visible dwelling of the soulthat from which the fire of passion darts, through which the languor of exhaustion is perceived, and. in which life and thought seem concentrated! Yet
the eye is nothing but a simple camera obscura: formed of the parts described above as essential to the camera obscura: but in its simplicity so
perfect that they who delight to find around them tangible evidences of the
existence of an all-wise and good Creator, point to this in the midst of thousands, as one of the most undeniable and triumphant proofs.  We shall now
describe the eye and its actions: and keeping present to us the idea of the
camera obscura, as already treated of, we shall find that the use of the various
parts will be declared by merely enumerating them. This paragraph should
be perused while the reader has the opportunity of observing either his own
eye reflected.in a mirror, or the eye of some companion near him.
The human eye, then is a globular chamber of the size of a large walnut,
having for its outer wall a very tough membrane called, from its hardness,




THE  EYE-STRUCTURE.                            347
the sclerotic coat, in the front of which there is one round opening or window, named, because of its horny texture; the cornea. The chamber is
lined with a finer membrane or web, the choroid (having relation to colour,)
which, to insure the internal darkness of the place, is covered with a black
paint, the pigmentum nigrum.  This lining is bordered at the edge of the
round window by a folded drapery, the ciliaryprocesses, hidden from without by being behind the curious contractile window-curtain the iris, (so named
for its rainbow variety of colour in different persons,) through the central
opening of which, called the pupil, the light enters. Immediately behind
the pupil is suspended, by attachments among the ciliary processes, the crystalline lens, a double convex, most transparent body of considerable hardness, which so influences the light passing through it from external objects,
as to form perfect images of these objects, in the way already described, on
the back wall of the eye, over which the optic nerve, there called the retina, is
spread as a second lining.  The eye is maintained in its globular condition
by a watery liquid which distends its external coverings, and which, in the
space before the lens, or the anterior chamber of the eye, being perfectly
limpid, is called the aquous humour, and in the remainder or larger posterior chamber, being enclosed in a pellucid spongy structure, so as to acquire
somewhat of the appearance of melted glass, is called the vitreous humour.
The annexed figure represents an eye of the common dimensions, supposed
to be cut through its middle, from above downwards, so as to show the edges
of the coats, &c.  C is the outer or sclerotic coat, known popularly, where
most exposed in front, as the white of the eye.  A is the transparent cornea
joined to the edge of the round opening of the sclerotic; it is more bulging
than the sclerotic, or forms a portion of a smaller sphere than the general
eyeball, so that, while it may be truly called a bow-window, it, or rather the
convex surface of its contained water, is also a powerful lens for acting on the
pencils of entering light. At B, the similarity all round the edge of the cornea, is attached to the window-curtain or iris, shown here edgeways immersed
in the aqueous humour, and hanging inwards from above and below towards
its central opening or pupil, through which the rays of light are passing to
the lens. The iris has in its structure two sets of fibres, the circular and the
radiating, which cross and act in opposition to each other; —when the circular
fibres contract, the pupil is lessened, when the radiating contract, it is enlarged; and the changes happen according to the intensity of light and the
state of sensibility of the retina, as may at any time be proved by closing the
eyelids for a moment to make the pupil dilate, and then opening them towards a strong light, to make it contract. Behind the pupil is seen the lens D
Fig. 148.
C
A~~~~~~~~~~~~~~~~~~~~~~.o-.




838                            LIGHT.
with its circumference attached to the ciliary processes E: it is more convex
behind than before. The disease of the eye called cataract (from the Greek
word implying obstruction,) is the circumstance of the lens become opaque,
and the cure is to extract the lens entirely, or to depress it to the bottom of
the eye, and then to substitute for it externally a powerful artifical lens or
spectacle-glass. The three lines marking here the boundary of the eye stand
for its three coats as they have been called, the strong sclerotic, and the double
lining of the choroi(d and retina. The figure of a cross is represented upon
the retina as formed by the light entering from the cross without (which cross
has to appear here small and near, although supposed to be large and distant.) The image of the cross is inverted, as explained for the camera obscura: but we shall learn below that the perception of an object may be
equally distinct in whatever position the image fall on the retina. It has been
explained above, that a lens can form a perfect image of considerable extent only on a concave surface,-and the retina is such a surface. The
present diagram further explains what is meant by the anterior and postes-ior chambers of the eye, namely, the compartments which are before and
behind the crystalline lens D.
The nature of the eye as a camera obscura is beautifully exhibited by
taking the eye of a recently killed bullock, and after carefully cutting away
the back part of the two outer coats, by going with it to a 4ark place and
directing the pupil towards any brightly illuminated objects; there may
then be seen though the semi-transparent retina, left as a screen at the
back of the eye, a minute but perfect picture of all such objects-a picture,
therefore, formed on the back of the little apartment or camera obscura by
the agency of the convex cornea and lens in front. The picture is inverted,
for reasons explained above.
Understanding from all this, that when a man is said to be looking at an
object, his mind is in truth only taking cognizance of the picture or impression made on his retina, it excites admiration in us to think of the exquisite
delicacy of texture and of sensibility which the retina must possess, that there
may be the perfect perception which really occurs of even the separate parts
of the minute images there formed. A whole printed sheet of newspaper,
for instance, may be portrayed on the retina on less space than the surface
of a finger-nail, and yet not only shall every word and letter be separately
perceivable, but in the centre of the picture at least, even an imperfection of
a single letter. Or, more wonderful still, when at night an eye is turned up
to the blue vault of heaven, there is portrayed on the little concave of the
retina the boundless concave of the sky, with every object in its just proportions. There a moon in beautiful miniature may be sailing among whiteedged clouds, and surrounded by a thousand twinkling stars, all in just proportion, so that to an animalcule within and near the pupil, the retina might
appear another starry firmanent decked in its glory. If the image in the
human eye be thus minute, what must they be in the little eye of a wren, or
of other animals smaller still! How wonderful are the works of nature!
Because the images formed on the retina are always inverted as respects
the true position of the objects producing them-just as happens in a simple
camera obscura-persons have wondered that things should appear upright,
or in their true situations.  The explanation is not difficult.  It is known
that a man with wry neck judges as correctly of thie position of the objects
around him as any other person-never deeming them to be inclined or
crooked, because their images are inclined in relation to the natural perpendicularity of his retina; and that a bed-ridden person, obliged to keep his head




THE  EYE-DOUBLE  SIGHT.                       349
upon his pillow, soon acquires the faculty of the person with wry neck; and
that an. affected girl inclining her head while trying her various attitudes,
learns from much practice, to judge of the manceuvres of a beau as conveniently in that way as in any other; and that boys who at play bend themselves
down to look backwards through their legs, although a little puzzled at first,
because the usual position of the images on the retina is reversed, soon see
as well in that way as in any other. It appears, therefore, that while the
mind studies the form, colour, &c., of external objects in their images projected on the retina, it judges of their position, not by the accidental position
of the images on the retina, but by the direction in which the light comes*
from the object towards the eye —no more deeming an object to be placed
low because its image is low in the eye, than a man in a room into which
a sunbeam enters by a hole in the window-shutter, deems the sun low because its image is on the floor. A candle carried past a key-hole, throws its
light on the opposite wall, so as to cause the luminous spot there to move in
a direction the opposite of that in which the candle is carried; but a child is
very young, indeed who has not learned to judge at once of the true motion
of the candle by the contrary apparent motion of the image. A boatman,
who, being accustomed to his oar, can direct its point against any object with
great certainty, has long ceased to reflect, that to move the point of the oar
in some one direction, his hand must move in the contrary direction. Now
the seeing things upright by images which are inverted, is a phenomena akin
to those which we have here reviewed.
Another question somewhat allied to the last is, why, as we have two eyes,
and an image of any object placed before them is formed in each-why the
object does not appear to us to be double. In answer to this, again, we shall
only state the simple facts of the case. As in two chess-boards there are
corresponding squares, so in the two eyes there must be corresponding points,
and when on those points a similar impression is made at the same time, the
sensation or vision is single; but if the impression be made on points which
do not correspond, owing to some disturbance of the natural position of the
eyes, the vision becomes double. Healthy eyes are so wonderfully associated,
that from earliest infancy they constantly move in perfect unison. By slightly
pressing a finger on the ball of either eye, so as to prevents its following the
motion of the other, there is immediately produced the double vision; and
tumours about the eye often have the same effect. Persons who squint have
always double vision: but they acquire the power of attending to the senslation in one eye at a time. Animals which have the eyes placed on opposite
sides of the head, so that the two can never be directed to the same point,
must have in a more remarkable degree the faculty of thus attending to one
eye at a time.
The corresponding points in the two eyes are equidistant and in similar
directions from the centres of the retinae, which centres are called the points
of distinct vision, and at them the imaginary lines named the axes of the eye
terminate-but it is worthy of remark that these points, in being both to
the right or both to the left of the centres, must be one of them on the inside
of the centre as regards the nose, and the other on the outside-that is to
say, a point of the left eye between the centre and nose, has its corresponding point in the right eye between the centre and the cheek-and from this
fact arises consequences meriting attention. When the two eyes were directed
to any object, their axes meet at it, and the centres of the two retinae are opposite to it, and all the other points of the eyes have perfect mutual correspondence as regards that object, giving the sensation of single vision; but the




3,50                          LIGHT.
images formed at the same time, of an object nearer to or farther from the
eye than the first supposed, cannot fall on correspondzing points for an object
nearer than where the axes meet would have both its images on the outsides
of the centres, and an object more distant would have both its images on the
insides of the centres, and in either case the vision would be double.  Thus
if a person hold up one thumb be'fore his nose, and the other in the same
direction, but farther off, by then looking at the nearest, the more distant
will appear double, and by looking at the more distant, the nearest will
appear double.
The reason for applying the term " point of distinct vision" to the centre
of the retina, is felt at once by looking at a printed page, and observing that
only the one letter to which the axes of the eye is directed, is distinctly seen;
and, consequently, that although the whole page be depicted on the retina
at once, the eye, in reading has to direct its centre successively to every part.
On examining a dead eye, the point of distinct vision is distinguishable
from the retina around by being more transparent. It might have been
expected that this point would have been where the optic nerve enters the
eye: but, in fact, the optic nerve enters considerably nearer to the nose than
the point of distinct vision is; and singularly, where it enters, the part is
altogether blind or insensible. Had the two optic nerves, therefore, entered,
at corresponding points of the retina, (in the sense explained above,) there
would have appeared a black spot on every object opposite to the insensible
points; but as the case really stands, the part of any object from- which the
light passes to the insensible or blind part of one eye must be opposite to a
sensible part of the other. The existence of the blind spot, where the nerve
of the eye enters, is discoverable by placing in a row three objects-wafers,
for instance-across a table, with intervals of about two inches between them.
and then looking with one eye, (the other being shut) from a distance of about
eight inches, at the wafer which is on the side of the nose; —the middle
wafer will be invisible, although the eye will see that on each side of it; and
if the eye be then directed to the middle wafer, the external one will disappear. Another proof is obtained by shutting one eye and looking with the
other at the points of two fingers held together before it;-if one of the fingers be then gradually moved away laterally, its point when at a certain distance from the other will disappear, but will be seen again when its distance
is still increased.
It appearing, from the explanations now given, that there canot be perfect
sight unless where a perfect image is formed on the retina, and the truth
having been formerly explained, that images behind any lens will be at different distances from it, according to the various distances of the objects in
front, that is to say, according as the pencils of light which fall upon it have
more or less of divergence in them, it follows, that the eye in being able, as
it is, to see distinctly objects at different distances, (the nearest is about five
inches,) possesses a power of altering the relation of its parts to accommodate
itself to the circumstances.  We do not yet perfectly know whether it does
this by lengthening or changing the form of the ball through the action of
the surrounding muscles, or by changing the place or the form of the lens,
but that one or more of these events occurs there can be no doubt.
Among the eyes of the myriads of mankind, however, it happens that all
do not originally possess these powers exactly in the requisite degree, and
that many lose them, as life advances, from a natural and usual decay.
Persons are called short-sighted whose eyes from too great convexity of
the cornea or lens, have so strong a bending or converging power, that the




THE EYE-SHORT SIGHT-LONG SIGHT.   351
rays of light entering them are brought
to a focus before reaching the retina-           Fig. 149.
at a, for instance, instead of at b: so
that the rays, by spreading again beyond
the focus, produce on the retina that
sort of indistinct image which is seen in
the camera obscura, of which the screen b. a  -...
is too distant from  the lens.  This
defect of sight obliges the individual
when using the -naked eye to hold objects very near it, that the consequent greater divergence of the rays may
be proportioned to the unusual refracting power of the eye-or the person
may find a remedy in placing concave lenses between the object and the
eyes, which lenses, by rendering light from objects at a usual distance
more divergent, (as explained page 340,) cause the perfect images in the
eye to be formed farther from the lens, and thereby on the retina itself.
Without concave spectacles-as the lenses are called when fixed together in
a frame —persons with the defect now under consideration cannot see distinctly any object that is distant, for the rays, coming nearly parallel, are
quickly gathered to a focus. This defect often diminishes with years, and
the person who in youth needed spectacles, in old age sees well without
them.
There is an opposite defect of deficient converging power in the eye,
dependent on a too great flatness of the cornea or lens, and which is much
more common than the last-mentioned defect; indeed, the great majority of
persons after middle age sooner or later begin to experience it. In this case,
the rays of light are not yet collected into a focus when they reach the
retina: they would only meet at b,
for instance, instead of as they should        Fig. 150.
do at c, and hence the image is indistinct, in the same manner as in a
camera obscura, when the screen is  -f
held too near the lens.  Persons.
suffering this defect cannot, when   l  \.
using the naked eye, see distinctly
to it, because the gathering or converging power of the eye cannot conquer
the great divergence of rays coming from a near point; and hence such persons always remove objects under consideration to a considerable distance,
often to that of arm's length, so as to receive from  them only the rays
nearly parallel. These persons in contradistinction to the last described,
are called long -sighted persons; and after middle age, most persons become
more or less long-sighted. Their defect is remedied by the common convex
spectacles, which do part of the converging work, so to express ourselves,
before the light enters the eye, leaving undone only that which the eye can
easily accomplish. As this defect, like the last, is met with in all degrees,
spectacles must be chosen accordingly. Certain curvatures or strengths of
these have been particularized and numbered as naturally belong to different ages or periods of life, but each person should choose under the
direction of an experienced judge, until that strength be found which enables him to read, without any straining of the eyes, at the common distance of from twelve to eighteen inches.  We cannot apply the mind to this
part of our subject without feeling admiration at what science has accomplished for man in assisting and restoring his sight. Now that in civilized




352                           LIGHT.
society, the common employments and enjoyments of life require a visual
power capable of distinguishing such minute objects as written or printed
characters, to deprive old men of their spectacles, would be to condemn
many of them  to useless inactivity and a listless blank of mind for the
remainder of their lives.
An eye much accustomed to examine near and minute objects, often loses
something of its pliancy, and becomes defective when tried at a distant
thing, as the watchmaker's eye, the engraver's; &c. On the other hand,
the old seamen's, which has so often and uninterruptedly been bent on the
distant horizon, straining to catch the view of an expected sail, or of land,
has a power of discovering distant things which is wonderful; but it often
experiences deficiency in regard to near things.
A man who uses his eyes under water sees very indistinctly, because the
difference of density between water and the eye not being so great as between air and the eye, the bending or refraction of light entering from the
water is not so great as of light entering from air, and the internal structure
of the human eye being adapted to the greater refraction, perfect images are
not formed on the retina. A man to see well under water, therefore, requires
to aid the usual power of his eyes by strong convex spectacles. It is to
meet the necessity now explained, that the lens of a fish's eye is extremely
convex, or almost round, as is every day seen in the white round bead which
issues from the eye of a boiled fish-that little globe being the crystalline
lens of the fish coagulated or hardened like the white of an egg during
cooking.
There are many important considerations connected with the sensibility of
the retina, which regard rather the laws of life than of light, but we must
here glance at a few of them.
Any impression of light made upon the retina lasts for about the sixth
of a second.  Hence when the burning end of a stick is made to sweep
rapidly across the view, its path appears to the eye a line of light: and if
it be made to revolve in a circle six times in a second, as when moved by the
hand or fixed to a turning wheel, that circle will appear to the eye a complete ring of fire.  The polished end of an elastic wire, of which the other
end is fixed in a block of wood, when caused to. vibrate, similarly forms a
line or a curve of light. A harp-string, while vibrating as it sounds, appears like a fiat transparent riband. Lightning or other meteor darting
across the sky, although in fact but a moving luminous point, is generally
thought of as a long line of light: the term  forked-lightning has reference
to this prejudice. The same remark applies in a degree to a sky-rocket in
its rapid ascent. Two or more colours painted separately on the rim of a
wheel which is made to turn rapidly, appear to a spectator to be as completely united as if they were really mixed;-it has been already explained
how patches of all the colours of the rainbow, when mixed in this way, form
white light. If on one side of a card a little bird be painted, and on a
corresponding part of the other side a cage, then, on making the card turn
rapidly by twisting between the fingers and thumbs two threads fixed to its
opposite edges, the bird and cage will be seen at once, and the bird will
appear to be imprisoned in the cage;-or, if a pensive Juliet, sitting in her
bower, occupy one side of the card, and a longing Romeo the other, by the
magic turn of the threads the lovers may instantly be brought together.
Dr. Paris displayed taste and an amiable ingenuity in designing this toy
with great variety of subjects.




THE EYE-JUDGING BY APPEARANCES.   353
A certain intensity of light is necessary to distinct vision, but the degree
varies with the previous state of the organ. A person passing from the bright
day into a shaded room, might for a time fancy himself in total darkness, and
to persons sitting in the room, and become so accustomed to the legs light as
to see well with it, he might appear to be almost blind. The dawn of morning
after the darkness of night appears much brighter than an equal degree of
light in the evening. When, as the night falls, our lamps or candles are first
introduced; the glare is often for a time offensive to the eye; and a similar
feeling but still stronger is experienced, when in the morning, bed-room
window shutters or close drawn curtains are suddenly opened   After the
repose of night, the sensibility of the eye, when first opened, is often such
that the globules of blood moving in the capillary vessels of the retina produce the impression there of little balls of light pursuing one another along
the tortuous vessels. To a prisoner after long confinement in a dark dungeon, the light of the sun is almost insupportable. And a dungeon, which to
common eyes is utterly dark, still to its long-held inmate has ceased to be
so;-there are various instances in the records of the barbarous ages of
prisoners confined for years in darkness, deemed absolute, but who after a
time could see in it, and make entertaining companions of the mice and
spiders which frequented their cells.  The darkness of a total eclipse after
bright sunshine, appears deeper than that of midnight, because of the sudden
contrast.  The long polar night of months ceases to appear very dark to the
polar inhabitants If an eye be directed for a time to a black wafer laid ona
sheet of white paper, and be then turned to another part of the sheet, a portion of the paper at that other part, of the size of the wafer, will appear
brilliantly illuminated; for the ordinary degree of light from it appears intense
to the part of the retina lately receiving almost none.  An eye directed long
and intensely upon any minute object-as when a sailor watches a speck in
the distant horizon, supposed to be a ship,' or when a hunter on the brown
heath keeps his eye fixed on a bird nearly of the colour of the heath or
when an astronomer gazes long at a little star-has the sensibility of its
centre at last exhausted, and ceases to perceive the object; but on directing
the axis of the eye a little to one side of the object, so that an image may
be formed only near'the centre, the object may be again perceived, and the
centre in the mean time enjoying repose, will recover its power.
But the most extraordinary fact connected with the sensibility of the retina
is, that if part of it be strongly exercised for a time by looking at some brightcoloured object, on the eye being then turned away or altogether shut, an
impression of spectrum will remain of the same form as the object lately contemplated, but of a perfectly different colour.  Thus if an eye be directed for
a time to a red wafer laid on white paper, and be then shut or turned to another part of the paper, a beautifully bright green wafer will be seen; and
vice versa, a green wafer will produce a red spectrum: an orange wafer will
similarly produce a blue spectrum; a yellow one a violet spectrum, &c., and
a cluster of wafers will produce a similar cluster of opposite colours.  Then
if the hand be held over the closed eye lids to prevent entirely the approach
of light to them, the spectrum of bright objects -will appear luminous surrounded by a dark ground, and when the hand is again removed, the contrary
will be true. Again, if the eye be in a degree fatigued by looking at the setting sun, or even at a window with a bright sky beyond it, or at any very
bright object, on then shutting it, the lately contemplated forms will be perceived, first of one vivid colour, and then of another, until perhaps all the
primary colours have passed in review. These extraordinary facts prove
23




354                            LIGHT.
that the sensations. of light and colour, although excitable by light are also
producible without it. This truth gave occasion to Darwin's ingenious
theory, that the sensation of.any particular colour, of red, for instance, is
dependent upon a certain state of contraction of the minute fibres of the
retina,-and that the fibres, when fatigued in that condition, seek relief when
at liberty, by throwing themselves into an opposite state,-as a man whose
back is fatigued by bending forward, relieves himself not by merely standing
erect, but by bending the spine backwards —which new conditiori in the eye,
whether produced by light or by any other cause, gives the sensation of
green.  He applied his explanation similarly to all other cases of colour.
It is remarkable that the colours which thus appear opposite to each other in
kind are those which, when the solar spectrum  produced by a prism, as
described a few pages back, is painted round a wheel or circle, are opposite
to each other in place.
There are persons who, although having distinct perceptions of form, and
of light and shade, have not the power of distinguishing colours. It is common for such persons to deem pink and pea-green (naturally opposites) the
same colour, and therefore, not to distinguish difference of colour in a red
berry and the leaves around it.  A man with this defect trusting to his own
judgment, has, without knowing it, dressed himself like a parrot.
The mninl judges of external objects by the relative size, blriyhtness, c41our,
&c., of'the minute but perfect images or pictures of them formed at the
back of the eye on the expansion of n erve called the retina; and the art of
painting is successful in proportion as it produces onl a larger scale such
a picture, which, when afJerwards held before the eye to reproduce itself izn
miniatuere upon the retina, may excite the same impression as on the original object." (Read the Analysis, page 325.)
We now understand how an admirable miniature resemblance of the objects
before us is produced upon the retina of the eye, by the light from them refracted in passing through the different parts of the eye; but after all, this is
only a picture, and the inquiry remains-which many persons would suppose
so simple as to be trifling, but which is in reality most curious and important
-how we are thereby enabled to judge of the magnitudes, distances, and
other particulars respecting the things examined? [ere it will be found, to
the surprise of persons first entering upon the subject, that we learn the
meaning of a scene or pictorial signs only gradually, as we do of any other
system of signs, and that a person whose eyes, although perfect, had been
kept covered from infancy up to maturity, would no more " see," in the
complete sense of the word, that is,. understood, any sense or prospect on
which he first opened his. eyes, so as to have a perfect picture of it on his
retina, than a child understands or can read a printed page, when he first
looks into a book.  Most interesting information has been obtained on this
subject, by observing the facts where blindness from birth has, by surgical
operation, been suddenly cured in persons arrived at maturity.
If a man were placed from infancy in an apartment fitted up as a camera
obscura, and had no means of becoming acquainted with external nature. but
by watching the images appearing on the screen, he could learn scarcely
any thing of the universe around him; but if after a time he were allowed to
walk out, and to examine by the touch and by the measurement the different
objects whose images he was in the habit of viewing, and to ascertain what
size, shape and distance of an object corresponded with a certain magnitude




THE  EYE-VISUAL  ANGLE.                         355
form, position, and brightness of image, the transient imagery might at last
be to him a very clear indication of the real particulars: making him in
imagination present to the objects, almost as if he went and examined them
with his hands.  In the same manner, in a degree, the mind may be considered as stationed in or about the little camera obscura of the eye whence
it cannot itself escape to examine external nature; but must learn the meaning of the images formed on the retina, by commanding the services of the
bodily limbs or members, and the other organs of sense.-The judging of
things by sight, then, is merely the interpreting one set of signs, as judging
by sounds or language is interpreting another, and judging by hieroglyphics
or any written character is interpreting a third. The common visual signs
on the retina, however, are of all signs the most readily learned or understood, from. having certain fixed relations in form, magnitude and position
to the things signified; while words, hieroglyphics, and written characters,
are quite arbitrary, and have no such relations.
Bodies differ and are distinguished among themselves chiefly by their
comparative dimensions, that is, their formn and magnitude, or shape and
size: and to ascertain these and the relative distances and positions, are the
great objects which by the eye the mind seeks to accomplish. Now it effects
its ends by considering collectively.
1st. The space and place occupied by objects in the field of view, measured by what is called the visual angle.
2d. The intensity of light, shade, and colour.
3d. The divergence of the rays of light.
4th. The convergence of the axes of the eyes.
We shall treat of these particulars separately in the order now stated.
Ist. The space and place occupied ia the field of view, measured by the
visual angle.
The term field of view is used to designate that open or visible space before the eyes, in which objects are seen: aud it may mean either the small
field visible in one position of the eyes, or that which is perceived on directFig. 150.




356                            LIGHT.
ing them  all round.  If a man as at e, were surrounded by a large globe
or sphere of glass as a, through which his eye at the centre might view the
several objects around occupying certain situations and certain proportions
of the circumference, and if the sphere had any equal divisions or degrees
marked upon it all round, he would be able at once to say exactly what
portion of his sphere or field of view was shadowed or occupied by any
single object, as the cross here shown at i, and thus to describe very intelligibly either for his own recollection, or to inform others, its relative magnitude and situation as then appearing to him,-just as he might say, on
looking at a tree in the garden through a common window which is a portion of the field of view really divided by the cross bars, whether he saw the
whole tree through one pane or through several, and through which pane or
panes he saw it. It may be remarked farther, that whether the supposed
sphere of glass were large or small, viz., were as b or c, the part of its surface
apparently occupied by any object either beyond or within it, would bear the
-same proportion to the whole surface;-if a d were a tenth of the small circle
or globe, c g would be a tenth of a larger. Now as men have found it convenient to consider a circle (and every circle) as divisible into 360 degrees,
(which are smaller, therefore, in a small than in a larger circle, although in
each having the same relation to the whole,) the ready mode of comparing
the apparent magnitude of objects is to say how many of these degrees of the
field of view each object occupies: and this is really what is meant by the
apparent size of an object.  And because the most convenient way of measuring a portion of a circle, of which the whole is not seen, is to measure by
a fit instrument the angle or corner formed at its centre by lines drawn from
the extremities of the portion, —as here the angle at e formed by the lines c e
and g e, the object is said either to occupy a certain number of degrees of the
circumference of the circle, or to subtend an angle of the same number of
degrees at its centre, and this angle is called the visual angle, the subject of
our present disquisition.
The visual angle, then, in regard to any object, is that included between
the lines or rays, as a u and t i, Wthich, from the extreme points of the object,
Fig. 151.
sa    metan  cos i    en o te ye ad0         iiAfte
as a cd, meet and cross in the lens of the eye, and go afterwards to form the
extremes of the image on the retina, and, as formerly explained, the angle is
the same on each side of the lens, viz, towards the object or towards the
image.
Now if all bodies were at the same distance from the eye, the magnitude
of their images formed on the-retina, or in other words, of the visual angles
subtended by them, would be an exact measure of their comparative real
magnitudes, as is seen in i u, the image of the great cross a d, and in i o the
image of the small cross b d: but it is evident here, that the cross c e, which




THE EYE-APPAIBENT SIZE OF OBJECTS.  357
is twice as large as b d, makes, because twice as far off, an image of only the
same size as b d, and an image therefore only half as large as that of a cross
a d equal in size with itself: and the same rule of proportion holds for all
other comparative distances-at a hundred times the distance, an object
appearing only the hundreth part as tall, and so forth. To judge, therefore
by the eye, of the true size of an object, we must know its distance as well
as its apparent size or visual angle.
Many familiar facts receive their explanation from the law of the visual
angle or apparent size being less always in proportion as the distance of
an object is greater.
A man (instead of the cross here shown) at d, standing near the outside of
a window, as b c (here represented edgeways) may to the eye of a spectator
within the window at h, subtend the samle visual angle, or appear as tall as
the window, the light from the man's head passing through the top of the
window, and that from his
feet passing through the                 Fig. 152.
bottom; but if the man
then moves away from the               
window, the eye of the
spectator will be able to see
his whole body through a
smaller and a smaller extent of the window,-as
through half its height or a c, when he is twice as distant, or atf, and through
the third or o c, when he shall be three times as distant, or at g, and so forth,
for any other distance; so that soon a small figure of a man cut in paper, if
applied upon the glass, would exactly cover the part of it through which the
light from him entered the spectator's eye, and would then, by completely
hiding him from view, be an exact measure of his apparent size: and at last
a fly passing over the pane might equally hide him, and the fly then would
subtend a larger visual angle than he, that is to say, would be forming on the
retina a larger image than the man. Thus it often happens in reality, that
a person sitting near a window, and intent upon some object of study or of
conversation, mistakes a fly on the glass for a man at a distance; or, on the
contrary, a man for a fly. It is ascertained that the eye, with an ordinary
degree of light, can see an object which in the field of view occupies only the
sixtieth of a degree (or one minute.) This space is about the 100th of any
inch in a circle of twelve inches in diameter, the eye being supposed in the
centre of the circle. Now a body smaller than this at six inches from the
eye, or any thiug, however large, placed so far from the eye as to occupy in
the field of view less space than this, is invisible to ordinary sight. At four
miles off, a man becomes thus invisible. A pin-head near will hide a house
on a distant hill-nay, will hide even the. planet Jupiter, although 1,000
times bigger than this earth.
In accordance with the principle now explained, a marine telescope has
been constructed, in which the field of view is divided by fine cross wires,
or otherwise, so that the person using it can say at once how much of its
field any object occupies.  When ships are in chase, it is common, by this
instrument, or some other which will detect a change of visual angle, or
apparent size, to view the fleeing or pursuing ship; and if the apparent
size be observed to increase, the conclusion follows that the ships are




358                            LIGHT.
nearing each other; if, on the contrary, the size diminishes, the chased ship
is escaping.
By applying this rule, whenever the real size of a distant object is known,
the distance is ascertainable, and, vice versa, where the distance is exactly
known, the size is determinable:-for it is evident that if a body, as a ship,
known to be 100 feet'tall, occupy or subtend in the field of vision the 360th
part of a whole circle, or one degree, the whole circle must be in circumference 360 times 100 (hundred) feet, or 36,000; and the diameter of any
circle being nearly one-third of its circumference, while, in the case supposed,
the distance of the ship is the half-diameter, we learn that distance. Again,
if we know the distance of a ship or other object to be a mile, and if we then
find the visual angle subtended by the object to be the 1,000th part of a
circle, we know its true size to be the 1,000th part of a circle, of which the
half-diameter or radius is one mile. It is by applying this rule in a manner
to be afterwards explained, that we determine the size of the heavenly
bodies.
We now perceive that if the rays of light coming to the eye through a
plate of glass, from objects seen beyond it, could leave marks in the glass,
at the points where they passed, and marks capable of giving out the same
kind of light-as the objects, there would be formed upon the glass such a
representation or picture of the 6bjects fornrd or viewed through it, that
when held before the eye, it would form on the retina, the image or images
the same in-almost all respects as the objects themselves; for from the.different points of the glass, light could dart to the eye of the same kinds and
in the very same directions as that originally coming from the objects.
Now the art of painting seeks so to dispose lights, shades and colours on
any plane surface, as to produce the sort of representation of objects here
contemplated, while the picture-frame has to recall the window-frame, or
edge of the plate of glass through which the true scene is supposed to be
viewed. It is admirable how perfectly this art now accomplishes its ends;
and although there are still trifling differences between the effect upon the
eye, of the picture and of the realities-which peculiarities we shall consider
presently, and how they may be combated so as to render the illusion
almost perfect,-it is not one of them, as might be supposed from the small
extent of the canvass, that the picture appears to the retina smaller than the
objects themselves.  Few people, before studying this subject, are aware
that in a good picture the size of the figures is always made exactly such,
that at the distance from the eye at which the picture is meant to be
viewed, they produce on the retina the yery same size of image as would
be produced by the realities seen under the aspect represented in the picture.
To become sensible of this, let a person look through a winsow-pane, with
the eye at the distance of eight inches from it, and let hlm trace with a
sharp point upon the glass, previously coated with gum, the outline of the
scene beyond-perhaps a street or square,-he will find that the outline of
a man seen there at the distance of twenty paces, and appearing perfectly to
coincide with the boundaries of the person, so'that, if opaque, it would just
hide the person, will be scarcely half an inch tall, while the figure of the
man'a few hundred paces off, will appear so small, that the minuter fractures
could not be distinguished, even if they could be drawn.
Now as a person who reads the description' of an elephant, does not
deem the animal larger or smaller because of the size of letter used in the
printing, or in the size of the accompanying engraved representation; and
as a man in a picture gallery viewing minatures and larger portraits, does




THE EYE-APPARENT SIZE OF OBJECTS.  359
not conceive of the originals according to the size of the representations:
and as a man viewing a well-executed picture of a Grecian temple, never
dreams,'unless his attention be particularly directed to the fact, that upon
the canvass the distant pillars of the rows are much smaller than the near
ones; but in all such cases the mind merely uses the signs to help it to conceive of the thligs according to previous knowledge, or to other principles
of judging:-so in any common case of seeing, the mind takes little account
of the apparent size of objects, but passes instantly from the types to the
realities, which are, generally, more or less known, and it soon ceases to be
aware that the apparent size of the same object ever changes.  Few persons,
for instance, are aware that when'two friends shake hands, each appears
to the mere eye of the other ten times taller than when he has walked ten
paces away; or that a chair at one end of a room appears to a person sitting
at the other, only half as large as a chair in the middle of the room, but
such are the facts; and they may be immediately proved by holding a
common eye.glass or ring at a certain distance from the eye, and then
looking through it at any similar objects placed at different distances; then,
while of a chair standing near, only a small part will be visible through the
ring-of a distant chair the whole may be seen; and so of any other case.
At five miles distance, the fleets which met on the great day of Trafalgar
might have been seen through a marriage-ring as the picture-fiame. There
are occasions, however, where the usual collateral helps to the immediate
recognition of objects being wanting, the observer's attention is strongly
aroused to the facts of their diminutive appearance produced by distance;
for instance, when a man on a long sea-voyage first approaches a land of
which the features are in a degree new to him; as when an Englishman
arriving in India, scarcely believes that the little specks which he sees scattered along the shore are commodious dwellings, or that what seem to him
only luxuriant herbs or bushes, are magnificent palm-trees.
For the same reason that a distant body to the mere eye appears diminutive,
namely, the smallness of the visual angle subtended by it, so-does a distant
motion to the mere eye appear slow. A carriage dashing past a pedestrian
in the street, may surprise him by its speed; but if viewed at the same time
by a spectator at the top of a lofty tower near, it seems to be but crawling
along the pavement. A ship driven before a tempest, seems to a sailor on
board almost to fly through the white foam which surrounds her; but if then
observed by a spectator on shore, as an object on the distant horizon, she is
scarcely perceived to change her place. A balloon high in the air, and borne
aiong on the wings of the wind at the rate of seventy or eighty-miles an hour,
may still for a time leave a spectator on earth doubtful as to whether it be in
motion, or in what direction'it moves. The moon in her orbit wheels round
the earth at the astonishing rate of about 2,000 miles an hour, yet, owing
to her distance from it, her motion is not visible to the naked eye of the
inhabitants of the earth, except by comparing her place at considerable
intervals. In respect to bodies still more distant than the moon, the truth
at-present under consideration is still more striking.
Having now explained how the apparent transverse measures or breadth of
bodies and of space, in other words, the visual angle subtended by them,
is affected by their distance from the eye, we proceed.to show how it is
affected also by their position.
A globe at a certain distance from the eye, however turned, preserves the
same appearance in the field of view, and its outline traced upon a plate of




360                            LIGHT.
glass held across between it and the eye, is, like its direct shadow upon a
wall, always a circle; but an egg which, held in one position, produces a
circular outline or image, when held in another, produces an image nearly
oval.  A wheel when viewed sideways appears a perfect circle, when viewed
edgeways it appears a broad straight band or line, and in any intermediate
position it appears oval. The apparent form, then,- of a body, is only a hint
to the mind from which, by former experience or instruction, it guesses at
the true form. If a man had never seen an egg but endways, he never
could have known that it was not a sphere.
If any long, straight object, as a beam, be placed with one of its ends
directly to the eye, that end only can Ble seen, and according to the case,
must appear a square or circle of the diameter of the beam; if it then be
placed with its side directly to the eye, its whole length will be seen; and if
placed in any intermediate position, it will appear more or less shortened;
-in all cases, its outline on the retina being similar to that of its shadow
on a wall directly behind the person. A man has advanced on a spear
pointed directly to his eye without seeing, or on the end of a bar of iron
carried on the shoulder of a porter in the street.  A common telescope held
with its end to the eye, appears a perfect circle, if then inclined a little, it
seems to jut out on one side, and as the inclination is increased, it juts out
more and more, until it displays its whole length. A great ship of war
whose stern is towards a spectator, appears a rounded building with its
rows of windows like those of a peaceful habitation; but as it turns, it
gradually reveals the long batteries of bristling cannon. A straight row of
a thousand similar objects, ads of soldiers in rank, pillars, trees, &c., may
appear to a person at the extremity as only one object of the kind, the
nearest individual completely hiding all the others; but if viewed from the
side and at a certain distance, the individuals may be counted.
The appearance now treated of is called foreshortening, and is to be noted
wherever surfaces or lines are not placed so as directly to face the spectator.
Perhaps the most important case of foreshortening is when the eye looks
more or less obliquely along an extensive plane surface, the general surface
of the earth, for instance, or of the sea, by estimating aright. the foreshortening
of which, we judge of the distance or situation of the objects placed upon it.
It will be readily perceived, that in all such cases the more distant portions
of the surface are progressively more foreshortened than the nearer; —for a
man standing at a on the plane as a b, and with his eye at c, looking down
before him, sees a por: ion of
Fig. 153.                   the surface a d almost directly, or with a little foreshorte                                 ening, and an extent, as a d,
equal to- the height of the
eye, will subtend in his eye,
an angle of 450, or half a
right angle, viz., the angle
d       2         M               a c d, and therefore rather
more than half of all that can
be subtended by a straight line or space from his feet to the horizon, however distant; the next equal spaces, viz., df, will subtend an angle of only
1 80, viz., d cf, tke next of 8~, viz., f c g, and so on; and as he carries his
view more and more forward, the surface becomes to it more and more oblique,
until at last the light coming seems more to skim along the level than to rise.
This explains why a person having a side view of a row of separate objects,




THE  EYE -FORESHORTENING.                           361
as of men in line, trees, pillars, &c., may see through or between the nearest
of them, but towards the extremes sees them as if standing in closest possible
array, or as if forming a continued surface. The same remark explains why
masses of cloud scattered uniformly- over the sky, may allow a spectator to
see wide intervals of the blue heaven over head, while all around there is a
dense cloudy mass appearing to rest on the horizon.
If a man standing on a hill look down upon a field or plain which is well
known to him, and if he see some objects near its side, and some near its
middle, and some near its distant border, he knows at once how far they are
from him and from one another. Similarly, when viewing the ocean from a
lofty cliff, and seeing ships scattered over its face, he may judge correctly of
their distance, for he can see only a certain extent of ocean which becomes
to him as a known field. The man stationed at the flag-staff on the High
Knowl of St. Helena, looks down upon the circular field of the Atlantic a
hundred miles broad, and can tell the distance of a sail in sight to within
a mile or'two.  Now, although the ground-plan of an extensive landscape
may not be so level as the face of the ocean, there is still an approximation,
which very considerably assists a spectator's judgment of distances.
Painters are not only careful to foreshorten, according to the proportion
explained above all, the objects which they portray, but they often avail
themselves of the principles to produce most striking effects. For instance,
Martin, in many of his beautiful designs, by judicious foreshortening, has
exhibited miles in extent of gorgeous architecture and of armed men, on a
space of canvass that would seem scarcely more than sufficient to receive a
very few figures; he has made a single magnificent pillar or accoutred warrior
in the foreground, become the type which first fills the mind with admiration,
and then sends it along the retiring lines of beautiful perspective, where
every tip or edge renews the first impression. A man lying on a high table
or bed, with his feet towards the spectator, is foreshortened into a roundish
heap, of which the soles of the feet hide the greater part. This is the description of the painting which has been called the " Miraculous Entombment," and it is because an unreflecting spectator moving sideways with the
expectation of seeing more of the body, still sees only the soles of the feet,
and may suppose the body turned round so as to keep the feet towards him,
that the painting has received its appellation. For nearly the same reason,
the eye of a common full-faced portrait may seem to follow a spectator to
whatever part of the room  he goes,-for by moving to one side he cannot
ie the side of the eye-balls. It is related of a murderer, that he was impelled to commit suicide by observing that the eyes of the portrait of his
victim were always fixed upon him. A rifleman portrayed as if taking aim
directly in front of the picture, will appear to every spectator in the room
to be pointing at him especially. To terrify young ladies, a little arch Cupid
has been similarly represented with his arrow pointed directly at them, and
just ready to let it slip from his bended bow.
As the painter, availihg himself of a lnowledge of the principles now explained, by which the eye usually judges of size and distance, may produce
on his canvass the most charming illusions, so may the tasteful landlord, in
his ornamental gardens and pleasure-grounds, by working his levels into
artificial undulation of hill and dale, and clothing these with tree and edifice
of magnitudes to correspond —make the eye of a spectator luxuriate in the
contemplation of the supposed extensive plains, lofty mountains, widespread
lakes, land distant padogas-all within the narrow space of an acre or two;
thus, by other means, producing on the retina the same impressions as Claude
Poussin, or Wilson has done by the finest pictures.




362                            LIGHT.
When any objects or mass of objects is foreshortened, by one part being
farther from the eye than another, that part appears also in a proportion
smaller than the other. For example, in a straight row of similar houses,
pillars, trees, &c., (see the next cut,) those nearest to the eye will, on a glass
held before the eye to receive their images, from the largest images, and there
will be a gradual diminution from the largest to the least, so that lines drawn
upon the glass along the tops and bottoms of the images would tend to a
point, called for a reason to be explained below, the vanishing point. Thus
a person looking from a window along a straight street, must, to see the
chimney of the nearest house, look through the top of the window, and to see
the street door must look through the bottom; but the most distant house,
both top and bottom, may be concealed from view by a little spot upon the
glass at the height of the eye. This remarkable tapering of foreshortened
objects may of course be strikingly observed on looking at any correctly
made drawing or engraving imeant to represent a retiring row of similar objects;-such drawing being in truth an attempt to realize by art, on the
surface of a sheet of paper, the appearance of the objects as seen through a
window or aperture the size of the paper.
The art which gives rules for tracing objects on a plane surface, as they
would appear to an eye looking at them through that surface, if transparent,
with their various degrees; first, of apparent diminution on account of distance; and, secondly, of foreshortening on account of the obliquity of view,
is called, from the Latin word signifying to look through the art of perspective.  It regards entirely the two particulars now mentioned; and notwithstanding the terror with which, in the imaginations of many young painters,
the study of it is clothed, by reason of the mathematical difficulties with which
it has usually been mixed up, it is in itself exceedingly simple. We hope
that a person capable of ordinary attention, will, after what we have already
said, and after the few additional remarks which we have still to make on the
appearances of nature, be able readily to understand the great rules of perspective.  Although, without a knrowledge of these rules, a quick eye soon
enables its possessor to sketch from nature with much truth; and although
the two instruments, the camera obscura, already described, and camera lucida, to be described in a future page, give almost mathematical accuracy to
drawings from nature, without requiring other skill in the draughtsman than
to trace and make permanent,
Fig. 154.                  with ink or pencil, the lines of
light which he sees on tl1
paper; still the subject is so
interesting to all who attempt
to sketch, and indeed to all
who wish to look intelligently
pc    ] ~either at nature or at the
6l 11          1.e -il.il    works of art, that none who;'....:-oi, I                have the opportunity of studyAls  jj ir!'.kll                     uing it should neglect it.
il~~~lllr"~                       Supposing a straight row of
similar objects, as of the stone
~I~!~.a        c       b" hLa   d    blocks or pillars represented
here from a or b to S, to be
viewed by a person stationed
near the side and end of the row, as over the point C, then, because, as
already explained, objects to the eye appear smaller in exact proportion to




THE  EYE-PROSPECTIVE.                           363
their increased distance from it, the second block, if twice as far off as the
first, would appear only half as large; the third if three times as far, would
be only one-third as large, and so on to any extent and for any other proportions; and if the 1;000th of any other block, owing to its distance, subtended to the eye an angle less than the sixtieth of a degree of the field of
view, it would be altogether invisible (as explained at page 333,) even if
nothing intervened between it and the eye. Then, where the row ceased to
be visible from the minuteness of the parts, or from the fact of the nearer
objects concealing the more remote, it might be said to have reached its
vanishing point. When a student of perspective has learned all that regards
the vanishing point in relation to a line, and the corresponding vanishing
line, in relation to a surface, he has learned half of his art. The above cut
considered as the representation of a street running directly south to S,
sketched from a window looking along its centre, will *serve as a useful
illustration.
It is important, first, to remark, that in any case of a straight line, or a
row of objects thus vanishing from sight, as here the line or row a S, in
whatever direction it points, wheth-er east, west, north, or south, &c., in that
direction, too, will its remote or vanishing extremity appear to be from the
eye. In this sketch the row a S is supposed to run directly south; and
although the eye to see the near end of it, would have to look towards the
left hand, or in a degree east, still every successive pillar would be more and
more nearly south, and the point in the heavens, or in the picture, or in a
transparent plane before the eye, where the line would vanish, would be so
nearly south from the eye, and not to the east, because the pillars happened
ko the east of the individual, that no ordinary measure would detect the
ttle want of correspondence; then similarly, if there were more rows of
objects, as of pillars, houses, trees, &c., parallel to the first, but considerably
apart from each, as the lines here a S, b S, d S, &c., still all would vanish
or seem to terminate in the very same point of the field of view. The reason of this is easily understood.  Let us suppose a line drawn directly south
from the eye to the point S, between the parallel lines of pillars, houses,
and trees, a S, b S, d S, &c., also pointing directly south; and let us suppose the two rows of pillars to be 100 feet apart, then evidently for the
same reason as the space between the top and bot.bm of the pillars, that is
to say, their height becomes apparently less and less as their distance from
the eye increases, so will the space between each pillar and its opposite, or
between it and the point corresponding to it in the visual ray along which
the eye looks, become apparently less, and therefore the lines of pillars
really 100 feet apart from each other, and 50 feet from the visual ray, will,
at a certain distance from the eye, (viz., where a space of 50 or 100 feet is
apparently reduced to a point,) appear to join, and the three lines will appear to meet in that point, beyond which none of them can be visible, and
which is therefore the vanishing point of all. The conception of this truth
may be facilitated by supposing a planet to be visible to the exact point of
the heavens at the moment of observation; then, if the three parallel lines
were continued on to the planet, and were visible all the way, they would
arrive there with the interval between them just as when they left the
earth'; but as a planet, although thousands of miles in diameter, owing to its
distance from the earth, appears on earth only as a point, much more would
two lines only 100 feet apart be there undistinguishable in place by human
sight.  And what is true of space of 100 feet between parallel lines, is
equally true of a mile or of thousands of miles. As a general rule, there



364                            LIGHT.
fore, it holds, that all lines in nature parallel to each other, when represented
in perspective, tend towards an end in the same vanishing point; and that
point is the situation where the line terminates, along which the eye looks
when directed parallel to any one of the real lines. And this is true only of
lines all in the same level or horizontal plane, viz., such as might lie along
the surface of the sea, but also of lines placed one above another, as those
running along the tops and bottoms of the pillars here, or along the walls,
roofs, and windows of the houses, or along the roots and, summits of the
trees, and indeed of all lines in whatever situation, provided they are parallel to the visual ray, and therefore to one another. And the' truth holds
equally with respect to lines which do not vanish at the " point of sight," or
centre of the picture, as with respect to those which do. When it is ascertained, therefore, that a line or boundary of any natural or artificial object
has a certain inclination to the axis of the picture, or to what we have described as the principal visual ray, then also is it known that all parallels
to that line have their vanishing point in the same spot of the field of view,
and a line supposed to be* drawn from the eye to the heavens, or really
drawn from the eye to the picture in that direction, marks upon the picture
the true vanishing point.
It will be understood why, in a long arched tunnel or a cathedral, with
many longitudinal lines on its floor, walls, roof, &c., all such lines seen by
an eye looking along from one end, appear to converge to a point at the
other, like the radii of a spider's web; and why similarly in the representation of a common room, viewed from one end, all the lines of the corners,
tops and bottoms of windows,
Fig. 155.                floor, stripes, on a carpet, corners of tables, &c., being parallel to each other, tend to the
same vanishing point as V.
The appearance of the lines in
the floor of this room may recall that of the furrows in a
L;2ii11          ploughed field as seen from
[JT;I   ]V[ |v  WE               one end, when they appear
-O'qt!'.:' --  I1" ] t~like the ribs of a fan spread
3 _?[".. l I V:..-'"[I " —iout towards the  spectator.
The same considerations will
A/ V//   I~'    \         \ \     vexplain the phenomenon often
to be observed, of two little
clouds seen near each other,
and almost motionless for a
time in the distant sky, but which on approaching the spectator with the
wind, appear gradually to separate, and in a corresponding degree to enlarge,
until one of them sweeps past considerably to the right hand, and the other
considerably to the left; after this, they lessen and approximate as they before enlarged and separated, and at last beyond the spectator, appear as
small and as near as when first observed. Clouds being so mutable and uncertain in their forms, persons have been led to deem all apparent changes
in them, of form, size and place, to be real changes, and not, as they generally are, mere optical or perspective illusion.
By far the most important vanishing point in common scenes is the middle
of the horizon or level line, and in a picture properly placed, it is at the exact
height of the eye. It is marked S in the figure before the last, and V in the




THE  EYE-PERSPECTIVE.                            365
last figure. Because in houses, the roofs, foundations, floors, windows, &c.,
are all horizontal, the vanishing points of their lines must be somewhere in
the horizon, and if the spectator be in the middle of a street or of a building,
and be looking in the direction of its walls, their vanishing point will be in
the centre of the scene or picture; if he be elsewhere, it will be to one side.
In holding up a picture-frame, through which to view a scene suitable for a
picture, it would be found most generally befitting to raise it until the line of
the horizon appeared to cross it at about one-third from the bottom:-this
fact becomes the reason of the rule in painting, so to place the horizontal line.
In beginning a picture, this line is usually the first line drawn on the canvas,
marking the place of the vanishing points of all level line and surfaces. And
the eye of the spectator is supposed to be placed before the middle of it and
generally about as far from the picture as the picture is itself long, such being
the extent of view which the eye at one time most conviently commands.
Understanding now that the apparent or perspective direction of all lines
in a scene is towards their vanishing points as above discovered, and the rule
having been given for determining these points in a drawing, we'proceed
to show how much of a line drawn to any vanishing'point belongs to the
known magnitude of any object through which it passes; in other words,
how much an object is in perspective foreshortened in consequence of its
distance and obliquity of position.
If we suppose A S P to represent a plate of glass seen edgeways, and that
towards the point S in it, an
eye is looking from  the                    Fig. 156.
point D, evidently then, a                        A.
line from P continued in the
direction of R and beyond                         S
until vanished from sight,  D 
would have as its perspective image or representation
on the glass a line reachinga            -                      c C R
from  P  to S,-S  being,
moreover, the point of sight here, and the pictorial vanishing point of the
line.  Now to divide the representative line P S so as to correspond with
any given portions of the original line P R, &c., it would only be necessary
to draw other linesfrom the place of the eye D to cut or touch the original line
in the situations desired, and these lines would cut the perspective line S P
as required: for instance, the portion of true line a b would be represented
by that portion of the image-line S P included between the two lines a D,
and b D, and so of any other portions. There are figures drawn on many
mathematical scales by which such problems as this can be at once opproximatively solved; and it would be possible, by trigonometrical calculation, to
solve them exactly in all cases: but the most generally convenient mode in
practice is to sketch on the intended drawing (as that of which the boundaries are given in the next cut)the kind of measure shown above, by setting
off from the point of sight S, a distance on the horizontalline, as at D, equal
to the distance of the eye from the picture, and then by ollique lines from D
drawn upon the base line IP R, to cut the perpendicularline P S in the situations desired-"-as is seen in the last figure, which differs from fig. 161
chiefly in having the point of distance marked before its point of sight, instead of, as here, laterally. And the line P S being always cut by the oblique
line from D in proportion to the length of base-line between P and the extremity of the oblique line, a horizontal line drawn thiough any point in it,




366                           L I GHT.
cuts in corresponding proportions all the other lines which have their vanishing
points in the horizontal line at S, for instance a S, b S, &c. Thus, to draw
in perspective, on the surface above represented and prepared, a chess-board
or board of squares, it is necessary to set off the breadth of the board on the
Fig. 157.
base-line to the right and left of P, viz., at b and a, and then to draw to the
point of sight as a vanishing point, the lines a S and b S, part of which lines
will, therefore, represent the sides of the board, and then to draw the diagonal b ID, which, for the reason above stated, will cut the lines P S and a S in
proportion to the length of base-line to the right of their extremities; a efb,
therefore, is a square seen in perspective, and any number of smaller included squares are made by drawing lines from the vanishing point to equal
divisions on the base, and making cross lines where the diagonal cuts these.
Much of the delight which the art of painting is calculated to afford is lost
to the world, because persons in general know not how to look at a picture.
Unless a spectator places himself where he can see the objects in true parspective, so that he may fancy himself looking at them through a window or
opening, every thing must appear to him false and distorted. The eye
should be opposite the point of sight of the picture, and, therefore, on a level
with the line of the horizon, and it should be at the required distance, which
is generally at least as great as the length of the picture.  But blame not
unfrequently rests also with the artist, from his having neglected the study of
perspective.  It is very common, for instance, to see miniature resemblances
of architectural structures so foreshortened and tapered, that the eye, to see
them in true perspective, would require to be within an inch of the paper;
whence at the usual distance of ten or twelve inches they are seen as hideous
distortions. The specimens, in the few preceding pages, necessarily exemplify in a degree this error, because the point of distance had to be marked
where there was but a small page.  These figures, therefore, by any person
studying the subject particularly, should be drawn on a scale so much larger
as to allow the eye really to view them at the distance supposed.
A means of judging of the dimensions of the bodies by the visual angle, but
which depends neither on the absolute size of the image, nor on the foreshortening of the ground plane on which the body stands, is to use known
objects in view as measures for others near them which are unknown.
If any person of our acquaintance be standing at some distance from us
near another person who is a stranger, we know how tall the stranger is by
taking the acquaintance as a measure.
In pictorial representations of objects little familiar as to many people are
the Egyptian pyramids, the bodies of the whale, the elephant, the camel, &c.,
human beings may be represented around them to serve as measures for the




THE  EYE-PERSPECTIVE.                           367
less known object. The Colossus of Rhodes seen from afar, might to a
stranger have appeared but an ordinary statue of a man, but the exact magnitude would have been known as soon as a ship of known dimensions were
seen sailing into port between his gigantic lignbs.
When an unpracticed eye is first directed from a distance to a great ship of
war, it will on many accounts dwell upon the object with wonder and admiration; but it may not judge truly of the enormous magnitude until it be
near enough to perceive the sailors climbing on the rigging, and appearing
there, by comparison, as flies or little birds appear among the branches of a
majestic tree.
By having a measure of this kind presented to us, the magnitude and elevation of great edifices are rendered more obvious. The magnificient pile of
St. Paul's in London becomes still more striking, when we discover visitors
looking from the balconies near the summit-cross. They appear so minute
among the surrounding huge masses that a person is for a while disposed to
doubt whether they be men: but the fact once ascertained, the grandeur of
the temple is most impressive.
Manny persons cannot distinguish between the little pilot balloon (sometimes dispatched before the great one to show the direction of the wind) and
the great balloon itself, until with the last they perceive the aeronauts as
little black points suspended under the globular clouds.
Strangers visiting Switzerland, on first entering the valleys there, are often
much deceived as to their extent. Being familiar generally with more lowly
hills and shorter valleys at home, which, however; from being near to the
eyes, form bulky images, and having at first no other measure, they almost
universally underrate the Alpine dimensions:-they will wonder, for instance,
in the valley of Chamouny, that they should be travelling swiftly for hours
without reaching the end, where on entering they did not anticipate a drive
of more than half an honr.
The author once sailed through the Canary Islands, and passed in view
of the far-famed Peak of Teneriffe. It had been in sight during the afternoon
of the preceding day, at a distance of more than 100 miles, disappointing
general expectation by appearing then only as an ordinary distant hill rising
out of the ocean, but next morning, when the ship had arrived within about
twenty miles of it, and while another ship of the fleet, holding her course six
miles nearer to the land, served as a measure, it stood displayed as one of
the most stupendous single objects which on earth, and at one view, human
vision can command. The ship in question, whose side, showing its tiers
of cannon, equalled in extent the front of ten large houses in a street, and
whose masts shot up like lofty steeples, still appeared but as a speck rising
from the sea, when compared with the huge prominence beyond it, towering
sublimely to heaven, and around which the masses of cloud, although as lofty
as those which sail over the fields of Britain, seemed still to be hanging low
on its sides.  Teneriffe alone of very high mountains, rises directly and
steeply out of the bosom of the ocean, to an elevation of 13,000 feet, and as
an object of contemplation, therefore is more impressive than even the still
loftier summits of Chimborazo or the Himalayas, which rise from elevated
plains, and in the midst of surrounding hills.




368                           LIGHT.
It is, because objects which are nearly on a level with us, as contrasted with,
such as are either much above or much below, are in general more numerously surrounded by other known objects which serve as measures of
comparison, that we judge so much more correctly of the size and distance
of those near our level than of others.
A man walking like ourselves on the sea-shore or other level, is at once
fully recognized; and probably it may not occur to us, that he appears smaller
on account of the distance; but if the same man be seen afterwards at an
equal distance above us, collecting the sea-fowl's eggs on the face of a cliff,
or below us, gathering shells on the beach when we ourselves have reached
the height, he appears no bigger than a crow; yet in all the cases he is where
the same bulk forms the same magnitude of image on the retina.
Even on a horizontal plane, if the general surface be bare and uniform,
single distant objects appear very diminutive. This is true, for instance, of
a man seen apart from his caravan, while journeying across a s:nldy desert;
but a man viewed at an equal distance, in the midst of a cultivated landscape, appears of his natural size.  The same is true of a boat or ship seen
out on the high sea, as constrasted with the like viewed in a harbor where
other known objects are near them.
We may now understand why the sun and moon, at rising or setting,
appear to us much larger than when they have attained meridian heightalthough, if we examine them by any measure of the visual angle, as simply
by looking at them through the same ring or tube, we find that there is
scarcely a difference; and singularly we find the difference to be, that the
orbs appear horizontally even less than when seen on the meridian, owing
to our being then about 4,000 miles more distant from them.  The sun
and moon as they appear from this earth are nearly of the same size,
each occupying in the field of view about the half of a degree, or as much as
is occupied by a circle of a foot in diameter when held 125 feet from the
eye-which circle, therefore, at that distance, and at any time, would just
hide either of them. Now when a man sees the rising moon apparently
filling up the end of a street, which he knows to be 100 feet wide, he very
naturally believes that the moon then subtends a greater angle than usual,
until the reflection occurs to him, that he is using as a measure, a street known
indeed to be 100 feet wide, but of which the part concerned, owing to its
distance, occupies in his eye a very small space.  The width of the street
near him may occupy 600 of his field of view, and he might see from between
the houses many broad constellation instead of the moon only, but the width
of the street far off may not occupy, in the same field of view, the twentieth
part of a degree, and the moon, which always occupies half a degree will
there appear comparatively large. The kind of illusion now spoken of is
yet more remarkable when the moon is seen rising near still larger known
objects, for instance, beyond a town, or a hill which then appears within the
luminous circle.  Any person who-from the river-side terraces of Greenwich
has observed the sun setting beyond London, with St. Paul's Cathedral
included in the glorious picture, will recollect a most interesting example of
our present subject.-That our ocular judgment of the size of the sun or
moon is thus influenced by the presence or absence of objects of comparison,
and not by the place of the bodies in the sky, is proved by the fact, that a
person viewing these bodies at any elevation from the bottom of some of the
Swiss valleys, where he might almost suppose himself placed at the centre
of the earth, and looking abroad along an endless extent of precipices —if he




THE  EYE-INTENSITY  OF LIGHT.                         369
can closely compare them with certain known magnitudes of ridge or forest
bounding his view, sees them as large as they appear from other situations
when rising beyond a low horizon. Another proof is afforded by the case
of a balloon at a great elevation seen crossing the disc of the sun or moon,
and then appearing, however large in reality, as an absolute speck within
the vast luminous area. In a future paragraph it will be explained, that
another circumstance contributes to cause the sun and moon, when low, to
sppear larger than when high, namely, their apparent dimness, owing to the
obstruction of their light in traversing the low dense atmosphere.
It may be remarked here, that the visual estimate formed of the great
size of the sun and moon when they are seen on the horizon, is not an
illusion, as is popularly supposed, but an approximation to truth] still prodigiously short of the reality.  VWhen we see a distant tree, or a house, or
a hill, apparently within the circumference of one of these orbs, it is really
true that the orb is larger than the tree, or house, or hill, just as another
more distant hill would be larger than nearer objects similarly surrounded
by its outline; but the celestial body is so much larger, that even if the
whole British Isles could be lifted away from the earth, and suspended near
the moon, as a map in the sky, they would bide from a spectator on earth
but a small part of the disc of the moon.
Having now shown that the visual angle or apparent size can be a measure
of the distance of any object only when the true size also is known, or
of the true size when the distance is known, we proceed to examine other
means which the eye commands for guessing at distances.
2d.  Intensity of light, shade and/colour.  (See the Analysis, pages 325
and 355.)
It has already been explained that light, like every other influence, radiating from a centre, becomes rapidly weaker as the distance from the centre
increases, being, for instance, only one-fourth part as intense, at double
distance, and in a corresponding proportion for other distances; while it is
still farther weakened by the obstacle of any transparent medium through
which it passes.  Now the eye soon becomes sufficiently familiar with these
truths, to judge from them, with considerable accuracy, of- the comparative
distances of objects.
The fine Gothic pile of Westminster Abbey may break upon the view in
some situations where nearer edifices, and perhaps some minor imitations of
its beauties, already fill the eye with their strong lights, but the misty or
less distinct outline of the venerable pile may warn the approaching stranger
of its true magnitude, and prepare him for the enjoyment which a nearer
inspection of its grandeur and perfection is to afford.
A small yacht or pleasure-boat may be built according to the same model
or with the same comparative dimensions as a first-rate vessel of war, and
may be in yiew from the shore at the same time, only so much nearer than
the ship, that both shall form images of the same magnitude on the retina of
a. spectator. In such a case, an unpractised eye might have difficulty to
discriminate, but to an old seamen, the bright lights of the little vessel,
contrasted with the softer or more misty appearance of the larger, would
declare the truth at once.  A haziness occurring in the atmosphere between
the little vessel and the eye might considerably favour the illusion.
In a fleet of ships, if the sui's direct rays fall upon one here and there
24




370                            LIGH f T.
through openings among the clouds, while the others remain in shade, the
former, in appearance, starts towards the spectator.  In like manner, the
mountains of an unknown coast, if the sunshine fall upon them, appear
comparatively near, but if the clouds again intervene, they recede and mock
the awakened hope of the approaching mariner.
A conflagration at night, however distant, appears to spectators, generally,
as if very near, and inexperienced persons often run towards it with the
hope of soon arriving, but find, after miles travelled, that they have made
but a little part of the way.
A person ignorant of astronomy deems the heavenly bodies so much
nearer to the earth than they are, merely because of their being so bright
or luminfus.  The evening star, for instance, seen in a clear sky, over some
distant hill-top, appears as if a dweller on the hill might almost reach itfor the most intense artificial light which could be placed on the height
would be dim in comparison with the beauteous star; yet to a dweller on
the hill it appears just as distant as to one on a plain; nay, at a thousand
miles farther west, and, therefore, nearer, the appearance would still be
nearly the same.
The concave of the starry heavens appears flattened above, or as if its
zenith were nearer to the earth than its sides or horizon, because the light
from above having to pass through only the depth or thickness of the atmosphere, is little obstructed, while of that which darts towards any place
horizontally through hundreds of miles of dense vapour-loaded air, only a
small part arrives.
The sun and moon appear larger at rising and setting than when midway
in heaven, partly, as already explained, because they can then be easily compared with other objects, of which the size is known, but partly, also, because
of the much less light arriving from them in the former situation, while
their diameters are nearly the same.
A fog or mist is said to magnify objects seen through it.  The truth is,
that because it diminishes the intensity of the light from them, it makes
them appear farther distant without lessening the visual angles subtended
by them; and because an object at two miles, subtending the same angle
as an object at one mile is twice as large, the conclusion is drawn that the dim
object is large.  Thus, a person in a fog may believe that he is approaching
a great tree, fifty yards distant, when the next step throws him into the bush
which had deceived himn-Two friends meeting in a fog have often mutually mistaken each other for persons of much greater stature.-A row of
fox-glove flowers on a neighbouring bank has been mistaken for a company
of scarlet-clad soldiers on the more distant face of the hill.-There are, for
similar reasons, frequent misjudgings in late twilight and early dawn.-The
purpose of a thin gauze screen interposed between the spectators in a theatre
- and some person or object meant to appear distant, is intelligible on the same
principle: a boy near, so screened, appears to be a man at a distance. —The
art of the painter uses sombre colours when his object is to produce in his
picture the effect of distance.  On the alarming occasion of a. very dense
fog coming on at sea, where the ships of a. fleet are near to each other,
without wind, and where there is considerable swell or rolling of the sea,
much damage is often done: but it is to be remarked in such a case, that the
size of each ship approaching to the shock, is always, in the apprehension
of the crew of the other ship, exaggerated.
The celebrated Spectre of the Brocken, among the Hartz Mountains, is a
good illustration of our present subject.  Oa a certain ridge, just at sunrise




THE  EYE-INTENSITY  OF LIGHT.                        371
a gigantic figure of a man had often been observed walking, and extraordinary stories were related of him. About the year 1800 a French philosopher
and a friend went to watch the apparition; but for many mornings they
paraded on the opposite ridge in vain. At last, however, the monster was
seen, but he was not alone; he had a companion, and singularly he and his
companion aped all the motions and atitudes of the observer and his companion; in fact the spectres were merely shadows of the observers, formed
by the horizontal rays of the rising sun falling on the morning fog which
hovered over the valley between the ridges; and because the shadows were
very faint, the figures were deemed distant, seeming men walking on the
opposite ridge, and because a comparatively small figure seen near, but supposed distant, appears of gigantic dimensions, these shadows were accounted
giants.
While the comparative intensities of light coming from bodies considered
as wholes, or from their sides similarly exposed to the source of light —furnish an indication of their different distances from the observer, the comparative intensities from their sides dissimilarly or unequally exposed to the
source of light, and which, therefore, reflect light to the eye, or are illuminated in different degrees, furnish an indication of the forms and attitudes
of the bodies.' In observing, for instance, a white house exposed to the sun,
it is seen that the side directly receiving the rays is highly illuminated or
bright, while the other sides are less so, and are said to be in the shade —a
shade which is more or less deep in proportion as there are few or many
sources of reflected light near it.  The different faces or walls of such a house.
are to the sense of the observer, as strongly distinguished from each other,
by the mere difference of shade, as if they were of different colours, or as if
they were examined by the touch, or by walking round them.  If the object
were a ball instead of a square house, there would still be as great differences
of shade in the half not receiving direct rays, but the parts, instead of forming
abrupt contrasts like the walls of a house, would appear to melt into each
other, marking the beautiful round contour of the object. The consideration
of all such cases forms the subject of chiaro-oscuro, so interesting to the
painter.
Had there not been in nature the provision of light and shade now described, the sense of sight would have been of comparatively little use, and a
mass of things in the light, if of the same colour, would have been as little
distinguishable from one another by a person looking directly at them, as
things forming a mass, or shadow are distinguishable by a person looking at
the shadow. It is this provision, therefore, which enables us, independently
of colour, to distinguish the profile or outlines of different bodies placed near
to each other, and to distinguish in the same body the protuberent or other
form of the surfaces which is towards the observer.  But for this, it would
have been impossible to distinguish, for instance, between a white wall when
naked and when having various white objects placed before it; and it would
have been impossible to distinguish between the rounded figures, if similarly
coloured, of a flat circle, a sphere and a cone, all directly opposed to the eye;
but in reality, by some difference of shade, the white objects are distinguished from the wall, and in the three geometrical figures, the uniformly
bright surface of the circle, the soft rounded shadowing of the sphere and
the shade coming to a point on the cone, at'once declare the true forms. But
for the shadowed parts, the facade of a white palace of varied architecture
would have been an unmeaning sheet of lights: the lights, however, and
shadows produced by the juttings and recesses, mark the variety of surface




3872                          LIGHT.
most completely; and the round pillar is distinguished from the square, and
every pediment, and capital, and architectural ornament, stands out pleasingly conspicuous. But for light and shade again, the " human face divine."
would have been an unmeaning patch of flesh, for there are few lines in it
but those made by different exposures to the lights, and yet its every
prominence and depression, and every momentary change, are so truly indicated to the eye that it becomes full of meaning or expression.  HIow well
mere light and shade serve to convey what the eye has to learn of a scene
or object, may be perceived by examining any of the admirable engravings
which now abound, and which, although made up entirely of degrees of
shade, or of black and white, are scarcely inferior in expression to finished
paintings.
The student of painting soon learns that the lines called outlines, by which
he first sketches subjects, do not exist at all in nature, and have to be again
effaced in his finished work: for they only mark the place where lights and
shades happen to meet.  Much may be-conveyed to the mind, however, by
a mere outline, and particularly of lines if different breadth and thickness
are used to mark the situation of the fainter and deeper shadows.
The subject of chiaro-oscuro is not so simple as, from the fact of the sun
being the great source of light, might at first be supposed; for although this
be true, still every body which reflects the sun's light becomes a new source
to those about it, and the shading of a picture must have reference to all such
sources, and to the colours of the body itself, and of the neighbouring bodies.
In looking at an extended landscape, it is seen that the near objects considered as wholes, are comparatively bright, that their shadows are strongly
marked, and that their peculiar colours are everywhere easily distinguishable
-as of flowers, fruit, foliage, &c., but of objects farther off, the colours, with
increasing distance become dimn, the lights and shadows melt into each other
or are confused, and the. illumination altogether becomes so faint that the
eye at last sees only an extent of distant blue mountain or plain-appearing
bluish, partly because the transparent air through which the light must pass
has a blue tinge, and partly because the quantity of light which can arrive
through the great extent of air is insufficient to exhibit the detail.  The
ridge called Blue Mountains in Australia, another of the same name in
America., and many others elsewhere, are not really blue, for they possess
all the diversity of scenery which their climates can give, but to the eye
which first discovered them, bent on them  generally from a distance: they
all at first appeared blue, and they have retained the name.
In a good picture, where, upon canvass stretched on a frame, the artist has
disposed the lights, shades and colours in the very situations and with the
intensities which they would have had on coming from the real scene to the
eyes, through a plate of glass filling up the frame, all that we have now
been saying is strictly exemplified.  In the foreground, the objects are large
and bright, but as the scene is supposed to be gradually more remote, the
size and brightness correspondingly diminish, until, at last, there is only a
dim mixture of bluish or grayish masses forming the horizon and sky.
A child during what may be called the education of the sense of sight,
has a strong perception of the vast differences of appearance which things
assume according to their accidental distance from the eye, their position,
their exposure to light, &c.; for many of these differences, being at first
calculated to deceive the young judgment, have, from  time to time, been
noted by him with surprise. Thus, a boy when he first discovers that a ship
which at the quay, with her white sails spread out, concealed from him




THE  EYE-PERSPECTIVE.                           373
half the heavens, is in an hour or two afterwards, seen by him on the distant horizon as a dark speck hardly big enough to hide one star, has his
attention strongly awakened, and he feels surprised; or, again, when he
discovers that the faint blue unchanging mass which he had always observed
bounding in one direction, the view from the house of his infancy, is a distant mountain side, thickly inhabited and corvered with fields and gardens,
where in succession, all -the bright colours of the different seasons predominate-he is equally struck. But as soon as experience has enabled him
to interpret readily and correctly, the visual signs under every variety of
circumstance, his attention passes so instantly firom them to the realitieswhich alone are interesting to him-just as it might pass from the paper and
printing of a newspaper to the important intelligence communicated by them
-that he very soon ceases to be aware that the sign, which, in every case,
similarly suggests the object, is not, also, in every case similar to itself, and
the very same true and complete representation of the reality. The prejudice
that the sign is of this nature becomes quickly so strong, that even a difficult effort has been made by a grown person again to attend to. the mere
appearances, in any scene of which the realities are known.
This attempt to analyze mere appearances, and which, in one sense, is an
attempt to unlearn something, or to retrograde, is called, as already'stated,
the study of perspective. When it regards the apparent reduction of size and,
the foreshortening of bodies under various circumstances, it is called linear
perspective-when it regards the fading of light and the modifying of colour
it is called aerial perspective.  As the art of painting depends entirely upon
the understanding of these two departments, the gradual progress which it
has made in different countries is a measure of the degree in which the common prejudice that things appear as they are has, in them, been overcome.
Where this prejudice exists, any untaught person conceives a good painting
to be merely a miniature representation drawn according- to a certain reduced scale,-as of an ipch to a yard,-and in which all the demensions of
things are to be measured as simply as in the reality-while the colour, as
to vividness, &c., should perfectly agree with the originals. This statement
is remarkably illustrated by the facts, that children in their rude atempts to
paint, always aim at realizing the notion of the art above detailed, and that
such has been the first stage of painting in every country. In Europe now,
owing to the labours of men of genius, art in painting may be said almost
to rival nature, producing scenes as lovely as the finest of nature's scenes,
and scarcely distinguishable from them: but in other countries, as in China
and India, among the native artists, the first stage of the art is still in existence.  In a Chinese picture, owing to the absence of perspective proportions, an extensive subject is only a collection of portaits of men and things
drawn all on the same scale, and placed near one another, and where all the
colours are as vividly shown as if the objects were only a few feet from the
eye; there, the figures at the bottom or fore-ground are supposed to represent the objects nearest to the spectator, while the figures higher up are supposed to be of more remote objects, all appearing as they might be seen in
succession by a person who had the power of flying over the country. This
kind of picture or representation, although not natural, if all viewed at once,
may communicate more information than a single common painting, for it is
equivalent to many such. In Europe, lately, the principle has been again
usefully acted upon for certain purposes, as for representing on one long
sheet or on a succession of sheets, connected in a suitable manner, the banks
of a river or a line of a road. The banks of the Rhine particularly have thus




374                            LIGHT.
been admirably portrayed, so that, the spectator directing his eye along the
paper feels almost as if carried in a balloon to view in detail the whole of
the real and enchanting scenery. The principle might, perhaps, with advantage, be acted upon still more extensively —for instance, to produce,
instead of common maps or charts of countries, true bird's-eye views, over
which the eye, moving from place to place, and at every new point of sight,
would see a certain portion of the country, as a bird or aeronaut would, the
sketch being supposed to be taken from that certain elevation deemed most
suitable for the ends in view.
3d Divergence of the rays of light.  (See Analysis,"page 355.)
- This is the next circumstance to be mentioned by which the eye judges
of distance.  Supposing the line E F to mark the place and breadth of the
pupil of the eye, the light entering from an object at a which is near (it is
here placed nearer than an object
Fig. 158.               could be seen in reality,) is very
divergent, or is spreading with a'Eg          _                       large angle; from b the pencil of
rays is less divergent, or opens
with a smaller angle; from c it is
less divergent still, and so on.
Now the eye, to form an image
on its retina requires to exert a bending power exactly proportioned to the
divergence of the received rays: and it appears to have a sense of the effort
made, which becomes to the person a kind of measure of the distance of the
object. This divergence of the rays entering the eye, is the chief circumstance in which the most perfect painting must still differ in its effect upon
the eye from a natural scene-for, first, in the natural scene, the objects are
generally more distant than their representation can be; and secondly, while,
in nature, every object, according to its distance, is sending rays which reach
the eye with different divergence, and which rays, therefore, can produce
distinct images on the retina at any one time, only of the objects which are
at the same distance from the eye, the rays from a picture, which is a
single plane surface, come from every part with the same divergence, and
the eye must feel a disappointment in not having to accommodate its power
of bending, to the different distances attempted to be portrayed on the canvass. It might be expected that this kind of disappointment would be more
felt on looking at a common picture placed a few feet from the eye, than at
the sort of picture called panorama, which is on a larger scale and proportionately more distant, but such is not the case: and the reason seems to be
that in the former the illusion is not intended to be complete, the fact of its
being but a picture not being at all concealed, and the eye is therefore at
once told to expect a difference of feeling; —but in the panorama, the whole
circumstances are arranged to deceive the eye entirely, if possible, and to
make it believe that the images on the retina are formed by light from the
objects themselves,-then to the eye, really deceived in all other particulars,
the non-accordance with nature in this one is strongly, and, by some persons,
even painfully felt, so as on their first entering the place to cause headache
or giddiness.-The illusion and consequently the pleasure from viewing any
picture may be made more complete by the spectator using lenses or spectacles, such that the focal distance shall be equal to the distance of the painting from the eye; because such lenses, as was formerly explained, would
render all the rays entering the eye nearly parallel, and therefore very nearly
such as would arrive from objects at a considerable distance.




THE  EYE-PERSPECTIVE.                  3' 75
4th.  Convergence of the axes of the eyes.  (See the Analysis, page 355.)
This is the last circumstance to'be mentioned, by which a person through
the eye, judges of the distance of objects.  In consequence of there being
in the two eyes corresponding parts which must be similarly affected by any
object, that the person may have a single vision of it, as was explained in a
former page, the axes of both eyes must point to the object, and if it happen
to be very near, they will meet and cross each other so near the face as to
produce the appearance of squinting,-seen when a person trying to look at
the point of his nose,-but if the object be more distant, the obliquity will be
less, until at last the eyes directed to a thing at a very great distance, will
have their axes almost parallel. The last figure may serve also to explain
this subject. Supposing E and F to mark the place of the two eyes if the
object looked at be near them, as at a, they must be very much turned
inwards, that their axes may meet at a; if it be at b, they will be less
turned, if at c less still, and so forth.
When the eyes are not directed to any thing in particular, the axes generally become parallel, or as if they were pointed to a very distant object; and
because this happens generally when persons are reflecting on things which
are absent and seen only by the mind's eye, it is an expression of countenance held to mark contemplation or thoughtfulness.
The direction of the visual axis is another particular, like the divergence
of light as to which a mere picture, can never produce upon the eye precisely the effect of the objectsathemselves.  To see a picture, the axes must
meet at it, and generally, therefore, at'a few feet from the eye; while to see
the objects of nature, they often do not meet nearer than at miles. By a
glass, however, as will be explained a little farther on, it is possible to correct
also this defect, and to render the optical illusion, as regards still objects,
absolutely complete.
When a picture has to represent objects supposed far from the eye, the
farther the picture itself is placed from the eye supposing the figures to be
made proportionately large, the more nearly perfect will the illusion become,
because the divergence of rays and convergence of the axes (the two circumstances in which the effect of a mere picture on the eye must always differ
from the effect of a real scene) will be in proportion more nearly natural.
This explains in part why the picture called panorama (from Greek words
signifying a viewo of the whole) is an exhibition so charming; for usually the
paintingis far removed from the eye, and is drawn on a proportionately large
scale, and the eyes feel that the light comes from a considerable distance, and
that their axes do not need to converge very much; and when in such a case,
the first impression of the want of absolute conformity to nature has passed
away, the illusion becomes nearly comnplete. But a not less important peculiarity in the panorama is, that instead of being a painting on a plane surface
like common pictures, and embracing only a small part of the field of view,
it is on a curved surface which entirely surrounds the spectator, and on which
all the objects visible in the various directions from the supposed place are
seen in the very situations which in nature they hold; and the spectator is
enabled to conceive much more distinctly of each particular by seeing it in
relation to others around. Few persons can forget the impression made on
them by the first panorama which they may have seen; and after increased
maturity of judgment, they will discover still more and stronger reasons for
admiring this almost miraculous mode of instantly transporting them to any
distance, beyond seas and other dangers, to contemplate at their ease the




376                           LIGHT.
most interesting scenes on earth, represented under the most favourable circumstances of position, light and weather.  Hence few persons of good taste
neglect the opportunity now, in most great towns so frequently offered, of
obtaining at little cost so high a gratification.
To correct the slight remaining optical defects of a common panorama, a
large lens may be used, of which the focal distance is equal to the distance
of the picture from the eye. This has the effect of diminishing the divergence of the rays until it becomes exactly that which belongs to the supposed
remoteness of the objects, and it also bends the light so that the axes of the
eyes may be nearly parallel. The author has found a convenient mode of
using the lens for such a purpose to be to cut out two round pieces from
opposite sides of it, and to form them into a pair of spectacles: —from one
lens three pairs may be formed. Panorama exhibitors should keep such
lenses or spectacles for the use of visitors.
The effect of the magnitude and distance of the ordinary large panoramic
views might, with the assistance of proper glasses, be had from even the
smallest picture on engraved representation embracing the same field; and it
is remarkable that some enterprising person has not undertaken to publish
sets of interesting views fitted to be used in that way. A common panorama,
occupying a circular wall of 150 feet in circumference and twenty feet high,
may be reduced-and still retaining the same truth of proportions, to appear
on a piece of paper five feet long and eight inches high or broad; and if
this were set up in a suitable frame, like a wall, round the head of a spectator,
while its edges were concealed by drapery or. otherwise, and the eye could
only view it through fit glasses placed in its centre and made to turn round
so as to command the whole, it could not by any ordinary spectator be distinguished from the large panorama. With the art of lithography, now so
well adapted for producing soft representations of scenery, the expense of
such views might be very moderate, allowing them to form a common part of
library furniture.  When we reflect upon the expansion of mind obtained by
travelling, and that not a few of the advantages would follow a familiarity
with a good selection of panoramic views, it is not perhaps too much to suppose that courses of instruction in geography, history, &c., may before long
be illustrated by this most interesting mode of aiding the conception and
memory.
Common paintings and prints may be considered as detached parts of a
panoramic representation, showing as much of that general field of view
which always surrounds a spectator, as can be seen by the eye kept in one
place, and looking through a window or other opening of moderate size.
The pleasure from contemplating these is much increased by using a lens or
such spectacles as above described. There is in the
Fig. 159.      shops such lens,'with the title of optical pillar machine, or diagonal mirror, fitted up so that the print
to be viewed is laid upon a table beyond the stand of
c1:1 7.        the lens, and its reflection in a mirror supported diagonally over it, is viewed through the lens. The illusion is rendered  more complete in such a case by
having a.box, as a b, on the bottom of which the painting is laid, and at the top of which the lens and mirror,
b _ -do fixed in a smaller box at a, are made to slide up and
down to allow of a ready adjustment of the focal distance. This box used in a reverse way becomes a
perfect camera obscura. The common show-stalls seen in the streets are




THE EYE-PAINTING REPRESENTING MOTION. 377
boxes made somewhat on this principle, but without the mirror; and
although the drawings or prints in them are generally very course, they are
Dot uninteresting.  To children whose eyes are not yet very critical, some
of the show-boxes afford an exceeding great treat.
A still more perfect contrivance of the same kind has been exhibited for
some time in London and Paris under the title of Casmorano, (from Greek
words signifying views of the world, because of the great variety of views.)
Pictures of moderate size are placed beyond what have the appearance of
common windows, but of which the panes are really large convex lenses
fitted to correct the errors of appearance which the nearness of the pictures
would else produce.  Then, by adding various subordinate contrivances,
calculated to aid and heighten the effects, even shrewd judges have been led
to suppose the small pictures behind the glasses to be very large pictures,
while all others have let their eyes dwell upon them with admiration, as
magical realizations of the natural scenes and objects. Because this contrivance is cheap and simple, many persons affect to despise it; but they do not
thereby show their wisdom: for to have made so perfect a representation
of objects, is one.of the noblest triumphs of art, whether we regard the
pictures as drawn in true perspective and colouring, or the lenses which
assist the eye in examining them.
It has already been stated that the effect of looking through such glasses
at near pictures, is obtainable, in a considerable degree, without a glass, by
having thepictures very large, and placing them at a corresponding distance.
The rule of proportion in such a case is, that a picture of one foot square at
one foot distance from the eye, appears as large as a picture of 60 feet square
at 60 feet distance.  The exhibition called the Diorama is merely a large
painting prepared in accordance with the principle now explained. In principle it has no advantage over the cosmorarno or the show-box, to compensate
for the greater expense incurred, but that many persons may stand before it
at the same time, all very near the true point of sight, and deriving the
pleasure of sympathy in their admiration of it, while a slight motion of the
spectator does not make his eye lose the right point of the view.
A round building of prodigious magnitude has lately been erected in the
Regent's Park in London, on the walls of which is painted a representation
of London and the country around, as seen from the cross on the top of St.
Paul's Cathedral. The real scene is unquestionably one of the most extraordinary which the world affords, and this representation of it combines the
several advantages of-the circular view of the panorama —the size and distance of the great diorama-and that from the details being so minutely
painted, distant objects may be examined by a telescope or opera-glass.
From what has now been said, it may be understood, that for the purpose
of representing still-nature, or mere momentary states of moving objects, a
picture truly drawn, truly coloured, and which is either very large to correct
the divergence of light and convergence of visual axes, or if small, is viewed
through a glass, would affect the retina exactly as the realities.  But the
desideratum still remained of being able to paint motion. Now this, too,
has been recently attempted, and in many cases with singular success, chiefly
by making the picture transparent. and throwing lights and shadows upon it
from behind.  In the exhibition of the diorama and cosmorama there have
been thus represented with admirable truth and beauty such phenomena asthe sunbeams occasionally interrupted by passing clouds, and occasionally
gilding the varied scene: perhaps darting through the windows of a venerable
cathedral and illuminating the interesting objects in its interior-the rising




378                           LIGHIT.
and disappearing of mist over a landscape-running water, as, for instance,
the cascades among the sublime precipices of Mount St. Gothod, in Switzerland;-and one of the most striking scenes of all, a great fire or conflagrr,tion. In the cosmoramo of Regent Street, the great fire of Edinburgh was
admirably represented; first, that noble city was seen sleeping in darkness
as the fire began, then the conflagration grew and lighted up the sky, and at
short intervals, as the wind increased, or as roofs fell in, there were bursts
of flame towering to heaven, and vividly illuminating every wall or spire which
caught the direct light-then the clouds of smoke were seen rising in rapid
succession and sailing northward upon the wind, until they disappeared in
the womb of distant darkness.  So naturally was all this represented, that
no stranger can have viewed the appalling scene with indifference, while on
those who knew the city, the effect can scarcely have been weaker, than if
they had witnessed the reality. The mechanism for producing such effects
is very simple; but spectators, that they may fully enjoy them, need not
particularly inquire about it.
It is remarkable, when the imagination is once excited by some beautiful
or striking view, how readily any visual hint produces clear and strong
impressions. One day in the cosmorama, a school-boy visitor exclaimed
with fearful delight that he saw a monstrous tiger coming from its den among
the rocks;-it was a kitten belonging to the attendant, which by accident
had strayed among the paintings. And another young spectator was heard
calling that he saw a horse galloping up the mountain side;-it was a
minute fly crawling slowly along the canvas. There is, in this department,
a very fine field yet open to the exercise of ingenuity, for the contemplation
of pictures representing motion or progressive events, may be made the
occasion of mental excitement the most varied and intense. For instance,
there are few scenes on earth calculated to awaken more interesting reflections on the condition of human nature than that beheld by a person who
sails along the river Thames from London to the sea, a distance of about
sixty miles, through the wonders which on every side there crowd on the
sight-the forests of masts from all parts of the world-the glorious monuments of industry, of philanthropy, of science-the endless indications of
the riches, the high civilization, and progressive happiness of the people.
Now this scene was lately in one of our theatres, strikingly portrayed by
what was called a moving panorama of the southern-bank of the Thames.
It was a very long painting, of which a part only was seen at a time gliding
slowly across the stage, and the impression made on the spectators was that
they themselves were sailing down the river in a steamboat, and viewing the
fixed realities. In the same manner might be most interestingly represented
the whole coast of Britain, or any other coast, or any line or road, or even a
line of balloon flight. There was another moving panorama exhibited about
the same time at Spring Garden, aiming at an effect of still greater difficulty,
viz., to depict a course of human life; and the history chosen was that of the
latter part of Bonaparte's career. Scenes representing the principal events,
were, in succession, made to glide across the field of view, and were so
designed that the real motion of the picture gave to the spectator the feeling
that the events were then in progress; and with the accompaniments of clear
narration and suitable music, they produced on those who viewed them the
most complete illusion. The story began by recalling the blow struck at
Bonaparte's ambition in the battle of Trafalgar; and to mark how completely,
by representations of various movements and situations of the battle, the spectators were in imagination made present to it, the author may mention that




THE EYE-PAINTING REPRESENTING MOTION. 379
on the occasion of his visiting the exhibition, a young man seeing a party of
British represented as preparing to board an enemy's ship, started from his
seat with a hurrah, and seemed quite confounded when he discovered that
he was not really in the battle.  To the views of Trafalgar succeeded many
others, similarly introduced and explained, in each of which the hero himself
appeared: there were his defeat, at Waterloo-his subsequent flight-his
delivery of himself to the British admiral-his appearing at the gangway of
the Bellerophon to thousands of spectators in boats around, while in Plymouh
harbour, previous to his departure for ever from the shores of Europe-his
house and habits during his exile, with various picturesque views of St.
Jfelena;-and last of all, that solemn procession, in which the bier with his
lifeless corpse was moving slowly on its way to the grave under the willowtree. The exhibition now spoken of might have been made better in all
respects, yet in its mediocrity it served to prove how admirably adapted such
unions of painting, music, and narration are to affect the mind, and therefore to become the means.of conveying most impressive lessons of historical
fact and moral principle.
Painting, whether employed to portray scenes of entirely still-nature, or
scenes involving some kind of motion as above described, has still, as its
great aim or end, merely to represent interesting subjects, and to give to the
spectator as much as possible that clear conception of them which is obtained
by ocular examination of realities; and thus, as a system of visual signs of
thought, it becomes, like language, which is a system of audible signs, a
means of expanding the boundaries of individual human existence into wider
space and time, and thus of elevating human nature. While it portrays only
strict matters of fact, whether of past or present time, as particular human
individuals, objects of natural history, the beautiful and magnificent scenes
of nature, interesting events which the artist has the means of faithfully
representing, &c., it may be called truly historical painting, embodying the
materials of true history, both natural and civil, and then it is of singular
value. But even when applied to other purposes, it may be fraught with
delight; and just as language, of which the grand object or use is to express
strict truths, has still been admirably employed in giving a permanent existence to a variety of fictions, from the wildest fables and rhapsodies to the
historical plays and novels of modern times, as those of Shakspeare and of
Scott —which plays and novels, although not furnishing true portraits of
individual human nature, are yet most correct portraits of general human
nature-so may painting be employed in embodying fictions adapted to its
peculiar powers, and it may do so in a manner to prove the artist endowed
with the highest degree of human genius. It should always be recollected,
however, that what is usually dignified with the name of historical painting,
really bears to historical truths only the kind of relation which novels and
plays bear to it, and often approaches even less nearly to the truth; for it
pretends to relate a thousand minute circumstances which no history has
preserved, and which, therefore, only the imagination of the artist can supply.
Thius when a painter, knowing that Lucretia stabbed herself in the presence
of her father and others, after the crime of Tarquin, exhibits a woman dying,
and a certain number of persons around her in horror and astonishment, he
no more represents the real Lucretia and her friends than he represents any
other particular young woman and her friends; for lie is quite assured that
not one of the figures in such a pictute is a portrait of the individual whose
name it bears; his picture, therefore, in so far, is as untruth or-fiction, while
it very probably has some of the additional errors and even absurdities so




380                           LIGHT.
common among historical painters, in respect of national usage in costume,
religion, manners, &c, and in respect to the general personal appearance,as when a Rubens, wishing to represent Sabine or other ladies, gave them
the Dutch corpulency deemed comely in his own country, although it
strikingly contrasted with the true forms of Italian or Grecian nymphs.
From all this it appears that historical pictures may often be regarded as
portraitures, not of the realities, but of comedians acting scenes in historical
plays intended to represent the realities.
In dealing with the events of ordinary history, there is no strong reason
why artists may not please themselves and their spectators as we have now
been describing; but it may admit of doubt whether similar liberties should
be allowed with respect to religion   Yet any painting of the last supper, for
instance, or of the ascension, is not more true than a theatrical representation.
To judge of the nature of such a picture we have only to suppose any of the
events recorded in the New Testament to be represented by a painter in
China with the countenances seen on Chinese tea-boxes; such a representation would appear in Europe revoltingly absurd; but the common practice
here is only a degree better, Italian countenances being usually substituted
for the Jewish; and twenty painters, undertaking the same subject generally put different persons into all the situations.  Then it can produce no
pleasing impression on a Christian's mind to be told, that an admired painting of the crucifixion was made chiefly from the body of an executed mur
derer; or that for a praised representation of the triumphal entry into Jerusalem, the painter had deemed his own physiognomy the most befitting for
the principal figure, while he copied the portrait of Voltaire as a specimen
o the bad Jews, of Newton as a specimen of the good, and of wife, cousins,
acquaintances, and old clothes-men, to make up the remaining groups. With
the knowledge that such things have often been, it need not surprise that
many persons of correct feeling turn with horror from all these mimicries
and falsehoods, to seek their idea of God and his providence in the sublime
descriptions of his attributes, which written language conveys and which all
creation, in a mute language not less impressive, so strongly confirms. When
men generally could not read, and as a mass were extremely ignorant, various
means of fixing their attention upon religious subjects might be useful, and
therefore proper, as sacred plays, certain processions, pictures, &c., which
have now in many countries ceased to be either; but a person of good sense
will continue to regard with a certain respect whatever at any time may have
contributed to reclaim portions of mankind from barbarism and wickedness
to the just appreciation as the divine charities of a pure religion.
There are in painting other classes of fictions, which pretend to nothing
beyond fiction, and which yet are truly admirable; such are personifications
of the virtues and vices, serving to recommend the practice of the former,
and to deter from that of the latter —almost all Hogarth's works are of this
character, and evince the highest mental acumen and genius:-then may be
mentioned the personifications of what have been called the elements and
powers of nature, including many of the personages of the heathen mythology-then other generalizations of the characteristics of human or other
nature, as scenes of domestic affection, of the play of the passions, &c., &c.;
and because many subjects when so sketched, are intelligible to the eye with
the suddenness of lightning, where longest verbal description would convey
the idea but imperfectly, the art of painting, in regard to them, possesses a
truly magical and inestimable power.
As painting, whether employed to represent matters of fact or of fiction,




THE  TELESCOPE.                            381'
can accomplish its ends only through the means of drawing or linear perspective, and of shading and colouring, or aerial perspective, these subjects
require to be studied by every Artist with great attention; but it is important
for all to be aware that the greatest mastery over these, which are merely the
mechanical parts of the art, will go a very short way towards producing good
performances, unless there be present also the genius to select or to compose
subjects worthy of being represented,-indeed, will go little farther to make
a painter than the learning of mere penmanship goes to make a historian or
a poet. This remark seems the more necessary, because there is in human
nature a disposition to value so much the means by which important ends are
attained, that often the end itself is forgotten in the contemplation of the
means-as when a person, perceiving that money will procure all desirable
things, at last becomes the insane miser, and dies from want of the common
necessaries of life rather than touch his hoarded treasures:-while among
painters, as among persons of other occupations, the talent for the inferior
or more mechanical departments of the art, is more common than for the
higher. Do we not see the subordinate accomplishments of the painter, by
not a few, both artists and pretended connoisseurs, supposed to be the prinpal? But this is evidently to value the dress or clothing instead of the
person; or like the bibliomaniac, to regard the type and binding of books
more than' the subject. To prove how unessential what is called highfinishing in painting, is to the complete attainment of the purposes of the
art, we may instance the cartoons of the immortal Raphael, which to the
mere mechanic in art appear almost daubs, although exciting such enthusiasm in the superior mind: and many of the mere sketches of genius are
to a true taste more precious than some of the most labored pieces in our
galleries. As it is of no importance to a man who sees approaching the
friend of his heart, whether it be by day-light or candle-light, or with the
source of light above or below, &c., provided there is light enough for him
to distinguish clearly the friend of his heart, so is it of no importance how
any interesting subject is represented, provided the picture vividly excite a
true conception of the subject. A painter will discover the difficulties which
a brother artist had to surmount in representing an object in some particular
predicament, as regards the light, &c., and may estimate the talent accordingly: but the great proportion, even of the most accomplished ordinary
spectators, will generally be looking beyond the sign to the thing signified,
heedless of the artist's difficulties.  In consequence, however, of the prejudice in favor of " a sweet or adorable bit of colourino," as it will sometimes
be called-and which in truth may have the merit of most natural colouring,
there are preserved in many galleries pictures disgusting in almost all other
respects, as of drunken Dutch boors, with fiery noses and physiognomies
degrading to human nature, &c., &c., on seeing which the man of taste
deplores that the art of painting should so often have been prostituted by
clever men to the vile purpose of representing things of worse than no interest.
" When the image formed, as above described, beyond a lens, is viewed in
the air by an eye placed still farther beyond in the same direction, the
arrangement, according to minor circumstances, constitutes either the
common TELESCOPE o0r MICROSCOPE."  Read the whole second paragraph
of the Analysis, page 325.
The name of telescope (a compound Greek term, signifying to see far, as
microscope signifies to see what is small,) applies to that wondrous instrument of modern invention by the use of which the intelligent soul may be




382                            LIGHT.
said, on the beams of light as its path, to dart widely into space for the purpose of contemplating the distant glories of creation; or again, by which it
can command distant objects instantly to approach, for the purpose of convenient inspection.  In ancient times, a man, while looking with admiration
on the bright face of the moon, might have exclaimed " How pleased would
I be, had I the power to fly upwards to that celestial orb, the better to understand its nature and beauties;" but he could little have anticipated that the
day was coming when human ingenuity would find means in. a great measure
to satisfy the wish:-now the telescope is this means, for one which merely
doubles apparent magnitudes, shows the moon exactly as she would appear
to a person who had ascended towards her from the earth a distance of 120,000
miles, while one of greater power produces effects correspondingly great.
-But to examine the heavenly bodies is by no means the only use of the
telescope, man being often extremely interested-to discover what is passing
at a distance on the surface of the earth around him.  Thus, by a telescope,
the military chief obtains a close view of approaching friends or foes, while
they are yet concealed from the naked eye, in the blue mist of distant mountain or plain-and similarly the sea-captain, while persons around him perceive only a little speck on the far horizon, discovers there a ship of class and
nation at once evident to him, and with the crew of which, by the additional
use of signal flags, he is enabled readily to converse  At midnight, a telescope directed to a distant cathedral, may so effectually call it into the presence
of the observer, that on the clock-turret may be watched the progress of the
slow hands which tell of the unceasing lapse of time. A man, in the midst
of a wide plain, or on a lofty hill-top, or far on the face of a lake, who might
suppose himself quite alone and unseen, would yet, by a telescope, be
instantly placed under the observation of whoever chose to watch him. And
the same might happen to a man within the high walls of his own garden,
or even within his house near an open window, if a straight line could reach
him from some station where an observer was.  Some remarkable cases of
actions, imagined by the parties to have been done in perfect secrecy, have
thus been brought to light.
Now the telescope, with its extraordinary powers, exhibits but another
nlodification of the simple case (described at page 341, and exemplified in
the camera obscura, &c.,) of an image formed for visual inspection beyond
a lens   And we shall here explain that its powers depend altogether on the
two circumstances, first of its large lens collecting, for the formation of the
image (subsequently transferred to the observer's retina) a thousand times
or more the quantity of light which the naked pupil could receive; and
secpnd, of its forming by this light an image, to which the eye may be brought
very near, so as to examine it with magnifying glasses of any power.
To understand this well, we must recall, that the nature of the bending of
light in passing through a lens is such, that all the rays reaching the lens
Fig. i60.
- T                        A
from any point of a visible object in front, and forming what is called a pencil
of light-as that spreading from the point A of the cross here represented, to




THE TELESCOPE.                             383
the lens L-are collected in a corresponding point, as a, at the focal distance beyond the lens, so as always to meet the central ray-of the pencil
here, (the direct line A a,)and, therefore, when the light comes from above
the centre of the lens, the focal meeting is below, as shown here; and when
it conies from below, the meeting is above; then the same happening as regards every visible point of the object (the rays from only the two -extreme
points A and B are here represented) at corresponding points beyond the
lens in the space between a and b, the collected light, if received on a white
screen placed there, as in the camera obscura, will make apparent to an eye
in any direction a beautiful inverted image of the object. Now in the place
where the rays meet to form this image, if no screen be interposed, the rays,
although not lost or destroyed, but merely cross each other in the air, without interference, nearly as they previously crossed in the lens, and spread
-again beyond the focal points, or towards c as here shown, as they originally spread from the several points of the object itself; an eye, therefore,
placed anywhere beyond c, must receive portions of the pencil from  every
point of the image, and may see the image in the air as it would see an
object situated w'here the image is, in the focus of the lens.  This may be
observed at once by holding a spectacle-glass or any lens at a proper distance
between an object and the eye.
Now a telescope is merely a long tube, blackened within to exclude and
destroy useless light, and having a la:ge lens, called the object-gltss, filling
its distant end, to gather the ligbt from the objects in front, and with that
light to form images towards the other or near end of the tube, where the
eye may conveniently inspect them. These images, for a purpose to be immediately explained, are examined through another lens called the eye-glass,
which is fixed in a small tube made to slide backwards and forwards in the
large, so as to admit of the focal distances being adjusted to the power of
different eyes, &c. The accompanying skletch shows the progress of the
Fig. 161.
light from  the object A, through the object-glass L, to form an image b a,
and afterwards to be bent by the eye-glass, D, so as to enter the pupil of
the eye at E, where the rays cross, to form the last image on the retina.
In the simple telescope with only two lenses, as above represented, called
the astronomical telescope, or the night-telescoepe, because chiefly used at night,
the image is inverted; but this is a circumstance of no importance in viewing the heavenly bodies; to fit the instrument, however, ior viewing terrestrial objects, it is necessary to place in the tube another simple or compound
lens, which shall form a second image from the first, and by inverting a
second time, shall produce an image equally upright.
To determine how much larger an object will appear when viewed through
a certain telescope,-for instance, through one with an object-glass of three
feet focus, —than when viewed by the naked eye, we must recollect that the
image is formed in the focus of the object-glass, or at b a, in the last figure,
and subtends from the centre of that glass or lens, the same visual angle as
the object itself (a fact explained at page 357,) and to an eye placed there




384                            LIGHT.
would appear of the same size as the object, but if the eye be brought nearer
to the image than the centre of the object-glass, the image will appear by so
much taller and broader, and thus, as cplpared with the object, may be
called so much magnified. Now as the naked eye cannot see distinctly an
object nearer to it than at about six inches, because of the great divergence
of light from a nearer radiant point, the telescope in question, without an
eye-glass, would allow the eye to come only six times nearer to the image
than when at the centre of the object-glass and would only magnify the
diameter six times; but if then an eye-glass, as D, of half. an inch focus
were placed half an inch from the image, so as to render the rays of every
pencil parallel, and therefore fitted to the powers of the eye, while the different parcels would cross each other a little way-beyond the glass, as shown
above, an eye placed to receive in its pupil the crbssing parcels, would see
the image as large as if at half an inch from it, and therefore 72 times nearer
than if viewed from the object-glass, and therefore again as of 72 times greater
diameter. Now, as in all cases, the image in a telescope is in the focus both
of the object-glass and eye-glass, and is therefore nearer to the latter than to
the former in proportion as their focal distances differ, the magnifying power
is measured by that difference-in the case at present supposed, the difference is as 72 to 1, and 72 is the magnifying power of the telescope. The rule
is generally thus expressed, " divide the focal distance of the object-glass by
that of the eye-glass, and the quotient is the magnifying power." It is always
to be remembered, that if the diameter of an object be magnified 10 times,
the surface is magnified 100 times, and so in proportion for other numbers.
With such means of aiding the sight, then, it is that we discover the
mountains of our moon, and can even measure their altitudes; that we can
see the four beautiful moons of the planet Jupiter; that we can perceive
marks and irregularities on the surfaces of the other planets, enabling us to
say at what rate they severally whirl round their axes, experiencing the
phenomena of day and night; —and that we can determine many other
interesting particulars.
The discovery of the telescope is said to have been first made accidentally
by the children of a Dutchl spectacle-lmaker, while playing with their father's
work; but it was turned to no use until Galileo, led by science, fell upon it
again, and with the knowledge of its worth, obtained from it the most sublinie results.  The human heart can rarely have throbbed with such delight
as Galileo's when he first directed his optic tube to the heavens, and through
it contemplated so many glorious objects before unseen by human eye; —as
the planet Venus, our' beautiful morning and evening star, appearing not a
circle, but a crescent like our moon in her quarters-as the satellites of Jupiter-the rings of Saturn-myriads of stars until then invisible to man;
and in a word, when he beheld the undoubted proofs of the true system of
the universe, as his genius had before conceived it, uniting the greatest
simplicity with unspeakable grandeur.
The Galilean telescope was simply a large object-glass to collect much
light, with a small concave eye-glass placed so as to intercept the converging rays before the  reach their focus, arind to change their convergency into
the parallelism  which the eye could command.  This telescope, although
magnifying less than that made of two convex glasses, as above described,
still, from occasioning no loss of light by the crossing of rays in forming an
image, was of considerable powdr.  The common opera-glass is a telescope
made on this principle.
It was explained at page 338, that a ray of light, in being bent or refracted




THE  TELESCOPE.                            385
ty transparent media, as by a lens, is also- divided into rays of the different
colours'seen in the rainbow. Hence an image formed behind a simple lens
has coloured edges or fringes. This fact rendered the images of small objects much magnified, in the first made telescopes, very indistinct: and, but
for the important discovery made by Dollond, the optician, that different
kinds of glass have dispersive and refractive powers of different relative
force, so that a concave lens of a certain curve applied to a convex lens
might completely counteract the dispersion of colour by the latter, while it
left enough of the convergence of the rays for the formation of an image —
refracting telescopes would have always been very imperfect.  Dollond
called his telescopes achromatic, or not-colouring. It is very remarkable,
that he had the fortune to obtain some glass for his purposes, more suitable
than any which has been procured since, or which could be made by known
rules, until the late improvements in the manufacture suggested by the ingenuity of Mr. Farraday.  The author of this work carried abroad with him
a small telescope of old Dollond's, which often gave more correct information
respecting minute colored objects at a distance, as signal flags at sea, than
much larger glasses of modern make.
The MIrcoscoPE of greatest power and with the form called compound,
in its structure approaches very closely to the telescope, the chief difference
being, that while in the telescope a large distant object forms in the focus of
the object-glass an image exactly as much smaller than itself as the distance
of the image from the glass is less-in the microscope conversely, a small
object placed near the focus of the object-glass produces a more distant
image, as much larger than itself as the image is more distant than it-and
in the one case as in the other, the image is viewed through an appropriate
eye-glass.  The object-glass in the telescope is large, in the microscope it is
generally very small. If, in the latter, an object-glass be used of one-eighth
of an inch focal distance, and the object be so placed that its image is formed
at six inches, the image will be of diameter 48 times as great as the object, or
will have nearly 2,500 times as much surface: and if that image be viewed
through an eye-glass of half an inch focus, the image will appear still twelve
times larger, or 30,000 times larger than the object.
A simple convex lens is called a single microscope, and it magnifies, as
already explained, chiefly by allowing the eye to be brought much nearer to
the object than the distance at which the object could be seen without the
glass; but even where the distance of the eye and object is not changed, a
lens interposed will still magnify by bending the light, as at d and f, making
that which comes to the eye at e from
the top of such an object as the little         Fig. 162.
cross a, to appear to come from  b,
and that'from the bottom  to come                         l......
from  c, thus magnifying the cross                       K
here represented by the black lines   e
to appear of the size represented by                               C
the dotted lines.  A concave lens
minifies for the contrary reason.
Perhaps there is not a greater treat for a person who has feeling for the
beauties of nature, than to explore with the microscope. While the telescope
lifts the mind to the contemplation of boundless space occupied by myriads
of suns, and exhibits this globe of ours as less, compared with the universe
around it, than a leaf is compared with a forest, or one grain of sand compared with all which lies on the ocean's shore, the microscope again excites
25




38i6                           LIGHT.
new astonishment by showing on a leaf, or in a single drop of some water in
which the leaf has been infused, thousands of living creatures, and of creatures not imperfect because thus small, but endowed with organs and parts
as complex and curious as those of an elephant. And he who admires the
curious structure of a honey-comb, may bend his eye through the microscope
upon the cut surface of a willow-branch, or of other wood, there to see a
similar structure more wonderful still; or he may compare the lace of a
fly's wing with the most perfect which human art can weave; or the beautiful proportions and perfection of the limbs and weapons of an insect, invisible, perhaps, to the naked eye, with any larger objects of the kind already
known to him.
Telescopes and microscopes might with propriety be both called microscopes, for often the telescopic object subtends to the naked eye eyen a smaller
angle than the objects which the microscope examines. The minutest visible
insect at hand may hide from the eye a planet at a distance.  The image in
the telescope, however, is always much smaller than in the microscope because the rays from a distance b]eing nearly parallel, must form the image
nearly in the principal focus of the object glass; while for the microscope,
the rays from the near object being very divergent ma.y be made to form the
image far beyond that focus, and therefore proportionally larger.
Light falling on very smooth or polished surfaces, is reflected so nearly
in the order in which it fills, as in m.an~y cases to appear to the eye as if
coming directly from  the objects originally emitting it,-and such surfices are callec MIRRORS; the surface of which is flart as well as polished,
is called a plane mirror."  (Read the Analysis, page 325.)
If, on a marble slab, or other flat surface, (represented here at M R, with
the edge supposed towards the spectator,) a ball
Fig. 163.        were projected from A perpendicularly towards'D, the ball would rebound directly back to A,
but if projected obliquely, as from B to I, it
C       would not return to the first situation B, but to:    b, a corresponding situation on the opposite side
of the perpendicular, thus making the angle of
cZ.....2:'. -_    A    the return or reflection equal to the angle of
approach or incidence; the same would be true
of a ball approaching obliquely from any other
point, as C, and rebounding to c. Now light
is reflected from polished surfaces according to
R              the same law, so that an eye at A would see
itself as if placed at d, an eye at b woald see an
object really at B, as if it were at e, and so forth.
Where the existence of a mirror is not suspected, the objects reflected from
it are held to be realities placed beyond where it is. A wild animal will
attack its image in a glass: and the fable says that a dog crossing a brook,
quitted the piece of meat in its mouth to catch the tempting image which he
saw in the water below. The reason that an object seen in a plane mirror
appears to be just as far beyond the mirror as its true distance on the side
of the spectator, is, that the diverging rays of a pencil of light have the
same divergence after as before reflection.
Any plane very smooth surface reflects light as now described, and is a
mirror; but different substances send back very different proportions of the




MIRRORS.                               387
light which falls on them. A highly polished metallic surface is the best
mirror, often returning three-fourths of the whole light.  Hence, in reflecting telescopes, the mirrors are made of polished metals.
Our common looking-glasses are really metallic mirrors, for it is the
smooth, clear surface of the quicksilvered tin foil behind the glass which reflects the light, the glass itself merely serving the purpose of preserving the
metallic surface perfectly clean and flat. There is always an imperfection
in such glass mirrors, when used for viewing objects obliquely, because the
external surface of the glass acts also as a mirror, although so much more
feebly than the metal behind, and forms a separate image not quite coinciding with the other, and therefore mixing with and confusing it.
The mirror-power of glass unaided is seen from the panes of a plate-glass
window, which make objects in front very visible, although by no means
with clearness comparable to that from a metallic surface. All common
panes of glass in windows, or in print frames, &c., reflect as much light as
plate-glass, but the reflection beihg irregular because tie surface is irregular,
scarcely attracts notice.
The smooth surface of a fluid is a mirror, which is, moreover, horizontal;
and when that surface is metallic, as of mercury, the mirror is most perfect.
In water, spirits, oil, or any other liquid, it is also perfect, but feebler.
The mirror of liquid quicksilver is sometimes used by astronomers in
observing the apparent altitudes of the heavenly bodies, for the image in the
miirror appearing exactly as much below the horizon as the object is really
above it, half the distance between them is the true height.
A varnished picture or any japanned surface, is a mirror: nay, even a
polished table of mahogany or other wood-as it is well known among playful children. The author, while writing this, has before him a table covered
with black leather, and in that covering, as a mirror, he clearly sees the
bright objects beyond the table. Polished stones, as marble slabs, &c., reflect
as much as glass. Even a surface of air may be a mirror, as where a cold
and dense stratum happens to lie in contact with a warmer and rarer stratum.
In such cases, where particular causes have unequally heated different levels
of the atmosphere, the trees, islands, &c., happening to be below, are reflected
from above, and appear as if in the sky. This phenomena is called mirage.
It is often to be observed over the burning sands of Africa, where the air is
much heated; and elsewhere certain kinds of mists and thin clouds produce
a similar effect, causing, for instance, a ship to appear as if suspended aloft,
with keel uppermost.
In certain cases, an object seen by the light reflected from a mirror appears
reversed, as when the right hand of a person standing before a glass becomes
the type for the left hand of the image; or when a tree, or rock, or mountain, seen in the mirror of a lake, has its top downwards.
It is on this account, that a man painting his own portrait from a mirror,
is apt to reverse all the accidental characteristics of the countenance or person, not the same on both sides; and if, as is generally true, one eye be
higher thana the other, or the ndse be a little to one side, a very incorrect
resemblance will be produced. Hence also a person whose countenance is
at all thus peculiar never sees himself in a mirror as he appears to others;
and a belle or beau, who has decided that a curl is more graceful on the left
temple, may unconsciously leave it on the right.
By an image, however reflected from a first mirror to a second, and from
that to the eye, persons may see the object, or themselves, if they choose,
as others see them. What a pity that there are not some moral mirrors to
answer an analogous purpose!




888                            LIGHT.
A candle placed between two parallel mirrors fixed on opposite sides of a
room, makes visible in either glass to a spectator near the middle of the
room an endless straight line of lights. If the glasses be inclined to each
other, the lights will appear as if placed in the circumference of a circle, of
which the centre is where the prolonged mirrors would meet: this fact is
well illustrated in the beautiful toy called the kaleidoscope.  It is possible
to place a few mirrors in such situations around an apartment, that a man
entering it, may see himself multiplied into a crowd, and a few ornamental
pillars may produce the effect of. thousands formed into long colonades of
retiring lines.
The sun or moon reflecting in a still lake, appear as they do in the sky;
but if the surface of the water become at all ruffled by the breeze, instead
of one distinct image, there will be a long line of bright tremulous reflection.
The reason of this appearance is, that every little wave, in an extent perhaps
of miles, has some part of its rounded surface with the direction or obliquity
which according to th$ required relation of the angles of incidence and reflection, fits it to reflect the light to the eye, and hence every wave in that extent
sends its momentary gleam, which is succeeded by others.
Although the external surface of glass reflects but a small part of the light
which falls upon it, being, therefore, a feeble mirror, still curiously, if light
which has entered a piece of glass, fall very obliqueFig. 164.        ly upon the back or internal surface, instead of
E~A              A    passing out there, it is more perfectly reflected than
it would be by the best metallic mirror. This light
from A entering a piece of glass at B, is entirely
D~  3B  reflected at C, the back of the piece, and escapes
C             at D towards E.  The back of a wedge of glass,
or common prism, thus becomes a perfect mirror.
It is this fact which enabled Dr.'Wallaston to construct that beautiful little
instrument called by him the Camera Lucida.  The two surfaces at the
back of the small prism of glass A become mirFig. 165.         rors, the first reflecting to the second, and the
second to the eye at E, the objects in the landscape before it, while the eye also sees through
the glass to the paper below at B, and may suppose the imagery to be feebly portrayed on the
paper: with a pencil that appearance is made
permanent, and a correctly-drawn outline of the
scene is at once obtained The instrument for:j3B              assisting draftsmen is still simpler than the camera obscura. Other modifications of the instrument have since been contrived.
The same fact of the internal surface of a transparent mass becoming a
mirror, gives us the explanation of that apparition or phenomenon so
admired before it was understood, and not less admired since-the rainbow,
or arc in the sky, as in France and elsewhere it is named-an object which
the poets of nature have almost worshipped for its beauty, and which few of
us can cease to remember as one of the delights of our boyish days, when
we saw it stretching over the haunts of our young pleasures, and may have
pursued it in the hope of catching some of the falling rubies and emeralds,
or bright coloured dew of which it might be composed.
When a partial shower of rain falls on the side of the landscape opposite
to where the sun is shining, there immediately appears in the shower a




I I R R 0 R S.                         889
variegated arch, red at its external border or confine, and then successively
orange, yellow, green, &c. (in the order of the colours of the prismatic spectrum
described at page 337,) towards its inner border. Its centre is directly opposite to the sun, or at the end of the straight line supposed to be drawn from
the sun through the eye of the spectator towards the opposite horizon; and
being, therefore, always under the horizon, the bow is less than a semicircle.
The diameter of the circle of which the bow is a part, occupies nearly 82~
of the field of view, that is to say, the bow always coincides with a hoop of
one foot diameter held eight inches from the eye.  There is a second bow of
much fainter light than the first, and with the colours in reverse order: it is
of 108~ diameter, and therefore external to the other.
Now the explanation of this miracle of beauty is simply as follows.
While the sun shines upon the spherical drops of falling rain, its light falls
upon the whole central part of any drop, passes completely through, but that
portion which enters near the edge of the drop, as at a, is refracted or bent,
and reaches the back surface of the drop at y so
slantingly, or an angle so great, that it suffers there   Fig. 166.
an entire reflection instead of being transmitted; the
ray, therefore is turned to b, where it escapes from
the drop, and as here shown, descends to the earth
or eye in tle direction b e.  Thus every drop of rain    y
on which the sun shines is a little mirror suspended
in the sky, and is returning at a certain angle all..
round it, viz., at an angle of 4~0, a portion of the
light which falls on it; and the eye placed in the re-             e
quired direction receives that reflected light.  If in this case, however, there
were reflection only, and not also refraction with separation of colours, the
rainbow would be only a very narrow resplendent arc of white light formed
of millions of little images of the sun; but
in truth, because the light, which enters            Fig. 167.
near the edge of the drop, traverses the
surface very obliquely, it is much bent or    2
refracted before its reflection, V seen at a,                   -:.........7 
and is divided into rays of its seven col-:  
ours, as it would be on passing through a.'
prism  (as explained at page 337;) and     7
this division or separation continuing after...2
the light again escapes from the drop at b,
instead of one white ray descending from                             m
each drop to a certain point of the earth,
seven rays descend (here marked by dotted lines from the figure 1 on the
left hand to 7, 6, 5, &c., on the right, and with separation greater than occurs in reality to make it very evident,) and of these rays, an eye can only
receive one at a time from the same drop, which drop will then appear of the
colour of the ray: but for the same reason that seven eyes placed in a line
from above downwards, as at 7, 6, 5, &c., on the right would be required to
see the seven colours from one drop in the centre of the bow, so one eye looking in the direction of seven drops situated in a corresponding row, as from
1 to 7 on the left, will catch the lower or red ray of the upper, the orange
or second ray of the next, the yellow or third ray of that which follows, and
so on, while it will lose all the others, and thus will see the several drops
as if they were each of one colour only. Of such elements, then found in
the same relative directions all around the eye, the glorious arch is formed.




390                             LIGHT.
No two eyes can see the same rainbow, that is, can receive light from the
same drops at the same time; and the same eye does not for two instants
receive light from the same drops. This rainbow can never appear to a person on a plain, unless when the sun is within 41~ of the horizon, for otherwise the centre of the rainbow would be more than 410 under the horizon,
and therefore the whole circumference would be below it too.
We have described above what is called the
Fig. 168.         principal bow, formed in the drops by two
refractions, and one reflection of light.  To
produce the fainter second or external bow,
mentioned above, and of which the colours are
in reverse order, the light which enters on the
under side of the drop as at a, is reflected first
at y, then again at b, and escapes at c towards
the eye: after two reflections as well as two
refractions. As the semi-diameter of the bow
is 540~ it may be visible when the internal bow is not.
An artificial rainbow may be produced in sunshine at any time by scattering water-drops from a bush or otherwise; and a rainbow is often seen among
the spray of a lofty waterfall, or of a stormy sea.  The cut-glass ornaments
of chandeliers, &c., produce colours on the same principle as rain.drops; as
do also mist and particles of frozen water between a luminous body, and the eye
exhibiting the circular coloured halos often observed around the sun and moon.
A white halo is light reflected from the external surfaces of drops or particles.
"'Mirrors may be plane, convex, or concave; and certain curvatures will
produce images by reflection, just as lenses produce images by refraction;
in consequence, there are reflecting telescopes, microscopes, &c., as there
are refracting instruments of the same names."  (See the Analysis,
page 325.)
While a plane surface reflects light, so that what is called the image in it
of a known object may readily be mistakenfor the reality, convex or cohcave mirrors reflect as if every distinct point of them were a separate small
plane mirror, and their effects on light correspond with a relative inclination
of the different parts. The only forms of much importance are the regularly
spherical or parobolic concave and convex mirrors.  We shall now find that
these produce on light similar effects with lenses, only the concave mirror
answers to the convex lens, and the convex mirror to the concave lens.  It
is the concave mirror which gathers the light to form images in the most
perfect telescopes that exist, as those of Herschel and others.  Admirable as
is the refracting telescope, it still falls short in certain respects of the telescope acting by reflection.
In a hollow sphere, or part of a sphere with
Fig. 169.         polished internal surface, if rays radiate from the
A.                   centre in all directions, they reach every part
perpendicularly, and therefore are thrown back
C    to the centre.  Thus if A  B were a concave
spherical mirror, of which C. were the centre,
rays issuing from C. would, in obedience to the
~3B                  law that the angles of incidence and reflection
are equal, again meet at C.
It can be proved also, that any ray parallel to




CURVED  MIRRORS.                             391
the axis, falling upon such a mirror, will be         Fig. 170.
reflected inwards so as to cut the axis halfway between the mirror and its centre, viz.,    /
at D, the centre being C.  Then as all pa-           D        C
rallel rays must meet in the same point,
that point becomes a focus, as already ex-                            d
plained for lenses, and there an image of the
sun will be formed when the mirror is held
directly towards the sun.  This point is called the principal focus of the
mirror.
For the same reason that parallel rays meet in the focus, so will rays,
issuing from the focus towards the mirror, become parallel, after reflection,
as seen above or in the figure at page 271; and if they be then caught in a
second and opposite mirror, as also represented at page 271, corresponding
effects will follow.
Now, for a concave mirror, as already explained for a lens, when rays fall
on it obliquely from one side of the axis, their focus will be on the opposite
side, and therefore the mirror will form an inverted image of any body
placed before it, just as the lens does; and the image will be near or distant
and large or small, according to the divergence of the approaching rays,
exactly as happens with lenses; and thus the camera obscura, magic lantern,
telescopes and microscopes, may all be formed by mirrors, as they may be
by lenses.  Moreover, concave mirrors magnify, as concave lenses of the
opposite names do. The two subjects of images by refraction and by reflection run so nearly parallel, that it would be useless repetition here to enter
upon the detailed consideration of the latter subject, and we shall therefore
content ourselves with showing why a concave mirror magnifies and why a
convex mirror minifes.
A concave mirror magnifies be-                 Fig. 171.
cause the light from the top of the
cross at A, reaching the mirror   C          --       -
where it can be reflected to an eye                                  A
placed at F, viz., at E, seems to!              l    -'
the eye to come from C, and the...i.;1.
light of B  similarly appears to
come from ID, so that the cross A
B, by the reflection, seems to the
eye to be of the greater dimensions C D.
In the convex mirror, again, for corresponding reasons, the cross A B
appears only as C D, and therefore much smaller than the reality.
Concave or magnifying mirrors are often
used by persons in shaving.                            Fig. 172.
A convex mirror is a common ornament of
our apartments, exhibiting a pleasing miniature of the room and its contents.
Any polished convex body is a mirror, and
therefore the ball of the human eye is one, in.. —...
which  we may contemplate  most perfect  D
miniatures of surrounding things. It is the
image of the window or of the sun in the
convex mirror of the eye, which painters usually represent by a spot of
white paint there; and a similar luminous spot or line must be made when




392                            LI~GIT.
they have to represent almost any of the pieces of furniture which have
rounded polished surfaces as bottles, glasses, smooth pillars, &c.,
"Convex lenses thus are also mirrors to all the objects around them, and
very strikingly so, owing to the perfection of the form of a lens. The
polished back of a watch, often; in the same way, attracts the attention of a
child, who wonders to see there so clearly "the little baby."
It has been mathematical amusement to calculate what kind of distortion mirrors of unusual forms will produce, and then to make distorted drawings, which, when reflected from such mirrors, might produce in the eye the
natural image of the objects.
When a concave mirror is used for a telescope, the image formed in front
of it, and to be examined through the magnifying eye-glass, may be viewed,
-first, as in Herschel's telescope, by the spectator turning his back to the
real object, and looking in at the mouth of the telescopic tube, near to the
edge of which the image is thrown by a slight inclination of the mirror at its
bottom:-or, secondly, as in the Newtonian telescope, through an opening
in the side of a tube, after being reflected by a small plane mirror, placed
diagonally in the centre of the tube:-or, thirdly, as in the Gregorian
telescope, through an opening cut in the principal mirror or speculum, after
being reflected towards that opening by a small mirror placed in the centre
of the tube; this last arrangement is that preferred for smaller telescopes,
because the spectator, while seeing the image, is also looking in the direction of the object.
Reflecting telescopes have the advantage of being perfectly achromatic,
that is, of producing no coloured or rainbow edges to the images; for compound light is reflected, although not refracted entire, all the colours following the same law of equal angles of incidence and reflection.
Herschel's largest telescope had a mirrow of 48 inches in diameter, and
therefore received about 150,000 times more light than an unassisted eye,
forming with that light, at a focal distance of 40 feet, a large image admira.
bly distinct. It was with such a telescope that, in the obscurity of remote
space, Herschel discovered the immense planet rolling along, which in honour
of his royal patron, he called the Georgium Sidus, but which now, by
the decision of the scientific world, bears his own name;-and with such
he discovered moons before unseen of other planets, and he unravelled the
celestial nebuloo and clustered stars of the milky way and, in a word, unveiled vastly more than had' before been done, the system of the boundless
universe. If this world were to last for millions of years, the discoveries
made by Herschel's telescope would mark a memorable epoch of its early
history.
"Light returned from, or passing through bodies of rougher or irregular
surface, or which have other peculiarities, is so modified as to produce all
those phenomena of colour and varied brightness seen among natural
bodies, and giving them their distinctive characters and beaut@y."  (See
the Analysis, page 325.)
General remarks on this part of our subject were made in the beginning
of the section, in the explanations of how objects not self-luminious become
visible by reflecting the light of other bodies, and of how the prismeseparates a ray of white light into rays of the several colours which are seen
also in the rainbow —which rays, on being again mixed, become white light




COMPARISON  OF LIGHT  AND  SOUND.                         983
as before: —and much beyond these remarks we have not the intention of
now proceeding   To give a full account of the matters that might come
within the scope of this department, would occupy the pages of a large
volume, for there would be to pass in review-the various opinions which
have existed on the intimate nature of light,-the facts connected with what
has been called the polarization of light,-the relation of light in its double
refraction, to the ultimate structure of material masses, &c., all which subjects are in certain respects highly interesting, but-as some of them are not
yet completely investigated-as respecting others various opinions prevail,as they involve few matters yet applied to common use, —as the reasonings
about them are far removed from ordinary trains of thinking, and refer to
-facts altogether unknown to common observation,-we hold them not to be
fit parts of a popular treatise on light.  We may state, however, that persons
who have the leisure and mathematical preparation necessary for pursuing
the study, will find their labour in it richly rewarded.
What we deem necessary here to add, is, that white light, in falling upon
any.transparent substance, as air, water, glass, &c., reduced to thin plates of
films, is so affected, that for certain degrees of thinness, different for each
substance, it is decomposed, and is reflected or is transmitted, not as white
light, but as some of the colours of the rainbow, and the colour reflected in
any case, is always the opposite or complement of that which is transmitted,
that is to say, is such that the two brought together make white light as
before. The facts may be studied as. Newton originally studied them, in the
thin plate of air which occupies the space between a convex lens and a plane
surface of glass upon which the lens is laid,-in which plate, as the distance
from the point of the apparent contact of the glasses increases, there are
all degrees of thinness, and with these appear successive rings of vivid colours. The same truth is exemplified in the colours of a soap-bubble, which
brighten as the bubble swells and becomes of thinner substance, and are
different as the thickness is different and greater from above downwards;and it is exemplified also in the colours seen in the fissures of cracked
ice or crystalline spars, and in numerous other common facts. Now, whatever be the reasons of such decomposition of light-and the explanation is
not yet complete-we cannot doubt that in natural bodies generally, the
colours, opacity, transparency, &c., depend entirely upon the volume and
arrangement of the minute fibres or plates, with included interstices, which
constitute the volume or structure of each mass  Accordingly, whatever
changes that arrangement may change also the colour of the mass  Thus,
by drawing a certain number of minute lines on a certain extent of any metallic surface, we may make it of what colour we please; and mother-of-pearl
owes its vivid colours and beauty entirely to its furrowed or striated surface,
as is proved by our making an impression of that surface on sealing wax and
perceiving that the wax exhibits similar colours.
The investigations in progress respecting the phenomena of light, are furnishing new proofs of the extreme simplicity of nature, amidst the boundless
extent and most curious variety. When men thought of the sense of touch
only as it exists at the tips of the fingers, or on the general surface of the
body they were far from suspecting that the sense of hearing had the near
relation to it which subsequent.discoveries have proved, and still less, that
the sense of sight was only yet a finer touch than hearing. But step by
step they have ascertained; 1st, in relation to sound, that the air through
which it usually reaches the organ of hearing, is a material fluid as much as




3894                           LIGHT.
water, consisting of the same or smaller particles, only more distant among
themselves,-and that a motion or trembling in the air, by affecting nerves
exposed in the ear, produces the sensation of sound by slight repeated pressures
on these nerves, as the trembling in a log of wood caused by the action of a
saw produces a peculiar sensation of touch in the nerves of a hand laid on the
log;-and, moreover, that sound in all its varieties, is merely such trembling
affecting a structure of nerve in the ear, which nerve is made as much more
readily excitable than the nerves in the fingers or general cutaneous surface,
as the action or impulse of trembling air is more delicate than the stronger
pressures of common occurrence.-And, 2dly, in the investigation respecting
light, this kind of comparison is carried a step farther, for it is become
matter almost of certainty that the sensation of light is produced in the'
suitable nervous tissue of the eye, called the retina by a trembling motion in
another fluid than air, which fluid pervades all space, and in rarity or subtilety of nature surpasses air yet more than air does water or solids;-and
that, while in sound, different tones or notes depend on the number of
vibrations in a given time, so in light do different colours depend on the
number and extent of the vibrations.  Can human imagination picture to
itself a simplicity more magnificent and fruitful of marvellous beauty and
utility than all this?-But yet farther, as air answers in the universe so many
important purposes besides that of conveying sounds-although this, alone,
comprehends language, which almost means reason and civilization-so also
does the material of light minister in numerous ways, in the phenomena of
heat, electricity and magnetism.
The truths now positively ascertained with respect to the nature of light
and vision, are among those in the wide field of human inquiry, which,
acting on ordinary apprehension, most forciby place the student, as it were,
in the very presence of Creative Intelligence, awakening in him  the most
elevated thoughts of which the human mind is capable. Had there been no
light in the universe, all its other perfections in regard to man had existed in
vain. This earth would have been to its human inhabitants what any
unknown shore would be to exiles abandoned upon it after their eyes were
put out: every movement might be to their destruction, for their perceptions,
being limited by the length of their arms, and of their fearful groping steps,
the wretched beings separating when impelled by hunger to search for food,
would probably scatter to meet no more. But the material of light exists,
pervading all space, and certain impressions made upon it in one place, extend rapidly over the universe, the progressive impression being called a ray,
or beam of light.  The beams of light, then from all parts coming to every
individual, may be regarded as millions of supplementary arms or feelers
belonging to the individual, and which reach to the end of the universe, so
that each person, instead of being as a blind point in space, becorries nearly
omnipresent; then these limbs or feelers have no weight, they are never in
the way, they impede nothing, and they are only known to exist when their
use is required!
But this miracle of LIGHT would have been totally useless, and the paradise of earth would have been to man still a dark and dreary desert, had there
not been farther the twin miracle of the EYE, an organ of commensurate delicacy to perceive the light. In the Eye we have to admire the round cornea,
of such perfect transparency, placed exactly in the anterior of the ball, (and
elsewhere it had been useless,) then exactly behind this, the beautiful curtain,
the iris, with its opening, called the pupil, dilating and contracting to suit the




PERFECTION OF THE EYE.                           395
intensity of light-and exactly behind the iris, again, the crystalline lens possessing important and remarkable properties, and which, by acting on the
entering light, forms of it on the retina beautiful pictures or images of the
objects in front;-the most sensible part of the retina being where the images
fall. Of these parts and conditions, had any one been otherwise than as it is;
the whole eye had been useless, and light useless, and the great universe useless to man, for he could not have existed in it.-Then, farther, we find that
this precious organ, the eye, is placed in the person, not as if by accident,
any where, but aloft on a befitting eminence, where it becomes the glorious
watch-tower of the soul; and, again, not so that to alter its direction the
whole person must turn, but in the head, which, on a pivot of admirable
structure, moves while the body is at rest; besides that, the ball of the eye
itself can roll in its place, and is furnished with muscles which, as the will
directs, turn it with the rapidity of lightning to sweep along the horizon, or
take in the whole heavenly concave;-then is the delicate orb secured in a
strong socket of bone, and there is over this the arched and padded eyebrow
as a cushion, to mitigate the shock of blows, and with its inclined hairs to
turn aside any descending perspiration or other moisture which might incommode;-then is there the soft and pliant eyelid with its beauteous fringes,
incessantly wiping the polished surface, and spreading over it the pure moisture poured out from the lachrymal glands above, of which moisture the
superfluity, by a fine mechanism, is sent into the nose, there to be evaporated by the current of the breath;-still farther, it is to be noted, that instead
of there being only one such precious organ, there are two, lest one, by accident, should be destroyed, but which two have so entire sympathy, that they
act together as only one more perfect; then the sense of sight continues
perfect during the period of growth from birth to maturity, although because
the eye then increases in size, the distance between the lens and the retina is
constantly increasing; —and the pure liquid which fills the eye, if rendered turbid by accident or disease, is by the actions of life, although its source be the
thick red blood, gradually restored to transparency.-The mind which can
suppose or admit that,. within any limits of time, one single such apparatus
of vision could have been produced by accident, or without design, must
surely be of extraordinary character, or must have received unhappy bias in
its education; but the mind which can still farther admit that the millions
of human eyes which now exist on earth, all equally perfect, can have sprung
from accident-and that the millions of millions of other eyes throughout
the almost innumerable species of the living creation, where each is adapted
to the peculiar nature and circumstances of the animal which bears it, can
be accident; and, lastly, that the countless millions of all these well adapted
kinds, which have existed in past ages, were all but accidents-the mind
which can admit this, must have some of its highest faculties either
benumbed or destroyed.
As a concluding reflection with respect to vision, we may remark, that all
the provisions above considered have mere utility in view, for any one of
them wanting would leave a necessary link in the chain of creation wanting:
but we have shown, in a preceding part of the work, that if there had been
white light only, susceptible as now of different degrees of intensity and
shade, the merely useful purposes of vision should have been answered about
as perfectly as with all the colours of the rainbow-a truth instanced in the
facts, that many persons do not distinguish colours, and that it imports not
whether a person view objects in the morning, or at mid-day, or at even-tide,




396                           LIGHT.
or through plain glass or coloured glass, provided there be light and shade
enough to show them clearly. While, therefore, the existence of light generally, and of the eye, speaks of Creative Power and Intelligence, the existence of colours, or of that lovely variety of hues exhibited in flowers, in the
plumage of birds, in the endless aspects of the earth and heavens-because
appearing expressly planned to give delight to animated beings, speaks of
Creative Benevolence, and may well excite in us towards the Being in whom
these attributes reside, the feelings associated in our minds during this
earthly scene, with the endearing appellation of " Father."




Fig. 173.
P.
iLc








MIECHANISM  OF  THE HUMAN  SKELETON.   399
PART V.
ANIMAL AND MEDICAL PHYSICS.
SECTION I.
MIechdanism of the Human Sceleton.
HAVING now completed our study of general mechanics, we shall proceed,
with the light thence derived, to examine that most interesting illustration
of many of the truths-the solid frame-work of the human body-a perfect
work of an unerring Engineer!
There is scarcely a part of the animal body, or an action which it performs, or an accident that can befall it, or a piece of professional assistance
which can be given to it, that does not furnish illustration of some truth of
natural philosophy; but were we here to enter into much detail, we should
be giving minute lessons in medical science, instead of explaining general
laws. We shall therefore only touch upon as many particulars as will make
the understanding of all the others easy; trying to conclude, among our
illustrations, such matters of importance as would be likely to escape the
notice of a hasty student.
The cranium or skull has been already mentioned as an instance of the
arched form answering the purpose of giving strength. The brain, in its
nature, is so tender or susceptible of injury, that slight local pressure disturbs its action. Hence a solid covering like the skull was required with
those parts made stronger and thicker which are most exposed to injury.
An architectural dome is constructed to resist one kind of force only, always
acting in one direction, viz., gravity; and therefore its strength increases
regularly towards the bottom, where the weight and horizontal thrust of the
whole are to be resisted; but in a skull, as in a barrel or egg-shell, the mere
tenacity of the substance is many times greater than sufficient to resist
gravity, and therefore the form and securities are calculated to resist forces
of other kinds operating in all directions. When we reflect on the strength
displayed by the arched film of an egg-shell, we need not wonder at the
severity of blows which the cranium can withstand.
In the early foetal state, that which afterwards becomes the strong bony
case of the brain exists only as a tough flexible membrane. Ossification
commences in this membrane long before birth, at a certain number of points
from which it spreads, and the portions of the skull formed around these
points soon acquire the appearance of so many scales or shells applied on
the surface of the brain, and held together by the remaining membrane not
yet ossified. They afterwards become firmly fixed together, by projections
of bone from each, shutting in among similar projections of the adjoining
ones, until all mutually cohere by perfect dove-tailed joints, like the work of
a carpenter. These joints are called sutures of the cranium, and are visible




400                       ANIMAL AND
to extreme old age. Through early childhood, the cranium remains to a
certain degree yielding and elastic, causing the falls and blows, so frequent
during the lessons of walking, &c., to be borne with comparative impunity.
The mature skull consists of two layers or tables, with a soft dploe between
them; the outer table being very tough, with its parts dove-tailed into each
other as tough wood is joined by human artificers; while the inner table is
harder and more brittle, (hence called vitreous) with its edges merely lying
in contact.
A very severe partial blow on the skull generally fractures and depresses
the part, as a pistol-bullet would: while one less severe, but with more extended contact, being slowly resisted by the arched form, often injures the
skull by what is correspondent to the horizontal thrust in a bridge, and
causes a crack at a distance from the place struck-generally half way
round to the opposite side.  The French in speaking of this effect, us6 the
term  contre-coup.  Sometimes in a fall with the head foremost, the skull
would escape injury, but for the trunk which falls upon it, and drives the
end of the spine against or even through its base.
In the lower jaw we have to remark the greater mechanical,advantage, or
lever-power, with which the muscles act, than in other parts of animals.
The temporal and masseter muscles pull almost directly, or at right angles
to the line of the jaw, while in most other cases, as in that of the deltoid
muscle lifting the arm, the muscles act very obliquely, and with power
diminished in proportion to the obliquity.  An object placed between the
back teeth is compressed with the whole direct power of the strong muscles
of the jaw. Hence the human jaw can crush a body which offers great
resistance, and the jaws of the lion, tiger, shark, and crocodile, &c., are
stronger still.
The teeth rank high among those parts of the animal body which appear
almost as if they were severally the results of distinct miraculous agencies
-so difficult is it to suppose a few simple laws of life capable of producing
the variety of form and fitness which they exhibit. They constitute a beautiful set of chisels and wedges so arranged as to be most efficient for cutting
and tearing, and grinding the food, with their exterior enamel so hard, that
few substances in nature can make an impression upon it. In early states,
of society, teeth were used for many purposes for which steel is used now.
It seems, however, as if the laws of life, astonishing to human intellect as
they are, had still been inadequate to cause teeth cased in their hard and
polished enamel, to grow as the softer bones grow; and hence has arisen a
provision more extraordinary still. A set of small teeth'appear soon after
birth, and serve the child until six or seven years of age: these then fall
out, and are replaced by larger ones,.which endure for life; the number of
the latter, however, being completed only when the man or woman is full
grown, by the four teeth, called wisdom teeth, from coming with the person's
maturity, to fill up the then spacious jaw.
The spine or back bone, in its structure, has as much of beautiful and
varied mechanism as any part of our wonderful frame. It is the central
pillar of support and great connecting chain of all the other parts; and has
at the same time, the office of containing within itself, and of protecting
from external injury, a prolongation of the brain, called the spiral marrow,
more important to animal life than the greater part of the brain itself.  It
has united in it the apparent incompatibilities of great elasticity, great
flexibility in all directions, and great strength, both to support a load and to
defend its important contents,-as we shall now perceive.




MEDICAL MECHANICS.                            401
Elasticity.-The head rests on the elastic column of the spine, as softly'
as the body of a carriage rests upon its springs.  Between each two of the
twenty.four vertebrae or distinct bones of which the spine consists, there is
a soft elastic inttervertebralsubstance, about half as bulky as a vertebrae, and
which yields readily to ahy sudden jar: then the spine is waved or bent like
an italic f, as if perceived on viewing it sideways, or in profile, and by this
reason, also, it yields to any sudden pressure operating against either end.
The bending might seem a defect in a column intended to support weight,
but the disposition of the muscles around is such as to leave all the elasticity
of that form, and a roomy thorax, without any diminution of strength.
F1exibility.-The spine has been compared to a chain, because it consists
of many distinct pieces (twenty-four.) They are in contact by smooth rubbing surfaces, which allow of a degree of motion in all directions; and a little
motion comparatively between each two adjoining pieces, becomes a great
extent of motion in the whole line.
The strength-of the spine as a whole, is shown in the fact of a man's
easily carrying upon his head or back a weight heavier than himself; and the
strength of each separate vertebrae surrounding the spinal marrow, is evident
in its being a double arch, or strong irregular ring.  The spine increases in
size towards the bottom, in the justest proportion, as it has more weight to
bear. The articulating surfaces of the spine are so many, and so exactly fitted
to each other, and are connected by such number and strength of ligaments,
that the combination of pieces, becomes, in reference to motion, a much
stronger column than a single bone of the same size would be.
Considering the great number of parts forming the spine, and their nice
mutual adaption, it might be expected that injuries and diseases of the structure would be very frequent. The reverse, however, under natural circumstances, is true; so that while hundreds and thousands of works have been
published on the diseases of almost every other part of the body, hardly any
have been written on spine-affections, and what have appeared are of very
recent date. One reason of this is, that whatever regards health and diseaqe
is now much more completely analyzed than formerly; but another and the
chief reason is, that from a change in modern times introduced into the system
of education for young ladies, a considerable proportion of them having grown
to womanhood with weakened and crooked spines.-The subject merits
further consideration here.
To the well-being of the higher classes of animals, a certain degree of exercise of their various parts is not less necessary than their nourishment, and
if, during the period of growth, such exercise be withheld by any cause, the
body never acquires its due proportions and strength. To prompt young
creatures to the required exertion, nature has given them an overflow, of life
and energy, as evinced in the ever-changing occupation of a child in the quick
succession of its ideas, in its jumping and skipping, and using all the modes
of roundabout action to expand muscular energy, instead of seeking, as in after
life, to accomplish its ends in the shortest ways: —and as seen among the
inferior animals, in the play of kittens, puppies, lambs, &c. But, strongly as
nature has thus expressed herself, tyrant fashion, with a usual perversion of
common sense, had of late times, in England, for young women of the higher
classes formed a school discipline, directly at war with nature's dictate; so
that a stranger arriving from China, might almost suppose it our design to
make crooked and weak spines by that discipline, as it is the design in China
to make little feet by the iron shoe. The result is the more striking, when the
brothers of the female victims, and who of course have similar constitutions,
26




-402                       ANIMAL AND
are seen to be robust, healthy, and well-formed. A peasant-girl, when her
spirits are buoyant, is allowed to obey her natural feeling, and at proper times
to dance, and skip, and run, until healthy exhaustion asks that repose which
is equally allowed; and thus she grows up strong and straight, but the youag
ladcy is receiving constant admonition to curb all propensity to such vulgar
activity, and often, just in proportion as she subdues nature, she receives the
praise of being well-bre(d.  The multifarious studies, also, of the latter come
powerfully in aid to the admonition, by fixing her for many hours every day
to sedentary employment; and the consequences soon follow, of weakness
in the body generally from the want of the natural quantity and variety of
musdular exertion, but weakness of the back particularly, from the manner
in which the sitting is usually performed. It would be accounted great cruelty
to make a delicate girl stand all day, because her legs would tire, but this
very cruelty is in almost constant operation against her back, as if backs could
not tire as well as legs.  When she is allowed to sit down because she has
been long standing, great care is taken that the muscles of the back, which
still remain in action as she sits, shall not be at all relieved; for, from the
idea that it is ungraceful to loll, she is either upon a stool which has no back
at all, or upon a very narrow chair with a perpendicular back. Now neither
of these seats relieve her spine, the stool, however, being less hurtful than
the chair, because it allows the spine to bend in different ways so as to rest
thile different sets of muscles alternately, while the chair keeps the spine
constantly upright and nearly unmoved.  The excessive fatigue soon causes
the spine, somewhere, to give way and to bend, and the curvature often
becomes permanent.  And, as when a bend takes places in one situation,
there immediately follows an opposite bend above or below, to keep the centre
of gravity of the body always directly over the base, the curve thus becomes
double, like an italic J; and the distortion is rendered complete.-In bending
the spine is sometimes also partially rotated or twisted, so as to show from
behind that waving profile which should be seen only from the side.
When owing to such discipline the inclination of the back has once been
begun, it is often rapidly increased by the means used to correct it.  Strong
stiff stays are put on to support the back, as is said, but which in reality, by
superseding the action of the muscles placed there by nature as the supports,
cause these to lose their strength, and to be unable, when the stays are withdrawn, to support the body.  Longer sittings in the narrow upright chair are
then recommended, and sometimes the back is forcibly stretched by pullies,
so the patient is kept all day and night lying on an inclined board, losing her
health, &c.;-the only things guarded against being, the patient should take
due exercise and air, and should rest properly when she is not taking exercise.  With many persons the prejudice had at last grown up, that strong
stays should be put on at a very early age, to prevent the first approach of
the mischief, and that children should always be made to sit on straightbacked chairs, or to lie on hard planes; and it is probable, that if these cures
and preventives had been adopted as universally and strictly as many deemed
them  necessary, we should now scarcely have in England a young lady of
healthful form.  What would be said of the person who should try to
improve the strength and shape of a young race-horse or greyhound, by
binding tight splints or stays round its beautiful young body, and then
tying it up in a stall! But this is the kind of absurdity and cruelty which
has been so commonly practised in this country towards beings than whom,
as nature offers them, the universe surely contains none more faultless.
A pernicious prejudice, with respect to such curvature or distortion of the




MEDICAL MECHA NICS.                           403
spine, long existed, namely that it was a scrofulous affection; and many
mothers concealed it as much as possible, and sought remedy from quacks
far from home. In consequence, until within a few years, the management,
of spine diseases was chiefly in the hands of some irregular members of the
profession,-and a rich source of wealth it became to them, from many of
their remedies being calculated rather to prolong than to cure the evil. The
practice in such cases, however, has now fallen into the hands of the profession generally; the science having detected the true cause of the evil, its
frequency is already diminished.  It has been shown that to prevent the
disease is easy, and that the best cures are those conducted on the general
principles of improving the health of the patient by fit regimen, of prescribing such. exercises as may directly strengthen the affected part, and of causing the patient,'when reposing to assume positions which directly counteract
the morbid tendency.
Some might expect here a long description of machines employed in the
treatment of spine affection: but the list of those which are useful or safe
is very short:-a sofa to rest upon during the day and a fit bed for the
night; (the " hydrostatic bed,") proposed by the author of this work, and
described in the next chapter, has certain advantages;) choice of pleasant
means of taking exercise, such as the skipping-rope, shuttle-cock, dum-bells,
a rope-ladder to climb, a winch to turn, &c.:-and where it is much desired
that the young lady should employ herself in the sitting attitude, as in
practising music, a chair may be used with crutches rising from its side, or
with straps descending from pullies in an overhanging canopy or crane, and
kept tight by proper weights at their distant ends, to support the head and
shoulders. The author has had a small crane of wood made, which well
answers the last mentioned purpose, and may be attached to a common
chair.  It would be out of place here to detail those particulars of constitutional treatment which, in peculiar habits, may be required to aid the effects
of the means above described
The ribs -Attached to twelve vertebrae in the middle of the back, there
are the ribs or bony stretchers of the cavity of the chest, constituting a structure which solves, in the most perfect manner, the difficult mechanical problem of making a cavity with solid exterior, which shall yet be capable of
dilating and contracting itself.  Each pair of corresponding ribs may be
considered as constituting a hoop: which hangs obliquely down from  the
place of attachment behind, so that when the forepart of all the hoops is
lifted by the muscles the cavity of the chest is enlarged.
~We have to remark the double connection of the rib behind, first to the
bodies of two adjoining vertebrae, and then to process or projection from
the lower, thus affecting a very steady joint, and yet leaving the necessary
freedom of motion: and we observe the forepart of the rib to be joined in the
breast-bone by flexible cartilage, which allows the degree of motion required
there without the'comiplexity of a joint, and admirably guards, by its elasticity against the effects of sudden blows or shocks.
The muscles, which have their origin on the ribs and their insertion into
the bones of the arm, afford us an example worth remembering of action and
reaction being equal and contrary. When the ribs are fixed,'these muscles
move the arm; and when the arm is fixed, as by resting on a chair or other
object, they with equal force move the ribs The latter occurrence is seen
in fits of asthma and dyspnoea.
The human skeleton, with its naked ribs, is so associated in the common
mind, with ideas of death and loss of friends, and all the terrors of doubtful




404                        ANIMAL AND
futurity, that to most persons it is an object of abhorrence; but to the phiIosophic mind, which rises superior to place and time, the so admirable adaptation of all the parts to their purposes, and of parts which, being purely
mechanical, are perfectly understood, makes it independently of all professional considerations, an object of the most intense interest. Such mechanism
reveals, by intelligible signs, the hand of the Creator; and a man may be
said sublimely to commune with his Maker, who contemplates and understands the structure aright.
The shoulder-joint is remarkable for combining great extent of motion
with great strength. The round head of the shoulder-bone, that it may turn
freely in all ways, rests upon a shallow cavity or socket in the shoulderblade; and the danger of dislocation from this shallowness is guarded against
by two strong bony projections above and behind. To increase the range of
motion to the greatest possible degree, the bone called the shoulder-blade,
which contains the socket of the arm, slides above itself upon the convex
exterior of the chest, having its motions limited in certain directions by its
connection, through the collar-bone or clavicle, with the sternum.
The scapula or blade-bone is extraordinary as an illustration of the mechanical rules for combining lightness with strength.  It has the strength of
the arch from being a little concave, like the dished wheel already described,
and its substance is chiefly collected in its borders and spines, with thin
plates between, as the strength of a wheel is collected in its rim, and spokes,
and nave.
The bones of the arms, considered as levers, have the muscles which
move them attached very near to the fulcra, and very obliquely, so that the
muscles, from working through a short distance, comparatively with the
displacement of the resistances at the extremities require to be of great
strength. It has been calculated that the muscles of the shoulder-joint, in
the exertion of lifting a man upon the hand, pull with the force of two
thousand pounds.
Notwithstanding all the securities of the shoulder-joint now described in
the infinite variety of twists, and falls and accidents to which men, in the
busy scene of society, are liable, the joint is frequently dislocated, that is,
the rounded head of the humerus or arm-bone slips from his socket, with
instant lameness as a consequence.
In the treatment of dislocations and fractures of the frame-work of the
human body, the surgeon cannot avoid displaying strikingly either his professional skill or ignorance.  With what ease does the displaced arm or thighbone return to its socket, under the guidance of the skilful hand; and to
what horrible, and often unavailing torture, is the patient subjected, when,
in such a case, ignorance dares to act! It is very painful to allow the imagination to dwell upen the records of apcient surgery, and to be made present,
as it were, to the stretching of patients on the rack with pullies and powerful engines, to do, what better information could have accomplished with
such gentleness. And would that the records of modern times contained
no instances of individuals crippled for life by bad practice. To a
practioner in this branch, impunity and a quiet conscience can now be secured onlyiby his having a perfect knowledge of anatomy, and familiarity
with the laws of mechanical philosophy.
With our present information on these subjects, we are suprised at the
detail of the practices and errors promulgated in former times, owing to imperfect knowledge of mechanics, even by authors of the highest credit.  It
would hardly be believed that so distinguished an ornament of English sur



MEDICAL MECHANICS.                            405
gery as Mr. Pott, should assign as one reason for not pulling by the hand
or foot, in reducing a dislocation of the shoulder or hip, that the intervening
joints prevented the strain from reaching the part desired.
Some surgeons, possessing a certain degree of knowledge in mechanics,
but only that degree which is dangerous, having heard that the lever was
a powerful-engine, have tried to replace bones solely by leverage, as it was
called. Thus, a man's dislocated arm has been placed over the back of a
chair as a fulcrum, or over the top of a door, and while the weight of the
suffering body was hanging to it on one side as the resistance, force has been
applied to the other side, enough sometimes to break the bone, or to tear
away the ligaments and soft parts about the joint.
Other surgeons, after learning in the same way the effects of the pulley,
have wished to do all by irresistible extension, and instead of borrowing the
moderate assistance which might be useful, have torn muscles and ligaments
from their attachments.
It is not the object of this work to enter into an extended examination of
the accidents which befall the body requiring mechanical skill for their proper
management, for this would be to deliver a course of instruction on practical
surgery; but it is wished to awaken the attention of the medical student to
those valuable general principles which may furnish direction in most difficulties. Knoyving these principles, and possessing good sense, he will often
be a more effective minister of his art than a man full of learned precedents,
who knows them not.  To make this lesson more impressive to his young
readers, the author may take the liberty of adducing his own experience.
When he was himself very young, and had not yet had extensive practical
experience, he was thrown into a situation where a heavy medical charge
developed upon him, and where, through accidents among a numerous crew,
during a very eventful voyage, which led to intercourse with the savage
inhabitants of unfrequented coasts, he had, within twenty-six months, more
practice in singular wounds, dislocations, and fractures, than falls to the lot
of many practitioners during a life:-in that time he became strongly im.
pressed with the importance to the medical man of such knowledge as he now
recommends: and he had reason to rejoice that although Natural Philosphy
was not then much insisted upon in the course of professional education, circumstances had led him to look carefully at the body through that medium.
The os humeri, or bone of the upper arm, is not perfectly cylindrical, but
like most of the other bones called cylindrical, it has ridges to give strength,
on the principle explained in the chapter " on strength of materials."
The elbow joint is a correct hinge, and so strongly secured that it is rarely
dislocated without fracture.
The fore-arm consists of two bones with a strong membrane between
them.  Its great breadth, from this structure, affords abundant space for the
origin of the many muscles which go to move the hand and fingers: and the
very peculiar mode of connection of the two bones, gives man that most
useful faculty of turning the hand round, into what are called the positions
of pronation and supination,-exemplified in the action of twisting or of
turning a gimblet.
The old surgeons, who acted frequently by rules of routine rather than
by reasons, in the accident of fracture to one or both bones of the fore-arm,
often applied a tight bandage, which pulled the bones at the fractured part
close to each other, and thus injured the future shape'and strength of the arm.
The wrist.  The many small bones forming the wrist have a signal effect




406                        ANIMAL AND
of deadening, in regard to the parts above, the shocks or blows which the
hand receives.
The annular ligament is a strong band passing round the joint, and
keeping all the tendons which pass from the muscles above to the fingers,
close to the joint. It answers the purpose of so many fixed pullies for
directing the tendons: without it, they would all, on action, start out like
bow-strings, producing deformity and weakness.
The human hand is so admirable, from  its numerous mechanical and
sensitive capabilities, that one opinion at one time prevailed, that man's
superior reason depended on his possessing such an instructor and such a
servant. Now, although reason, with hoofs instead of fingers, could never.
have raised man much above the brutes, and probably could not have secured
the continued existence of the species,-still, the hand is no more than a
fit instrument of the Godlike mind which directs it.
The pelvis, or strong, irregular ring of bone on the upper edge of which
the spine rests, and from the sides of which the legs spring, forms the centre
of the skeleton. A broad bone was wanted here to connect the central
column of the spine with the lateral columns of the legs, and a circle was
the lightest and strongest.  If we attempt still farther to conceive how the
circle could be modified so as to fit it-for the spine to rest on, for the thighs
to roll in, for muscles to spring from, both above and below, for the person
to be able to sit, &c., we shall find, on inspection, that all our anticipations
are realized in the most perfect manner.  In the pelvis, too, there are the
thyroid hole and ischiatic notches, furnishing subordinate instances of contrivance to save material and weight:-they are merely deficiencies of bone
where solidity could have given no additional strength.  The broad ring of
the pelvis protects most securely the important organs placed within it.
The hip joint exhibits the perfection of the ball and socket articulation.
It allows the feet to move round in a circle, as well as to have the great
range of backward and forward motion, exhibited in the action of walking,
When we see the elastic, tough, smooth cartilage which lines the deep socket
of this joint, and the similar glistening covering of the ball or head of the
thigh-bone, and the lubricating synovia poured into the cavity by appropriate secretaries, and the strong ligaments giving strength all around, we
feel how far the most perfect of man's work falls short of the mechanism
exhibited in nature.
The thi'gh-bone is remarkable for its two projections near the top, called
trochanters, to which the moving muscles are fixed; and which lengthen
considerably the lever by which the muscles work. The shaft of the bone
is not straight, but has a considerable forward curvature.  Short-sightedness
might suppose this a weakness, the bone being a pillar to support a weight;
but the bent gives it in reality the strength of the arch, to bear the action
of the mass of muscles called vastus, which lies and swells upon its fore part.
The knee is a hinge joint of complicated structure, claiming the mnost
attentive study of the surgeon.  The rubbing parts are fiat and shallow, and,
therefore, the joint has little strength from form; but it derives security from
tile numerous and singularly strong ligaments which surround it. The ligaments on the inside of the knees resemble, in two circumstances, the annular
ligaments of joints, viz., in having a constant and great strain to bear, and
yet in becoming stronger always as the strain increases. The line of the leg,
even in the most perfect'shapes, bends inward a little at. the knee, requiring
the support of the ligaments; and in many persons it bends very much; but




MEDICAL MECHANICS.                               407
the inclination does not increase with age.  The legs of many weakly inkneed children beconme straight by exercise alone.  This inclination at the
middle joint of the leg, by throwing a certain strain on the ligamnents, gives,
in such actions as jumping, running, &c., an increase of elasticity to the limb.
In the knee there is a singular provision of loose cartilages between the
ends of the bones.  They have been called friction-cartilages, from  a
supposed relation in use to friction-wheels, but their real effect seems to be,
to accommodate, in the different positions of the joint, the surfaces of the
rubbing bones to each other.
Under the head Phcumat/cs, we shall find that the bones forming the
joints, are held together, independently of their ligaments, by a constant
pressure of the atmosphere, amounting in the knee, for instance, to upwards
of sixty pounds.
The great, muscles on the fore-part of the thigh are contracted into a tendon a little. above the knee, over and in front of which the tendon has to
pass to reach the top of the leg, where its attachment is.  The part of the
tendon over the joint becomes bony, and forms the patella or knee-pan,
often called the pulley of the knee.  This peculiarity enables the muscles
to act more advantageously, by increasing the distance of the rope from the
centre of the motion. The petella is, moreover, a sort of shield or protection
to the fore-part of this important joint.
The leg below the knee, like the fore-arm already described, has two
bones.  They offer spacious surface of origin for the numerous muscles
required for the feet, and they form a compound pillar of greater strength
than the same quantity of bone as one shaft would have had.  The individual bones also are angular instead of round, hence deriving greater power
to resist blows, &c.
The antkle-joint is a perfect hinge of great strength.  There is in front of
it an angular ligament, by which the greater part of the tendons passing
downwards to the foot and toes are kept in their places.  One of these tendons passes behind and under the bony projection of the inner ankle, in a
smnooth, appropriate groove, exactly as if a little fixed pulley were there.
The heel, by projecting so far backwards, is a lever for those strong muscles to act by, which form the calf of the leg and terminate in the tendoaehillis.  The muscles by drawing at it, lift the4 body, in the actions of
standing on the toes, walking, dancing, &c.  In the foot of the negro, the
heel is so long as, in European estimiation, to appear ugly; and its great
length rendering the effort of smaller muscles sufficient tfr the various purposes, the calf of the negro's leg is smaller thlan of other races of men.
In a graceful human step the heel is always raised before the foot is lifted
from the ground, as if the foot were part of a wheel rolling forward; and
the weight of the body, supported by the muscles of' the calf of the leg, as
just described, rests for the time on the fore-part of the foot and toes. [There
is at that time a bending of the foot in a certain degree. But where strong
wooden shoes are used, or any slhoe so stiff that it will not yield and allow
this bending of the foot, the heel is not raised at all until the whole foot rises
with it, so that the muscles of the calf are scarcely used, and in consequence
soon dwindle in size, and almost disappear.  Mlany of the English farmservants wear heavy stiff shoes; and in London may constantly be seen as
the drivers of country wagons, with fine robust body and arms, but with legs
which are fleshless spindles, producing a gait most awkward and unmanly.
The brothers of these men, otherwise employed, are not so mis-shapen;
and even they themselves, when they choose to become soldier, and are




408                       ANIMAL AND
trained in military exercises, lose their peculiarity. What a pity that, for
the sake of a trifling saving, graceful nature should be thus deformed. An
example of an opposite kind is seen in Paris, where, as the streets have no
side pavements, and the ladies are obliged consequently to walk almost constantly on tiptoe, the great action of the muscles of the calf has given a
conformation of the leg and foot, to match which the Parisian belles proudly
challenge all the world,-not aware, probably, that it is a defect of their
city to which the boasted peculiarity is mainly due.
A person confined to his bed for a week or two by sickness, has generally'
to remark a much greater wasting of the legs than of the arms: the reason
of which is, that the muscles of the leg, in ordinary cases, being more in
use than those of the arms, their ordinary bulk is more dependent on use,
and they suffer a corresponding change from inaction.
Such facts as now mentioned, bear directly on the subject so near the
hearts of many English mothers, viz., the weak and crooked backs of their
daughters. From such they may understand that strong stays, which in
part supercede the action of the muscles placed by nature around the spine
to support it, cause these muscles to dwindle, and afterwards, when the support of the stays fails or becomes unequal, leave the back to bend or twist.
Stays, therefore, can neither help to make strong and well-formed backs.
originally, nor can they be a remedy after the weakness has commenced.
A healthy young woman from the country, with spine lying deep between
the firm cushions of muscles which support it, if, according to town fashion,
braced up in tight stays, will frequently, at the end of a short time, exhibit
such a wasting Qf the flesh, that the points of bone in the spine may be
counted by the eye, all the way down.
The arch of the foot is to be noticed as another of the many provisions
for saving the body from shocks by the elasticity of the supports. The heel
and the ball of the toes are the two extremes of the elastic arch, and the
leg rests between them.
Connected with elasticity, it is interesting to remark how imperfectly a
wooden leg answers the purpose of a natural one. The centre of the body,
when supported by the wooden leg, which always remains of the same
length, must describe, at each step a portion of a circle of which the bottom
nob of the leg is the centre; and the body is, therefore, constantly rising and
falling;-but with the natural legs, which, by gentle flexture at the knee;
are made shorter or longer in different parts of the step as required, the body
is carried along in a manner perfectly or nearly level. In like manner, a
man riding on horseback, if he kept his back upright and stiff, has his head
jolted by every step of the trotting animal; but the experienced horseman,
even without rising in the stirrups, by letting the back yield a little at each
movement, as a bent spring yields during the motion of a carriage, can
carry his head quite smoothly along.
In a general review of the skeleton, we have to remark, 1st, the nice
adaptation of all the parts to one another, and to the strains which they have
respectively to bear; as-in the size of the spinal vertebrae increasing from
above downwards-the bones of the leg being larger than those of the arm,
and so on.  2dly, the objects of strength and lightness combined; as by the
hollowness of the long bones-their angular fbrm-their thickening and
flextures in particular places where great strain has to be borne —the enlargement of the extremities to which the muscles are attached, lengthening the
lever by which these acts, &c. 3dly, we have to remark the nature and




MEDICAL MECHANICS.                             409
strength of material in different parts, so admirably adapted to the purposes
which the parts serve: there is bone for instance, in one place nearly as
hard as iron, where, covered with enamel, it has, the form of teeth, with the
office of chewing and tearing all kinds of matter used as food; in the cranium
again, bone is softer but tough and resisting; in the middle of the long bones
it is compact and little bulky, to leave room for the swelling of the muscles
lying there; while at either end, with the same quantity of matter, it is
large and spongy, to give a broad surface for articulation; and in the spine,
the bodies of the vertebra, which rest on an elastic bed of intervertebral substance are light and spongy, while their articulating surfaces and processes
are very hard. In the joints we see the tough elastic smooth substance called
cartilage, covering the ends of the bones, defending and padding them, and
destroying friction. In infants we find all the bones soft or gristly, and therefore calculated to bear with impunity the falls and blows incidental to their
age; and we see certain parts, where elasticity is necessary or useful, remaining cartilage or gristle for life, as at the anterior extremities of the ribs.
About the joints we have to remark the ligaments which bind the bones together, possessing a tenacity scarcely equalled in any other known substance;
and we see that the muscular fibres whose contractions move the bones and
thereby the body,-because they would have rendered the limbs clumsy
even to deformity had they all passed over the joints to the parts which
they have to pull,-attach themselves, at convenient distances, to a strong
cord called a tendon, by means of which, like a hundred sailors at a rope
they make their effort effective at any distance. The tendons are remarkable for the great strength which resides in their slender forms, and for the
lubricated smoothness of their surfaces.  Many other striking particulars
might be enumerated, but these may suffice.  Such, then, is the skeleton
or general frame-work of the human body; less curious and complicated,
perhaps, than some other parts of the system which we have yet to examine,
but so perfect and so wonderful, that the mind which can attentively consider it without emotion is in a state not to be envied.*
This living force of man has been used as a working power in various
ways, as in turning a winch-pulling a rope-walking in the inside of a
large wheel to move it, as a squirrel or turnspit dog moves his little wheel,
&c. Each of these'has some particular advantage: but the mode in which
for many purposes, the greatest effect may be produced, is for a man to carry
up to a height his body only, and then to let it work by its weight in descending. A bricklayer's labourer would be less fatigued to lift twice as
many bricks to the top of a house in the course of a day, by ascending the
ladder without a load, and raising bricks of nearly his own weight over a
pully each time in descending, than by carrying fewer bricks and himself
up together, and descending again without a load, as is still usually done.
Reflection, independently of experiment, would naturally anticipate the
above stated result, for the load which a man should be best able to carry,
is surely that from which he can never free himself, —the load of his own
body.  Accordingly the strength of muscles and disposition of parts are all
such as to make his body appear very light to him.
The question which was agitated with such warmth some time ago as to.-In the second and third editions of this work a criticism was introduced at this
place of a treatise on "Animal Mechanics," published as a part of the Library of
Useful Kinowledye: in which treatise the author from imperfect acquaintance with
Natural Philosophy had fallen into many grave errors. That note, having answered its
purpose is not repeated here.




410                       ANIMAL AND
the propriety of condemning men and women to work on the tread-mill receives an easy decision here. They work by climbing on the outside of a
large wheel or cylinder, which is turned by their weight, and on which, to
avoid falling from their proper situation, they must advance just as fast as it
turns.  There are, on the outside of the cylinder, projections or steps for the
feet, and the action to the workers is exactly that of ascending an acclivity.
Now, as nature has fitted the human body for climbing hills as well as for
walking on plains, the work of the tread-mill, under proper restrictions as
to duration, must be nearly as natural and healthful as any other.  Its
effects have ultimately proved it to be so.
Animal power being exhausted in proportion, as well to the time during
which it is acting, as to the intensity of force exerted, there may often be a
great saving of power by doing work quickly,. although with a little more
exertion during the time.  Suppose two men of equal weight to ascend the
sanme stair, one of whom takes only a minute to reach the top, and the other
takes four minutes, it will cost the first but a little more than a fourth part
of the fatigue which it costs the second, because the exhaustion has relation
to the time during which the muscles are acting.  The quick mover may
have exerted, perhaps, one-twentieth more force in the first instant, to give
his body the greater velocity which was afterwards continued, but the sloth
supported his load four times as long.
A healthy man will run rapidly up a long stair, and his breathing will
scarcely be quickened at the top; but if he walk up slowly, his legs will
feel fatigued, and he will have to wait some time before he can speak calmly.
For the same reason coach-horses are much spared by being made to
gallop up a short hill, and then being allowed to go more slowly for a little
time, so as to rest at the top.
The rapid waste of muscular strength which arises from continued action,
is proved by keeping the arm  extended horizontally for some time.  Few
persons can continue the exertion beyond a minute or two.  In animals
which have long horizontal necks,, there is a wonderful provision of nature
in a strong elastic substance on the back or upper part of the neck which
nearly supports the head independently of muscular exertion.
In farther illustration of the truth that strength is saved in many cases by
doing work quickly, we may recall the fact explained at page 60, that a body
thrown or shot upwards with double velocity, rises four times as far when
that with a single velocity, or half the other.
" Jstruments."
The following remarks regard some instruments used by medical men, and
which range under the present division of "M3Iechanics."
The obstetric forceps-As the blades beyond the joint or fulcrum  are
longer than the handles, the pressure on the head included in them is less
than that exerted by the hand that uses them, but its degree should always
be kept present to the mind of the operator.
The vectis, or lever used instead of the forceps just mentioned, is a dangerous instrument in unskilful hands.  In fact whenever it is used as a
lever, in the common acceptation of the term, with some part of the pelvis
as the fulcrum, the use of it is a piece of unskilful cruelty; for the soft parts
between the bone and the instrument are bruised not only with the whole
force of the hand, but with twice or thrice as much, according as the resist



MEDICAL MECHANICS.                             411
ance is nearer to the fulcrum  than to the hand.  The instrument is safely
used, only when the operator makes one of his hands the fulcrum, and uses
the other as the power, or makes different parts of the same hand answer
both purposes; and then there is a resemblance between the action of the
vectis and of a hook.
The levator, or lever for rising the broken and depressed portion of the
skull in trepanning, has a fulcrum  attached to it by a joint.  Care should
be taken to place this fulcrum where its pressure cannot be injurious.
The circular-saw or crown-saw of the trephine, should be worked with a
quick motion and gentle pressure, for the reason given at page 107, when
treating of cutting instruments.  The purpose is thereby attained, both
sooner and better, and the head of the patient is less shaken.
For the same reason, in amputation, a light and quick motion of the
straight saw causes less jarring, and effects an easier division of the bone.
In using the amputation knife, the speed, neatness and success of the
operation are all favoured by blending the drawing or saw motion of the knife,
with the pressure towards the bone.
These last observations are of a hundred similar, which might be made to
prove the extreme importance to a surgeon of having familiarity with the
use of tools and instruments.  Perhaps a person cannot better acquire this
than by practising, while young, some amusing work of carpentry. Manual
dexterity, and a little readiness at mechanical contrivance, so frequently
prove of importance to persons in all stations, that it is a gre at defect in the
prevailing system of general education, not to cultivate them with greater
attention.
The tooth-key is an instrument found in many hands, and persons who do
not pretend to more than the lowest degree of skill in the healing art, are
not afraid to use it.  The consequence is, that perhaps scarcely a day passes
in which teeth are not broken and jaws splintered, and gums bruised even
to sloughing, by the unskilful or awkward use of it. The common tooth-key
may be compared to wheel and axle, the hand of the operator acting on two
spokes of the wheel to work it, while the tooth is fixed to the axle by the
claw, and is drawn out as the axle turns. The gum and alveolar process of
the jaw form the support on which the axle rolls. The common errors in
tooth-drawing by the key, are these:
1st. Turning the key towards that side where the adjoining teeth are so
close that the tooth to be drawn cannot pass, without either breaking one of
them, or being itself broken.  Sometimes two or even three teeth are thus
unfixed instead of one.
2d. Neglecting the natural inclination of the tooth. By winding it round
in the direction in which it already inclines, and in accordance with a bend
which is generally found in it, the operation is easy and safe; but by drawing it in the opposite way, it not unfrequently is broken, or it splinters the
part of the jaw-bone in which it was set.
3d. If the tooth-claw be blunt, its point may slip upon the tooth, so as
to produce a jar which is very apt to break the tooth
4th. Unless the axle or fulcrum of the key be made to rest as evenly as
possible on the gum, it will tear or otherwise injure the gum. It should
rest, if possible, qver the part of the bone in which the tooth is set, for if
not-as when a back tooth is drawn by an instrument resting on a part considerably anterior to it-the twist produced is painful, and there is danger
of splintering.




412                       ANIMAL AND
A man who, after making these reflections, operates leisurely a few times
on the dead subject, will be able to give instant and safe relief to very common and most intense suffering. And it is hardly excusable in any medical man who may be placed where a professed dentist cannot be procured,
to neglect acquiring a talent so easy.
Some dentists, by a strong' forceps made for the purpose, pull teeth
directly out; others by a simple sharp-pointed lever push them out; and
others use a forceps in the manner of the tooth-key, by resting one side of
it on the gum as a fulcrum, and thee giving it a twisting motion: in the
latter case, the resting side of the forceps is formed like the bolster of a
tooth-key. But much more in all cases depends on the dexterity of the
operator than on the form of the instrument.
Steel trusses for ruptures are among the blessings to suffering humanity
which modern ingenuity has supplied. From the unhealthy employments
of some men, and the early dissipations or unnatural modes of life of others,
debilitated constitutions are frequent, and are'bften transmitted to offspring;
and one of the lamentable effects is that weakness of the flesh forming the
sides of cavities which, and on occasions of strong effort, allows, at particular
points the living parts to protrude from within, so as to form tumours under
the skin. A person perfectly healthy may suffer the same injury from a
very violent strain or pressure on the abdoman.  The occurrence is called
hernia or rupture; the most common hernia is that of the small intestine
through the groins.
Formerly this occurrence disabled for life. A man who had hernia was
discharged from the army or navy; he could not ride on horseback, or take
usual exercise; he could not lift a weight; and in a word he often became
a miserable burden to himself and others. Now, by fitting the pad of a
good steel truss to the part, the rupture is as perfectly restrained, as if the
hand of a slKilful surgeon were constantly there. The truss may be put on
and off, with as little reflection or trouble, as a part of the ordinary dress,
and the man becomes again almost as fit for all the duties of life as if he
were without his ailment.
The old and still common form of the steel truss is that of a half or threequarter hoop, so bent and tempered, that when put upon the patient, one
end which has a pad upon it, presses with a given force, over the opening by
which the rupture protrudes, and the remainder tightly embraces the body.
The objections to this kind of truss are, the difficulty of making it to fit
exactly; its being rather troublesome to put on and off; and its pressing
disagreeably round the body.
Another kind of truss, free from these objections, consists of a little more
than half a hoop, with a pad at each end: one of the pads supports the
weakness, and the other rests upon the centre of the back, to bear all the
strain there, while the hoop itself passing round the hip far.thest from the
hernia, reposes loosely on the hip.  This truss may be called self-adjusting,
for it almost of itself falls into its place, and needs no fastenings; the same
truss fits all persons of one size, whatever their shape; and the strength may
be adjusted by changing the number of plates in the spring-hoop.
Tourniquets, crutches, splints, &c., &c., are so simple in all respects as not
to merit special notice here.                        *
This section contains some of the reflections which, in contemplating the
human skeleton, occur to a person familiar with mechanical philosophy: and
the more complete such a person's knowledge is of anatomy, physiology,




MEDICAL MECHANICS.                         413
surgery and medicine, the more numerous will be the professlnal objects on
which this philosophy will shed a light, dissipating doubt and error. The
author has not entered into more minute detail, because it would have been
encroaching upon the office of the teachers of particular departments, and
because he thinks that any one who is not enabled, by the examples here
given, to make the applications of the general laws to all possible cases,
may account the study of the healing art unsuited to the faculties with
which he is endowed.




414      FLUIDITY IN  RELATION  TO  ANIMALS.
PART V.
(CONTINUED.)
SECTION II. —DOCTRINES  OF  FLUIDITY  IN  RELATION  TO
ANIMALS.
In the preceding sections on the laws of fluidity, occasional illustrations
have been taken from the animal economy; but there are many other particulars of the same class and of great interest, which it is convenient to
consider apart under the four heads of, 1st. The circulation of the blood;
2d. The reespiration and voice; 3d.'The tdigestion, and 4th. The pelvic
phenomena.  It is important to remark here, that this section cannot be
understood by a person ignorant of those which precede.
THE CIRCULATION OF THE BLOOD.
PERHAPS there are few points more remarkable in the history of the progress by which man has arrived at his present knowledge of the universe,
than that he was so long ignorant of the fact that the blood in his own and
in other animal bodies is constantly circulating.  England claims as one of
her sons, the man whose powerful intellect at last established this truth, in
opposition to strong appearances, and to the most fixed prejudices.  Dr.
Harvey published his proofs in the year 1619.  A person who tries to imagine what the science of medicine could have been while it took no account
of this fact, on which, as a basis, much of the certain reasoning about the
phenomenon of life must rest, is prepared for what old medical books exhibit
of the writhings of human reason, in attempts to explain and to form theories,
while a fatal error was mixed with every supposition. —The chief circumstances which prevented the earlier discovery of the circulation was, that on
examining dead bodies, the arteries were always found empty of blood;which was the reason, also, of those vessels being called arteries or airtubes.
We now know, that as the Thames water spreads over London in pipes
to supply the inhabitants generally, and to answer the particular purposes of
brewers, bakers, tanners, and others, and is then in great part returned to
where the current sweeps away the impurities, so, nearly in the human
body, does the blood spread from the centre, through the arteries, to nourish
all the parts, and to supply material of secretion to the liver, the kidneys,
the stomach, and other viscera; and returns from these by the veins towards
the heart and lungs, to be purified and to have its waste replenished, that it
may again renew its course.
The circulation may be more particularly described thus: From the left




THE CIRCULATION.                            415
chamber or ventricle of the strong muscular mass, the heart, a large tube
arises, called the aorta; and by a continued division or ramification, opens
a way for the bright scarlet blood to the very minutest part of the living
frame,-the extreme divisions of twigs being so small that they are called
capillary or hair-like tubes.  At the termination of these vessels, the blood,
after answering the purposes of general nutrition, &c., by which it loses its
bright colour, enters the comnmencement of the venous tree, or returning
channel; and gliding successively from smaller to larger branches, returns
towards the right chamber or ventricle of the heart requiring purification and
partial renewal.  Considering the great arterial and venous systems of the
body as tlwin trees-the scarlet and the purple, with corresponding and
meeting branches. and with trunks which touch each other at the heart —it
will appear that they again similarly meet or inosculate by their extreme
roots, and thus form a continued or circular channel.  The root of the venous
tree, by which the blood spreads from the right chamber of the heart through
the lungs is called the pulmonary artery, and that of the arterial tree, by
which the blood returns to the left chamber, is called the pulmonary vein.
Both of these ramify in the spongy masses of the lungs forming a great part
of the pulmonary substance.  Fresh material for the blood is brought froml
the di(estive organs by the l cteal absorbents and thoracic duct, and is constantly pouring into a large vein near the heart, to be completely mixed with
the dark or returning blood by a violent agitation or churningy during its
passage through the heart.  The mixture, on leaving the right ventricle, is
strained through the minute ramifications, the vessels in the lungs, and at
the same time is exposed to the action of the air entering the cells of the
lungs in respiration, by which exposure the dark purple blood becomes again
pure scarlet, and when it reaches the left chamber or ventricle, is ready to set
out on its journey as before, charged with new life and nutriment.  The
two chambers or ventricles of the heart have each an anti-chamber or auricle,
(so called from an external resemblance to a dog's ear,) into which the blood
is first received from the veins; and there are valvular doors between the
auricle and ventricle, which allow the blood to pass readily into the ventricle,
but oppose its recoil during the ventricular contraction.  Similarly acting
valves are placed between the ventricles and great arteries. There are valves,
also, in many of the veins, over the body, to secure the natural course of
the circulation.  Besides the important change of purification which the
blood undergoes in passing through the lungs, its composition is much
influenced by the action of the kidneys, of the exhalent of the skin, and of
the liver, —the two former relieving it of superfluous moisture and salts, the
last from a large quantity of matter in the formn of bile.
The description given above of the circulation of the blood is only an outline; and yet by showing the manner in which fresh material enters, it contains more than Harvey knew of the subject. In this department of knowledge,
as in most others, we have advanced from the very general and vague, to the
more particular and precise,-and just as the general nature of steam was
known long before it served in steam-engines, and as the period of the moon's
revolutions had been accurately observed for thousands of years before the
fluctuations in her velocity could be calculated so as to make her the mariner's best guide in his courses across the ocean, so, when Harvey had proved
the general fact of the circulation of the blood, he had left much yet to be
done, by observing and raising from subordinate facts, to render the knowledge available for the many useful purposes which it is calculated to serve.
Within a few years only, has the importance of the subordinate circum



416   FLUIDITY IN RELATION TO ANIMALS.
stances been fully appreciated,-as is evinced, by the numerous works
composed to elucidate them; but many of which works have served only to
prove, that if the difficulties were to be solved by natural philosophy, medical
men in general had not yet studied it sufficiently to be able to use it successfully. In this section it will be attempted to place certain important
points of the subject in a clear light: and by referring directly to the general
laws of nature, as explained in the body of the work, to settle existing
disputes on some of these points, to remove remaining doubts on others,
and to suggest sbme important new applications.
The fact of the circulation of the blood being once admitted, an inquirer
who contemplates the apparatus by which it is effected, is led by the general
analogies of nature to conclude-I-st. That the ventricle of the heart, at each
contraction, empties itself into the great artery, as a forcing-pump at each
stroke empties itself into its pipe;-2d. That the consequent jet causes a
wave to pass along the extremities of the arterial tree, (accounted simply
elastic,) so as to produce every where, what is called the pulse; 3d. That
the force of the heart acting along the arteries, forces the blood through
their open capillary extremities into the commencing veins, and along the
veins back to the heart again. Now these assumptions, which Harvey
believed completely to describe the circulation, are all nearly true: but are
still so far from being either the exact or the whole'truth, that they leave
important facts unexplained. Thus: —st. The pulse, instead of being the
wave, as slowly progressive as the view above given anticipates, is almost as
instantaneous over the whole body as a shock of electricity; 2d. The arteries
are all found empty after death, although they have no power of emptying
themselves; and if an artery be tied in the living body, the part beyond the
ligature, and cut off, therefore, from the influence of the heart, is equally
emptied; —3d. The rapidity of the blood's passage through the capillaries
varies very much, but it does not vary in exact accordance with the changes
in the rapidity or force of the heart's action.-These and other facts ascertained since Harvey's day, not exactly squaring with his views, have rendered
further investigation necessary; and it is now additionally known-ist.
That the coats of the arteries are not simply elastic but actively contractile;
and 2dly. That the capillary vessels can move the blood independent of the
heart.  In analyzing this subject, it is convenient to follow the blood round
from the heart to the heart again, through the three stages of, 1st. The
arteries; 2d. The capillaries; 3d. The veins.
Motion. of the blood in the arteries.
The contractions of the heart inject the blood into the arteries with a force
maintaining such a tension in them, that according to the interesting experiments of Dr. Hales, recorded in his Statistical Essays, if any artery of a
large animal like a horse be made to communicate with an upright tube, the
blood will ascend in the tube to a height of about ten feet above the level of
the heart and will there continue, rising and falling a few inches with each
pulsation of the heart. Now a column of ten feet, as explained at page 120,
indicates a pressure of about four and a half'pounds on the square inch of
surface: this, therefore, is the force of the heart urging the blood along the
arteries into the veins.-The opposing tension of the veins is much less, because, as will be explained under the proper head, the blood readily escapes
from them into the heart; Hales found that in a tube communicating with a
vein, the blood stood only a few inches higher than the level of the heart.




THE CIIRCULATION.                           417
In small animals he ascertained the tension of artery and vein to be less than
in large ones; and the ratios deduced for the human body, under ordinary circumstances, were an eight feet column, or nearly four pounds per
inch for the arteries; and half a foot column, or a quarter of a pound per
inch for the veins.
Arteries examined after death are found to consist of —ist, an outer coat
of strong elastic substance; 2d, a middle coat of circular fibres; and 3d, an
invex inner coat of smooth lining membrane.  Their elasticity or power of
resisting change of dimension, and of returning to a middle state from either
dilatation or compression, because remaining in the dead artery, was the most
obvious property, and was that first attended to. Minute observation of the
phenomena of life has since determined the following facts, proving and illustrating a contractility resident in the fibrous coat.
1. A small living artery, cut across, soon contracts so as to close its canal
and arrest hmemorrhage.
2. While an animal is bleeding to death, the arteries, in accommodating
themselves to the decreasing quantity of blood, contract far beyond the degree to which their simple elasticity would carry them; and they relax again
after death. Dr. Hales took seventeen quarts of blood from a horse before
it died, in whose body only three quarts more were found altogether, and yet
the moment before death the tension of the arteries sustained a column of
two feet of blood in his experimental tube.
3. The artery of a living animal, if exposed by dissection to the air, sometimes will contract in a few minutes to a great degree: and in such a case,
only a single fibre of the artery may be affected, narrowing the channel like
a thread tied round it.  (See Parry on- the Pulse.)
4. When a living artery is tied, the part between the ligature and the
nearest branch on the side of the heart gradually contracts, and becomes at
last a solid or impervious cord.
5. Fluctuation in the vital action of parts, is often attended with sudden
increase or diminution of caliber in the arteries concerned.
Although these facts prove indubitably a contractility in the coats of arteries distinct from their elasticity, still, because the circular fibres do not
resemble common muscles in colour or in chemical composition, or in being
immediately obedient to the stimuli of electricity, pricking, great heat, &c.,
their contractility was by many persons for a long time denied. The dispute, however, was often more about the words contractility and muscularity,
than about facts.
The pulse in the arteries, chiefly as regards its almost instantaneous occurrence over the whole system, in all states of arterial dilatation, and its great
strength and sharpness in very small and remote branches, points also to the
active contractility of the arterial coats: for,
1. Were the arterial tree in the living body a system of simply elastic tubes
as readily admitting of further dilatation as in the dead body, the first part
or trunk would affect the motion of the blood beyond it, nearly as the airvessel (see page 126) placed at the commencement of artificial arrangements
of water-pipes affects the motion of the water in them;-that is to say, as the
air-vessel converts the sudden and interrupted jets of water from pumps of
fire-engines, town suppyaying pipes, &c., into a uniform stream with scarcely
a remnant of shock, so, in the arterial branches, simple elasticity would
cause a more tranquil flow than indicated by the remarkable gushes from a
wounded artery, and a quieter beat than that bounding pulse of life felt in the
remote artery of the wrist, as sensibly, in proportion, as near the heart itself.
27




418   FLUIDITY IN RELATION T'O ANIMALS.
2. Were the pulse a wave advancing in tubes that yielded as readily as the
dead arteries in their middle states of dilatation, it would be more slowly
progressive from the heart to the extremities; but it is felt so instantly over
the whole body, as to be commonly compared to a shock of electricity.
3. A pulse may be produced artificially in the arteries of a body recently
dead by filling them with water to the tension of life, and then injecting at
intervals, by a syringe, as much water as the heart throws off blood at a beat;
but although the artery is then distended nearly to the limit of its dilatability,
and is, therefore, rendered rigid, the beats are weaker than those of the living pulse. A similar experiment, tried by connecting the artery of a dead
animal -with the corresponding artery of a living one, has a similar result.
4. A tube extensively elastic, that it might convey a wave of liquid with
a velocity approaching to that of the pulse, would require to be so tense,
from fulness, as to be discernible always by the touch, through any imbeding medium, such as flesh, like a hard cylinder or cord; and it would be acting constantly as a spring tending to straighten itself, and, therefore, would
be stiffening the parts through which it passed. Now the living arteries,
between their pulsations, are almost as soft and compressible as the surrounding flesh, and they offer no perceiveable opposition to bending, in any movement of the parts. This may be verified by examination of the lips, for instance, or of the fingers; but when a person sits cross-legged, the well-known
shaking of the suspended foot, in unison with the pulse, shows the recurring
efforts of the artery to straighten itself, during the moments of greater tension.
5. A bulky wave in elastic vessels would have to recoil from the extremities, or to pass through them as a gush; and the recoil would be particularly
observea'le near the ligature of the tied artery; but examination has not
detected such effects in the living body. The operation for aneurism, —in
which the artery is tied beyond the tumour, instead of, as usual on the side
next the heart,-if it checked a strong wave, would almost certainly produce
bursting; yet Mr. Wardrope and others have lately performed this operation
with successful issue.
6. The wave would be more interrupted by the bandage in the operation
of bleeding, than the living pulse is.
7. The pulse of a paralytic limb often seems more affected, than mere
change of size in the artery will account for. The same is true, in an
opposite way, of the pulse in an artery leading to an inflamed part.
8. If the abdomen of a living animal be opened, the mesenteric artery, in
all its ramifications, is seen stiffened and raised up suddenly with every
pulsation, in a manner which the spreading of newly received blood in a
very yielding vessel does not account for.
9. In the interesting experiments of Bic'hat, Parry and others, to ascertain the exact extent of the supposed dilatation and contraction of arteries,
during a pulse, not the slightest degree of either was discernible, even when
sought for with microscopes.
To explain these and other phenomena, then, it seems necessary to admit,
~as occurring throughout the whole body, and almost simultaneously with the
contraction of the heart itself, such an action of the contractile fibres of the
arteries, so as to modify the elasticity of the arteries, and to render them rigid
enough, in all degrees of dilation, for the heart to produce its effects through
them  almost as it would through tubes of metal.-Dr. Young, in a paper
published in the Philbsophical Transactions for 1809, and characterized by
the usual elegance and precision of his writings, has adduced experiments




THE CIRCULATION.                               419
and calculations, to show that waves in elastic vessels advance more quickly
than was before imagined: but the spreading of the pulse seems to be yet
more rapid than his calculation anticipates. —It is evident, that when arteries,
in consequence of depletion, are contracted beyond the middle station of
their elasticity, their tension and power of quickly conveying the pulse must
be dependent altogether on the condition of their contractile fibres.
The careful experiments which could detect no change of size in the
arteries during the pulse, while they disprove the ancient belief of a considerable tumefaction or wave passing along, or of a considerable filling and
emptying of arteries, like what occurs in the heart, might also be supposed
positively to disprove the occurrence of any general constriction of the vessels
on their contents-but erroneously: —for if a man's arterial system, considered as one cavity, besupposed to contain five pounds of blood (which is
near the truth,) and if the vessels be thought to embrace their contents, even
between the pulses, with force enough to have all a rounded or cylindrical
form, although.remaining soft and yielding to the pressure of the finger;
and if we suppose their coats, during the pulse to be thrown into a sudden
contraction, as if in obedience to an electrical shock, still, because blood is incompressible, and because just as much enters the arteries with every pulse
as escapes from them before the next, their bulk would not sensibly diminish
by the strongest conceivable action of their coats; of which action the only
sensible effects would be, that the soft, yielding, and, in some places, compressed tubes would be suddenly converted into hard or resisting cylinders;
and that wherever, by any accidental pressure, an artery had been flattened,
it would, in regaining its cylindrical form, strike or pulsate against the compressing body.-Whether such an action as this contributes to produce the
arterial pulse will be considered under the head of "the pulse," after we
have seen how the blood moves in the capillaries and veins
In any admissible view, however, of arterial agency, we find that the
arteries contribute to the circulation of the blood, but as tubes which convey
It, their own permanent tension, and, therefore, the force with which the
blood is pressed into the capillaries, being derived from the heart alone.
Even if there be a momentary arterial contraction, such as alluded to above,
at the instant of the pulse, it is of too short duration to have an appreciable
effect, abd probably any effect would be counterbalanced by the same action
pervading the capillaries.  Many physiologists have had a confused belief
that the arteries aided very actively in propelling the blood; but a little
reflection would have shown, that as they have no vermicular or progressive
contraction, like the intestines, they no more prapel the fluid within them,
than any other tubular conduits do.-Although they be thus in no degree
instrumental in the propulsion of the blood, still, by more permanently
enlarging or diminishing their caliber, that is, by merely becoming larger or
smaller conduits, they may much influence its local distribution, and the
speed of its transmission.*
* It has long been a subject of dispute whether the arteries exercise any active power in
the circulation of the blood, and many ingenious experiments have been instituted to determine the question. Our author, in denying that the arteries are in any degree instrumental
in the propulsion of the blood, is not borne out by recent investigations. M. Poiseuille,
indeed, seems satisfactorily to have proved that the contractility of the arteries does assist
in the propulsion of the blood. By a series of well devised, and apparently accurate experiments, M. Poiseuille arrived at the unexpected result, that the force of the blood in the
arteries will support a column of mercury of the same height with whatever part of the
course of the arterial circulation the column is placed in connection-whether for example
it is connected with the origin of the carotid, or with a branch derived by repeated subdivi.
sion from the crural artery: and he, therefore, concludes that the force with which a mole



420   FLUIDITY IN RELATION TO ANIMALS.
The nature of this work does not allow us to record historically the various errors into which even able men have fallen, in attempting to explain
the office of the arteries, but we shall glance at the following -Dr. Mlllonro
and John Hunter, two of the most able physiologists that the world has
seen, believed that the arteries did almost as much in propelling the blood
as the heart itself. We need not repeat the refutation of this opinion. The
ingenious Bichat, again, unable to detect either momentary contraction or
dilatation in the arteries, thought that the blood was pushed along them by
cule of blood moves, is the same throughout the whole arterial circulation. Following out
these researches, he has investigated the manner in which the original impulse communicated by the heart, is transmitted unimpared to distant parts of the circulation, notwithstanding the retarding tendency of friction, and the yielding of the parieties of the vessels.
His first object here was to ascertain whether the arteries are dilated by the stroke of the
heart, and impulse communicated to the blood, and what the amount of the dilatation may
be. By many physiologists a very extravagant idea used to be entertained of the amount
of their dilatation; on the other hand, the latter researches of Parry, and other experimentalists have assigned exceedingly narrow limits to it; nay, by one eminent physiologist,
Bichat, it has been denied altogether. M. Poiseuille has determined the point by means of
a very satisfactory set of experiments with an apparatus of his own invention, and has
ascertained that dilatation is affected; but that it is so small as certainly to be indistinguishable in an artery by the unaided senses. This apparatus cannot be thoroughly described without a diagram; it will be sufficient, therefore, for us to mention, that it is so
contrived as to contain about eight inches of the carotid artery of the horse in:a vessel
filled with water and made water tight, except at one point, from which a small horizontal
glass-tube issues, about an eighth of an inch in diameter. At each contraction oflthe
animal's heart it was found that the water in the small tube advanced two inches and
eight-tenths, and that it retired to its former place during the diastole of the heart. The
diameter of the artery was seven-twentieths of an inch. Hence it may be calculated tliat
at each pulsation its capacity was increased about a thirtieth part.
Having ascertained this fact, M. Poiseuille goes on to inquire, whether the power which
is expended by the blood in causing this dilatation is restored by the subsequent contraction of the artery. For this purpose a portion of the common carotid artery of the horse,
ten inches long, and seven-twentieths of an inch in diameter, taken immediately after death,
was connected with the end a, of the tube, (see Figure;) a stop-cock, however, being previously fitted between aand b. The other end of the artery was fixed on a tube of analogous
construction, different in fact only in so far asthe limb c d was inclined at about half a right
angle instead of being vertical, and the stop-cock was placed near the end d. The whole of
the first tube, the artery and part of the descending limb b c of the second tube was filled
with water, a little mercury then filled the curvature of the second tube, and the ascending
inclined limb of that tube above the mercury was filled with water. The stop-cock of the
last tube being closed, and that on the first tube being opened, mercury
Fig. 174.     was poured into the former by its end d, till the pressure on the artery
amounted to ninety five millimetres or about 3.8 inches. The stopcock of the first tube was then closed and thAt on the second tube was
opened; upon which the water rose instaneously in the latter, a
portion flowed out at the top, and the remainder then sankalittle, and
assumed a fixed level. On making the necessary computations, M.
Poiseuillc?ound that the point to which the mercury wasraised in the
second tube at the moment of the contraction of the artery indicated
rtT              an elevation of one hundred and ten millimetres or 4.4 inches. Hence
thepoower with which the arterial coats contract upon themselves after
being dilated, exceeds that which is expended in dilating them. In the
present experiment the excess was equivalent to six-tenths of an inch
of mercury, or three nineteenths more than the dilating force. In three
e  other experiments, the excess of the column of mercury was 9.20, 14.
20, 19.20 of an inch. When repeated with the artery of an animal
which had been killed four days before, the excess was less than 4.20
It is evident, therefore, that whatever force the blood after issuing from
C      the heart loses in consequence of its acting on yielding vessels, is completly restored by the elastic contraction of their parietes. The memoirs of M. Poiseuille will be found in the Repertoir General d'Anatomie et de Physiologie, Tom, VII.
Dr. Badham in an interesting paper in the London Medical Gazette (vol. viii. p. 549)
has adduced some strong evidences of the existence of independent arterial action, and
gives a sketch of various pathological phenomena, which appears explicable on such an
admission, and inexplicable without it.-Am. Ed.




THE CIRCULATION.                            421
the heart instantly through their whole extent, as a solid rod of metal or
wood is advanced by an impulse at one end. Dr. Parry took nearly the
same view of the subject, and illustrated his idea by referring to the experiment of moving a whole line of billiard-balls by striking the extreme one.
Both these authors erred by neglecting the hydrostatical truth, that pressure
in a fluid operates equally in all directions, and, therefore, that fluid pressed
into a tube tends to dilate the tube, just as powerfully as to drive the fluid
forward; and they did not advert to the fact tit the progress of the blood in
the small arteries is not by waves or successive jets, but is nearly a uniform
stream.  The blood could only advance, as they supposed, by the arteries
becoming, for an instant, absolutely rigid, in consequence of a strong action
of their contractile fibres.
It merits notice here, although not strictlya mechanical fact, that arteries
permanently increase or diminish in size when a permanent change takes
place in the demand for their service. The arteries of the gravid uterus, or
of an increasing tumour, grow with the part supplied, while on the contrary,
those of the stump left after amputation soon remarkably diminish. If the
chief arte ry of a limb be obliterated by any cause, as after the operation for
aneurism, the small collateral anastomozing branches increase in size to do
its duty.
It is farther remarkable, that when arteries are called upon to carry an
increased quantity of blood, they often become tortuous or serpentine, as
well as larger; and that arteries leading to parts whose actions are naturally
intermitting or fluctuating, have generally the tortuous form. Of these
truths, the arteries leading to rapidly growing tumours, or to varicose
aneurisms, and the arteries of the uterus and testes, may serve as instances.
This bending of arteries, and the very curious divisions into many branches
which again re-unite, found in those leading to the brains of some animals,
do not seem intended to slacken the rapidity of the sanguineous current,
but to give the artery a greater control over the supply.
Passage of the blood through the capillaries.
We have seen that the heart keeps up a tension or-pressure in the arteries
of about four pounds on the square inch of their surface; and with this force,
therefore, is propelling the blood into the capillaries. If these lastwere passive tubes, constantly open, such force would be suffi6ient to press the blood
through them with a certain uniform velocity; but they are vessels of great
and varying activity; it is among them that the nutrition of the different textures of the body takes place, as of muscle, bone, membrwne, &c.; and that
all the secretions from the blood are performed, as of bile, gastric juice, or
saliva, &c.; and to perform such varied and often fluctuating offices, they
require to be able to control, in all ways, the motion of the blood passing
through them. The capillaries of the cheek, under the influence of shame,
dilate instantly, and admit more blood producing what is called a blush;under the influence of anger or fear, they suddenly empty themselves, and
the countenance becomes pallid-tears or saliva, under certain circumstances,
gush in a moment, and in a moment again are arrested-if a person having
inflammation in one hand be blooded from corresponding veins in both arms
at the same time, twice or thrice as much blood will flow from the diseased
side as from the other. Similar changes occur in many other instances.
Now the only action of cylindrical vessels, capable of causing these phenomena, is contraction and dilatation of their coats; and with reference to such




422   FLUIDITY IN RELATION TO ANIMALS.
action it merits notice, that arterial branches have more of the fibrous or
contractile coat in proportion as they are smaller.
A muscular capillary tube strong enough to shut itself against the arterial
current from the heart, is also strong enough to propel the blood to the heart
again through the veins, even if the resistance on the side of the veins were
as great as the force on the side of the arteries. For if we suppose the first
circular fibre of the tube to close itself completely, it would, of course, be
exerting the same repellent force. on both sides, or as regarded both the
artery and vein. If, then, the series of such fibres forming the tube were to
contract in succession towards the vein, as the fibres of the intestinal canal
contract in propelling the contents of that canal, it is evident that all the
blood in the capillary would thereby be pressed into the vein towards the
heart.  If after this the capillary again relaxed on the side of the artery, so
as to admit more blood, and again contracted towards the vein as before, it
would produce a forward motion of the blood, first towards the vein, and then
in it independently of the heart. As capillary action, however, is not visible, its nature has not yet been positively ascertained:-some persons have
deemed it electrical.
It is capillary action which absorbs and moves the fluids of the classes of
animals which have no heart.  It must also be the power which moves the
blood in warm-blooded monsters formed without hearts. There are cases of
apparent death among human beings, where the heart remains inactive for
days and yet a degree of circulation sufficient to preserve life is carried on
by the capillaries. In illustration of capillary action, we have also the absorption, by the lacteals, of nutriment from the alimentary canal; and, perhaps,
t2 a certain extent, the circulation of blood in the livers of animals.  In
is last case, the blood collected by veins from the abdominal viscera, instead
of going directly to the heart, is again distributed through the liver by the
branches of the vena portas, and is then again collected by the ordinary veins
of the liver, and carried to the heart: it thus moves through two sets of
capillaries in passing from the arteries to the heart again.
The action of the capillaries is the cause of that singular fact which prevented the ancients from discovering the circulation of the blood, viz., the
empty state of the arteries after death. All the muscular parts of an animal,
including, therefore, the contractile coats of vessels, retain their life, or power
of contracting, for a considerable time after respiration has ceased —as is
seen in the recovery of persons apparently drowned or suffocated: in the
leaping of a heart taken from an animal recently killed; in the actions resembling life which can be produced, by the agency of galvanism, in a body
recently dead: but the fact is seen still more amply for our purpose, in the
total disappearance of a local inflammation after the death of the patient,for inflammation involves a gorging or over-tension of the capillaries, into
which, when the heart has ceased to press blood, the contractile force remaining in them, even under disease and in a dead animal, is sufficient to squeeze
the blood out of them, and often to remove all trace of the malady which has
been fatal. In ordinary cases, then, the capillaries throughout the body remain alive and active for a considerable time after breathing has ceased, working like innumerable little pumps,. and empyting the arteries into the veins.
As the red blood is their proper sustenance as well as stimulus, they work as
long as there is any of it coming into them from the arteries behind; except,
however, the capillaries of the lungs, which soon cease to act, because, after
breathing has ceased, they receive only black blood, and are moreover oompressed by the collapse of the chest; and all the blood accumulates behind




THE CIRCULATION.                            423
them. The capillaries may continue to be filled from the arteries, either in
consequence of their elasticity opening them with what is called a suction
power, or of an absorbant power depending on life, like that of the lacteals
and of the absorbents all over the body, and, perhaps, of the vessels in the
roots of vegetables.  When death is produced by lightning, or by the poisons
which destroy all muscular irritability, and therefore that of capillaries, the arteries. after death are found to contain blood like the veins.  In a living body,
if an artery be tied, the part beyond the ligature is soon emptied into the veins,
and becomes fiat.-The experiment has be made even upon the aorta itself.
The empty state of the arteries after death has been ascribed, by some
teachers, to the momentum with which they supposed the blood to be thrown
out from the heart in its last contraction-sufljcient, said they, to squirt it fairly
through the most distant capillaries; a doctrine exemplifying the carelessness
with which men sometimes receive and repeat opinions, to which their attention has never been fully awakened. Such an effect would not follow, even
if the action of a dying heart were the strongest possible; while, in reality,
it is in most cases so feeble, that the pulse for sometime ceases to be perceptible at the extremities, and the diminished circulation lets them become
cold.-Other physiologists have taught that an artery is capable of contracting directly upon its contents, so as to expel even the last drop;-but large
arteries, when emptying, do not contract roundly like an intestine; they
become flat like elastic tubes of leather sucked empty, and no contractile
action of the vessel itself could bring its sides together in such a manner.
If arteries emptied themselves by their own action, tle pulmonary artery
should be more certainly empty than the aorta, because it is shorter; yet it
is always full; for the reason already stated, that the pulmonary capillaries
cease to act after respiration has ceased, on account of the blood in them
being venous or dark blood, and therefore not life-supporting or stimulant to
them.
Passage of blood through the veins.
The veins have much thinner coats than the arteries, and if taken altogether
have much greater capacity: for besides being larger than the corresponding
arteries, they, exist in many situations, as double sets, an exterior and an
interior: they have also very frequent inosculations or communications with
each other throughout their whole course.
the simple weight of the column of blood in any descending artery is just
suffient tb raise the blood through open capillaries to an equal height in
the corresponding vein, according to the hydrostatical law, that fluids attain the
same level in all communicating vessels; and therefore, as the arch of the
aorta rises considerably above the heart, the gravitating pressure of the descending arterial column of blood would be sufficient to lift that in the veins
not only up to the heart, but considerably beyond it. In addition to this influence of gravity on the venous current, the blood is pressed into the arteries,
and from them, therefore, towards the veins, with a force from the heart itself,
as stated above, of about four pounds to the square inch, or, in other words,
as if there were a column of blood eight feet higher than the heart urging the
current. It might be expected from the law of equal diffusion of pressure in
fluids, that these causes would soon produce a tension in the veins as great
as in the arteries: and this does not happen, only because the blood has a
ready escape from the veins through the right ventricle of the heart. Under
ordinary circumstances, there can be no greater tension in the veins than what
is sufficient to lift the blood to the heart and to overcome the friction;-as in




424   FLUIDITY IN RELATION TO ANIMALS.
an upright leathern tube, open at top, and receiving water at its bottom from
a powerful forcing pump, there never can be a greater tension or pressure than
what corresponds to the height of the fluid column in the tube, and to the
friction between the fluid and tube. In Dr. Hale's experiments, already
alluded to, a tube connected with a vein so as to receive its blood, became
filled with blood to a height only of about six inches above the level of the
heart.  As Dr. H. generally cut the vein completely across, and inserted the
tube into the portion leading from the capillaries, he would have discovered
the whole power with which the blood is pushed along the veins from the
capillaries, but for the free lateral communication of veins with each other,
which reduces the tension even in an obstruct branch, to the degree existing in the system generally. When, from agitation of the animal, or any
straining exertion, the passage of the blood into the heart was impeded, all
the veins became tense, and a tube inserted into the returning jugular had
blood running over, at a height of three feet above the heart.
If the blood did not escape from the. veins, as above described, the only
cause which could prevent the venous tension from becoming as great as the
arterial, would be obstruction in the connecting capillaries: but the following
facts and considerations prove that these vessels, which, in the dead body,
allow the passage of injections, in the living body freely allow the passage
of blood.  1st. Magendie laid bare the chief artery and vein of a living limb,
and at the part, detached them from the flesh underneath, so that he could
apply a tight ban-age round the limb without including them, and could thus
render them the only channels of circulation for the limb beyond the bandage. He then found, that when a separate ligature was put upon the vein,
to prevent the return of its blood to the heart, and a puncture was made
beyond the ligature, the flux of blood from the puncture was rapid or slow
according as the heart was allowed to produce a greater or less degree of
tension in the artery: —this tension was regulated by his compressing the
artery between the fingers. 2d. After a similar preparation of the parts, the
blood will ascend in a tube from the obstructed vein very nearly as high as
fromn the artery   3d. In the common operation of bleeding at the moment
of puncturing the vein, the blood often jets from it as from an artery, staining even the top of the bedstead. 4th. The microscope discovers in the
capillaries, a uniform forward motion of the blood, as if it were obeying the
steady pressure of the arterial tension, and not any intermitting action. 5th.
Disturbed action of the heart, by obstructing the passage of the blood through
it, is very soon attended with a tumefaction of all the veins leading toi the
heart: the tumefaction becomes very visible about the neck and head, and
in the liver produces swelling and acute pain. 6th. Dr. Young, from experiments made by him, and reported in the philosophical transactions for 1809,
concluded that perfectly open capillaries, of the size existing in the living
body, should just retard a flow of blood urged by the usual arterial tension,
in the degree which really occurs:-a correspondence proving that they must
be open; and open vessels, however small, and how slowly soever they
transmit the blood, still, if the escape of blood from the veins were arrested,
would transmit the arterial tension without diminution.  7th. The fact that
after death the capillaries empty the arteries into the veins, proves that, under
certain circumstances, the venous tension may become even greater than the
arterial. —These facts then and others that might be mentioned, prove incontestably, that the blood is pressed into the veins from the arteries and capillaries, with force sufficient to lift it, not only to the heart again, but many feet
farther, viz., about as far as it would ascend in a tube rising from the tense




THE CIRCULATION.                             425
arteries themselves. So little, however. has this important truth been understood, that in elementary works of authority lately published, the venous
current is treated of as a very obscure subject; and some authors, in their
anxiety to explain it, have assigned causes for it, which, as will appear hereafter, are positive absurdities in physics. The difficulty in the question
seems to have arisen from the great disparity observed between the tension'
in the arteries and in the veins, while the reflection did not occurr, that the
disparity was owing to there being a free passage or outlet from the veins
through the heart.
The illustrious Bichat, with an inattention to facts, extraordinary in him,
persuaded himself that the influence of the heart ceased entirely at the capillaries and that the blood was returned through the veins by the action of the
capillaries, alone. How could he avoid the single reflection, that, if the purpose of the arteries had been merely to convey the blood to the capillaries,
and not also to bear the force which pressed it into and through them, the
extraordinary strength of the arterial coats, and the great power of the heart
to fill them and keep up the tension described, would have been quite superfluous?-and he knew that nature does nothing in vain.* The reflection
applies strikingly to the pulmonary artery, of which no branch exceeds a
few inches in length.
The uniform current of blood along the veins, so apparent in the operation
of bleeding, and produced, as now explained, by the combined influence of
the heart and capillaries, suffers a considerable disturbance in the neighbourhood of the heart from three causes. 1st. As there is no valve between
the veins and the auricles of the heart, each contraction of the right auricle
tends to throw the blood back into the veins, as well as forward into the
ventricle, and thus produces the venous pulse often felt in the neighbourhood of the chest. 2d. When the chest is expanded by inspiration, it is
more roomy than during the collapse of expiration, and the blood then enters
it more readily. 3d. While the chest is inhaling or drawing in air; that is
to say, expanding so as to diminish the tension or pressure of the air within
it, (see Pneumatics,) it is by the same action favouring the entrance of blood
through the veins towards the heart placed in it;-on the contrary, while it
is exhaling or throwing out air, it is, with equal force, resisting the entrance
of blood, and slackening, or even causing recoil of the inward current. This
favouring or resisting force, however, as will be hereafter shown, is only
such as to lift or support a column of blood of about half an inch in height.
It appears, then, that the entrance of blood into the- chest fluctuates by reason of the respiration, &c., as the entrance of a river stream into the sea
fluctuates by reason of the ebbing and flowing of the tide. An eye watching the jugular vein, under favourable circumstances, may see it tense or
slack in accordance with the opening and shutting of the chest.
It still remains to be ascertained whether or not veins have in themselves
any contractile power, such as can partially empty a lower portion into a
higher portion beyond an adjoining valve. If so, the valve by then bearing
the pressure would let more blood be easily raised from below into the portion so relieved: and the action, without being equal to the office of completely emptying any portion of a vein, would still have the effect of dividing
* This incorrect and inconclusive mode of reasoning is so common that we may be permitted to protest against it. The influence of the heart may cease with the capillaries,
and yet nature has done nothing in vain. Before we would be justified in making such a
charge against nature we must possess an infinitely more precise knowledge of the circulatory forces and of the functions of the arterial system than we do at present. Am. Ed.




426   FLUIDITY IN RELATION TO ANIMALS.
a long heavy column into a number of short columns of comparatively little
resistance.  It is certain, at least, that the valves in the veins, by preventing
the falling back of blood which has once passed towards the heart, must
affect its flow during bodily exercise; for every time that pressure is made
on a vein by a swelling muscle or otherwise, the blood in the part must be
bforced forward, and cannot return.
The veins which are surrounded by muscles are thinner and weaker than
those supported only by the skin. The external veins of the legs are
almost as strong as arteries. Proving, however, that the fabric of veins is
much weaker than that of arteries, any vein in the living body, made to
communicate directly with an artery, soon exhibits what is called a varicose
aneurism, and swells to bursting. Veins possess power, to a great extent,
of adapting themselves to the varying quantity of blood.
Some recent authors, as stated above, either not aware of the facts which
prove that the blood is everywhere pressed into the veins with force much
more than sufficient to raise it to the heart again; or, being unable, from
their little familiarity with mechanical science, to draw exact conclusions
from the facts, or to avoid errors in their own hypotheses, have promulgated
fhe opinion that the progression of the blood in the veins is greatly owing
to a partial vacuum or a suction power in the heart or chest; that is to say,
to the atmospheric pressure remaining constant on the body generally, while
it is, at intervals, lessened about the heart. Now the whole influence of
this effect or circumstance, as stated above, is merely a slight disturbance of
the uniformity of the venous current near the chest. Such a doctrine could
not be proposed or entertained for a moment by a person understanding the
principle of a common household pump; and its having been published, and
tolerated by certain professional men in the present time, will remain a proof
to posterity of the deficiency, as regards fundamental science or natural
philosophy, now existing in the ordinary medical education. Much ingenuity
has been wasted upon it, particularly by Drs. Carson and Barry, the latter
of whom, after making laborious experimental investigations on living animals, has even attempted to build upon it a superstructure of medical theory
and practice! To say that the influence of the heart or chest is the power
which draws the blood to the heart from the general system, is as if one
asserted that the rising and falling of the tide at the mouth of a river is the
power which collects the tributary streams in the interior country.
We shall enter into a little detail on this subject, because the discussion
will elucidate some minor points connected with the circulation.
Presuming, then, that the reader perfectly understands the theory of
pumps, and therefore of atmospheric pressure, as explained under Pneumatirs, he will readily understand the two following propositions, either of
which proves it to be a phy'sical impossibility, That a sucking action of the
heart or chest can be a cause of the blood's motion along the veins. Ist.
The veins are pliant tubes free to collapse, and no pump can lift liquid
through such.  2d. The suction-power of the chest in healthy respiration is
too weak to lift liquid even one inch through tubes of any kind.
A practical illustration of the first proposition is afforded by putting the
point of a syringe into a piece of gut, or eel-skin, or vein filled with water,
and then trying to pump up the water. The result will be, that the fluid
close to the mouth of the syringe will enter it, and then the sides of the
pliant tube will collapse against the syringe, making an end of the experiment. In exact proportion to the rigidity of the tube will be the distance to
which the influence of the syringe, will extend in it; if, for instance, half an




THE CIRCULATION.                           427
ounce of pressure on the square inch of its surface be required to make it
collapse, then the pump will draw up one inch of water, and so for other
proportions. If, during the action of the syringe, the tube were allowed to
open freely at the bottom into a vessel of water, instead of the syringe then
drawing any more water from the vessel into the tube, the original contents
of the tube would straightway be discharged downwards into the vessel.
The explanation of all these facts is found in the pressure of the afmosphere, (see from page 153 to page 158) seeking entrance everywhere at the
surface of the earth, with a force of fifteen pounds per square inch, and
overcoming any opposing force less than this;-a pressure which is sufficient, therefore, to push a column of water thirty-four feet in height, up
through a rigid tube into the vacuum of a pump, but will cause the sides of
the tubes to collapse, unless able to sustain its force. When nature intends
a tube to resist any degree of suction, the tube ts made rigid in proportion;
— witness the wind-pipe and its branches, which are the only instances in
the human body.  And if tubes prepared for sucking light air only have
received such rigidity, how much stronger would tubes have been made for
sucking blood.
Some bad reasoners on this subject have believed, that if a suction power
exist, capable of lifting one inch of a column of liquid, any column, however
long, must follow the first inch when acted upon by the power; for, say
they, the atmospheric pressure, by preventing a vacuum, will prevent separation of the liquid. Now, in the first place, this reasoning is quite inapplicable to pliant tubes, because the ready collapse of their sides will both
allow the separation and prevent the vacuum; and, in the second place,
with respect to rigid tubes, it is equivalent to asserting that a force just
capable of lifting one link, of a chain, must, therefore, be able to lift any
number of connected links.  Water, in a rigid tube, to which the air has
no admittance, may truly be considered as a chain, for it is held together by
a force of fifteen pounds per inch, pressing inwards at the two ends; and by
force inferior to this, cannot lift one portion of it away from another, and,
therefore, cannot draw out a drop but by lifting the whole. A man cannot
suck any water from a rigid tube which is closed at the bottom; and if the
bottom be open, and he has not power to support the whole contained fluid,
it will sink from his tantalized lips to stand at an elevation corresponding
to his suction power.
To illustrate the second proposition respecting the trifling suction power
really residing in the chest, we may state that a person of ordinary strength
using the whole power of the chest, (but not of the mouth separated, which
is a smaller and much more powerful pump than the chest,) cannot, through
a rigid tube, suck water from more than about two feet below his lips, and
therefore not half way so far as from the extremities of his body; while,
in the opposite action of blowing outwards, as in the attempt to blow
through a tube which is dipping into water, he finds nearly the same limit.
But in ordinary breathing, instead of force corresponding to a liquid column
of two feet, or afifteenth of the atmospheric pressure, the increase and diminution of air-density in the chest are measured by a column of less than
one inch or about a five-hundredth of the atmospheric pressure. This fact
is easily shown by breathing through the nose, while holding in the mouth
one end of a glass tube, the other end of which is immersed in water, and
then noting how much the water in the tube.rises above the surrounding
level during inspirqtion, and sinks below it during expiration. The mouth,
during this experiment, may be considered as a part of the general cavity of




428   FLUIDITY IN RELATION TO ANIMALS.
the chest, to and from which air is passing by the narrow openings of the
nostrils. In tranquil breathing, with both nostrils open, the fluctuation in
the tube is less than half an inch each way; with one nostril closed and the
other a little compressed, it may amount to a whole inch; and with hurried
or convulsive breathing, like that of an animal in terror and in pain, it may
exceed twelve inches. Although the measures thus obtained from the
mouth are somewhat too small for the changes in the chest itself, because
the chest is more remote from the opening by which the external air enters,
the difference is very trifling, as may be proved during such experiments, by
stopping the nostrils altogether, while the same respiratory efforts are continued; and as is also proved by the agreement of the results with strict
calculation founded on the inertia and velocity of the air respired-a calculation similar to that required in adjusting the index to the machine mentioned at page 215, for measuring water-currents. In common healthy
breathing, then, while the mouth is open, the fluctuation of pressure in the
chest would be measured by less than half an inch motion each way of the
liquid column.  Dr. Barry, not aware that this point could be so easily
determined by the bloodless experiment described above, or even by a simple
calculation, sought the solution by numerous trials on living animals, into
some part of whose chest he forced a tube. But even if farther experiments
had been at all necessary, those of Dr. B. could not have decided the question, first, because the pain and agitation of the dying animals rendered the
breathing violent or unnatural; and, secondly, because his experimental
tube often or always became a syphon, and he, not adverting to this fact,
has not recorded the difference of level in the liquids at the two ends. That
the external level was, for the most part, higher than the internal, is proved
by his having noticed, almost solely, the inhaling action of the chest, although
the exhaling is often a more powerful effort.
Calling au inch column of blood, then, the measure of the greatest sugescent and repellent powers of the chest during ordinary respiration, we see
that the force which really sends the blood from below to the heart, may
have to lift a column one inch shorter during inspiration, and one inch
longer during expiration: but this is the full and true measure and nature
of the influence of the inspiration on the blood's return to the heart. To
say, then, that the' atmospheric pressure, modified by respiration, is the
great power which moves the venous blood, is just as if we said, that a boy,
standing near the fly-wheel of a steam engine of a hundred horses' power,
and giving it his Lilliputian thrust, alternately backward and forward, were
the prime mover of the machinery.
The truth explained above, that no kind of pump can lift fluid through
pliant tubes, free to collapse, like the veins, renders it unnecessary farther to
speak here of the pumping action of the heart itself, insisted on by Dr. Car.
son, or of that other action, mentioned in a subsequent part of this work, to
which, also, he attributes great influence. viz., the tendency towards a vacuum
external to the lungs and around the heart, produced by the disposition of
the lungs to collapse. It may be remarked, however, that this last influence
is more considerable than the simple inspiratory action dwelt on by Dr.
Barry, and that it operates during expiration nearly as much as during
inspiration, varying in force with the degrees of expansion of the chest. It
is weaker in the living than in the dead body, because the rigidity of the
distended pulmonary arteries helps to support the weight of the living lungs.
Were it necessary to give proofs to persons unableto follow the above
argument, that a suction-power in the heart or chest is not the force which




THE CIRCULATION.                                 4'9
draws the blood from the extreme veins, reference might be made to many
notorious facts quite incompatible with that supposition; such for instance,
as those recorded at page 424, and others. A vein tied, fills tensely below
the ligature-a vein cut across bleeds from the orifice which is distant from
the heart, and will fill a lofty tube connected with it-the circulation goes on
in persons holding their breath-the veins of fishes, which do not breathe,
return the blood as well as those of men, &c., &c.*
After the explanations now given, it is almost superfluous to remark that
absorption in animals cannot depend on atmospheric pressure, and that the
effect of cupping-glasses applied to extract blood, or to prevent the absorption
of poison in wounds, in no way depends upon the fluctuating density of the
air in the chest.t  Dr. Barry's reasonings upon these subjects involve the
same fallacies as his reasonings on the venous current. With respect to
absorption, they neglect the facts of fluids having weight; and with respect
to cupping-glasses, of which the true action is explained at page 175, they
are equivalent to asserting that the action of pumps drawing water from a
river among the hills is influenced by tides, or pumps operating at its mouth
in the sea.
If the fluids in animal vessels had no weight, it is true, that in absorption
at external atmospheric pressures of fifteen pounds per inch might force new
matter into a receiving orifice, at the instant during inspiration, when the
opposing pressure in the chest, at the other ends of the vessels, were half an
ounce per inch less,-there would be no physical absurdity in opposing this,
although there are physiological facts that disprove it-but when we reflect,
that in all vessels under the level of the heart, the weight of the contained
fluids causes an additional outward pressure of about half an ounce troy for
every perpendicular inch of fluid column, making an excess of outward pressure at the toes, for instance, even at the morp favourable time of absorption,
* The influence of inspiratipn of the cavity in the chest exterior of the lungs, and the expansive power of the heart, off the circulation of the blood in the veins, have no doubt been
greatly overestimated by Drs. Barry, Carson and others, but our author appears to us to
have undervalued their effect. Their joint power is more considerable than the reader
might be led to suppose from the perusal of the preceding pages.
The influence of inspiration has been estimated by our author, perhaps justly, as only
sufficient to raise a column one inch; if this force acted through rigid tubes of the length
of the veins, it would produce no movement of the contained'fluid; but acting through pliant
tubes, it would rise one inch of the blood out of the vein nearest the heart, and if this power
acted alone, its effect would here cease. But the vis a tergo, produced by the propulsive
power of the capillaries, and, perhaps, also of the heart, prevents the collapse; the vein is
kept full, and at every inspiration this power is renewed.
The influence of the tendency towards a vacuum external to the lungs, and around the
heart, from the contractile disposition of the resilience of the lungs, is admitted by our
author to be more considerable than the inspiratory effort, and it in fact is, we think, greater
than is suspected. There are reasons for believing that the lungs do not entirely fill the
cavity in which they are contained; the influence of this space is therefore constant, though
greater during inspiration, and of course diminished during expiration.
The capillaries, our author has most satisfactorily shown, have a vital expansive power;
and though he does not 4ssert that the heart has no such power, he denies that it can have
any influence on the movement of the venous blood, since it must act through pliant tubes.
This would be the fact if the expansion of the heart were the only moving power, but the
vis a tergo prevents their collapse, and the effect of the expansive power of the heart,
whatever that may be, is allowed to act.
While, therefore, the action of the capillaries and perhaps of the left ventricle of the
heart must be considered as the main forces by which the blood is propelled through the
veins, the expansive power of the heart-respiration and the resilience of the lungs, or
atmospheric pressure-ought to be viewed as necessary forces, though their precise power
cannot readily be estimated.-Am. Ed.
t The effect of cupping-glasses in preventing the absorption of poisons has been shown
by Dr. Pennock to be owing to mechanical pressure. See the interesting experiments in
the American Journal of the Medical Sciences, Vol. II.-Am. Ed.




430   FLUIDITY IN RELATION TO ANIMALS.
of about two pounds per inch, we see that absorption must be a strong action
of life, able to overcome a great excess of mechanical resistance, instead of
a passive phenomenon obeying an excess of mechanical force. If a mere
balance of pressures acted at the orifices, as Dr. B. supposes, thle blood and
other fluids would be constantly oozing out from all orifices below the heart,
as blood really does from an artificial opening, with force that would fill a
tube reaching as high as the heart. It would be good news for proprietors
of mines, and other persons having to raise water, if by taking off an ounce
or two per inch of the atmospheric pressure at the top of a full pipe, the atmospheric pressure continuing elsewhere would then force water in at openings
below and cause the upward current:-but in truth to make the atmospheire
efficient below, powerful steam-engines or other means must be used to take
off a pressure above, of at least half an ounce per square inch, for every inch
in height the water has to rise.
Another erroneous conception of atmospheric pressure, akin to that which
we have been considering, is expressed in the following reasoning on the
progress of blood in the veins. " The atmosphere presses 15 lbs. per square
inch on all things; the blood therefore, in a vein which has 20 inches of surface is pressed upon through the flesh, with a force of 20 times 15, or 300
lbs., while a cross section of the vein near the heart would measure less than
one inch. The blood, therefore, is always running towards the heart, to
escape from a powerful excess of atmospheric pressure."-This paradox is
solved by the law of fluid pressure, explained at page 131. The same reasoning would prove that an eel-skin suspended by its lip, and filled with water,
when exposed to the pressure of the atmosphere, should quickly be emptied;
and nearly the same would prove that a long sharp wedge thrown into water,
should be always moving in a direction away from its point; and that a ship
formed like the wedge, should nBake quick speed across the sea without either
oar or sail.
A knowledge of the facts detailed under the three heads of arteries, capillaries
and veins, prepares us for the discussion of the following subjects.
The force of the heart.
The arterial tension of four pounds to the square inch, marked by its supporting in a tube connected with the arteries, a column of blood eight feet
high, (see page 417,) is produced by the action of the heart; but as the
heart, while injecting the blood against this resistance, has moreover to overcome the inertia both of the quantity injected and of the mass in the great
artery, first moved by the injection, as also the resisting elasticity of the
vessel which yields to momentary increase of pressure, the heart must act
with a force exceeding four pounds on the inch.  And as the left ventricle
of the human heart, when distended, has about ten square inches of internal
surface, the whole force exerted by it is a matter of simple calculation.  It
is remarkable, as there is this easy means of solving the question, that the
accurate Magendie, in his recent elements of physiology, should speak of it
as undetermined; and should cite, as the best approximation, an estimate
made from the obscure circumstance of a loaded foot shaking in unison with
the pulse, when suspended in the cross-legged sitting attitude.
Some physiologists have expressed surprise that the force of the heart
should he so great as it is, remarking that much less would have sufficed to
propel the blood to the most distant capillaries; but they did not reflect that
the heart, besides carrying on the general circulation, has to force blood into




THE CIRCULATION.                            431
those parts of the flesh which, in the various positions of sitting, lying,
standing, &c., are for the time compressed by the whole weight of the body;
for that, if it were not strong enough fJr this purpose, either the compressed
parts, deprived of their nourishment, would quickly die, or the person obliged
to be every moment changing position, could obtain no lengthened repose.
In illustration of this point, we may advert to the frequent occurrence, in
diseases where the power of the heart is for the time weakened, for sloughings, or bed-sores in. the bearing parts, causing many cases of illness to
terminate fatally which would otherwise soon have terminated in health.
The author of this work has had great satisfaction in suggesting a means
of entirely preventing deplorable termination, namely, that which he is
now about to describe under the title of
The HYDROSTATIC BED for Invalids.
In many of the diseases which afflict humanity, more than half of the suffering and danger is not really a part of the disease, but the effect or consequence of the confinement to which the patient is subjected. Thus a fracture
of a bone of the arm is as serious a local injury as a fracture of one of the
bones of the leg; but the former leaves the patient free to go about and amuse
himself, or attend to business as he wills, and to eat and drink as usual-in
fact, hardly renders him an invalid; while the latter imprisons the patient
closely upon his bed, and brings upon him, first, irksomeness of the continued position, and then the pains of the unequal pressures borne by the parts
on which the body rests. These, in many cases of confinement, disturb the
sleep and the appetite, and excite fever, or such constitutional irritation as
much to retard the cure of the original disease, and not unfrequently to produce new and more serious disease. That complete inaction should prove
hurtful to the animal system, may by all be at once conceived; the operation
of the continued local pressures will be understood from the following statements. The health, and even life, of every part of the animal body, depend
on the sufficient circulation through it of fresh blood, driven in by the force
of the heart. Now when a man is sitting or lying, the parts of his flesh compressed by the weight of the body, do not receive the blood so readily as at
other times; and if from any cause the action of his heart has become weak,
the interruption will both follow more quickly and be more complete.  A
peculiar uneasiness soon arises where the circulation is thus obstructed, impelling the person to change of position; and a healthy person changes as
regularly, and with as little reflection, as he winks to wipe and moisten his
eyeballs. A person weakened by disease, however, while he generally feels
the uneasiness sooner, as explained above, and therefore becomes what is
called restless, makes the changes with much fatigue; and should the sensations after a time become indistinct, as in the delirium of fever, in palsy, &c,,
or should the patient have become too weak to obey the sensation, the compressed parts are kept so long without their natural supply of blood that they
lose their vitality, and become what are called sloughs or mortified parts.
These have afterwards to be thrown off, if the patient survive, by the process of ulceration, and they leave deep holes, requiring to be filled up by
new flesh during a tedious convalescence. Many a fever, after a favourable
crisis, has terminated fatally from this occurrence of sloughing on the back
or sacrum; and the same termination is common in lingering consumptions,
palsies, spine diseases, &c., and generally in diseases which confine the
patients long to bed.




432   FLUIDITY IN RELATION TO ANIMALS.
It is to mitigate all, and entirely to prevent some of the evils attendant on
the necessity of remaining in a reclining posture, that the hydrostatic bed is
intended. It was first used under the following circumstances.
A lady after her confinement, which occurred prematurely, and when her
child had been for some time dead, passed through a combination and succession of low fever, jaundice, and slight phlegmasia dolens of one leg. In
her state of extreme depression of strength and sensibility, she rested too
long in one posture, and the parts of the body on which she had rested all
suffered: a slough formed on the sacrum, another on the heel; and in the
left hip, on which she had lain nmuch, inflammation began, which terminated
in abscess. These evils occurred while she was using preparations of bark,
and other means, to invigorate the circulation, and while her ease and comfort were watched over by the affectionate assiduity of her mother with
numerous attendants. After the occurrence, she was placed upon the bed
contrived for invalids by Mr. Earle, furnished for this case with pillows of
down and of air of various sizes, and out of its mattrass portions were cut opposite to the sloughing parts; and Mr. Earle himself soon afforded his
valuable aid. Such, however, was the reduction of the power of life, that
in spite of all endeavors, the mischief advanced, and about a week later,
during one night, the chief slough on the back was much enlarged, another
had formed near it, and a new abscess was proceeding in the right hip An
air-pillow had pressed where these sloughs appeared. The patient was at
that lime so weak that she generally fainted when her wounds were dressed;
she was passing days and nights of uninterrupted suffering, and as all known
means seemed insufficient to relieve her, her life was in imminent danger.
Under these circumstances, the idea of the hydrostatic bed occurred to me.
Even the pressure of an air-pillow had killed her flesh; and it was evident
that persons in such condition could not be saved unless they could be supported without sensible inequality of pressure.  I then reflected, that the
support of water to a floating body is so uniformly diffused, that every thousandth of an inch of the inferior surface has, as it were, its own separate
liquid pillar, and no part bears the load of its neighbour —that a person
resting in a bath is nearly thus supported-that this patient might be laid
upon the surface of a bath over which a large sheet of the water-proof Indiarubber cloth were previously thrown, she being rendered sufficiently buoyant
by a soft mattress placed beneath her-thus would she repose on the face of
the water, like a swan on its plumage, without sensible pressure anywhere,
and almost as if the weight of her body were annihilated. The pressure of
the atmosphere on our bodies is of fifteen pounds per square inch of its surface, but because uniformly diffused, is not felt.  The pressure of a waterbath or depth to cover the body, is less than half a pound per inch, even on
the under side where it is greatest, and is similarly unperceived. A bed
such as then planned, was immediately made. A trough of convenient dimensions (6 feet long, 2 feet 8 inches wide, and 11 inches deep, are good commnon dimensions) was lined with metal to make it water-tight; it was about
half filled with water, and over it was thrown a sheet of the India-rubber
cloth as large as would be a complete lining to it if empty. Of this sheet
the edges, touched with spirit varnish to prevent the water creeping round
by capillary attraction, were afterwards secured in a water-tight manner all
round to the upper border or top of the trough, shutting in the water as
closely' as if it had been in bottles, the only entrance left being through an
opening at one corner, which could be perfectly closed. Upon this beautiful
dry sheet a suitable mattress was laid, and constituted a bed ready to receive




THE  CIRCULATION.                           433
its pilow and bed-clothes, and not distinguishable from a common bed but
by its most surpassing softness or yielding. The bed was carried to the
patient's house, and she was laid upon it; she was instantly relieved in a
remarkable degree: sweet sleep came to her; she awoke refreshed; she
passed the next night much better than usual; and on the following day,
Mr. Earle found that all the sores had assume# a healthy appearance; the
healing from that time went on rapidly, and no new sloughs were formed.
When the patient was first laid upon the bed, her mother asked her where
the down pillows, which she before had used, were to be placed; to which
she answered, that she knew not, for that she felt no pain to direct; in fact,
she needed them no more.
It may be here recalled to mind, that the human body is nearly of the
specific gravity of water, or of the weight of its bulk of water, and therefore,
as is known to swimmers, is just suspended or upheld in water without exertion when the swimmer rests tranquilly on his back with his face upwards.
He then displaces water equal to his own body in weight as well as in bulk,
and is supported as the displaced water would have been. If his body be
two and a half cubical feet in bulk, (a common size,) he will just displace two
and a half cubic feet of water, equal in weight to his body. If, however,
instead of displacing the water with his mere body, he choose to have something around or under him which bulky with little weight, as the mattrass
of the bed above described, then, after his weight is forced two cubical feet
of that under the level of the water around, he will float with four-fifths of his
body above the level, and will sink much less into his floating mattiass than
a person sinks in an ordinary feather-bed. It thus appears that by choosing
a certain thickness of mattrass, and if unusual positions are required by
having different thicknesses in different parts, or by placing a bulk of folded
blanket or of pillow over or under the mattrass in certain situations, any
desired position of the body may be easily obtained. If the water be about
six inches deep, which in general will suffice, the person standing upon any
part of the bed, or sitting with the knees raised, will cause the part of the
mattrass on which he rests gently to touch the bottom, because a narrow end
of the body cannot displace water equal to the bulk of the whole, but even
then the person is as if standing or sitting on, a soft sofa. If it be desired
to prevent the mattrass, when used as a seat, from touching the bottom, the
object may be attained by having under its middle a broad band or strap
fixed to one edge of the trough, and connected with the other by buttons or
otherwise,- so as to be tightened to allow the mattrass to descend just so far,
and no farther.
This bed is a warm bed, owing to'water being nearly an absolute nonconductor of heat from above downwards, and owing to its allowing no passage of cold air from below. From this last fact, however, less of the perspiration, sensible and insensible, is carried off by the air than in a common
bed, and unless the patient can leave the bed daily to let it be aired like a
common bed, there will be a necessity for ventilation to prevent the perspiration from being condensed on the water-sheet below. This ventilation is
perfectly obtained by placing under the mattrass, arranged like the bars of a
gridiron, small flexible tubes of tinned wire, wound spirally, with their ends
open to the atmosphere, either directly or through two larger tubes crossing
and connecting their extremities near the ends of the mattrass, and then
issuing at the corners of the bed from under the clothes. This bed is in
itself as dry as a bed can be, for the India-rubber cloth (of which bottles can
be made) is quite impermeable to water, and the maker is now preparing
28




434   FLUIDITY IN RELATION TO ANIMALS.
cloth expressly for this purpose. Then, as Sir Humphrey Davy recommended
that his safety lamp should be double, some persons may prefer a double
sheet, to obviate the possibility of accident. Unlike any other bed that ever
was contrived, it allows the patient, when capable of only feeble efforts, to
change his position, almost like a person swimming, and so to take a degree of exercise, affording jibe kind of relief which, in constrained positions,
is obtained by occasional stretching, or which an invalid seeks by driving out
in a soft-springed carriage.  It exceedingly facilitates turning for the purpose of dressing wounds, for by raising one side of the mattrass or depressing the other, or merely by the patient's extending a limb to one side, he is
gently rolled over, nearly as if he were simply suspended in water; and it is
possible even to dress wounds, apply poultcies, or place vessels under any
part of the body, without moving the body at all: for there are some inches
of yielding water under the body, and the elastic mattrass may at any part
be pushed down, leaving vacant space there, without the support being lessened for the other parts. Then, with all the advantages which other invalid beds posses, and with those which are entirely its own, it may yet be
made so cheaply, that even in hospitals, where economy must prevail, it may
at once be adopted for many of the bed-ridden. Mr. Earle within a few days
of seeing the first one, had others made for patients in St. Bartholomew's
Hospital, and has been as much pleased with the results of them as of the
first. The bed has since been introduced into St. George's Hospital by Mr.'Keate, and elsewhere.-The author has now seen enough of the effects of
this bed to make him feel it a duty at once to publish a notice of it. With
it, evidently, the fatal termination called sloughing, now so common, of fevers, and other diseases need never occur again. And not only will it prevent that termination, but by alleviating the distress through the earlier
stages, it may prevent many cases from even reaching the degree of danger.
Then it is peculiarly applicable to cases of fractured bones, and other surgical
injuries; to palsies, diseases of the hip joint, and spine; and universally,
where persons are obliged to pass much time in bed.  And in all cases of
curvature of the spine, either actually existing or threatened, it affords a
means of laying a patient in any desired position, and with any degree of
pressure incessantly urging any part of the spine back to its place. If used
without the mattrass, it becomes a warm or a cold bath, not allowing the
body, however, to be touched by the water, and in India, it might be made
a cool bed for persons sick or sound, during the heats which there prevent
sleep and endanger health. There are numerous other professional adaptions
and modifications of it, which will readily occur to practitioners sufficiently
versed in the department of natural philosophy (hydrostatics) to which it belongs. Before reflection, a person might suppose a resemblance between it
and an air-bed or pillow, calling this a water-bed or pillow; but the principles of the two are perfectly distinct or opposite. An air-pillow supports
by the tension of the surface which encloses the air, and is therefore like a
hammock or the tight sacking under the straw mattrass of a common bed,
and really is a hard pillow; but in the hydrostatic bed, there is no tense
surface or web at all; the patient is floating upon the water, on which a.
loose sheet is lying, merely to keep the mattrass dry, and every point of his
body is supported by the water immediately beneath it. To recall the difference here described, and which is of great importance, the bed is better
described by the appellations of hydrostatic-bed, or floating-bed, than of
water-bed.
The author has given no exclusive right or privilege to any person to




THE  CIRCULATION.                          435
make this bed. He has hitherto employed the carpenter nearest to him,
Mr. Smith, 2583 Tottenham-court Road, at the back of Bedford Square;
Mackintosh & Co., 58 Charing Cross, the manufacturers of the cloth; and
Mr. Williams, 25 Cleveland.Street, Fitzroy Square; but any carpenter or
upholsterer may learn to supply them, and he gives free permission to all.
He hopes, for the sake of the poor, that a trough, without metallic lining,
and with a cheaper water-proof cloth, may be found to answer satisfactorily.
The principle of the hydrostatic bed, is applicable, also, to couches for
invalids, and with certain considerable modifications, to the construction,
also, of chairs; and there are other means than the water-proof sheet of
adapting the hydrostatic principle for all-but the subject has already
occupied its full share of this volume.
The preceding paragraphs are intended as much to direct in the choice and
use of common beds for the sick, as to announce and describe the bydrostatic bed for the cases in which it may be required. At present, the medical attendant generally leaves whatever regards the bed to the judgment of
friends or nurses; but evidently, he who has been led to reflect how much
the course and event of a maladay may depend on the patient's being supported, so that no pain shall arise from local pressure, and as little muscular
weariness as possible from constrained position, will deem the bed-management worthy of his own attention, and will be able more judiciously both to
choose and to use beds. There is a bed constructed of spiral springs, which
may be made so as to diffuse the support more equally than any except the
hydrostatic bed; and bad professional men generally been acquainted with
it, it would have been more used than it is, and would have received various
modifications, of which it is susceptible, for medical purposes. It has long
been known, chiefly, however, as a mechanical curiosity, or an object of
luxury, and was introduced into this country about seventy years ago by
Mr. Merlin; but it has been so little known, that a few years ago an English
tradesman thought he might appropriate the manufacture by taking a patent
for it. It is now made by upholsterers generally, and the same principle is
applied in the construction of sofas, chairs, and carriage cushions.
The velocity of the circulating blood.
This has been much overrated. 1st. By assuming that the ventricles of
the heart are both completely filled from the auricles and emptied -towards
the arteries at each pulsation:-an assumption disproved by inspections of
the exposed heart of a living body, and by the fact of the valves between the
auricles and ventricles not closing so perfectly as quite to prevent regurgitation. 2d. By supposing the issue of blood from a wounded artery or vein
to be the measure of the usual velocity. Now it would be as reasonable to
suppose the issue of water from a wounded pipe connected with any reservoir to be the measure of a continued current in that pipe, although, in truth,
the issue would be the same even if the water in the pipe were usually at
rest. 3d. By supposing thefi'equency of the pulse to be a measure. Now
we know, that in diseases of debility, and in animals bleeding to death, the
pulse usually becomes more frequent as it becomes more feeble, and as there
is less blood moving: viz.) the heart very partially discharging its contents
at each contraction.  4th, and lastly.  By supposing the strength of the
pulse to be the measure. Now we find that the pulse in an artery just tied,
and in which, consequently, there is,no current at all, is scarcely weaker
than in an open artery. The common fact of a person's feet remaining
stone-cold for hours, although the arteries leading to them pulsate nearly




436   FLUIDITY IN RELATION TO ANIMALS.
as usual, is a proof that exceedingly little blood is passing through the
capillaries at the time, and that the strength of the pulse, therefore, is no
measure of the speed of the blood.
The ventricles of the heart appear, under common circumstances, to throw
out about an ounce and a half of blood at every contraction-or about seven
pounds per minute. Now if the body contain about twenty pounds altogether, as seems to be the case, the whole would circulate twenty times in
an hour. This would give an average velocity of about eight inches per
second in the aorta, but gradually less in the smaller arteries, because whenever a vascular channel subdivides, the branches taken collectively have
considerably greater area than the trunk from which they arise, and the
current diminishes in a corresponding proportion, just as the speed of a
river is always less in the parts of the channel which are deeper and broader.
The velocity in the extreme capillaries is found to be often less than one
inch per minute. In the veins, the blood must move more slowly than in
corresponding arteries, in proportion as the veins are more capacious than
the arteries.
The pulse.
The opinion which the ancients held, that the arteries contained vital
spirits or air and not blood, rendered the pulse, to them, a very mysterious
phenomenon; and many curious hypotheses were framed to explain it.
These it would now be unprofitable to detail. Even Harvey's grand discovery of the circulation, however, has not rendered the subject so simple
as might have been anticipated. The following opinions now exist, or have
lately existed with respect to the pulse.
ist. The great majority of physiologists have believed that a tumefaction
is produced in the aorta by each jet of blood from the hear, and spreads
afterwards as a wave into all the arterial branches. 2d. Many have supposed
an extensive contractile action of the arteries themselves, corresponding to
that of the heart. 3d. Bichat, unable by any means to detect the slightest
change of diameter in the arteries during pulsation, but perceiving that in
many situations they were at the time somewhat lengthened, so that straight
portions became bent, and portions originally bent, were bent still more, held
that this locomotion or changing of place in the arteries was the cause.
4th. Others have supposed the impulse of the heart's contraction to be transmitted through the fluid blood, somewhat as sound is transmitted through
bodies generally, or as a blow struck on one end of a log of wood, is felt
distinctly by a hand applied to the other, although there be no'visible locomotion. 5th. Dr. Young, in the paper in the Philosophical Transactions
already alluded to, has shown that a sudden rush forward of the blood in
the artery, such as might be produced by injection at one end of a rigid
tube, would be felt by a finger applied to the artery, quite as distinctly as
a tumefaction; and he deems this occurrence to be a chief cause of the
pulse. Dr. Parry, in his work on the pulse, points to this almost as exclusively
the cause.
Now the truth is, that the pulse in the living body does not depend
exclusively upon -any one of the particulars just noticed, but has all of them
as elements; and its fluctuations and varieties depend upon proportions in
which these elements are combined. We shall review them again to prove
this.
1st. At each jet of blood thrown into the aorta, a tumefaction or wave
must spread from the heart to the extremities; for it is evident, that if blood




THE CIRCULATION.                             437
be at all pushed into the arterial system, it either must dilate it, or cause an
equal quantity to be expelled at the same instant from the distant extremities:
now as the passage of blood through the capillaries appears perfectly uniform,
there must be an intermediate dilatation. Dr. Parry and others should not
have denied this dilatation because they could not see it: for even if its advancing front were more considerable than it is, it passes with such velocity
that like a cannon-ball crossing before the face, it would not be perceived.
2d. Contraction of the arterial coats certainly does not take place in the
manner and to the extent supposed by some, who have spoken of it as resembling the contraction of the heart itself, and as what might be a substitute for
the action of the heart in propelling the blood; but as shown at page 414,
the rigidity of tube which in all degrees of'arterial dilatation causes the pulse
to be transmitted so quickly, can depend on nothing but a contractile action
of the fibres. There are some reasons for doubting whether this rigidity
may not increase at the moment of the pulse.
3d. Unless the arterial tubes were absolutely inelastic, which they are far
from being, they must be lengthened a little by a sudden injection of blood,
and, therefore, at all the curvatures particularly, there must be a degree of
the locomotion described by Bichat, often sensible to a finger applied.
4th. That a tangible shock is conveyed thiugh a fluid without any apparent accumulation of the fluid or change of velocity, and much in the manner
of sound, is proved by the facts, that we may discover the working of a
water-pump at very great distances, through iron pipes connected with it, and
even through elastic pipes of leather, as those of a common fire-enine, from
which the water is spouting, nevertheless, in a uniform stream.  The pulse
in a tied artery, in which there is no current or rushing wave, must be chiefly
from this cause and from the locomotion of the artery.
5th. That any additional quantity of fluid injected into elastic vessels
already full, must spread all over with a forward rush, affecting the finger
of an examiner, as described above, is also most certain. As the heart, however, often beats without discharging much of its blood, and as in many
arteries from inaction of the capillaries, or pressure of the blood for a time
makes little or no progress, while the pulse, however, remains very distinct,
the pulse in such cases must be produced independently of the forward rush.
An animal intestine prepared, and filled with water or air, and laid upon a
table-or a full vein in the living body, carries a rapid and distinct pulse to
a great distance when gently tapped by the finger. t'he cause of the sensation, then, cannot be the simple forward rush without tumefaction, described
by Dr. Young and Dr. Parry.
In whatever proportions these particulars combine to form the pulse, its
force will be proportioned to the size of the artery. Hence as an artery leading to an inflamed part becomes of greater calibre, its pulse also becomes
stronger.
It is a remark respecting the pulse, appearing to the author worthy of deep
consideration, that if the purpose of the heart and arteries were merely the
propulsion and conveyance of the blood, their structure and action would
form most signal deviations from the ascertained rules of fitness in mechanics.
In machines of human contrivance, it is one of the most important maxims
"to avoid shocks, or jerking motions;" and in former parts of this work, we
have described fly-wheels, air-vessels, springs, &c., as means of accomplishing
this object, and thereby of preventing the tearing and straining of parts which
would else happen. In the human body, also we have to describe the admirable elasticity of the spine, of the arch of the foot, of the cartilages of joints,




438      FLUIDITY  IN  RELATION  TO  ANIMALS.
&c., as contrivances answering the same end; and to remark that, in other
cavities than the heart, which are alternately filled and emptied like it, as the
stomach, bladder, uterus, &c., there is a smooth and gradual action.  The
heart alone is the rugged anomaly, which, from before birth unto the dying
moment, throbs unceasingly, and sends the bounding pulse of life to every
part; and which, moreover, instead of being secured and tied down to its
place, is attached at the extremity of the aorta, like a weight at the end of
an elastic branch of a tree, and every time that it fills the aorta, is thrown
with violence, by the consequent sudden tendency of that vessel to become
straighter, against the ribs, in the place where the hand applied, feels it so
distinctly beating.
Now one use of the pulsation of the heart probably is, by the agitation and
churning which the blood suffers in passing through it, to keep in complete
mixture all the heterogeneous parts of the blood, which so readily separate
when left to repose: —but this cannot be the only use, for the object might
have been more simply detained; and we may conclude that the phenomenon
has relation to some important laws of life still hidden from us.  The cause
commonly assigned for the heart'scontraction is the peculiar stimulus of the
blood; yet if we reflect that the heart will beat after removal from the body
and when it contains only air, Od that during life it beats with extraordinary
regularity, whether the state of the circulation allow it to empty itself at each
beat or not, we perceive that the case is more obscure.  We cannot contemplate this subject attentively without perceiving a strong analogy between
the action of the heart and some electrical phenomena in which there are
successive accumulations and exhaustion of power; and, recollecting the
important relations which late researches have shown to exist between electricity and certain other actions of life, the inquiry becomes very interesting.
Galvanism can excite the muscles to their usual actions; it powerfully affects
the secretions and the digestive function; and the-breathing in asthma; strong
animal passion seems to produce electrical excitement: and certain animals
have the faculty of stunning their enemies by an electrical discharge. The
pulse, then, in its sudden, strong, and regular recurrence, may be a kindred
phenomenon. In this view, there would be less difficulty in supposing a
momentary stiffening or slight contraction of the whole arterial system, such
as the sudden rising of the mesenteric arterial tree so readily suggests: if
there be such, however, it is still closely connected with, and proportioned
to, the action of the lheart; for it occurs only with that action, it indicates
ny disturbance in the action, and as death approaches, it ceases in the
remote extremities first.
The preceding considerations exhibit the pulse as a complex subject, and
one on which professional opinions are not yet settled. By showing its close
relation to the powers of life, they also prove it to be an object of high importance to the medical practitioner. This last truth has scarcely been questioned but by persons either utterly uninformed or singularly deficient in the
power of tactile discernment; yet, because no simple and good analysis of
the pulse, and detail of its relation to morbid states, has been made and published, the degrees of skill acquired by individual practitioners with respect
to it are very various, and in a great measure accidental. Some practitioners
try the pulse merely for form's sake, because patients expect it; many examine it only to count its frequency; but others read in it, with confidence,
much of the history and probabilities of the disorder, and decide on the treatment accordingly. Few who have attended to the subject at all, can confound
the pulses of such diseases, as acute rheumatisw, gastric inflammation, the




THE CIRCULATION.                            439
fits of ague, &c. The author remembers to have conversed with a Chinese
practitioner who had only the scanty medical information of his countrymen,
but who judged by the pulse with a singular penetration.
The changing circumstances in the state of the circulatory system, connected with health and disease, and discoverable by a finger watching the
pulse, seem to be chiefly the following; and the epithets added in italics,
are those which seem best to indicate the sensations perceived. The artery
at the wrist is that generally chosen for examination, because it is not like
others imbedded in soft parts, having only the skin over it, and nothing between it.and the bone below.
Ist. The number of the contractions of the heart in a given time, and
the regularity of their recurrence.-Pulse, frequent, slow, intermittent, equal,
regular, of varying force.
2d. The degree of the heart's contraction, or the quantity of blood ejected
at each time; an{ the corresponding state of the capillaries as to the quantity of blood passing through them.-Pulse, full, long, labouring, boundiny, feeble..
3d. The force of the heart's action, with the correspondent arterial tension
or rigidity. —Pulse, hard, sharp, strong, wiry, weak, soft, yielding.
4th. The suddenness of the individual contractions of the heart, and the
rigidity of the vessels in conveying the shock.-Pulse, quick, tardy.
5th. The size of the artery for the time, whether larger or smaller than
usual.-Pulse, large, small.
Superficial as is this sketch, it may show that a good treatise on the subject of the pulse, as connected with disease, is yet a desideratum in medicine.  The sort Of empirical but useful tact which many persons acquire,
is not fitted to satisfy the physician who reasons deeply, and whose mind
should have always present to it the various constituents of the pulse, and
all the important circumstances of health or disease related to its indications.
The laboured treatise of Solano,.Bordue, Boerhaave, &c., may treat of
what were clear ideas to their authors, but by not referring the physical
causes of many varieties, they become so obscure to others, that many of
the divisions and denominations appear altogether fanciful. Dr. Young's
excellent paper in the Phiosophical Transactions, details important facts, as
far as it goes, but it was not intended to point out all the pathological relations. Dr. Y., guided by general principle, asserted a progressive motion
of the pulse, while other authorities were holding it to be quite simultaneous
over the whole system. He m.ight have mentioned in proof, that careful
examination can practically detect a succession of beats at different distances,
particularly at the four stations; 1st, of the heart; 2d, in the lip; 3d, at
the wrist; 4th, at the ankle: —but the interval of time, even between the
extremes, being only a small part of a second, persons will often fail to
make their first experiment satisfactorily. Dr. Parry's treatise on the pulse.
which is the last one of note, although having excellencies, errs-in attributing the phenomenon to one cause so exclusively-in denying arterial
dilatation, because it was not discovered by his mode of searching for it, in
supposing that a liquid column in an elastic tube, can be made to advance
like a solid rod, or line of billiard-balls. The too common neglect of mechanical philosophy by medical, men is signally proved, by our finding in
works of authority, puljshed at the present day, such statements as that the
arterial pulse may be more frequent or less frequent than the beatings of the
heart.  Dr. Good (Studny of Medicine) says, that there may be various frequency of pulse in various parts of the body at the same time: Richerand




440   FLUIDITY IN RELATION TO ANIMALS.
(Ph/ysiologie) says, the pulse is more frequent in the artery leading to a
whitlow than at the same time elsewhere; and many practitioners share
these notions.  What a satire on the medical profession is this disagreement
on a point which, to common observers, seems, obove all others, to occupy
the attention of the attendant on the sick!
Having now explained the circulation of the blood in general, we proceed to
consider some cases where mechanical circumstances modify it.
Circulation in the head.
The head may be considered as an air-tight vessel or cavity of bone, containing chiefly brain and blood, and having openings occupied by bloodvessels, leading to and from the heart. The atmospheric pressure, therefore, always keeps the head full, as it keeps the top of a syphon full; and
because the substance of the brain itself does not more than water,' sensibly
change in bulk by any ordinary degrees of pressure, there must always be
the same quantity of blood.in the head, how much soever the quantity may
vary in the body generally. Regard to this important truth, a knowledge of
which has followed the discovery of the true nature of atmospheric pressure,
enables us to explain many hitherto obscure facts, both in health and disease;
-as the following instances will show.
If, from any cause, the arteries in the head become too full of blood, in
the same proportion the veins must become too empty; or, if the veins
become too full, the arteries must be too empty; and in either case, the circulation in the head will be in a coresponding degree imupeded, because,
when one part of a channel is narrowed or diminished, the current throughout the whole is slackened. Now, as insensibility supervenes when the
supply of fresh blood to the brain is interrupted, and death follows if the
interruption continue long, it seems evident that in many of the cases of
apoplexy, where, on inspection, there is found nothing but a fulness of the
arterial or of the venous system of the head, death has happened merely
because the circulation was arrested in this way. In other parts of the
body, not circumstanced like the brain, an excess of blood in one set of
vessels may happen without inducing deficiency in another, and therefore
with perfect impunity to the individual.
Simple increase of pressure produced by the blood on the brain, provided
the proper balance exist between the quantity in veins and arteries, has no
injurious effect. This is proved by the safe descent of a person in a divingbell, where, at thirty-four feet under the surface of the water, the body is
bearing an additional pressure. of fifteen pounds on a square inch (see page
163,) which pressure through the blood-vessels affects the brain as much as
any other part.  On the other hand, when a man climbs a mountain, or is
lifted up in a balloon, the brain is less pressed than usual; but the proper
balance in artery and vein being maintained, no inconvenience is felt. The
inhabitants of some of the valleys among the Andes are as far above the sea
as they would be at the top of Mont Blanc, where the atmosphere presses
only half as much as on the sea-shore; but they enjoy good health.
As the box of the cranium encloses the brain so as to leave no vacant
space, it is evident, that when the heart injects blow with unusual violence,
the strain at first is borne chiefly by the cranium, and not by the coats of
the blood-vessels. Hence, the arteries of the brain need not to be, and are
not, nearly so strong as those of other parts of the body.




THE CIRCULATION.                           441
The veins of the brain are also peculiar.  Common veins in the head
would, for the reasons above given, collapse by any sudden tension of the
arteries there, and if they did, insensibility or death would ensue, on account
of the consequent stoppage of the circulation. The chief channels, therefore, for the refluent blood, instead of being common compressible veins, are
what have been called sinuses, or grooves in the bone itself, with exceedingly strong membranous coverings, supported so that the channelhs become
in strength, and as to maintenance of their capacity, a little inferior to complete channels of bone. This singular deviation in the structure of the
cerebral veins from what is found elsewhere, and without which deviation,
animal existence could not be continued, is one of those particulars which
powerfully affect the contemplative mind, as proofs of the designing intelligence which has planned this glorious universe.
From not adverting sufficiently to the fact now explained, of the cranium
being a vessel always full, and which will hold only a certain quantity,
misconception has prevailed among medical men with respect to many of
the affections of the brain.
It has been said, for instance, that the substance of the brain cannot bear
pressure with impunity,'for that stupor immediately follows pressure, however produced. Now the truth is, that pressure produces stupor only when
it interferes with the circulation. In wounds with loss of a large piece of
the cranium, the brain will bear very rough handling, because if compressed
at one part, it may bulge at another, and leave the circulation free; but if the
wound be small, pressure made through it instantly affects the whole brain,
and the blood is prevented from entering from the heart. Let one reflect, for
an instant, on what happens to the feetal head during parturition-how often
it escapes elongated and bent, almost as if it were of soft clay-yet the child
lives and thrives, and the natural form is soon recovered.  The reason is,
that the foetal skull is soft, and pressure in one part is compensated for by
a bulging or extension in another, and the blood is not expelled.
Water in the head, again, is said to kill by this fatal pressure on the tender
brain; but, in reality, it kills by keeping out of the blood, and so mechanically
arresting the circulation. Accordingly we see, that where the fontanelle still
remains open, or where the sutures or joinings of the skull will yield, water
may accumulate to a great degree without causing much disturbance.
A tumour in the brain, which would be of no consequence if the brain
were unconfined, soon becomes fatal by occupying room in the skull, and to
the extent of its Size excluding or checking the supply of blood.
If the substance of the brain at all increase and diminish in bulk, as muscles,
&c., under certain.circumstances, do, in the body below, all such changes
must produce a considerable effect on the'cerebral circulation and functions.
Effects of position on the circulation.
While a man is in a standing attitude, the heart and arteries have to
send the blood up the head against gravity; but in the horizontal position,
the blood, if equally propelled, must arrive with greater force, because gravity
then does not resist. Hence headache, no other symptom arising from fulness of blood in the arteries of the head, is often relieved by the upright
position, and is increased by lying down.
Many people who have had a slight degree of toothache during the day,
find it intolerable when they lie down at night. and are relieved again by
rising and walking about. Commonly they suppose that it is the cold of the




442   FLUIDITY IN RELATION TO ANIMALS.
night which then lulls the pain; but it is in fact the change of position.
The author knew a lady who was obliged to sleep for months in the sitting
posture, because she had a tic doloureux in the face whenever she lay down;
and another who was under the same necessity for a considerable period after
an inflammatory affection of the brain, because if her head fell low during
sleep she was immediately assailed by a terrific dream of swords driven into
the brain.
Delirium in fever is sometimes checked at once by elevating the head.
On account if the great relief thus obtained, some continental practitioners
had proposed to support the patients occasionally in an upright posture.
Apoplexy has often been brought on by a man bending his head down in
the act of tying his shoe, or of pulling on his boot.
Children and professed tumblers being much in the habit of placing their
bodies in all positions, feel no inconvenience from having the head downwards; apparently, because arteries and veins usually become strong enough
to bear the pressure to which they are habitually exposed; but to many old
people, accustomed to keep the head always up, the attempt would be fatal.
Ulcers on the legs are often obstinate and will bleed, because the veins
about them are too weak to support the lofty columns of blood above. Hence
the frequent counsel given in such cases to keep the feet raised upon a
chair, and the utility of certain modes of bandaging.
Many inflammations of the legs and feet become exceedingly painful when
the limbs are in a hanging position, and the pain is relieved by laying them
horizontally.
Many anasarcous or dropsical affections of the legs increase towards night,
because, during the dependent position of the legs through the day, the
absorbents have not power to lift the fluid. The s.welling disappears again
before morning.
When the heart has to send blood upwards, it requires to act more strongly
than when the body is horizontal, and the pulse increases five or six beats
in the minute; hence the common rule to make a patient with beamorrhage
lie in the horizontal position, that the heart may become tranquil and allow
the bleeding to cease.
Fainting from diminished arterial tension.
Fainting, which is a temporary cessation of the action of the heart, and
hence, as explained above, of the action of the brain for want of blood, is
produced by several causes, and among others, by any occurrence which
renders the blood.vessels about the heart suddenly less full of tense than
usual. It would appear that the heart being accustomed, when it contracts,
to a certain degree of resistance, has its action disturbed when the resistance
is much diminished.
Thus heemorrhage, from any cause, by.lessening the general tension of the
sanguiferous system, often causes fainting. The state is relieved by lying
down; probably because the still remaining, weaker action of the heart is
sufficient to send blood to the head along a horizontal course, until the gradual
contraction of the whole vascular system reproduces the tension necessary to
perfect action.  A small quantity of blood taken away suddenly, affects the
circulation as much as a larger quantity taken gyadually, apparently because
a certain space of time is required for the gradual lessening of the vessels.
The operation of tapping for dropsy in the abdomen would often bring on
fainting, but for the precaution of tightening a broad bandage upon the body




THE CIRCULATION.                            443
as the water flows. The reason is, that the sudden removal of a large
quantity of fluid which had been compressing all the abdominal vessels, and
keeping them perhaps only half full of blood, allows them again suddenly
to receive their natural quantity, and thus produces a relaxation of the other
parts of the vascular system.
Sudden parturition often causes faintness for the same reasons.
Even rising up suddenly from a horizontal position will cause an approach
to fainting in weak people, or in those who have been long bed-ridden: probably because the heart having for a time been accustomed to send blood
only in a horizontal direction to the head, does not at an instant exert the
additional power required to lift an upright column with equal force;-'besides, that the blood does not then return to the heart by the veins, from
the inferior parts of the body so readily as before.
These various facts, now easily understood, form the reason of a rule
which is a great modern improvement in the practice of the healing art, viz.,
in bleeding for the cure of inflammation, to take the blood away as quilckly
as possible. This subject deserves a little farther consideration.
A great proportion of dangerous diseases involve inflammation of some
vital organ; an inflammation consists chiefly, as already stated at page 422,
of a gorging or over-distension of the capillary vessels in the part. The
nature of the capillaries, again, is such (page 422) that when not maintained
constantly full by the pressure of the heart behind them, they gradually by
their own action, empty themselves towards the veins-as is seen in the
disappearance of a local inflammation soon after the death of the person, or
in the fact of the arteries being emptied of blood after breathing ceases, &c.
Now ever since medicine deserved the name of an art, practitioners have
accounted the lancet their sheet-anchor in inflammatory disease; but it is
only in late times, since the circulation of the blood was understood, that
they have known the rationale of the remedy, viz., that it acts by diminishing
vascular tension, and hence the action of the heart, and so allowing the small
vessels to empty themselves by their own force, and to recover sufficiently
to resist the return of an excessive load.  It is still more lately that they
have understood how much more suddenly and completely the disease is
cured by abstraction of a small quantity of blood so rapidly as to produce
fainting, than of a much larger quantity so slowly that only weakness follows. Judicious treatment now cures inflammation much more certainly
and completely than was done formerly, yet with much smaller loss of the
precious blood, and with less danger of those diseases of weakness, or of
that complete breaking up of the constitution, which often follow great
depletion. To induce faintness, large openings are to be made into the
veins-sometimes into two veins at once, and the patient is kept in the
upright attitude, Often thus an inflamed eye, which was as red as scarlet
before bleeding, in a few minutes is rendered nearly of the natural appearance; and intense internal inflammations, as of the brain, lungs, bowels,
&c., which, if neglected, would be shortly fatal, are removed in the same
manner. In all these cases, the faintness seems to be almost equally efficacious, whether it happens after the loss of ten ounces of blood, or of fifty;
or even, as sometimes occurs, when it happens without bleeding at all, after
merely tying the arm in preparation.
Reflection upon these circumstances led the author to think that, in certain
cases, the beneficial effects of blood-letting might be attainable by the simple
means of extensive dry cupping, alluded to at page 176; that is to say, by
diminishing the atmospherical pressure on a considerable part of the body,




444     FLUIDITY IN  RELATION  TO  ANIMALS.
on the principle of the cupping-glass used very-gently, and thus suddenly
removing for a time from about the heart, a quantity of blood, sufficient by
its absence to produce faintness.  The results of trial have been such as to
give great interest to the inquiry, and the author's leisure will be devoted
to the prosecution of it.-An air-tight case of copper or tin-plate, or of airtight cloth kept extended by hoops, being put upon a limb, and made close
by a suitable collar tied at the same time round its mouth and the limb,on part of the air being then extracted by. a suitable syringe, in an instant
the vessels all over the limb become gently distended with blood; and as the
blood is suddenly taken from the centre of the body, faintness is produced,
just as by bleeding from a vein. The excess of blood may be retained in
the limb as long as desired; for the circulation is not impeded.  To produce'
a powerful effect with a slight diminution of pressure, more than one limb
must be operated upon at the same time.
An instrument resembling the contrivance now described, was proposed
about twenty years ago by a non-professional person, as a means of drawing
all sorts of diseases out of the body through the pores of the skin.  He
enclosed a leg in an air-tight case; he then admitted steam to heat the limb,
and relax the pores of the skin, as he said, and then he worked an air-pump
to draw out the disease.  He called the engine the air-pump vapour bath.
In various cases where its true action was desirable, although not understood
by the proposer, nor judiciously managed, it proved beneficial.
The operation of applying tourniquets or bandages round the limbs, so as
to influence the transmission of the blood, affects the action of the heart. It
is said sometimes to have prevented the accession of the ague. It is a means
akin to those above described.
Because arteries are stronger and more tense than veins, a bandage may
be put round a limb tight enough to close the veins but not the arteries, and
the limb will then swell beyond the ligature. By thus putting tight elastic
bandages round all the limbs at once, and immersing them in warm water to
favour the dilatation of their vessels, so much blood may be suddenly detained
in them as to cause the person to faint.  Such a means, therefore, might
also be used remedially.
In the same way, a tight handkerchief, or stock round the neck, will often
retain the venous blood in the head, and cause apoplexy.-Strong pressure
made on a jugular vein kills as certainly as if made on the windpipe.
When a hernia, or other tumor, is strangulated, it swells in the manner
above described, and if not relieved, soon mortifies.
DIffusedpressure, like that made by rolling a bandage round a whole limb,
or by immersing the limb in fluid, must affect the circulation. The veins
will be more compressed than the arteries, by reason of the distending force
in them being less.  Varicose veins, therefore, are usefully supported by a
bandage or laced stocking. The reason why this manner of supporting
assists so powerfully in the healing of ulcers on the legs, may be, that the
support affects the capillaries and absorbents, as well as the larger vessels.
Poultices, by their weight, produce a soft compression of the parts on
which they are applied; and in certain cases, may benefit by mechanically
squeezing the excess of blood out of the weakened vessels.
The author has relieved the chronic inflammation of a sprained ankle, by
ordering the foot and leg, covered with an oiled-silk stocking, to be enclosed
in a boot Strong enough to support the pressure of quicksilver, which was
then poured into the boot. The effect is a pressure by the fluid metal on
the weak vessels, of one pound to the square inch, for every two inches of the




RESPIRATION AND VOICE.                           445
depth of metal above the part.-A  height of four or five inches gives the
relief expected. A much greater elevation would stop the circulation altogether. No bandage can press with uniformity approaching to this action
of a fluid.
The effect of continued pressure, in removing tumours of various kinds,
is explicable on the same principle. The author doubts not that in such
cases, pressure properly managed, would prove a more valuable remedy than
is at present generally supposed. The elastic steel half-hoop with one
cushion before and another behind, lately introduced for the relief of hernia,
affords an admirable mode, in certain cases, of producing a uniform pressure
of the nature.spoken of, and of any desired force.
When a man stands in a bath, with the water up to his chin, there is a
pressure of the water upon his body, proportioned everywhere to the depth,
(see page 130.)  This pressure must produce a considerable effect on the
blood-vessels of the lower parts of the body. We see in this that a bath
must propel the blood from all other parts of the body, towards the cavity
of the chest, which the pressure cannot reach.  It is this effect which in
part causes the feeling of thoracic oppression experienced by persons on
first plunging into water, which feeling is usually attributed altogether.to
the cold.
The old practice of placing a patient in a pit, and surrounding his body
with earth and sand, must have had a mechanical action of the kind now
contemplated, in addition to any other influence.
Transfusion of blood from  a vein of a healthy person into that of one
fainting or dying from haemorrhage, is an operation the converse of some of
those mentioned above. It has been frequently performed with success.
The cases to which it seems best fitted, are those of flooding after parturition,
and of wound; and there can be no doubt that many of the lives lost from
these so frequently recurring causes) might be saved by its adoption. The
blood to be injected is received into a vessel, as in common bleeding, from
which vessel, by a fit syringe, (to be described in a future page,) it is transferred, as it flows, into an opened vein of the patient. The admission of air
with the blood would be fatal, and has therefore to be most carefully guarded
against. The last interesting report upon this subject is that of Dr. Blundell,
in his Physiological Essays.
RESPIRATION AND VOICE,
The doctrines of fluidity, illustrating and illustrated by the animal respiration and voice.
As the motion of a windmill depends altogether on the breeze to which its
vanes are exposed, so does the motion and the life of that most wonderful of
structures, the animal body, depend on the supply of air for its breathing.
If this supply be withheld but for a few moments, painful convulsions ensue;
and if for a still longer period, the body, however perfect and beautiful, is
made a lifeless corpse, soon to putrify and be decomposed.
The mechanical nature of air, as to its lightness, elasticity, &c., and the
fact of its forming an ocean around the earth of about fifty miles high, are
now well understood, and have been fully explained under pneumatics; but
the precise nature of its life-sustaining action has yet to be elucidated by
farther research of chemists and physiologists. Thus far, however, we




446   FLUIDITY IN RELATION TO ANIMALS.
know-that the ingredient called oxygen, constituting a fifth of the atmosphere, is the most essential part-that air, by being breathed once is rendered
unfit for farther respiration at the time-and that a man requires about a
gallon per minute. The enterprising Mr. Spalding, who introdnuced the use
of the diving-bell, descended for the last time with a companion on the coast
of Ireland, when owing to the signal cord becoming entangled round the
great rope of the bell, which had turned in descending, he could not make
their want of air known above, and both were found dead when the bell was
drawn up soon after, although the water had not touched them. Of a hundred and forty-six Englishmen, who, in the year, 1750, were made prisoners
at Calcutta, and where thrown into the close dungeon, since called the blackhole, only twenty-three survived the few hours of their confinement, and one
of the most appalling recitals of human suffering existing on record, is what
these persons had afterwards to make.
We know generally of the life-supporting action of air, that it consists in
some change operated by the air on the blood; and we know that the function of respiration has merely to bring air and blood together in the cavity of
the chest, that this change may take place. The blood while in the chest is
moving along a part of its circle, in vessels of extreme minuteness and thinness, and the air at each inspiration rushes in among these, so that every
globule of blood passes within its influence. And the blood, which, after
having served the purposes of the body, arrives at this part of its course,
black and impure, immediately after its exposure to the air enters the left
chamber.of the heart, of a beautiful scarlet colour, and thence departs to
carry new life to the general system.
The minute vessels through which the circulating blood is strained in the
chest, do not hang loose in the cavity, but are supported by running through
spungy masses, called the lungs, which consist chiefly of these vessels and
of thin membrane formed into cells. The cells at every inspiration, receive
fresh air through the cartilaginous windpipe which branches into them, and
at every inspiration they return the changed air by the same channels to the
atmosphere. The lungs of a child, before birth, are perfectly collapsed, or
without the least air in their structure, and hence are dense enough to sink
in water; but after breathing, they retain a portion of air, and will float.
This fact has been accounted a test of whether a child had been born dead
or alive; but because putrefaction, &c., will cause air to be in lungs which
have never breathed, the criterion may be fallacious.
The chest is a large cavity, of form approaching to that of a common beehive, bounded laterally by the encircling ribs, behind the spine, and before
by the sternum, and divided below, from the abdomen or belly, by a strong
membranous and muscular expansion, called the diaphragm. The ribs, in
the natural state, hang obliquely downwards from their posterior attachments,
and on being raised in front, they widen or increase the size of the cavity, as
already explained at p. 403. The cavity is farther enlarged by the descent of
the diaphragm, which may be regarded as both the floor of the chest and the
roof of the abdomen, and which being convex upwards like a dome, by contracting itself to a more flat condition, sinks -out of, and enlarges the chest,
while it descends into, and diminishes the abdomen, or at least causes
protrusion of its sides.
Now on the chest being enlarged by the rising of the ribs and descent of
the diaphragm, or by either singly, the air rushes into it through the mouth
and windpipe, exactly as air rushes into a common bellows through its pipe,
when the valve is shut and the two boards are drawn apart; and air is again




RESPIRATION AND VOICE.                           447
expelled from the lungs by the contraction of the chest, as from the bellows
by the approximation of the boards. Into both cavities air enter, because
with the enlarging dimensions, the air which was witl  dilates, and
becomes less powerfully tense or resisting against the external pressure of
the atmosphere, and,so allows more air to rush in to restore the equilibrium.
The air is expelled agMn by the contraction of the cavities, because by being
compressed, its elastic force or tension' becomes greater than that bf the external air, which it therefore easily repels, and so in part escapes.  By immersing in water an india rubber bottle, and then opening and shutting it,
the entrance and exit of fluid in this manner may be rendered very apparent.
That the air admitted into the chest should have the fullest action on the
blood passing there, it was necessary that the spongy mass of lungs in which
the blood-vessels ramify, should occupy the whole of the cavity, and be
equally distributed. Now while the equable distribution is effected by the
uniform elasticity or resilience which belongs to the structure of the lung the
complete filling of the cavity is obtained, not by general attachment between
the lungs and the ribs or sides of the chest, as might be expected, but by the
following means, equally simple, and still more perfect. The spongy mass
of the lungs, is completely covered by a strong adherent membrane, called the
pleura, through which air cannot pass; between this membrane and a similar
lining of the chest there is no air or empty space, and therefore in the raising
and falling of the ribs during respiration, this membrane remains always in
contact with the lining of the ribs, just as a bladder put into a bellows as a
lining, with its mouth secured around the nozzel, is filled and emptied, and
remains in contact with the interior of the bellows, in all the states of dilatation, as if there were attachments in a thousand places. This construction
allows the lungs to have a singular freedom of play during all the motions
of the body; a freedom further provided for by their being divided into five
portions or lobes, which slide upon one another: of these, three occupy the
right side of the chest, and two with the heart occupy the left. The right
and left sides of the chest are rendered cavities quite distinct from each other
by the mediastinum, a strong membranous partition.  The mechanical disposition of the contents of the chest, as now described, is productive of
certain consequences which it is important to understand;-for instance,
If a wound be made in one side of the chest so asto admit air, the lungs of
that side collapse in obedience to theirweight and elasticity; and as the chest
afterwards enlarges and diminishes in respiration, air more easily enters and
leaves the space around the collapsed lung, through the wound, than it can
enter or leave the lung itself through the windpipe; because, in the first case,
it has no force to overcome, and in the second, the elasticity, weight and
inertia of the lung oppose   Thus the lungs of the wounded side become collapsed and useless. If such a wound; therefore, were made in both sides of
the chest at once, even without hurting any part within, the person, unless
assisted, would die of suffocation. The kind of resistance required in such a
case, is first to press the ribs down so as to empty the chest of air as imuch
as possible, and then to keep the wounds close or covered while the ribs rise
again; the air, of course, will then enter by the natural road, the only one left,
to fill the chest, and will distend the lung, and reach the blood in the pulmonary vessels as usual. Teen by straining with the muscles or the chest, as
in the action of blowing, and at the same time preventing the breath from
escaping by the mouth or nose, all the air which had entered by any wound
in the chest may be expelled. In Benjamin Bell's system of surgery, which
was long the manual practitioners, counsel on this head was given the very




448   FLUIDITY IN RELATION TO ANIMALS.
contrary of that required, and, of course, any patient treated according to it
must have been lost.
In cases of ngerous hemorrhage from a lung, caused by a wound in the
side, the proper practice is to allow the lung to collapse, as now explained,
that the haemorrhage may be checked: and when the danger is past, the
treatment above described is to be adopted to restorelthe natural play of the
lung. Life may be supported for a long time by the lung in one side of the
chest.
In cases of hremoptysis, or spontaneous bleeding from the lungs, a disease
so often fatal,. life might sometime be saved or prolonged by making an
opening between two of the ribs, and allowing the lung to collapse. The
affected lung is often pointed out by the circumstances; and the opening,
when properly made, would be no more dangerous than in the case where,
by a similar opening, water or pus is discharged from the chest.
The same operation has been tried as a forlorn hope in pulmonary consumption. This disease is often limited to the lung of one side, and as the
alternate stretching and collapse of the deceased lung during respiration,
together with the contact of the air, powerfully prevent an ulcer there from
healipg, or inflammation from subsiding, a new chance of recovery is given
by allowing the deceased lung to collapse and remain at rest. Some cases
are recorded where cure is said to have followed this operation, and certainly
where the circumstances are favourable for it, and where death must ensue
unless it can save, it is worth trying.
When ribs are fractured, it is the practice to put a bandage round the chest,
so as for the time to prevent almost entirely the respiratory motion of the
ribs, and the breathing is then performed chiefly by the rising and falling of
the diaphragm or floor of the chest as above described. Although a person
with broken ribs is wisely for a time subjected to the unnatural restraint, it
is surely the height of folly to inflict the same on healthy beings, as is yet,
however, so commonly done among young women, and often to the destruction of their health, by the fashion of bracing the body in tight stays.
The force of a healthy chest's action in blowing is equal, as stated in last
section, to be about one pound on the inch of its surface; that is to say, the
chest can condense its contained air with that force, and can, therefore, blow,
through a tube, the mouth of which is two feet under the surface of the
water. In the opposite action of sucking or drawing in air, the power, is
nearly the same.  In both actions it is possible to use the cavity of the
mouth separately from that of the chest; and the mouth, being smaller, with
stronger muscles about it in proportion to its size, it can act more strongly.
Some men can suck with the mouth so as to make nearly a perfect vacuum,
or to lift water nearly thirty feet. An expert operator with the blow-pipe
can keep up an uninterrupted blast by shutting the mouth behind, while he
inhales, and replenishing it as is required in the intervals.
In coughing, the glottis or top of the windpipe, by a curious sympathy
of parts, is first closed for an instant, during which the chest is compressing
and condensing its contained air, and on the glottis being then opened, a
slight explosion as it were, of the compressed air takes place, and blows out
any irritating matter that may be in the air-passages; just, only with
inferior force, as the burst from the chamber of~an air-gun discharges its
bullet. This shutting of the glottis to allow the compression of the air, and
the subsequent opening to allow the discharge, may recur at every minute
intervals, and many times for one fill of the chest, as is instanced in hoopingcough. The action of coughing is often produced by irritation from a cause




RESPIRATION AND VOICE.                         449
which cannot be'removed by coughing, as inflammation of the clhest, or
tubercles; or even by irritation in a distant part, as when children are
teething, or when the stomach is overloaded.
Sneezing is a phenomenon resembling cough, only the chest empties itself
at one throe, and chiefly through the nose, instead of through the mouth,
as in coughing. The irritation that produces sneezing is generally in the
nose; but, as in the case of cough, sneezing may occur from distant sympathies: witness that from worms in the bowels.
Laughing consists of quickly repeated expulsions of air from the chest,
the glottis being at the time in a condition to produce voice; but there is not
between the gusts, as in coughing, complete closure of the glottis.
Crying differs from laughing almost solely in the circumstances of the
intervals between the gusts of air being longer. Children laugh and cry
in the same breath, and it is often difficult to mark the moment of change.
Hiccup is the sudden stopping, by a closure of the glottis, of a strong
inspiration at its commencement.  If the inspiratory effort be afterwards
continued, it may cause air from the atmosphere, or half-digested food from
the stomach, to enter the cesophagus.
In straining to lift weights, or to make any powerful effort, the air is shut
up in the lungs, that there may be steadiness and firmness of the person.
At such a time, by the compression and condensation of air around the heart
and large blood-vessels, the blood is determined violently outwards from the
chest, and often rises to the head, with force that produces giddiness, or even
apoplexy,-and the eye will sometimes become suddenly bloodshot from a
small vessel giving way; and leech-bites will break out afresh.-The force
of this pressure outwards is measured, as already stated, by a column of
about two feet of blood; and this is, therefore, the measure of the additional
arterial and venous tension in the body generally.
Suffocation is the name given to what happens when the supply of air to
the lungs is, any way, prevented. The blood, not then refreshed by the
approach of the air, rises in the brain unfit for its purpose, and confusion of
thought is immediately produced, soon followed by convulsion and death.
When this happens from mechanical obstruction at the narrow entrance
of the windpipe, as in croup, by the tenacious films thrown off from the
inflamed lining of the air-passages, life may be saved by making a new
entrance for air through the windpipe, lower down in the neck, and keeping
it free by a little tube inserted, until the obstruction above be removed.
Where children die with croup, it is fiequently not from the violence of the
constitutional disease, but from detached films thus accidentally sticking in
the narrow entrance of the air-passage.
In the cases of strangling and hanging, the tight binding of the rope or
ligature crushes inwards the cartilaginous rings of the windpipe, and shuts
the air-passage.  It may also cause apoplexy, by arresting the passage of
blood to and from the head: and there may be dislocation of the cervical
vertebrae of the spine.
In drowning, communication with the atmosphere is cut off altogether
by the supernatent water. If, during submersion, the chest expands, it can
receive water only, instead of air. The nerves and muscles, however, at the
entrance of the windpipe, are so irritable, as to be immediately excited by
the contact of any unusual matter, and, for a considerable time, they keep
the passage shut against the liquid seeking entrance. It is partly on this
account that the body of a person, after submersion in water and apparent
death, may, often, if recovered within a noderate time, be restored to life.
29




450   FLUIDITY IN RELATIO.N TO ANIMALS.
The apparatus of the Humane Society for the recovery of persons apparently drowned, includes a bellows for producing artificial respiration. This
bellows resembles a common bellows, except that its flap or valve, instead
of being internal, is external like a large flute-key, and has a spring to close
it, obedient to the finger of the operator. The bellows receives its charge
of fresh air on being expanded, while the valve is open; it sends the charge
into the lungs on being compressed while the valve is shut; it withdraws
the charge again on being expanded with the valve shut; and the impure
air is thrown out to the atmosphere on its being compressed with the valve
open. These changes, repeated and continued, produce the artificial respiration required. It is most important to remark here, that if air be injected
into the lungs, either in too large quantity or very suddenly, instead of
recalling or sustaining life, it is as certain a means of killing as a dagger
driven through the heart. This truth has been but lately known, and ignorance of it has probably decided the fate of many persons, treated with a
view to recovery after submersion. The operator should reflect that he is
dilating the delicate air-cells of the lungs with the force of an hydraulic
press; and that if he does so very suddenly, although to a small extent, he
still may rupture many small blood-vessels, before they can empty themselves so as to:yield. In a bellows for the purpose of artificial respiration,
there should be the means of checking its opening to suit the capacity of the
patient's chest, and there should be a cock in the pipe or nozzle to regulate
the speed of the passing air.
In addition to the artificial breathing for the recovery of suspended animation, it is often necessary to restore natural warmth of the body, to rub
the limbs in aid of the circulation, to administer stimulants by the mouth,
to excite by galvanism, &c.
It seems to be an error, and probably often a fatal error, in the present
mode of treating persons apparently drowned, to use cold instead of warm
air for the artificial respiration. Thus while the important object of restoring
the temperature of life is sought by all external means, the great inconsistency is committed of blowing cold air upon an internal surface of the body
more extensive than the external; and until that reciprocal action of the
air and blood begins, which constitutes the slow combustion of natural
respiration, every bellows-full of cold air admitted, brings back with it a
portion of the remaining central warmth, and may thus chill so as to make
the recovery impossible:-as a fire which has fallen very low may be immediately extinguished by the same action Qf a bellows, which a little before
would have made it blaze. Air might easily be heated for the purpose of
respiration by pouring boiling water into a vessel containing it, and then
connecting the bellows with that vessel by a fit pipe, or by making the
bellows draw through a pipe partially immersed in hot water:-a quart of
boiling water has heat enough in it to warm many gallons of air to bloodheat. This plan would not only avoid the mischiefs arising from the cold
air, but by affording the means of applying warmth even higher than that
of life, it might probably furnish the most useful of all stimulants to the
parts about the heart. A healthy man can breathe with impunity air that
is much hotter than boiling water.
Late physiological investigations have shown that the breathing, or mechanical action of the chest in respiration, is so dependent upon the influence
of the brain, as to be disturbed and even stopped when the brain is embarrassed; they have shown, farther, that the action of the heart is dependent on
the breathing, but not on the brain, e,xcept as the cause of the breathing



THE  VOICE  AND  SPEECH.                       451
for that respiration kept up artificially, will preserve the circulation and the
life for a considerable time after the brain has altogether ceased to act, or
even has been removed from the body. Now, some interesting experiments
of Mr. Brodie have proved, that certain poisons are fatal, merely because
they suspend for a time the action of the brain-through which suspension,
the actions, first of the chest, and then of the heart, cease, and death ensues:
but that in such cases, if the action of the chest be maintained artificially,
the circulation and life of the body will be for a time continued, and the
brain may gradually recover from the.effect of the poison, so as to resume
its office.  Thus certain cases of poisoning, which formerly would have been
fatal may now end in recovery.
An important application of this discovery is to the treatment of cases of
convulsion, particularly those occurring from teething or other irritations in
infancy. The respiration ceases in these cases often only because the action
of the brain is suspended; and if the respiration be continued artificially, the
circulation and life will also continue for a time, during which the brain may
recover itself, either spontaneously, or in consequence of remedies employed,
and life may be saved. —The chest of an infant is comparatively so small,
that it may be filled from the mouth and windpipe of a grown person, with
air which has not descended to that person's lungs, and therefore has not been
rendered unfit by respiration; and on the little chest being afterwards compressed by the hand, the air will return. The air may be blown directly into the child's mouth through a thin handkerchief laid over the mouth, or it
may pass through a tube inserted into the nostril or treachea:-to prevent it
from passing into the stomach, the larynx must be pressed against the (esophagus during its entrance.  Let all who try this remedy, keep present to
their minds. the danger of inlating too much.
Any medicated air is generally inhaled by a patient from an oiled-silk,
or other air-tight bag, or from a light gasometer (see page 211.) When the
compound nature of our atmosphere was first discovered, great advantages
were anticipated to medicine froml the use of pneumatic or aerial mixtures.
These expectations have not been realized, but the subject still remains
highly deserving of research.
THE VOICE AND SPEECH.
The chest and air-passages, with certain additional parts, constitute the
organs of voice and speech.
An inquirer into the constitution of the universe around him, meets with
few things calculated more to surprise him than that faculty in the human
mind by which it can associate the ideas of objects with any arbitrary signs,
so closely that the ideas are afterwards excited by the signs almost as vividly
as by the objects themselves. The inhabitants of China, for instance, have
contrived many thousand grotesque characters, and determined what object
each should recall; when a Chinese by study becomes familiar with these,
he may have his bodily eye poring over pages of crooked and unseemly
scratches, while his mental eye through them sees only a pleasing succession
of the most beautiful imagery of nature;-and the characters may be rendered
intelligible to the deaf and dumb man, as well as to him who speaks; and
they serve as a media of thought and communication through many provinces
and countries of which the spoken languages have no common resemblance.




452   FLUIDITY IN RELATION TO ANIMALS.
But if the ready resemblance of visible signs be wonderful, which have
a permanent existence, and which often may have some resemblance in form
to the things signified, how much more wonderful is it that an audible sign,
that is, a passing sound or fugitive breath, called by man a word, should
serve as well; and that by a succession of mere sounds, having so- little
natural connection with the things signified, that they are totally different in
different countries, and are changing with fashion from age to age, any train
of thought may be made to pass through the minds of an audience, so as to
excite and to leave impressions almost as vivid as from realities! Such,
however, is the fact, and it is greatly owing to this and to a correspondent
faculty of producing at will a sufficient number of distinguishable sounds,
that man owes his elevation above the brutes of the field. His godlike
powers of intellect would have remained dormant and unknown, had he
wanted the faculty of comparing invisible thoughts with those of his fellow
men, and of arranging and recording them by means of signs.-Written
language is a double remove from the objects themselves/;being visible signs
not of things, but of the audible signs.
The admirable apparatus by which man is enabled to produce a sufficient
variety of sounds to answer his purposes, passes generally under the denomination of the organs of speech; because the act of using sounds which have
meanings assigned to them is called speech. It consists of the chest for containing air; of the larynx or cartilaginous box, with its narrow aperture called
the glottis, at the top of the windpipe, for producing the voice, and varying
its pitch; and of the short tube of the mouth, with the tongue and lips, for
farther modifying the voice.
In the chapter on acoustics, we explained that sound is the name given to
the effect produced upon the ear by certain tremblings conveyed to it g(nerally through the medium of the air; and we explained how air, forced from
the human lungs through the opening at the top of the windpipe, causes the
elastic lips of that opening to vibrate, and to excite the tremblings. We have
now to show that this sound, in passing forward from the top of the windpipe, may be modified at the will of the individual, in a great variety of ways
-a variety, however, which is still very simple.
The modifications of voice easily made, and easily distinguishable by the
ear, and therefore fit elements of language, are about fifty in number; but
no single language contains more than about half of them. They are divisible into two very distinct and nearly equal classes, called, for reasons now to
be explained, vowels and consonants.
Those of the first class are the simple voices issuing through the open mouth,
and influenced only by the degrees in which the mouth is opened and elongated. They may be continued as long as there is breath to issue from the
chest, and it is for this reason that they are named vowels or calling sounds.
The Roman letters, A, E, I, O, U, as generally pronounced on the continent
of Europe, and of which the sounds correspond nearly to those aw, a, e, o,
and oo, of English writing, indicate the most easily distinguishable vowels.
Sounds passing through the mouth in its most natural state of relaxation, is
heard as the modifications expressed by the Italian E, (or the a of the English
word care;) if the mouth be then widened, the sound becomes A (as in the
English word bar;) if the mouth be narrowed, we hear the I (or the e of the
English tedious;) if the mouth be elongated and at the same time widened,
we hear the 0 (as in the English word bore;) and if more elongated but
narrowed, we hear the U (as in the English rude.) The possible number of
vowels, however, is as great as the possible degrees in which the dimensions




THE  VOICE  AND  SPEECH.                      453
of the mouth may be altered. About twenty of them are sufficiently distinguishable, but few languages comprehend more than twelve. Modern art
can produce the vowel sounds mechanically by means of tubes of certain
dimensions.
The alphabets of Europe are very faulty in not all using the same characters for the same sounds, and in not having, according to the true intent of
an alphabet, a character for each distinct sound. In English, one letter is
used for several sounds, as A, in water, far, fat, fate, where it indicates four
sounds perfectly distinct. In repeating the English alphabet, the A is pronounced as the broad E of the Italians and of continental Europe, and the E
as the I; and the I (in tide, for instance,) as the dipthong AI of more coriect alphabets; and the U (in muse,) as the dipthong IU. In consequence
of the changes which the English have made in the meaning of the Roman
letters, they now experience increased difficulty in learning modern continental languages; and their own pronunciation of the ancient languages, to
all but themselves, is ridiculous, and almost unintelligible. The same cause
renders the pronunciation of English difficult to foreigners, and thus restricts
much in other countries, the cultivation of English literature.
To explain the second class of the modifications of sound, called conso-?nants, we may remark, that while any continued or vowel sound is passing
through the mouth, if it be interrupted, whether by a complete closure of
the mouth, or only by an approximation of parts, the effect on the ear of a
listener is very remarkable, and is so exceedingly different, according to the
situation in the mouth where the interruption occurs, and to the manner in
which it occurs, that many most distinct modifications thence arise. Thus
any continued sound, as A, if arrested by a closure of'the mouth at the external confine or lips, is heard to terminate with the modification which.we
choose to express by the letter P, that is, the syllable AP has been pronounced; but if, under similar circumstances, the closure be made towards
the back of the mouth by the tongue rising against the palate, we hear the
modification expressed by the letter K, and the syllable AK has been pronounced; and if the closure be made in the middle of the mouth by the tip
of the tongue rising against the roof, the sound expressed by T, is produced
and the syllable AT is heard-and so of others.  It is to be remarked, also,
that the ear is equally sensible of the peculiarities whether the closure precedes the continued sound or follows it: that is to say, whether the syllables
pronounced are as above, AP, AT, AK, or on the contrary, PA, TA, KA.
-The modifications of which we are now speaking, appear, then, not so
much to be sounds, as distinguishable manners of beginning and ending
sounds; and it is because they are thus only perceivable in connection with
vocal sounds that they are called consontants.
Now in the mouth, considered as a vocal tube, there are three situations,
in which interruption of voice or breath may most conveniently be made,
and there are six modes of making it at each; so that eighteen distinct interruptive modifications or consonants hence arise. These we shall now describe.
The three great oral positions, as they may be called, are,
1st. At the external confine of the mouth, or lips, giving the labial articulations, of which is an example.
2d. In the middle of the mouth, where the tip of the tongue approaches
the palate behind the teeth, producing the palatal articulations, of which
T is an example..




454   FLUIDITY IN RELATION TO ANIMALS.
3d. Near the back of the mouth, where the body of the mouth approaches
the palate, giving th guttural articulations, of which K is an example.
And the six modes in which the voice or breath may be affected in passing
through each of the three positions of the mouth, are,
1st. A sudden and complete stoppage, producing what may be called a
Qte articulation: viz., P, in the labial position; T, in the palatal; and K,
in the guttural. (See here the general table of articulations next page
which table may be considered as representing the tube of the mouth, with
the letters so placed as to show in what situations in the mouth the sounds
represented by them are severally produced.) A mute may also be made by
stopping the breath exactly at the teeth, viz., a dental-mute; but it is
hardly distinguishable from the p1alatal mute, produced just behind it, and
being less perfect is not used.-Some awkward speakers, substituting it for
the proper mute, are said to speak thick.
2d. A sudden shutting, as in the last case, but the voice being allowed
to continue until the. part of the mouth behind the closure be distended with
air.-This produces the semi-mutes, B, D, and G, (as heard in the syllables
AB, AD, AG,) for the three positions.  There might be a dental half-mute,
but it is not more used than the dental-mute, and for the same reasons. If
the sides of the tongue be depressed, after it has taken the position required
for T or D, the sound L is produced; and the letter is, in the table, placed
below D, although the sound, from being continuable, Is not in any sense a
inute.
3d. The positions-closed as for the mutes, while sound is allowed to pass
by. the nose.-Thus arise the semi-vowels or nasals, M, N, NG, for the
three positions,-NG (as in king) is a simple sound, although our imperfect
alphabet has no single letter for it. The nasal sound of the French language,
which gives it so great a peculiarity, approximates to the English NG, but
differs from it in sound being allowed to pass by the mouth, as well as by
the nose.  It is pointed at by the small 9n in the table, and like the other
sounds which do not occur in the English language, is here pointed in the
Italic character.
4th. Breath only (or whisper) allowed to pass at the three oral positions,
nearly closed.-Hence come the sounds which we call aspirates, viz., F, for
the labial position, TH and S, for the palatal, and CH (heard in the Scottish
word loch,) for the guttural; the TH and CH, are simple sounds, although
each expressed in Britain by two letters.  The TH is heard in the word
bath, and is the sound expressed by the single letter 0 of the Greeks. The
CH is heard in the German ich, and is the x of the Greeks.  The soft aslpirate TH is more easily made by pressing the tongue gently against the
teeth, and allowing the breath to pass all round, than by the true palatal
approximation of parts, and the soft dential aspirate, therefore, is used in
preference to the palatal.  The letter S is the hard palatal aspirate, and
differs from  the soft palatal aspirate TH, in the breath being made to
issue with greater force, and only by a narrow space over the centre of a
rigid tongue, instead of on all sides of a soft tongue, as for TH. French
people on first attempting to pronounce TH, substitute for it the D, or the
S, or the Z (which is nearly related to S, as explained below.) The author
has found it easy to enable them to pronounce the TH at once, and perfectly,
by explaining its nature as above. If we depress the sides of the tongue while
pronouncing S, we produce the simple sound expressed by the English double




THE  VOICE  AND. SPEECH.                      455
letter S E, just as by depressing the sides, of the tongue while making D,
we produce L.
5th. Using voice in the same manner as breath or whisper is used for the
aspirates.-This produces the sound called vocal aspirates, viz., V, TH, Z,
J, and GH. Til vocal aspirate, is heard in bathe, as contrasted with the
simple aspirate in bath; Z comes from the S position, only with sound
instead of breath; SH pronounced with voice, becomes the J of the French
in the word je, or the sound heard in the middle of the English word vision.
GH is a simple sound in German, but not in English.
Table of Articulations.
0o        c
oj         a           o
P   B   M    F   V   pr LABIAL....   th  th.  Dental.
T   D   N    S   Z   R   PALATAL.. L      sh   J.    with the edges of
the tongue depressed.
K    G   ng  ch   gh  ghr GUTTURAL.
6th. Shaking the approaching parts in the three positions. —We thus
make vibratory sounds, of which the middle position gives the common R,
the only one of them used in England. Some bad speakers of English, however, make the labial vibratory by shaking the P in such words as property;
and many use the guttural, which is the burr of Northumberland, and the
common affectation in Parisian speech, termed prarler gras, or grasseyer.
Additional Remarks.
The sound of iH is an aspirate produced even behind the situation of the
guttural aspirate ch; it is, indeed, merely a forcible passing of the breath
through the very back part of the mouth or throat.




456   FLUIDITY IN RELATION TO ANIMALS.
CH, in such words as chain, means T before sh.
J, as heard in the English name John, is a compound sound, viz., D before the simple J of the table, which is the S of vision.
LL. The liquid or double LL of the French as heard in the word paille,
is merely L with the letter Y begun to be pronounced after it. It is heard
in the English words billiard and halyard, and would be their terminating
liquid were the syllable ard not pronounced.  The double LL of the Welch,
as in the name of Lloyd, has the first L pronounced as an aspirate, that is,
as a.whisper, and the second in the ordinary way.
GN. The soft G N of the Italians and French, is the English N with Y
begun to be pronounced after it.  It is heard in our word tanyard; and in
the Italian words pegnio bagnio; and in the French word craignent.
C, in English, stands always either for S or K, as in the words certain
and car, and has no sound proper to itself.
Q, in English, expresses the sound of the letter K, with U following it:
and yet, uselessly, U is always written after Q.
X, in English, means either KS, as in the word axle, or GZ as in the word
example.
The consonants are best heard by sounding them with voice before them:
that is to say, by making them rather terminate a syllable than begin it:
pronouncing B, D, G, thus eb, ed, ey, rather than their common alphabetical
names be, de, ge.
The labial sounds may be made either by the two lips, or by one lip and
the opposite teeth.  F may be pronounced, for instance, by the lips only, or
by the lips and teeth: and some persons awkwardly make it by the under
teeth and upper lip.
The letters Y and I, in most modern languages, stand for nearly the same
sound.  in English, for instance, bullion and minion might be written
bulllyon and minyon, without suggesting a change of pronunciation.  In the
words, yard, you, yes, &c., the Y is a short.I, very closely joined to the
following sound. —W is also thus a short U, as perceived in the words war,
we, &c.
The author believes the analysis of articulation to be the best basis for a
system of short-hand written characters.  IIe has tried such a system, and
found it exceedingly convenient.
Lispiny is chiefly the habitual substitution of the aspirate TH for the S
and SH.
1Whispering is articulation without voice; that is to say, articulation while
breath only is passing.
Stuttering, stammering or hesitation of speech, are terms implying an
nt.errupted articulation, accompanied generally with more or less of straining and distortion of feature.  It is remarkable with respect to this defect,
that when the present work was first published, scientific or regular medicine
had taught as yet no certain cure for it, although the frequent success of nonprofessional, and often ignorant individuals, by a mode of treatment which
they solemnly bound their patients not to divulge, proved the cure, in certain
cases, to be both possible and not difficult.-The author's attention had been
drawn to the subject some years before, by an interesting case submitted to
him of stuttering connected with other disease; and it was in analysing the
subject with a view to the treatment of that case, that he framed the analysis
of articulation contained in the preceding pages, and drew up a part of the
additional observations which are now to follow. A cure was obtained;
but ax the case possessed a favourable peculiarity in the powerful mind of




THE VOICE AND SPEECH.                           457
the individual, to which the author attributed great importance, as he had
little leisure from his ordinary professional duties, to pursue the subject, or
to ascertain in what respects his plan might differ from that employed by
the most successful of the practitioners who concealed their proceedings, he
gave his remarks in former editions of this work, merely as continued elucidation of the subject of speech.  He is now, however, enabled to state, that
his analysis has completely detected the nature of the morbid affection, and
that it directs simple and effectual means of relief. He declined meddling
with many cases offered to him  after the original publication of his work,
from the impression that the cure in the instance mentioned above, was
owing at least as much to the ingenuity and perseverance of the patient, as
to his suggestions, and, therefore, that his professional superintendence of
the discipline required for ordinary cases would demand care and attention
which he could not spare; but subsequent experience has proved to him
that the business is altogether very simple and easy, and as regards children,
may be managed by any intelligent instructor of youth who chooses to devote attention to it, while grown individuals will often be able to relieve
themselves by the study of the present section; and he hopes that in very
few cases will the counsel of a person familiar with the anatomy and actions
of the organs be found to fail.
Command over the organs of speech is acquired in the same way as over
all the other muscular organs of the body; those, for instance, used in walking, skating, fencing, performing on musical instruments, &c.: that is to say,
at first, a distinct act of volition is required for every individual movement;
but the law of association or habit of rendering the actions easier with each
successive repetition, they are at last formed into connected tribes or trains,
which appear as obedient to a single wish as the separate elements originally
were.  A child at first exerts as distinct and powerful a volition to pronounce the syllable pa, as after some practice to double the syllable and
make it papa; or after. still more practice, to pronounce the longest and
hardest word of the language: —nay, at last, where there is strong and
healthy power of association, complete sentences, and even rounded periods
of eloquence, are poured out like single words, the mind of the speaker seeming at liberty, after each sentence or period is begun, to meditate and prepare
that which is next to follow. As the faculties of locomotion and of speech
are acquired in infancy and early childhood, persons no more recollect how
they gradually acquired them than how their limbs grew; but the progress,
described above, may be watched by any individual of mature years in his
own person, while he is learning such an art as that of playing on a musical
instrument.  He will find, that at first, every finger which is moved to produce a note, obeys a distinct thought and volition; that soon short trains of
connected notes become obedient to the will almost like a single note; that
then by degrees, longer and longer trains or passages become familiar, until
at last the instrument is obedient to the practised player, as voice is to the
singer or speech to the orator.
There is a great original diversity among individuals as to their powers of
muscular association, and therefore, also, as to their aptitude for acquiriug
the various faculties of which we have been speaking.  Thus some children
walk well before a year, others require a much longer time, and some never
succeed perfectly until they have had lessons from the dancing-master or
drill serjeant. So, again, many people, by ear and imitation alone, learn
easily to play on musical instruments; but others must begin by studying
the written notes, and the precise fingering by which each note is produced




4,58     FLUIDITY  IN  RELATION  TO  ANIMALS.
on the instrument; and many, unless the notes be constantly before them
cannot play at all. So, again, all persons may be said to learn to speak at
first by ear and imitation; but many grow up to a certain age with defects,
which judicious lessons from parents or other tutors are required to remove;
and there are some as stutterers, who, owing to a naturally weak or irregular association, or to some accident in early life, which has strongly affected
their nervous system, retain defects which no ordinary teaching can correct.
It appears, then, that an analysis and scale of articulate sounds, with minute
description of the organic actions required to produce them, like the scale
which we possess for music, in the gamut and rules for fingering, should
give nearly the same assistance to the speaker which the gamut gives to the
player. The table and analysis contained in the preceding pages is intended
to supply this information.  It is constructed from minute consideration of
the organs of speech while in action. It agrees in many respects with the
common grammatical divisions of elementary sounds, but in others it pursues
the analysis in a different way, and considerably farther. A person who
understands it.well, will have, while he speaks, an intelligent perception of
what he is doing, in addition to the parrot-like faculty of habit, or of repeating by rote, and will thus command any desired sound by two powers instead
of one. And, as a musician, when his musical memory fails him, finds
help by thinking of his written notes and their relation to his instrument,
so may a stutterer, when hesitating at any sound, receive benefit by thinking
of the letter which represents it, and of the position of the organs required
for that letter. Then, by frequent practice in making the particular combinations of sound which are difficult to him, he may strengthen the useful
habit, and ultimately overcome his defect.
The most common case of stuttering, however, is not, as has been almost
universally believed, where the individual has a difficulty in respect to some
particular letter or articulation, by the disobedience of the parts of the mouth
which should form it to the will or power of association, but where the spasmodic interruption occurs altogether behind or beyond the mouth, viz., in
the glottis, so as to affect all the articulations equally.  To a person ignorant
of anatomy, and therefore knowing not what or where the glottis is, it may
be sufficient explanation to say, that it is the slit or narrow opening at the
top of the windpipe, by which the air passes, to and from-the lungs, being
situated just behind the root of the tongue.  It is that which is felt to close
suddenly in hiccup, arresting the ingress of air, and that which closes, to
prevent the egress of air from the chest of a person lifting a heavy weight or
making any straining exertion; it is that, also, by the repeated shutting of
which a person divides the sound in pronouncing several times, in distinct
and rapid succession, any vowel, as o, o, o, o. Now the glottis, during
common speech, needs never to be closed, and an ordinary stutterer is instantly cured, if, by having his attention properly directed to it, he can keep it open.
Had the edges or thin lips of the glottis been visible, like the external lips of
the mouth, the nature of stuttering would not so long have remained a nmystery,
and the effort necessary to the cure would have been suggested to the most
careless observer: but because they were hidden, and professional men had
not detected in how far they were concerned, and the patient himself had
only a vague feeling of some difficulty, which, after straining, grimace, gesticulation, and sometimes almost general convulsions of the body, gave way,
the uncertainty with respect to the subject has remained. Even many persons who, by attention and much labor, had overcome the defect in themselves,
as Demosthenes did, have not been able to describe to others the nature of




THE VOICE AND SPEECH.                           459
their efforts, so as to ensure imitation; and evidently the quacks who have
succeeded in relieving many cases, but in many also have failed, or have
given only temporary relief, have not really understood what precise end in
the action of the organs their imperfect directions were accomplishing.
Now, a stutterer, understanding of anatomy only what is stated above, will
comprehend what he is to aim at, by being farther told, that when any contiuued sound is issuing from his mouth, as when he is humming a single note
or tune the glottis is necessarily open, and therefore, that when he chooses
to begin pronouncing or droning what we have already described to be the
simplest of vocal sounds, namely the vowel e, and in its less distinct modification, as heard in the English word certain, or in the French word que (to
do what at once no stutterer has difficulty,) he thereby opens the glottis, and
renders the pronunciation of any other sound easy:-or if, when speaking or
reading, he joins his words together, nearly as a person joins them in singing,
(and this may be done without its being at all noted as a peculiarity of speech,
for many persons do it in their ordinary conversation,) the voice never stops,
the glottis never closes, and there is of course no stutter.  The author has
given merely this explanation or lesson, with examples, to persons who before would have required half an hour to read a page, but who immediately
afterwards read it quite smoothly; and who then, on transferring the lesson
to the speech, by continued practice and attention, obtained the same facility
with respect to it. There are many persons not accounted peculiar in their
speech, who in seeking words to express themselves, or while coming to a
decision, often rest between their words on the simple sound of e mentioned
above, saying, for instance, hesitatingly, " e...... I e.....  think
e... I.. I stlall," the sound never ceasing until the end of a sentence,
however long it may be delayed. Now a stutterer, who, to open his glottis
at the begining of a phrase, or in the middle after any interruption, uses
such a sound, would not even at first be more remarkable than a drawling
speaker, and he would only require to drawl for a little while, until practice
facilitated his command of the other sounds. Although producing the simple
sound mentioned is a means of opening the glottis, which by stutterers is
found very generally to answer, there are cases in which such other means
may be more suitable, as the intelligent preceptor will soon discover. —Were
it possible to divide the nerves of the muscles which close the glottis, without at the same time destroying the faculty of producing voice, such an
operation would be an immediate and certain cure of stuttering.
While the spasmodic closure of the glottis, as above described, is the common cause of stuttering, there are also cases in which the cause is a spasmodic
prolongation of some of the aspirates or semivowel sounds, as of s m 1, &c.,
Fortunately, however, the substitution of the simple sound is equally the
cure for all.
While the cure of many stutterers has been accomplished by their own
efforts, after the study of what i, written in this section, for others, and particularly for young people, the following have been found to be farther useful
rules or forms of direction; and a commentary upon them making them
perfectly intelligible, would seem to comprehend all that can be communicated
upon the subject -I1. Familiarize yourself with the idea of a continued sound
as of the roar of the sea or waterfall, or the note of an organ-pipe, and feel
that your speech is to be as uninterrupted. —2d. Then never stutter more, but
substitute always the simple continued sound for any threatened defect, and
rest upon it until power be felt to overcome the difficulty. —3d. Never repeat
words or syllables. —-4. The simple sound must become the first syllable




460   FLUIDITY IN RELATION TO ANIMALS.
(closely joined) of every difficult word, until the morbid habit be weakened.
The object of all these directions is to enable the patient, first to substitute
universally the drawl for the stutter, and then, as soon as possible, to discard
the drawl too.
The view given above of the nature of stuttering and its cure, explains the
following facts, which to many persons have hitherto appeared extraordinary.
-Stutterers often can sing well, and without the least interruption; for the
tune being continued the glottis does not close.-Many stutterers also can
read poetry well, or any declamatory composition, in which the uninterrupted
tone is almost as remarkable as in singing.-A person who draws a deep
breath before beginning to speak, as he cannot long retain the air, and the
glottis must be open to let it escape, is to a degree securedagainst the occurrence of stuttering. The secret remedy of an American quack, who years
ago got much money from Englishmen, was the direction thus to fill the chest
before beginning to speak. A Dr. McCormac, also, who published a work
on this subject, founded on the erroneous idea, that stuttering was an effort
to speak while inhaling air, instead of while exhaling, gave the same direction.-The cause of stuttering being a weak and easily disturbed association
of certain muscular actions, we have the reason why any degree of anxiety
or dread as to speaking well, exceedingly increases the defect; and why many
stutterers, who cannot make themselves intelligible' in society, still, when
alone can speak and read as perfectly as any otlier person. This explains
also why many stutterers, who have gone to live for a time at the houses of
pretended curers of their defect, have felt themselves singularly relieved from
the moment of entering the house; because knowing that they were expected
to speak ill, they had no fear of disagreeably attracting attention, and therefore
had their powers much more' at command. These persons, on returning to
the world, have generally stuttered as badly as ever, but many of the asserted
cures of stuttering, with certificates obtained from the parties at the time have
been of the nature now described.-The cause of stuttering being so simple,
as above described, one rule given and explained may, in certain cases, instantly cure the defect, however aggravated, as has been observed in not a
few instances; and this.explains also why an ignorant pretender may occasionally succeed in curing, by a given rule of which he knows not the reason,
and which he cannot modify to the peculiarities of other cases.-The same
view of the subject explains why the speech of a stutterer has been correctly
compared to the escape of liquid from a bottle with a long, narrow neck,
coming —" either by hurried gushes or not at all:" for when the glottis is
once opened, and the stutterer feels that he has the power of utterance, he is
glad to hurry out as many words as he can, before the interruption recurs.
The study of the table of articulations lead to the immediate correction of
many minor defects in utterance, and is calculated to facilitate the acquirement of foreign languages.  A lisping person, for instance, is cured at once,
by being told that the tongue must not touch the teeth in pronouncing the
letter S; and a Frenchman who deems it impossible for him to pronounce
the English sound of TH, discovers that lie cannot avoid doing so if he rests
his tongue softly against his teeth, opened a little, and then forces breath or
sound to pass between the tongue and teeth.
Several of the modern languages of Europe consist of nearly the same elementary or radi6al words, and differ among themselves chiefly by the prevalence in each of certain terminations and of one or other of the related and
convertible sounds classified in the analysis given above. A student, therefore, who, by analytical investigation, or considerable practice, has become




THE VOICE AND SPEECH.                         461
impressed with the peculiar genius of a language, may invent, or determine
by analogy, even before minute study, the majority of those words belonging
to each which have sprung from a common origin. This remark is so true
with respect to the languages of Italy, Spain, Portugal, and even France, that
to persons familiar with them, they are -at last listened to rather as the same
language spoken by different individuals, than as languages in themselves
different.
Ventriloquism is the name commonly given to the art by which an individual can assume characters of voice and speech which are not natural to
him, and thus, although alone, can:imitate closely a conversation held
between two or more persons.
The most remarkable diversity is obtained by speaking during inspiration,
instead of, as usual, during expiration. The voice so produced is more feeble
than the -ordinary voice, and when accompanied by other circumstances favouring the illusion, it may suggest very completely the idea of a boy calling
from the bottom of a pit, or from the interior of a chimney, &c. An unsuspecting peasant may be tricked into unloading his hay-wagon by an expert
ventriloquist, who makes him believe that there is a poor child packed under.the heap and ready to be smothered there.
4. pelson, by a little practice, may acquire the power of producing, without the slightest apparent motion of the lips or countenance, all the articulations except the labial, and of them theF, V and M may be tolerably imitated by parts behind; hence by avoiding words in which P and B occur,
such person may speak without visible movements of the organs, and if he
assume the attitude of a listener, he may make the deception of ventriloquism
complete.  The idea which some authors have had (see Good's Study of
Medicine, &c.) that the articulations of the ventriloquist are not produced by
the tongue and mouth, as in common speech, is altogether an error. The art,
carried to a certain degree, is not very difficult, as any person may ascertain
who tries it after considering minutely the nature of common speech.
There are also striking varieties of voice producable by speaking with a
more acute or grave pitch than usual, and with different degrees of contraction of the mouth; but these may be more properly called imitations than
ventriloquisn.
The variety of effect in sound which the human organs are capable of producing is truly surprising. There are adepts in the art of imitations, who
not only mimic the speech of all ages and condition of the human race, but
the songs of birds, the cries of animals, and even not a few of the sounds producing among inanimate things.  Many of these performances become in the
highest degree ludicrous, and furnish favourite amusements in our theatres.
A Mr. Henderson, of London, about the end of tile eighteenth century, used
to kill his calf, as he called it, to crowded houses every night.  After dropping a screen between him and the audience, he caused to issue from behind
it all the sound, even to the minutest particular, which may be heard while
a calf is falling a victim in the slaughter-house;-the conversation of the
butchers, the struggling and bellowing and quick breathing of the frightened
animal, the whetting of the knife, the plunge, the gush, the agony; —and,
revolting as the occasion is in itself, the imitation was so true to nature, that
thousands eagerly went to see the art of the mimic.
The following cases of inanimate sound may be closely imitated by the
mouth: The working of a grindstone, including the noise of the water into
which it dips, the rough attrition of the steel upon it, and the various changes
occuring with the change of pressure; —the working of a saw cutting wood;




462      FLUIDITY  IN  RELATION  TO  ANIMALS.
-the uncorking of a bottle, and the gurgling noise of decanting its contents;
— the sound of air rushing into a rwom in a winter night by a crevice or
key-hole-and many others.
It has already been explained, that voice depends on the vibration of the
two edges or lips of the slit-like opening of the glottis, by which the air
passes to and from the chest. The number of vibrations in a given time, or
the pitch of voice depends, of course, on the length and tension of these
edges. The length is varied by the position sof the arytoenoid cartilages, and
the tension by the action of small muscles which act on these; and the
cavity of the mouth is enlarged or lessened to accord with the number of
vibrations, by the rising or falling of the tongue and larynx which form its
bottom. The peculiarities of individual voices must depend chiefly on the
size and firmness of the cartilaginious box of the larynx, the strength of the
muscles of the chest which force the air through the glottis, and the pliancy
of -the moving parts.
The glottis is smaller in women than in men, and hence their pitch of
voice is higher:-with reference to music, the difference is generally of an
octave, or eight notes.
The voice of a boy, in regard to pitch, is generally the same as that of a
woman; but at the age of puberty, the sounding organs in the male enlarge
suddenly, and render the voice stronger than before, and by nearly an octave
graver. The voice of a eunuch is the voice of the boy continued, because
the change called puberty does not take place in him.
Complete loss of voice, for longer or shorter periods, is often experienced
by persons while in feeble states of health.  The vibrating, and, therefore,
sounding edges of the glottis, which are usually kept tense by the operation
of certain muscles, on these ceasing to act, owing to the state of their nerves,
will not vibrate as required, and the voice is lost. Slight colds suffice in
many people to produce this effect: in others of morbidly sensitive or delicate nervous temperament, it follows fatigue, or any other cause of debility.
Articulation is not destroyed by loss of voice; and whispering answers
passably the end of vocal speech.
No intelligent mind can meditate on human speech and its influence in the
world, without being roused to vivid admiration. But for speech, the most
gifted individuals who have lived, had they existed at all, could have been
little superior ini their worldly state to the leading oxen of our herds, or to
leading monkeys in the woods. As regarded the rest of mankind, Homer
and Newton would have lived in vain. At the present day, among the,natives of Australasia, where language may be said scarcely yet to exist,
human nature is seen thus brutishly debased; while, on the other hand, in
the history of the world, we may trace, as a consequence of more perfect
speech, all the progress which has been made in arts and civilization. By
language, fathers have communicated their gathered experience and reflections to their children, who in their turn become fathers, have transmitted
them to succeeding children, with new accumulation; and when, in the
course of ages, the precious store had increased, until mere memory could
retain no more, the art of writing arose, making language visible and pernanent, and enlarging without limit the receptacles of knowledge; and then
the art of printing came, which now rolls the still swelling flood into every
hamlet and every hut. Language thus, at the present moment of the world's
existence, may be said to bind the whole human race of uncounted millions,into one gigantic rational being, whose memory reaches to the beginnings
of written record, and retains imperishably the important events that have




THE  DIGESTION.                          463
occurred; whose judgment, analysing the treasures of memory, has already
discovered many of the sublime and unchanging laws of nature, and has
built, on them the arts of life, and through them, piercing far into futurity,
sees distinctly many events that are to come; and whose eyes, and ears, and
observant mind are, at this inoment, in every corner of the earth, watching
and recording new phenomena, for the purpose of still better compreheniding
the'magnificence and simplicity and beauty of creation.
THE DIGESTION.
The doctrines of fluidity, illustrating and illustrated by certain phenomena
of digestion.
The animal body may be seen at first, in the maternal ovary, as a single
speck of mucus; but from possessing life —wonderful life-the little nucleus,
placed in new circumstances, begins to gather itself substance from around,
and it increases in bulk.  For a certain time it remains attached to the body
of its parent, and draws the material of its increase from its parent's blood;
but after that time it is alone and entirely dependent on its own resources.
Then we see brought into play that extraordinary apparatus now to be
described under the name of the digestive or assimilating organs; which,
under the direction of a nervous energy, can, out of almost any kinds of dead
animal or vegetable matter build up the beautiful living body to perfect
maturity of size, and form, and faculty.  And it is not only while their
bodies are growing that animals require to take in and assimilate new matter,
but also after maturity, in order to repair the waste of constant action.
Supply of fuel and water to the steam-engine is not more necessary than of
aliments to the living body.
Some of the less perfect animals take in sustenance almost like vegetables,
by absorbent tubes that open on. their surface; but by far the greater part
receive it first into an interior cavity, where it undergoes certain preparation,
and is then offered to internal absorbents, which drink up what is required
and carry it into the circulating blood. This internal cavity is called a
stomach. Its form and appendages differ exceedingly in different animals,
according to the nature of the substances which serve for their sustenance,
and to various other circumstances.
In man, the process of digestion has the following steps. The food is
first received by the mouth.  It is there broken or torn into small portions
by the cutting and grinding wedges, called teeth, with which the jaws are
armed; at the same time a fluid called saliva is mixed with it, poured out
from glands around, so as to reduce it into a pulpy mass: this mass is then
pushed backwards by the tongue to enter the long tube called the gullet or
oesophagus, which by successive contraction of circular fibres, propels it
down to the pouch of the stomach, placed under the edge of the left ribs.
From the internal surface of the stomach a liquor oozes, called the gastric
juice, the most general solvent in nature, and which, attacking the received
food, soon reduces it, of whatever kind, to the state of a pultaceous mass,
named chyme; in this state it enters the narrow intestinal canal which is
continued from the stomach, where it almost immediately receives a mixture
of bile and pancreatic juice poured out from the liver and pancreas. After
this mixture, as it gradually passes on, a chemical decomposition and separation of parts takes place, and the pure nutriment of the body assumes the




464      FLUIDITY  IN  RELATION  TO ANIMALS.
state of a milky fluid floating among refuse. This milky fluid, called c71le,i
is taken up all along the canal by the numberless absorbent mouths of the
vessels called lacteals, and is then carried to the thoracic duct, and by it into
the blood to supply the waste. The intestinal canal is about six times as
long as the body, affording, therefore, a very extensive surface from which
absorbtion may take place. That remnant of the chyme which the absorbents refuse, mixed with various depositions or secretions, continues its
journey onwards, and is periodically discharged.
Much of the process which we have now described is mechanical, as will
appear immediately; other parts of it are chemical, such as the solution of
the food by the gastric juice, the separation of the milky chyle, &c.; and
parts are vital, such as the afflux, just when wanted, of saliva, gastric juice,
bile, &c., and the muscular and absorbent actions. He who neglects the
study of any one of these three classes of particulars, must have a very
incomplete acquaintance with the function.-We proceed now to explain the
mechanical or physical circumstances connected with digestion.
The abdomen may be considered as a vessel full of liquid, in which. therefore, there is pressure in all directions, increasing with the depth, (see
hydrostatics,) and increased also by the action of the surrounding muscles
which form the sides of the cavity.
The justness of this view of the abdomen becomes evident, when we consider that only moistened or semifluid food descends into the stomach, that
drink follows, and that gastric and other juices are poured out to mix- with
the food as it passes on to occupy the long intestinal canal; and that then
the intestines externally are perfectly-smooth, and are moistened by the
constant secretion of lubricating serum, so that they slide among each other,
without sensible impediment from friction. The abdomen, therefore, is in
fact a roundish smooth vessel filled with a thick fluid, which is farther contlined in a perfectly pliant and smooth-coated tube.
Thus any part of the contents of the stomach and bowels, in a living man.
is supported like water in surrounding water, and therefore, if the whole
contents be of equal specific gravity, no part can descend or advance by its
weight.  Neither can any general pressure, nor contraction of the surrounding parietes, hasten, except at the moment of expulsion, the motion of any
contained matter-as has, however, often been supposed; nor can it help to
empty one part into another-the stornach, for instance, or the gall-bladder,
into the small intestine.
For the same reason, however, the very slightest contractile action of any
containing part is sufficient to dislodge its contents-gravity as a resistance
being neutralized by the surrounding fluid.  And when the gall-bladder, or
stomach, or any part of the intestinal tube, becomes so full as to put the,elasticity of the coats ever so little upon the stretch, this circumstance alone,
unless some muscular action oppose, will cause a discharge of the contents.
-The natural action of the intestinal canal is a successive contraction of its
circular fibres from above downwards propelling the contents, just as if a
small ring or tube were put round the canal and pushed forwards.
These considerations make evident the common error of supposing that
vomiting can by the sudden compression of the abdominal viscera mechani.
cally emulge or clear the obstructed biliary ducts. If general pressure of the
abdomen could produce this and similar effects, a descent in the diving-bell
should be a powerful remedy in human maladies; for nearly fifteen pounds




THE DIGESTION.                           465
on the inch are added to the ordinary abdominal pressure, at a depth of
thirty feet in water.
We hence see also the kind of error into which our predecessors fell so
generally, when they attributed much of the digestive power of the stomach
to its simple pressure upon the food. The idea probably arose from the
contemplation of the stomach or gizzard of a fowl, which is a powerful gristly
substance, answering the purpose almost of a mouth and teeth, as well as of
a stomach.
It is an error also to suppose that quicksilver, which is sometimes swallowed to remove obstruction, runs through the bowels simply by its weight.
On first entering the loose small intestine, it must drag the part containing
it to the bottom of the abdomen, and in that situation, the whole intestine
must pass round it, nearly as a rope passes through a ring fixed to the floor.
When the mercury arrives at the part of the intestine called the cwocum,
where the farther course lies upward along the fixed arch of the colon, it probably can be dislodged only by the patient's lying down. Any useful operation of quicksilver in such cases, may be in its stimulating the bowels, by
dragging or displacing them, in the-manner above described.
When the abdominal muscles, which are the containing sides of the cavity,
become tense, whether from unusual fulness of the cavity, or from their
own action in any of the straining exertions, a variety of important mechanical effects ensue. Thus,
A full stomach produces-tension and projection of the belly-projection
of the diaphragm into the chest, causing hurried breathing, and impeding
speech and singing-expulsion of blood from the abdominal vessels, and,
therefore, congestions elsewhere, as in the arteries of the head, sometimes
producing apoplexy.
Abdominalfulness, as in dropsy, tympanitis, corpulency, pregnancy, &c.,
produces most of the effects now mentioned in an aggravated degree. If
dropsy be allowed to proceed too far without tapping, the patient will die of
suffocation from the rise of the diaphragm.-The extbrnal veins of the legs
and abdomen of a dropsical person are generally turgid, because the blood is
pressed into them out of the abdominal cavity, and because the passage of
blood through the abdomen is impeded. In tympanitis, or windy dropsy,
as it has been called, the viscera hang down in the abdominal cavity, while
the air occupies the upper part. In common dropsy the viscera float about
and are supported.
Straining or strong action of the abdominal muscle, and therefore, also
pressure on the abdominal contents, occur with almost every considerable
bodily exertion; for the abdominal muscles are the antagonists of the great
muscles on the back and about the spine, and must always come into play
with them to give firmness and rigidity to the trunk of the body. This
may be seen remarkably in the actions of lifting, running, wrestling, &c.
As the abdominal muscles cannot act in a continued way and strongly, unless
the ribs, from which they arise, become nearly fixed, the ribs are supported
during exertion by the intercostal muscles, and by the air in the chest, then
confined by the closure of the air-passages: hence there is generally compression in the chest also when the abdomen is compressed, and the blood is
squeezed towards the extremities from both cavities at once. It is important
to remark also, that in what are called the strong actions of the chest, as
coughing, sneezing, blowing, &c., the abdominal muscles are at least as
30




466   FLUIDITY IN RELATION TO ANIMALS.
active as the pectoral: by pulling down the ribs to which they are attached,
they narrow the chest, and by compressing the abdominal contents, and thus
raising up the diaphragm, they shorten the chest.
The following cases exemplify the effects of straining -The lifting of a
great weight, or making any great exertion, drives the blood up to the head;
as is marked by the sudden redness of the face.-Coughing or vomiting will
cause closed leech-bites to bleed afresh, and sometimes will overcome the
action of the sphincter of the bladder or rectum: coughing will also produce
vomiting,-Straining to empty the bladder, rectum, or womb, or the effort
of vomiting, will cause the rupture of a blood-vessel in the white of the eye,
with consequent effusion of blood there.-Apoplexy often happens under
the same circumstances, from the breaking of a vessel in the brain.-The
rupture of a varicose vein, or of aneurism, generally happens during exertion.-And during exertion, the protrusion is likely to occur at any weak
part of the abdominal cavity, of some portion of its contents, producing what
is called hernia or rupture.
Vomiting is produced, not by the forcible contraction of the stomach, as
was long supposed, but chiefly by the action of the parietes of the abdomen.
This is proved by the fact that the stomach has been removed from a living
animal, and a sheep's bladder containing liquid has been substituted for it,
in connection with the gullet above and the intestines below; and on then
injecting an emetic drugjinto the veins of the animal vomiting has taken
place, as if the stomach had been there and unhurt.* From this we see why,
to prevent regurgitation of the food, during exertion, the upper orfice of the
stomach requires to be almost as strongly closed as the sphincters below.
A small pump-in this application called the stomach-pump-has lately
been used in medical practice, for removing poisons from the stomach in
cases where the action of vomiting could not be excited. It has already saved
many lives. It resembles the common small syringe, except that there are
two appertures near the end, instead of one, which, owing to the valves in
them, opening different ways, become what are called a sucking and aforcing
passage. When the object is to extract from the stomach, the pump is
worked while its sucking orifice is in connection with an elastic tube passed
into the stomach, and the discharged matter passes by the forcing orifice.
When it is desired, on the contrary, to throw the cleansing water or liquid
into the stomach, the connection of the tube is reversed.
As a pump may not be always procurable when the occasion for it arises,
the profession should be aware that in many cases a simple tube will answer
the purpose as well, if not better. Such a tube being introduced, and the
body of the patient being so placed that the tube forms a downward channel
from the stomach, all fluid matter will escape from the stomach by the tube,
as water escapes from a funnel by its pipe; and if the outer end of the tube
* The mechanism of vomiting is still a moot point in physiology. Mr. Haighton, a celebrated English physiologist, opened several animals during the effort of vomiting, and he
asserts that he distinctly saw the contractions of the stomach. The more recent experiments
of M. Magendie, which were repeated in the presence of a committee of the French Institute, are, however, entirely contradictory of those of Mr. Haighton, and seem to show that
the stomach is entirely quiescent in the act of vomiting. M. Maingault, nevertheless, has
been led to results opposed to those of M. Magendie, and he is supported by Professor
Portal and M. Bourdon, both of whom appeal to experiments, and to some pathological
facts, which are very imposing. It appears to us probable that vomiting is usually the
joint effect of the contraction of the stomach, and of the diaphragm and abdominalmuscles,
though either is of itself occasionally sufficient for that purpose.-Am. Ed.




THE  DIGESTION.                             467
be kept immersed in liquid, there will be during the discharge a syphon action
of considerable force. On then changing the posture of the body, water may
be poured in through the tube to wash the stomach, and may, by the same
channel, be again discharged. Such a tube, made long enough, might, if
desired, be rendered a complete bent syphon) the necessary preliminary
suction being produced by a syringe, or by an assistant, who acts through
an interposed vessel.
But there is still an easier mode than either of these now described, of dislodging poison from a torpid stomach, viz., merely to place the patient so
that the mouth shall be considerably lower than the stomach, —as when the
body lies across a chair or a sofa, with the face near the floor, —and then, if
necessary, to press on the stomach with the hand. The cardiac orifice opens
readily in such a case, and the stomach is emptied like any other inverted
vessel.
Useful as the pump may prove, upon occasions, in evacuating the stomach,
its more ancient office of injecting the enema is still the more important-and
recent experience seems to show that such injection may become a remedy
of more extensive utility than had yet been suspected.  From an erroneous
opinion, that what had been called the valve of the cwecurn acts as a perfect
valve, allowing passage downwards only, few practitioners have ventured to
order much liquid to be injected, for fear of overstretching the lower part of
the intestine; and the possibility of thus, by injection, relieving disease situated above the supposed valve, has scarcely been contemplated. It is now
ascertained, however, that fluid may be safely thrown in even until it reach
the stomach.-Perhaps few, if any cases of obstruction of the bowels, could
resist the gentle force of penetrating water, so that a mechanical remedy of
certain effect may, in many cases, be substituted for the drastic purgatives
and pernicious bleedings now used, and often used in vain.-Fromn what has
been said above of the abdomen and the intestinal canal, it appears that an
injection tends to spread itself with singular uniformity over the whole. This
tendency may be rendered obvious to sight by throwing a sheep's intestine,
recently extracted, into a bucket of water, and then pumping water in at one
end; a stream will issue strongly at the other end, although several feet distant, almost immediately, and without any intermediate part having become
very sensibly tense.-Of course, in the living body, in cases of spasms or
obstruction, the liquid must be thrown in against resistance very gradually.
That case is called intro-susception of the bowel, in which an upper portion
falls, or is received into a portion below, (as one part of the finger of a glove
may be received into another part,) and the receiving portion of the bowel,
mistaking the received for descending food, holds it fast.  This occurrence
forms a complete obstruction, and generally proves fatal. Many infants with
irritable bowels, die of it. —Now a copious enema, such as we have described
above, is almost a certain cure. The liquid advances until it reaches the
part where the portion of gut has been swallowed by gut below; and as it
cannot pass without pushing the intro-suscepted portion back to liberty, it
effects the cure.*
The perpetual syr'inge or little valved pump, of which we have been
speaking as lately used in applications to the animal body, can inject or with* It should be remarked, however, that this measure can succeed onlywhilst the introsusception is recent; at least before inflammation has occurred and adhesions formed
between the intro.suscepted portion and that portion of the bowel in which it is received.
Common or atmospheric air, from its great elasticity, lightness, &c., is the best fluid for
the injection.-Am. Ed.




468   FLUIDITY IN RELATION TO ANIMALS.
draw any quantity, and is therefore very superior, for almost every purpose,
to the large old syringes which had no valves, and which, without being
removed could inject only once their fill. With well-adapted additional
apparatus, the same instrument will answer for many purposes, as for throwing up the enema, clearing the stomach, transfusion of blood exFig. 175.    hausting cupping-glasses, relieving the over-distended breasts;
for the lotio vesicee, vagina vel urethrm, &c. No surgical
apparatus is now complete without one.  The annexed outline represents such a syringe.  The aperature c is rendered asucking orifice, by a valve at it, opening inwards; and a is
the forcing orifice, in consequence of having its valve opening
outwards: b is the piston, with its handle. -The valves may
be variously made, or a single double-waoc   cock may be userl
Cba -— r  s instead of both.  Convenient dimensions for the syringe are
from four to six inches for the length, and from three-quarters
of an inch to an inch and a quarter for the diameter.
For a case of diseased rectum, where it was necessary to
use an enena daily, or oftener, the enema funnel, represented
here, (from the funnel-shaped mouth a downwards, and exclusive of the part
above a) was contrived, and was found more manageable by the patient than
any other instrument. If the tube a b be about two feet long, (it may be of
metal, oiled silk, caoutchouc cloth, &c.,) the liquid column contained in it
suffices to overcome the ordinary abdominal resistance;
Fig. 176.     but if a very short tube be used, the open funnel a must
be converted into a close vessel, as represented here by
t  Oz. ~   the dotted line above the funnel, having a bladder, or
other air-tight bag, d, connected with it, and a bottleneck and cork, or a cock, at c, for admitting the enema.
On pouring in the liquid at c, the air in the vessel at c a
is forced into the bag, and on then closing the opening
at c, and compressing the bag, it is evident, that any
desired degree of injecting pressure may be exerted on
the enema. This apparatus is both cheaper and more
simple than a syringe, and is equally effectual; and
the bag never being wetted, lasts long: b is a cock kept
shut until the moment of injection.-The principle of
substituting, in an injecting apparatus, the pressure of a
-b          liquid column for that of a piston, was first suggested in
this work; and yet since the publication of the work,
more than one patent has been taken for it, for parties
seeking to convert it to their profit.
By viewing the abdomen in the true light of a vessel or bag filled with
liquid which is seeking to escape in all directions, we have the explanation
of several circumstances connected with hernia or rupture; in which accident,
the containing sides of the abdomen in some part have given way, allowing
a portion of the viscera to escape, so as to form a tumour under the Ekin.
Hernia may be produced by all causes which strain or weaken the muscles;
as by leaping, lifting, great weights, coughing and sneezing, lying with the
belly across a bench or yard, as sailors do on ship-board, over distention of
the belly by eating or drinking, corpulency, dropsy, pregnancy; debility of
muscle from dissipation, &c.
The reason that a rupture increases so rapidly after it has once begun, is,
that the protruding part is truly a fluid wedge, of which, therefore, the opening




PELVIC APPARATUS.                           469
force increases with the diameter. (See Hydrostatics.) This shows the
singular importance of arresting the accident at its very commencement.
The truss used to repress rupture were described at page 412.
In attempting to return any part of the abdominal contents which may have
escaped as rupture, we should recollect, that a soft uniform compression or
squeezing exerted upon the tumour by the hands of the operator, if greater
than the internal pressure of the abdomen, is slowly pushing back again any
fluid matter that can ooze inward from the tumour; and by thus gradually
lessening the size of the tumour, may effect the desired object, without the
adoption of the last resource, of cutting parts to widen the inlet. When, in
such a case, the operator sees clearly with the mind's eye what is passing
under his fingers, his efforts may often be successful, where a less intelligent
individual would fail. No man practices medicine long, whatever his nominal department, without having opportunities of saving life, or of preventing
a serious operation, by judicious management of recent hernia. The barbarous old fashion of lifting the patient by the heels and shaking him, that the
weight of the bowels might drag back again the part which had escaped, was
founded on ignorance of the fact, that the weight of the bowels in all positions of the body, is supported almost entirely, not by their attachments, but
by the surrounding parts.
The function of digestion or assimilation sketched in the preceding paragraphs, by which the animal body assumes foreign matters from around, and
converts them into his own substances, is a subject of study little inviting in
some of its details, but taken altogether is one of the most wonderful which
can engage the human attention. It points directly to the curious and yet
unanswered question —what is LIFE? The student of nature may analyze
with all his art those minute portions of matter called seeds and ova, which
he knows to be the rudiments of future creatures, and the links by which
endless generations of living creatures hang to existence: but he cannot disentangle and display apart their mysterious LIFE! that something under the
influence of which each little germ, when placed in due circumstances, swells
out, to fill an invisible inould of maturity which determines its forms and proportions. One such substance thus expands into a beauteous rose-bush;
another into an noble oak; a third into an eagle; a fourth into an elephantyea, in the same way, out of the rude materials of broken seeds and roots,
and leaves of plants, and bits of animal flesh, is built up the human frame
itself, whether of the active man, combining gracefulness with strength or of
the gentler woman, with beauty around her as light. How passing strange,
that such should be the origin of the bright eye, whose glance pierces as if
the invisible soul were shot with it-of the lips which pour forth sweetest
eloquence-of the larynx, whose vibrating fills the surrounding air with
music; and, more wonderful than all, of that mass shut up within the body
fortress of the skull, whose delicate and curious texture is the abode of the
soul, with its reason which contemplates, and its sensibility which delights
n these and endless other miracles of creation.
PELVIC APPARATUS.
The Secretion of the Kidneys, &c.
OF the large quantity of fluid daily taken into the human body, much
escapes with the breath, as is provided by the visible condensation of it in
frosty air, or on any cold polished surface held near the mouth; part escapes




470   FLUIDITY IN RELATION TO ANIMALS.
by the skin in perspiration; but the greatest part after having answered the
purposes of the constitution, is separated from the blood by the two secreting
organs, called the kidneys, and from them by fit channels, is carried off,
holding in solution various other matters, which the system does not require.
The kidneys are situated in the loins, one on each side of the spine; and
the constant drain of liquid from them passes down by two membranous
canals called ureters into the bladder, from which the liquid is again expelled
through the urethra, at considerable intervals, according to the rapidity of
accumulation.
The bladder is a curious membranous and muscular reservoir, of which
the fibres can contract so as to expel the last drop, and yet can yield so as
to admit a quart or more.
The passage of fluid downwards through the ureters from the kindeys to
the bladder resembles, in some respects, the passage of blood in the veins.
Authors have erroneously supposed that the weight of the fluid suffices to
cause its descent: but the bladder and ureters are enclosed in a common
cavity with the intestinal canal; and while this is full of a semi-fluid mass
of greater specific gravity than the urine, the latter Is not only supported by
the surrounding pressure, as water would be supported by water, but is forced
upwards or resisted, as water would be in honey or treacle: in descending,
therefore, it obeys some other force than gravity, namely, that of the secreting vessels and heart.
The ureters, bladder and urethra are the seats of some of the most distressing diseases, to which the human frame is liable. Two classes of these
being relievable chiefly by mechanical means, require to be shortly considered here. They are, obstructions in the urethra; and concretions, or stones,
as they are called, in the bladder.
Obstructions or strictures in the urethra are generally consequences of an
inflammation, which has destroyed the dilatability of a part of the canal.
They appear as if a thread or a bit of tape were tied round the canal, so as
to narrow its caliber. Constant irritation, which destroys the general'health,
fits of fever, broken rest, and even death from total' suppression of urine,
have been common consequences of stricture.
Until within a recent period, the treatment of such obstructions was pursued very generally according to a blind routine.  The attempt was made
either to bore them open by wedges, called bougies, often of doubtful and
tedious operation, or to destroy them by caustic passed down to them in the
end of a bougie, which caustic often hurt the part of the canal anterior to
them, or eat out false passages about the stricture, or open blood-vessels
so as to cause dangerous hbemorrhage.
Struck by the defective state of this branch of the healing art, the author
of the present work, while abroad, and situated where he had interesting
opportunities of observation, had bestowed considerable attention upon it,
and he then contrived and tried several new means of relief. These were
afterwards brought more extensively into use and improved, and others were
added, by his brother Dr. James Arnott, superintendent surgeon in the service of the Hon. East India Company, who gave a minute account of them
in a treatise on urethral diseases, and a supplement published in the years
1818 and 1820, They have become, perhaps, still better known in Francee
than in England, through the work of Dr. Ducamp, which describes them,
and which, having been submitted to the French Institute, and most favourably reported upon by the appointed authorities, soon became a standard treatise in the country; —in France, also, the philosophy of mechanics had been




PELVIC APPARATUS.                           471
studied by surgeons more generally than in England. It is painful to be
obliged to add, that Dr Ducamp as regarded these instruments and the
views of disease and treatment which had suggested them, concealed the fact
of his being only a translator. The imposition was not discovered at the
time of his death, which happened two years afterwards, hastened apparently by the fatigues of the extensive practice which the report of the
Acadamy brought upon him. The auther has had so much pleasing intercourse with enlightened and honourable Frenchmen, that it pains him to
have this fact to relate.
The objects aimed at by the new means were,-to ascertain the exact condition of the diseased canal-to facilitate the passing of instruments in cases
of difficulty —and to effect a permanent cure.  The following seven of these
means may here be particularized:
1st. An examining sound; being a bougie with the point formed of a softer
tenacious material; in which fibres of cotton or silk are mixed to prevent
any portion from being broken off or detached during use.  This sound,
pressed against the obstruction, takes a correct impression of its anterior face,
and shows the magnitude and exact position of the still remaining opening.
2nd An expanding or dilator sound which is a small tube with a dilatable
button at its extremity. The button consists of a little bag, which is passed
through the structure empty, and is filled with fluid after it has passed. It
readily discovers any other strictures beyond the first, and, to a certain degree,
the state of each.
3d. A conducting canula or tube, open at both ends. It is passed down
to the stricture, for the purpose of supporting and directing small bougies
seeking entrances through very narrow strictures, or of guarding the caustic
bougie in its approach to the place of its action.
4th. In cases where the attempt to open the passage has failed by all common means, a conducting tube is first introduced, and through it six or-more
small bougies are passed side by side, so as to probe the whole face of the
stricture'at the same time.  It is thus scarcely possible that the opening
should not be found.
5th. Were even this means to fail, the cpndueting tube may be filled with
water, under any degree of pressure, which water will either open the passage for the small bougies, or will itself act as the sharpest and most insinuating of all instruments. The stricture, by which ever means opened will
then allow the urine to escape. As patients might fear that water forced
towards a bladder already too full would'only increase the evil, J. Arnott
waited for more numerous proofs of the utility and safety of the practice,
before strongly recommending it: Dr. Amussat, of Paris, has since published a statement of numerous cases of retention thus relieved.
6th. A dilator for widening the stricture, after a small instrument can be
passed through it. It is intended as a substitute for the boutgies and sounds
of former times. The chief objections to these last are, the painful friction,
the danger of making false passages, the tediousness and imperfection of the
cure, and that they cannot dilate any part of the canal beyond the size of its
orifice, through which they have to pass, and which during health, is the
narrowest part of the canal.,
The dilator consists of a tube of thin membrane introduced while empty
into the stricture, on a ball-pointed. wire and then filled with fluid by a
syringe, so to dilate the stricture, with any degree of force, from the mere
filling of the part to the strain of the hydrostatic press, sufficient to tear the
strongest texture that disease can form. The dilating tube is about two inches




472   FLUIDITY IN RELATION TO ANIMALS.
long, and its end next to the operator is fixed to the point of a small catheter,
through which the distending fluid is injected. The tube is formed of thin
silk riband of various sizes, with the edges joined. It is lined with prepared
gut of the cat or dog, which is almost as thin as gold-beater's skin, although
very strong and water-tight; and it is covered with the same to give the
smoothest and softest possible external surface. When complete and enclosing its blunt wire, it is still much less bulky than the bougie which would
be required for the same case. Thus, it passes easily; it cannot tear the canal
or make false passages; it can enter through a small orifice, and then dilate
to any desired extent; and its greatest advantage is, that by swelling so as
to follow the yielding of the stricture, it can effect at one application, what
only a succession of hard bougies, during long treatment, could accomplish.
In one day it has often removed disease which had resisted other means for
months or even years.
Some practitioners and critics, not understanding the law of fluid pressure
(explained at p. 128,) objected at first to the dilator, that a little water or air
pressed into it by a syringe, would be unable to overcome much resistance.
Had they seen the instrument lifting so readily as it does, a heavy weight
laid upon it, or snapping a strong ligature tied round it they would not have
had this prejudice. It was objected, also, that the instrument would do mischief by dilating the urethra before and behind the stricture more than the
stricture itself; now its dimensions being determined and fixed by those of
its silken tunic, it never can distend beyond the diameter chosen, and, therefore, if of the proper size, it can only press on the stricture itself. It was
also said, that this instrument requires, in the operator, greater manual dexterity and acquaintance with mechanical philosophy than many surgeons
possess; but this is merely saying that the arts are progressive, and that the
accomplished surgeon of the present day is more dexterous and intelligent
than his predecessors of the last centuary. It is not accounted a reason why
the delicate apparatus of the oculist should fall into disuse, that all surgeons
are not able to apply it.
Some attempts had been made before, to construct a dilator offluid pressure, but they produced nothing of value. For urethral purposes, a simple
gut or intestine is worse than useless, for, being yielding in its texture, the
surgeon can never know truly the size of his instrument, and therefore may
do much mischief by it.  Dr. Ducamp, in speaking of the dilator, allows that
he did not first invent it, but then, from ignorance of what constitutes its true
value, he takes praise to himself for simplifying and improving it, by throwing away the silk, and using the gut only. —A variety of metallic dilators
have been contrived and used by English surgeons since the publication of
Arnott's Treatise on Strictures, but although manageable with less trouble
than the fluid dilator, they want its chief merits.
The dilator is applicable to many other purposes in surgery besides that
now mentioned,-as for removing stricture of the gullet, and of the rectum,
for checking hemorrhage in deep wounds, for dilating wounds as a tent, &c.
And the operation of lithotomy was saved to a gentleman, whom Sir Astley
Cooper and the auther of this work were attending together, by the dilator
openupg a fistula inperineo, so that a large stone was extracted without cutting. The dilator has also served in removing stones from the female bladder.
7th. Another improved means for the treatment of stricture, described in
the treatise, is a mode of applying caustic for its entire destruction, but so as
not to touch any other part of the canal.  Formerly the caustic was applied
to the face or anterior part of the stricture, and, therefore, had almost always




PELVIC APPARATUS.                                   473
to destroy a portion of the healthy canal before it could reach the narrowest
fibres: the extent of such portion depending on the distance from  these
fibres of the part where the lining of the canal began to be drawn inwards by
them. This explains why not unfrequently a hundred applications of caustic
were made in a single case, and why, during such treatment, false passages
were often bored, and other mischiefs produced. Now by applying the caustic
within the stricture at once a single application generally suffices. To accomplish this, a ring of caustic is placed (as described in the Treatise, and
in the Cases,) on a bougie of peculiar construction, about an inch from its
extremity; and the bougie being then passed down to the stricture through
a tube or conductor, and the point being passed beyond the stricture, the
caustic is guided to the very spot where it is desired to act.*
X Dr. Ducamp incurred a singular risk in giving himself out as the first proposer of the
instruments and practice described above; for he was already known as a translator of
English medical books, and the Treatise on Strictures of J. Arnott had been held up to
public attention two years before by the various medical reviews, in terms such as the following: " We have carefully perused this little volume, and are of opinion that it is by
far the best systematic work on the subject in the English language."-It is a judicious
compilation, interwoven with much original and acute observation; and it gives publicity
to instruments which promise to be of essential benefit to operative surgery."-Medico:
Chirurgical Review, January, 1819.
Perhaps Dr. Ducamp imagined that the slight alteration proposed by him in the construction of three of the new instruments, might be a shield to him when detected; but as
the chief merit was in the analysis of the subject which suggested such instruments, and
not in the mere mechanical fulfilment of intentions, even a considerable improvement in
the instruments would not have been a sufficient excuse. His changes, however, were either
trifling or retrograde. His metallic dilating sound is less perfect than metallic sounds contrived by J. A., but not described, because the fluid dilating sound was found to be preferable. His porte-caustique is defective in not distending the stricture at the moment of
applying the caustic; and his mode of making a dilator without the silken tunic, renders
it not only a useless, but a dangerous instrument:-indeed, such as obliged him to use the
caustic in almost every case. His silence with respect to the liquid probe favors the conclusion that he did not understand it, although Dr. Amussat of Paris has since used it with
much success:-and the same remark applies to the double catheter (see Arnott's cases,)
or sonde a double current, as it has been called by those who have since used it in Paris.
The following are extracts from the report made by the commissioners of the French
Institute, Doctors Deschamps and Percy, in May, 1822, on the subject of Ducamp's work
entitled Traite des retentions d'urine.
" This treatise concerning a most important malady, because one of the most common
and painful which affects humanity, has appeared to us to merit more than ordinary
attention.
"When, some years ago, your same commissioners had to express their opinion of another work on this subjet, they commended the zeal and industry of its estimable author
(Dr. Petit;) but they could not conceal that there were still imperfections in his modes of
treatment; and also that they were almost entirely either borrowed or imitated from the
English.
"The work of Dr. Ducamp now leaves us, however,-nothing more to desire, and we have
no longer reason, as regards this subject, to envy our neighbours. Although a volume of
moderate size, it is incomparably more complete and full of matter than the bulky treatises lately published in other countries.
" * * Ducamp leaves all these authors far behind him, whether as to the soundness of
his doctrines, the superiority of his trials, or the invention of instruments.
"He takes a print or model of the stricture by an instrument of his invention, called
Sonde Exploratrice. (Arnott's examining sound, page 471.)
" For introducing bougies in difficult cases, he uses an elastic gum tube, which he calls
conductor. (Described above, page 471.)
"Mr. D. has invented for measuring the length of strictures, &c., an instrument which,
when introduced, enlarges beyond the stricture. (The dilating sound, page 471.)
" The nitrate of silver, or common daustic, is what he uses for destroying strictures, but
he employs it in a new manner, which appears to us to give it new powers, and to deprive
it of all its former dangers. * * He carries the caustic into the stricture by means of his
porte caustitue. (See above, page 473. No. 7 of Mr. Arnott.)
".......... To enlarge the canal at the morbid part of its true caliber, he uses an instrument
which he names a dilatateur. (Dilator, page 471.) He does not conceal that this instru



474   FLUIDITY IN RELATION TO ANIMALS.
Stone in the bladder is another disease relievable chiefly by mechanical
means
The urine as secreted in the kidneys, contains dissolved in it, a variety of
substances which, under certain circumstances, separate and assume the solid
form, —as sugar separates in small crystals from cooling syrup, or salt from
cooling brine:-and it is thius that those minute grains are produced which
we call urinary gravel.  A single particle of gravel remaining by an accident in the bladder, soon attracts to itself more matter of the same kind, and
becomes the nucleus or centre of an increasing mass, which is what we call
the stone in the bladder.
In a second Tract by the author's brother, published in 1820,* the follow
ing paragraph appears:' From the severe suffering of the patient labouring under stone in the
bladder, and the remedy being an operation so painful and dangerous, that
many wear out their lives in certain misery, rather than submit to it, it has
arisen that no part of surgery has excited more attention, either in the medical
profession or out of it.t No very important change in the treatment of this
disease has now been made for upwards of a century; and, indeed, it has
appeared to be the opinion of modern surgeons, that the manner of operating
practised in Cheselden, about a century ago, and which has been called the'glory of English surgery' was so nearly perfect as to leave little room for
improvement. The hopes which the rapid progress of chemistry, and the
grand discoveries relating to stone of Scheele, Wollaston, Fourcroy, and
others some time ago gave birth to, that we should be able to dissolve stone
by lithontriptics, and thus save the horrors of lithotomy, had again died
away,.and the researches of many ingenious men who have been, and still
are employed about the question, have for their end, more to prevent the
formation of stone by remedies and regimen, than to improve the manner of
removing it when once formed. I trust, however, notwithstanding the supposed exhausted nature of the subject, that the following essay will prove
that much was still possible in the improvement of this department of the
healing art."
The publication from which the above paragraph is taken, and the " Treatise" which preceded it, in both of which new instruments and new processes were described, and interesting facts were detailed, aroused the public
attention in England to the possibility of improving the treatment of stone;
and about the same time, a similar spirit awoke with more decided effects in
France.  The results have now become of great importance to humanity.
In the medical publications since that time, cases soon began to be recorded in lithotomy superseded by new means, and lately such cases form
the majority.  We shall now briefly animadvert to the principal of these
ment had been imagined before him, but he has the merit of perfecting it, and of reducing
to practice what before had only -existed as a project.
"......... In rendering justice to the able men who have preceded Ducamp, we must still
say, that no one has displayed so much industry, dexterity, and talent, and we think that
he has high, claims to the confidence of patients and the gratitude of the profession, and
that his work merits the eulogium of the Academy,
(Signed " DESCHAMPS,-PERCt, Reporters.
CUVIER,....... Secretary.
* Cases illustrative of the Treatment of Urethral Obstructions and of Stone. By James
Arnott.-Longman and Co., 1820.
t The Catalogue of authors who have written upon stone occupies in Plocquet's Litera.
tura Medica, no less than twenty-nine very closely printed quarto pages.




PELVIC APPARATUS.                           475
means, intending, however, only to interest the reader in a manner that may
lead him to the persual of the original works, where more minute information
is to be found. They shall be named in the order in which they have come
into use.
The dilator, as applied to the treatment of stone, has already been spoken
of in the preceding pages.
The double catheter. This instrument, with its applications to causes of
stone and other affections of the bladder, is described in Arnott's Cases. It
has two channels, by one of which a fluid may pass into the bladder, while
by the other there is a returning current mixed with urine. It is equipped
with two pliant tubes, of which one leads, from a su~pplyin  reservoir, and
the other to a waste vessel. It will soothe irritation of the bladder, whether
arising from stone or not, by keeping the acrid urine in a dilated state, or by
applying bland and medicated liquids directly to the internal surface of the
bladder. Not being larger than a common catheter, it may be worn for any
period as the common catheter now is. It need prevent no sedentary occupation, and may be used during sleep.  It will act powerfully to dilate a
contracted bladder, if the reservoirs be placed high, and the fluid be caused to
distend with the pressure of a lofty column. It also affords by far the best
means of admitting to the bladder any solvent of stone. Even pure water is
a weak solvent of most animal calculi, as is proved by placing them in a running stream; but the living bladder bears with impunity a diluted acid or
alkali.
The syphon catheter (also first described in Artott's Cases) is merely a
catheter of a length that will allow its external part to descend, so as to constitute the long leg of a syphon. (See Pneumatics.) Its outer extremity
is turned up a little, or has a portion of soft animal gut tied upon it to act
as a valve, for preventing the entrance of air.  The most useful application
of this instrument is to keep the bladder empty after operationis, until the
healing process has made a certain advance. The diffusion of urine among
the surrounding parts after lithotomy, particularly after the high operation,
is often a cause of death; and the syphon catheter, by providing a channel
by which the urine must immediately pass away as secreted, obviates the
danger. This instrument is sometimes useful in very irritable bladders, by
preventing the repeated distensions of the bladder, with the consequent
excruciating contraction. It has also relieved in the deplorable case of the
bladder torn or opened by sloughing in parturition, as it can keep the unhappy patient quite dry.
Aforceps, calculated to pass through a tube into the bladder, and to open
there, for the purpose of seizing any small stone or other solid object offered
to it, was described long ago in the Armamentum Chirurgicum of Scultetus,
but was again forgotten until John Hunter's investigations led him to a
second invention of it. Such an instrument had for a considerable time passed
under the appellation of Bunter's urethra or bladder forceps, answering for
extracting small stones, and therefore, if used in time, preventing occasionally, the necessity of lithotomy. Soon after the publication of Arnott's
Essay, it was modified and much more extensively used by Sir Astley Cooper
and other surgeons in England.
But a new and intense interest has now been excited with respect to the
forceps, as a means for removing stone, by the discovery-also an old discovery revived-that a straight tube may be passed to the bladder, as a conductor
instead of the bent tubes or catheters commonly used.  A door is thus, as it
were, opened directly into the bladder, tlirough which a stone might even be




476   FLUIDITY IN RELATION TO ANIMALS.
seen, if light were directed upon it, and through which it easily may be caught
and broken to pieces, and brought away without doing injury to the living
parts. Dr. Civiale, of Paris, had the merit first of contriving good instruments for this operation, and of himself operating with complete success in
many cases.. But the praise of carrying the operation of Lithotrity (stonewearing-down,) as it is now named, to its present state of perfection, is shared
by various other ingenious surgeons, as Gruithuisen (.who first used the
straight sound,) Amussat, Leroy, Heurteloup, (who proposed the mode of
percussion,) &c. The operator introduces a strong forceps,- which seizes
and holds fast the stone, he then weakens the stone by boring it in various
directions with a simple drill, which passes through the handle of the forceps,
and is turned rapidly by a drill-bow acting on its external end, or with a drill
of which the point can be bent to one side, so as to excavate to any desired
extent; after which weakening, the stone is crushed, either by the forceps
which first held it, or by another instrument called brisecoque, made on purpose; or without boring at all, Heurteloup and others at once break the stone
to pieces by blows of a small hammer acting on a sliding limb of the forceps.
Dr. Darwin, in his Zoonomia, published in 1790, proposed to seize stones
by forceps passed into the bladder, and then to break them down or destroy
them mechanically; but the supposed necessity of working through a long
bent tube prevented trials from being made. The author of this work also
showed some years ago, before any of the above-described improvements
were made (see Cases, page 93,) that it was possible to pass a bag into the
living bladder, and to enclose a stone there, so that any solvent might be
injected into the bag, and again withdrawn without coming into contact with
the bladder. This was shown rather to excite attention to the possibility of
operating with the living bladder with great precision, than to recommend
that precise means of destroying stone.
To all the ingenions instruments above spoken of for breaking down the
stone, there is still this objection, that it is broken into such fragments, that
many of them require to be afterwards treated as distinct stones, and thus
the painful operation has to be repeated again and again, and whole months
may pass before the operation be completed. —The author deems it possible
to make a forceps of several claws or ribs, which should surround the stone
so loosely as to leave it freedom of motion within the claws, like a loose
kernal in a shell, and so that on making the. forceps itself whirl backwards
and forwards, like the drill in Civiale's apparatus, the stone might be quickly
rubbed to dust by the friction or file action of the roughened interior of the
claws. The bladder would be filled during the operation, with water, or even
air, to secure plenty of room for the turning instrument:-or a slender external forceps might be used as a guard, to prevent contact of the bladder with
the moving instrument.  Out of the body, a stone harder than urinary
calculus, placed in such a cage with rough interior, and subjected to the
action described, is soon reduced to dust. There are various ways of making
a forceps or cage for this operation, which will readily suggest themselves
to persons knowing what has already been achieved in this department of
practice, and having the ingenuity likely to engage them in such a pursuit.
The high operation of lithotomy possesses over the common lateral operation such advantages as the following: —thinness of the parts cut through
-distance of the knife from important arteries-stones of very large size may
be more easily extracted-the prostate gland is not wounded. But the high
operation has not become general,-because-there was difficulty in avoiding the peritoneum while making the opening into the bladder-there was




PELVIC APPARATUS.                            477
danger of effusion of urine among the cut parts, after the operation —and
where the bladder was contracted, the incision had to be very deep. Now
these objections are obviated by, 1st, the double catheter, which will dilate
the contracted bladder; 2d, by the s/phon catheter, which will prevent the
effusion of urine; and 3d, by the jointed sliding sound, (see Cases, page
104,) which will ensure the accurate cutting in the desired place. Had we
possessed then, for the removal of stone, no less hazardous means than
cutting, the high operations with the new securities might have been best.
When a catheter has to be retained in the bladder after any operation, in
cases where, if it slipped out, it might with difficulty be replaced, something
should be passed through it like a small spring forceps to expand and
become an internal button preventing its escape. (See Cases, page 94.)
UTERINE PHENOMENA.
Although so many of the uterine phenomena are mechanical, there are
few of them which could be treated of with advantage, except in connection
with particulars, of which the consideration does not belong to a work like
this. We shall, however, cite the following particulars as examples.
The protection given to the tender fretus by the liquor amnii in which it
floats, is such, that a blow from without is expended on the surrounding
water, and cannot reach the fcetus.
The head of the foetus, because ossification begins in it first, becomes of
greater specific gravity than the other parts of the body, and therefore generally lies at the bottom of its liquid bed.  It is thus ready to appear first in
parturition, according to the safest course of delivery
The membranes distended by the liquor amnii descend before the head,
as a soft but powerful wedge preparing the way, according to the principle
explained in a previous page.
We have spoken, at page 168, under the name of pneumatic tractor, of a
circular piece of leather or similar soft substances, kept extended by included
solid rings or radii, as being adapted to some purposes of surgery. Now it
seems peculiarly adapted to a purpose of obstetric surgery, viz., as a substitute for the steel forceps, in the hands of men who are deficient in manual
dexterity, whether from inexperience or natural inaptitude. The forceps, to
be well and safely used, requires address, which'even the naturally dexterous
man cannot possess without a certain degree of continued practical familiarity
with it, and except in large towns, a man must be very unfortunate in his
practice who often requires it: hence the really small number of persons, who
use it well. The consideration of the tractor as a substitute for it belongs
properly to the present section: but as the true mode of action of the tractor
is not very readily conceived by persons who either have never been instructed in the general laws of physics, or who have ceased to be familiar
with them, such persons are advised to read this paragraph in continuation
of that at page 168, and to weigh well the following remarks. A tractor of
three inches in diameter, would act upon any body, to lift or draw it, with a
force of about a hundred pounds-with more therefore, than is ever required
or allowable in obstetric practice. In lifting a stone, the tractor does act
as if it were glued or nailed to the stone, but merely bears or takes off the
atmospheric pressure from one part, and allows the pressure on the opposite
side, not then counterbalanced, to push the stone in the direction of the tractor;-so when placed upon the heafd, it would not pull by the skin, in the




478   FLUIDITY IN RELATION TO ANIMALS.
manner of a very strong adhesive plaster applied there, as uninformed persons would be apt to suppose, but by taking off a certain atmospheric pressure from the part of the head on which it rested, it would allow the pressure
on the other side or behind to urge the head forward on its way. Of course
the forwarding pressure in such a case would not operate on the head directly,
but through the intervening parietes and contents of the maternal abdomen.
It would be much better to have a gentle and diffused action of the tractor
over a large surface, than an intense action on a small surface, and therefore
a tractor for the purpose now contemplated should not be very small, and
should have a little air underneath it in a slight depression or cavity at its
centre. —The forceps must be more effective than the tractor for rectifying
malposition of the head, and diminishing its transverse diameter; but the
tractor will answer both these purposes in a degree greater than many would
expect.* The author proposes to publish on this matter, and on some other
strictly professional subjects which are lightly touched upon in the present
general work, such a practical detail, as for the dilator, syphon catheter,  &c.,
is found in his brother's " Treatise" and " Cases."
Conclusion.
It is almost superfluous to remark here, that, for the practice of general
and obstetric surgery, learning and judgment are of little avail unless accompanied by manual dexterity: and it is one of the improvements yet to be
made in our system of education for various professions, to cultivate more
methodically the use of the hands. Children and young people, in obtaining practical familiarity with ingenious toys, tools of carpentry, games of
address, musical instruments, &c., are often fitting themselves for the
important business of their future life.
* We have been already compelled on one or two occasions to differ from the able author
of this work, in relation to the practical application of some of his principles, and we must
be again permitted to record our dissent from his opinion that the pneumatic tractor, under
certain circumstances is peculiarly adapted as a substitute for the obstetric forceps. Our
author cannot be a practical accoucheur or he would at once perceive that the various manceuvres by which labour is assisted with the forceps, cannot be accomplished with the
tractor. That address and knowledge are requisite to apply the forceps properly, is no
objection to their use; it only shows the necessity of the operators's acquiring this dexterity
and knowledge before attempting to apply the instruments; and these acquirments are not
so difficult as our author seems to think, nor do we believe that the number who possess
them is so very small. It is not contended even by the author that the tractor is superior to
the forceps; he only recommends it as being less dangerous in the hands of the unskilful.
Now it might be supposed fromthis that the tractor is readily applied and cannot effect injury, both of which are erroneous. Every instrument is dangerous in the hands of ignorance. If a person deficient in dexterity could succeed in applying the traction, (of which
we have strong doubts, believing it would require, in most instances, even Dr. Arnott's skill
and knowledge) it is quite as probable that he would produce injury as benefit. In certain
states of labour, the tractor may be applied to the neck of the uterus instead of the head
of the child, or to both, drawing out the uterus thus as well as the child; it may be applied
before the uterus is sufficiently dilated, or the force may be applied in the wrong direction; indeed, there are but few cases in which force could be applied in the proper direction with the tractor, &c. These accidents cannot happen to the well instructed; but in the
hands of such, the forceps are more effectual and equally safe. The tractor, then, requiring
skill for its proper application, and being a less efficient instrument than the forceps, ought
not, independent of many otherreasons, to be recommended. It is not to those who devise
imperfect substitutes for valuable instruments, or temporary palliatives for important operations, in order that the awkward and ignorant may imperfectly perform what the skilful
or instructed should only attempt, or are capable of accomplishing, that praise is to be
awarded. It is the just meed of those who furnish proper instructions for the use of instruments and for performing operations, and present the means of gaining information and
skill.-Am. Ed.




PELVIC APPARATUS.                           479
While the author directs the attention of the profession to the important
physical considerations set forth in the preceding pages, he deems it necessary most pointedly to remark, that in the living body mechanical principles
are generally associated in their operation with the more recondite principles
of chemistry and of life; and that the man who allows his mind to dwell
boo exclusively on any one of the three classes, must be a very bad reasoner
in questions either of health or disease. It is within a very recent period,
however, that just views on this subject have begun to prevail, and that the
titles of the peculiarly mechanical physician, or chemical physician, or physician attending only to the influence of nerves or life, are likely to be no
longer justly applicable. The light of true philosophy is at last breaking in
upon the very complex and difficult subjects of medical inquiry; and where
formerly keen penetration beheld only confusion, even common minds now
begin to see clear divisions and beautiful arrangement.








THEICAL TA
ANALYTICAL TABL E.
INTRODUCTION.
Progress of man and stationary condition of inferior aniimals.
The progress more rapid at present than ever.
The divisions of human knowledge.
Natural philosophy particularly considered.
SYNOPSIS.
The fundamental truths of natural philosophy explained under the terms
atoms, attraction, repulsion, and inertia-the divisions of this work,
page 19.
PART I.-The FOUR FUNDAMENTAL TRUTHS minutely examined
and used to explain the general constitution of material substances, and
of the motions going on among them, page 22.
SECT. I.-CONSTITUTION OF MATERIAL MASSES, 22.
ATOMS-minute-indestructible-occupying space, 22.
ATTRACTION of atoms is mutual, 25.
Gravitation, 26-Cohesion, 27-Capillary attraction, 28 —Chemical attraction, 28 —Definite proportions, 29.
REPULSION —The influence of heat on masses, 30.
Solid-liquid —air, 31.
Repulsion of surfaces, 32.
Mlod',fication of Masses: 33-Crystal, 33-Porosity, 34-Density, 35,-Hardness, 36-Elasticity, 37 —Brittloness.   38 —
Malleability, 38-Ductility, 38-Pliancy, 39 -Tenacity, 39.
APPENDIX, Properties of Matter, 41.
SECT. II. —IOTIONS AMONG BODIES, 41.
Motion and Rest;
INERTIA of matter, 42.
Motion is naturally permanent, 46 —uniform,  47- straighllt, 47.
Centripetal and centrifugal forces, 49.
Quantity of motion and force, 53 —momentum, 54-.
Direction of forces and composition of motion, 55.
The two forces of nature are Attraction and Fliepalsion, 58.
Accelerated motion, 58.
Retarded motion, 60.
Pendulum and balance wheel, 60.
Projectiles, 64.
Tides, winds, currents, &c.., obey atrfract;n, 66.
Explosion, steam, &c., obey repulsion, 66.
31




482                  ANALYTICAL TABLE.
All great velocities are results of continued action, and are destroyed by continued action, 67.
Action and reaction equal and contrary, 70.
APPENDIX, -Definitions, 74-Laws of motion, 75-Impulsive
force and rectilinear motion, 75-IConstant force and constantly
accelerated motion, 76-Of Gravity, 77-Motion produced by
joint forces, 77-Equilibriums, 78-Joint action of an impulsive
and constant force, 78-Laws of central forces, 80-Joint effect
of active and inactive forces, 80 —Pendulum, 80 —Impact of
bodies, 83.
PART II. —DOCTRINES OF SOLIDS or MECHANICS, 84.
Centres of inertia and gravity, 85.
In animal bodies, 90-Sea sickness, 91.
Centres of percussion and oscillation, 93.
Solids moving round a centre, or so that different parts may have
different speed, 94.
Simple machines, 95.
Lever, 95-Wheel and Axle, 103-Inclined plane, 105 —
Wedge, 106, —Screw, 107 — Pulley, 108-Engine of oblique
action, 109.
Fly wheels, 111.
Complex machines, 113.
Friction, 114.
Wheel carriages, 115-Rail-ways, 11]6.
Strength of materials. 119.
Influence of form —Arches, &c., 122.
PART III.-DOCTRINES OF FLUIDS or HYDRODYNAMICS, 126.
SECT. I.-lHYDROSTATICS, or fluids in repose, 126.
Pressure in a fluid extends equally through the whole, 127.
Hydrostatic press, &c., 129.
Pressure in a fluid increases with the depth, 129.
Compressibility of water, &c., 131.
Not influenced by shape of vessel, 132.
Level surface of fluids, 133.
Spirit level, 133-Canals, 134-Running streams, 135gradual change of the earth's surface produced by running
water, 136.
Same level in communicating vessels, 139.
City water works, 140.
Springs and wells, 141.
Support of bodies floating in fluids, 143.
Specific gravities, 144.
Floating bodies, 148.
Swimming of.man and inferior animals, 149.
Ballast, 152.
Fluids of different density, 153.




ANALYTICAL  TABLE.                         483
SECT. II.-PNEUMATICS, or phenomena of air, 156.
Lightness, 158.
Elasticity, 158.
Air-pumps, 159-Diving-bell, 162-Water-balloon,, 163Hiero's fountain, 164.
Pressure in all directions, 165.
Pressure as the depth, 165.
Weight of the atmosphere, 167.
Atmospheric pressure on solids, 167.
Magdeburgh hemispheres, 168.
Pneumatic tractor, 168.
Atmospheric pressure on liquids, 169.
Pumps, 171-Syphon, 172 —intermitting fountains, 173Bird-glass, 174-Vent-plugs, 174.
Atmospheric pressure on animal body, 175.
Cupping, &c., 176.
The barometer, 176.
Atmospheric pressure determines the liquid or aeriform state of
aeriform substances, 182.
Boiling, 182.
-at different heights, 183.
-in vacuo and distilling, 184.
Elastic force of steam, 186.
Steam-engines, 187.
Explosions, 1907 193.
Atmospheric pressure affecting combinations of bodies, 194.
Effervescence-sparkling liquids, 194.
Atmospheric pressure affecting the density and temperature of the
air, 195.
Climate depending on elevation, 196.
Atmospheric pressure. affecting the humidity of the air, 196.
Rain, mist, snow, hail, dew, 197-Hygrometer, 198.
Rain and clouds among mountains, 199.
Fluid support of floating in air, 200.
Balloons, 200.
Ascent of flame and smoke, 202.
Chimneys, 203.
Warming and ventilating of houses, 205.
Apartments for consumptive patients, 207.
Winds, 208.-Trade winds, 208.
Land and sea breezes, 209.
Monsoons,.210.
Pneumatic trough, 210.
Gasonieter, 211.
Pneumatic chemistry, 212.
SECT. III.-HYDRAULICS, or fluids in motion, 213.
Fluids moving in channels or issuing from vessels, 213.'Aqueducts, 216.
Fountains and jets, 216.
WVaves, 216.
biMnlemtum and resistance of fluids, 220.




484                 ANALYTICAL TABLE.
Resistance to ships, &c., increases much more rapidly than the
velocity, 220.
Steam-boats, 221.
Paddle-wheels, 221.
Resistance to bodies in air, 222.
Fluid resistance limits many velocities, 22~2.
is influenced by shape of solid, 223.
Water-wheels, 224.
Fluid resistance proportioned to surface of contact, and not to
qulantity of matter, 224.
Projectiles, 225-levigating,   225-Winnowing, 226-Washing
gold dust, 226.
Oblique action of fluids, 226.
Navigation-Sails, 226-Rudder, 227.
Windmills, 229-Feathered arrows, 229-Paper kites, 230.
Lifting fluids, 231.
Buckets-Pumps-Wheels-Water-screw, 231 —Water.ram, 232.
SECT. IV.-AcouSTICS, or doctrines of sound, 234.
Nature of simple sound, 235.
Continued sound or tone, 265 —Grave and sharp sounds, 238.
Musical sounds, 238.
Musical scale, 239.
MIelody-Harmony-Accompaniment-Time, 241.
Tuning-forks, 242.
Musical ear, 245.
Spreading of sound-in solid and fluid, 246-Stethoscope, 247.
Velocity of sound, 248 —Many examples, 248.
Reflection of sound, 248.
Echo-whispering galleries-Ear trumpets-Speaking-trumpets,
* 249.
Animal ear, 252.
I'ART IV. —Doctrines of imponderable substance, 254.
HEAT. 256.
SECT. 1-HEAT, phenomena which it produces, 257.
Properties, 258 —Means of measuring quantity in bodies,'259.]Diffuses itself equally, 259.
Cold, 260.
Conducting power of bodies, 260-Natural covering of animals,
262 —Clothing 262.
Mode in which heat spreads, 266.
By the motion of the particles in fluids, 266.
By radiation, 269.
Expansion of bodies by heat, 274.
Capacity of bodies for heat, 275-Influence of bulk on this,'277 —Of density, 280.




ANALYTICAL TABLE.                          485
Each substance expands in a degree peculiar to itself, 281Expansion of solids, 282-Expansion of fluids, 284-Expansion of gases, 284.
Latent heat, 290.
Boiling, 295.
Distillation, 295.
Evaporation, 295.
Table of temperature, 800-Thermometer, 300.
Absolute quantity of heat, 304.
Influence of heat on chemical combinations, 305.
Influence of heat on vegetables and animals, 307.
Sources of heat, 309-The sun, 309 —Electricity, 311-Combustion, 311-Fuel, 317 —Condensation and friction, 322Functions of animal life, 323.
SEC. II —LIGHT, 325 —SoulSes of, 327.
Becomes less intense as it spreads, 327-Falling on bodies renders them visible, 328.
Shadows, 329, 331.
Velocity of light, 329.
Direction of light, 330.
Transmission of bodies, 332.
Refraction, 333.
Lenses, 339.
Camera obscura, solar microscope, magio lantern, 341.
Eye, 346.
Vision, 348, 352.
Distinct vision, 350.
Short-sightedness, 350
Long-sightedness, 350.
Visual angle, 355.
Apparent size of objects, 357.
Foreshortening, 360.
Perspective, 362, 373.
Intensity of light, shade and colour, 369.
Divergence of rays of light, 374.
Convergence of axes of the eyes, 375.
Cosmorama, 377.
Painting representing motion, 378.
Art of painting, 379.
Telescope, 381.
Mirrors, straight, 386-Curved, 390.
Comparison of light and sound, 392.
Perfection of the eye, 395.
PART V. —ANIMAL AND MEDICAL-PHYSICS.
SECT. I.-ANIMAL MECHANICS, 399.
Skull, &c., 399.
Spine and its distortions, 400.
Limbs and mechanical surgery, 401.




486                 ANALYTICAL TABLE.
Living force, 409-Treadmill, 410.
Surgical instruments, 410.
SECT. II. —ANIMAL HYDROSTATICS AND HYDRAULICS, or Fluidity in
relation to animals, 414.
1. Circulation of blood.
In arteries, 416.
In capillaries, 421.
In veins, 423.
Force of the heart, 430.
Hydrostatic or floating bed, 431.
Velocity of blood, 435.
The pulse, 436.
Circulation in the head, 440.
Effects of position on the circulatibn, 441.
Fainting from diminished arterial tension, 442.
Diffused pressure, 444.
Mercurial bath, 445.
Transfusion of blood, 445.
2. Respiration and voice, 445.
Action of chest, 446.
Wounds of chest, 447.
Hemoptysis, 448.
Coughing, 448-Sneezing-Hiccup, &c., 449.
Suffocation, 449.
Humane Society's apparatus, 450.
Artificial respiration, 450.
Speech, 451.
Modifications of voice, 452.
Table of articulations, 455.
Stuttering, 456.
Ventriloquism, 461.
3. Digestion, 463.
Mechanism of the organs, 463.
Effects of abdominal pressure, 465.
Vomiting, 466.
Stomach pump, &c., 466.
Enema funnel, 468.
4. Secretion of the kidneys, 469.
The apparatus.
Obstructions in urethra, 470.
New instruments and means for treatment, 471.
Stone in the bladder, 474.
New instruments and means, 475.
5. Uterine phenomena.
Protection of footus by the liquor amnii, 477.
Position of ditto, 477.
Importance of physical knowledge and manual dexteirty, 478
THE END.












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Practice," "Todd and Bowman's P1hysiology," "Malgaigne's Surgery," "West on Children,"
West on Females, Part I.,"'" Habershon on the Alimeiitary Canal," &c.
While in the number for January, 1860, is commenced a new and highly important work,
CLINICAL LECTURES ON THE DISEASES OF WOMEN.
BY PROFESSOR J. Y. SIMIPSON, of Edinburgh.
WNITH NUMEROUS IHANDSOMIE ILLUSTRATIONS.
These Lectures, published in England under the supcrvision of the Author, carry with them all
the weight of his wide experience and distinguished reputatioii. Their enminently practical nature,
and the importanlce of the subject treated, catunot all to render them in the highest degree satistactory to subscribers, who can thus secure them without cost. The present is therefore a particularly eligible time for gentlemen to commence their subscriptions.
Lt will thus be seen that for the small sum of FIVE DOLLARS, paid in advance, the subscr.ber
will obtain a Quarterly and a Monthly periodical,
EMBRACING NEARLY SIXTEEN HUNDRED LARGE OCTAVO PAGES,
mailed to any part of the United States, free of postage.
Those subscribers who do not pay in advance will near in mind that their subscription of Five
Dollars will entitle them to the Journal only, without the News, and that they will be at the expense
of their own postage on the receipt of each number. The advantage of a remittance when ordering the Journal will thus be apparent.
As tne Medical News and Library is in no case sent without advance payment, its subscribers
will always receive it free ot postage.
Remittances of subscriptions can be mailed at our risk, when a certificate is taken from the Postmaster that the money is duly inclosed and forwarded.
Address      BLANCHARD) & LEA, PHILADELPHIA.




AND SCIENTIFIC PUBLICATIONS.                                         3
ASHTON (T. J.),
Surgeon to the Blenheim Dispensary, &c.
ON THE DISEASES, INJURIES, AND MALFORMIATIONS OF THE
RECTUM AND ANUS; with remarks on Habitual Constipation. From the third and enlarged
London edition. With handsome illustrations. In one very beautifully printed octavo volume,
of about 300 pages. (Now Ready.)  $2 00.
INTRODUCTION. CHAPTER I. Irritation and Itching of the Anus. II. Inflammation and Excoriation of the Anus. IlI. Excrescences of the Anal Region. IV. Contraction of the Arus.  V.
Fissure of the Anus and lower part of the Rectum.  V[. Neuralgia of the Anus and extretnity
of the Rectum. VII. Inflammation of the Rectutn. V1[I. Ulceration of the Rectum. IX. HIemorrhoidal Affections. X. Enlargement of Hemorrhoidal Veins. XI. Prolapsus of the Rectum.
XII. Abscess near the Rectum. X[II. Fistula in Alio. XIV. Polypi of the Rectum. XV. Stricture of the Rectum. XVI. Maligltant Diseases of the Rectumn. XVII. Injuries of the Rectum.
XVIII. Foreign Bodies in the Rectum. X[X. Malformationls of the Recttumn. XX. Habitual
Constipation.
The most complete one we possess on the subject. reach of American practitioners. WVe are satisfied
Medico-Chirurgical Review.                    after a careful examination of the volunle, and a
comnparison of its contents with those of its leading
Its merits as a practical instructor, well arranged, predecessors and contemporaries, that the best way
abundantly furnished with illustrative cases, and for the reader to avail himself of the excellent adclearly anti comprehensively, albeit too diffusely, vice given in theconcludingparagraph above, would
written, are incontestable. They have been suffi- be to provide himself with a c:)py of tie book from
ciently endorsed by the verdict of his countrymen which it has been taken, and diligently to con its
in the rapid exhaustion of the first edition, and thev instructive pages. They may secure to him many
would certainly meet with a similar reward in the a triumph and fervent blessing.-Am. Journal Med.
United States were the volume placed within the Sciences, April, 1858.
ALLEN   (J. M.), M. D.,
Professor of Anatomy in the Pennsylvania Medical College, &c.
THE  PRACTICAL  ANATOMIST; or, The Student's Guide in the DissectingROOM. With 266 illustrations. In one handsome royal 12mo. volume, of over 600 pages, leather. $2 25.
However valuable may be the c" Dissector's ally recommend it to their attention.-Western LanGuides" which we, of late, have had occasion to  ce.
notice, we feel confident that the work of Dr. Allen   We believe it to be one of the most useful works
is superior to any of them. We believe with the upon the subject ever written. It is handsomrely
author, that none is so fully illustrated as this, and illustrated, well printed, and will be found of conthe arrangement of the work is such as to facilitate  venient size for use in the dissecting-room.-tIted.
the labors of the student in acquiring a thorough  Examiner.
practical knowledge of Anatomy. We most cordiANATOMICAL ATLAS.
By Professors H. H. SMITH and W. E. HORNEKI,  of the University of Pennsylvania. 1 vol. 8vo., extra cloth, with nearly 650 illustrattons. 1F' See S.IITH, p. 27.
ABEL (F. A.), F.C.S. AND C. L. BLOXAM.
HANDBOOK  OF  CHEMISTRY, Theoretical, Practical, and Technical; with a
Recommendatory Preface by Dr. HOFMANN. In one large octavo volume, extra cloth, of 662
pages, with illustrations. $3 25.
ASHWELL (SAMUEL), M. D.,
Obstetric Physician and Lecturer to Guy's Hospital, London.
A PRACTICAL TREATISE ON THE DISEASES PECULIAR TO WOMEN.
Illusstrated by Cases derived from Hospital and Private Practice. Third American, from the Third
and revised London edition. In one octavo volume, extra cloth, of 528 pages. $3 00.
The most useful practical work on the subject in   The most able, and certainly the most standard
the English language. — Boston AVed. and Surg. and practical, work on female diseases that we have
Journal.                                      yet seen.-IvMedico-Chirurgical Review.
ARNOTT (NEILL), M. D.
ELEMENTS   OF   PHYSICS;  or Natural Philosophy, General and  Medical.
Written for universal use, in plain or non-technical language. A new edition, by ISAAC HAYS,
M. D. Complete in one octavo volume, leather, of 484 pages, with about two hundred illustrations. $2 50.
BIRD (GOLDING), A. M., M. D., &c.
URINARY  DEPOSITS: THEIR  DIAGNOSIS, PATHOLOGY, AND
THERAPEUTICAL INDICATIONS. Edited by EDMUND LLOYD BIRKETT, M. D. A new
American, from the fifth and enlarged London edition. With eighty illustrations on wood. In one
handsome octavo volume, of about 400 pages, extra cloth. $2 00. (Jlist Issued.)
The death of Dr. Bird has rendered it necessary to entrust the revision of the present edition to
other hands, and in his performance of the duty thus devolving on him, Dr. Birkett has sedulously
endeavored to carry out the author's plan by introducing such new matter and modifications of
the text as the progress of science has called for. Notwithstanding the utmost care to keep the
work within a reasonable compass, these additions have resulted in a considerable enlargement.
It is, therefore, hoped that it will be found fully up to the present condition of the subject, and that
the reputation of the volume as a clear, complete, and compendious manual, will be fully maintained.




4                      BLANCHARD & LEA'S MEDICAL
BUDD (GEORGE), M. D., F. R. S.,
Professor of Medicine in King's College, London.
ON DISEASES OF THE LIVER. Third American, from the third and
enlarged London edition. In one very handsome octavo volume, extra cloth, with four beautifully colored plates, and numerous wood-cuts. pp. 500. $3 00.
iH-as fairly established for itself a place among the is not perceptibly changed, the history of liver disclassical medical literature of England.-British  eases is made more complete, and is kept upon a level
and Foreign Medico-Chir. Review, July, 1857.  with the progress of modern science. It is the best
Dr. Budd's Treatise on Diseases of the Liver is work on Diseases of the Liver in any language.now a standard work in Medical literature, and dur- London l Med. Times and Gazette, June 27, 1S57.
ing the intervals which have elapsed between the   This work, now the standard book of reference on
successive editions, the author has incorporated into the diseases of which it treats, has been carefully
the text the most striking novelties which have cha- revised, and many new illustrations of the views of
racterized the recent progress of hepatic physiology  the learned author added in the present edition.and pathology; so that although the size of the book Dublin Quarterly Journal, Aug. 1857.
BY THE SAME AUTHOR.
ON THE ORGANIC DISEASES AND FUNCTIONAL DISORDERS OF
THE STOMACH. In one neat octavo volume, extra cloth. $1 50.
BUCKNILL  (J. C.), M. D.,
Medical Superintendent of the Devon County Lunatic Asylum; and
DANIEL  H. TUKE, M. D.,
Visiting Medical Officer to the York Retreat.
A MANUAL OF PSYCHOLOGICAL MEDICINE; containing the History,
Nosology, Description, Statistics, Diagnosis, Pathology, and Treatment of INSANITY. With
a Plate. In ore handsome octavo volume, of 536 pages. $3 00.
The increase of mental disease in its various forms, and the difficult questions to which it is
constantly giving rise, reader the subject one of daily enhanced interest, requiring on the part of
the physician a constantly greater familiarity with this, the most perplexing branch of his profession. At the same time there has been for some years no work accessible in this country, presenting the results of recent investigations in the Diagnosis and Prognosis of Insanity, and the greatly
improved methods of treatment which have done so much in alleviating the condition or restoring
the health of the insane. To fill this vacancy the publishers present this volume, assured that
the distinguished reputation and experience of the authors will entitle it at once to the confidence
of both student and practitioner. Its scope may be gathered from the declaration of the authors
that c" their aim has been to supply a text book which may serve as a guide in the acquisition of
such knowledge, sufficiently elementary to be adapted to the wants of the student, and sufficiently
modern in its views and explicit in its teaching to suffice for the demands of the practitioner."
BENNETT  (J. HUGHES), M. D., F. R. S. E.,
Professor of Clinical Medicine in the University of Edinburgh, &c.
THE PATHOLOGY AND TREATMENT OF PULMONARY TUBERCULOSIS, and on the Local Medication of' Pharyngeal and Laryngeal Diseases frequently mnistaken
fobr or associated with, Phthisis. One vol. 8vo.,extra cloth, with wood-cuts. pp. 130. $1'25.
BENNETT (HENRY), M. D.
A PRACTICAL TREATISE ON INFLAMMATION OF THE UTERUS,
ITS CERVIX AND APPENDAGES, and on its connection with Uterine Disease. To whi ch
is added, a Review of the present state of Uterine Pathology. Fifth American, from the third
English edition. In one octavo volume, of about 500 pages, extra cloth. $2 00. (Now Reeady.)
The ill health of the author having prevented the promised revision of this work, the present
edition is a reprint of the last, without alteration. As the volume has been for some time out (f
print, gentlemen desiring copies can now procure them.
BOWMAN (JOHN E.), M.D.
PRACTICAL HANDBOOK OF MEDICAL CHEMISTRY. Second American, from the third and revised English Edition. In one neat volume, royal 12mo., extra cloth,
with numerousillustrations. pp. 288. $1 25.
BY THE SAME AUTHOR.
INTRODUCTION TO PRACTICAL CHEMISTRY, INCLUDING ANALYSIS. Second American, from the second and revised London edition. With numerous illustrations. In one neat vol., royal 12mo., extra cloth. pp. 350. $1 25.
BEALE ON THE LAWS OF HEALTH IN RE- BUCKLER ON THE ETIOLOGY,PATHOLOGY,
LATION TO MIND AND BODY. A Series of  AND TREATMENT OF FIBRO-BRONCHILetters from an old Practitioner to a Patient. In   TIS AND RHEUMATIC  PNEUMONIA.  In
one volume, royal 12mo., extra cloth. pp. 296.  one 8vo. volume, extra cloth. pp. 150. $1 25.
80 cents.                                    BLOOD  AND  URINE (MANUALS ON). BY
B3USHNAN'S PHYSIOLOGY OF ANIMAL AND   JOHN  WILLIAM  GRIFFITH, G. OWEN
VEGETABLE LIFE; a Popular Treatise on the   REESE, AND ALFRED  MARKWICK.  One
Functions and Phenomena of Organic Life. In   thick volume, royal 12mo., extra cloth, with
one handsome royal 12mo. volume, extra cloth,   plates. pp. 460. $1 25.
with over 100 illustrations. pp. 234. 80 cents.   BRODIE'S CLINICAL LECTURES ON SURGERY. 1vol.Svo. cloth. 350pp. $1 25.




AND SCIENTIFIC PUBLICATIONS.                                          5
BARCLAY (A. W.), M. D.,
Assistant Physician to St. George's Hospital, &c.
A MANUAL OF MEDICAL DIAGNOSIS; being an Analysis of the Signs
and Symptoms of Disease. In one neat octavo volume, extra cloth, of 424 pages. $2 00. (Lately
issued.)
Of works exclusively devoted to this important j The task of composing such a work is neither an
branch, our profession has at command, compara- easy nor a light one; but Dr. Barclay has performed
tively, but few, and, therefore, in the publication of it in a manner which meets our most unqualified
the present work, Messrs. Blanchard & Lea have approbation. He is no mere theorist; he knows his
conferred a great favor upon us. Dr. Barclay, from  work thoroughly, and in attempting to perform it,
having occupied, for a long period, the position of has not exceeded his powers.-British Med. Journal,
Medical Registrar at St. George's Hospital, pos- Dec. 5, 1857.
sessed advantages for correct observation and reli-We venture to predict that the work will be deable conclusions, as to the significance of symptoms, servedly popular, and soon become, like Watson's
which have fallen to the lot of but few, either in Practice, an indispensable necessity to the practihis own or any other country. He has carefully  tioner.-N. A. Med. Journal, April, 1858.
systematized the results of his observation of over
twelve thousand patients, and by his diligence and   An inestimable work of reference for the young
judicious classification, the profession has been practitioner and student.-Nashville Med. Journal,
presented with the most convenient and reliable May, 1858.
work on the subject of Diagnosis that it has been   We hope the volume will have an extensive cirour good fortune ever to examine; we can, there- culation, not among students of medicine only, but
fore, say of Dr. Barclay's work, that. from his sys- practitioners also. They will never regret a faithtematic manner of arrangement, his work is one of ful study of its pages. —Cincinnati L-ancet, Mar.'58.
the best works " for reference" in the daily emer-   An important acquisition to medical literture.
gencies of the practitioner, with which we are ac- It is a work of high merit, both from the vast imquainted; but, at the same time, we would recom-portance of the subJect upon which it treats, and
mend our readers, especially the younger ones, to also from the real ability displayed in its elaboraread thoroughly and study diligently thewhole work, tion. In conclusion, let us bespeak for this volume
and the "emergencies" will not occur so often.- that attention of every student of our art which it
Socuthern Med. and Suerg. Journ., March, 1858.  so richly deserves - that place in every medical
To give this information, to supply this admitted library which it can so well adorn.-Peninsular
deficiency, is the object of Dr. Barclay's Manual. Medical Journal, Sept. 1858.
BARLOW (GEORGE H.), M. D.
Physician to Guy's Hospital, London, &c.
A MANUAL OF THE PRACTICE OF MEDICINE. With Additions by D.
F. CONDIE, M.D., author of" A Practical Treatise on Diseases of Children," &c. In one handsome octavo volume, leather, of over 600 pages. $2 75.
WTe recommend Dr. Barlow' s Manual in the warm- will be found hardly less useful to the experienced
est manner as a most valuable vade-mecum. We physician. The American editor has added to tile
have had frequent occasion to consult it, and have work three chapters-on Cholera Infantum, Yellow
found it clear, concise, practical, and sound. It is Fever, and Cerebro-spinal Meningitis. These addieminently a practical work, containing all that is tions, the two first of which are indispensable to a
essential, and avoiding useless theoretical discus- work on practice destined for the profession in this
sion. The work supplies what has been for some country, are executed with great judgment and fitime wanting, a manual of practice based upon mo- delity, by Dr. Condie, who has also succeeded hapdern discoveries in pathology and rational views of pily in imitating the conciseness and clearness of
treatment of disease. It is especially intended for style which are such agreeable characteristics of
the use of students and junior practitioners, but it the original book.-Boston Mled. and Surg. Journal.
BARTLETT (ELISHA), M. D.
THE HISTORY, DIAGNOSIS, AND TREATMENT OF THE FEVERS
OF THE UNITED STATES. A new and revised edition. By ALONZO CLARK, M. D., Prof.
of Pathology and Practical Medicine in the N. Y. College of Physicians and Surgeons, &c. In
one octavo volume, of six hundred pages, extra cloth. Price $3 00.
It is the best work on fevers which has emanated logy. His annotations add much to the interest of
from the American press, and the present editor has the work, and have brought it well up to the condlcarefully availed himself of all information exist- tion of the science as it exists at the present (lay
ing upon the subject in the Old and New World, so in regard to this class of diseases.-Southern Iled.
that the doctrines advanced arebrought down to the and Surg. Journal, Mar. 1857.
latest date in the progress of this department of   It is a work of great practical value and interest
_M.e,.d.ic              It is a  work of great practical value ane interest,
Medical  7ience.-London Med. Times and Gazette, containing much that is new relative to the several
May'2, lh,7.                                  diseases of which it treats, and, with the additions
This excellent monograph on febrile disease, has of the editor,is fully up to the times. The distinctstood deservedly high since its first publication. It ive features of the different forms of fever are plainly
will be seen that it has now reached its fourth edi- and forcibly portrayed, and the lines of demarcation
tion under the supervision of Prof. A. Clark, a gen- carefully and accurately drawn, and to the Ameritleman who, from the nature of his studies and pur- can practitioner is a more valuable and safe guide
suits, is well calculated to appreciate and discuss than any work on fever extant,-Ohio Med. and
the many intricate and difficult questions in patho- Surg. Journal, May, 1857.
BROWN (ISAAC BAKER),
Surgeon-Accoucheur to St. Mary's Hospital, &c.
ON SOME DISEASES OF WOMEN ADMITTING OF SURGICAL TREATMENT. With handsome illustrations. One vol. 8vo., extra cloth, pp. 276. $1 60.
Mr. Brown has earned for himself a high reputa- and merit the careful attention of every surgeontion in the operative treatment of sundry diseases accoucheur.-Association Journal.
and injuries to which females are peculiarly subject.
We can truly say of his work that it is an important   We have no hesitation in recommending this book
addition to obstetrical literature. The operative to tlre careful attention of all surgeons who make
suggestions and contrivances which Mr. Brown de- female complaints a part of their study and practie.
scribes, exhibit much practical sagacity and skill, -Dublin Quarterly Journal.




6                     BLANCHARD & LEA'S MEDICAL
CARPENTER (WILLIAM B.), M. D., F. R. S., &c.,
Examiner in Physiology and Comparative Anatomy in the University of London.
PRINCIPLES  OF HUMAN  PHYSIOLOGY; with their chief applications to
Psychology, Pathology, Therapeutics, Hygiene, and Forensic Medicine. A new American, from
the last and revised London edition. With nearly three hundred illustrations. Edited, with additions, by FRANCIS GURNEY SMITH, M. D., Professor of the Institutes of Medicine in the Pennsylvania Medical College, &c. In one very large and beautiful octavo volume, of about nine hundred
large pages, handsomely printed and strongly bound in leather, with raised bands. $4 25.
In the preparation of this new edition, the author has spared no labor to render it, as heretofore,
a complete and lucid exposition of the most advanced condition of its important subject. The
amount of the additions required to effect this object thoroughly, joined to the former large size of
the volume, presenting objections arising firom the unwieldy bulk of the work, he has omitted all
those portions not bearing directly upon HUMAN PHYSIOLOGY, designing to incorporate them in
his forthcoming Treatise on GENERAL PHYSIOLOGY. As a full and accurate text-book on the Physiology of Man, the work in its present condition therefore presents even greater claims upon
the student and physician than those which have heretofore won for it the very wide and distinguished favor which it has so long enjoyed. The additions of Prof. Smith will be found to supply
whatever may have been wanting to the American student, while the introduction of many new
illustrations, and the most careful mechanical execution, render the volume one of the most attractive as yet issued.
For upwards of thirteen years Dr. Carpenter's  To eulogize this great work would he superfluous.
work has been considered by the profession gene- WVe should observe, however, that in this edition
rally, both in this country and England, as the most the author has remodelled a large portion of the
valuable compendium on the subject of physiology former, and the editor has added much matter of inin our language. This distinction it owes to the high terest, especially in the form of illustrations. We
attainments and unwearied industry of its accom- may confidently recommend it as the most complete
plished author. The present edition (which,like the work on Human Physiology in our language.last American one,was prepared by the author him- Southern Med. and Surg. Journal.
self), is the result of such extensive revision, that it  The most complete work on the science in our
may almost be considered a new work. We need language.-Am. led. Journal.
hardly say, in concluding this brief notice, that while   m
the work is indispensable to every student of medi-   The most complete work now extant in our lan
sine in this country, it will amply repay the practi-guage.-N. 0. Med. Register.
tioner for its perusal by the interest and value of its   The best text-book in the language on this excontents.-Boston Med. and Surg. Journal.     tensive subject.-London Med. Times.
This is a standard work-the text-book used by all  A complete cyclopaedia of this branch of science.
medical students who read the English language. -N. Y. Med. Times.
It has passed through several editions in order to   The profession of this country, and perhaps altse
keep pace with the rapidly growing science of Phy- of Europe, have anxiously and for some time awaited
siology. Nothing need be said in its praise, for its the announcement of this new edition of Carpenter's
merits are universally known; we have nothing to Human Physiology. His former editions have for
say of its defects, for they only appear where the many years been almost the only text-book on Physcience of which it treats is incomplete.-Western siology in all our medical schools, and its circulaLancet.                                      tion among the profession has been unsurpassed by
The most complete exposition of physiology which any work in any department of medical science.
any language can at present give.-Brit. and For.  It is quite unnecessary for us to speak of this
Med.-Chirurg. Review.                        work as its merits would justify. The mere announcement of its appearance will afford the highest
The greatest, the most reliable, and the best book pleasure to every student of Physiology, while its
on the subject which we know of in the English perusal will be of infinite service in advancing
language.-Stethoscope.                        physiological science.-Ohio Med. and Surg. Journ.
BY THE SAME AUTHOR.
PRINCIPLES OF COMPARATIVE PHYSIOLOGY. New American, from
the Fourth and Revised London edition. In one large and handsome octavo volume, with over
three hundred beautiful illustrations. pp. 752. Extra cloth, $4 80; leather, raised bands, $5 25.
The delay which has existed in the appearance of this work has been caused by the very thorough
revision and remodelling which it has undergone at the hands of the author, and the large number
of new illustrations which have been prepared for it. It will, therefore, be tbund alnost a new
work, and fully up to the day in every department of the subject, rendering it a reliable text-book
for all students engaged in this branch of science. Every effort has been made to render its typographical finish and mechanical execution worthy of its exalted reputation, and creditable to the
mechanical arts of this country.
This book should not only be read but thoroughly no man, we believe, could have brought to so sucstudied by every member of the profession. None cessful an issue as Dr. Carpenter. It required for
are too wise or old, to be benefited thereby. But its production a physiologist at once deeply read in
especially to the younger class would we cordially the labors of others, capable of taking a general,
commend it as best fitted of any work in the English critical, and unprejudiced view of those labors. and
language to qualify them for the reception and coin- of combining the varied, heterogeneous materials at
prehension of those truths which are daily being de- his disposal, so as to form an harmonious whole.
veloped in physiology.-Medical Counsellor.   We feel that this abstract can give the reader a very
Without pretending to it, it is an encyclopedia of imperfect idea of the fulness of this work, and no
the subject, accurate and complete in all respects- idea of its unity, of the admirable iianner in which
a truthful reflection of the advanced state at which material has been brought, from the most various
the science has now arrived.-Dublin Quarterly sources, to conduce to its completeness, of the lucidJournal of Medical,Science.                 ity of the reasoning it contains, or of the clearness
of language in which the whole is clothed. Not the
A truly magnificent work-in itself a perfect phy- profession only, but the scientific world at large,
siological study.-Ranking's Abstract.         must feel deeply indebted to Dr. Carpenter for this
This work stands without its fellow. It is one great work. It must, indeed, add largely even to
few men in Europe could have undertaken; it is one his high reputation. —~ledical Times.




AND SCIENTIFIC PUBLICATIONS                                        7
CARPENTER (WILLIAM B.), M. D., F. R. S.,
Examiner in Physiology and Comparative Anatomy in the University of London.
THE MICROSCOPE AND ITS REVELATIONS. With an Appendix containing the Applications of the Microscope to Clinical Medicine, &c. By F. G. SMITH, M. D.
Illustrated by four hundred and thirty-four beautiful engravings on wood. In one large and very
handsome octavo volume, of 724 pages, extra cloth, $4 00; leather, $4 50.
Dr. Carpenter's posltlon as a mlcroscopist and physiologist, and his great experience as a teacher,
eminently qualify him to produce what has long been wanted-a good text-book on the practical
use of the microscope. In the present volume his object has been, as stated in his Preface, (' to
combine, within a moderate compass, that information with regard to the use of his'tools,' which
is most essential to the working microscopist, with such an account of the objects best fitted for
his study, as might qualify him to comprehend what he observes, and might thus prepare him to
benefit science, whilst expanding and refreshing his own mind "  That he has succeeded in accomplishing this, no one acquainted with his previous labors can doubt.
The great importance of the microscope as a means of diagnosis, and the number of microscopists who are also physicians, have induced the American publishers, with the author's approval. to
add an Appendix, carefully prepared by Professor Smith, on the applications of the instrument to
clinical medicine, together with an account of American Microscopes, their modifications and
accessories. This portion of the work is illustrated with nearly one hundred wood-cuts, and, it is
hoped, will adapt the volume more particularly to the use of the American student.
Every care has been taken in the mechanical execution of the work, which is confidently presented as in no respect inferior to the choicest productions of the London press.
The mode in which the author has executed his intentions may be gathered from the following
condensed synopsis of' the
CONTENTS.
INTRODUCTION-History of the Microscope.  CHAP. I. Optical Principles of the Microscope.
CHAP. II. Construction of the Microscope. CHAP. III. Accessory Apparatus. CHAP. IV.
Management of the Microscope CHAP. V. Preparation, Mounting, and Collection of Objects.
CHAP. VI. Microscopic Forms of Vegetable Life-Protophytes. CHAP. VII. Higher Cryptogamia. CHAP. VIII. Phanerogamic Plants. CHAP. IX. Microscopic Forms of Animal Life-Pro.
tozoa —Animalcules. CHAP. X. Foraminifera, Polycystina, and Sponges. CHAP. XI. Zoophytes.
CHAP. XII. Echinodermata. CHAP. XIII. Polyzoa and Compound Tunicata. CHAP. XIV.
Molluscous Animals Generally. CHAP. XV. Annulosa. CHAP. XVI. Crustacea. CHAP. XVII.
insects and Arachnida. CHAP. XVIII. Vertebrated Animals. CHAP. XIX. Applications of the
Microscope to Geology. CHAP. XX. Inorganic or Mineral Kingdom-Polarization. APPENDIX.
Microscope as a means of Diagnosis-Injections-Microscopes of American Manufacture.
Those who are acquainted with Dr. Carpenter's medical work, the additions by Prof. Smith give it
previous writings on Animal and Vegetable Physio- a positive claim upon the profession, for which we
logy, will fully understand how vast a store of know- doubt not he will receive their sincere thanks. Inledge he is able to bring to bear upon so comprehen- deed, we know not where the student of medicine
sive a subject as the revelations of the microscope; will find such a complete and satisfactory collection
and even those who have no previous acquaintance of microscopic facts bearing upon physiology and
with the construction or uses of this instrument, practical medicine as is contained in Prof. Smith's
will find abundance of information conveyed in clear appendix; and this of itself, it seems to us, is fully
and simple language. —lMed. Times and Gazette. worth the cost of the volume.-Louisville Mediecal
Although originally not intended as a strictly  Review, Nov. 1856.
BY THE SAME AUTHOR.
ELEMENTS (OR MANUAL) OF PHYSIOLOGY, INCLUDING PHYSIOLOGICAL ANATOMY. Second American, from a new and revised London edition. With
one hundred and ninety illustrations. In one very handsome octavo volume, leather. pp. 566.
$3 00.
In publishing the first edition of this work, its title was altered from that of the London volume,
by the substitution of the word "Elements" for that of "1 Manual," and with the author's sanction
the title of " Elements" is still retained as being more expressive of the scope of the treatise.
To say that it is the best manual of Physiology   Those who have occasion for an elementary treanow before the public, would not do sufficient justice tise on Physiology, cannot do better than to possess
to the author. —Buffalo Medical Journal.    themselves of the manual of Dr. Carpenter.-Medica l
In his former works it would seem that he had Examiner.
exhausted the subjectof Physiology. In the present,  The best and most complete expos6 of modern
he gives theessence, as it were, of the whole.-N. Y. Physiology, in one volume, extant in the English
$oucrnal of Medicixe.                        language.-St. Louis Medical Journal.
BY THE SAME AUTHOR. (Preparing.)
PRINCIPLES OF GENERAL PHYSIOLOGY, INCLUDING ORGANIC
CHEMISTRY AND HISTOLOGY. With a General Sketch of the Vegetable and Animal
Kingdom. In one large and very handsome octavo volume, with several hundred illustrations.
The subject of general physiology having been omitted in the last editions of the author's "( Comparative Physiology" and "Human Physiology," he has undertaken to prepare a volume which
shall present it more thoroughly and fully than has yet been attempted, and which may be regarded
as an introduction to his other works.
BY THE SAME AUTHOR.
A PRIZE ESSAY ON THE USE OF ALCOHOLIC LIQUORS IN HEALTH
AND DISEASE. New edition, with a Preface by D. F. CONDIE, M. D., and explanations of
scientific words. In one neat 12mo. volume, extra cloth. pp. 178. 50 cents.




8                      BLANCHARD & LEA'S MEDICAL
CONDIE (D. F.), M. D., &c.
A PRACTICAL TREATISE ON THE DISEASES OF CHILDREN. Fifth
edition, revised and augmented. In one large volume, 8vo., leather, of over 750 pages. $3 25.
(Just Issued, 1859.)
In presenting a new and revised edition of this favorite work, the publishers have only to state
that the author has endeavored to render it in every respect " a complete and faithful exposition of
the pathology and therapeutics of the maladies incident to the earlier stages of existence-a full
and exact account of the diseases of infancy and childhood." To accomplish this he has subjected
the whole work to a careful and thorough revision, rewriting a considerable portion, and adding
several new chapters. In this manner it is hoped that any deficiencies which may have previously
existed have been supplied, that the recent labors of practitioners and observers have been thoroughly incorporated, ard that in every point the work will be found to maintain the high reputation
it has enjoyed as a complete and thoroughly practical book of reference in infantile affections.
A few notices of previous editions are subjoined.
Dr. Condie's scholarship, acumen, industry, and   We pronounced the first edition to be the best
practical sense are manifested in this, as in all his work on the diseases of children in the English
numerous contributions to science.-Dr. Holmes's language, and, notwithstanding all that has been
Report to the American Medical Association.   published, we still regard it in that light.-Medical
Taken as a whole, in our judgment, Dr. Condie's Examiner.
Treatise is the one from the perusal of which the   The value of works by native authors on the dispractitioner in this country will rise with the great- eases which the physician is called upon to combat,
est satisfaction.-Western Journal of Medicine and will be appreciated by all; andithe work of Dr. ConSurgery.                                      die has gained for itself the character of a safe guide
One of the best works upon the Diseases of Chil- for students, and a useful work for consultation by
dren in the English language.-Western Lancet.   those engaged in practice.-N. Y. Med. Times.
W\Te feel assured from actual experience that no   This is the fourth edition of this deservedly popuphysician's library can be complete without a copy
of this work.-N. Y. Journal of Medicine.      tion, it has been subjected to a thorough revision
by the author; and all new observations in the
A veritable pediatric encyclop edia, and an honor pathology and therapeutics of children have been
to American medical literature.-Ohio Medical and included in the present volume. As we said b(fore,
Surgical Journal.                             we do not know of a better book on diseases of chilWe feel persuaded that the American medical pro- dren, and to a large part of its recommendations we
fession will soon regard it not only as a very good, yield an unhesitating concurrence.-Buffalo Med.
but as the VERY BEST " Practical Treatise on the Journal.
Diseases of Children." —American Medical Journal.  Perhaps the most full and complete work now beIn the department of infantile therapeutics, the fore the profession of the United States; indeed, we
work of Dr. Condie is considered one of the best may say in the English language. It is vastly supewhich has been published in the English language. rior to most of its predecessors.-Transylacnia Med.
-The Stethoscope.                             Journal.
CHRISTISON  (ROBERT), M. D., V. P. R. S. E., &c.
A  DISPENSATORY; or, Commentary on the Pharmacopoeias of Great Britain
and the United States; comprising the Natural History, Description, Chemistry, Pharmacy, Actions, Uses, and Doses of the Articles of the Materia Medica. Second edition, revised and improved, with a Supplement containing the most important New Remedies. With copious Additions, and two hundred and thirteen large wood-engravings. By R. EGLESFELD GRIFFITH, M. D.
In one very large and handsome octavo volume, leather, raised bands, of over 1000 pages. $3 50.
COOPER (BRANSBY B.), F. R. S.
LECTURES ON THE PRINCIPLES AND PRACTICE OF SURGERY.
In one very large octavo volume, extra cloth, of 750 pages. $3 00.
COOPER  ON  DISLOCATIONS AND FRAC- COPLAND ON THE CAUSES, NATURE, AND
TURES OF THE JOINTS.-Edited by BRANSBY   TREATMENT OF PALSY AND APOPLEXY.
B. COOPER, F. R. S., &c. With additional Ob-   In one volume, royal 12mo., extra cloth. pp. 326.
servations by Prof. J. C. WARREN. A new Ame-  80 cents.
rican edition. In one handsome octavo volume,
extra cloth, of about 500 pages, with numerous   MER ON FEVERS; TREATMENT  In one
illustrations on wood. $3 25.                 PATHOLOGY, AND TREATMENT  In one
tCOOPER ON TwE ANATOMY AND DISEASES   octavo volume, leather, of 600 pages. $1 50.
COOPER ON TIlE ANATOMY AND DISEASES
OF THE BREAST, with twenty-five Miscellane- COLOMBAT DE L'ISERE ON THE DISEASES
ous and Surgical Papers. One large volume, im-   OF FEMALES, and on the special Hygiene of
perial 8vo., extra cloth, with 252 figures, on 36  their Sex. Translated, with many Notes and Adplates. $2 50.                                ditions, by C. D. MEIGS, M. D. Second edition,
COOPER  ON  THE STRUCTURE AND DIS-   revised and improved. In one large volume, ocEASES OF THE TESTIS, AND ON  THE   tavo, leather, with numerous wood-cuts. pp. 720.
THYMUS GLAND. One vol. imperial 8vo., ex-   $3 50.
tra cloth, with 177 figures on 29 plates. $2 00.
CARSON (JOSEPH), M. D.,
Professor of Materia Medica and Pharmacy in the University of Pennsylvania.
SYNOPSIS OF THE COURSE OF LECTURES ON MATERIA MEDICA
AND PHARMACY, delivered in the University of Pennsylvania. Second and revised edition. In one very neat octavo volume, extra cloth, of 208 pages. $1 50.
CURLING   (T. B.), F.R.S.,
Surgeon to the London Hospital, President of the Hunterian Society, &c.
A PRACTICAL TREATISE ON DISEASES OF THE TESTIS, SPERMATIC CORD, AND SCROTUM. Second American, from the second and enlarged English edition. In one handsome octavo volume, extra cloth, with numerous illustrations. pp. 420. $2 00.




AND SCIENTIFIC PUBLICATIONS.                                         9
CHURCHILL (FLEETWOOD), M. D., M. R. I. A.
ON THE THEORY AND PRACTICE OF MIDWIFERY. A new American
from the fourth revised and enlarged London edition. With Notes and Additions, by D. FRANCIS
CONDIE, M. D., author of a "Practical Treatise on the Diseases of Children," &c. With 194
illustrations. Tn one verly handsome octavo volume, leather, of nearly 700 large pages. $3 50.
(1Vow Ready, October, 1860.)
This work has been so long an established favorite, both as a text-book for the learner and as a
reliable aid in consultation for the practitioner, that in presenting a new edition it is only necessary
to call attention to the very extended improvements which it has received. Having had the benefit
of two revisions by the author since the last American reprint, it has been materially enlarged, and
Dr. Churchill's well-known conscientious industry is a guarantee that every portion has been thoroughly brought up with the latest results of European investigation in all departments of the science and art of obstetrics. The recent date of the last Dublin edition has not left much of novelty
for the American editor to introduce, but he has endeavored to insert whatever has since appeared,
together with such matters as his experience has shown him would be desirable fo)r the American
student, including a large number of illustrations. With the sanction of the author he has added
in the form of an appendix, some chapters from a little "Manual for Midwives and Nurses," recently issued by Dr. Churchill, believing that the details there presented can hardly fail to prove of'
advantage to the junior practitioner. Tne result of all these additions is that the work now contains fully one-half more matter than the last American edition, with nearly one-half more illustrations, so that notwithstanding the use of a smaller type, the volume contains almost two hundred
pages more than before.
No effort has been spared to secure an improvement in the mechanical execution of the work
equal to that which the text has received, and the volume is confidently presented as one of the
handsomest that has thus far been laid before the American profession; while the very low price
at which it is offered should secure for it a place in every lecture-room and on every office table.
A better book in which to learn these important   The most popular work on midwifery ever issued
points we have not met than Dr. Churchill's. Every  from the American press.-Charleston Med..Tournal.
page of it is full of instruction; the opinion of all Were we reduced to the necessity of having but
writers of authority is given on questions of diffi-  We work  on midwifery, and permitted to choose
culty, as well as the directions and advice of the we would unhesitatingly take Churchill.-Western
learned author himself, to which he adds the result Med. and Surg. Journal.
of statistical inquiry, putting statistics in their pro.
per place and giving them their due weight, and no   It is impossible to conceive a more useful and
more. hWe have never read a book more free from  elegant manual than Dr. Churchill's Practice of
professional jealousy than Dr. Charchill's. It ap- Vidwifery.-Provincial Medical Journal.
pears to be written with the true design of a book on   Certainly, in our opinion, the very best work on
medicine, viz: to give all that is known on the sub- he subject which exists.-N. Y. Annalist.
ject of which he treats, both theoretically and prac-   No work holds a higher position, or is more detically, and to advance such opinions of his own as serving of being placed in the hands of the tyro,
he believes will benefit medical science, and insure the advanced student, or the practitioner.-Medical
the safety of the patient. WVe have said enough to Examiner.
convey to the profession that this book of Dr. Churchill's is admirably suited for a book of reference   Previous editions, under the editorial supervision
for the practitioner, as well as a text-book for the of Prof R. MI. Huston, have been received with
student, and we hope it may be extensively pur- marked favor, and they deserved it; but this, rechased amongst our readers. To them we most printed from a very late Dublin edition, carefully
strongly recommend it.-Dublin  Medical Press, revised and brought up by the author to the present
June 20, i8G0.                                time, does present an unusually accurate and able
exposition of every important particular embraced
To bestow praise on a book that has received such in the department of midwifery. * * The clearn&,s.
marked approbation would be superfluous. Ve need directness, and precision of its teachings, togetnier
only say, therefore, that if the first edition was with the great amount of statistical research which
thought worthy of a favorable reception by the its text exhibits, have served to place it already in
medical publiq, we can confidently affirm that this the foremost rank of works in this department of rewill be found much more so. The lecturer, the medial science.-N. 0. Med. and Surg. Journal.
practitioner, and the student, may all have recourse   In our opinion, it forms one of the best if not the
to its pages, and derive from their perusal much in- very best text-book and epitome of obstetric science
terest and instruction in everything relating to theo- which we at present possess in the English lanretical and practical midwifery.-Dublin Quarterly guage.-Monthly Journal of Medical Science.
Journal of Medical Science.
The clearness and precision of style in which it is
A work of very great merit, and such as we can written, and the great amount of statistical research
confidently recommend to the study of every obste- which it contains, have served to place it in tlhe first
tric practitioner.-London Medical Gazette.    rank of works in this departmentof medical science.
This is certainly the most perfect system extant. -N. Y. Journal of Medicine.
It is the best adapted for the purposes of a text-   Few treatises will be found better adapted as a
book, and that which he whose necessities confine text-book for the student, or as a manual for the
him to one book, should select in preference to all frequent consultation of the young practitioner. —
others.-Southern Medical and Surgical Journal.   American Medical Journal.
BY THE SAME AUTHOR. (Lately Pzblished.)
ON THE DISEASES OF INFANTS AND CHILDREN. Second American
Edition, revised and enlarged by the author. Edited, with Notes, by W. V. KEATING, M. D. In
one large and handsome volume, extra cloth, of over 700 pages. $3 00, or in leather, $3 25.
In preparing this work a second time for the American profession, the author has spared no
labor in giving it a very thorough revision, introducing several new chapters, and rewriting others,
while every portion of the volume has been subjected to a severe scrutiny. The efforts of the
American editor have been directed to supplying such information relative to matters peculiar
to this country as might have escaped the attention of the author, and the whole may, therefore, be safely pronounced one of the most complete works on the subject accessible to the American Profession. By an alteration in the size of the page, these very extensive additions have
been accommodated without unduly increasing the size of the work.
BY THE SAME AUTHOR.
ESSAYS ON THE PUERPERAL FEVER, AND OTHER DISEASES PECULIAR TO WOMEN. Selected from the writings of British Authors previous to the close of
the Eighteenth Century. In one neat octavo volume, extra cloth, of about 450 pages. $2 50.




10                    BLANCHARD & LEA'S MEDICAL
CHURCHILL (FLEETWOOD), M. D., M. R. I. A., &c.
ON  THE  DISEASES  OF WOMEN; including those of Pregnancy and Childbed. A new American edition, revised by the Author. With Notes and Additions, by D. FRANcIs CONDIE, M. D., author o1 " A Practical Treatise on the Diseases of Children."  With numerous illustrations. In one large and handsome octavo volume, leather, of 768 pages. $3 00.
This edition of Dr. Churchill's very popular treatise may almost be termed a new work, so
thoroughly has he revised it in every portion. It will be found greatly enlarged, and completely
brought up to the most recent condition of the subject, while the very handsome series of' illustrations introduced, representing such pathological conditions as can be accurately portrayed, present
a novel feature, and afford valuable assistance to the young practitioner. Such additions as appeared desirable for the American student have been made by the editor, Dr. Condie, while a
marked improvement in the mechanical execution keeps pace with the advance in all other respects
which the volume has undergone, while the price has been kept at the former very moderate rate.
It comprises, unquestionably, one of the most ex- I extent that Dr. Churchill does. His, indeed. is the
act and comprehensive expositions of the present only thorough treatise we know of on the subject;
state of medical knowledge in respect to the diseases and it ma) be commended to practitioners and stuof women that has yet been published.-Am. Journ. dents as a masterpiece in its particular department.
Afed. Sciences, July, 1857.                  -The Western Journal of Medicine and Surgery.
This work is the most reliable which we possess   As a comprehensive manual for students, or a
on this subject; and is deservedly popular with the work of reference for practitioners, it surpasses any
profession.-Charleston Med. Journal, July, 1857.  other that has ever issued on the same subject from
We know of no author who deserves that appro- I the British press. —Dublin Quart. Journal.
bation, on " the diseases of females,"' to the same
DICKSON  (S. H.), M. D.,
Professor of Practice of Medicine in the Jefferson Medical College, Philadelphia.
ELEMENTS  OF  MEDICINE; a Compendious View  of Pathology and Therapeutics, or the History and Treatment of Diseases. Second edition, revised. In one large and
handsome octavo volume, of 750 pages, leather. $3 75. (Just Isseed.)
The steady demand which has so soon exhausted the first edition of this work, sufficiently shows
that the author was not mistaken in supposing that a volume of this character was needed-an
elementary manual of practice, which should present the leading principles of medicine with the
practical results, in a condensed and perspicuous manner. Disencumbered of unnecessary detail
and fruitless speculations, it embodies what is most requisite for the student to learn, and at the
same time what the active practitioner wants when obliged, in the daily calls of his profession, to
refresh his memory on special points. The clear and attractive style of the author renders the
whole easy of comprehension, while his long experience gives to his teachings an authority everywhere acknowledged. Few physicians, indeed, have had wider opportunities for observation and
experience, and few, perhaps, have used them to better purpose. As the result of a long life devoted to study and practice, the present edition, revised and brought up to the date of publication,
will doubtless maintain the reputation already acquired as a condensed and convenient American
text-book on the Practice of Medicine.
DRUITT (ROBERT), M. R. C. S., &c.
THE PRINCIPLES AND PRACTICE OF MODERN SURGERY. A new
and revised American from the eighth enlarged and improved London edition. Illustrated with
four hundred and thirty-two wood-engravings. In one very handsomely printed octavo volume,
leather, of nearly 700 large pages. $3 50. (Now Ready, October, 1860.)
A work which like DRUITT'S SURGERY has for so many years maintained the position of a leading favorite with all classes of the profession, needs no special recommendation to attract attention
to a reviced edition. It is only necessary to state that the author has spared no patis to keep the
work up to its well earned reputation of' presenting in a small and convenient compass the latest
condition of every department of surgery, considered both as a science and as an art; and that the
services of a competent American editor have been employed to introduce whatever novelties may
have escaped the author's attention, or may prove of' service to the Americait practitioner. As
several editions have appeared in London since the issue of the last Americanu reprint, the volume
has had the benefit of repeated revisions by the author, resulting in a very tnorough alteration and
improvement. The extent of these additions may be estimated from the fact that it now contains
about one-third more matter than the previous American edition, and that notwithstanding the
adoption of a smaller type, the pages have been increased by about one hundred, while nearly two
hundred and fifty wood-cuts have been added to the former list of illustrations.
A marked improvement will also be perceived in the mechanical and artislical execution of the
work, which, printed in the best style, on new type, and fine paper, leaves little to be d sired as
regards external finish; while at the very low price affixed it will be found one of the cheapest
volumes accessible to the profession.
This popular volume, now a most comprehensive nothing of real practical importance has been omitwork on surgery, has undergone many corrections, ted; it presents a faithful epitome of everything reimprovements, and additions, and the principles and lating t ) surgery up to the present hour. It is dethe practice of the art have been brought down to servedly a popular manual, both with the student
the latest record and observation. Of the operations and practitioner.-Lendon Lancet, Nov. 19, 1859.
in surgery it is Impossible to speak too highly. The
descriptions are so clear and concise, and the iltus- In closing this brief notice, we recommend as cortrations so accurate and numerous, that the student dially as ever this most useful and comprehensive
can have no difficulty, with instrument inhand and hand-book. It must prove a vast assistance, not
book by his side, over the dead body, in obtaining     to t he student of surgery, hut also to tie busy
a proper knowledge and sufficient tact in this much practitioner whe may nothave the leisure to devote
neglectedoeparlmentofmedmcaleducation.-British  himself to the study of more lengthy volumes.-.
and Foreign Medico-Chirurg. Review, Jan. 18960.   London Med. Times and Gazette, Oct. 22, 1859.
In the present edition the author has entirely re-   In a wordl, this eighth edition of Dr. Druitt's
written many sof the chapters, and has incorporated Manual of Surgery is all that the surgical student
the various improvements and additions in modern or practitioner could desire. —Dublin Quarterly
surgery. On carefully going over it, we find that Journal of Med. Sciences, Nov. 1859.




AND SCIENTIFIC PUBLICATIONS.                                            11
DALTON, JR. (J. C.), M. D.
Professor of Physiology in the College of Physicians, New York.
A TREATISE ON HUMAN PHYSLOLOGY, designed for the use of Students
and Practitioners of Medicine. With two hundred and fifty-four illustrations on wood. rn one
very beautiful octavo volume, of over 600 pages, extra cloth, $4 00; leather, raised bands, $4 25.
(Juist Issued.)
This system of Physiology, both from the ex- its author; to his talents, his industry, his training;
cellence of the arrangement studiously observed  to the institution with which he is connected, and
throughout every page. and the clear, lucid, and in- to American science. —Boston Med. and Surgical
structive manner in which each subject is treated, Journal, Feb. 24, 1859.
promises to form one of the most generally received
class-books in the English language. It is, in fact,   A NEW  ook and a first rate one; an original book,
a most admirableepitoneof ail the really iportnt and one which cannot be too highly appreciated,
discoveries that have always been received as incon- and which we are proud to see emanating from our
testable truths, as well as of those which have been   ountry's press. It is by an author who, though
recently added to our stock of knowledge on this sub- young, s considerably famous for physiological rej et.     will, however, proceed to ive a few  x-search, and who in tlis work has erected for himjeeL.I. C      self an enduring monument, a token at once of his
tracts from the hook itself, as a specimen of its style          monument, a token at once of his
labor and his success. —Nashville Medical Journal,
and composition, and this, we conceive, will be quite laor and his success-Nashille Medical Jorna.
sufficient to awaken a general interest in a work    ach, 189.
which is immeasuralbl   superior in its details to the   Throughout the entire work, the definitions are
majority of those of the same class ti which it he- clear and precise, the arrangement admirable, the
longs. In its purity of style and elegance of com- argument briefly and well stated, and the style
position it may safely take its place with the very  nervous, simple, and concise. Section third, treatbest of our English classics; while in accuracy of ing of Reproduction, is a monograph of unapdescription it is imrpossible that it could be surpass- proached excellence. upon this subject, in the Enged. In every line is beautifully shadowed forth the lish tongue. For precision, elegance and force of
enmanations of the polished scholar, whose reflec- style, exhaustive method and extent of treatment,
tions are clothed in a garb as interesting as they are fulness of illustration and weight of personal reimpressive; Aith. the one predoiminant feeling ap- search, we know of no Ameri an contribution to
pearing to pervade the whole-an anxious des.re to medical science which surpasses it, and the day is
please and at the same time to instruct.-Dublin  far distant when its claims to the respectful attenQuarterly Journ. of Med. Sciences, Nov. 1859.    tion of even the best informed scholars will not be
cheerfully conceded by all acquainted with its range
The work before us, however, in our humble judg- ana depth.-Charleston Med. Journal, May, 1859.
merit, is precisely what it purports to be, and will   A new elementary work on Human Physiology
answer admirably the purpose for which it is in- lifting up its voice in the presence of late and sturdy
tended. It is par excellence, a text-book; and the editions of Kirke's, Carpentir's, Todd ard Bowbest text-book in t. is department that we have ever man's, to say nothing of Dur glison's and Draper's,
seen. We have carefully read the book, and speak  should have something superior in the matter or the
of its merits from a more than cursory perusal. manner of its utterance in order to win for itself
Looking back upon the work we have just finished, deserved attention and a name. That matter and
we must say a word concerning the excellence of its that manner, ater a candid perusal, we think disillustrations. No department is so dependent upon  tinguish this work, and we are proud to welcome it
good illustrations, and those which keep pace with  not merely for its nativity's sake, but for its own
our knowledge of the subject, as that of physiology. intrinsic excellence.  Its language we find to be
The wood-cuts in the work before us are the best plain, direct, unanbitious, and falling with a just
we have ever seen, and, being original, serve to  conciseness on hypothetical or unsettled questions,
illustrate precisely what is desired.-Buffalo Med. and yet with sufficient fulness on those living topics
Joufrnal, March, 1859.                           already understood, or the path to whose solution
A book of genuine merit like this deserves hearty is definitely marked out. It does not speak exhaustpraise before subjecting it to any minute criticism. i iely upon every subject that it notices, but it does
We are not prepared to find any fault with its design  speak suggestively, experimentally, and to their
until we have had more time to appreciate its merits main utilities.  Into the subject of Reproduction
as a manual for daily consultation, and to weigh  our author plunges with a kind of loving spirit.
its statements and conclusions more deliberately. Throughout this interesting and obscure department
Its excellences we are sure of; its defects we have he is a clear and admirable teacher, sometimes a
yet to discover. It is a work highly honorable to  brilliant leader. —Am. Med. Monthly, May, 1859.
DUNGLISON, FORBES, TWEEDIE, AND CONOLLY.
THE CYCLOP1EDIA OF PRACTICAL MEDICINE: comprising Treatises on
the Nature and Treatment of Diseases, Materia Medica, and Therapeutics, Diseases of Women
and Children, Medical Jurisprudence, &c. &c.  In four large super-royal octavo volumes, of
3254 double-columned pages, strongly and handsomely bound, with raised bands. $12 00.
*4k* This work contains no less than four hundred and eighteen distinct treatises, contributed by
sixty-eight distinguished physicians, rendering it a complete library of reference for the country
practitioner.
The most complete work on Practical Medicine I titioner. This estimate of it has not been formed
extant; or, at least, in our language.-Buffalo from a hasty examination, but after an intimate acMedical and Surgical Journal.                    quaintance derived from frequent consultation of it
For reference, it is above all price to every prac- during the past nine or ten years. The editors are
titioner.-Western Lancet.                        practitioners of established reputation, and the list
of contributors embraces many of the most eminent
One of the most valuable medical publications of professors and teachers of London, Edinburgh, Dubthe day-as a work of reference it is invaluable.-  lin, and Glasgow. It is, indeed, the great merit of
Western Journal of Medicine and Surgery.         this work that the principal articles have been furIt has been to us, both as learner and teacher, a nished by practitioners who have not only devoted
work for ready and frequent reference, one in which especialattention to the diseases about which they
modern English medicine is exhibited in the most have written, but have also enjoyed opportunities
advantageous light.-Medical Examiner.            for an extensive practical acquaintance with them,
and whose reputation carries the assurance of their
We rejoice that this work is to be placed within competency justly to appreciate the opinions of
the reach of the profession in this country, it being others, while it stamps their own doctrines with
unquestionably one of very great value to the prac- high and just authority.-American Medical.Tourn.
DEWEES'S COMPREHENSIVE SYSTEM OF  AND MEDICAL TREATMENT OF CHILDMIDWIFERY. Illustrated by occasional cases   REN. The last edition. In one volume, octavo,
and many engravings. Twelfth edition, with the   extra cloth, 548 pages. $2 80
author's last improvements and corrections  In  DEWEE'S   TREATIE  ON  THE DISEAE
one octavo volume, extra cloth, of 600 pages. $320.   OF EALES  TREAT ISE  ON   THE DISEASES
DEWEES'S TREATISE ON THE PHYSICAL    octavo extra cloth, 532 pages, with plates. $3 00




12                      BLANCHARDD & LEA'S MEDICAL
DUNGLISON (ROBLEY), M.D.,
Professor of Institutes of Medicine in the Jefferson Medical College, Philadelphia.
NEW AND ENLARGED EDITION.
MEDICAL  LEXICON; a Dictionary of Medical Science, containing a concise
Explanation of the various Subjects and Terms of Anatomy, Physiology, Pathology, Hygiene,
Therapeutics, Pharmacology, Pharmacy, Surgery, Obstetrics, Medical Jurisprudence, Dentistry,
&c. Notices of Climate and of Mineral Waters; Formulte for Officinal, Empirical, and Dietetic
Preparations, &c. With French and other Synonymes. Revised and very greatly enlarged.
In one very large and handsome octavo volume, of 992 double-columned pages, ia small type;
strongly bound in leather, with raised bands. Price $4 00.
Especial care has been devoted in the preparation of this edition to render it in every respect
worthy a continuance of' the very remarkable favor which it has hitherto enjoyed. The rapid
sale of FIFTEEN large editions, and the constantly increasing demand, show that it is regarded by
the profession as the standard authority. Stimulated by this fact, the author has endeavored in the
present revision to introduce whatever might be necessary " to make it a satisfactory and desirable-if not indispensable-lexicon, in which the student may search without disappointment for
every term that has been legitimated in the nomenclature of the science."  To accomplish this,
large additions have been found requisite, and the extent of the author's labors may be estimated
from the fact that about Six THOUSAND subjects and terms have been introduced throughout, rendering the whole number of' definitions about SIXTY THOUSAND, to accommodate which, the number of pages has been increased by nearly a hundred, notwithstanding an enlargement in the size
of the page. The medical press, both in this country and in England, has pronounced the work indispensable to all medical students and practitioners, and the present improved edition will not lose
that enviable reputation.
The publishers have endeavored to render the mechanical execution worthy of a volume of such
universal use in daily reference. The greatest care has been exercised to obtain the typographical
accuracy so necessary in a work of the kind. By the small but exceedingly clear type employed,
an immense amount of matter is condensed in its thousand ample pages, while the binding will be
found strong and durable. With all these improvements and enlargements, the price has been kept
at the former very moderate rate, placing it within the reach of all.
This work, the appearance of the fifteenth edition  tells us in his preface that he has added about six
of which, it has become our duty and pleasure to thousand terms and subjects to this edition, which,
announce, is perhaps the most stupendous monument before, was considered universally as the best work
of labor and erudition in medical literature. One of the kind in any language.-Silliman's Journal,
would hardly suppose after constant use of the pre- March, 1858.
ceding editions, where we have never failed to find   HIe has razed his gigantic structure to the foundaa sufficiently full explanation of ever) medical term, tions, and remodelled and reconstructed the entire
that in this edition "' about six thousand subjects pile. No less than six thousand additional subjects
and terms have been added," T with a careful revision  and terms are illustrated and analyzed in this new
and correction of the entire work. It is only neces- edition, swelling the grand aggregate to beyond
sary to announce the advent of this edition to make sixty thousand! Thus is placed before the profesit occupy the place of the preceding one on the table sion a complete and thorough exponent of medical
of every medical man, as it is without doubt the best terminology, without rival or possibility of rivalry.
and most comprehensive work of the kind which has -Nashville Journ. of Med. and Surg., Jan. 1858.
ever appeared.-Buffalo Med. Journ., Jan. 1858.  It is universally acknowledged, we believe, that
The work is a monument of patient research, this work is incomparably the best and most coinskilful judgment, and vast physical labor, that will plete Medical Lexicon in the English language.
perpetuate the name of the author more effectually The amount of labor which the distinguished author
tlan any possible device of stone or metal. Dr. has bestowed upon it is truly wonderful, and the
Dunglison deserves the thanks not only of the Ame- learning and research displayed in its preparation
ricani profession, but of the whole medical world.-  are equally remarkable. Comment and commendaNorth Am. AIedico-Chir. Review, Jaii. 1858.   tion are unnecessary, as no one at the present day
A Medical Dictionary better adapted for the wants thinks of purchasing any other Medical Dictionary
of the profession than any other with which we are than this.-St. Louis Mled. and Srg. Journ., Jan.
acquainted, and of a character which places it far
above comparison and competition.-Am. Journ.   It is the foundation stone of a good medical libraMIed. Sciences, Jan. 1858.                    ry, and should always be included in the first list of
We need only say, that the addition of 6,000 new  books purchased by the medical student.-Am. Med.
terms, with their accompanying definitions, may be Monthly, Jan. 1858.
said to constitute a new work, by itself. We have   A very perfect work of the kind, undoubtedly the
examined the Dictionary attentively, and are most most perfect in the English language.-M-ed. and
happy to pronounce it unrivalled of its kind. The Surg. Reporter, Jan. 1858.
erudition displayed, and the extraordinary industry   It is now emphatically the Medical Dictionary of
which must have been demanded, in its preparation the English language, and for it there is no substiand perfection, redound to the lasting credit of its tute.-N. H. Med. Journ., Jan. 1858.
author, and have furnished us with a volume indis- t is scarcely necessary to remark that any mdi
pensable at the present day, to all who would find s scarcely necessary o remark ha any medi
themselves au niveau with the highest standards of cat library wanting a copy of Dunglison's Lexicon
medical information.-Boston Mledical and Surgical must be imperfect.-Cn. Lancet, Jan. 1858.
Journal, Dec. 31, 1857.                         We have ever considered it the best authority pubGood lexicons and encyclopedic works generally, lished, and the present edition we may safely say ihas
are the most labor-saving contrivances which lite- ni equal in the world,-Peninsular Med. Journal,
rary men enjoy; and the labor which is required to Jan. 1858.
produce them in the perfect manner of this example   The most complete authority on the subject to be
is something appalling to contemplate. The author found in any language. —Va. Mtied. Journal, Feb.'58.
BY THE SAME AUTHOR.
THE  PRACTICE  OF  MEDICINE.  A  Treatise on Special Pathology and The.
rapeutics. Third Edition. In two large octavo volumes, leather, of 1,500 pages. $6 25.




AND SCIENTIFIC PUBLICATIONS.                                           13
DUNGLISON (ROBLEY), M. D.,
Professor of Institutes of Medicine in the Jefferson Medical College, Philadelphia.
HUMAN   PHYSIOLOGY.   Eighth  edition.   Thoroughly revised  and extensively modified and enlarged, with five hundred and thirty-two illustrations. In two large and
handsomely printed octavo volumes, leather, of about 1500 pages. $7 00.
In revising this work for its eighth appearance, the author has spared no labor to render it worthy
a continuance of the very great favor which has been extended to it by the profession. The whole
contents have been rearranged, and to a great extent remodelled; the investigations which of late
years have been so numerous and so important, have been carefully examiled and incorporated,
and the work in every respect has been brought up to a level with the present state of the subject.
The object of the author has been to render it a concise but comprehensive treatise, containing the
whole body of physiological science, to which the student and man of science can at all times refer
with the certainty of finding whatever they are in search of, fully presented in all its aspects; and
on no former edition has the author bestowed more labor to secure this result.
We believe that it can truly be said, no more com- ) The best vwork of the kind in the English lanplete repertory of facts upon the subject treated, $ guage.-Sillimnan's Journal.
can anywhere be found. The author hlas, moreover,   The present edition the author has made a pclicc t
that enviable tact at description and that facility  mirror of the science as it is at the present hour.
and ease of expression which render him peculiarly  As a work upon physiology proper, the science of
acceptable to the casual, or the studious reader. the functions performed by the body, the student will
This faculty, so requisite in setting forth many find it all ie wisaes.-Nashville Journ, of Med.
graver and less attractive subjects, lends additional
charms to one always fascinating.-Boston Med.. That he has succeeded, most admirably succeeded
and Surg. Journal.                             in his purpose, is apparent from the appearance of
an eighth edition. It is now the great encyclopuedia
The most complete and satisfactory system  of on the subject, and wortlly if a place in every phyPhysiology in the English language.-Amer. Med  sician's library.-Western Lancet.
Journal.
BY THE SAME AUTHOR. (A new edition.)
GENERAL THERAPEUTICS AND MATERIA MEDITCA; adapted for a
Medical Text-book. With Indexes of Remedies and of Diseases and their Relnedies. SIXTH
EDITIoN, revised and improved. With one hundred and ninety-three illustrations. In two large
and handsomely printed octavo vols., leather, of about 1100 pages. $6 00.
In announcing a new edition of Dr. Dunglison's   The work will, we have little doubt, be bought
General Therapeutics and Materia Medica, we have  and read by the majority of medical students; its
no words of commendation to bestow upon a work  size, arrangement, and reliability recommyend it to
whose merits have been heretofore so often and so  all; no one, we venture to pre(iict, will study it
justly extolled. It must not be supposed, however, without profit, and there are few to whom it will
that the present is a mere reprint of the previous not be in some measure useful as a work of referedition; the character of the author for laborious ence. Tihe young practitioner, more especially, will
research, judicious analysis, and clearness of ex- find the copious indexes append(d to this edition of
pression, is fully sustained by the numerous addi- Ireat assistance in the selection and preparation of
tions he has made to the work, and the careful re- suitable forinula.-Charleston Med. Journ. and Revision to which he has subjected the whole. —N. A. qview Jan. 1858.
Medico-Chir. Review, Jan. 1858.
BY THE SAME AUTItOR. (A new Edition.)
NEW REMEDIES, WITH FORMULE   FOR THEIR PREPARATION AND
ADMINISTRATION. Seventh edition, with extensive Additions. In one very large octavo
volume, leather, of 770 pages. $3 75.
Another edition of the " New Remedies" having been called for, the author has endeavored to
add everything of moment that has appeared since the publication of the last edition.
The articles treated of in the former editions will be fiound to have undergone considerable expansion in this, in order that the author might be enabled to introduce, as lfar as practicable, the
results of the subsequent experience of others, as well as of his owni observation and reflection;
and to make the work still more deserving of the extended circulation with which the preceding
editions have been favored by the profession. By an enlargement of the page, the numerous additions have been incorporated without greatly increasing the bulk of the volume.-Prefacce.
One of the most useful of the author's works.-   The great learning of the author, and his remarkSouthern Medical and Surgical Journal.         able industry in pushing his researches into every
Tlhis elaborate and useful volume should be source whence information is derivable,have enabled
found in every medical library, for as a book of re- him to throw together an extensive mass of facts
ference, for physicians, it is unsurpassed by any and statements, accompanied by full reference to
other work in existence, and the double index for authorities; which last feature renders the work
diseases and for remedies, will be found greatly to practically valuable to investigators who desire to
enhance its value.-Nedwr  York Med. Gazette,   examine the original pIapers.-The American Journsal
of Pharmacy.
ELLIS (BENJAMIN), M. D.
THE  MEDICAL  FORMiIULARY: being a Collection of Prescriptions, derived
from the writings and practice of many of the most eminent physicians of America and Europe.
Together with the usual Dietetic Preparations and Antidotes for Poisons. To which is added
an Appendix, on the Endermic use of Medicines, and on the use of Ether and Chloroform. The
whole accompanied with a few brief Pharmaceutic and Medical Observations. Tenth edition,
revised and much extended by ROBERT P. THO.IAS, M. D., Professor of Materia Medica in the
Philadelphia College of Pharmacy. In one nea't octavo volume, extra cloth, of 296 pages. $1 75.
r   lli V11~~~ 11T~L VC~~L5I~a) VV




14                       BLANCHARD & LEA'S MEDICAL
ERICHSEN (JOHN),
Professor of Surgery in University College, London, &c.
THE SCIENCE AND ART OF SURGERY; BEING A TREATISE ON SURGICAL
INJURIEIS, DISEASES, AND OPERATIONS. New and improved American, from the second enlarged
and carefully revised London edition. Illustrated with over four hundred engravings on wood.
In one large and handsome octavo volume, of one thousand closely printed pages, leather,
raised bands. $4 50.  (Just Issued.)
The very distinguished favor with which this work has been received on both sides of the Atlantic has stimulated the author to render it even more worthy of the position which it has so rapidly
attained as a standard authority. Every portion has been carefully revised, numerous additions
have been made, and the most watchful care has been exercised to render it a complete exponent
of the most advanced condition of surgical science. In this manner the work has been enlarged by
about a hundred pages, while the series of engravings has been increased by more than a'hundred,
rendering it one of the most thoroughly ill ustrated volumes before the profession. The additions of
the author having rendered unnecessary most of the notes of the former American editor, but little
has been added in this country; some few notes and occasional illustrations have, however, been
introduced to elucidate American modes of practice.
It is, in our humble judgment. decidedly the best step of the operation, and not deserting him until the
book of the kind in the English language. Strange  final issue of the case is decided.-Sethoscope.
that just such books are notoftener produced by pub.   Embracing, as will be perceived, the whole surgilic teachers of surgery in this country and Great cal domain, aild each division of itself almost comtlritain. Indeed, it is a matter of great astonishment. plete and perfect, each chapter full and explicit, each
but no less true than astonishing, that of the many  subject faithfully exhibited, we can only express oui
works on surgery republished in this country within  estimate of it in the aggregate. We consider it an
the last fifteen or twenty years as text-books for excellent contribution to surgery, as probably the
medical students, this is the oddly one that even ap. best single volume now extant on the subject, and
proximates to the fulfilment of the peculiar wants of with great pleasure we add it to our text-books.youngmen just ci;tteringupon the study of thlishranch  Nashville Journal of Medicine and Surgery.
of the profession. — Wester~nJour.of Med. anal Surgery.
Prof. Erichsen's work, for its size, has not been
Its value is greatly enhanced by a very copious surpassed; his nine hundred and eight pages, prowell-arranged index. We regard this as one of the fusely illustrated, are rich in physiological, patholomost valuable contributions to modern surgery. To gical, and operative suggestions, doctrines, details,
one entering his novitiate of practice, we regard ii and processes; and will prove a reliable resource
the most serviceable -gide which he can consult. He for information. both to physician and surgeon, in the
will find a fulness of detail leading him through every ( hour of peril.-N. O. MIed. and Surg. Journal.
FLINT (AUSTIN), M. D.,
Professor of the Theory and Practice of Medicine in the University of Louisville, &e.
PHYSICAL EXPLORATION AND) DIAGNOSIS OF DISEASES AFFECTING  THE RESPIRATORY  ORGANS.  In one large and handsome octavo volume, extra
cloth, 636 pages. $3 00.
We regard it, in point both of arrangement and of   A work of original observation of the highest merit.
the marked ability of its treatment of the subjects, We recommend the treatise to every one who wishes
as destined to take the first rank in works of this to become a correct auscultator. Based to a very
class. So far as our information extends, it has at large extent upon cases numerically examined, it
present no equal. To the practitioner, as well as carries the evidence of careful study and diseriminathe student, it will be invaluable in clearing up the tion upon every page. It does credit to, the author'?
diagnosis of doubtful cases, and in shedding light and through him, to the profession in this country
upon difficult phenomena.-Buffalo MAbed. Journal.  It is, what we cannot call every book upon anscultation, a readable book.-Am. Jour. Mled. Sciences.
BY THE SAME AUTHOR.  (NOW Ready.)
A PRACTICAL TREATISE ON THE DIAGNOSIS, PATHOLOGY, AND
TREATMENT OF DISEASES OF THE HEART.  In one neat octavo volume, of about
500 pages, extra cloth. $2 75.
We do no* know that Dr. Flint has written any- diseases of the chest. lWe have adopted his work
thing which is not first rate; but this, his latest con- upon the heart as a text-book, believing it to be
tribution to rredical literature, in our opinion, sur- more valunble for that purpose than any work of the
passes all the others. The work is most comprehen- kind that, has yet appeared.-laslhville Med. Jowrn.,
sive in its scope, and mo(st sound in the views it enun- Dec. 1859.
cinats. The descriptions are clear and methodical;   With more than pleasure do we hail the advent of
the statemrents are substantiated by facts, and are this work, for it fills a wide gap on the list cf textmade with such simplicity and sincerity, that with- books for our schools, and is, iir the practitioner,
out them they would carry conviction. The style  the most valuable practical work of its kind.-N. 0.
is admirably clear, direct, and free from dryness  Med. NTews, Nov. 1859.
With Dr. Walslle's excellent treatise before us, we
have no hesitation in saying that Dr. Flint's book is   In regard to the merits of the work, we have no
the best work on the heart in the English language. hesitation in pronouncing it full, accurate, and ju-Boston  lIed. and Surg. Journal, Dec. 15, 1859.   dicious. Considering the pres nt state of science,
AVe have thus endeavored to present our readers such a work was much needed. It should be in the
with a fair analysis of this remarkable work. Pre- iinds of every practitioner-Chicago  led, Joursal,
ferring to employ the very words of thedistinguished  April, 1860.
author, wherever it was possible, we have essayed    Bat these are very trivial spots, and in no wise
lto condense into the briefest spacea general view of prevent us from declaring our most healrty approval
his observations and suggestions, and to direct the of the author's ability, industry, and eonscientiousattention of our brethren to the abounding stores of ness.-Dublin Quarterly Journal of Iled. Sciences,
valuable matter here collected and arranged for their Feb. 1860.
use and instruction. No medical library will here-   He has labored on with the same industry and care,
after be considered complete without this volume; and his place among the first authors of our country
and we trust it will promptly find its way into the is hecomning fully established. To this end, the work
lhands of every Amet ican student and physician.-  whose title is given above, contributes in no small
NA Am. Med. Chir. Review, Jan 1860.               degree. Our spae will not admit of ani extended
This last work of Prof. Flint will add much to  analysis, and we will close this Grief notice by
his previous well-earned celebrity, as a writer of commending it without reserve to every class of
great force and bieauty, and, with his previous work,  readers in the profession.-Peninsular 11ed. Journ.,
places him at the head of Anieric.n writers upons Feb. 1860.




AND SCIENTIFIC PUBLICATIONS.                                          15
FOWNES (GEORGE), PH. D., &c.
A MANUAL OF ELEMENTARY CHEMISTRY; Theoretical and Practical.
From the seventh revised and corrected London edition. With one hundred and ninety-seven
illustrations. Edited by ROBERT BRIDGES, M. D. In one large royal 12mo. volume, of 600
pages. In leather, $1 65; extra cloth, $1 50. (Just Issued.)
The death of the author having placed the editorial care of this work in the practised hands of
Drs. Bence Jones and A. W. Hoffman, everything has been done in its revision which experience
could suggest to keep it on a level with the rapid advance of chemical science. The additions
requisite to this purpose have necessitated an enlargement of the page, notwithstanding which the
work has been increased by about fifty pages. At the same time every care has been used to
maintain its distinctive character as a condensed manual for the student, divested of all unnecessary
detail or mere theoretical speculation.  The additions have, of course, been mainly in the department of Organic Chemistry, which has made such rapid progress within the last few years, but
yet equal attention has been bestowed on the other branches of the subject-Chemical Physics and
Inorganic Chemistry-to present all investigations and discoveries of importance, and to keep up
the reputation of the volume as a coniplete manual of the whole science, admirably adapted for the
learner. By the use of a small but exceedingly clear type the matter of a large octavo is compressed
within the convenient and portable limits of a moderate sized duodecimo, and at the very low price
affixed, it is offered as one of the cheapest volumes before the profession.
Dr. Fownes' excellent work has been universally   The work of Dr. Fownes has long been before
recognized everywhere in his own and this country, the public, and its merits have been fully apprecias the best elementary treatise on chemistry in the ated as the best text-book on chemistry now in
English tongue, and is very generally adopted, we existence. WVe do not, of course, place it in a rank
believe, as the standard text- book in all ur colleges, superior to the works of Brande, Graham, Turner,
both literary and scientific.-Charleston Med Journ. Gregory, or Gmelin, but we say that, as a work
and Review, Sept. 1859.                        for students, it is preferable to any of them.-LonA standard manual, which has long enjoyed the don Journal of Medicine.
reputation of embodying much knowledge in a small   A work well adapted to the wants of the student
space. The author has achieved the difficult task of It is an excellent exposition of the chief doctrines
condensation with masterly tact. His book is con- and facts of modern chemistry. Tile size of the work.
else without being dry, and brief without being too and still more the condensed yet perspicuous style
dogmatical or gencral. —Virginia Med. and Surgical in which it is written, absolve it from the charges
Journal.                                       very properly urged against most manuals termed
popular.-Edinburgh Journal of Medical Science.
FISKE  FUND  PRIZE  ESSAYS  - THE EF-   EDWARD WtARREN, M. D,of Edenton, N. C. ToFECTS OF CLIMATE ON TUBERCULOUS   gether in one neat 8vo volume, extra cloth. $1 00.
DISEASE. Byn EDwTN LEE, M. R. C. S,London, FRICK ON RENAL AFFECTIONS; their Diagand THE INFIUENCE ()F PREGNANCY ON    nosis and Pathology. With illustrations.  Orne
THE DEVELOPIM'ENT OF TUBERCLES  By   volume, royal l2imo., extra cloth.;5 cents
FERGUSSON (WILLIAM), F. R. S.,
Professor of Surgery in King's College, London, &c.
A SYSTEM OF PRACTICAL SURGERY. Fourth American, from the third
and enlarged London edition. In one large and beautifully printed octavo volume, of about 700
pages, with 393 handsome illustrations, leather. $3 00.
GRAHAM  (THOMAS), F. R. S.
THE ELEMENTS OF INORGANIC CHEMISTRY, including the Applications of the Science in the Arts. New and much enlarged edition, by HENRY WATTS and ROBERT
BRIDGES, M. D. Complete in one large and handsome octavo volume, of over SC0 very large
pages, with two hundred and thirty-two wood-cuts, extra cloth. $4 00.:+*  Part II., completing the work from p. 431 to end, with Index, Title Matter, &c., may be
had separate, cloth backs and paper sides. Price $2 50.
From Prof. E. N. Horsford, Harvard College.   afford to be without this edition of Prof. Graham's
It has, in its earlier and less perfect editions, been Elements.-Sillimnan's Journal, MAarch, 1858.
famil;iar to me, and the excellence of its plan and   From Prof. Wolcott Gibbs N. Y. Free Academy
the clearness and completeness of its discussions,
have long been sty admiration.                   The work is an admirable one in all respects, and
I its republication here cannot fail to exert a positive
No reader of English works on this science can influence upontheprogress ofscience n this country.
GRIFFITH  (ROBERT  E.), M. D., &c.
A UNIVERSAL FORMULARY, containing the methods of Preparing and Administering Otlicinlal and other Medicines. The whole adapted to Physicians and Pharmaceon.
tists. SECOND EDITION, thoroughly revised, with numerous additions, by ROBaErT P. THOMAS.
M. D., Professor of Materia Medica in the Philadelphia College of Pharmacy. In one large and
handsome octavo volume, extra cloth, of 650 pages, double columns. $3 00; or in sheep, $3 25.
It was a work requiring much perseverance, and   This is a work of six hundred and fifty-one pages.
when published was looked upon as by far the best embracing all on the subject of preparing and admiwork of its kind that had issued from the American  iistering medicines that can be desired by the physipress. Prof Thomas has certainly " improved," as  i an and pharmaceutist.- Western Lancet.
well as added to this Formulary, and has rendered it  T
additionally deserving of the confidence of pharma- ticing plysicial, is really immense. s mate r aprac.
ceutists anid physicians.-Am. Journal of Pharmacy.  and Surg. Journal.
WSe are happy to announce a new and improved   This edition has been greatly improved by the reedition of this, one of the most valuable and useful vision and ample additions of Dr Thomas, and is
works that have emanated from an American pen. now, we believe, one of the most complete works
It would do credit to any country, and will be found of its kind il any language.  The additions amount
of daily usefulness to practitioners of medicine; it ishas ee spared
better adapted to their purposes than the dispensato- to include in tem all the recent improvemenls. A
ries.-,Sto iutlude il hm all the reent im.arovemerns. A
ries.-Southern ~lled. acrd Surg. Journal,      work of this kind aqppears to us indispensable   lo the
It is one of the most useful books a country practi- physician, and therie is notle we call nmore cordially
tioner can possibly have.-Medical Chronicle.   recort mend.-N. Y. Journal of Medicinle.




16S                     BLANCHARD &; LEA'S MEDICAL
GROSS (SAMUEL D.), M. D.,
Professor of Surgery in the Jefferson Medical College of Philadelphia, &c.
Just Issued.
A  SYSTEM.  OF  SURGERY: Pathological, Diagnostic, Therapeutic, and Operative. Illustrated by NINE HUNDRED AND THIRTY-SIX ENGRAVINGS. In two large and beautifully
printed octavo volumes, of nearly twenty-four hundred pages; strongly bound in leather, with
raised bands. Price $12.
FROM THE AUTHOR'S PREFACE.
c" The object of this work is to furnish a systematic and comprehensive treatise on the science and
practice of surgery, considered in the broadest sense; one that shall serve the practitioner as a
thithful and available guide in his daily routine of duty. It has been too much the custom of modern writers on this department of the healing art to ominit certain topics altogether, and to speak of
others at undue length, evidently assuming that their readers could readily supplyr the deficiencies
from other sources, or that what has been thus slighted is of no particular practical value. My aim
has been to embrace the whole domain of surgery, and to allot to every subject its legitimate claim
to notice in t he great ftm ily of external diseases and accident,.  How far this object has been accomplished, it is not for me to determine.  It may satfely be affirmed, however, that there is no topic,
properly appertaining to surgery, that will not be found to be discussed, to a greater or less extent,
in these vollimes.  If a larger space than is customary has been devoted to the consideration of
inflammation and its results, or the great principles of surgery, it is because of the conviction,
grounded upon long and close observation, that there are no subjects so little understood by the
general practitioner. Special attention has also been bestowed upon the discrimination of diseases;
and an elaborate chapter has been introduced on general diagnosis."
That these intentions have been carried out in the fuillest and most elaborate manner is sufficiently
shown by the great extent of the work, and the  length of time during which the author has been
concentrating on the task his studies and his experience, guided by the knowledge which twenty
years of lecturing on surgical topics have given him of the wants of the profession.
Of Dr. Gross's treatise on Surgery we can say   At present, however, our object is not to review
no more than that it is the most elaborate and corn- tle work (this we purpose doing hereafter), but
plete work on this branch of the tealing art which simply to announce its appearance, that in the
has ever been published in any country. A svs- meantime nor readers may procu'e arnd examine it
tematic work, it admits of no analytical review; for themselves. But even this much we cannot do
but, did our space permit, we should gladly give without expressing the opinion thtat, in putting forth
some extracts from it, to enable our readers to judge these two volumes, Dr. Gross has reared for himof the classical style of the author, and the exhaust- self a lasting monument to his skill as a surgeon,
ing way in which each subject is treated.-Dublin  and to his Industry and learning as an author.-St.
Quarterly Journal of Med. Science, Nov. 1859.    Louis lled. and Surg. Journal, Nov. 1859.
The work is so superior to its predecessors in   WTith pleasure we record the conpletinn of this
matter and extent, as well as in illustrations and long-anticiputed work. The reputation  thich the
style of publication, that we can honestlr recom- author has for muav  years sustained, both as a surmend it as the best work of the kind to be taken geon and as a writer, had prepared us to expect a
home by the young practitioner. —Am. M2ed. Journ., treatise of great excellence and originality; but we
Jan. 1S60.                                       confess we were by no means prepared for the work
The treatise of Prof. Gross is not, therefore, a which is before us-the most complete treatise upon
mere text-book for undergraduates, but a systema-  surgery ever published. either in this or any other
tic record of more than thirty years' experience, cuntry, alnd we might, perhaps, safelv say, the
reading, and reflection by a man of observation, most original. There is no subject helonging  prosound judgment, and jare practical tact, and as such perly to surgery which has not received from the
deserves to take rank with the renowned produc- author a due sliare of attention. Dr. Gross has suptions of a similar character, by Vidal and Boyer, of Plied a want in surgical literature w}r.ich has long
France, or those of Chelius, Blasius, and Langen- been felt by practitioners; he has furnished us with
beck, of Germany. Hence, we do not hesitate to a complete practical treatise upion surgery ill all its
express the opinion that it will speedily take the departments  As Aaericans, we are proud of the
same elevated position in regard to surgery that has achievement; as surgeons, we are most sincerely
been given by colmmonl consent to the masterly work thankful to him for his extraord nary labors in our
of Pereira in MIateria Medica, or to Todd and Bow- I belal.-N. Y MlTonthly Review and Bsuffualo  Med.
man in Physiology.-N. O. Med. and SUrg. Journal, Journae, Oct. 1850.
Jan. 1860.
BY THE SAME AUTHOR.
ELE MENTS OF PATHOLOGICAL ANATOMY. Third edition, thoroughly
revised and greatly improved. In one large and very handsome octavo volulne, with about three
hundred and fifty beautiful illustrations, of which a large number are from original drawings.
Price in extra cloth, $4 75; leather, raised bands, $5 25. (Lately Pucblis!ledl.)
The very rapid advances in the Science of Pathological Anatomy during the last few years have
rendered essential a thorough modification of this work, with a view of mlaking it a correct exponent of the present state of the subject. The very careful manner in which this task has been
executed, alnd the amount of alteration which it has undergone, have enabled the author to say that
4"with the many changes and improvements now introduced, the work may be regarded almost as
a new treatise," while the efforts of the author have been seconded as regards the mechanicai
execution of the volume, rendering it one of the handsomest productions of the American press.
We most sincerely congratulate the author on the WVe have been favorably impressed with the genesuccessful manner in which he has accomplished his ral manner in which Dr. Gross has executed his task
prbposed object. His book is most admirably cal- of affording a comprehensive digest of the present
culated to fill up a blank which has long been felt to  state of the literature of Patlological Anatomy, and
exist in this department of medical literature, and  have much pleasure in recommending his work to
as such must become very witely circulated amongst our readers, as we believe one well deserving of
all classes of the profession.-Dublin Quarterly  diligent perusal and careful study.-Montreal hIed.
Journ. of sIed. Science, Nov. 1857.              Chron., Sept. 1S57.
BY THE SAME AUTHOR.
A PRACTICAL TREATISE ON FOREIGN BODIES IN THE AIR-PASSAGES. In one handsome octaveo volume, extra cloth, with illustrations. pp. 468. $2 75.




AND SCIENTIFIC PUBLICATIONS.                                            17
GROSS (SAMUEL D.), M. D.,
Professor of Surgery in the Jefferson Medical College of Philadelphia, &c.
A PRACTICAL TREATISE ON THE DISEASES, INJ URIES, AND
MALFORMATIONS OF THE URINARY BLADDER, THE PROSTATE GLAND, AND
THE  URETHRA. Second Edition, revised and much enlarged, with one hundred and eightyfour illustrations. In one large and very handsome octavo volume, of over nine hundred pages.
In leather, raised bands, $5 25; extra cloth, $4 75.
Philosophical in its design, methodical in its ar- I agree with us, that there is no work in the English
rangement, ample and sound in its practical details, language which can make any just pretensions to
it may in truth be said to leave scarcely anything to i be its equal.-1N. Y. Journal of lMedicine.
be desired on so important a subject.-Boston Med.   A volume replete with truths and principles of the
cand Sorre.Jourenal.                            utmost value in the investigation of these diseases. -
Whoever will peruse the vast amount of valuable American Medical Journal.
practical information it contains, will, we think,
GRAY (HENRY), F. R. S.,
Lecturer on Anatomy at St. George's Hospital, London, &e.
ANATOMY, DESCRIPTIVE AND SURGICAL.  The Drawings by H. V.
CARTER, M. D., late Demonstrator on Anatomy at St. George's Hospital; the Dissections jointly
by the AIuTHOR and Dr. CARTER. In one magnificent imperial octavo volume, of nearly 810
pages, with 363 large and elaborate engravings on wood. Price in extra cloth, $6 25; leather
raised bands, $7 00. (Just Issued.)
The author has endeavored in this work to cover a more extended range of subjects than is
customary in the ordinary text-books, by giving not only the details necessary for the student, but
also the application of those details in the practice of medicine and surgery, thus rendering it both
a guide for the learner, and an admirable work of reference for the active practitioner.  The
engravings form a special feature in the work, many of them  being the size of nature, nearly all
original, and having the names of the various parts printed on the body of the cut, in place of figures
of reference with descriptions at the foot. They thus form a complete and splendid series, which
will greatly assist the student is obtaining a clear idea of Anatomy, and will also serve to refresh
the memory of those who may find in the exigencies of practice the necessity of recalling the details
of the dissecting room; while combining, as it does, a complete Atlas of Anatomy, with a thorough
treatise on systematic, descriptive, and applied Anatomy, the work will be found of essential use
to all physicians who receive students in their offices, relieving both preceptor and pupil of much
labor in laying the groundwork of a thorough medical education.
The work before us is one entitled to the highest to exist in this country. MTr. Gray writes throughpraise, end we accordingly welcome it as a valu- out with both branches of his subject in view. His
able addition to medical literature. Intermediate  description of each particular part is followed by a
in fulness of detail between the treatises of SAar  notice of its relations to toe parts with which it is
pey and of Wilson, its characteristic merit lies in  connected, and this, too, sufficiently ample for all
the number and excellence of the engravings it the purposes of the operative surgeon. After decontains.  Most of these are original, of much  scribing the bones and muscles, he gives a concise
larger than ordinary size, and admirably executed. statement of the fractures to which the bones of
The various parts are also lettered after the plan the extremities are most liable, together with the
adopted in Holden's Osteology. It would be niffi- amount and direction of the displacement to which
cult to over-estimate the advantages offered by this the fragments are subjected by muscular action.
mode of pictorial illustration.  Bones, ligaments, The section on arteries is remarkably full and acmuscles, bloodvessels, and nerves are each in turn  curate. Not only is the surgical anatomynv given to
figured, and marked with their appropriate names; every important vessel, with directions for its ligathusenabling thestudenttoccmprehend,ataglance, tion, but at the end of the description of each artewhat would otherwise often be ignored, or at any  rial trunk we have a useful summnary of the irregurate, acquired only by prolonged and irksome ap- larities which may occur in its origin, course, and
plication. In conclusion, we heartily commend the termination.-N. A. Med. Chir. Review, Mar. 1859.
work of Mr. Gray to the attention of the medical
profession, feeling certain that it should be regarded   Mr. Gray's book, in excellency of arranement
as one of the most vall;able contributions ever made  nd comhleteness of executioen, exceeds any wsork
to educational literature. —N. Y. Monthly Review. on anatomy hitherto published in the English lanDec. 1859.                                      1 guage, affording a complete view of the structure of
the human t)ody, with especial reference to practical
In this view, we regard the work of Mr. Gray as surgery. Thus the volume constitutes a perfect book
far better adapted to the wants of the profession, of reference for the practitioser, demanding a place
and especially of the student, than any treatise on in even the most limited library of the physician or
anatomyyetpubllshedinthiscountry. It isdestined. surgeon, and a work of necessity for the student to
we believe, to supersede all others, both as a manual fix in his mind what he has learned by the dissecting
of dissections, and a standard of reference to the  knife from the book of nature.-The Dublin Quarstudent of general or relative anatomy.-N. Y. terly Journal of Med. Sciences, Nov. 1858.
Journal of Medicine, Nov. 1859.
This is by all comparison the most excellent work   In our judgment the moe  of illustral ion adopted
on Anatoriy extant. It is just the thing that has vantages to the student ofanntobuy. To the zealous
been long desired l)y the profession. \'ith such a disciple of Vesalius. earnestly desirous of real imguide as this, the student of anatomy, the practi- provement, the book will certainly le of immense
tionner of medicine, and the surgical devotee have value; but, at the same time, we must also confess
all a newer, clearer, and more radiant light thrown  that to those simply desirous of "r arlsolning  it
upon the intricacies and mysteries of this wondtr-odsend   The peculiar alue
fu scliesce, and are thus enabled to accomplish re- of Mr. Gray's mode of illustration is nowhere more
suItsl which hitherto seemed possible only to the markedly evident than in the chapter on osteology,
specialist. The plates, which are copied from re-and especially in those pirtions which treat of the
cent dissecti[ns, are so well executed, that the most bones of the head and of their development. The
superficial observer cannot fall toperceive the posi- study of these parts is thus made one of comprative
tions, relations, and distinctive features of the vari- y  of                th
ease, if not of positive pleasure; and those bugbears
ous parts, and to take in more of anatomy at a glance, of the student, the temporal anmi solenoii bones are
than by many long hours of diligent study over the  shorn of half their terrors. It is, in our estimation,
most erudite treatise, or, perhaps, at the dissecting  an admirable and complete text-book for tile student,
table itself.-Med. Journ. of N\. Caro~lisa, Oct. 1859. and a useful work of reference for the practitioner;
For this truly admirable work the profession is its pictorial character forming a novel element, to
indebted to the distinguished author of "Gray on which we have already sufficiently alluded.-Am.
the Spleen.1"  The vacancy it fills has been long felt Journ. iMed. Sci., July, 1859.




IS                     BLANCHARD1 & LEA'S MEDICAL
GIBSON'S INSTITUTES AND PRACTICE OF   tions. by JOSEPH LEIDY, AM. D. In one volume,
SURGERY. Eighth edition, improved and al-   very large Imperial quarto, extra cloth, witl 320
tered. With thirty-fourplates. In twohandsome   copper. plate figures, plain and colored, $5 00.
octavo volumes, containing about 1,000 pages,
leather, raised band   $G 50.      HUGHES' INTRODUCTION TO TIlE PRACleather, raised band. $6 0.        TICE OF AUSCULTA'ION  AND  OTHER
GARDNER'S MEDICAL CHEMISTRY, for the   MIODES OF PHYSICAL DIAGNOSIS IN DISuse of Students and the Profession. In one royal   EASES OF THE LUNGS AND HEART. Se12mo. vol., cloth, pp. 396, with wood-cuts.  1.  cond edition   I vol. royal L2mo., x. cloth, pp.
GLUGE'S ATLAS OF PATHOLOGICAL HIS-  304. $1 00.
TOLOGY. Translated, with Notes and AddiHAMILTON (FRANK H.), M. D.,
Professor of Surgery in the Uiiversity of Buffalo, &c.
A PRACTICAL TREATISE ON FRACTURES AND DISLOCATIONS. In
one large and handsome octavo volume, of over 750 pages, with 289 illustrations. $4 25. (Noaw
Ready, January, 1860.)
This is a valuable contribution to the surgery of illustrated, which will be a desideratum for those
most important affections, ana is the more welcome, practitioners who cannot conveniently see the moinasmuch as at the present time we do not possess dels applied. —New York ltied. Press, Feb. 4, 1860.
a single complete treatise on Fractures and Dislo- We regard this work as an honor not only to its
cations in the English language. It has remained for author, hbt to the profession of our country. Were
our American brother to produce a complete treatise  e to review it thoroughly we could not convey to
upon the subject, and bring together in a convenient the mind of the reader more forcibly our honest
form. those alterations and improvements that have opinion expressed in the few words-we think it the
been madle from time to time in the treatment of these best book ot its kind extant. Every man interested
affections. One great and valuable feature in the
work before us is the fact that it comprises all the He  who does not, uiill be the loser.-kVew Orheans
improvements introduced into the practice of hoth Medical News, March, 1860.
English and American surgery, and though far from 
omitting mention of our continental neighbors, the   Now that it is before us, we feel bound to say that
author by no means encourages the notion-but too much as was expected from it, and onerous as was
prevalent in some quarters-that nothing is good tie undertaking, it has surpassed expectation, and
unless imported from France or Germany.  Tne achieved more than was pledged in its behalf; for
its title does not express in full the richross of its
latter half of the work is devoted to the considera- its title does not express in full the riedhress of its
tion of the various dislocations and their appropri-     On the whole, we are prouder of tis
ate treatment. and its merit is fully equal to that of i work than of any which has for years emacated
the preceding portion.-The London Lancet, May 5,  from the American medical press; its sale will cer1860.' tainly be very large in this country, and we anticiI pate its eliciting much attention in Europe. —YashIt is emphatically the book upon the subjects of viille 3Medical Record, Mar. 1860.
which it treats, and we cannot doubt that it will
continue soi to be for an indefinite period of time.   Every surgeon, young and old, should possess
When we say, however, that we believe it will at himself of it, and give it a careful perusal in doing
once take its place as the best book for consultation which he will be richly repaid.-St. Louis Med.
by the practitioner; and that It will form the most and Surg. Journal, March, 1860.
complete, available, and reliable guide in emergen-   Dr. Hamilton is fortunate in having succeeded in
cies of every nature connected with its subjects; and filling the void, so long felt, with what cannot fail
also that the student of surgery may make it his text- to be at once accepted as a model monograph in some
book with entire confidence, and with pleasure also, respects, and a work of classical authority. WVe
fron its agreeable and easy style —we think our own sincerely congratulate the profession of the United
opinion may be gathered as to its value. —Boston States on the appearance of such a publication front
Medical and Surgical Joursal, March 1, 1860.   one of their number. We have reason to be proud
The work is concise, judicious, and accurate, and  of it as an original work. both in a literary and seiadapted to the wants of the student, practitioner,  entific point of view, and to esteem it as a valuable
and investigator, honorable to the author and to the guide in a most diffcult ant important branch of
profession.t-Chicago Mbed. Journal, March, 1800.  study and practice. On every account, therefore,
p  we hope that it may soon be widely known abroad
We venture to say that this is not alone the only as an evidence of genuine progress on this side of
complete treatise on the subject in the language, the Atlantic, and further, that it may be still more
but the best and most practical we have ever read. widely known at home as an authoritative teacher
The arrangement is simple and systematic, the die- from which every one may profitably learn, and as
tion clear and graphic, and the illustrations nume- I affording an example of honest, well-directed, and
rous and remarkable for accuracy of delineation. un tiring industry in authorship which every surgeon
The various mechanical appliances are faithfully may eimuiate.- Am. Med. Journal, April, 1860.
HOBLYN (RICHARD D.), M. D.
A DICTIONARY OF THE TERMS USED IN MEDICINE AND THE
COLLATERAL SCIENCES. A new American edition. Revised, with numerous Additions,
by ISAAC HAYS, M. D., editor of the cc American Journal of the Medical Sciences."  In one large
royal 12mo. volume, leather, of over 500 double columned pages. $1 50.
To both practitioner and student, we recommend use; embracing every department of medical science
this dictionary as being convenient in size, accurate down to the very latest date.-Western Lancet.
in definition, and sufficiently full and complete for   Hoblyn's Dictionary has long been a favorite with
ordinary consultation.-Charleston Med. Journ.  us. It is the best book of definitions we have, and
WCe know of no dictionary better arranged and ought always to be upon the student's table.adapted. It is notencumbered'with the obsolete terms Southern Med. and Surg. Journal.
of a bygone age, but it contains all that are now in
HOILLAND'S  MEDICAL  NOTES  AND  RE-   TOLOGY. Eighth edition. Extensiv-ly revised
FLECTIONS. From the third London edition.  and riodified. in two large octavo volumes, exIn one handsome octavo volume, extra cloth. $3.  tra cloth, of more than 1000 pages, with over 300
HORNER'S SPECIAL ANATOMY AND HIS-   illustratiois. $G 00.
HABERSHON (S. O.), M. D.,
Assistant Physician to and Lecturer on Materia Medica and Therapeutics at Guy's Hospital, &c.
PATHOLOGICAL AND P'RACTlCAL OBSERVATIONS ON DISEaSES
OF THE ALIMENTARY CANAL, (ESOPHAGUS, STOMACH, ClECUM, AND INTESTTNES. With illustrations on wood.  In one handsome octavo volume of 312 pages, extra
cloth  $1 75. (INow Ready.)




AND SCIENTIFIC PUBLICATIONS.                                         19
HODGE (HUGH L.), M.D.,
Professor of Midwifery and the Diseases of Women and Children in the University of Pennsylvania, &c.
ON   DISEASES  PECULTIAR  TO  WOMEN, including Displacements of the
Uterus. W\Vith original illustrations. In one beautifully printed octavo volume, of nearly 500
pages. (N1ow Ready.)
The profession will look with much interest on a volume embodying the long and extensive experierce of Professor Hodge on an important branch of practice in which his opportunities for
investigation have been so extensive. A short summary of the contents will show the scope of
the work, and the manner in which the subject is presented. It will be seen that, with the exception of Displacements of the Uterus, he divides the Diseases peculiar to Women into iwo great
constitutional classes-those arising from irritation, and those arising from sedation.
CONTENTS.
PART I. DISEASES OF IRRITATION.-CHAPTER I. Nervous Irritation, and its Consequences — I.
Irritable Uterus.-II. Local Symptoms of Irritable Uterus: Menorrhagla and Heemorrhdgia;
Leucofrhcea; Dysmenorrhcea -IV. Local Symptoms of Irritable Uterus; Complications. —\.
General Symptoms of Irlitable Uterus: Cerebro-spinl Irritations.-VI. General Symptoms of
Irritable Uterus.-VII. Progress and Results of Irritable Uterus.-VIII. Causes and Pathology
of Irritable Diseases -IX. Treatment of Irritable Uterus; Removal or Palliation of the Cause.
-X. Treatment of' Irritable Uterus: To Diminish or Destrov the Morbid Irritability -Xl.
Treatment of the Complications of Irritable Uterus.-XII. Treaitment of the Complications of
Irritable Uterus.
PART II. DIsPLACEIENTS OF THE UTERUS.-CHAPTER I. Natural Position and Supports of the
Uteru,. —II. Varieties of Displacements of the Uterus. and their Causes.-II. Symptoms of
Displacements of the Uterus.-IV. Treatment of Displacements of the Uterus.-V. Treatment
of Displacements; Internal Supports.-VI. Treatment of Displacements; Lever Pessaries.VII. Treatment of the Varieties of Displacements.-VIII. Treatment of' Complications of Displacements of the Uterus.-IX. Treatment of Enlargements and Displacements of the Ovaries, &c.
PART III. DISEAsES OF SEDATION.-CHAPTER I. Sedation and its Consequences: Organic and
Nervwous Sedation; Passive Congestion; Reaction; Treatment -II. Sedation of the Uterus;
Amenorrhcea: Sedation of the Uterus from Aloral Causes; Sedation of the Uterus from Physical
Causes. —III. Diagnosis and Treatment of Setdation of the Uterus.
The illustrations, which are all original, are drawn to a uniform scale of one-half the natural size.
JONES (T. WHARTON), F. R. S.,
Professor of Ophthalmic Medicine and Surgery in University College, London, &c.
THE PRINCIPLES AND PRACTICE OF OPHTHALMIIC MEDICINE
AND SURGERY. WVith one hundred and ten illustrations. Second American from the second
and revised London edition, with additions by EDWARD HARTSHORNE, M. D., Surgeon to Wills'
Hospital, &c. In one large, handsome royal 12mo. volume, extra cloth, of 500 pages. $1 50.
JONES (C. HANDFIELD), F. R. S., & EDWARD H. SIEVEKING, M.D.,
Assistant Physicians and Lecturers in St. Mary's Hospital, London.
A MANUAL OF PATHOLOGICAL ANATOMY. First American Edition,
Revised. With three hundred and ninety-seven handsome wood engravings. In one large and
beautiful octavo volume of nearly 750 pages, leather. $3 75.
A s a concise text-book, containing, in a condensed obliged to glean from a great number of monographs,
form, a comp ete outline of what is known in the and the field was so extensive that but few cultivated
domain of Pathological  Anatomy, it is perhaps the it with any degree of success. As a simple work
best work in the Einglish language. Its great merit of reference, therefiore, it is of great value to the
consists in its complleteness and brevity, and in this student of patlhological anatomy, and should be in
respect it supplies a great desideratum in our lite- every physician's library.-lT'estern Lancet.
rature. Heretofore thie student of pathology was
KIRKES (WILLIAM SENHOUSE), M. O.,
Demonstrator of Morbid Anatomy at St. Bartholomew's Hospital, &c.
A  MANUAL OF PHYSIOLOGY.  A new American, from the third and
improved London edition. With two hundred illustrations. In one large and handsome royal
12mo. volume, leather. pp. 586. $2 00. (Lately Published.)
This is a new and very much Improved edition of   One of the very best handbooks of Physiology we
Dr. Kirkes' well-known Handbook of Physiiology. possess-presenting just sueh an outline of the sciIt combines conciseness with completeness, and is, ence as the student requires during his attendance
therefore. admirably adapted for consultation by the upon a course of lectures, or for reference whilst
busy pracritioner.-Dublin Quarterly Journal.  preparing for examination.-Am. Medical Journal.
Its excellence is in its compactness, its clearness,   For the student beginning this study, and the
and its carefully cited authorities. It is the most practitioner who has but leisure to refresh his
convenientoftext-books. These gentlemen, Messrs. memory, this book is invaluable, as it contains all
Kirkes and Paget, have really an immense talent for that it is important to know, without special details,
silence, which is not so common or so cheap as prat- which are read with interest only by those who
ing people fancy. They have the gift of telling us would make a specialty, or desire to possess a critiwhat we want to know, without thinking it neces- cal knowledge of the subject-Charleston Mled.
sary to tell us all they know.-Boston Med and Journal.
Surg. Journal.




20                     BLANCHARD & LEA'S MEDICAL
KNAPP'S TECHNOLOGY; or, Chemistry applied LAYCOCK'S LECTURES ON  THE PRINCIto the Arts and to Manufactures. Edited by Dr.   PLES AND METHODS OF MEDICAL OBRONALDS, Dr. RICHARDSON, and Prof. W. R.   SERVATION AND RESEARCH. For the Use
JOHNSON. In two handsome 8vo. vols., with about   of Advanced Students and Junior Practitioners.
500 wood engravings. $6 00.                    In one royal 12mo. volume, extra cloth. Price $1.
LALLEMAND AND WILSON.
A PRACTICAL TREATISE ON THE CAUSES, SYMPTOMS, AND
TREATMENT OF SPERMATORRH(EA. By M. LALLEMAND. Translated and edited by
HENRY J MCDOUGALL. Third American edition. To which is added -    ON DISEASES
OF THE VESICUL2E SEMINALES; AND THEIR ASSOCIATED ORGANS. With special reference to the Morbid Secretions of the Prostatic and Urethral Mucous Membrane. By MARRIS
WILSON, M. D. In one neat octavo volume, of about 400 pp., extra cloth. $2 00. (Jecst Issued.)
LA ROCHE (R.), M. D., &c.
YELLOW   FEVER, considered  in its Historical, Pathological, Etiological, and
Therapeutical Relations. Including a Sketch of the Disease as it has occurred in Philadelphia
from 1699 to 1854, with an examination of the connections between it and the fevers known under
the same name in other parts of temperate as well as in tropical regions. In two large and
handsome octavo volumes of nearly 1500 pages, extra cloth. $7 00.
From Professor S. H. Dickson, Charleston, S. C., nant and unmanageable disease of modern times,
September 18, 1855.             has for several years been prevailing in our country
A monument of intelligent and well applied re- to a greater extent than ever before; that it is no
search, almost without example. It is, indeed, in longer confined to either large or small cities, but
itself. a large library, and is destined to constitute penetrates country villages, plantations, and farmthe special resort as a book of reference, in the houses; that it is treated with scarcely better sucsubject of which it treats, to all future time.  cess now than thirty or forty years ago; that there
TW/Te have not time at present, engaged as we are, is vast mischief done by ignorant pretenders to knowby da~y andl by night, in the work of combating this ledge in regard to the disease, and in view of the probability that a majority of southern ph ysicians will
very disease, now prevailing in our city, to do more ability th at   a majority of sou thern ph ians will
than give this cursory notice of what we consider becalledupontotreatthedisease,wetrustthatthis
as undoubtedly the most able and erudite medical able and comprehensive treatise will he very genepublication our country has yet produced. But n rally read in the south.-emphis Med. Recorder.
view of the startling fact, that this, the most maligBY THE SAME AUTHOR.
PNEUMONIA; its Supposed Connection, Pathological and Etiological, with Autumnal Fevers, including an Inquiry into the Existence and Morbid Agency of Malaria. In one
handsome octavo volume, extra cloth, of 500 pages. $3 00.
LUDLOW   (J. L.), M. D.
A MANUAL OF EXAMINATIONS upon Anatomy, Physiology, Surgery,
Practice of Medicine, Obstetrics, Materia Medica, Chemistry, Pharmacy, and Therapeutics. To
which is added a Medical Formulary. Third edition, thoroughly revised and greatly extended
and enlarged. With 370 illustrations. In one handsome royal 12mo. volume, leather, of 816
large pages. $2 50.
The great popularity of this volume, and the numerous demands for it during the two years in which
it has been out of print, have induced the author in its revision to spare no pains to render it a
correct and accurate digest of the most recent condition of all the branches of medical science. In
many respects it may, therefore, be regarded rather as a new book than a new edition, an entire
section on Physiology having been added, as also one on Organic Chemistry, and many portions
having been rewritten. A very complete series of illustrations has been introduced, and every
care has been taken in the mechanical execution to render it a convenient and satisfactory book for
study or reference. The arrangement of the volume in the form of question and answer renders it
especially suited for the office examination of students and for those preparing for graduation.
We know of no better companion for the student crammed into his head by the various professors to
during the hours spent in the lecture room, or to re- whom he is compelled to listen.-Vestern Lance:,
fresh, at a glance, his memory of the various topics May, 1857.
LEHMANN (C. G.)
PHYSIOLOGICAL  CHEMISTRY.   Translated from  the  second  edition  by
GEORGE E. DAY, M. D., F. R. S., &c., edited by R. E. ROGERS, M. D., Professor of Chemistry
in the Medical Department of the University of Pennsylvania, with illustrations selected from
Funke's Atlas of Physiological Chemistry, and an Appendix of plates. Complete in two large
and handsome octavo volumes, extra cloth, containing 1200 pages, with nearly two hundr'ed illustrations. $6 00.
The work of Lehmann stands unrivalled as the   The most important contribution as yet made to
most comprehensive book of reference and informa- Physiological Chemistry.-Am. Journal Med. Scition extant on every branch of the subject on which [ ences, Jan. 1856.
it treats.-Edinburgh Journal of Medical Science.
BY THE SAME AUTHOR. (Lately Pzeblished.)
MANUAL OF CHEMICAL PHYSIOLOGY.  Translated from the German,
with Notes and Additions, by J. CHESTON MORRIS, M. D., with an introductory Essay on Vital
Force, by Professor SAMUEL JACKSON, M. D., of the University of Pennsylvania. With illustrations on wood. In one very handsome octavo volume, extra cloth, of 336 pages. $2 25.
FroEm Prof. Jackson's Introdscctory Essay.
In adopting the handbook of Dr. Lehmann as a manual of Organic Chemistry for the use of the
students of the University, and ill recommending his original work of PHYSIOLOGICAL CHEMISTRY
for their more mature studies, the high value of his researches, and the great weight of his authority in that important department of medical science are fully recognized.




AND SCIENTIFIC PUBLICATIONS.                                         21
LAWRENCE (W.), F. R. S., &c.
A TREATISE ON DISEASES OF THE EYE. A new edition, edited,
with numerous additions, and 243 illustrations, by ISAAc HAYS, M. D., Surgeon to Will's Hospital, &c. In one very large and handsome octavo volume, of 950 pages, strongly bound in leather
with raised bands. $5 00.
MEIGS (CHARLES D.), M. D.,
Professor of Obstetrics, &c. in the Jefferson Medical College, Philadelphia.
OBSTETRICS: THE SCIENCE AND THE ART. Third edition, revised
and improved. With one hundred and twenty-nine illustrations. In one beautifully printed octavo
volume, leather, of seven hundred and fifty-two large pages. $3 75.
The rapid demand for another edition of this work is a sufficient expression of the favorable
verdict of the profession. In thus preparing it a third time for the press, the author has endeavored
to render it in every respect worthy of the favor which it has received. To accomplish this he
has thoroughly revised it in every part. Some portions have been rewritten, others added, new
illustrations have been in many instances substituted for such as were not deemed satisfactory,
while, by an alteration in the typographical arrangement, the size of the work has not been increased,
and the price remains unaltered. In its present improved form, it is, therefore, hoped that the work
will continue to meet the wants of the American protession as a sound, practical, and extended
SYSTEM OF MIDWIFERY.
Though the work has received only five pages of   The best American work on Midwifery that is
enlargement, its chapters throughout wear the im- accessible to the student and practitioner-N. W.
press of careful revision. Expunging and rewriting, Med. and Surg. Journal, Jan. 1857.
remodelling its sentences, with occasional new ma-   This is a standard work by a great American Ohterial, all evince a lively desire that it shall deserve stetrician. It is the third and last edition, and, in
to be regarded as improved in manner as well as the language of the preface,theauthorhas"brought
matter. In the matter, every stroke of the pen has the subject up to the latest dates of real improveincreased the value of the book, both in expungings ment in our art and Science."-Nashville Journ. of
and additions -Western Lancet, Jan. 1857.      Med. and Surg., May, 1857.
BY THE SAME AUTHOR. (Juist Issued.)
WOMAN: HER DISEASES AND THEIR REMEDIES. A Series of Lectures to his Class. Fourth and Improved edition. In one large and beautifully printed octaveo
volume, leather, of over 700 pages. $3 60.
In other respects, in our estimation, too much can-   Full of important matter, conveyed in a ready and
not be said in Draise of this work. It abounds with  agreeable manner.-St. Louis Med. and Surg. Jour.
beautiful passages, and for conciseness, for originality, and for all that is commendable in a work on   There is an off-hand fervor, a glow, anld a warmthe diseases of females, it is not excelled, and pro- heartedness infecting the effort of Dr. Ieigs, which
bubly not equalled in the English language. On the  is entirely captivating, and which absolutely hurwhole, we know of no worli on the diseases of wo-ries the reader through from beginning to end. Bemen which we can so cordially commend to the  sides, the book teems with solid instruction, and
student and practitioner as the one before us.-Ohio  it shows the very highest evidence of ability, viz.,
Med. and Surg. Journal.                        the clearness with which the information is presented. We know of no better test of one's underThe body of the book is worthy of attentive con- standing a subject than the evidence of the power
sideration, and is evidently the proiduction of a of lucidly explaining it. The most elementary, as
clever, thoughtful, and sagacious physician. Dr. well as the obscurest subjects, under the pencil of
Meigs's letters on the diseases of the external or-  Prof. Meigs, are isolated and made to stand out in
gans, contain many interesting and rare cases, and  such bold relief, as to produce distinct inipressiona
many instructive observations. We take our leave upon the mind and memory of the reader. - Tie
of Dr. Meigs, with a high opinion of his talents and Charleston Med. Journal.
originality.-The British and Foreign  Medico-Chi.
rurgical Reaieew.                                Professor Meigs has enlarged and amended this
great work, for such it unquestionably is, having
Every chapter is replete with practical instruc- passed the ordeal of criticism at home and abroad,
tion, and bears the impress of being the composition  hut been improved thereby; for in this new edition
of an       and experienced mind. There is aterse- the author has introduced real improvements, and
ness, and at the same time an accuracy in his de- increased the value and utility of the book imscription of symptoms, and in the rules for diagnosis, measurably. It presents so many novel, bright
which cannot fail to recommend the volame to the and sparkling thoughts; such an exuberance of new
attention of the reader.-Ranking's Abstract.   ideas on almost every page, that we confess ourIt contains a vast amount of practical knowledge, selves to have become enamored with the book
by one who has accurately observed and retained and its author; and cannot withhold our congratuthe experience of many years. —Dublin Quarterly lations from our Philadelphia confreres, that such a
Journal.                                       teacher is in their service.-N. Y. Med. Gazette.
BY THE SAME AUTHOR.
ON THE NATURE, SIGNS, AND TREATMENT OF CHILDBED
FEVER. In a Series of Letters addressed to the Students of his Class. In one handsome
octavo volume, extra cloth, of 365 pages. $2 50.
The instructive and interesting author of this lectable book. *  * *   This treatise upon childwork, whose previous labors have placed his coun- bed fevers will have an extensive sale, being destrymen under deep and abiding obligations, again  tined, as it deserves, to find a place in the library
challenges their admiration in the fresh and vigor- of every practitioner who scorns to lag in the rear.ous, attractive and racy pages before us. It is a de- Nashville Journal of Medicine and Surgery.
BY THE SAME AUTHOR; WITH COLORED PLATES.
A TREATISE ON ACUTE AND CHRONIC DISEASES OF THE NECK
OF THE UTERUS. With numerous plates, drawn and colored from nature in the highest
style of art. In one handsome octavo volume, extra cloth. $4 50.
MAYNE'S DISPENSATORY  AND  THERA- MALGAIGNE'S OPERATIVE SURGERY, based
PEUTICAL REMEMBRANCE;R. WVith every   on Normal and Pathological Anatomy.  TransPractical Formula contained in the three British   lated from the French by FREDERICK BRITTAN,
Pharmacopoeias. Edited, with the addition of the   A. B.,M.D. Vith numerous illustrations onwood.
Formulae of the U. S. Pharmacop(eia, by R. E.   In one handsome octavo volume, extra cloth, of
GRIFFITH,M. D. 1 12mo. vol. ex. cl.,300 pp. 75 c.   nearly six hundred pages. $2 25.




22                     BLANCHARD & LEA'S MEDICAL
MACLISE (JOSEPH), SURGEON.
SURGICAL  ANATOMY.  Forming  one volume, very large  imperial quarto.
With sixty-eight large and splendid Plates, drawn in the best style and beautifully colored. Containing one hundred and ninety Figures, many of them the size of life. Together with copious
and explanatory letter-press. Strongly and handsomely bound in extra cloth, being one of the
cheapest and best executed Surgical works as yet issued in this country. $11 00.
*** The size of this work prevents its transmission through the post-office as a whole, but those
who desire to have copies forwarded by mail, can receive them in five parts, done up in stout
wrappers. Price $9 00.
One of the greatest artistic triumphs of the age F A work which has no parallel in point of accuin Surgical Anatomy.-British American Medical racy and cheapness in the English language.-N. Y.
Journal.                                        Journal of Medicine.
No practitioner whose means will admit should
fail to possess it. —Ranking's Abstract.          We are extremely gratified to announce to the
profession the completion of this truly magnificent
Too much cannot be said in its praise; indeed,       which, as  whole, certainly stands unriweavenot lan eto do it justice-Ohio Medi- work, which, as a whole, certainly stands unriwe have nut languaige to do it Justice.-Oho Med   valled, both for accuracy of drawing, beauty of
cal and Surgical Journal.                       coloring, and all the requisite explanations of the
The most accurately engraved and beautifully  subject in hand.-The New Orleans Medical and
colored plates we have ever seen in an American  Surgical Journal.
book-one of the best and cheapest surgical works
ever published.-Buffalo Medical Journal.          This is by far the ablest work on Surgical Anatomy that has come under our observation. We
It is very rare that so elegantly printed, so well know of no other work that would justify a stuillustrated, and so useful a work, is offered at so dent, in any degree, for neglect of actual dissecmoderate a price.-Charleston Medical Journal.    tion. In those sudden emergencies that so often
arise, and which require the instantaneous command
Its plates can boast a superiority which places of minute anatomical knowledge, a work of this kind
them almost beyond the reach of competition.-Medithem almonstbeyond ther reac k ofo ompetition. —Meai- keeps the details of the dissecting-room perpetually
cal Examqiner.                                  fresh in the memory.-The Western Journal of MediCountry practitioners will find these plates of im- cine and Surgery.
mense value.-N. Y. Medical Gazette.
M I LLER (H EN RY), M. D.,
Professor of Obstetrics and Diseases of Women and Children in the University of Louisville.
PRINCIPLES ANi) PRACTICE OF OBSTETRICS, &c,; including the Treatment of Chronic Inflammation of the Cervix and Body of the Uterus considered as a frequent
cause of Abortion. With about one hundred illustrations on wood. In one very handsome octavo volume, of over 600 pages. (Lately Published.)  $3 75.
The reputation of Dr. Miller as an obstetrician is too widely spread to require the attention ot
the profession to be specially called to a volume containing the experience of his long and extensive
practice. The very favorable reception accorded to his " Treatise on Human Parturition," issued
some years since, is an earnest that the present work will fulfil the author's intention of providing
within a moderate compass a complete and trustworthy text-book for the student, and book of reference for the practitioner.
We congratulate the author that the task is done. tion to which its merits justly entitle it. The style
We congratulate hitn that he has given to the medi-  is such that the descriptions are clear, and each subcal public a work which will secure for him a high  ject is discussed and elucidated with due regard to
and permanent position among the standard autho- its practical bearings, which cannot fail to make it
rities on the principles and practice of obstetrics. acceptable and valuable to both students and pracCongratulations are not less due to the medical pro- titioners.  We cannot, however, close this brief
fession of this country, on the acquisition of a trea- notice without congratulating the author and the
tise embodying the results of the studies, reflections, profession on the production of such an excellent
and experience of Prof. Miller. Few men, if any, treatise. The author is a western man of whom we
in this country, are more competent than he to write  feel proud, and we cannot but think that his book
on this department of medicine. Engaged for thirty- will find many readers and warm admirers wherever
five years in an extended practice of obstetrics, for obstetrics is taught and studied as a science and an
many years a teacher of this branch of instruction  art.-The Cincinnati Lancetand Observer, Feb. 1858.
in one of the largest of our institutions, a diligent   A most respectable and valuable addition to our
student as well as a careful observer, an original and home medical literature, and one reflecting credit
independent thinker, wedded to no hobbies, ever alike on the author and the institution to which he
ready to consider without prejudice new views, and  is attached. The student will find in this work a
to adopt innovations if they are really improvements, most useful guide to his studies; the country pracand withal a clear, agreeable writer, a practica  titioner, rusty in his reading, can obtain from its
treatise from his pen could not fail to possess great pages a fair resume of the modern literature of the
value.-Buffalo Med Journal, Mlar. 1858.         science; and we hope to see this Americen producIn fact, this volume must take its place among the  tion generally consulted by the profession.-Va.
standard systematic treatises on obstetrics; a posi- Mled. Journal, Feb. 1858.
MACKENZIE (W.), M. D.,
Surgeon Oculist in Scotland in ordinary to Her Majesty, &c. &c.
A PRACTICAL TREATISE ON DISEASES AND INJURIES OF THE
EYE. To which is prefixed an Anatomical Introduction explanatory of a Horizontal Section of
the Human Eyeball, by THOMAS WHARTON JONES, F. R. S. From the Fourth Revised and Enlarged London Edition. With Notes and Additions by ADDINELL HEWSON, M. D., Surgeon to
Wills Hospital, &c. &c. In one very large and handsome octavovolume, leather, raised bands, with
plates and numerous wood-cuts. $5 25.
The treatise of Dr. Mackenzie indisputably holds able manner in which the author's stores of learning
the firstplace, and forms, in respect of learning and and experience were rendered available for general
research, an Encyclopuedia unequalled in extent by use, at once procured for the first edition, as well on
any other work of the kind, either English or foreign. the continent as in this country, that high position
-Dixon on Diseases of the Eye.                  as a standard work which each successive edition
Few modern books on any department of medicine has more firmly established.  We consider it the
or surgery have met with such extended circulation, duty of every one who has the love of his profession
or have procured for their authors a like amount of and tie welfare of his patient at heart, to make himEuropean celebrity. The immense research which  self familiar with this the most complete work in
it displayed, tihe thorough acquaintance with tlhe the English language upon the diseases of the eye.
subject, practically as well as...ati.oly, and the — Med. Tisnes and Gazette.




AND SCIENTIFIC PUBLICATIONS.                                           23
MILLER  (JAMES), F. R. S. E.,
Professor of Surgery in the University of Edinburgh, &c.
PRINCIPLES OF SURGERY. Fourth American, from the third and revised
Edinburgh edition. In one large and very beautiful volume, leather, of 700 pages, with two
hundred and forty illustrations on wood. $3 75.
The work of Mr. Miller is too well and too favor-   The work takes rank with Watson's Practice of
ably known among us, as one of our best text-books, Physic; it certainly does not fall behind that great
to render any further notice of it necessary than the work in soundness of principle or depth of reasonannouncement of a new edition, the fourth in our ing and research. No physician who values his recountry, a proof of its extensive circulation among putation, or seeks the interests of his clients, can
us. As a concise and reliable exposition of the sci- acquit himself before his God and the world without
ence of modern surgery, it stands deservedly high-  making himself familiar with the sound and philowe know not its superior.-Boston 3Med. and Surg. sophical views developed in the foregoing book.Journal.                                        New Orleans Med. and Surg. Journal.
BY THE SAME AUTHOR. (Just Issued.)
THE PRACTICE OF SURGERY.  Fourth American from the last Edinburgh edition. Revised by the American editor. Illustrated by three hundred and sixty-four
engravings on wood. In one large octavo volume, leather, of nearly 700 pages. $3 75.
No encomium of ours could add to the popularity  his works, both on the principles and practice of
of Miller's Surgery. Its reputation in this country  surgery have been assigned the highest rank. If we
is unsurpassed by that of any other work, and, when were limited to but one work on surgery, that one
taken in connection with the author's Principles of should be Miller's, as we regard it as superior to all
Surgery, constitutes a whole, without reference to others.-St. Louis Med. and Surg. Journal.
to which no conscientious surgeon would be willing
practice his art  d Sug. ourn   The author has in this and his " Principles," prepractice his mart.-aSothe rn vme. and Seg. Journald. sented to the profession one of the most complete and
It is seldom that two volumes have ever made so reliable systems of Surgery extant. His style of
profound an impression in so short a time as the writing is original, impressive, and engaging, eer" Principles" and the " Practice" of Surgery by getic., concise, and lucid. Few have the faculty of
Mr. Miller-or so richly merited the reputation they  condensing so much in small space, and at the same
have acquired. The author is an eminently sensi- time so persistently holding theattention. Whether
ble, practical, and well-informed man, who knows as a text-book for students or a book of reference
exactly what he is talking about and exactly how to for practitioners, it cannot be too strongly recontalk it.-Kentucky Medical Recorder.             mended.-Southern Journal of Mled. and Physical
By the almost unanimous voice of the profession, Sciences.
MORLAND  (W. W.), M. D.,
Fellow of the Massachusetts Medical Society, &c.
DISEASES OF TIlE URINARY ORGANS; a Compendium of their Diagnosis,
Pathology, and Treatment. With illustrations.  In one large and handsome octavo volume, of
about 600 pages, extra cloth. (Just Issued.) $3 50.
Taken as a whole, we can recommend Dr. Mor- yet in a succinct, narrational style, such as to render
land's compendium as a very desirable addition to  the work one of great interest, and one which will
the library of every medical or surgical practi- prove in the highest degree useful to the general
tioner.-Brit. and For. Med.-Chir. Rev., April, 1859. practitioner. To the members of the profession in the
Every medical practitioner whose attention has country it will be peculiarly valuable, on account
been to any extent attracted towards the class of of the characteristics which we have mentioned,
diseases to which this treattise retl tes, must have and the one broad aim of practical utility which is
often and sorely experienced the want of some full, kept in view, and which shines out upon every page,
yet concise r:eent compendiumn to which he could together with the skill which is evinced in the comrefer. This desideratum has been sunplied by Dr. bination of this grand requisite with the utmost
Morland, and it tias been ably done. He has placed  brevity which a just treatment of the subjects would
before us a full, judicious, and reliable digest. admit.-N. Y. Journ. of Medicine, Nov. 1858.
Each subject is treated with sufficient minuteness,
MONTGOMERY  (W. F.), M. D.$ M. R. I. A., &c.,
Professor of Midwifery in the King and Q(ueen's College of Physicians in Ireland, &c.
AN EXPOSITION OF THE SIGNS AND SYMPTOMS OF PREGNANCY.
With some other Papers on Subjects connected with Midwifery. From the second and enlarged
English edition. With two exquisite colored plates, and numerous wood-cuts.  In one very
handsome octavo volume, extra cloth, of nearly 600 pages. (Lately Published.) $3 75.
A book unusually rich in practical suggestions.-  has been weighed and reweighled through years of
Am. Journal Med. Sciences, Jan. 1857.           preparation; that this is of all others the book of
These several subjects so interesting in them- Obstetric Law, on each of its several topics; on all
selves, and so important, every one of them, to the points connected with pregnancy, to be everywhere
most delicate and precious of social relations, con- received as a manual of special jurisprudence, ht
trolling often the honor and domestic peace of a once announcing fact, affording argument, establishfamily, the legitimacy of offspring, or the life of its ing precedent, and governing alike the juryman, adparent, are all treated with an elegance of diction, vocate, and jude. It is not merely in its legal refulness of illustrations, acuteness and justice of rea-lations that we ind this work so interesting  Hardly
soning, unparalleled in obstetrics, and unsurpassedin  a page bitt that has its hints or facts important to
medicine. The reader's interest can never flag, so the general practitioner; and not a chapter without
fresh, and vigorous, and classical is our author's especial matter for the anatoniist, physiologt, or
style; and one forgets, in the renewed charm of patllologist.-N. A. MIed.-Chir. Review, March,
every page, that it, and every line, and every word 1857.
MOHR (FRANCIS), PH. D., AND REDWOOD (THEOPHILUS).
PRACTICAL PHARMACY. Comprising the Arrangements, Apparatus, and
Manipulations of the Pharmaceutical Shop and Laboratory. Edited, with extensive Additions,
by Prof. WILLIAM  PROCTER, of the Philadelphia College of Pharmacy. In one handsomely
printed octavo volume, extra cloth, of 570 pages, with over 500 engravings on wood. $2 75!




24                    BLANCHARD & LEA'S MEDICAL
NEILL (JOHN), M. D.,
Surgeon to the Pennsylvania Hospital, &c.; and
FRANCIS GURNEY SMITH, M. D.,
Professor of Institutes of Medicine in the Pennsylvania Medical College.
AN ANALYTICAL COMPENDIUM OF THE VARIOUS BRANCHES
OF MEDICAL SCIENCE; for the Use and Examination of Students. A new edition, revised
and improved. In one very large and handsomely printed royal 12mo. volume, of about one
thousand pages, with 374 wood-cuts. Strongly bound in leather, with raised bands. $3 00.
The very flattering reception which has been accorded to this work, and the high estimate placed
upon it by the profession, as evinced by the constant and increasing demand which has rapidly exhausted two large editions, have stimulated the authors to render the volume in its present revision
more worthy of the success which has attended it. It has accordingly been thoroughly examined,
and sluch errors as had on former occasions escaped observation have been corrected, and whatever
additions were necessary to maintain it on a level with the advance of science have been introduced.
The extended series of illustrations has been still further increased and much improved, while, by
a slight enlargement of the page, these various additions have been incorporated without increasing
the bulk of the volume.
The work is, therefore, again presented as eminently worthy of the favor with which it has hitherto
been received. As a book for daily reference by the student requiring a guide to his more elaborate
text-books, as a manual for preceptors desiring to stimulate their students by frequent and accurate
examination, or as a source from which the practitioners of older date may easily and cheaply acquire
a knowledge of the changes and improvement in professional science, its reputation is permanently
established.
The best work of the kind with which we are the students is heavy, and review necessary for an
acquainted.-Med. Examiner.                    examination, a compend is not only valuable, but
Having made free use of this volume in our ex-it is almost a sine qua non. The one before us is
aminations of pupils, we can speak from experi- in most of the divisions, the most unexceptionable
ence in recommending it as an admirable compend  of all books of the kind that we know of.  The
for students, and as especially useful to preceptors newest and soundest doctrines and the latest imwho examine their pupils. It will save the teacher provements and discoveries are explicitly, though
much labor by enabling him readily to recall all of concisely, laid before the student. There is a class
the points upon which his pupils should be ex- to whom we very sincerely commend this cheap book
amined. A work of this sort should be in the hands as worth its weight in silver-that class is tile graduof every one who takes pupils into his office with a ates in medicine of more than ten years' standing,
view of examining them; and this isunquestionably  who have not studied medicine since. They will
the best of its class.-Transylvania  Med. Journal,  perhaps find out from it that the science is not exactly
now what it was when they left it off.-The StethoIn the rapid course of lectures, where work for scope.
NELIGAN  (J. MOORE), M. D., M. R. l.A., &c.
(A splendid work. Just Issued.)
ATLAS OF CUTANEOUS DISEASES. In one beautiful quarto volume, extra
cloth, with splendid colored plates, presenting nearly one hundred elaborate representations of
disease. $4 50.
This beautiful volume is intended as a complete and accurate representation of all the varieties
of Diseases of the Skin. While it can be consulted in conjunction with any work on Practice, it has
especial reference to the author's " Treatise on Diseases of the Skin," so favorably received by the
profession some years since. The publishers feel justified in saying that few more beautifully executed plates have ever been presented to the profession of this country.
Neligan's Atlas of Cutaneous Diseases supplies a give, at a coup d'ceil, the remarkable peculiarities
long existent desideratum much felt by the largest of each individual variety. And while thus the dis
class of our profession. It presents, in quarto size, ease is rendered more definable, there is yet no loss
16 plates, each containing from 3 to 6 figures, and of proportion incurred by the necessary concentraforming in all a total of 90 distinct representations tion. Each figure is highly colored, and so truthful
of the different species of skin affections, grouped has the artist been that the most fasted ous observer
together in genera or families. The illustrations could not justly take exception to the correctness of
have been taken from nature, and have been copied the execution of the pictures under his scrutiny.with such fidelity that they present a striking picture Montreal Med. Chronicle.
of life; in which the reduced scale aptly serves to
BY THE SAME AUTHOR.
A PRACTICAL TREATISE ON DISEASES OF THE SKIN. Third
American edition. In one neat royal 12mo. volume, extra cloth, of 334 pages. $1 00.
~        The two volumes will be sent by mail on receipt of Five Dollars.
OVWEN ON  THE DIFFERENT  FORMS OF I One vol. royal 12mo., extra cloth with numerous
THE SKELETON, AND OF THE TEETH.   illustrations. $1 25.
PIRRI E (WILLIAM), F. R. S. E.,
Professor of Surgery in the University of Aberdeen.
THE PRINCIPLES AND PRACTICE OF SURGERY. Edited by JOHN
NEILL, M. D., Professor of Surgery in the Penna. Medical College, Surgeon to the Pennsylvania
Hospital, &c. In one very handsome octavo volume, leather, of 780 pages, with 316 illustrations.
$3 75.
We know of no other surgical work of a reason- rately discussed the principles of surgery, and a
able size, wherein there is so much theory and prac- safe and effectual practice predicated upon them.
tice, or where subjects are more soundly or clearly Perhaps no work upon this subject heretofore issued
taught.-The Stethoscope.                      is so full upon the science of the art of surgery. —
Prof. Pirrie, in the work before us, has elabo. Nashville Journal of Medicine and Surgery.




AND SCIENTIFIC PUBLICATIONS.                                        25
PARRISH (EDWARD),
Lecturer on Practical Pharmacy and Materia Medica in the Pennsylvania Academy of Medicine, &c.
AN INTRODUCTION TO PRACTICAL PHARMACY. Designed as a TextBook for the Student, and as a Guide for the Physician and Pharmaceutist. With many Formulee and Prescriptions.  Second edition, greatly enlarged and improved.  In one handsome
octavo volume of 720 pages, with several hundred Illustrations, extra cloth. $3 50. (1TVow
Ready.)
During the short time in which this work has been before the profession, it has been received
with very great favor, and in assuming the position of a standard authority, it has filled a vacancy
which had been severely felt. Stimulated by this encouragement, the author, in avaJliln himself
of the opportunity of revision, has spared no pains to render it more worthy of the confidence bestowed upon it, and his assiduous labors have made it rather a new book than a new edition, many
portions having been rewritten, and much new and important matter added. These alterations and
improvements have been rendered necessary by the rapid progress made by pharmaceutical science
during the last few years, and by the additional experience obtained in the practical use of the
volume as a text-book and work of reference. To accommodate these improvements, the size of
the page has been materially enlarged, and the number of pages considerably increased, presenting
in all nearly one-half more matter than the last edition. The work is therefore now presented as a
complete exponent of the subject in its most advanced condition. From the most ordinary matters
in the dispensing office, to the most complicated details of the vegetable alkaloids, it is hoped that
everything requisite to the practising physician, and to the apothecary, will be found fiillv and
clearly set forth, and that the new matter alone will be worth more than the very moderate cost of
the work to those who have been consulting the previous edition.
That Edward Parrish, in writing a book upon there is no production of the kind in the English
Practical Pharmacy some few years ago-one emi- language so well adapted to the wants of the pharnently original and unique-did the medical and maceutist and druggist. To physicians, also, it canpharmaceutical professions a great and valuable ser- not fail to be highly valuable, especially to those
vice, no one, we think, who has had access to its who are obliged to prepare and compound many of
pages will deny; doubly welcome, then, is this new  their own medicine. —N. Am. Med. Chir. Review,
edition, containing the added results of his recent Jan. 1860.
and rich experience as an observer, teacher, and
practic l operator in the pharmaceutical laboratory.   Of course, all apothecaries who have not already
The excellent plan of the first is more thoroughly, a copy of the first edition will procure one of this;
and in detail, carried out in this edition.-Peninsular it is, therefore, to physicians residing in the country
Med. Journal, Jan. 1960.                      and in small towns, who cannot avail themselves of
W~e know of no work on the subject which would the skill of an educated pharmaceutist, that we
be more indispensable to the physician or student would especially commend this work. In it they
desiring information on the subject of which it treats. will find all that they desire to know, and should
With Griffith's " Medical Formulary" and this. the know, but very little of which they do really Know
practising physician would be supplied with nearly in reference to this important collateral branch of
or quite all the most useful infortation on the sub- their profession; for it is a well established fact,
ject.-Charleston Med. Journal and Review, Jan. that, in the education of physicians, while the sci1860.'      ence of medicine is generally well taught, very
1860.                                       little attention is paid to the art of preparing them
This edition, now much enlarged, is one of the for use, and we know not how this defect can be so
most useful works of the past year.-N. 0. Med. well remedied as by procuring and consulting Dr.
aned Surg. Journal, Jan. 1860.                PaTrish's excellent work.-St. Louis Med. Journal.
The whole treatise is eminently practical; and Jan. 1860.
PEASLEE  (E. R.), M. D.,
Professor of Physiology and General Pathology in the New York Medical College.
HU3MAN  HISTOLOGY, in its relations to Anatomy, Physiology, and Pathology;
for the use of Medical Students. With four hundred and thirty-four illustrations. In one handsome octavo volume, of over 600 pages. (Lately Published.) $3 75.
It embraces a library upon the topics discussed   We would recommend it to the medical student
within itself, and is just what the teacher and learner and practitioner, as containing a summary of all that
need. Another advantage, by no means to be over- is known of the important subjects wvlich it treats;
looked, everything of real value in the wide range of all that is contained in the great works of Simon
which it embraces, is with great skill compressed and Lehmann, and the organic chenists in general.
into an octavo volume of but little more than six Master this onevolume, wewould say to the medical
hundred pages. We have not only the whole sub- student and practitioner-master this book and you
ject of Histology, interesting in itself, ably and fully know all that is known of the great fundamental
discussed, but what is of infinitely greater interest principles of medicine, and we have no hesitation
to the student, because of greater practical value, in saying that it is aii honor to the American mediare its relations to Anatomy, Physiology, and Pa- cal profession that one of its members should have
thology, which are here fully and satisfactorily set produced it.-St. Louis MIed. and Surg. Journal,
forth.- Nashville Journ. of Med. and Surgery, Dec. March, 1858.
1857.
PEREIRA (JONATHAN), M. D., F. R. S., AND L. S.
THE ELEMENTS OF MATERIA MEDICA AND THERAPEUTICS.
Third American edition, enlarged and improved by the author; including Notices of most of the
Medicinal Substances in use in the civilized world, and forming an Encyclopmdia of Materia
Medica. Edited, with Additions, by JosEPH CARSON, M. D., Professor of Materia Medica and
Pharmacy in the University of Pennsylvania. In two very large octavo volumes of 2100 pages,
on small type, with about 500 illustrations on stone and wood, strongly bound in leather, with
raised bands. $9 00.
~'~ Vol. II. will no longer be sold separate.
PARKER (LANGSTON),
Surgeon to the Queen's Hospital, Birmingham.
THE MODERN TREATMENT OF SYPHILITIC DISEASES, BOTH PRIMARY AND SECONDARY; comprising the Treatment of Constitutional and Confirmed Syphilis, by a safe and successful method. With numerous Cases, Formulae, and Clinical Observations. From the Third and entirely rewritten London edition. In one neat octavo volume,
extra cloth, of 316 pages. $1 75.




26                     BLANCHARD & LEA'S MEDICAL
RAMSBOTHAM (FRANCIS H.), M.D.
THE PRINCIPLES AND PRACTICE OF OBSTETRIC MEDICINE AND
SURGERY, in reference to the Process of Parturition. A new and enlarged edition, thoroughly
revised by the Author. With Additions by W. V. KEATING, M. D. In one large and handsome
imperial octavo volume, of 650 pages, strongly bound in leather, with raised bands; with sixtyfour beautiful Plates, and numerous Wood-cuts in the text, containing in all nearly two hundred
large and beautiful figures. $5 00.
From Prof. Hodge, of the University of Pa.
To the American public, it is most valuable, from its intrinsic undoubted excellence, and as being
the best authorized exponent of British Midwifery. Its circulation will, I trust, be extensive throughout
our country.
It is unnecessary to say anything in regard to the truly elegant style in which they have brought it
utility of this work. It is already appreciated in our out, excelling themselves in its production, especountry for the value of the matter, the clearness of cially in its plates. It is dedicated to Prof. ltIeigs,
its style,.and the fulness of its illustrations. To the and has the emphatic endorsement of Prof. Hodge,
physician's library it is indispensable, while to the as the best exponent of British Midwifery. We
student as a text-book, from which to extract the knew of no text-book which deserves in all respects
material for laying the foundation of an education on to be more highly recommended to students, and we
obstetrical science, it has no superior.-Ohio Med. could wish to see it in the handsof every practitioner,
and Surg. Tournal.                            for they will find it invaluable for reference.-Med.
The publishers have secured its success by the Gazette.
RICORD (P.), M.D.
A TREATISE ON THE VENEREAL DISEASE. By JOHN HUNTER, F. R. S.
With copious Additions, by PH. RICORD, M.D. Translated and Edited, with Notes, by FREEMA-N
J. BUMSTEAD. M. D, Lecturer on Venereal at the College of Physicians and Surgeons, New York.
Second edition, revised, containing a risumi of RICORD'S RECENT LECTURES ON CHANCRE. In
one handsome octavo volume, extra cloth, of 550 pages, with eight plates. $3 25. (Just Issued.)
In revising this work, the editor has endeavored to introduce whatever matter of interest the recent investigations of syphilographers have added to our knowledge of the subject. The principal
source from which this has been derived is the volume of "Lectures on Chancre," published a few
months since by M. Ricord, which affords a large amount of new and instructive material on many
controverted points. In the previous edition, M. Ricord's additions amounted to nearly one-third
of the whole, and with the matter now introduced, the work may be considered to present his views
and experience more thoroughly and completely than any other.
Every one will recognize thie attractiveness and secretaries, sometimes accredited and sometimes not.
value which this work derives from thus presenring   In the notes to Hunter, the master substitutes himthe opinions of these two masters side by side. But, selfforhis interpreters, and gives hisorgiaial thoulghts
it must be admitted, what has made the fortune of to the world in a lucid and perfectly imlteligible manthe book, is the fact that it contains the " most comn- her. In conclusion we cain say that this is inconplete embodiment of the veritable doctrines of the testably the best treatise on syphilis with which we
H'Ipital du AMidi," which has ever been made public. are acquainted. and, as we do not often employ the
The doctrinial ideas of M. Ricord, ideas which, if not phrase, we may be excused for expressin* the hope
universally adopted, are incontestably dominant, have that it may find a place ill the library of every phyheretofore only been interpreted by more or less skilful I sician.-V irginia fed. and Surg. Journal.
BY THE SAME AUTHOR.
RICORD'S LETTERS ON SYPHILIS. Translated by W. P. LATTIMORE, M. D.
In one neat octavo volume, of 270 pages, extra cloth. $2 00.
ROYLE'S MATERIA MEDICA AND THERAPEUTICS; including the
Preparations of the Pharmacopoeias of London, Edinburgh, Dublin, and of the United States.
With mlany new medicines. Edited by JOSEPH CARSON, M. D. With ninety-eight illustrations.
In one large octavo volume, extra cloth, of about 700 pages. $S. 00.
ROKITANSKY (CARL), M.D.,
Curator of the Imperial Pathological Museum, and Professor at the University of Vienna, &c.
A MANUAL OF PATHOLOGICAL ANATOMY. Four volumes, octavo,
bound in two, extra cloth, of about 1200 pages. Translated by W. E. SWAINE, EDWARD SIEVRKING, C. H. MOORE, and G. E. DAY. $5 50
The profession is too well acquainted with the re- so charged his text with valuable truths, that any
putation of Rokitansky's work to need our assur- attempt of a reviewer to epitomize is at once paraance that this is one of the most profound, thorough  lyzed, and must end in a failure.-Western Lancet.
and valuable books ever issued from the medical   As this is the highest source of knowledge upon
press. It is suigeneris, and hasno standard of cornm- the important subject of which it treats, no real
parison. It is only necessary. to announce that it is student can afford to be without it. The American
issued in a form as cheap as is compatible with its publishers have entitled themselves to the thanks of
size and preservation, and its sale follows as a the profession of their country, for this timeous and
matter of course. No library can be called com- beautiful edition.-Nashaille Journal of Medicine.
plete without it.-Buffalo MArled. Jouraal.      As a book of reference, therefore, this work must
An attempt to give our readers any adequate idea prove of inestimable value, and we cannot too highly
of the vast amount of instruction accumulated in recommend it to the profession.-Charleston Med.
these volumes, would be feeble and hopeless. The Journal and Review.
effort of the distinguished author to concentrate   This book is a necessity to every practitioner.in a small space his great fund of knowledge, has Am. Med. Monthly.
RIGBY (EDWARD), M. D.,
Senior Physician to the General Lying-in Hospital, &c.
A   SYSTEM   OF  MIDWIFERY.  With Notes and  Additional Illustrations.
Second American Edition. One volume octavo, extra cloth, 422 pages. $2 50.
BY THE SAME AUTHOR. (Lately Published.)
ON THE CONSTITUTIONAL TREATMENT OF FEMALE DISEASES.
In one neat royal 12mo. volume, extra cloth, of about 250 pages. $1 00.




AND SCIENTIFIC PUBL1CATIONS,                                         27
STILLE (ALFRED), M. D.
THERAPEUTICS AND MIATERIA MEDICA; a Systematic Treatise. on the
Action, and Uses of Medicinal Agents, including their Description and History. In two large and
handsome octavo volumes, of 1789 pages. (Now Ready, 1860.) $8 00.
This work is designed especially for the student and practitioner ofmedicine, and treats the various
articles of the Materia Medica from the point of view of the bedside, and not of the shop or of the
lecture-room. While thus endeavoring to give all practical information likely to be useful with
respect to the employment of special remedies in special afiections, and the results to be anticipated
from their admilnistration, a copious Index of Diseases and their Remedies renders the work eminently fitted for reference by showing at a glance the different means which have been employed,
and enablitg the practitioner to extend his resources in difficult cases with all that the experience
of' the profession has suggested. At the same time particular care has been given to the subject
of General Therapeutics, and at the commencement of each class of medicines there is a chapter
devoted to the consideration of their common influence upon morbid conditions. The action of
remedial agents upon the healthy economy and on animals has likewise received particular notice,
from the ccnviction- that their physiological effects will afford frequent explanations of their pathologisal influence, and in many eases lead to new and important suggestions as to their practical use
in disease. Within the scope thus designed by the author, no labor has been spared to accumulate
all the facts which have accrued lrom the experience of the profession in all atges and all countries;
and the vast amount of recent researches recorded in the periodical literature of both hemispheres
has been zealously laid under contribution, resulting in a mass of practical informlation scarcely
attempted hitherto in any similar work in the language.
Our expectations of the value of this work were the practical utility of his book by passing briefly
based on the well-known reputation and character over the physical, botani al, aild commercial history
(o the author as a man of scholarly attainments, an of medicines, and directing attention chiefly to their
elegant writer, a candid inquirer after truth, and a physiological action, and their application for the
philosophical thinker; we knew that the task would  amelioration or cure of disease. He ignares hypothebe conscientiously performed, and that few, if any, sis and theory wh ich are so alluring to many medical
among the distinguished medical teachers in this writers, and so liable to lead them astray, and concountry are better qualified than te to prepare a fines him-elf to such facts as have been tried in the
systen atic treatise on therapeutics in accordance crucible of experience.-Chicago Miedical Journal,
with the present requirements of medical science. March, 1860.
Our preliminary examination of the work has satis-   The plan pursuel by the author in these very elafied us that we were not mistaken in our anticip3t- borate volumes is not strictly one of scientific unity
tions. In cmnratulating the author on the comple- and precision; he has rather subordinated these tio
tion of the great labor which such a work involves, practical utility. Dr. Stille has produced a work
we are hap,)y in expressing the conviction that its which will be valuable equally to the student of
merits will receive that reward which is above all medicine and the busy practitioner.-London  Lanprice- the grateful appreciation of his medical bre- cet, March 10, 1860.
thren. —New  Orleans Medical Neuws, March, 1S60.
thre-New O s Medical Newsarch,      With Pereira, Dunglison, Mitchell, and Wood beXVe think this work will do much to obviate the fore us, we may well ask if there was a necessity
reluctance to a thorough investigation of this branch for a new book on the subject. After examining this
of scientific study, for in the wide range of medical work with some care, we can answer affirmatively.
literature treasured in the English tongue, we shall Dr. Wood's book is well adapted for students, while
hardly find a work written ill a style more clear and I)r. Stillh's will be more satisfactory to the practisimple, conveying forcilly the facts taught, and yet tioner, who desires to study the action of medicines.
free from turgidity and redundancy. There isa fas- The author needs no encomiunts fromn us, for he is
cination in its pages that will insure to it a wide well known as a ripe scholar and a man of the most
popularity and attentive perusal, and a degree of extensive reading in his profession. This work bears
tsefulness not often attained through the influence evidence of this fact on every pgce.-Cincinnlati
of a single work. The author has much enhanced  Lancet, April, lt60.
SMITH (HENRY H.), M.D.
MINOR  SURGERY; or, Hints on the Every-day Duties of the Surgeon.  With
247 illustrations. Third edition. 1 vol. royal 12mo., pp. 456. In leather, $2 25; cloth, $2 00.
BY THE SAME AUTHOR, AND
HO RNER (WILLIAM  E.), M.D.,
Late Professor of Anatomy in the University of Pennsylvania.
AN  ANATOMICAL  ATLAS, illustrative of the Structure of the Human Body.
In one volume, large imperial octavo, extra cloth, with about six hundred and fifty beautiful
figures. $3 00.
These figures are well selected, and present a late the student upon the compruletion of this Atlas,
complete and accurate representation of that won- as it is the most convenient work of the kind that
derful fabric, the human body. The plan of this has yet appeared; and we must add. the very beauAtlas, which renders it so peculiarly convenient tiful manner inwhich it is " got up" is so creditabie
for the student, and its superb artistical execution, to the country as to be flattering to our national
have been already pointed out. We must congratu- pride.-American Medical Journal.
SHARPEY (WILLIAM), M. D., JONES QUAIN, M. D., AND
RICHARD QUAIN, F. R. S., &c.
HUMAN ANATOMY. Revised, with Notes and Additions, by JosEra  LEIDY,
M. D., Professor of Anatomy in the University of Pennsylvania. Complete in two large octavo
volumes, leather, of about thirteen hundred pages. Beautifully illustrated with over five hundred
engravings on wood. $6 00.
SIMPSON  (J. Y., M. D.,
Professor of Midwifery, &c., in the University of Edinburgh, &c.
CLINICAL LECTURES ON THE DISEASES OF FEMALES. With numcrous illustrations.
This valuable series of practical Lectures is now appearing in the "'MEDICAL NEUws AND
LIBRARY" for 1860, and can thus be had without cost by subscribers to the "AMERICAN JOURNAL
OF THE MEDICAL SCIENCES."  See p. 2.




28                     BLANCHARD & LEA'S MEDICAL
SARGENT (F. W.), M. D.
ON BANDAGING AND OTHER OPERATIONS OF MINOR SURGERY.
Second edition, enlarged. One handsome royal 12mo. vol., of nearly 400 pages, with 182 woodcuts. Extra cloth, $1 40; leather, $1 50.
Sargent's Minor Surgery has always been popular,   A work that has been so long and favorably known
anddeservedly so. It furnishesthatknowledgeofthe to the profession as Dr. Sargent's Minor Surgery,
most frequently requisite performances of surgical needs no commendation from us. We would remark,
art which cannot be entirely understood by attend- however, in this connection, that minor surgery seling clinical lectures. The art of bandaging, which dom gets that attention in our schools that its imis regularly taught in Europe, is very frequently portance deserves. Our larger works arealso very
overlooked by teachers in this country; the student defective in their teaching on these small practical
and junior practitioner, therefore, may often require points. This little book will supply the void which
that knowledge which this little volume so tersely all must feel who have not studied its pages.-TVestand happily supplies.-Charleston Med. Journ. and ern Lancet, March, 1856.
Review, March, 1856.
SMITH (W. TYLER), M. D.,
Physician Accoucheur to St. Mary's Hospital, &c.
ON PARTURITION, AND THE PRINCIPLES AND PRACTICE OF
OBSTETRICS. In one royal 12mo. volume, extra cloth, of 400 pages. $1 25.
BY THE SAME AUTHOR.
A PRACTICAL TREATISE ON THE PATHOLOGY AND TREATMENT
OF LEUCORRHCEA. With numerous illustrations. In one very handsome octavo volume,
extra cloth, of about 250 pages. $1 50.
SOLLY ON THE HUMAN BRAIN; its Structure,   handsome octavo volume, extra cloth, of over 650
Physiology, and Diseases. From the Second and   pages, with about one hundred wood-cuts. $3 25.
much enlarged London edition. In one octave  SIMON'S GENERAL PATHOLOGY, as conducvolume, extra cloth, of 500 pages, with 120 wood-   ive to the Establishnment of Rational Principles
cuts. $2 00.                                  for the prevention any Cure of Disease. In one
SKEY'S OPERATIVE SURGERY. In one very   octavo volume, extra cloth, of 212 pages. $1 25.
TODD (R. B.), M. D., F. R. S., &c.
CLINICAL LECTURES ON CERTAIN DISEASES OF THE URINARY
ORGANS AND ON DROPSIES. In one octavo volume, 284 pages. $1 50.
BY THE SAME AUTHOR. (NowU Ready.)
CLINICAL LECTURES ON CERTAIN ACUTE DISEASES. In one neat
octavo volume, of 320 pages, extra cloth. $1 75.
The subjects treated in this volume are-RHEUMATIC FEVER, CONTINUED FEVER, ERYSIPELAS,
ACUTE INTERNAL INFLAMMATION, PYIEMIA, PNEUMONIA, and the THERAPEUTICAL ACTION OF ALCOHOL. The importance of' these matters in the daily practice of every physician, and the sound
practical nature of Dr. Todd's writings, can hardly fail to attract to this work the general attention
that it merits.
TANNER (T. H.), M. D.,
Physician to the Hospital for Women, &c.
A MANUAL OF CLINICAL MEDICINE AND PHYSICAL DIAGNOSIS.
To which is added The Code of Ethics of the American Medical Association.  Second
American Edition. In one neat volune, small 12mo., extra cloth, 871 cents.
TAYLOR (ALFRED S.), M. D., F. R. S.,
Lecturer on Medical Jurisprudence and Chemistry in Guy's Hospital.
MEDICAL JURISPRUDENCE. Fourth American Edition. With Notes and
References to American Decisions, by EDWARD HARTSHORNE, M. D. In one large octavo volume,
leather, of over seven hundred pages. $3 00.
No work upon the subject can be put into the   It is not excess of praise to say that the volume
hands of students either of law or medicine which before us is the very best treatise extant on Medical
will engage them more closely or profitably; and Jurisprudence. In saying this, we do not wisil to
none could be offered to the busy practitioner of be understood as detracting from the merits of the
either calling, for the purpose of casual or hasty excellent works of Beck, Ryan, Traill, Guy, and
reference, that would be more likely to afford the aid others; but in interest and value we think it must
desired. We thereforerecommend it as the best and be conceded that Taylor is superior to anything that
safest manual for daily use.-American Journal oJ has preceded it.-N. W. Medical and Surg. Journal.
lhedical Sciences.,
BY THE SAME AUTHOR. (New Edition, just issued.)
ON POISONS, IN RELATION TO MEDICAL JURISPRUDENCE AND
MEDICINE. Second American, from a second and revised London edition. In one large
octavo volume, of 755 pages, leather. $3 50.
Since the first appearance of this work, the rapid advance of Chemistry has introduced into
use many new substances which may become fatal through accident or design —while at the
same time it has likewise designated new and more exact modes of counteracting or detecting those
previously treated of. Mr. Taylor's position as the leading medical jurist of England, has during
this period conferred on him extraordinary advantages in acquiring experience on these subjects,
nearly all cases of moment being referred to him for examination, as an expert whose testimony
is generally accepted as final. The results of his labors, therefore. as gathered together in this
volume, carefully weighed and sifted, and presented in the clear and intelligible style for which
he is noted, may be received as an acknowledged authority, and as a guide to be followed with
implicit confidence.




AND SCIENTIFIC PUBLICATIONS.                                        29
TODD (ROBERT BENTLEY), M. D., F. R. S.,
Professor of Physiology in King's College, London; and
WILLIAM BOWMAN, F. R. S.,
Demonstrator of Anatomy in King's College, London.
THE PHYSIOLOGICAL ANATOMY AND PHYSIOLOGY OF MAN. With
about three hundred large and beautiful illustrations on wood. Complete in one large octavo
volume, of 950 pages, leather. Price $4 50.
[    Gentlemen who have received portions of this work, as published in the " MEDICAL NEWS
AND LIBRARY," can now complete their copies, if immediate application be made. It will be furnished as follows, free by mail, in paper covers, with cloth backs.
PARTS I., II., III. (pp. 25 to 552), $2 50.
PART IV. (pp. 553 to end, with Title, Preface, Contents, &c.), $2 00.
Or, PART IV., SECTION II. (pp. 725 to end, with Title, Preface, Contents, &c.), $1 25.
A magnificent contribution to British medicine, so well adapted to the wants of the medical student.
and the American physician who shall fail to peruse Its completion has been thus long delayed, that the
it, will have failed to read one of the most instruc- authors might secure accuracy by personal observative books of the nineteenth century.-N. 0. Med. tion.-St. Louis Med. and Surg. Journal, Sept.'57.
and Surg. Journal, kept. 18577.
end Surg. Journal, Sept. 1857.  Our notice, though it conveys but a very feeble
It is more concise than Carpenter's Principles, and and imperfect idea of the magnitude and importance
more modern than tile accessible edition of Mitller's of the work now under consideration, already tranElements; its details are brief, but sufficierlt; its scends our limits; and, with the indulg(nce of our
descriptions vivid; its illustrations exact and copi- readers, and the hope that they will peruse the book
ous; and its language terse and perspicuous.-  for themselves, as we feel we can with confidence
Charleston Med. Journal, July, 1857.          recommend it, we leave it in their hands. - The
We know of no work on the subject of physiology Northwestern Med. and Surg. Jouzrnal.
TOYNBEE (JOSEPH), F. R. S.,
Aural Surgeon to, and Lecturer on Surgery at, St. Mary's Hospital.
A PRACTICAL TREATISE ON DISEASES OF THE EAR; their Diagnosis, Pathology, and Treatment. Illustrated with one hundred engravings on wood. In one
very handsome octavo volume, extra cloth, $3 00. (vow Ready.)
Mr. Toynbee's name is too widely known as the highest authority on all matters connected with
Aural Surgery and Medicine, to require special attention to be called to anything which he may
communicate to the profession on the subject. Twenty years' labor devoted to the pIresent work
has embodied in it the results of an amount of experience and observation which perhaps no other
living practitioner has enjoyed. It therefore cannot fail to prove a complete and trustworthy guide
on all matters connected with this obscure and little known class of diseases, which so frequently
embarrass the general practitioner.
Tile volume will be found thoroughly illustrated with a large number of original wood-engravings, elucidating the pathology of the organs of hearing, instruments, operations, &c., and in every
respect it is one of the handsomest specimens of mechanical execution issued from the American
press.
The following condensed synopsis of the contents will show the plan adopted by the author, and
the completeness with which all departments of the subject are brought under consideration.
CHAPTER I. Introduction-Mode of Investigation-Dl)issection.  I[. The External Ear-Anatomy-Pathology-Malformations - Diseases.  III. The External Meatus —Its Exploration.
IV. The External Meatus —Foreign Bodies and Accumnulations of Cerumen. V. The External
Meatus-The Dermis and its Diseases. VI. The External Nieatus-Polypi. VII. The External
Meatus-Tumors.  VIII. The Membrana Tympani-Structure and Functions. IX. The Memnbrana Tympani-Diseases.  X. The Membrana Tympani-Diseases.  XI. The Eustachian
Tube-Obstructions.  XII. The Cavity of the Tympanum —Anatomy-Pathology —Diseases.
XIII. The Cavity of the Tympanum-Diseseases. XIV. The Mastoid Cells-Diseases.  XV.
The Diseases of the Nervous Apparatus of the Ear, producing what is commonly called " Nervous Deafiness."  XVI. The Diseases of the Nervous Apparatus, continued. XVIL. Malignant
Disease o' the Ear.  XVIII. 011 the Deaf and Dumb.  XIX. Ear-Trumpets and their uses.
APPENDIX.
WILLIAMS (C. J. B.), M.D., F. R. S.,
Professor of Clinical Medicine in University College, London, &e.
PRINCIPLES OF MEDICINE. An Elementasy View of the Causes, Nature,
Treatment, Diagnosis, and Prognosis of Disease; with brief remarks on Hygienics, or the preservation of health. A new American, from the third and revised London edition. In one octavo
volume, leather, of about 500 pages. $2 50. (Just Issuzed.)
We find that the deeply-interesting matter and expressed. It is at judgment of almost unqualified
style of this book have so far fascinated us, that we praise.-London Lancet.
have unconsciously hung upon its pages, not too   A text-book to which no other in our language is
long, indeed, for our own profit, but longer than re- comparable.-Charleston Medical Journal.
viewers can be permitted to indulge. XWe leave the
further analysis to the student and practitioner. Our   No work has ever achieved or maintained a more
judgment of the work has already been sufficiently deserved reputation.- Va. Med. and Surg. Journal.
WHAT TO OBSERVE
AT THE BEDSIDE AND AFTER DEATH, IN MEDICAL CASES.
Publishedunder the authority of the London Society for Medical Observation. A new American,
from the second and revised Londo;i edition. In one very handsome volume, royal 12mo., extra
cloth. $1 00.
To the observer who prefers accuracy to blunders I One of the finest aids to a young practitioner we
and precision to carelessness, this little book is ia- have ever seen. —eninsular Journal of Medici.a.
valuable.-N. IH. Journal of Medicine.




30                    BLANCHARD & LEA'S MEDICAL
New  and much enlarged edition-(Just Issued.)
WATSON (THOMAS), M. D., &c.,
Late Physician to the Middlesex Hospital, &c.
LECTURES ON THE PRINCIPLES AND PRACTICE OF PHYSIC.
Delivered at King's College, London. A new American, from the last revised and enlarged
English edition, with Additions, by D. FRANCIS CONDIE, M. D., author of " A Practical Treatise
on the Diseases of Children," &c. With one hundred and eighty.five illustrations on wood. In
one very large and handsome volume, imperial octavo, of over 1200 closely printed pages in
small type; the whole strongly bound in leather, with raised bands. Price $4 25.
That the high reputation of this work might be fully maintained, the author has subjected it to a
thorough revision; every portion has been examined with the aid of the most recent researches
in pathology, and the results of modern investigations in both theoretical and practical Oubjects
have been carefully weighed and embodied throughout its pages. The watchful scrutiny of' the
editor has likewise introduced whatever possesses immediate importance to the American physician
in relation to diseases incident to our climate which are little known in England, as well as those
points in which experience here has led to different modes of practice; and he has also added largely
to the series of illustrations, believing that in this manner valuable assistance may be conveyed to
the student in elucidating the text. The work will, therefore, be found thoroughly on a level with
the most advanced state of medical science on both sides of the Atlantic.
The additions which the work has received are shown by the fact that notwithstanding an enlargement in the size of the page, more than two hundred additional pages have been necessary
to accommodate the two large volumes of the Lontlon edition (which sells at ten dollars), within
the compass of a single volume, and in its present form it contains the matter of at least three
ordinary octavos. Believing it to be a work which should lie on the table of every physician, and
be in the hands of every student, the publishers have put it at a price within the reach of all, mnaking
it one of the cheapest books as yet presented to the American profession, while at the same time
the beauty of its mechanical execution renders it an exceedingly attractive volume.
The fourth edition now appears, so carefully re-   TiThe lecturer's skill, his wisdom, his learning, are
vised, as to add considerably to the value of a book equalled by the ease of his graceful diction, his eloalready acknowledged, wherever the English lan- 1 quence, and the far higher qualities of candor, of
guage is read, to be beyond all comparison the best courtesy, of modesty, and of generous appreciation
systematic work on the Principles and Practice of of merit in others. May he long remain to instruct
Physic in the whole range of medical literature. us, and to enjoy, in the glorious sunset of his deEvery lecture contains proof of the extreme anxiety clining years, the honors, the confidence and love
of the author to keep pace with:he advancing know- gained during his useful life.-N. A. Med.-Chir.
ledge of the day, and to bring the results of the Review, July, 1858.
labors, not only of physicians, but of chemists and
histologists, before his readers, wherever they can    Vatson's unrivalled, perhaps unapproachable
be turned to useful account. And this is done with work on Practice-the copious additions tade to
such a cordial appreciation of the merit due to the which (the fourth edition) have given it all the noindustrious observer, such a generous desire to en-velty aid much of the interest of a new book.
courage younger and rising men, and such a candid Charleston Med. Journal, July, 1&58.
acknowledgment of his own obligations to them,   Lecturers, practitioners, and students of medicine
that one scarcely knows whether to adnmire most the will equally hail the reappearance of the work of
pure, simple, forcible English —the vast amount of Dr. Watson in the form of a new —a fourth-edition.
useful practical information condensed into the  Ve merely do justice to our own feelings, and, we
Lectures-or the manly, kind-hearted, unassuming are sure, of the whole profession, if we thank him
character of the lecturer shining through his work. for having, in the trouble and turmoil of a large
-London 3Med. Times and Gazette, Oct. 31, 1857.  practice, made leisure to supply the hiatus caused
Thus these admirable volumes come before the by the exhaustion of the publisher's stock of the
profession in their fourth edition, abounding in those third edition, which has been severely felt for the
distinguished attributes of moderation, judgment, last three years. For Dr. Watson has not merely
erudite cultivation, clearness, and eloquence, with caused the lectures to be reprinted, but scattered
which they were from the first invested, but yet through the whole work we find additions or alteraricher than before in the results of more prolonged tions which prove that the author has in every way
observation, and in the able appreciation of the sought to bring up his teaching to the level of the
latest advances in pathology and medicine by one most recent acquisitions in science.-Brit. and For.
of the most profound medical thinkers of the day.-  Medico-Chir. -Review, Jan. 1858.
London Lancet, Nov. 14, 1857.
WALSHE (W. H.), M. D.,
Professor of the Principles and Practice of Medicine in University College, London, &c.
A PRACTICAL TREATISE ON DISEASES OF THE LUNGS; including
the Principles of Physical Diagnosis. A new American, from the third revised and mluch enlarged London edition. In one vol. octavo, of 468 pages. (Just Issuted, June, 1860.) i2 25.
The present edition has been carefully revised and much enlarged, and may be said in the main
to be rewritten. Descriptions of several diseases, previously omitted, are now introduced; the
causes and mode of production of the more important affiections, so far as they possess direct practical significance, are succinctly inquired into; an effort has been made to bring the description ot
anatomical characters to the level of the wants of the practical physician; and the diagnosis and
prognosis of each complaint are more completely considered. The sections on TREATMENT and
the Appendix (concerning the influence of climate on pulmonary disorders), have, e-pecially, been
largely extended.-Author's Preface.
t   To be followed by a similar volume on Diseases of the Heart and Aorta.
WILSON  (ERASMUS), F. R. S.,
lecturer on Anatomy, London.
THE   DISSECTOR'S  31ANUAL; or, Practical and Surgical Anatomiy.  Third
American, from the last revised and enlarged English edition. Modified and rearranged, by
WILLIAM HUNT, M. 1)D., Demonstrator of Anatomy in the University of Pennsrrylvanuia. In one
large and handsome royal 12mo. volume, leather, of 582 pages, with 154 illustrations. $2 00.




AND SCIENTIFIC PUBLICATIONS                                        31
New and much enlarged edition-(Just Issued.)
WILSON (ERASMUS), F. R. S.
A SYSTEM OF HUMAN ANATOMY, General and Special. A new and revised American, from the last and enlarged English Edition. Edited by W. H. GOBRECHT, M.D.,
Professor of Anatomy in the Pennsylvania Medical College, &c. Illustrated with three hundred
and ninety-seven engravings on wood. In one large and exquisitely printed octavo volume, of
over 600 large pages; leather. $3 25.
The publishers trust that the well earned reputation so long enjoyed by this work will be more
than maintained by the present edition. Besides a very thorough revision by the author, it has been
most carefully examined by the editor, and the efforts of both have been directed to introducing
everything which increased experience in its use has suggested as desirable to render it a complete
text-book for those seeking to obtain or to renew an acquaintance with Human Anatomy. The
amount of additions which it has thus received may be estimated from the fict that the present
edition contains over one-fourth more matter than the last, rendering a smaller type and an enlarged
page requisite to keep the volume within a convenient size. The author has not only thus added
largely to the work, but he has also made alterations throughout, wherever there appeared the
opportunity of improving the arrangement or style, so as to present every fact in its most appropriate manner, and to render the whole as clear and intelligible as possible. The editor has
exercised the utmost caution to obtain entire accuracy in the text, and has largely increased the
number of illustrations, of which there are about one hundred and fifty more in this edition than
in the last, thus bringing distinctly before the eye of the student everything of interest or importance.
It may be recommended to the student as no less beauty of its mechanical execution, and the cleardistinguished by its accuracy and clearness of de- ness of the descriptions which it contains is equally
scription than by its typographical elegance. The evident. Let students, by all means examine the
wood-cuts are exquisite.-Brit. and For. Medical claims of this work on their notice, before they purReview.                                       chase a text-book of the vitally important science
which this volume so fully and easily unfolds.An elegant edition of one of the most useful and which thi volume so fully and easily unfolds.-.
accurate systems of anatomical science which has
been issued from the press  The illustrations are   We regard it as the best system now extant for
really beautiful. In its style the work is extremely students.-estern Lancet.
concise and intelligible. No one can possibly take   It therefore receives our highest commendation. —
it) this volume without being struck with the great Southern Mled. and Surg. Journal.
BY THE SAME AUTHOR. (Just Issued.)
ON  DISEASES  OF THE SKIN.  Fourth and enlarged American, from  the last
and improved London edition. In one large octavo volume, of 650 pages, extra cloth, $2 75.
Thewritings of Wilson, upondiseases of the skin, at some of the more salient points with which it
are by far the most scientific and practical that atiounds,and whichmakeitilncomparal)uysuperiorin
have ever been presented to the medical world on excellence to all other treatises on the subject of derthis subject. The present eition isa great improve- matology. No mere speculative views are allowed
ment on all its predecessors. To dwell upon all the a place in this volume, which, without a doubt, will,
great merits and high claims of the work before us, for a very long period, be acknowledged as the chief
seriatim, would indeed be an agreeable service; it standard work on dermatology. The principles of
would be a mental homage which we could freely an enlightened and rational tlierapeia are introduced
offer, but we should thus occupy an undue amount on every appropriate occasion.-Am. Jour. Med.
ofrspace in this Journal. WVe will, however, look  Science, Oct. 1857.
ALSO, NOW READY,
A SERIES OF PLATES ILLUSTRATING WILSON ON DISEASES OF
THE SKIN; consisting of nineteen beautifully executed plates, of which twelve are exquisitely
colored, presenting the Normal Anatomy and Pathology of the Skin, and containing accurate representations of about one hundred varieties of disease, most of them the size of nature. Price
in cloth 94 25.
In beauty of drawing and accuracy and finish of coloring these plates will be found equal to
anything of the kind as yet issued in this country.
The plates by which this edition is accompanied   We have already expressed our high appreciation
leave nothing to be desired, so far as excellence of of Mr. Wilson's treatise on Diseases of the Skin.
delineation and perfect accuracy of illustration are The plates are comprised in a separate volume,
concerned.-Medico-Chirurgical Review.         which we counsel all those who possess the text to
Of these plates it is impossible to speak ton hihly  purchase. It is a beautiful specimnen of color printThe representations of the various forms of cutane- Li, and the representations of the various forms of
ous disease are singularly accurate, and the color- skin disease  are -s faithful as is possible in plates
ing exceeds almost anything we have met with in of the slze.-Bostoa MIed. and Ssurg. Journal, April
point of delicacy and finish.-British and Foreign  8, 1858.
Medical Review.
BY THE SAME AUTHOR.
ON CONSTITUTIONAL AND HEREDITARY SYPHILIS, AND OIN
SYPHILITIC ERUPTIONS. In one small octavo volume, extra cloth, beautifully prinited, with
four exquisite colored plates, presenting more than thirty varieties of syphilitic eruptions. $2 25.
BY THE SAME AUTHOR.
HEALTtHY SKIN; A Popular Treatise on the Skin and Hair, their Preservation and Management. Second American, from the fourth London edition. One neat volume,
royal 12mo., extra cloth, of about 300 pages, with numerous illustrations. $1 00; paper cover,
75 cents.
WHITEI:EAD ON THE CAUSES AND TREAT- I Second American Edition. In one volume, octaMIENT OF ABORTION  AND STERILITY. I vo extra cloth, pp. 308. $1 75.




32    BLANCHARD & LEA'S MEDICAL PUBLICATIONS.
WINSLOW  (FORBES), M. D., D. C. L., &c.
ON OBSCURE DISEASES OF THE BRAIN AND DISORDERS OF THE
MIND; their incipient Symptoms, Pathology, Diagnosis, Treatment, and Prophylaxis. In one
handsome octavo volume, of nearly 600 pages. (Ju.st Issued, June, lbO60.) $3 00.
The momentous questions discussed in this volume have perhaps not hitherto been so ably and
elaborately treated. Dr. WVinslow's distinguished reputation and long experience it everything relating to insanitv invest his teachings with the highest authority, and in this carefully considered
volume he has drawn upon the accumulated resources of a life of observation. His deductions
are founded on a vast number of cases, the peculiarities of which are related in detail, rendering
the work not only one of sound instruction, but of lively interest; the author's maitl object being
to point out the connection between organic disease and insanity, tracing the laiter thrcough all its
stages from mere eccentricity to mania, and urging the necessity of early mneasures of prophylaxis
and appropriate treatment. A subject of greater importance to society at large could.carcely be
named; while to the physician who may at any moment be called upon for interference in the most
delicate relations of life, or for an opinion in a court of justice, a work like the present tray be considered indi pensable.
The treatnient of the subject may be gathered from the following summary of the contents:CHAPTER I. Introduction.-II.  Morbid Phenomena of Intelligence. III. Premonitory Symptoms
of Insanity.-IV. Confessions of Patients after Recovery.-V. State of the Mind during Recovery.-VI. Anomalous and Mask-ed Affections of the Mind.-VII. The Stage of Consciousness.
-VIII. Slage of Exaltation.-IX. Stage of Mental Depression.-X. Stage of Aberration.-XI.
Impairment of Mind.-XII. Morbid Phenomena of Attention.-XIII. Morbid Ph'enomena of
Memory -XIV. Acute Disorders of Memory.-XV. Chronic Affections of' Melmory.-XVI.
Perversion and Exaltation of Memory.-XVIL. Psychology an(d Pathology of  lemnory.-XVIII.
Morbid Phenomena of Motion.-XIX. Morbid Phenomena of Speech.-XX. Morbid Phenomena
of Sensation.-XXI. Morbid Phenomena of the Special Senses.-XXII. Morbid Phenomlena of
Vision, Hearing, Taste, Touch, and Srnel.-XXIII. Morbid Phenomena of Sleep and Dreaming.
-XXIV. Morbid Phenomlena of Organic and Nutritive Life.-XXV. General Princip!es of Pathology, Diagnosis, Treatment, and Prophylaxis.
WEST (CHARLES), M. D.,
Accoucheur to and Lecturer on Midwifery at St. Bartholomew's Hospital, Physician to the Hospital for
Sick Children, &c.
LECTURES ON THE DISEASES OF WOCMEN. Now complete in one handsome octavo volume, extra cloth, of about 500 pages; price $2 50.
Also, for sale separate, PART II, being pp. 309 to end, with Index, Title matter,
&c., 8vo., cloth, price $1.
We must now conclude this hastily written sketch  and children is not to be found in any country.with the confident assurance to our readers that the Southern Med. and Surg. Jouranal, January 1858.
work will well repay perusal. The conscientious,   We gladly recommend his Lectures as in the highpainstaking,practicalphysician isapparent(n every  est degree instructive to all who are interested in
page.-N. Y. Journal of Medicine, March, 1858.  obstetric practice. —London Lancet.
We know of no treatise of the kind so complete   We have to say of it, briefly and decidedly, that
and yet so compact.-Chicago Med. Journal, Janu- it is the best work on the subject in any language;
ary, 1858.                                     and that it stamps Dr. WVest as the facile princeps
A fairer, more honest, more earnest, and more re- of British obstetric authors.-Edinb. I.Med. Journ.
liable investigator of the many diseases of women
BY THE SAME AUTHOR. (Now Ready.)
LECTURES ON THE D ISEASA3ES OF INFANCY AND CHILDHOOD.
Third American, from the fourth enlarged and improved London edition. In one handsome
octavo volume, extra cloth, of about six hundred and fitty pages. $2 75.
The continued favor with which this work has been received has stimulated the author to render it in every respect more complete and imore worthy the confidenice of the profession. Containing nearly two hundred pages mlore than the last American edition, with -everal additional
Lectures and a careful revision and enlargement of those formerly comprised in it, it call hardly
fail to maintain its reputation as a clear and judicious text-book for the student, and a afet and
reliable guide for the practitioner. The fact stated by the auttior that these Lectures,  now emlbody
the results of 900 observations and 288 post-mortem  examinatiolis made anmoill nearly 30,000
children, who, during the past twenty-years, have come under my care," is sufficient to show their
high practical value as the result of ani amount of' experience which few physiclans enj ty.
The three.former editions of the work now before diseases it omits to notice altogether. But those
us have placed the author in tine foremost rank cf who know anything of the present condition (of
those physicians who have devoted special attention paediatrics will readily adimit mhat it would be next
to the diseases of early life  We attempt no ana- to impossible to effect more, or effect it better, thai)
13 sis of this edition, but may refer the reader to some the accoucheur of St. Bartholohmew's has done in a
of the chapters to which the largest additions have single volume. The lecture (XVI.) upon Disorcers
been made-those on Diphtheria, Disorders of the of the Mind in children is an adlmirabie specimen of
Mind, and Idiocy, for instance-as a proot that the the value of the later iniformmation conve) ed in the
work is really a new edition; not a mere reprint. Lectures of Dr. Charles West. —London Lancet,
In its pre.ent shape it will be mound of the greatest Oct. 22, 1859.
possible service in the every-day practice of nine-   Since the appearance of the first edition, about
tenths of the profession.-Med. Times and Gazette, eleven years ago, the experience of the autlior has
London, Dec. 10, 1839.                         doubled; so that, whereas tile lectures at first were
All things considt red, this book of Dr. West is founded on six hundred observations, and one hunby far tile best treatise in our language upon such dred and eighty dissections made among learly fourmodifications of morbid action and disease as are teen thousand children, they now embody the results
witnessed when we have to deal with infancy and of nine hundred observations, and two inun-dred and
childhood. It is true that it confines itself to such eigllty-eight post-morten  examinations made among
disorders as come within the province of the phy- nearly thirty thousand children, who, during the
sician, and even with respect to tsese it is unequal past twenty years, have been under his care. -
as regards minuteness of consideration, and some British Nled. Jcurnal, Oct. 1, 1859.
BY THE SAME AUTHOR.
AN ENQUIRY INTO THE PATHOLOGICAL IMPORTANCE OF ULCERATION OF THE OS UTTERI. In one neat octavo volume, extra cloth. $1 00.