A
CATECHISM
OF THE
STEAM ENGINE,
ILLUSTRATIVE OF THE
SCIENTIFIC PRINCIPLES UPON WHICH ITS OPERATION DEPENDS9
AND THE PRACTICAL DETAILS OF ITS STRUCTURE, IN
ITS APPLICATION TO MINES, MILLS, STEAM
NAVIGATION AND RAILWAYS.
1'tDt barious %tuggesttono of 3Inr4obemtecnt.
BY  JOHN  BOURNE, C. E.
A NEW EDITION.
NEW-YORK:
D. APPLETON & COMPANY,
346 & 848 BROADWAY.
M.DCCC.IIV.








PREFACE.
THE present work is not intended as a subsli'tu^
for the quarto Treatise on the Steam  Engine,
which I lately published, but is rather to be regarded as an introduction and in some measure
also as a supplement to that work.  Notwithstanding the existence, therefore, of the larger
Treatise, it appeared to me that a work upon the
steam engine, which in a moderate compass should
give an outline of the whole subject in its practical
aspect, would still be of much utility. There are
no doubt many compendiums already existing
which profess to accomplish this object; but I
have not met with any which were calculated to
satisfy even the most moderate expectations.
Most of them are mere compilations from theoretical authors, and abound even with scientific
errors, whilst indicating the absence of any practical acquaintance with the subject; so that they
possess but slender claims upon the attention of
the engineer, or indeed of any one desirous of ob



4                 PREFACE.
taining accurate information on the subject of
which they treat.  I hold it to:be thebfirst.quality
of an introductory work that it shouldfat least be
sound-that the doctrines it inculcates, and the
lessons it conveys, shall not all have to be unlearned
again at a subsequent stage of progress —and
whatever be its other characteristics, I believe
that the present work will at least be found to
conform to this standard of utility. It embodies,
I believe, the best information now existing upon
the subjects of which it treats-not taken from
books, nor deduced from mere theoretical considerations, but derived from my own practice or
from the personal communications of the most
experienced engineers of the present time.
JOHN BOURNE.




A CATECHISM
OF
THE STEAM ENGINE.
PART I.
MECHANICAL PRINCIPLES.
1. Q.-What is meant by a vacuum?
A.-A vacuum means an empty space; a space in
which there is neither water nor air, nor any thing else
that we know of.
2. Q.-Wherein does a high pressure differ from a
low pressure engine?
A.-In a high pressure engine the steam, after having pushed the piston to the end of the stroke, escapes
into the atmosphere, and the impelling force is therefore that due to the difference between the presslr,;e of
the steam and the pressure of the atmosphere.  In the
condensing engine the steam, after having pressed the
piston to the end of the stroke, passes into the condenser, in which a vacuum is maintained, and the impelling
force is that due to the difference between the pressure
I*




6                A CATECHISM OF
of the steam above the piston, and the pressure of
the vacuum  beneath it, which is nothing; or, in other
words, you have then the whole pressure of the steam
urging the piston, consisting of the pressure shown by
the safety valve on the boiler, and the pressure of the
atmosphere besides.
3. Q.-How can the pressure of a vacuum be said
to be nothing, when the existence of a vacuum occasions a pressure of 15 lbs. on the square inch?
A. -Because it is not the vacuum which exerts this
pressure, but the atmosphere, which, like a head of
water, presses on every thing immerged beneath it. A
head of water, however, would not press down a piston,
if the water were admitted on both its sides; for an
equilibrium would then be established, just as a balance
retains its equilibrium though an equal weight be added
to each scale; but take the weight out of one scale,
or empty the water from one side of the piston, and
motion or pressure is produced; and in like manner
pressure is produced on a piston by admitting steam or
air upon the one side, and withdrawing the steam or
air from the other side.  It is not, therefore, to a vacuum, but rather to the existence of an unbalanced
plenum, that the pressure made manifest by exhaustion
is due, and every one knows that a vacuum  of itself
would not work an engine.
4. Q.-How is the vacuum  maintained in a condensing engine?
A.-The steam, after having performed its office in
the cylinder, ig permitted to pass into a vessel called the
condenser, where a shower of cold water is discharged
upon it, The steam is condensed by the cold water, and




THE STEAM ENGINE.                7
falls in the form of hot water to the bottom of the condenser.  The water, which would else be accumulated
in the condenser, is continually being pumped out by
a pump worked by the engine.
5. Q.-If a vacuum be an empty space, and there
be water in the condenser, how can there be a vacuum
there?
A.-There is a vacuum above the water, the water
being only like so much iron or lead lying at the
bottom.
6. Q.-Is the vacuum in the condenser a perfect
vacuum?
A.-Not quite perfect; for the cold water entering
for the purpose of condensation is heated by the steam,
and emits a vapor of a tension represented by about
three inches of mercury; that is, when the common
barometer stands at 30 inches, a barometer with the
space above the mercury communicating with the condenser will stand at about 2-7 inches.
7. Q.-Is a barometer sometimes applied to the
condensers of steam engines?
A.-Yes; and it is called the vacuum gauge, because it shows the degree of perfection the vacuum has
attained. Another gauge, called the steam gauge, is
applied to the boiler, which indicates the pressure of the
steam by the height to which the steam forces mercury
up a tube.
8. Q.-Can a condensing engine be worked with a
pressure less than that of the atmosphere?
A.-Yes, if once it be started; but it will be a difficult thing to start an engine; if the pressure of the
steam be not greater than that of the atmosphere. Be



8                 A CATECHISM OF
fore an engine can be started, it has to be blown through
with steam to displace the air within it, and this cannot
be effectually done if the pressure of the steam be very
low. After the engine is started, however, the pressure
in the boiler may be lowered, if the engine be lightly
loaded, until there is a partial vacuum in the boiler.
Such a practice, however, is not to be commended, as
the gauge cocks, which are cocks applied for telling the
height of water within the boiler, become useless when
there is a partial vacuum in the boiler; inasmuch as,
when they are opened, the water will not rush out, but
air will rush in. It is impossible, also, under such circumstances, to blow out any of the sediment collected
within the boiler, which, in the case of the boilers of
steam vessels, requires to be done every two hours.  In
some cases, in which the boiler applied to an engine is
of inadequate size, the pressure within the boiler will
fall spontaneously to a point considerably beneath that
of the atmosphere..; but it is preferable in such cases
partially to close the throttle valve in the steam pipe,
whereby the issue of the steam is resisted; and the
pressure in the boiler is thus maintained, though the
cylinder only receives its former supply.
9. Q.-In what way would you class the various
kinds of condensing engines?
A. -Into single acting, rotative, and rotatory engines.
Single acting engines are engines without a crank,
such as are used. for pumping water. Rotative engines
are engines provided with a crank, by means of which
a rotative motion is produced; and in this important
class stand marine and mill engines, and all engines,
indeed, in which the rectilinear motion of the piston




THE STEAM ENGINE.                 9
is changed into a circular motion. In rotatory engines
the steam acts at once in the production of circular
motion, either upon a revolving piston or otherwise,
but without the use of any intermediate mechanism,
such as the crank, for deriving a circular from a rectilinear motion.  Rotatory engines have not hitherto
been successful, so that only the single acting or
pumping engine, and the double acting or rotative engine, can be said to be in actual use. For some purposes, such, for example, as forcing air into furnaces
for smelting iron, double acting engines are employed,
which are nevertheless unfurnished with a crank; but
engines of this kind are not sufficiently numerous to
justify their classification as a distinct species, and, in
general, those engines may be considered to be single
acting, by which no rotatory motion is imparted.
10. Q.-Is not the circular motion derived from a
cylinder engine very irregular, in consequence of the
unequal leverage of the crank at the different parts of
its revolution?
A.-No; rotative engines are generally provided
with a fly-wheel to correct such irregularities by its
momentum; but where two engines, with their respective cranks set at right angles, are employed, the
irregularity of one engine corrects that of the other
with sufficient exactitude for many purposes. In the
case of marine and locomotive engines, a fly-wheel is
not employed; but for cotton spinning, and other purposes requiring great regularity of motion, its use with
common engines is indispensable, though it is not impossible to supersede the necessity by new contrivances.
11. Q.-You implied that there is some other dif



10               A CATECHISM OF
ference between single acting and double acting engines, than that which lies in the use or exclusion of
the crank?
A.-Yes; single acting engines act in only one
way by the force of the steam, and are returned by a
counter weight; whereas double acting engines are
urged by the steam in both directions.  Engines, as I
have already said, are sometimes made double acting
though unprovided with a crank; and there would be
no difficulty in so arranging the valves of ordinary
pumping engines, as to admit of this action; for the
pumps might be contrived to raise water both by the
upward and downward stroke, as indeed in some mines's already done.  But engines without a crank are
almost always made single acting, perhaps from the
effect of custom as much as from any other reason,
and are usually spoken of as such, though it is necessary to know that there are some deviations from the
usual practice.
12. Q.-With what velocity does air rush into a
vacuum?
A.-With the velocity which a body would acquire
by falling from the height of a homogeneous atmosphere. The weight of air being known, as well as the
pressure it exerts on the earth's surface, it becomes
easy to tell what height a column of air an inch
square, and of the atmospheric density, would require
to be to weigh 15 lbs. The height would be 27,818
feet.
13. Q.-And what velocity would the fall of a body
from such a height produce?
A.-About 1,338 feet per second. All bodies fall




THE STEAM ENGINE.                  11
with the same velocity when there is no resistance
from the atmosphere, as is shown by the experiment
of letting fall a feather and a guinea from the top of a
tall exhausted receiver, when they reach the bottom at
the same time.  The velocity of falling bodies is one
that is accelerated uniformly, according to a known
law; and when the height from which a body falls is
given, the velocity acquired at the end of the descent
can be easily computed.  The square root of the
height in feet multiplied  by  8'021 will give the
velocity.
14. Q.-But the velocity in what terms?
A.-In feet per second.  The  distance through
which a body falls by gravity in one second is 16 -
feet; in two seconds, 64T  feet; in three seconds, 1449
feet; in four seconds, 2574  feet, and so on.  If the
number of feet fallen through in one second be taken
as unity, then the relation of the times to the spaces
will be as follows:Number of seconds....  1 2 3  4  5  6
Units of space passed through  1 4  9162536
so that it appears that the spaces passed through by a
falling body are as the squares of the times of falling.
The velocity acquired by a falling body at the end of
the 1st second is 32- feet per second, at the end of
the 2d second 641- feet, at the end of the 3d, 961 feet,
at the end of the 4th, 128- feet, and so on.  These
numbers proceed in the progression 1, 2, 3, 4, &c., so
that it appears that the velocities acquired by a falling
body at different points, are simply as the times of
falling. But if the velocities be as the; times, and the




12               A CATECHISM OF
total space passed through be as the squares of the
times, then the total space passed through must be as
the squares of the velocity; and as the vis viva or mechanical power inherent in a falling body, of any given
weight, is measurable by the height through which it
descends, it follows that the vis viva is proportionate
to the square of the velocity. Of two balls therefore,
of equal weight, but one moving twice as fast as the
other, the faster ball has four times the energy or
mechanical force accumulated in it that the slower ball
has. If the speed of a fly-wheel be doubled, it has
four times the energy it possessed before-such energy
being measurable by a reference to the height through
which a body must have fallen, to acquire the velocity
given.
15. Q.-By what considerations is the momentum
proper for the fly-wheel of an engine determined?
A.-By a reference to the power produced every
half-stroke of the engine, joined to the consideration
of what relation the energy of the fly-wheel rim must
have thereto, to keep the irregularities of motion
within the limits which are admissible. It is found
in practice, that when the power resident in the
fly-wheel rim, when the engine moves at its average
speed, is from two and a half to four times greater
than the power generated by the engine in one halfstroke —the variation depending on the momentum
inherent in the machinery the engine has to drive
and the equability of motion required —the engine
will work with sufficient regularity far all ordinary
purposes.




THE STEAM ENGINE.                13
16. Q.-What do you understand by the centre of
gravity of a body?
A.-That point within it, in which the whole of
the weight may be supposed to be concentrated, and
which continually endeavors to gain the lowest possible position. A body hung in the centre of gravity
will remain at rest in any position.
17. Q.-What do you understand by centrifugal
and centripetal forces?
A.-By centrifugal force, I understand the force
with which a revolving body tends to fly from  the
centre; and by centripetal force, I understand any
force which draws it to the centre, or counteracts the
centrifugal tendency.  In the conical pendulum, or
steam engine governor, which consists of two metal
balls suspended on rods hung from the end of a vertical revolving shaft, the centrifugal force is manifested
by the divergence of the balls when the shaft is put
into revolution; and the centripetal force, which in
this instance is gravity, predominates so soon as the
velocity is arrested; for the arms then collapse and
hang by the side of the shaft.
18. Q.-What is meant by the centre of gyration?
A.-The centre of gyration is that point in a revolving body in which the whole momentum may be
conceived to be concentrated, or in which the whole
effect of the momentum  resides.  If the ball of a
governor were to be moved in a straight line, the
momentum  might be said to be concentrated at the
centre of gravity of the ball; but inasmuch as, by
its revolution round an axis, the part of the ball
furthest removed from the axis moves quicker than
2




14                A CATECHISM OF
the part nearest to it, the momentum cannot be sup.
posed to be concentrated at the centre of gravity, but
at a point further removed from  the central shaft,
and that point is what is called the centre of gyration.
19. Q.-What is the centre of oscillation?
A.-The centre of oscillation is a poi.nt in a pendulum or any swinging body, such, that if all the matter
of the body were to be collected into that point the
velocity of its vibration would remain unaffected. It
is in fact the mean distance from the centre of susv
pension of every atom, in a ratio which happens not
to be an arithmetical one.  The centre of oscillation
is always in a line passing through the centre of suspension, and the centre of gravity.
20. Q.-By what circumstance is the velocity of
vibration of a pendulous body determined?
A.-By the length of the suspending rod only, or,
more correctly, by the distance between the centre of
suspension and the centre of oscillation. The length
of the arc described does not signify, as the times of
vibration will be the same, whether the arc be the
fourth or the four hundredth of a circle; or at least
they will be nearly so, and would be so exactly, if
the curve described were a portion of a cycloid. In
the pendulums of clocks, therefore, a small arc is preferred, as there is, in that case, no sensible deviation
from the cycloidal curve, but in other respects the
size of the arc does not signify.
21. Q.-If then the length of a pendulum be given,
can the number of vibrations in a given time be determined?
A.-Yes; the time of vibration bears the same re



THE STEAM ENGINE.                 15
lation to the time in which a body would fall through
a space equal to half the length of the pendulum, that
the circumference of a circle bears to its diameter. The
number of vibrations made in a given time by pendulums of different lengths, is inversely as the square
roots of their lengths.
22. Q.-Then when the length of the second's pendulum is known, the proper length of a pendulum to
make any given number of vibrations in the minute can
readily be computed?
A.-Yes; the length of the second's pendulum being known, the length of another pendulum, required
to perform  any given number of vibrations in the
minute, may be obtained by the following rule: multiply the square root of the given length by 60, and
divide the product by the given number of vibrations
per minute; the square of the quotient is the length
of pendulum required. Thus if the length of a pendulum  were required that would  make 70 vibrations per minute in the latitude  of London, then
-V/39'1393 X  60
V39 9I3 X  60 =  5'3632 =  28'75 in., which is the
70
length required.
23. Q.-What measures are there of the centrifugal
force of bodies revolving in a circle?
A.-The centrifugal force of bodies revolving in a
circle increases as the diameter of the circle, if the
number of revolutions remain the same. If there be
two fly-wheels of the same weight, and making the
same number of revolutions per minute, but the diameter of one be double that of the other, the larger
will have double the amount of centrifugal force. The




16               A CATECHISM OF
centrifugal force of the same wheel, however, increases
as the square of the velocity; so that if the velocity of
a fly-wheel be doubled, it will have four times the
amount of centrifugal force.
24. Q.-Can you give a rule for determining the
centrifugal force of a body of a given weight moving
with a given velocity in a circle of a given diameter?
A.-Yes.  If the velocity in feet per second be
divided by 4 01, the square of the quotient will be
four times the height in feet from which a body must
have fallen to have acquired that velocity. Divide this
quadruple height by the diameter of the circle, and the
quotient is the centrifugal force in terms of the weight
of the body; so that multiplying the quotient by the
actual weight of the body, we have the centrifugal
force in pounds or tons. Another rule is to multiply
the square of the number of revolutions per minute
by the diameter of the circle in feet, and to divide the
product by 5,8' 0. The quotient is the centrifugal force
in terms of the weight of the body.
25. Q.-Can you explain how it comes that the
length of a pendulum determines the number of vibrations it makes in a given time?
A.-Because the length of the pendulum determines
the steepness of the circle in which the body moves,
and it is obvious, that a body will descend more rapidly over a steep inclined plane, or a steep arc of a
circle, than over one in which there is but a slight
inclination.  The impelling force is gravity, which
urges the body with a force proportionate to the distance descended; and if the velocity due to the descent
of a body through a given height be spread over a




THE STEAM ENGINE.                17
great horizontal distance, the speed of the body lmust
be slow in proportion to the greatness of that distance.
It is clear, therefore, that as the length of the pendulum determines the steepness of the arc, it must also
determine the velocity of vibration.
26. Q.-If the motions of a pendulum be dependent
on the speed with which a body falls, then a certain
ratio must subsist between the distance through which
a body falls in a second, and the length of the second's
pendulum?
A.-And so there is; the length of the second's
pendulum at the level of the sea in London, is 39'1393
inches, and it is from the length of the second's pendulum that the space through which a body falls in a
second has been determined.  As the time in which
a pendulum vibrates is to the time in which a heavy
body falls through half the length of the pendulum, as
the circumference of a circle is to its diameter, and as
the height through which a body falls is as the square
of the time of falling, it is clear that the height through
which a body will fall, during the vibration of a pendulum, is to half the length of the pendulum as the
square of the circumference of a circle is to the square
of its diameter; namely, as 9'8696 is to 1, or it is to
the whole length of the pendulum as the half of this,
namely, 4'9348 is to 1; and 4-9348 times 39'1393 in. is
16 -  ft. very nearly, which is the space through which
a body falls by gravity in a second.
27. Q.-Are the motions of the conical pendulum
or governor reducible to the same laws which apply to
the common pendulum?
A.-Yes; the motion of the conical pendulum may
2*




18                A CATECHISM OF
be supposed to be compounded of the motions of two
common pendulums, vibrating at right angles to one
another, and one revolution of a conical pendulum will
be performed in the same time as two vibrations of a
common pendulum, of which the length is equal to the
vertical height of the point of suspension above the
plane of revolution of the balls. A steam engine governor may, it is true, be driven round with any speed,
and therefore it may be supposed that the length of
the arms cannot affect the time of revolution; but as
the speed is increased the balls expand, and the height
of the cone described by the arms is diminished, until
its vertical height is such that a pendulum of that
length would perform  two vibrations for every revolution of the governor.  If, therefore, a certain expansion of the balls be desired, and a certain length be
fixed upon for the arms, so that the vertical height of
the cone is fixed, then the speed of the governor must
be such, that it will make half the number of revolutions in a given time that a pendulum equal in length
to the height of the cone would make of vibrations. The
rule is, multiply the square root of the height of the
cone in inches by 0'31986, and the product will be the
right time of revolution in seconds.  If the number of
revolutions and the length of the arms be fixed, and it
is wanted to know what is the diameter of the circle
described by the ball, you must divide the constant
number 187-68 by the number of revolutions per
minute, and the square of the quotient will be the vertical height in inches of the centre of suspension above
the plane of the ball's revolution. Deduct the square
of the vertical height in inches from the square of the




THE STEAM ENGINE.                  19
length of the arm in inches, and twic  the qqlr-e root
of the remainder is the diameter of the circle in which
the centres of the balls revolve.
28. Q.-Cannot the operation of a governor be deduced merely from the consideration of centrifugal and
centripetal forces?
A.-It can, and by a very simple process.  The
horizontal distance of the arm from the spindle divided
by the vertical height, will give the amount of centripetal force, and the velocity of revolution requisite to
produce an equivalent centrifugal force may be found
by multiplying the centripetal force of the ball in terms
of its own weight by 70,440, and dividing the product
by the diameter of the circle made by the centre of the
ball in inches: the square root of the quotient is the
number of revolutions per minute.  By this rule you
fix the length of the arms, and the diameter of the
base of the cone, or, what is the same thing, the angle
at which it is desired the arms shall revolve, and then
you make the speed or number of revolutions such
that the centrifugal force will keep the balls in the
desired position.
29. Q —Does not the weight of the balls affect the
question?
A.-Not in the least; each ball may be supposed
to be made up of a number of small balls or particles,
and each particle of matter will act for itself.  Heavy
balls attached to a governor are only requisite to overcome the friction of the throttle valve which shuts off the
steam, and of the connections leading thereto. Though
the weight of a ball increases its centripetal force, it
icreases its centrifugal force in the same proportion,




20                A CATECHISM OF
30. Q. —What do you understand by the mechanical
powers?
A. —The  mechanical powers  are certain  contrivances, such as the wedge, the screw, the inclined
plane, and other elementary machines, which convert
a small force acting through a great space into a great
force acting through a small space.  In the school
treatises on mechanics, a certain number of these devices are set forth as the mechanical powers, and each
separate device is treated as if it involved a separate
principle; hut not a tithe of the contrivances which
accomplish the stipulated end are represented in these
leaAred works, and there is no very obvious necessity for considering the principle of each contrivance
separately when the principles of all are one and the
same.  Every pressure acting with a certain velocity,
or through a certain space, is convertible into a greater
pressure acting with a less velocity, or through a
smaller space; but the quantity of mechanical force
re. ains unchanged  by this transformation, and all
that implements called mechanical powers accomplish,
is to effect this transformation.
31. Q.-Is there no power gained by the lever?
A.-Not any.; the power is merely put into another
shape, just as the contents of a hogshead of porter are
the same, whether they be let off by an inch tap or by
a hole a foot in diameter. There is a greater gush in
the one case than in the other, but it will last a shorter
time: when a lever is used there is a greater force
exerted, but it acts through a shorter distance.  It
requires just the same expenditure  of mechanical
power to lift 1 lb. through 100 ft., as to lift 100 lbs.




THE STEAM ENGINE.                2]
through one foot.  A cylinder of a given cubical capacity will exert the same power by each stroke,
whether the cylinder be made tall and narrow, or
short and wide; but in the one case it will raise a
small weight through a great height, and in the other
case, a great weight through a small height.
32. Q.-Is there no loss of power by the use of the
crank?
A.-Not any.  Many persons have supposed that
there was a loss of power by the use of the crank, because at the top and bottom centres it is capable of
exerting little or no power; but at those times there
is little or no steam consumed, so that no waste of
power is occasioned by the peculiarity. Those who
imagine that there is a loss of power caused by the
crank, confuse themselves by confounding the vertical
with the circumferential velocity.  If the circle of
the crank, be divided by any number of equidistant
horizontal lines, it will be obvious that there must be
the same steam  consumed, and the same power expended, when the crank-pin passes from the level of
one line to the level of the other, in whatever part of
the circle it may be, those lines being indicative of
equal ascents or descents of the piston. But it will
be seen that the circumferential velocity is greater
with the same expenditure of steam when the crankpin approaches the top and bottom centres; and this
increased velocity exactly compensates for the diminished leverage, so that there is the same power given
out by the crank in each of the divisions.
33. Q.-Have no plans been projected for gaining
power by means of a lever?




22                A CATECHISM OF
A.-Yes, many plans-some of them  displaying
much ingenuity, but all displaying a complete ignorance of the first principles of mechanics, which teach
that power cannot be gained by any multiplication of
levers and wheels. I have occasionally heard persons
say: " You gain a great deal of power by the use of
a capstan: why not apply the same resource in the
case of a steam  vessel, and increase the power of
your engine by placing a capstan motion between
the engine and paddle-wheels?"  Others I have heard
say: "By the hydraulic press you can obtain unlimited power: why not then interpose a hydraulic
press between the engines and the paddles?"  To
these questions the reply is sufficiently obvious. Whatever you gain in force you lose in velocity; and it
would benefit you little to make the paddles revolve
with ten times the force, if you at the same time
caused them to make only a tenth of the number of
revolutions. You cannot, by any combination of mechanism, get increased force and increased speed at
the same time, or increased force without diminished
speed; and it is from the ignorance of this inexorable
condition, that such myriads of schemes for the realization of perpetual motion, by combinations of levers,
weights, wheels, quicksilver, cranks, and other mere
pieces of inert matter, have been propounded. Any
such combination can never increase power, nor diminish it either, except by friction. Power is not
measurable by force, but by force and velocity conbined,
34. Q.-What is friction?
A.-Friction is the resistance  experienced when




THE STEAM ENGINE.                  23
one body is rubbed upon another body, and is the result of the natural attraction bodies have for one
another, and of the interlocking of the impalpable
asperities upon the surfaces of all bodies, however
smooth.  There is also, no doubt, some electrical
action involved in its production, not yet recognized
nor understood.  When motion in opposite directions
is given to smooth surfaces, the minute asperities
of one surface must mount upon those of the other,
and both will be abraded and worn away: in which
act power must be expended.  The friction of smooth
rubbing substances is less when the composition of
those substances is different, than when it is the same,
the particles being supposed to interlock less when
the opposite prominences or asperities are not coincident.
35. Q. —Does friction increase with the extent of
rubbing surface?
A.-No; the friction, so long as there is no violent
heating or abrasion, is simply in the proportion of the
pressure keeping the surfaces together, or nearly so.
It is, therefore, an obvious advantage to have the
bearing surfaces of steam engines as large as possible,
as there is no increase of friction by extending the
surface, while there is a greater increase in the durability. When the bearings of an engine are made too
small, they very soon wear out.
36. —Q. Does friction increase in the same ratio as
velocity?
A.-No; friction does not increase with the velocity at all, if the friction over a given amount of surface be considered; but it increases as the velocity, if




24                A CATECHISM OF
the comparison be made with the time during which
the friction acts. Thus the friction of each stroke of
a piston is the same, whether it makes 20 strokes in
the minute, or 40: in the latter case, however, there
are twice the number of strokes made, so that, though
the friction per stroke is the same, the friction per
minute is doubled.  The friction, therefore, of any
machine per hour varies as the velocity, though the
friction per revolution remains, at all ordinary velocities, the same. Of excessive velocities we have not
sufficient experience to enable us to state with confidence whether the same law continues to operate
among them.
37. Q.-Can you give any approximate statement
of the force expended in overcoming friction?
A.-It varies with the nature of the rubbing bodies.
The friction of iron sliding upon iron, has generally
been taken at about one-tenth of the pressure, when
the surfaces are oiled and then wiped again, so that
no film of oil is interposed. The friction of iron rubbing upon brass has generally been taken at about
one-eleventh of the pressure under the same circumstances; but in machines in actual operation, where
a film of some lubricating material is interposed between the rubbing surfaces, it is probably not more
than one-third of this amount.  Indeed, where unguents are interposed, the friction depends in a great
measure upon the nature of the unguent, the viscidity
of which may constitute a greater retarding force than
the friction; and in watchwork and other fine mechanism, it is therefore necessary to keep the bearing
surfaces small, as the resistance by the viscidity of the




THE STEAM ENGINE.                25
unguent increases with the extent of the rubbing surface. In some experiments upon the friction of shafts
by Mr. G. Rennie, he found that with a pressure of
from 1 to 5 cwt. the friction did not exceed -31th of
the pressure when tallow was the unguent employed;
with soft soap it became -2th. The nature of the unguent, proper for different bearings, appears to depend
in a great measure upon the amount of the pressure
to which they are subjected,-the hardest unguents
being best where the pressure is greatest.
38. Q.-What do you understand by a horse power?
A.-An amount of mechanical force that will raise
33,000 lbs. one foot high in a minute. This standard
was adopted by Mr. Watt as the average force exerted by the strongest London horses: the object of
his investigation being to enable him to determine the
relation between the power of a certain size of engine,
and the power of a horse; so that when it was desired
to supersede the use of horses by the erection of an
engine, he might, from  the number of horses employed, determine the size of engine that would be
suitable for the work.
39. Q.-Then, when we talk of an engine of 200horse power, it is meant that the impelling efficacy
is equal to that of 200 horses, each lifting 33,000 lbs.
one foot high in a minute?
A.-No, not now; such was the case in Watt's
engines, but the capacity of cylinder answerable to a
horse power has been increased by most engineers
since his time, and the pressure on the piston has been
increased also, so that what is now called a 200-horse
power engine exerts, almost in every case, a greater
3




26               A CATECHISM OF
power than was exerted in Watt's time, and a horse
power has become a mere conventional unit for expressing a certain size of cylinder, without reference
to the power exerted.
40. Q.-Then each nominal horse power of a modern engine may raise much more than 33,000 lbs. one
foot high in a minute?
A.-Yes; some raise 50,000 lbs., others 60,000 lbs.
and others 66,000 lbs., one foot high in a minute by
each nominal horse power; and therefore no comparison can be made between the performances of different
engines, unless the power actually exerted be first discovered.
41. Q.-How is this discovery made?
A.-By means of an instrument called the indicator,
which consists of a small cylinder, about an inch in
diameter, fitted with a piston, which is pressed down
by a spring. This piston, by the height to which it
rises against the spring, indicates the pressure within
the cylinder of the engine; and the number of pounds'
pressure on the square inch multiplied by the number
of square inches in the area of the cylinder, and by
the number of feet travelled through by the piston per
minute, gives the amount of impelling force. From
this a trifling deduction-about a tenth in the case of
large engines is enough-is to be made for friction,
&c., and the remainder is the effective moving force
which, divided by 33,000 lbs., gives the actual horse
power.
42. Q.-What quantity of steam is supposed to be
consumed by engines per horse power?
A.-About 33 cubic feet per minute, according to




THE STEAM ENGINE.                27
Mr. Watt's rule, or a cubic foot of water raised into
steam per hour, which is the proportion assumed by
many persons, and which nearly  agrees with  that
adopted by Mr. Watt.  Any such rules, however, can
only have a very partial application in modern engines,
as it is now the common practice to work engines more
or less expansively; and, in such cases, less than the
quantity offteam formerly necessary, is used.
43. Q.-What is meant by working engines expansively?
A.-Adjusting the valves, so that the steam is shut
off from the cylinder before the end of the stroke,
whereby the residue of the stroke is left to be completed by the expanding steam.
44. Q.-And what is the benefit of that practice?
A.-It accomplishes an important saving of steam,
or, what is the same thing, of fuel, but it diminishes the
power of the engine, while increasing the power of the
steam.  A larger engine will be required to do the
same work; but the work will be done with a smaller
consumption of fuel. If, for example, the steam be shut
off when only half the stroke is completed, there will
only be half the quantity of steam used. But there
will be more than half the power exerted; for although
the pressure of the steam  decreases after the supply
entering from the boiler is shut off, yet it imparts,
during its expansion, some power, and that power, it is
clear, is obtained without any expenditure of steam
or fuel whatever.
45. Q.-Can you give any rule' for ascertaining the
amount of benefit derivable from expansion?
A.-Divide the length of stroke through which the




28                 A CATECHISM OF
steam expands, by the length of stroke performed with
full pressure, which call 1; the hyperbolic logarithm
of the quotient is the increase of efficiency due to expansion.  According to this rule it will be found, that
if a given quantity of steam, the power of which working at full pressure is represented by 1, be admitted
into a cylinder of such a size that its ingress is concluded when one-half the stroke has beeniperformed,
its efficacy will be raised by expansion to 1 69; if the
admission of the steam be stopped at one-third of the
stroke, the efficacy will be 2'10; at one-fourth, 2-39;
at one-fifth, 2'61; at one-sixth, 2'79; at one-seventh,
2'95; at one-eighth, 8'08.  The expansion, however,
cannot be carried beneficially so far as one-eighth, unless the pressure of the steam in the boiler be very
considerable, on account of the inconvenient size of
cylinder or speed of piston which would require to
be adopted, the friction of the engine, and the resistance of vapor in  the condenser, which  all become
relatively greater with a smaller urging force.
46. Q.-What is meant by latent heat?
A.-By latent heat is meant the heat existing in
bodies which is not discoverable by the touch or by
the thermometer, but which manifests its existence by
producing a change of state. Heat is absorbed in the
liquefaction of ice, and in the vaporization of water,
yet the temperature does not rise during either process,
and the heat absorbed is therefore said to become
latent.  The term  is somewhat objectionable, as the
effect proper to the absorption of heat has in each case
been made visible; and it would be as reasonable to call
hot water latent steam. Latent heat, in the present




THE STEAM ENGINE.                29
acceptation of the term, means sensible liquefaction or
vaporization; but to produce these changes heat is as
necessary as to produce an expansion of the mercury
in a thermometer tube, and it is hard to see on what
ground heat can be said to be latent when its presence
is made manifest by a change of state.  It is the temperature only that is latent, and latent temperature
means sensible something else.
47. Q.-But when you talk of the latent heat of
steam, what do you mean to express?
A.-I mean to express the heat consumed in accomplishing the vaporization compared with that necessary
for producing the temperature.  The latent heat of
steam is usually reckoned at about 1000 degrees; by
which it is meant that there is as much heat in any
given weight of steam as would raise its constituent
water 1000 degrees if the expansion of the water
could be prevented, or as would raise 1000 times that
quantity of water one degree.  The boiling point is
180 degrees above the freezing point; so that it requires 1180 times as much heat to raise a lb. of water
into steam, as to raise 1180 lbs. of water one degree;
or it requires about as much heat to raise a pound of
boiling water into steam as would raise 5~ lbs. of water
from the freezing to the boiling point; 5~ multiplied
by 180 being 990, or 1000 nearly.
48. Q.-What do you understand by specific heat?
A. —By specific heat, I understand the relative
quantities of heat in bodies at the same temperature,
just as by specific gravity I understand the relative
quantities of matter in bodies of the same bulk. Equal
weights of quicksilver and water at the same tempera3*




30                A CATECHISM OF
ture do not contain the same quantities of heat, any
more than equal bulks of those liquids contain the
same quantity of matter.  The absolute quantity of
heat in any body is not known; but the relative heat
of bodies at the same temperature, or in other words
their specific heats, have been ascertained and arranged
in tables, the specific heat of water being taken as
unity.
49. Q.-What expansion does water undergo in its
conversion into steam?
A.-A cubic inch of water makes about a cubic foot
of steam of the atmospheric pressure.
50. Q.-And how much at a higher pressure?
A.-That depends upon what the pressure is.  The
higher the pressure, the smaller will be the volume of
steam from a given quantity of water; for high pressure steam is just low pressure steam forced into a less
space.
51. Q.-If this be so, the quantity of heat in a given
weight of steam is the same, whether it is high or low
pressure steam?
A.-Yes; the heat in steam is a constant quantity,
or nearly so, at all pressures, if the steam be saturated
with water. Steam, to which an additional quantity of
heat has been imparted after leaving the boiler, or as
it is called "surcharged steam," comes under a different
aw, for the elasticity of such steam may be increased
without any addition being made to its weight; but
surcharged steam is not employed for working engines,
and it may therefore be considered in practice that a
pound of steam contains the same quantity of heat at
all pressures.




TILE STEAM ENGINE.                 31
52. Q.-But the temperature of steam  varies with
the pressure?
A.-Yes; the temperature rises with the pressure,
but the latent heat becomes less in the same proportion. The latent heat of high pressure steam is less
therefore than that of low pressure steam, while the
sensible heat is greater, and the two taken together
make up the same sum at all temperatures.
53. Q.-Is there any benefit arising from the use of
high pressure steam in steam engines?
A.-In high pressure, as contrasted with condensing
engines, there is always the loss of the vacuum, which
will generally amount to 12 or 13 lbs. on the square
inch.
54. Q.-But in high pressure engines, is there any
benefit arising from the use of a very high pressure
over a pressure of a moderate account?
A.-Yes, there is an advantage; for, in all high
pressure engines, there is a diminution in the power
caused by the counteracting pressure of the atmosphere
on the educting side of the piston: for the force of the
piston in its descent would obviously be greater, if
there was a vacuum beneath it; and the counteracting
pressure of the atmosphere is relatively less when the
steam used is of a very high pressure. It is clear, that
if you bring down the pressure of the steam in a high
pressure engine to the pressure of the atmosphere, it
will not exert any power at all, whatever quantity of
steam may be expended, and if the pressure be brought
nearly as low as that of the atmosphere, the engine
will exert only a very small amount of power; whereas,
if a very high pressure be employed, the pressure of




32                A CATECHISM OF
the atmosphere will become relatively as small in
counteracting the impelling pressure, as the attenuated
vapor in the condenser of a condensing engine is in
resisting the lower pressure which is there employed.
55. Q.-In the case of condensing engines, is there
any benefit derivable from the use of steam of a high
pressure?
A.-Setting aside loss from friction, and supposing
the vacuum  to be a perfect one, there would be no
benefit arising from the use of steam of a high pressure
in condensing engines, for the same weight of steam
used without expansion, or with the same measure of
expansion, would produce at every pressure the same
amount of mechanical power.  A piston, with a square
foot of area, and a stroke of three feet, with a pressure
of one atmosphere, would obviously lift the same weight
through the same distance, as a cylinder with half a
square foot of area, and a stroke of three feet, with a
pressure of two atmospheres. In the one case, we have
three cubic feet of steam of the pressure of one atmosphere, and in the other case 1  cubic feet of the pressure of two atmospheres. But there is the same weight
of steam, or the same quantity of heat and water in it,
in both cases; so that it appears a given weight of
steam  would, under such  circumstances, produce a
definite amount of power, without reference to the
pressure. In the case of ordinary engines, however,
these conditions do not exactly apply; the vacuum is
not a perfect one, and the pressure of the resisting
vapor becomes relatively greater as the pressure of
the steam  is diminished; the friction also becomes
greater from the necessity of employing larger cylinders,




THE STEAM ENGINE.                  33
so that even in the case of condensing engines, there
is a benefit arising from the use of steam of a considerable pressure. Expansion cannot be carried beneficially
to any great extent, unless the initial pressure be considerable; for if steam of a low pressure were used, the
ultimate tension would be reduced to a point so nearly
approaching that of the vapor in the condenser, that
the difference would not suffice to overcome the friction
of the piston; and a loss of power would be occasioned
by carrying expansion to such an extent. In some of
the Cornish engines, the steam is cut off at one-twelfth
of the stroke; but there would be a loss arising from
carrying the expansion so far, instead of a gain, unless
the pressure of the steam were considerable. It is clear,
that in the case of engines which carry expansion very
far, a very perfect vacuum in the condenser is more
important than it is in other cases. Nothing can be
easier than to compute the ultimate pressure of expanded steam, so as to see at what point expansion ceases
to be productive of benefit; for as the pressure of expanded steam  is inversely as the space occupied, the
terminal pressure when the expansion is twelve times, is
just one-twelfth of what it was at first, and so on, in all
other proportions. The total pressure should be taken as
the initial pressure-not the pressure on the safety valve,
but that pressure plus the pressure of the atmosphere.
56. Q.-Then in high pressure engines working at
from 70 to 90 lbs. on the square inch, as in the case of
locomotives, the efficiency of a given quantity of water
raised into steam may be considered to be about the
same as in condensing engines?
A.-Yes, there will not be much difference in the




34                A CATECHISM OF
case supposed: if the pressure of steam  in a high
pressure engine be 120 lbs., or 105 lbs. above the atmosphere, then the resistance occasioned by the atmosphere will cause a loss of 8th of the power.  If the
pressure of the steam  in a low pressure engine be
16 lbs. on the square inch, or 1 lb. above the atmosphere,
and the tension of the vapor in the condenser be equivaleut to four inches of mercury, or 2 lbs. of pressure
on the square inch, then the resistance occasioned by
this rare vapor will also cause a loss of — th of the power.
A high pressure engine, therefore, with a pressure of
105 lbs. above the atmosphere, works with only the
same loss from resistance to the piston, as a low pressure engine with a pressure of 1 lb. above the atmosphere; and with these proportions the power produced
by a given weight of steam will be the same, whether
the engine be high pressure or condensing.
57. Q.-Is there any limit to the force of steam?
A.-Yes; there is a limit regulated by the modulus
of elasticity of water, which at a temperature of 600 is
22,100 atmospheres.  The modulus of elasticity, of
any substance, is the measure of its elastic force, and
if water be enclosed in a close vessel which it exactly
fills, and be then subjected to heat, the expanding force,
it is clear, is that of compressed water-for no steam
can be produced under such circumstances-and we
have this proportion: as the volume of expanded water
is to the amount of expansion, so is the modulus of
elasticity of water to the elastic force of steam of the
same density as water.
58. Q. —Have experiments been made to determine
the elasticity of steam at different temperatures?




rHE STEAM ENGINE.                 35
A. — Yes; very careful experiments.  The following rule expresses the results obtained by Mr.
Southern: - To the given temperature in degrees of
Fahrenheit add 51 3 degrees; from  the logarithm  of
the sum, subtract the logarithm of 135'767, which is
2'1327940; multiply the remainder by 5'13, and to the
natural number answering to the sum, add the constant fraction -1, which will give the elastic force in
inches of mercury.  If the elastic force be known, and
it is wanted to determine the corresponding temperature,
the rule must be modified thus: -From  the elastic
force, in inches of mercury, subtract the decimal'1,
divide the logarithm of the remainder by 5'13, and to
the quotient add the logarithm 2'1327940; find the
natural number answering to the sum, and subtract
therefrom the constant 51'3; the remainder will be
the temperature sought.  The French Academy, and
the Franklin Institute, have repeated Mr. Southern's
experiments on a larger scale: the results obtained by
them are not widely different, and are perhaps nearer
the truth, but Mr. Southern's results are generally
adopted  by engineers, as sufficiently  accurate  for
practical purposes, and those results being identified
with engineering practice, it appears expedient to retain them.
59. Q.-What law is followed by surcharged steam
on the application of heat?
A.-The same as that followed by air, in which the
increments in volume are very nearly in the same proportion as the increments in temperature. A volume
of air which, at the temperature of 320, occupies 100
cubic feet, will at 2120 fill a space of 1371 cubic feet,




36               A CATECHISM OF
The volume which air or steam out of contact with
water, of a given temperature, acquires by being heated
to a higher temperature, the pressure remaining the
same, may be found by the following rule:-To each
of the temperatures before and after expansion, add
the constant number 459: divide the greater sum by
the less, and multiply the quotient by the volume at
the lower temperature; the product will give the expanded volume.
60. Q.-If the relative volumes of steam and water
are known, is it possible to tell the quantity of water
which should be supplied to a boiler, when the quantity of steam expended is specified?
A.-Yes; at the atmospheric pressure, a cubic inch
of water has to be supplied to the boiler for every cubic
foot of steam abstracted; at other pressures, the relative bulk of water and steam may be determined as
follows:-To the temperature of steam in degrees of
Fahrenheit, add the constant number 459, multiply the
sum by 75'7, and divide the product by the elastic
force of the steam in inches of mercury, and the quotient will give the volume required. In practice, however, it is necessary that the feed pump should be
able to supply the boiler with a much larger quantity
of water than what is indicated by these proportions,
from the risk of leaks, priming, or the accidental subsidence of the water to a dangerously low level, which
it is necessary as speedily as possible to remedy. It
appears expedient that the feed pump should be capable
of raising 3~ times the water evaporated by the boiler;
it is therefore made about 240th of the capacity of the
cylinder for low pressure engines, supposing the cylinder




THE STEAM ENGINE.                37
to be double acting, and the pump single acting.  In
high pressure engines, however, the capacity of the
pump should be greater, in proportion to the pressure
of the steam.
61. Q.-How do you estimate the quantity of watei
requisite for condensation?
A.-Mr. Watt found that the most beneficial temperature of the hot well of his engines was 100 degrees.
If, therefore, the temperature of the steam be 212~, and
the latent heat 1,0000, then 1,212~ may be taken to
represent the heat contained in the steam, or 1,112~
if we deduct the temperature of the hot well. If the
temperature of the injection water be 50~, then 50 degrees of cold are available for the abstraction of heat;
and as the total quantity of heat to be abstracted, is
that requisite to raise the quantity of water in the
steam 1,112 degrees, or 1,112 times that quantity one
degree, it would raise one-fiftieth of this, or 22'24 times
the quantity of water in the steam, 50 degrees. A
cubic inch of water therefore raised into steam, will
require 22'24 cubic inches of water at 50 degrees for its
condensation, and will form therewith 23-24 cubic inches
of hot water at 100 degrees. Mr. Watt's practice was
to allow about a wine pint (28 9 cubic inches) of injection water, for every cubic inch of water evaporated
from the boiler. The usual capacity for the cold water
pump is -I- th of the capacity of the cylinder, which
allows some water to run to waste.
62. Q.-Is not a good vacuum in an engine conducive to increased power?
A.-It is.
63. Q.-And is not the vacuum  good in the pro4




38               A CATECHISM OF
portion in which the temperature is low, supposing
there to be no air leaks?
A.-Yes.
64. Q.-Then how could Mr. Watt find a temperature of 1000 in the water drawn from the condenser,
to be more beneficial than a temperature of 700 or
80o, supposing there to be an abundant supply of cold
water?
A.-Because the superior vacuum due to k temperature of 70~ or 800 involves the admission of so much
cold water into the condenser, which has afterwards
to be pumped out in opposition to the pressure of the
atmosphere, that the gain in the vacuum does not equal
the loss of power occasioned by the additional load upon
the pump; and there is therefore a clear loss by the re.
duction of the temperature below 1000, if such reduction be caused by the admission of an additional quantity
of water. If the reduction of temperature, however, be
caused by the use of colder water, there is a gain produced by it, though the gain will within certain limits
be greater, if advantage be taken of the lowness of the
temperature to diminish the quantity of injection.
65. Q.-Cannot the condensation of the steam be
accomplished by any other means, than by the admission of cold water into the condenser?
A.-It may be accomplished by the method of external cold, as it is called, which consists in the application of a large number of thin metallic surfaces to the
condenser, on the one side of which the steam circulates,
while on the other side there is a constant current of
cold water, and the steam is condensed by coming into
contact with the cold surfaces, without mingling with




THE STEAM ENGINE.                39
the water used for the purpose of refrigeration. The
first kind of condenser employed by Mr. Watt was constructed after this fashion, but he found it in practice
to be inconvenient from its size, and to become furred
up or incrusted when the water was bad, whereby the
conducting power of the metal was impaired. He therefore reverted to the use of the jet of cold water, as being
upon the whole preferable. The jet entered the condenser instead of the cylinder as was the previous practice, and this method is now the one in common use.
Some few years ago, a good number of steam vessels
were fitted with Hall's condensers, which operated on
the principle of external cold, and which consisted of a
fagot of small copper tubes surrounded by water; but
the use of those condensers has not been persisted in,
and most of the vessels fitted with them have returned
to the ordinary plan.
66. Q.-How much water will a pound of coal raise
into steam?
A.-From 6 to 8 lbs. of water in the generality of
land boilers of medium quality, the difference depending
on the kind of boiler, the kind of coal, and other circumstances. Mr. Watt reckoned his boilers as capable
of evaporating 10'08 cubic feet of water with a bushel
or 84 lbs. of coal, which is equivalent to T7 lbs. of water
evaporated by one pound of coal, and this may be taken
as the performance of common land boilers at the present time. In some of the Cornish boilers, however, a
pound of coal raises 10 lbs. of water into steam, or a
cwt. of coal evaporates about 19 cubic feet of water.
The quantity of fuel burned on each square foot of fire
grate per hour, varies very much in different boilers:




40                A CATECHISM OF
in wagon boilers it is from 10 to 13 lbs.; in Cornish
boilers from 3~ to 4 lbs.; and in locomotive boilers from
80 to 100 lbs.  The number of square feet of surface
required to evaporate a cubic foot of water per hour is
about 70 square feet in Cornish boilers, 9 to 11 square
feet in land and marine boilers, and about 6 square feet
in locomotive boilers. The number of square feet of
heating surface per square foot of fire grate, is from
13 to 15 square feet in wagon boilers; about 40 square
feet in Cornish boilers; and from 50 to 70 sjuare feet
in locomotive boilers.
67. Q.-But what is the heating surface per horse
power?
A.-About 9 square feet per horse power is the
usual proportion in wagon boilers, reckoning the total
surface as effective surface, if the boilers be of a considerable size; but in the'case of small boilers the proportion is larger. The total heating surface of a twohorse power wagon boiler is, according to Boulton and
Watt's proportions, 30 square feet or 15 ft. per horse
power; whereas, in the case of a 45-horse power boiler
the total heating surface is 438 square feet or 9'6 ft. per
horse power.  The capacity of steam room is 84- cubic
feet per horse power, in the two-horse power boiler,
and 53- cubic feet in the 20-horse power boiler; and
in the larger class of boilers, such as those suitable for
30 and 45 horse power engines, the capacity of the
steam room does not fall below this amount, and indeed
is nearer 6 than 53 cubic feet per horse power.  The
content of water is 1 8  cubic feet per horse power in the
two-horse power boiler, and 15 cubic feet per horse
power in the 20-horse power boiler. In marine boilers




THF STEAM ENGINE.                 41
about the same proportions obtain in most particulars.
The original boilers of the " Great Western" steamer,
by Messrs. Maudslay, were proportioned with about
half a square foot of fire grate per horse power, and ten
square feet of flue and furnace surface, reckoning the
total amount as effective; but in the boilers of the
" Retribution," by the same makers, a somewhat smaller
proportion of heating surface was adopted. Boulton
and Watt have found that in their marine flue boilers
9 square feet of flue and furnace surface are requisite
to boil off a cubic foot of water per hour, which is the
proportion that obtains in their land boilers; but inasmuch as in modern engines the nominal considerably
exceeds the actual power, they allow 11 square feet of
heating surface per nominal horse power in  their
marine boilers, and they reckon as effective heating
surface, the tops of the flues, and the whole of the
sides of the flues, but not the bottoms. They have
been in the habit of allowing for the capacity of the
steam space in marine boilers 16 times the content of
the cylinder; but as there are two cylinders, this is
equivalent to 8 times the content of both cylinders,
which is the proportion commonly followed in land engines, and which agrees very nearly with the proportion
of between 5 and 6 cubic feet of steam room per horse
power.  Taking, for example, an engine with 23 inches
diameter of cylinder and 4 feet stroke-which will be
18'4 horse power —the area of the cylinder will be
415'476 square inches, which multiplied by 48, the
number of inches in the stroke, will give 19942'848 for
the capacity of the cylinder in cubic inches; 8 times
this is 159542'784 cubic inches, or 92-3 cubic feet; 92'3
4*




42                A CATECHISM OF
divided by 18 4 is rather more than 5 cubic feet per
horse power. There is less necessity, however, that the
steam space should be large when the flow of steam from
the boiler is very uniform, as it will be where there are
two engines attached to the boiler at right angles with
one another, or where the engines work at a great
speed, as in the case of locomotive engines. A high
steam chest too, by rendering boiling over into the
steam pipes, or priming as it is called, more difficult,
obviates the necessity for so large a steam space; and
the use of steam of a high pressure, worked expansively,
has the same operation; so that in modern marine
boilers, of the tubular construction, where the whole
of these modifying circumstances exist, there is no necessity for so large a proportion of steam room as 5 or
6 cubic feet per horse power, and about half that amount
more nearly represents the general practice. In many
of the marine tubular boilers, however, the steam space
is made too small; in some cases only 13 cubic feet
per hose power; but the operation of boilers thus proportioned is not satisfactory. The proportion of 11
square feet of heating surface per nominal horse power,
reckoning all the surface as effective, half a square foot
of fire grate, and 3 cubic feet of steam room, seems to
answer very well for tubular boilers, where the steam
chest rises above the boiler, and the engines work expansively through one-third or one-fourth of the stroke.
Boulton and Watt allow 0'64 of a square foot per
nominal horse power of grate bars in their marine
boilers, and a good effect arises from this proportion;
but sometimes so large an area of fire grate cannot be
conveniently got, and the proportion of half a square




THE STEAM ENGINE.                43
foot per horse power seems to answer very well in engines working with some expansion, and is now very
widely adopted.  With this allowance, there will be
about 22 square feet of heating surface per square-foot
of fire grate, and if the consumption of fuel be taken
at 6 lbs. per nominal horse power per hour, there will be
12 lbs. of coal consumed per hour on each square foot
of grate. The furnaces should not be more than 6 ft.
long, as, if much longer than this, it is impossible to fire
them effectually.
68. Q.-Can the consumption be made as small as
6 lbs. per nominal horse power per hour?
A.-It can by working expansively, and in some
engines the consumption per nominal horse power per
hour does not exceed this amount.
69. Q.-Can the consumption be made as low as
6 lbs. per actual horse power per hour without resorting to expansion?
A.-An actual horse power, or 33,000 lbs. raised one
foot high in the minute, is represented by the evaporation of a cubic foot of water in the hour; and if a cubic
foot of water has to be evaporated by 6 lbs. of coal,
then 1 lb. of coal must be capable of evaporating 10~
lbs. of water, which is more than the usual proportion,
and engines, therefore, with such a rate of consumption,
may be set down as deriving a part of their economy
from expansion.
70. Q.-Would it not be beneficial to introduce the
Cornish boilers into steam vessels, since they operate
with a superior economy in fuel?
A.-No; it would not.  The superior economy of
the Cornish boilers is not derived from any peculiarity




44                A CATECHISM OF
of form or arrangement, but from the immense extension of heating surface, which, though productive of
economy in fuel, is attended with an increased expense
of boiler, while the space occupied is such as in the
case of a steam vessel would be quite inadmissible. If
a marine boiler be made with the same quantity of
surface as a Cornish boiler, the marine boiler is found
to be the more economical of the two.
71. Q.-Is not the combustion in the furnaces of
the Cornish boilers very slow?
A.-Yes, very slow; and there is in consequence
very little smoke evolved. It is an important part of
smoke burning to make the combustion slow; and the
chief instigator of smoke, is an insufficient size of furnace.  The coal used in Cornwall is Welsh coal, which
evolves but little smoke, and is therefore more favorable for the success of a smokeless furnace; but in the
manufacturing districts, where the coal is more bituminous, it is found that smoke may be almost wholly prevented by careful firing and a large capacity of furnace.
72. Q.-What is the nature of combustion?
A.-Combustion is nothing more than an energetic
chemical combination; or, in other words, it is the
mutual neutralization of opposing electricities. When
coal is brought to a high temperature it acquires a
strong affinity for oxygen, and combination with oxygen
will produce more than sufficient heat to maintain the
original temperature.
73. Q.-Does air consist of oxygen?
A.-Air consists of oxygen and nitrogen mixed together, in the proportion of 31 lbs. of nitrogen to 1 lb.
of oxygen.  Every pound of coal requires about two




THE STEAM ENGINE.                45
pounds of oxygen for its saturation, and therefore for
every pound of coal burned seven pounds of nitrogen
must pass through the fire, supposing all the oxygen
to enter into combination. In practice, however, this
perfection of combination does not exist: from onethird to one-half of the oxygen will pass through the
fire without entering into combination; so that from
12 to 14 lbs. of air are required for every pound of?oal burned. 12 lbs. of air are about 160 cubic feet,
and 200 cubic feet may be taken as the quantity of air
required for the combustion of a pound of coal in practice.
74. Q.-Had Mr. Watt any method of consuming
smoke?
A.-He tried various methods, but eventually fixed
upon the method of coking the coal on a dead plate at
the furnace door, before pushing it into the fire. That
method is perfectly effectual where the combustion is so
slow that the requisite time for coking is allowed, and it
is much preferable to any of the methods of admitting
air at the bridge or elsewhere, to accomplish the combustion of the inflammable parts of the smoke. But the
best arrangement for smoke burning is, it appears to me,
that of the revolving grate, which feeds the fire at the
same time by a self-acting mechanism. In this plan, the
fire grate is made like a round table capable of turning
upon a centre; a shower of coal is precipitated upon the
grate through a slit in the boiler near the furnace mouth,
and the smoke evolved from the coal dropped at the front
part of the fire is consumed by passing over the incandescent fuel at the back part, from which all the smoke
must have been expelled in the revolution of the grate
before it can have reached that position.




46                A CATECHISM OF
75. Q.-Is a furnace of this kind applicable to a
steam vessel?
A.-I see nothing to prevent its application.  The
arrangement of the boiler should perhaps be changed,
to facilitate its application, and I should prefer to
combine its use with the employment of vertical tubes,
which are found to be more efficacious in practice
than horizontal tubes; but this innovation is not indispensable. The introduction of any effectual automatic contrivance for feeding the fire in steam vessels,
would bring about an important economy, at the same
time that it would give the assurance of the work being
better done.  It is very difficult to fire furnaces by
hand effectually at sea, especially in rough weather and
in tropical climates; whereas machinery would be un.
affected by any such impediments, and would perform
with little expense the work of many men.
76. Q. - The introduction  of some mechanical
method of feeding the fire with coals would enable a
double tier of furnaces to be adopted in steam vessels
without inconvenience?
A.-Yes, it would have at least that tendency; and
as the space available for area of grate is limited in a
steam vessel by the width of the vessel, it would be a
great convenience if a double tier of furnaces could
be employed without a diminished effect. It appears
to me, however, that the objection would still remain
of the steam raised by the lower furnace being cooled
and deadened by the air entering the ash-pit of the
upper fire, for it would strike upon the metal of the
ash-pit bottom; but this surface could be covered with
fire-brick to diminish the refrigeratory effect.




THE STEAM ENGINE.                47
77. Q.-What method of firing furnaces is the best?
A.-The coals should be broken up into small
pieces, and sprinkled thinly and evenly over the fire a
little at a time.  The thickness of the stratum  of coal
upon the grate should depend upon the intensity of
the draft: in ordinary land or marine boilers it should
be thin, whereas in locomotive boilers it requires to be
much thicker.  If the stratum of coal be thick while
the draft is sluggish, the carbonic acid resulting from
combustion combines with an additional atom of carbon
in passing through the fire, and is converted into carbonic oxide, which may be defined to be invisible
smoke, as it carries off a portion of the fuel: if, on the
contrary, the stratum of coal be thin while the draft is
very rapid, an injurious refrigeration is occasioned by
the excess of air passing through the furnace. The
fire should always be spread of uniform thickness over
the bars of the grate, and should be without any holes
or uncovered places, which greatly diminish the effect
of the fuel by the refrigeratory action of the stream of
cold air which enters thereby.
78. Q.-What is the method of consuming smoke
pursued in the manufacturing districts?
A.-In Manchester, where some stringent regulations for the prevention of smoke have latterly been
enacted, it is found that the readiest way of burning
the smoke is to have a very large proportion of furnace room, whereby slow combustion may be carried
on, and the operation of the dead plate, prescribed by
Mr. Watt, may be made more effectual.  In some
cases, too, a favorable result is arrived at by raising
a ridge of coal across the furnace lying against the




48                A CATECHISM OF
bridge, and of the same height: this ridge speedily
becomes a mass of coke, which promotes the combustion of the smoke passing over it.
78. Q.-Is the method of admitting a stream of air
into the flues regarded favorably?
A.-No; it is found to be productive of injury to
the boiler by the violent alternations of temperature
it occasions, as at some times cold air impinges on the
iron of the boiler, and at other times flame, just as
there happens to be smoke or no smoke emitted by
the furnace.  Boilers, therefore, operating upon this
principle, speedily become leaky, and are much worn
by oxidation, so that, if the pressure is considerable,
they are liable to explode. It is very difficult to apportion the quantity of air admitted, to the varying
wants of the fire, and as air may at some times be
rushing in when there is no smoke to consume, a loss
of heat, and an increased consumption of  fuel may be
the result of the arrangement; and, indeed, such is
the result in practice, though a carefully performed
experiment usually demonstrates a saving of 10 or 12
per cent.
80. Q.-What other plans have been contrived for
obviating the nuisance of smoke?
A. —They are too various for enumeration, but
most of them either operate upon the principle of admitting air into the flues to accomplish the combustion
of the uninflammable parts of the smoke, or seek to
attain the same object by passing the smoke over or
through  the  fire, or other incandescent materials.
Some of the plans, indeed, profess to burn the inflammable gases as they are evolved from the coal, without




THE STEAM ENGINE.                 49
permitting the admixture of any of the uninflammable
products of combustion which enter into the composition of smoke; but this object has been very imperfectly fulfilled in any of the contrivances yet brought
unler the notice of the public, and as the revolving
grate, without aspiring to a theoretic perfection, seems
to achieve the combustion of the smoke when the fire
is not urged with vehemence, and as it saves in addition the labor of firing, it seems to be entitled to a
preference in practice. Jucke's furnace, which consists of an arrangement of the fire bars in the form of
an endless chain, whereby the coal is gradually carried
forward as it burns, and the ashes and clinkers are
precipitated into the ash-pit at the extremity of the
furnace where the chain turns down over the extreme
carrying roller, is also well spoken of; but it is more
expensive to construct than the revolving grate, and
more difficult to keep in repair.  The patent-right of
the revolving grate expired some years ago, which
may, perhaps, be in some cases a further suasory to
its adoption.
81. Q.-You have stated  that an actual horse
power is a force capable of raising 33,000 lbs. one foot
high in the minute, or a dynamical effort such as is
produced by an expenditure of 33 cubic feet of steam
per minute, or the evaporation of a cubic foot of water
per hour, but you have given no definition of the value
of a nominal horse power.  How do you ascertain the
power of an engine in nominal horse power?
A.-The nominal power of an engine may be ascertained by the following rule: multiply the square
of the diameter of the cylinder in inches by the ve-.5




50                A CATECHISM OF
locity of the piston in feet per minute, and divide the
product by 6000; the quotient is the number of nominal horse power.  In using this rule, however, it is
necessary to adopt the speed of piston described by
Mr. Watt, which varies with the length of the stroke.
The speed of piston with a two feet stroke is, according to his system, 160 per minute; with a 2 ft. 6 in.
stroke, 170; 3 ft., 180; 3 ft. 6 in., 189; 4 ft., 200;
5 ft., 215; 6 ft., 228; 7 ft., 245; 8 ft., 256 ft.
82. Q.-By ascertaining the ratio in which the velocity of the piston increases with the length of the
stroke, could not the element of velocity be cast out
altogether, and the nominal power be determined by a
reference merely to the dimensions of the cylinder?
A.-Yes, and this for most purposes is the most
convenient method of procedure: multiply the square
of the diameter of the cylinder in inches by the cube
root of the stroke in feet, and divide the product by
47; the quotient is the number of nominal horse power
of the engine. This rule supposes a uniform effective
pressure upon the piston of 7 lbs. per square inch. Mr.
Watt estimated the effective pressure upon the piston
of his 4-horse power engines at 6 8 lbs. per square inch,
and the pressure increased slightly with the power,
and became 6-94 lbs. per square inch in engines of
100-horse power; but it appears to be more convenient
to take a uniform pressure of 7 lbs. for all powers.
Small engines, indeed, are somewhat less effective in
proportion than large ones, but the difference can be
made up by slightly increasing the pressure in the
boiler; and small boilers will bear such an increase
without inconvenience.




THE STEAM ENGINE.                51
83. Q.-Can nominal be transformed into actual
horse power?
A.-No, that is not possible in the case of common condensing engines; the actual power exerted by
an engine cannot be deduced from its nominal power,
neither can the nominal power be deduced from the
power actually exerted, nor from any thing else than the
dimensions of the cylinder.  The actual horse power
is a dynamical unit, and the nominal horse power is a
measure of capacity of the cylinder, which are obviously incomparable things.
84. Q.-That is, the nominal horse power expresses
the size of an engine, and the actual horse power the
number of times 33,000 lbs. it will lift one foot high in
a minute?
A.-Precisely; and to find the number of times
33,000 lbs., or 528 cubic feet of water, it will raise one
foot high in a minute-or, in other words, the actual
power-you first find the pressure in the cylinder by
means of the indicator, from which you deduct a pound
and a half of pressure for friction, the loss of power in
working the air pump, &c.; multiply the area of the
piston in square inches by this residual pressure, and
by the motion of the piston, in feet per minute, and
divide by 33,000; the quotient is the actual number
of horses power. The same result is attained by squaring the diameter of the cylinder, multiplying by the
pressure per square inch, as shown by the indicator,
less a pound and a half, and by the motion of the piston, in feet, and dividing by 42,017. The quantity
thus arrived at will, in the case of nearly all modern
engines, be very different from that obtained by mul



52                A CATECH ISM OF
tiplying the square of the diameter of the cylinder by
the cube root of the stroke, and dividing by 47, which
expresses the nominal power; and the actual and nominal power must by no means be confounded, as they
are totally different things.
85. Q.-What is meant by the duty of an engine?
A.-The work done in relation to the fuel consumed.
86. Q.-And how is the duty ascertained?
A.-In ordinary mill or marine engines it can only
be ascertained by the indicator, as the load upon such
engines is variable, and cannot readily be determined;
but in the case of engines for pumping water, where
the load is constant, the number of strokes performed
by the engine will represent the work done, and the
amount of work done by a given quantity of coal represents the duty. In Cornwall the duty of an engine
is expressed by the number of millions of pounds
raised one foot high by a bushel, or 94 lbs. of Welsh
coal.  A  bushel of Newcastle coal will only weigh
84 lbs.; and in comparing the duty of a Cornish engine
with the performance of an engine in some locality
where a different kind of coal is used, it is necessary
to pay regard to such variations.
87. Q.-Can you tell the duty of an engine when
you know its consumption of coal per horse power per
hour?
A.-Yes, if the power given be the actual, and not
the nominal power. Divide 166'32 by the number of
pounds of coal consumed per actual horse power per
hour; the quotient is the duty in millions of pounds.
If you already have the duty in millions of pounds,




THE STEAM ENGINE.                53
and wish to knot/ the equivalent consumption in pounds
per actual horse power per hour, divide 166'32 by
the duty in millions of pounds; the quotient is the
consumption per actual horse power per hour.  The
duty of a locomotive engine is expressed by the weight
of coke it consumes in transporting a ton through the
distance of one mile upon a railway; but this is a very
imperfect method of representing the duty, as the
tractive efficacy of a pound of coke becomes less as
the speed of the locomotive becomes greater, and the
law of variation is not accurately known.
88. Q. —What amount of power is generated in
good engines of the ordinary kind by a given weight of
coal?
A.-The duty of different kinds of engines varies
very much, and there are also great differences in the
performance of different engines of the same class. In
ordinary rotative condensing engines of good construction, 10 lbs. of coal per nominal horse power per hour
is a common consumption; but such engines exert
nearly twice their nominal power, so that the consumption per actual horse power per hour may be taken
at from 5 to 6 lbs.  Engines working very expansively,
however, attain an economy much superior to this. The
average duty of the pumping engines in Cornwall is
about 6'0,000,000 lbs. raised 1 ft. high by a bushel of
Welsh coals, which weighs 94 lbs.  This is equivalent to a consumption of 3'1 lbs. of coal per actual
horse power per hour; but some engines reach a duty
of above 100,000,000, or 1'74 lbs. of coal per actual
horse power per hour.  Locomotives consume from  8
to 10 lbs. of coke in evaporating a cubic foot of water,
0




54                A CATECHISM OF
and the evaporation of a cubic foot of water per hour
may be set down as representing an actual horse
power in locomotives as well as in condensing engines.
Measuring the consumption of fuel by the nuniber of
tons a locomotive will draw through a given distance, it
appears that passenger engines consume from I to 4 lb.
of coke per ton per mile, and the goods engines from -4
to I lb. When the locomotive is worked expansively,
there is of course a less consumption of water and
fuel per horse power, or per ton per mile, than when
the full pressure is used throughout the stroke, and
most locomotives now operate with as much expansion
as can be conveniently given by the slide valves.
89. Q.-But some kinds of coal are more effective
than other kinds, and there will be a difference of
effect from this cause?
A.-Yes. In a boiler so proportioned, that a pound
of the best Welsh coal will evaporate 9'493 lbs. of
water; a pound of anthracite will evaporate 9 014
lbs. of water; a pound of the best small Newcastle
8-524 Ibs.; a pound of coke from gas works 79908
lbs.; a pound of Welsh and Newcastle, of medium
quality, mixed half and half, 7'897 lbs.; a pound of
Derbyshire 6'772 lbs.; and a pound of Blyth Main,
Northumberland, 6-6 lbs. of water. There thus appears to be a difference of at least one-third in the
efficacy of the different coals of commerce in raising
steam.  Coal and coke may be reckoned equal to one
another in evaporative power.  A  pound of wood in
its ordinary state evaporates about 4'72 lbs. of water,
and a pound of turf 5'45 lbs. A pound of wood charcoal, however, is said to evaporate 13'37 lbs., and a




THE STrEAM ENGINE.                 55
pound of turf charcoal 11 63 Ibs.  The number of
pounds of air consumed by a pound of any combustible
is very nearly the same as the number of pounds of
water it is capable of evaporating, though twice that
quantity of air must pass through the fire. A pound
of wood in its ordinary state requires 4'47 lbs. of air
for its combustion, and a pound of turf 4'6 lbs. If one
pound of coal evaporate 9k lbs. of water, then a cubic
foot of water will be evaporated by 6-58 lbs. of coal.
6  lbs. of water evaporated  by a pound of coal is
equivalent to a cubic foot of water evaporated by
9 61 lbs. of coal; and 41 lbs. of water evaporated by
a pound is equivalent to a cubic foot of water evaporated by 13 88 lbs. of coal.  In such of the Cornish
boilers as evaporate 10 lbs. of water with a pound of
coal, a cubic foot of water will be evaporated by 6f
lbs. of coal; but the usual performance of the best
Cornish boilers is about 9 lbs. of water evaporated by
a pound of coal of the best quality.
90. Q.-By what process do you ascertain the
dimensions of the chimney?
A.-By a reference to the volume of air it is necessary in a given time to supply to the burning fuel, and
to the velocity of motion produced by the rarefaction
in the chimney; for the area of the chimney requires
to be such, that with the velocity due to that rarefaction, the quantity of air requisite for the combustion
of the fuel shall pass through the furnace in the specified time.  Thus if 200 cubic feet of air of the atmospheric density are required for the combustion of
a pound of coal, and 10 lbs. of coal per horse power
per hour are consumed by an engine, then 2000 cubic




56                A CATECHISM OF
feet of air must be supplied to the furnace per horse
per hour; and the area of the chimney must be such,
as to deliver this quantity at the increased bulk due
to the high temperature of the chimney when moving
with the velocity the rarefaction within the chimney
occasions, and which is usually such as to support a
column of half an inch of water.  The velocity with
which a denser fluid flows into a rarer one is equal to
the velocity a heavy body acquires in falling through
a height equal to the difference of altitude of two
columns of the heavier fluid such as will produce the
respective pressures; and, therefore, when the difference of pressure or amount of rarefaction in the
chimney is known, it is easy to tell the velocity of
motion which ought to be produced by it.  In practice, however, these theoretical results are not to be
trusted, until they have received such modifications as
will make them representative of the practice of the
most experienced constructors.  Boulton and Watt's
rule for the dimensions of the chimney of a land
engine is as follows:-multiply the number of pounds
of coal consumed under the boiler per hour by 12, and
divide the product by the square root of the height of
the chimney in feet; the quotient is the area of the
chimney in square inches in the smallest part.  A
factory chimney suitable for a 20-horse boiler is commonly made about 20 in. square inside, and 80 ft. high,
and these dimensions are those which answer to a consumption of 15 lbs. of coal per horse power per hour,
which  is a very common consumption  in factory
engines.  If 15 lbs. of coal be consumed per horse
power per hour, the total consumption per hour in




TIE STEAM ENGINE.                 57
a 20-horse boiler will be 300 lbs., and 300 multiplied
by 12=-3600, and divided by 9 (the square root of the
height)-400, which is the area of the chimney in
square inches.  It will not answer well to increase the
height of a chimney of this area to more than 40 or
50 yards, without also increasing the area, nor will it
be of utility to increase the area much without also
increasing the height.  The quantity of coal consumed
per hour in pounds, multiplied by 5, and divided by
the square root of the height of the chimney, is the
proper collective area of the openings between the
bars of the grate for the admission of air to the fire.
91. Q.-Is this rule for the dimensions of the chimney also applicable to steam vessels?
A.-In steam  vessels Boulton and Watt allow 8square inches of area of chimney per horse power, and
in marine flue boilers they allow 18 square inches of
sectional area of flue per horse power; but this proportion appears to be about one-third greater than
what is allowed by many other makers, whose boilers,
however, are scarcely so conspicuous for an abundant
supply of steam.  The sectional area of the flue in
square inches is what is termed the calorimeter of the
boiler, and the calorimeter divided by the length of
the flue in feet is what is termed the vent.  In marine
flue boilers of good construction the vent varies between the limits of 21 and 25, according to the size of
the boiler and other circumstances-the largest boilers
having generally the largest vents; and the calorimeter divided by the vent will give the length of
the flue in feet. The collective area for the escape
of the smoke and flame over the furnace bridges in




58                A CATECHISM OF
marine boilers is 19 square inches per horse power, according to Boulton and Watt's proportion.
92. Q.-Are these proportions applicable to tubular
and wagon boilers?
A.-No.  In wagon and tubular boilers very different proportions prevail, yet the proportions of every
kind of boiler are determinable on the same general
principle.  In wagon boilers the proportion of the
perimeter of the flue which is effective as heating surface, is to the total perimeter as 1 to 3, or, in some
cases, as 1 to 2 5; and with any given area of flue,
therefore, the length of the flue must be from 3 to 2'5
times greater than would be necessary if the total surface were effective.  If then the vent be the calorimeter
divided by the length, and the length be made 3 or 2'5
times greater, the vent must become 3 or 2'5 times less;
and in wagon boilers accordingly the vent varies from
8 to 11 instead of from 21 to 25, as in the case of marine flue boilers. In Boulton and Watt's 45-horse wagon boiler the area of flue is 18 square inches per horse
power; but the area per horse power increases very
rapidly as the size of the boiler becomes less, and amounts
to about 80 square inches per horse power in a boiler of
two-horse power.  Some such increase is obviously inevitable if a similar form of flue be retained in the larger
and smaller powers, and at the same time the elongation
of the flue in the same proportion as the increase of any
other dimension is prevented; but in the smaller class of
wagon boilers the consideration of facility of cleaning
the flues is also operative in inducing a large proportion of sectional area.  Boulton and Watt's 2-horse
power wagon boiler has 30 square feet of surface,




THE STEAM ENGINE.                  59
and the flue is 18 in. high above the level of the boiler
bottom, by 9 in. wide; while their 12-horse wagon
boiler has 118 square feet of heating surface, and the
dimensions of the flue similarly measured are 36 in.
by 13 in. The width of the smaller flue, if similarly
proportioned to the larger one, would be 6-  in., instead
of 9 in.; and, by assuming this dimension, we should
have the same proportion of sectional area per square
foot of heating surface in both boilers. The length of
flue in the 2-horse boiler is 19'5 ft., and in the 12-horse
boiler 39 ft., so that the length and height of the flue
are increased in the same proportion.
93. Q.-Will you extend your illustrations to the
case of a marine boiler?
A.-The Nile steamer, with engines of 110-horse
power, by Boulton and Watt, is supplied with steam
by two boilers, which are, therefore, of 55-horse power
each.  The height of the flue winding within the boiler
is 60 in., and its mean width 161- in., making a sectional
area or calorimeter of 990 square inches, or 18 square
inches per horse power of the boiler. The length of
the flue is 39 ft., making the vent 25, which is the
vent proper for large boilers.  In the Dee and Solway
steamers, by Scott and Sinclair, the calorimeter is only
9'72 square inches per horse power; in the Eagle, by
Caird, 11'9; in the Thames and Medway, by Maudslay,
11-34, and in a great number of other cases it does not
rise above twelve square inches per horse power; but
the engines of most of these vessels are intended to operate to a certain extent expansively, and the boilers are
less powerful in evaporating efficacy on that account.
94. Q.-Then the chief difference in the proportions




60                A CATECHISM OF
established by Boulton and Watt, and those followed
by the other manufacturers you have mentioned is,
that Boulton and Watt set a more powerful boiler to
do the same work?
A.-That is the main difference.  The proportion
which one part of the boiler bears to another part is
very similar in the cases cited, but the proportion ol
boiler relatively to the size of the engine varies very
materially. Thus the calorimeter of each boiler of the
Dee and Solway is 1,296 square inches; of the Eagle,
1,548 square inches; and of the Thames and Medway,
1,134 square inches; and the length of flue is 57, 60,
and 52 ft. in the boilers respectively, which makes the
respective vents 22~, 25, and 21 in.  Taking then the
boiler of the Eagle for comparison with the boiler of
the Nile, as it has the same vent, it will be seen that
the proportions of the two are almost identical, for 990
is to 1,548 as 39 is to 60, nearly; but Messrs. Boulton
and Watt would not have set a boiler like that of the
Eagle to do so much work.
95. Q.-Then the evaporating power of the boiler
varies as the sectional area of the flue?
A.-The evaporating power varies as the square
root of the area of the flue, if the length of the flue
remain the same; but it varies as the area simply, if
the length of the flue be increased in the same proportion as its other dimensions.  The evaporating
power of a boiler is referable to the amount of its
heating surface, and the amount of heating surface in
any flue or tube is proportional to the product of the
length of the tube and the square root of its sectional
area, multiplied by a certain quantity that is constant




THE STEAM ENGINE.                   61
for each particular form.  But in similar tubes the
length is proportional to the square root of the sectional area; therefore, in similar tubes, the amount of
heating surface is proportional to the sectional area.
On this area also  depends the  quantity of hot air
passing through the flue, supposing the intensity of
the draft to remain unaffected, and the quantity of hot
air or smoke passing through the flue should vary in
the. same ratio as the quantity of surface.
96. Q.-A boiler, therefore, to exert four times the
power should have four times the extent of heating
surface, and four times the sectional area of flue for
the transmission of the smoke?
A.-Yes; and if the same form of flue is to be retained it should be of twice the diameter and twice the
length; or twice the height and width if rectangular,
and twice the length.  As then the diameter or square
root of the area increases in the same ratio as the
length, the square root of the area divided by  the
length ought to be a constant quantity in each type
of boiler, in order that the same proportions of flue
may  be retained; and in wagon boilers without an
internal flue, the height in inches of the flue encircling
the boiler divided by the length of the flue in feet will
be I very nearly. Instead of the square root of the
area, the effective perimeter, or outline of that part of
the cross section of the flue which is effective in generating steam, may be taken; and the effective perimeter divided by the length ought to be a constant
quantity in similar forms of flue, and with the same
velocity of draft, whatever the size of the flue may be.
It is clear, that with any given area of flue, to increase
6




62                 A CATECHISM OF
the perimeter by adopting a different shape, is to diminish the length of the flue; and, if the extent of
the perimeter be diminished, the length of the flue
must at the same time be increased, else it will be
impossible to obtain the necessary amount of heating
surface.  In Boulton and Watt's wagon boilers the
sectional area of the flue in square inches per square
foot of heating surface, is 5'4 in the two-horse boiler;
in the three-horse it is 4-74; in the four-horse, 4-35;
six-horse, 3'75; eight-horse, 4'33; ten-horse, 3'96;
twelve-horse, 3 63; eighteen horse, 3'17; thirty-horse,
2'52, and in the forty-five horse boiler, 2'05 square
inches.  Taking the amount of heating surface in the
forty-five horse boiler at 9 square feet per horse power, we obtain 18 square inches of sectional area of flue
per horse power, which is also Boulton and Watt's proportion of sectional area for marine boilers with internal flues.
97. Q.-If to increase the perimeter of a flue is
virtually to diminish the length, then a tubular boiler
where the perimeter is in effect greatly extended ought
to have but a short length of tube?
A.-The flue of the Nile, if reduced to the cylindrical form, would be 35- in. in diameter to have the
same area; but it would  then  require to be made
473- ft. long, to have the same amount of heating surface. Supposing that with these proportions the heat
is sufficiently extracted from the smoke, then every
tube of a tubular boiler in which the same draft existed ought to have very nearly the same proportions,
so that a tube 3 in. in diameter ought to be about 4 ft.
long, supposing the conducting power of the metallic




THE STEAM ENGINE.                 63
surface through which the heat is transmitted, to be
in each case identical. But the metal of small tubes
being thinner than that of flues must conduct better,
and a tube 3 in. in diameter should therefore be less
than 4 ft. long, provided the draft remains such as is
due to an area of 18 square inches per horse power.
If the thinness of the metal attainable by the tubular
form be supposed to increase the efficacy of the heating surface in the same proportion as the increase
of surface due to the rectangular form, the length of
a tube 3 in. diameter ought to be 3 ft. 3 in., and it
would be of no service to extend its length beyond
this point, supposing the flue boiler to be properly
proportioned, as by the time the hot air had traversed
a length of 3 ft. 3 in. of tube, the heat of the air
would have been as thoroughly extracted as in ordinary boilers appears to be beneficial. The tubes of
tubular boilers, however, are usually about 6 ft. 6 in.
long; but to make this excess of length influential in
generating steam, the draft has to be made nearly
twice greater than in flue boilers having a sectional
area of 18 square inches per horse power; or, in other
words, the sectional area of tubular boilers must not
much exceed 9 square inches per horse power when
the tubes are of the length stated. The smaller the
tubes are, the shorter they should be made, or the less
the sectional area ought to be; and with a sectional
area of 10 square inches per horse power there will
be no advantage in making the length of the tube
more than from 26 to 32 times its diameter, which will
make the tube from  6 ft. 6 in. to 8 ft. long when the
diameter is three inches, and give from 7'4 to 8 square




64                A CATECHISM OF
feet of heating surface of tubes per horse power. If
the sectional area per horse power be increased, the
length of tube should be diminished in the same proportion; for the velocity of the draft varies with the
sectional area of tube per horse power, and on the velocity of draft the length of the tube ought to depend.
In locomotive boilers where the velocity of draft is
very great, long tubes are employed; but it is preferable to have the tubes of moderate length and a draft
of a moderate intensity, as in maintaining a fierce draft
by any process, there is a considerable expenditure of
power.  If, however, with  the view of making the
draft very slow, a proportion of sectional area approaching that of flue boilers be provided, the result
will not be satisfactory, as the smoke will all pass
through a few of the tubes, leaving the rest inoperative; though this defect may be in a great measure
corrected by partially closing up the ends of the tubes,
or even by partially closing the damper. The length
of tube multiplied by the diameter, and divided by
the area, is a constant quantity both in flue and tubular
boilers, or at least nearly so; and when any of the
elements are given the rest can easily be computed by
the aid of this proportion, the precaution being of
course taken to keep within the limits which have approved themselves most eligible in practice.
98. Q.-You stated that the capacity of the feed
pump was a 240th of the capacity of the cylinder in
the case of condensing engines-the  engine being
double acting and the pump single acting-and that
in high pressure engines the capacity of the pump
should be greater in proportion to the pressure of the




THE STEAM ENGINE.                65
steam. Can you give any rule that will express the
propel capacity for the feed pump at all pressures?
A.-That will not be difficult.  In low pressure engines the pressure in the boiler may be taken at 5 lbs.
above the atmospheric pressure, or 20 lbs. altogether;
and as high pressure steam is merely low pressure steam
compressed into a smaller compass, the size of the feed
pump in relation to the size of the cylinder must obviously vary in the direct proportion of the pressure;
and if it be a 240th of the capacity of the cylinder when
the total pressure of the steam is 20 lbs., it must be a
120th of the capacity of the cylinder when the pressure
is 40 lbs. per square inch, or 25 lbs. per square inch
above the atmospheric pressure. This law of variation
is expressed by the following rule:-multiply the capacity of the cylinder in cubic inches by the total pressure of the steam in lbs. per square inch, or the pressure
per square inch on the safety valve plus 15, and divide
the product by 4800; the quotient is the capacity of
the feed pump in cubic inches, when the feed pump is
single acting and the engine double acting. If the
feed pump be double acting, or the engine single acting, the capacity of the pump must just be one-half of
what is given by this rule.
99. Q.-How do you ascertain the power of high
pressure engines?
A.-The actual power is readily ascertained by the
indicator, by the same process by which the actual
power of low  pressure engines is ascertained.  The
friction of a locomotive engine when unloaded is found
by experiment to be about 1 lb. per square inch on tlfe
surface of the pistons, and the additional friction caused
6*




66               A CATECHISM OF
by any additional resistance is estimated at about'14
of that resistance; but it will be a sufficiently near approximation to the power consumed by friction in high
pressure engines, if we make a deduction of a pound
and a half from the pressure on that account, as in the
case of low pressure engines. High pressure engines,
it is true, have no air pump to work; but the deduction
of a pound and a half of pressure is relatively a much
smaller one where the pressure is high than where it
does not much exceed the pressure of the atmosphere.
The rule, therefore, for the actual horse power of a high
pressure'engine will stand thus:-square the diameter
of the cylinder in inches, multiply by the pressure of
the steam in the cylinder per square inch less 1  lbs.,
and by the speed of the piston in feet per minute, and
divide by 42,017; the quotient is the actual horse
power.
100. Q.-But how do you ascertain the nominal
horse power?
A.-The nominal horse power of a high pressure
engine has never been defined; but it should obviously
hold the same relation to the actual power as that
which obtains in the case of condensing engines, so
that an engine of a given nominal power may be capable of performing the same work, whether high pressure or condensing. This relation is maintained in the
following rule, which expresses the nominal horse power
of high pressure engines:-multiply the square of the
diameter of the cylinder in inches by the pressure on
the piston in pounds per square inch, and by the speed
of the piston in feet per minute, and divide the product
by 120,000; the quotient is the power of the engine




THE STEAM ENGINE.                 67
in nominal horses power.  If the pressure upon the
piston be 80 lbs. per square inch, the operation may be
abbreviated by multiplying the square of the diameter
of the cylinder by the speed of the piston, and dividing
by 1500, which will give the same result.
101. Q.-This rule for nominal horse power, however, is rot representative of the dimensions of the cylinder. Cannot you give a rule for the nominal power
of high pressure engines which shall discard altogether
the element of velocity, and, as in the rule you have
already given for the nominal power of low pressure
engines, express merely the dimensions of the engine?
A.-Nothing is easier, if you maintain an unvarying pressure of steam; but as different pressures are
used in different engines, the pressure must become an
element in the computation. The rule for the nominal
power will therefore stand thus: —multiply the square
of the diameter of the cylinder in inches by the pressure on the piston in pounds per square inch, and by
the cube root of the stroke in feet, and divide the product by 940; the quotient is the power of the engine
in nominal horses power, the engine working at the ordinary speed of 128 times the cube root of the stroke.
102. Q.-Is 128 times the cube root of the stroke
in feet per minute the ordinary speed of all engines?
A.-Locomotive engines travel at a quicker speed,
-an innovation brought about, not by any process of
scientific deduction, but by the accidents and exigencies
of railway transit. All other engines, however, travel
at about the speed of 128 times the cube root of the
stroke in feet; and condensing engines, as at present
constructed, cannot travel much quicker, as the valves




68               A CATECHISM OF
of the air pump strike very hard and wear themselves
quickly out if the engine is driven at a great velocity.
To mitigate the shock in cases in which a higher speed
has been desirable, as in the case of marine engines
employed to drive the screw propeller without intermediate gearing, canvas air pump valves, resting on a
perforated metal plate, have sometimes been adopted
-the plies of canvas being either stuck together with
India rubber or riveted with copper rivets, and being
multiplied so as to make the valve half an inch in
thickness; but valves of this kind, though they diminish the noise and tremor, wear rapidly away, and
require to be renewed very often.  It is very desirable,
however, that this impediment should be completely
surmounted, not merely as a means of meeting the
new exigencies created by the application of the screw
propeller to steam vessels, but because all engines will
then  be capable of working at a greatly increased
speed, and with a proportionate increase of power.
The most feasible way of enabling condensing engines
to work at a high speed, appears to lie in the application of a slide valve to the air pump, instead of valves
actuated by the pressure within the pump, and by performing the condensation in the air pump, instead o$
in the condenser, whereby the air pump valves wil
operate with as little noise at any speed as the cylindet
valves, and the condenser may be dispensed with altogether. Engines constructed upon this plan may h.
driven at four times the speed of com:) on enginee,
whereby an engine of large power may be purchased
for a very moderate price, and be capable of being
put into a very small compass; while the motion,




THlE STEAM ENGINE                69
from being more equable, will be better adapted for
most purposes for which a rotary motion is required.
Even for pumping mines and blowing iron furnaces,
engines of this kind appear likely to come into use; for
they are more suitable than other engines for driving
the centrifugal pump, which in many cases appears
likely to supersede other kinds of pumps for lifting
water; and they are also conveniently applicable to
the driving of fans, which, when so arranged that the
air condensed by one fan is employed to feed another,
and so on through a series of 4 or 5, have succeeded
in forcing air into a furnace with a pressure of 2~ lbs.
on the square inch, and with a far steadier flow than
can be obtained by a blast engine with any conceivable
kind of compensating apparatus.
103. Q.-Then, if by this modification of the air
pump you enable an engine to work at four times the
speed, you also enable it to exert four times the power?
A.-Yes; always supposing it to be fully supplied
with steam.  The nominal power of this new species
of engine, if condensing and if working at four times
the ordinary speed, may be ascertained by the following rule:-multiply the square of the diameter of the
cylinder in inches by the cube root of the stroke in
feet, and divide the product by 12; the quotient is the
power of the high speed condensing engine in nominal
horses power.  To ascertain the nominal power of a
high pressure high speed engine, multiply the square
of the diameter of the cylinder by the pressure on the
piston per square inch less a pound and a half, and by
the cube root of the stroke in feet, and divide the product by 235; the quotient is the power of the high




70                A CATECHISM OF
pressure high speed engine in nominal horses power.
104. Q.-The high speed engine does not require
so heavy a fly wheel as common engines?
-1.- No: the fly wheel will be lighter, both by
(I;ei~ of its greater velocity of rotation, and because
the -impuls:e (communicated by the piston is less in
amount and more frequently repeated, so as to approach more nearly to the condition of a uniform pressure. To find the proper quantity of cast iron for the
rim of a fly wheel:-multiply the mean diameter of
the rim by the number of its revolutions per minute,
and square the product for a divisor; divide the number of nominal horses power of the engine by the
number of strokes the piston makes per minute, multiply the quotient by the constant number 5,000,000,
and divide the product by the divisor found as above;
the quotient is the requisite quantity of cast iron in
cubic feet to form the fly wheel rim.
105. Q.-What is the maximum speed with which
a cast iron fly wheel may be driven without being burst
asunder by its centrifugal force?
A.-The velocity of the rim should not exceed 60
feet per second, and at that speed the bursting force
will amount to 1,100 lbs. on the square inch of section,
setting aside the support derived from the arms. In
engines for rolling iron, the fly wheel is driven usually
at a very great speed, and it requires to be put together very firmly to resist the bursting force.  The best
practice in such cases is to cast the rim in a single
piece, and to introduce malleable iron arms.
106. Q.-The steam  is admitted to and from the
cylinder by means of a slide or sluice valve?




THE STEAM ENGINE.                    71
A.-Yes; and of the slide valve there are many
varieties; but the kinds most in use are the D  val e,
so celled from its resembllance to a half cylinder o1 D
ii its cross section, and the three ported val:e, lwhi:i-.
i.~iisi ts )of a brass box set o'er the twe o ports o op,::;.
ings iito the cvlilder and a ccltnt i  pot which condi::
away the steam  to the atmosphere or condenser; but
the length of the box is so adjusted, that it can only
cover one of the cylinder ports and the central or
eduction port, at the same time. The effect, therefore,
of moving the valve up and down, as is done by the
eccentric, is to establish a connection alternately between each cylinder port and the passage whereby the
steam escapes; and while the steam is escaping from
beneath the piston, the position of the valve is such,
that a free communication exists between the space
above the piston and the steam  in the boiler.  The
piston is thus urged alternately up and down, the valve
so changing its position before the piston arrives at
the end of the stroke, that the pressure is by that time
thrown on the reverse side of the piston,.so as to urge
it into motion in the opposite direction. The valve
does not move down when the piston moves down, nor
does it move down when the piston moves up; but it
moves from its mid position to the extremity of its
throw, and back again to its mid position, while the
piston makes an upward or downward movement, so
that the motion is as it were at right angles to the
motion of the piston; or it is the same motion that the
piston of another engine, the crank of which is set at
right angles with that of the first engine, would acquire.
107. Q.-What is meant by the lead of the valve?




72                A CATECHISMAI 01
A. —The amount of opening the valve presents for
the admission of the steam, when the piston is just
beginning its stroke. It is found expedient that the
valve should have opened a little to admit steam on
the reverse side of the piston before the stroke terminates; and the amount of this opening, which is given
by turning the eccentric more or less round upon the
shaft, is what is termed the lead.
108. Q.-And what is meant by the lap of the
valve?
A.-It is an elongation of the valve face to a certain
extent ever the port or towards the port, whereby the
port is closed sooner than would otherwise be the case.
This extension is chiefly effected at that part of the
valve where the steam is admitted, or upon the steam
side of the valve, as the technical phrase is; and the
intent of the extension is to close the steam passage
before the end of the stroke, whereby the engine is
made to operate to a certain extent expansively. In
some cases, however, there is also a certain amount of
lap given to.the eduction side, to prevent the eduction
from being performed too soon when the lead is great;
but in all cases there is far less lap on the eduction
than on the steam side: very often there is none, and
sometimes less than none, so that the valve is incapable of covering both the ports at once.  The common stroke of the valve in rotative engines is twice
the breadth of the port, and the length of the valve
face will then be just the breadth of the port when
there is lap on neither the steam nor eduction side.
Whatever lap is therefore given, makes the valve face
just so much longer.  In some engines, however, the




THE STEAM ENGINE.                73
stroke of the valve is a good deal more than twice the
breadth of the port; and it is by the stroke of the
valve that the amount of lap is properly measurable.
109. Q.-Can you tell what amount of lap will accomplish any given amount of expansion?
A. —Yes, when the stroke of the valve is known.
From the length of the stroke of the piston, subtract
that part of the stroke which is intended to be accomplished before the steam is cut off; divide the remainder by the length of the stroke of the piston, and
extract the square root of the quotient, which multiply
by half the stroke of the valve, and from the product
take half the lead; the remainder will be the lap required.  To find how much before the end of the
stroke the eduction passage will be closed:-to the
lap on the steam side add the lead, and divide the
sum by half the stroke of the valve; find the arc whose
sine is equal to the quotient, and add 90~ to it; divide
the lap on the eduction side by half the stroke of the
valve, and find the arc whose cosine is equal to the
quotient; subtract this arc from the one last obtained,
and find the cosine of the remainder; subtract this cosine from two, and multiply the remainder by half the
stroke of the piston; the product is the distance of the
piston from the end of the stroke when the eduction
passage is closed. To find how far the piston is from
the end of its stroke when the steam that is propelling
it by expansion is allowed to escape to the atmosphere
or condenser:-to the lap on the steam  side add the
lead, divide the sum by half the stroke of the valve,
and find the arc whose sine is equal to the quotient;
find the arc whose cosine is equal to the lap on the educ.
7




74                A CATECHISM OF
tion side divided by half the stroke of the valve: add
these two arcs together and subtract 90~; find the cosine
of the residue, subtract it from one, and multiply the
remainder by half the stroke of the piston; the product
is the distance of the piston from the end of its stroke
when the steam that is propelling it is allowed to escape into the atmosphere or condenser. In using these
rules, all the dimensions are to be taken in inches, and
the answers will be found in inches also.
110. Q.-To what extent can expansion be carried
beneficially by means of lap upon the valve?
A.-To about one-third of the stroke; that is, the
valve may be made with so much lap, that the steam
will be cut off when two-thirds of the stroke have been
performed, leaving the residue to be accomplished by
the agency of the expanding steam; but if more lap
be put on than answers to this amount of expansion,
a very distorted action of the valve will be produced,
which will impair the efficiency of the engine. If a
further amount of expansion than this is wanted, it may
be accomplished by wire-drawing the steam, or by so
contracting the steam passage, that the pressure within
the cylinder must decline when the speed of the piston
is accelerated, as it is about the middle of the stroke.
Thus, for example, if the valve be so made as to shut
off the steam by the time two-thirds of the stroke have
been performed, and the steam  be at the same time
throttled in the steam pipe, the full pressure of the
steam within the cylinder cannot be maintained except
near the beginning of the stroke where the piston travels
slowly; for, as the speed of the piston increases, the
pressure necessarily subsides, until the piston  ap



THE STEAM ENGINE.                  75
proaches the other end of the cylinder, where the
pressure would rise again but that the operation of
the lap on the valve by this time has had the effect of
closing the communication between the cylinder and
steam pipe, so as to prevent more steam from entering.
By throttling the steam, therefore, in the manner here
indicated, the amount of expansion due to the lap may
be doubled, so that an engine with lap enough upon
the valve to cut off the steam  at two-thirds of the
stroke, may, by the aid of wire-drawing, be virtually
rendered capable of cutting off the steam  at one-third
of the stroke.  The usual manner of cutting off the
steam, however, is by means of a separate valve, termed
an expansion valve; but such a device appears to be
hardly necessary in ordinary engines. In the Cornish
engines, where the steam is cut off in some cases at
one-twelfth of the stroke, a separate valve for the admission of steam, other than that which permits its
escape, is of course indispensable; but in common rotative engines, which may realize expansive efficacy by
throttling, a separate expansive valve does not appear
to be required.
111. Q.-What size of orifice is allowed for the escape of the steam through the safety valve?
A.-About 0'8 of a circular inch per horse power
in condensing engines, or a circular inch per 1  horse
power.  The following rule, however, will give the
dimensions suitable for all kinds of engines, whether
high or low pressure: —multiply  the square of the
diameter of the cylinder in inches by the speed of the
piston in feet per minute, and divide the product by
375 times the pressure on the boiler per square inch;




76                A (CATEIICHISM OF
the quotient is the proper area of the safety valve in
square inches.  This rule of course supposes that the
evaporating surface has been properly proportioned to
the engine power?
112. Q.-Will you state the proper dimensions of
the air pump and condenser and of the steam passages?
A.-Mr. Watt made the air pump of his cngine half
the diameter of the cylinder and half the stroke, or oneeighth of the capacity, and the condenser was usually
made about the same size as the air pump; but as the
pressure of the steam has been increased in all modern
engines, it is better to make the air pump a little larger
than this proportion. 0-6 of the diameter of the cylinder and half the stroke answers very well, and the
condenser may be made as large as it can be got with
convenience, though the same size as the air pump will
suffice.  The common size of the cylinder passages is
one-twenty-fifth of the area of the cylinder, or one-fifth
of the diameter of the cylinder, which is the same thing.
This proportion corresponds  very nearly  with one
square inch per horse power when the length of the
cylinder is about equal to its diameter; and one square
inch of area per horse power for the cylinder ports
and eduction passages answers very well in the case of
engines working at the ordinary speed of 220 feet per
minute.  The area of the steam  pipe is usually made
less than the area of the eduction pipe, especially when
the engine is worked expansively, and with a considerable pressure of steam. In the case of ordinary
condensing engines, however, working with the usual
pressure of from 4 to 8 lbs. above the atmosphere, the
area of the steam pipe is not less than a circular inch




THE STEAM ENGINE.                77
per horse power; in such engines the diameter of the
steam pipe may be found by the following rule:-divide
the number of nominal horse power by 0'8, and extract
the square root of the quotient, which will be the
internal diameter of the steam pipe. The area of the
injection orifice should be about one-fifteenth of a
square inch per nominal horse power; but the area of
the injection pipe should be a little larger: a tenth of
a circular inch per horse power answers very well, or
tke diameter of the injection pipe may be found by
dividing the nominal horse power by 10, and extracting the square root of the quotient, which will be the
diameter in inches.  The area through the foot and
discharge valves is usually made equal to one-fourth
of the area of the air pump, and the diameter of the
waste water pipe is made one-fourth of the diameter
of the cylinder, which gives an area somewhat less than
that of the foot and discharge valve passages.
113. Q.-Will you explain by what process of computation these proportions are arrived at?
A.-The size of the steam pipe is so regulated that
there will be no material disparity of pressure between
the cylinder and boiler, and in fixing the size of the
eduction  passage the same object is kept in view,
When the diameter of the cylinder and the volocity
w-ith which the piston travels are known, it is easy to
tell what the velocity of the steam in the steam pipe
will be; for if the area of the cylinder be 25 times
greater than that of the steam pipe, the steam in the
steam pipe must travel 25 times faster than the piston,
and the difference of pressure requisite to produce this
velocity of the steam can easily be ascertained, by
7*




78                 A CATECHISM OF
finding what height a column of steam must be to give
that velocity, and what the weight or pressure is of
such a column.  The proper area of the injection
orifice per horse power can easily be told when the
quantity of water requisite to condense the steam is
known, and the pressure specified  under which the
water enters the condenser.  The vacuum  in the condenser may be taken at 26 in. of mercury, which is
equivalent to a column of water 29'4 ft. high, and the
square root of 29'4 multiplied by 8-021 is 43-15, which
is the velocity in feet per second that a heavy body
would acquire in falling 29'4 ft., or with which the
water would enter the condenser.  Now, if a cubic foot
of water evaporated per hour be'equivalent to an actual
horse power, and 28'9 cubic inches of water be requisite
for the condensation of a cubic inch of water in the
form of steam, 28'9 cubic feet of condensing water per
actual horse power per hour for the engine, or 13'905
cubic inches per second will be necessary, and the size.of the injection orifice must be such that this quantity
of water flowing with the velocity of 43'15 ft. per
second, or 517'8 inches per second, will gain admission
to the condenser.  Dividing, therefore, 13'905, the
number of cubic inches to be injected, by 517'8, the
velocity of influx in inches per second, we get 0'02685
for the area of the orifice in square inches; but inasmuch as it has been found by experiment that the
actual discharge of water through a hole in a thin
plate is only six-tenths of the theoretical discharge on
account of the contracted vein, the area of the orifice
must be increased in the proportion of such diminution
of effect, or be made 0'04475, or -ad of a square inch




THE STEAM ENGINE.                   79
per horse power.  This, it will be remarked, is the
area required per actual horse power; in most of the
modern engines there is more than a cubic foot of
water evaporated per hour per nominal horse power,
and it is therefore found expedient to make the area
of the injection orifice a third larger than the area due
to the actual power, or - th of a square inch per
nominal horse power, as already prescribed.
114. Q.-If the relation you have mentioned subsist
between the area of the steam  passages and the velocity of the piston, then the passages must be larger
when the piston travels very rapidly?
A.-And they are so made.  The area of the ports
of locomotive engines is usually so proportioned as to
be from -Lth to 1-th the area of the cylinder-in some
cases even as much as I-th; and in all high speed
engines the ports should be very large, and the valve
should have a good deal of travel. The area of port
which it appears advisable to give to modern engines
of every description, is expressed by the following
rule:-multiply the  area of the cylinder in square
inches by the speed of the piston in feet per minute,
and divide the product by 4,000; the quotient is the
area of each cylinder port in square inches. This rule
gives rather more than a square inch of port per nominal horse power to condensing engines working at
the ordinary speed; but the excess is but small, and is
upon the right side.  In locomotive engines the eduction pipe passes into the chimney, and the force of the
issuing steam has the effect of maintaining a rapid
draught through the furnace. The orifice of the waste
steam  pipe, or the blast pipe as it is termed, is much




80                 A CATECHISM OF
contracted in some engines with a view of producing
a fiercer draught; but an area of — d of the area of the
cylinder is a common proportion.
115. Q.-If the area of the blast pipe be much diminished, the escaping steam must so far resist the motion of the piston as to diminish considerably the power
of the engine?
A.-At a speed of 10 miles an hour the resistance
to the piston occasioned by the blast pipe may be taken
on the average at about 2 lbs. per square inch, and the
resistance increases directly as the speed of the engine.
In some cases, however, the blast pipe is so much contracted, that as much as half the power of the engine
is spent in forcing the steam  through it.  The object
of this undue contraction is to increase the intensity
of the draught in the furnace, and as a consequence
to increase the production of steam; but engines in
which such a waste of power exists from this single
cause, are altogether unsuitable for the work they have
to do, in consequence of their insufficient evaporating
power.  The question of draught is only a question of
area of fire grate, and it is much preferable to enlarge
the fire grate than to increase the intensity of the
draught.  The area of the chimney in locomotives is
usually made 1ath of the area of the fire grate, and its
height must not exceed 14 ft. above the level of the
rails, else it will come in contact with the bridges
which cross the line.  A given area of heating surface
in locomotive boilers is more effective than the same
area of heating surface in other boilers, on account of
the greater heat to which it is subjected.  If, at a
speed of 20 miles an hour, a locomotive boils off a




THE STEAM ENGINE.                  81
cubic foot of water in the hour for every six square
feet of heating surface, as appears to be an average
performance, then 1 square foot of heating surface must
boil off 10'4 lbs. of water per hour, at a speed of 20
miles an hour, and the rate of evaporation Will vary
nearly as the fourth root of the speed.
116. Q. —What is the amount of tractive force requisite to draw carriages on railways?
A.-Upon well formed railways, with carriages of
good construction, the average tractive force required
for low speeds is about 72 lbs. per ton, or g-1th of the
load; though in some experimental cases, where particular care was taken to obtain a favorable result, the
tractive force has been reduced as low as S-oth of the
load.  At low speeds the whole of the tractive force is
expended in overcoming the friction, which is made
up partly of the friction of attrition in the axles, and
partly of the rolling friction, or the obstruction to the
rolling of the wheels upon the rail. The rolling friction is very small when the surfaces are smooth, and
in the case of railway carriages does not exceed -T-oth
of the load; whereas the draught on common roads of
good construction, which is chiefly made up of the
rolling friction, is as much as -th of the load.
117. Q.-In reference to friction you have already
stated, in your answer to Question 33, that the friction
of iron sliding upon brass, which has been oiled and
then wiped dry, so that no film of oil is interposed, is
about AI-th of the pressure, but that in machines in actual operation, where there is a film of oil between the
rubbing surfaces, the friction is only about one-third
of this amount, or g-Sd of the weight.  How then can




82                A CATECHISM OF
the tractive resistance of locomotives at low speeds,
which you say is entirely made up of friction, be so
little as 5  th of the weight?
A.-I did not state that the resistance to traction
was Sgth of the weight upon an average-to which
condition the answer given to Question 33 must be
understood to apply-but I stated that the average
traction was about    6th of the load, which nearly
agrees with my former statement. If the total friction
be 3 o-th of the load, and the rolling friction be T-yoth
of the load, then the friction of attrition must be -  gth
of the load; and if the diameter of the wheels be 36 in.,
and the diameter of the axles be 3 in., vhich are common proportions, the friction of attrition must be increased in the proportion of 36 to 3, or 12 times, to
represent the friction of the rubbing surface when
moving with the velocity of the carriage.  -j-2ths are
about 3th of the load, which does not differ much
from the proportion of x-d, as previously determined.
While this, however, is the average result, the friction
is a good deal less in some cases.  Mr. Southern, in
some experiments upon the friction of the axle of a
grindstone-an account of which may be found in the
65th volume of the "Philosophical Transactions "found the friction to amount to less than — th of the
weight; and Mr. Wood, in some experiments upon the
friction of locomotive axles, found that by ample lubrication the friction might be made as little as — th of
the weight, and the traction, with the ordinary size of
wheels, would, in such a case, be about  o-th of the
weight.  The function of lubricating substances is to
prevent the rubbing surfaces from coming into contact,




THE STEAM ENGINE.                 83
whereby abrasion would be produced, and unguents
are effectual in this respect in the proportion of their
viscidity; but if the viscidity of the unguent be
greater than what suffices to keep the surfaces asunder,
an additional resistance will be occasioned; and the
nature of the unguent selected should always have reference, therefore, to the size of the rubbing surfaces,
or to the pressure per square inch upon them. With
oil the friction appears to be a minimum when the
pressure on the surface of a bearing is about 90 lbs.
per square inch: the friction from too small a surface
increases twice as rapidly as the friction from too large
a surface, added to which, the bearing, when the surface is too small, wears rapidly away.
118. Q.-What is the amount of adhesion of the
wheels upon the rails?
A.-The adhesion of the wheels upon the rails is
about 1-th of the weight when the rails are clean, or
either perfectly wet or perfectly dry; but when the
rails are half wet or greasy, the adhesion is not more
than  1          or IAth of the weight or pressure upon the
wheels.  The weight of a locomotive of modern construction varies from 20 to 25 tons.
119. Q.-And what is its cost and average performance?
A.-The cost of a common narrow gauge locomotive
at the present time varies from 19001. to 22001.; it
will run on an average 130 miles per day, at a cost for
repairs of 2Id. per mile; and the cost of locomotive
power, including repairs, wages, oil, and coke, does not
much exceed 6d. per mile run, on economically managed
railways.  This does not include a sinking fund for the




84                A CATECHISM OF
renewal of the engines when worn out, which may be
taken as equivalent to 10 per cent. on their original
cost.
120. Q.-Does the expense of traction increase much
with an increased speed?
A.-Yes; it increases very rapidly, partly from the
undulation of the earth when a heavy train passes over
it at a high velocity, but chiefly from the resistance of
the atmosphere, which constitutes the greatest of the
impediments to motion at high speeds. At a speed of
30 miles an hour, the atmospheric resistance amounts
to about 12 lbs. a ton, and in side winds the resistance
even exceeds this amount, partly in consequence of the
additional friction caused from the flanges of the wheels
being forced against the rails, and partly because the
wind catches to a certain extent the front of every
carriage, whereby the frontage exposed to the wind is
virtually increased. At a speed of 30 miles an hour,
an engine evaporating 200 cubic feet of water in the
hour, and therefore exerting about 200 horses power,
will draw a load of 110 tons.  Taking the friction of
the train at 7- lbs. per ton, or 825 lbs. operating at the
circumference of the driving wheel,-which, with 5 ft.
6 in. wheels and 18 in. stroke, is equivalent to 4757 lbs.
upon the piston,-and taking the resistance of the blast
pipe at 6 lbs. per square inch of the pistons, and the
friction of the engine unloaded at I lb. per square inch,
which, with pistons 12 in. in diameter, amount together
to 1582 lbs., and reckoning the increased friction of
the engine due to the load at I-th of the load, as in
some cases it has been found experimentally to be,
though a much less proportion than this would proba



THE STEAM ENGINE.                  85
bly be a nearer average, we have 7018'4 lbs. for the
total load upon the pistons.  At 30 miles an hour the
speed of the pistons will be at 45'78 feet per minute,
and 7018-4 lbs. multiplied by 457-8 ft. per minute., are
equal to 3213023"5 lbs. raised 1 foot high in the minute, which, divided by 33,000, gives 97-3 horses power as the power which would draw 110 tons upon a
railway at a speed of 30 miles an hour, if there were
no atmospheric resistance. The atmospheric resistance,
with a load of 110 tons, is at the rate of 12 lbs. a ton,
equal to 1320 lbs., moving at a speed of 30 miles
an hour, which, when reduced, becomes 105'8 horses
power, and thi~, added to 97'3, makes 203'1, instead of
200 horses power, as ascertained by a reference to the
evaporative power of the boiler, which small excess of
efficiency is probably produced by a certain amount of
expansive action in the engine.. The atmospheric resistance is found to increase as the square of the velocity,
while the other resistances do not increase with the
velocity at all per unit of space passed over, but increase as the velocity per unit of time during which
they act. It is not difficult, therefore, to approximate
to the power requisite for the propulsion of a train at
any rate of speed, when the distribution of power at
any one speed has been ascertained.  At a speed of
60 miles an hour, for example, the power required to
overcome the friction of a train weighing 110 tons,
will be 97'3 X  2 =  194-6 horses power; while the
power requisite to overcome the atmospheric resistance
will be 105'8 x 4 =  423-2 horses power, if the same
distance be passed over at the speed of 30 miles an
hour; but as twice the distance will be passed over in
8




86                A CATECHISM OF
the hour when the speed is twice as great, the power
requisite to overcome the atmospheric resistance will
be twice this amount, or 846'4 horses power, making
together a power of 1041 horses to draw a train weighing 110 tons, at a speed of 60 miles an hour. A locomotive, to perform this duty, should have 6246 squa e
feet of heating surface in the boiler, and the cylinders
should be sufficiently capacious to permit 320,628 cubic
feet of steam to pass through them in the hour, though
it will be preferable to make the capacity considerably
greater than this, and to work the steam expansively.
121. Q.-How comes it, that the resistance of the
air to the motion of a locomotive increases as the square
of the velocity, instead of the velocity simply?
A.-Because the height necessary to generate the
velocity with which the train strikes the air, or the air
strikes the carriage, increases as the square of the velocity, and the resistance or the weight of a column of
air, or of any other fluid, varies as the height. A falling body, as has been already explained in the answer
to Question 14, to have acquired twice the velocity,
must have fallen through four times the height; the
velocity generated by a column of any fluid is equal to
that acquired by a body falling through the height of
the column; and it is therefore clear, that the pressure
due to any given velocity, must be as the square of that
velocity, the pressure being in every case directly as the
altitude of the column.  The work done, however, by
a stream of air or other fluid in a given time, will
vary as the cube of the velocity; for if the velocity of
a stream of air be doubled, there will not only be four
times the pressure exerted per square foot, but twice




THE STEAM ENGINE.                  87
the quantity of air will be employed; and in windmills,
accordingly, it is found, that the work done varies
nearly as the cube of the velocity of the wind. If, however, the work done by a given quantity of air moving
at different speeds be considered, it will vary as the
squares of the speeds.
122. Q.-But in a locomotive there is no work done
by the wind, and as the resistance of the air upon the
train varies as the square of the speed, should not the
power requisite to overcome that resistance vary as the
square of the speed?
A.-It should, if you consider the resistance over a
given distance, and not the resistance during a given
time.  It will take four times the power, so far as
atmospheric resistance is concerned, to accomplish a
mile at the rate of 60 miles an hour that it will take
to accomplish a mile at 30 miles an hour; but in the
former case there will be twice the number of miles
accomplished in the same time, so that when the velocity of a train is doubled, we require an engine that
is capable of overcoming four times the resistance at
twice the speed; or, in other words, that is capable of
exerting eight times the power, so far as regards the
element of atmospheric. resistance.
123. Q.-When a train moves at the rate of 60
miles an hour, what will be the centrifugal force of the
wheels?
A.-60 miles an hour is 88 ft. per second, and if the
centrifugal force of a wheel travelling at this speed be
computed by the rule given in Answer 24, it will be
found to amount to 80'4 times the weight of the wheel,
or 4124'5 lbs. per square inch of sectional area of the




88                 A CATECHISM OF
rim of the wheel, supposing any one point of the rim
to sustain all the centrifugal force produced by its own
revolution. About 4000 Ibs. per square inch of sectional
area is the utmost strain to which iron should be exposed in machinery, and even if we take the centrifugal
force as divided between the two sides of the rim, railway wheels can scarcely be considered safe at a speed
of 60 miles an hour, unless so constructed that the centrifugal force of the rim  will be counteracted, to a
material extent, by the centripetal action of the arms.
Hooped wheels are very unsafe, unless the hoops are,
by some process or other, firmly attached to the arms.
It is of no use to increase the dimensions of the rim of
a wheel with a view  of giving increased strength to
counteract the centrifugal force, as every increase in
the dimensions of the rim will increase the centrifugal
force in the same proportion.
124. Q.-Does the law of resistance to the motion
of railway trains extend to the case of vessels moving
in water?
A.-The resistance of vessels moving in water varies
as the square of the velocity, and the power required to
overcome that resistance as the cube of the velocity; so
that the resistance to ships moving in water varies in
the same ratio as the resistance presented by the atmosphere to the carriages in the case of railway trains.
To double the velocity of a steamer it is necessary that
the engines should exert eight times the power; but a
double velocity may be gained without any increase of
power by such a modification of the form of the vessel
as will enable it to pass through the water with greater
facility. It is impossible, therefore, in the case of a




THE STEAM ENGINE.                 89
steam vessel, to tell what velocity with a given load a
given power of engine will produce, unless the shape
of the vessel be also known; and the shapes of vessels
are so various and inconsistent, that it is' impossible
to find any one expression by which they may all be
represented.
125. Q.-But may not the different shapes be so
classified that the speed answerable to a given power
in a specified class can be approximately predicted?
A.-Yes, that may be done, and has been done, by
Boulton and Watt, who have ascertained, experimentally, the speed realized by different forms of vessels
with different powers of engine, and have deduced
from thence rules which enable them to tell with great
precision the speed which any new vessel, of a particular form and power, will achieve.  The first set of
experiments was  made in 1828, upon the vessels
Caledonia, Diana, Eclipse, Kingshead, Moordyke, and
Eagle-vessels of a similar form  and all with square
bilges and flat floors; and the result was to establish
the number 925 as the co-efficient of performance of
such vessels.  This co-efficient is obtained by multiplying the cube of the velocity of the vessel in miles
per hour by the sectional area of immersed midshipsection in square feet, and dividing by the nominal
horse power; and its use is to enable us to determine
the speed of any similar vessel with any other area of
midship-section, and any other number of nominal
horses power.   The better the shape of the vessel is,
the larger the co-efficient becomes, as appears by the
second set of experiments, which were made upon the
superior vessels Venus, Swiftsure, Dasher, Arrow,
8*




90                A CATECHISM OF
Spitfire, Fury, Albion, Queen, Dart, Hawk, Margaret,
and Hero-all vessels having flat floors and round
bilges, where the co-efficient became 1160.  The third
set of experiments was made upon the vessels Lightning, Meteor, James Watt, Cinderella, Navy Meteor,
Crocodile, Watersprite, Thetis, Dolphin, Wizard, Escape, and Dragon-all vessels with rising floors and
round bilges, and the co-efficient of performance was
found to be 1430.   The fourth set of experiments
was made in 1834, upon the vessels Magnet, Dart,
Eclipse, Flamer, Firefly, Ferret, and Monarch, when
the co-efficient of performance was found to be 1580.
The velocity of any of these vessels, with any power
of sectional area, may be ascertained by multiplying
the co-efficient of its class by the nominal horse power,
dividing by the sectional area in square feet, and extracting the cube root of the quotient, which will be
the velocity in miles per hour;  or the number of
nominal horses power requisite for the accomplishment
of any required speed may be ascertained by multiplying the cube of the required velocity in miles per
hour, by the sectional area in square feet, and dividing
by the co-efficient: the quotient is the number of
nominal horse power requisite to realize the speed. In
the whole of these experiments the pressure of steam
in the boiler varied between 2- lbs. and 4 lbs. per square
inch, and the effective pressure on the piston varied
between 11 lbs. and 13 lbs. per square inch, so that the
average ratio of the nominal to the actual power may
be easily computed; but it will be preferable to state
the nominal power of some of the vessels, and their
actual power as ascertained by experiment.  Of the




THE STEAM ENGINE.                   91
Eclipse, the nominal power was 76, and the actual
power 144-4, horses; of the Arrow, the nominal power
was 60, and the actual 1195; Spitfire, nominal 40,
actual 64;  Fury, nominal 40, actual 65-6; Albion,
nominal 80, actual 135'4; Dart, nominal 100, actual
1524; Hawk, nominal 40, actual 73; Hero, nominal
100, actual 1714; Meteor, nominal 100, actual 160;
James Watt, nominal 120, actual 204; Watersprite,
nominal 76, actual 1576; Dolphin, nominal 140, actual 238; Dragon, nominal 80, actual 131; Magnet,
nominal 140, actual 238;  Dart, nominal 120, actual
237; Flamer, nominal 120, actual 234; Firefly, nominal 52, actual 866; Ferret, nominal 52, actual 88;
Monarch, nominal 200, actual 378.  In the case of
swift vessels of modern construction, such as the Red
Rover, Herne, Queen, and Prince of Wales, the coefficient appears to be about 2550; but in these
vessels there is a still greater excess of the actual over
the nominal power than in the case of the vessels previously enumerated, and the increase in the co-efficient
is consequent upon the increased pressure of the steam
in the boiler, as well as the superior form of the ship.
The nominal power of the Red  Rover, Herne, and
City of Canterbury is, in each case, 60 horses; but
the actual power of the Red Rover is 147, of the
Herne 177, and of the City of Canterbury 153, and in
some vessels the excess is still greater; so that with
such variations it becomes necessary to adopt a coefficient derived from the introduction of the actual
instead of the nominal power.   In the first class of
vessels experimented  upon, the  actual power was
about 16 times greater than the nominal power; in




92                A CATECHISM OF
the second class, 1'67 times greater; in the third class,
1 7  times greater; and  in  the fourth, 1 96 times
greater; while in such vessels as the Red Rover and
City of Canterbury, it is 2'65 times greater; so that
if we adopt the actual instead of the nominal power in
fixing the co-efficients, we shall have 554 as the first
co-efficient, 694 as the second, 832 for the third, and
806 for the fourth, instead of 925, 1160, 1130, and
1580, as previously specified; while for such vessels
as the Red Rover, Herne, Queen, and Prince of Wales,
we shall have 962 instead of 2550. These smaller coefficients then express the relative merits of the different vessels without reference to any difference of
efficacy in the engines, and it appears preferable, with
such a variable excess of the actual over the nominal
power, to employ them instead of those first referred
to.  From  the circumstance of the third of the new
co-efficients being greater than the fourth, it appears
that the superior result in the fourth set of experiments
arose altogether from a greater excess of the actual
over the nominal power.
126. Q.-In what way is it that the shape of a vessel influences her speed, since vessels of the same sectional area must manifestly put in motion a column
of water of the same magnitude, and with the same
velocity?
A.-A  vessel will not strike the water with the
same velocity when the bow lines are sharp as when
they are otherwise, for a very sharp bow has the effect
of enabling the vessel to move through a great distance
while the particles of water are moved aside but a
small distance; or, in other words, it causes the velocity




THE STEAM ENGINE.                  93
with which the water is moved to be very small relatively with the velocity of the vessel; and as the
resistance increases as the square of the velocity with
which the water is moved, it is conceivable enough in
what way a sharp bow  may diminish the resistance.
In vessels in which a high rate of speed is intended to
be realized, it will generally be advantageous to put
the midship frame abaft the centre of the vessel; or, in
other words, to make the bow end sharper than the
stern end, although the stern end must be sharp also,
else the water will be unable to fill the vacuity behind
the vessel with sufficient rapidity to obviate the resistance due to a difference of level, and the vessel also
will steer badly. It appears expedient, in most cases,
to make the shape of the bow lines such, that in equal
times the particles of water shall occasion equal increments of resistance; and it is also most important to
keep vessels intended to be swift as light as possible,
as the difference even of a few tons in the weight may
naterially affect the speed.
127. Q.-Will a vessel experience more resistance
in moving in salt water than in moving in fresh?
A.-If the immersion be the same in both cases a
vessel will experience more resistance in moving in
salt water than in moving in fresh, on account of the
greater density of salt water; but as the flotation is
proportionably greater in the salt water, the resistance
will be the sam  with the same weight carried.  The
resistance opposed to any body moving in a fluid is
directly porportional to the quantity of matter moved,
and the height necessary to generate the velocity of
motion; but the quantity of matter moved in any given




94                A CATECHISM OF
distance is directly as the density or specific gravity of
the fluid; so that the density multiplied by the height
requisite to generate the velocity, or, what is the same
thing, by the velocity squared, will express the resistance universally.  It has already been stated, in the
answer to Question 13, that the square root of the height
from which a body falls in feet multip ied by 8'021, will
give its velocity of motion in feet per second; therefore
the height multiplied by the square of 8'021, or 641,
will give the square of the velocity in feet per second,
and the square of the velocity divided by 64- will give
the height.
128. Q. —Then the resistance experienced  by a
vessel in passing through the water, will be less than
that of a flat board of the same area as the cross section
of the vessel?
A.-Yes, very much less, as is illustrated by the
circumstance that steam vessels are propelled by a very
small area of paddle float in proportion to their sectional area.  When a flat board is moved through a
fluid with its flat side foremost, the resistance it suffers
is equal to the weight of a column of the fluid with
a base of the same area as the board, and with the
altitude due to the velocity of motion with which the
board moves; and the resistance will be the same
whether the board moves through a quiescent fluid or
a moving fluid strikes against a stationary board.  As
the altitude in feet due to the velocity of motion in feet
per second, is equal to the square of the velocity divided
by 64-, and as a cubic foot of fresh water weighs
621 lbs., the pressure per square foot in lbs. upon a
board moved through river water will be 62  times




THE STEAM ENGINE.                  95
the square of the velocity in feet per second divided
by 64~; or it will be the square of the velocity multiplied by'9715.  To express the pressure or resistance
in actual horses power, we must multiply this quantity
by 60 times the velocity in feet per second, which will
give the velocity in feet per minute, and divide by
32,000 for horses power, which makes the resistance
per square foot in horses power equal to the cube of
the velocity in feet per second multiplied by'00176637.
If the water strikes the board obliquely, the resistance
will follow  a different law.  When a body impinges
obliquely upon a plane, the force of impact with any
given velocity varies as the sine of the angle of incidence; and therefore the force with which the particles
of water strike against a board will vary as the sine
of the angle at which they strike it-becoming of
course less and less as the board is turned more upon
its edge.  But the number of particles striking the
board will also vary as the sine of the angle of incidence, or, in other words, as the perpendicular height
of the inclined board; so that the resistance, as it
varies both with the force with which the particles
strike the board, and the number of particles which
strike it, must vary as the square of the sine of the
angle of incidence.  The horizontal resistance, therefore, per square foot in horses power of a board which
is struck by the water obliquely, such-as the float of
a water wheel, or which strikes the water obliquely,
such as the float of a paddle wheel, will be found by
the following rule:-multiply the cube or third power
of the velocity in feet per second by the area of the
plane in feet, by the square of the sine of the angle of




)96               A CAT'iECIJII,^M OF
incidence, and by the constant co-efficient'00176637:
the result will be the resistance in actual horses power
of a board striking the water obliquely or obliquely
struck by the water.
129. Q.-But in a paddle wheel the floats do not
move in a horizontal line?
A.-There are two kinds of paddle wheels in extensive use, the one being the ordinary radial wheel, in
which the floats are fixed on arms radiating from  the
centre, and the other the feathering wheel, in which
each float is hung upon a centre, and is so governed
by suitable mechanism as to be always kept in nearly
the vertical position. In the radial wheel there is
some loss of power from  oblique action, whereas in
the feathering wheel there is little or no loss from this
cause; but in every kind of paddle there is a loss of
power from the recession of the water from the float
boards, or the slip as it is commonly called; and this
loss is the necessary condition of the resistance for the
propulsion of the vessel being created in a fluid.  The
slip is expressed by the difference between the speed
of the wheel and the speed of the vessel; and the larger
this difference is, the greater the loss of power from
slip must be-the consumption of steam in the engine
being proportionate to the velocity of the wheel, and
the useful effect being proportionate to the speed of
the ship. In the feathering wheel, where every part
of any one immerged float moves forward with the
same horizontal velocity, the pressure or resistance
may be supposed to be concentrated near the centre of
the float; whereas in the common radial wheel this
cannot be the case; for as the outer edge of the float




TIIE STEAM ENGINE.                 97
moves more rapidly than the edge nearest the centre
of the wheel, the outer part of the float is the most
effectual in propulsion. The point at which the outer
and inner portions of the float just balance one another
in propelling effect, is called the centre of pressure;
and if all the resistances were concentrated in this
point, they would have the same effect as before in
propelling the vessel.  The resistance upon any one
moving float board totally immersed in the water will,
when the vessel is at rest, obviously vary as the square
of its distance from  the centre of motion-the resistance of a fluid varying with the square of the velocity;
but, except when the wheel is sunk to the axle or
altogether immersed in the water, it is impossible,
under ordinary circumstances, for one float to be totally
immersed without others being  immersed partially,
whereby the arc described by the extremity of the
paddle arm will become greater than the arc described
by the inner edge of the float; and consequently the
resistance upon any part of the float will increase in
a higher ratio than the square of its distance fiom the
centre of motion-the position of the centre of pressure
being at the same time correspondingly affected. In
the feathering wheel the position of the centre of pressure of the entering and emerging floats is continually
changing from  the lower edge of the float-where it
is when the float is entering or leaving the waterto the centre, of the float, which is its position when
the float is wholly immersed; but in the radial wheel,
the centre of pressure can never rise so high as the
centre of the float.
130. Q.-All this relates to the action of the paddle
9




98                A CATECHISM OF
when the vessel is at rest; will you explain its action
when the vessel is in motion?
A.-When the wheel of a coach rolls along the
ground, any point of its periphery describes in the air
a curve which is termed a cycloid; any point within
the periphery traces a prolate or protracted cycloid,
and any point exterior to the periphery traces a curtate
or contracted cycloid-the prolate cycloid partaking
more of the nature of a straight line, and the curtate
cycloid more of the nature of a circle. The action of a
paddle wheel in the water resembles in this respect
that of the wheel of a carriage running along the
ground: that point in the radius of the paddle, of
which the rotative speed is just equal to the velocity
of the vessel, will describe a cycloid; points nearer the
centre, prolate cycloids; and points further from the
centre, curtate cycloids. The circle described by the
point whose velocity equals the velocity of the ship,
is called the rolling circle; and the resistance due to
the difference of velocity of the rolling circle and centre
of pressure, is that which operates in the propulsion
of the vessel.  The resistance upon any part of the
float, therefore, will vary-as the square of its distance
from the rolling circle, supposing the float to be totally
immersed; but, taking into account the greater length
of time during which the extremity of the paddle acts,
whereby the resistance will be made greater, we shall
not err far in estimating the resistance upon any point
at the third power of its distance from the rolling circle
in the case of light immersions, and the 2 5 power in
the case of deep immersions.  With this assumption,
which accords sufficiently with experiment to justify its




THE STEAM ENGINE.               99
acceptation, the position of the centre of pressure may
be found by the following rule: —from the radius of
the wheel subtract the radius of the rolling circle; to
the remainder add the depth of the paddle board,
and divide the fourth power of the sum by four times
the depth; from the cube root of the quotient subtract
the difference between the radii of the wheel and rolling circle, and the remainder will be the distance of
the centre of pressure from  the upper edge of the
paddle.  The diameter of the rolling circle is very
easily found, for we have only to divide 5280 times
the number of miles per hour, by 60 times the number
of strokes per minute, to get an expression for the circumference of the rolling circle: or the following rule
may be adopted:-divide 88 times the speed of the
vessel in statute miles per hour, by 3'1416 times the
number of strokes per minute; the quotient will be the
diameter in feet of the rolling circle.  The diameter
of the circle in which the centre of pressure moves, or
the effective diameter of the wheel being known, and
also the diameter of the rolling circle, we at once find
the excess of velocity of the wheel over the vessel,
62'
which, multiplied by 64, or,'9715, will give the
resistance per square foot upon the vertical paddle.
131. Q.-Will you illustrate these rules by an example?
A.-A  steam vessel of moderately good shape, and
with engines of 200 horses power, realizes, with 22
strokes per minute, a speed of 10'62 miles per hour.
To find the diameter of the rolling circle, we have 88
times 10'62, equal to 934-66, and 22 times 3'1416, equal




100               A C ATrECHISMN OF
to 69'1152; then 934-66 divided by 69-1152, is equal
to 13-52 feet, which is the diameter of the rolling circle.
Thd diameter of the wheel is 19 ft. 4 in., so that the
diameter of the rolling circle is about ads of the diameter of the wheel, and this is a frequent proportion.
The depth of the paddle board is 2 feet, and the difference between the diameters of the wheel and rolling
circle will be 5-8133, which will make the difference
of their radii 2-9067; and, adding to this the depth of
the paddle board, we have 4-9067, the fourth power of
which is 579'74, which, divided by four times the depth
of the paddle board, gives us 72-455, the cube root of
which is 4-1689, which, diminished by the difference
of the radii of the wheel and rolling circle, leaves
1-2622 feet for the distance of the centre of pressure
from the upper edge of the paddle board in the case
of light immersions. The radius of the wheel being
9-6667, the distance from  the centre of the wheel to
the upper edge of the float is 7'6667, and adding to
this 1-2622, we get 8-9299 feet as the radius, or 17-8598
feet as the diameter of the circle in which the centre
of pressure revolves. With 22 strokes per minute, the
velocity of the centre of pressure will be 20'573 feet
per second, and with 10-62 miles per hour for the speed
of the vessel, the velocity of the rolling circle will be
15'576 feet per second. The effective velocity will be
the difference between these quantities, or 4-997 feet
per second; and taking the length of the floats at 10
feet, which makes the area of each float 20 square feet,
the resistance in lbs. encountered by the vertical float
will be 20 multiplied by 4-9972, multiplied by 0'9715,
equal to 485'17 lbs., which being doubled for the ver



THE STEAM ENGINE.                101
tical float of the other wheel, gives us 970'34 lbs. as
the pressure on the vertical floats in their motion
through the water. The velocity with which the floats
move is that of the centre of pressure, so that to find
the resistance in horses power, we must multiply the
velocity of the centre of pressure in feet per second by
60, to bring it to feet per minute, then by 970'34 lbs.,
and divide by 33,000, which will give about 36 horses
power as the power expended on the vertical paddles.  If the nominal power of the engines be 200
horses, the actual horses power will be greater in
engines of this class in the proportion of 33,000 to
50,000; so that the actual power exerted by the engines
will be about 315 horses, and 0'114 of the power of
the engines will be the proportion of power expended
upon the vertical paddles.
132. Q.-That is in the case of a common radial
wheel: what will be the proportion in the feathering
wheel?
A.-It will be the same in the feathering wheel,
if the float be of the same area, and all other circumstances the same; but in the radial wheel the vertical
float sustains the least resistance in propulsion, and in
the feathering wheel the most resistance; and as the
power of the engine is a fixed quantity, which has to
be expended among all the floats, and as in the radial
wheel it is chiefly distributed among the oblique floats
which encounter a resistance that has to be thrown in
a great measure upon the vertical float in the case of
the feathering wheel, a larger area of float-board is
necessary in the feathering wheel than in the radial
one.  To understand how the entering and emerging
9*




102               A CATECHISM OF
paddles of the radial wheel sustain more resistance in
propulsion than the vertical one, it is necessary to
remember that the only resistance upon the vertical
paddle is that due to the difference of velocity of the
wheel and the ship; but if the wheel be supposed to
be immersed to its axle, so that the entering float
strikes the water horizontally, it is clear that the
resistance on such float is that due to the whole
velocity of rotation; and that the resistance to the
entering float will be the same whether the vessel is
in motion or not.  The resistance opposed to the
rotation of any float increases from the position of the
vertical float-where the resistance is that due to the
difference of velocity of the wheel and vessel-until it
reaches the plane of the axis, supposing the wheel to
be immersed so far, where the resistance is that due to
the whole velocity of rotation; and although in any
oblique float the total resistance cannot be considered
operative in a horizontal direction, yet the total resistance increases so rapidly on each side of the vertical
float, that the portion of it which is operative in the
horizontal direction, is in all ordinary cases of immersion very great when compared with the whole
resistance upon the vertical float. In the feathering
wheel, where there is none of this oblique action, the
resistance will be simply in the proportion of the
square of the horizontal velocities of the several floats,
which may be represented by the horizontal distances
between them; and in the feathering wheel, the vertical
float having the greatest horizontal velocity will have
the greatest propelling effect.
133. Q.-Can you give a rule for ascertaining the




THE STEAM ENGINE.                 103
resistance encountered by a paddle board at any part
of its motion?
A.-In the common radial wheel the resistance or
pressure upon any float may be ascertained as follows:
-from  the velocity of the centre of pressure in feet
per second, subtract the velocity of the rolling circle
multiplied by the cosine of the angle of inclination the
paddle board makes with a vertical line; square the
remainder, and multiply it by'9715 times the area of
the float in feet, which will give the tangential resistance of the float in lbs. avoirdupois. The horizontal
resistance may be obtained by multiplying this quantity
again by the cosine of the angle of inclination the float
makes with a vertical line.  In the feathering wheel
the resistance may be ascertained by multiplying the
velocity of the centre of pressure by the cosine of the
angle of inclination, subtracting from this the velocity
of the rolling circle, squaring the remainder, and multiplying it by'9715 times the area of float in feet,
which will give the resistance of the float in pounds.
By thus calculating the resistance of the different floats
immersed, taking the mean of these resistances and
multiplying by the number of floats in the water, we
readily ascertain the resistance of the wheel; and by
finding what area of vertical paddle board moving at
the velocity of the vessel would occasion the same
resistance as all the floats immersed, and comparing
this area with the sectional area of the vessel, we may
find the power necessary to propel a board through
the water of equal area with the cross section of the
vessel, which will show the diminution of resistance
consequent upon the sharpening away of the bow and




104                A CATECHISM OF
stern.  These relations, with many others connected
with the paddle wheels, have been investigated at considerable length by Mr. Barlow in a paper published
in the Philosophical Transactions, and reprinted in the
Appendix to Tredgold's Treatise on the Steam Engine;
but some of Mr. Barlow's deductions are erroneous,
and he has vitiated nearly all his conclusions by confounding the actual with the nominal horse power.
134. Q.-Can you give any practical rules for proportioning paddle wheels?
A.-A common rule for the pitch of the floats is to
allow one float for every foot of diameter of the wheel;
but in the case of fast vessels a pitch of 2  feet, or even
less, appears preferable, as a close pitch occasions less
vibration.  If the floats be put too close, however, the
water will not escape freely from between them; and if
set too far apart, the stroke of the entering paddle will
occasion an inconvenient amount of vibratory motion,
and there will also be some loss of power. To find the
proper area of a single float: —divide the number of
actual horses power of both engines hb the diameter of
the wheel in feet; the quotient is the area of one paddle
board in square feet proper for sea-go;-g vessels, and
the area multiplied by 0-6 will give the lcngth of the
float in feet. In very sharp vessels, which offer less
resistance in passing through the water, thr area of
paddle board is usually one-fourth less than ta,: above
proportion, and the proper length of the float iray in
such case be found by multiplying the area by ('7.
In sea-going vessels about four floats are usually ir —
mersed, and in river steamers only one or two floats
There is more slip in the latter case, but there is als(




THE STEAM ENGINE.                105
more engine power exerted in the propulsion of the
ship, from the greater speed of engine thus rendered
possible. If to permit a greater speed of the engine the
floats be diminished in area instead of being raised out
of the water, no appreciable accession to the speed of
the vessel will be obtained; whereas there will be an
increased speed of vessel if the accelerated speed of the
engine be caused by diminishing the diameter of the
wheels.  In vessels intended to be very fast, therefore,
it is expedient to make the wheels small, so as to
enable the engine to work with n high velocity; and
it is expedient to make such wheels of the feathering kind to obviate loss of power from oblique action.
In no wheel must the rolling circle fall below the water
line, else the entering and emerging floats will carry
masses of water before them.  The slip is usually equal
to about one-fourth of the velocity of the centre of pressure in well-proportioned wheels; but it is desirable
to have the slip as small as is possible consistently with
the observance of other necessary conditions.  The
speed of the engine and also the speed of the vessel
being fixed, the diameter of the rolling circle becomes
at once ascertainable, and adding to this the slip, we
have tLe diameter of the wheel.
135. Q.-Is the screw propeller as effectual an instrument of propulsion as the radial or feathering paddle?
A.-In all cases of deep immersion it appears to be
quite as effectual as the radial paddle; but it is scarcely
as effectual as the feathering paddle, with any amount
of immersion, and scarcely as effectual as the common
paddle in the case of light immersions.
136. Q.-In what way are the dimensions of the




106               A CATECHISM OF
screw propeller proportioned to the power of the engines and area of midship-section of vessel?
A.-It has been found experimentally, that when
the screw is so proportioned as to make the slip about
Loth of the speed of the screw, a maximum effect is
produced, the speed of the vessel being then -1-ths of
what it would be if the screw were working in a solid.
Find, by some of the rules previously given, what speed
is due to the form and power as if the vessel were to
be propelled by paddles, which speed call y9ths, and
adding #lth for slip, we shall have the number of feet
per minute to be travelled by the screw, and this number
divided by the number of revolutions per minute, will
give the pitch of the screw in feet. The length of screw
that is found most beneficial is about ith of a convolution, and the diameter should be as large as it can be
got.  A screw with two arms, or a portion of a doublethreaded screw, has been found as effectual a propeller
as any other; but a screw  with three blades, or a
portion of a three-threaded screw, has been found to
act with a more equable and regular motion.  The
stern post carries behind it a certain quantity of dead
water which moves with the vessel, and when the
screw gets into this water, which happens in the case
of a double-threaded screw when the two arms of the
screw are vertical, the resistance becomes nearly equal
to that which would be due to the whole velocity of the
screw, instead of the difference of velocity of screw
and vessel; and the forward thrust at that point is so
much increased thereby, that an uneasy motion is
given to the hinder part of the ship, while a waste of
power is at the same time occasioned.  Where three




THE STEAM ENGINE.                107
blades are employed, this vibratory motion is by no
means so conspicuous, as the impulse is equally divided
among the three blades; and a three-bladed screw
therefore appears, on the whole, to be preferable. It is
very important to make the run of the ship very fine,
so that the vessel may carry as small a quantity as
possible of dead water behind her.  As regards the
form of screw, it is found that the common screw with
a uniform pitch is as effectual as any: screws of an
increasing pitch have been tried at various times, but
without the realization of any appreciable advantage.
The water thrown backwards by the screw assumes a
conical form, and the obliquity and consequent loss of
power will be greater when the pitch of the screw is
coarse and the diameter small; so that it is expedient
to have as large a diameter as possible, a quick speed,
and a fine pitch.
PART II.
STRENGTHS, CONSTRUCTIVE DETAILS, AND MANAGEMENT.
137. Q.-In what way are the strengths of the different parts of a steam engine determined?
A.-By a reference to the cohesion of iron, or its
power to resist a tensile, twisting, breaking, or crushing force-which has been fixed by numerous experiments, and by the computation of the amount of strain
to which the several parts are subjected, to the end
that the quantity of material may be made propor



108              A CATECHISM OF
tionate to the strain. The breaking strain of a bar of
malleable iron, of medium  quality, one inch square,
when pulled in the direction of its length, is from
50,000 to 60,000 lbs., and of a bar of cast-iron from
17,000 to 20,000 lbs.; but a much smaller strain than
this will damage the structure of the iron, and finally
break it, if the strain be permitted to act without interruption. The tensile strain to which a bar of malleable iron an inch square may be subjected, without
permanently deranging its structure, is 17,800 lbs.;
and in the case of cast-iron 15,300 lbs.: but it would
not be safe to apply such a strain in practice, as there
are so many different qualities of iron, and so many
irregularities of structure even in iron of the same
quality, from unequal conversion, imperfect welds, and
other circumstances, that it would be unwarrantable
to reckon the maximum  strength as at all times subsisting. The greatest strain to which malleable iron
should be subjected in machinery is 4000 lbs. on the
square inch of section, though in locomotive boilers
this strain is sometimes exceeded.  In the case of
beams subjected to a breaking force, the strength with
any given cohesion of the material will be proportional
to the breadth, multiplied by the square of the depth;
and in the case of revolving shafts exposed to a twisting
strain, the strength with any given cohesive power of
the material will be as the cube of the diameter.  If
the force acting at the end of an engine beam be taken
at 18 lbs. per circular inch of the piston, then the force
acting at the middle will be 36 lbs. per circular inch of
the piston, and the proper strength of the beam at the
oentre will be found by the following rule: —divide




THE STEAM ENGINE.                 109
the weight in lbs. acting at the centre by 250, and
multiply the quotient by the distance between the
extreme  centres.  To find the depth, the breadth
being given: divide this product by the breadth in
inches, and extract the square root of the quotient,
which is the depth.  The depth of an engine beam at
the ends is usually made one-third of the depth at the
centre, which is equal to the diameter of the cylinder
in the case of low pressure engines, while the length
is made equal to three times the length of the stroke,
and the mean thickness T-th of the length-the
width of the edge bead being about three times the
thickness of the web.   In low pressure engines the
diameter of the end studs of the engine beam  are
usually made -th of the diameter of the cylinder when
of cast-iron, and iVth when of wrought-iron, which
gives a load with low steam of about 500 lbs. per circular inch of transverse section; but a larger size is
preferable, as with large bearings the brasses do not
wear so rapidly and the straps are not so likely to be
burst by the bearings becoming oval.  To find the
proper size of a cast-iron gudgeon adapted to sustain
any given weight: multiply the wighght in lbs. by the
intended length of bearing expressed in terms of the
diameter; divide'he product by 500, and extract the
square root of the quotient, which is the diameter in
inches. Experiments upon the force requisite to twist
off cast-iron necks show that if the cube of the diameter of neck in inches be multiplied by 880, the product will be the force of torsion which will twist them
off when acting at 6 inches radius. To find the diameter of a cast-iron fly wheel shaft: multiply the
10




110               A CATECHISM OF
square of the diameter of the cylinder in inches by the
length of the crank in inches, and extract the cube root
of the product, which multiply by 0-3025, and the
result will be the diameter of the shaft in inches at
the smallest part when of cast-iron.  To find the
diameter of the paddle shaft of a steam vessel when of
wrought-iron: multiply the square of the diameter of
cylinder in inches by the length of the crank in inches,
and extract the cube root of the product, which multiply by 0'242; the result is the diameter of the shaft
in inches at the smallest part when of malleable iron.
The diameter of the crank pin is usually made 6th of
the diameter of the cylinder when of cast-iron, and
th of the diameter of the cylinder when of malleable
iron.  The diameter of the piston rod is usually made
I th of the diameter of the cylinder, or the sectional
area of the piston rod is To-oth of the area of the cylinder.  The sectional area of the main links in land
beam engines is T}Lth of the area of the cylinder, and
the length of the main links is usually half the length
of the stroke. In land engines the connecting rod is
usually of cast-iron, with a cruciform section: the
breadth across the arms of the cross is about l-th of
the length of the rod, the sectional area at the centre
Iath of the area of the cylinder, and at the ends 3-1th
of the area of the cylinder: the length of the rod is
usually 3-  times the length of the stroke.  To find
the proper dimensions for the  teeth  of a cast-iron
wheel: multiply the diameter of the pitch circle in
feet by the number of revolutions to be made per
minute, and reserve the product for a divisor; multiply the number of actual horses power to be trans



THE STEAM ENGINE.                11I
mitted by 240, and divide the product by the above
divisor, which will give the strength. If the pitch be
given to find the breadth, divide the above strength
by the square of the pitch in inches; or if the breadth
be given, then to find the pitch, divide the strength by
the breadth in inches, and extract the square root of
the quotient, which is the proper pitch in inches. The
length of the teeth is usually about 5-ths of the pitch.
Pinions to work satisfactorily should not have less than
30 or 40 teeth, and where the speed exceeds 220 feet
in the minute, the teeth of the larger wheel should be
of wood, made a little thicker to keep the strength unimpaired.  These rules are for the most part applicable
only in the case of condensing engines working with
steam of a few pounds pressure above the atmosphere;
whereas in many modern engines, and especially in
marine engines since the introduction of tubular boilers,
the pressure of the steam has been so much increased
as to make the force urging the piston twice greater
than formerly.
138. Q.-Cannot you give some rules of strength
which will be applicable whatever pressure may be
employed?
A. —I  the rules already given, the pressure may
be reckoned at from 18 to 20 lbs. upon every square
inch of the piston; and if the pressure upon every
square inch of the piston be made twice greater, the
dimensions must just be those proper for an engine of
twice the area of piston. It will not be difficult, however, to introduce the pressure into the rules as an
element of the computation, whereby the result will
be applicable both to high and low pressure engines.




112               A CATECHISM OF
The method of computation will then be as follows: to
find the dimensions of a malleable iron paddle shaft, so
that the strain shall not exceed 5-ths of the elastic
force, or 5-ths of the force iron is capable of withstanding without permanent derangement of structure,
which in tensile strains is 17,800 lbs. per square inch:
multiply 0'08264 times the pressure in lbs. per square
inch on the piston by the square of the diameter of
the cylinder in inches, and the length of the crank in
inches, and extract the cube root of the product,
which will be the diameter of the paddle shaft journal
in inches when of malleable iron, whatever the pressure
of the steam may be.  The length of the paddle shaft
journal should be 1 - times the diameter; and the
diameter of the part where the crank is put on is often
made equal to the diameter over the collars of the
journal or bearing. To find the exterior diameter of
the large eye of the crank when of malleable iron:to 1'561 times the pressure of the steam  upon the
piston in lbs. per square inch, multiplied by the square
of the length of the crank in inches, add 0 00494 times
the square of the diameter of the cylinder in inches,
multiplied by the square of the number of lbs. pressure
per square inch on the piston; extract the square root
of this quantity; divide the result by 75'59 times the
square root of the length of the crank in inches, and
multiply the quotient by the diameter of the cylinder
in inches; square the product, and extract the cube
root of the square, to which add the diameter of the
hole for the reception of the shaft, and the result
will be the exterior diameter of the large eye of the
crank when of malleable iron.  The diameter of the




THE STEAM ENGINE.               113
small eye of the crank may be found by adding to the
diameter of the crank pin 0'02521 times the square
root of the pressure on the piston in lbs. per square
inch, multiplied by the diameter of the cylinder in
inches; the diameter of the crank pin may be found
by multiplying 0'02836 times the square root of the
pressure on the piston in lbs. per square inch by the
diameter of the cylinder in inches. The length of the
pin is usually about 9th times its diameter, and the
strain if all thrown upon the end of the pin will be
equal to the elastic force; but in ordinary working,
the strain will only be equal to -3d of the elastic force.
The thickness of the web of the crank, supposing it to
be continued to the centre of the shaft, would at that
point be represented by the following rule:-to 1'561
times the square of the length of the crank in inches,
add 0'00494 times the square of the diameter of the
cylinder in inches, multiplied by the pressure on the
piston in lbs. per square inch; extract the square
root of the sum, which multiply by the diameter of
the cylinder squared in inches, and by the pressure on
the piston in lbs. per square inch; divide the product
by 9000, and extract the cube root of the quotient,
which will be the proper thickness of the web of the
crank when of malleable iron, supposing the web to
be continued to the centre of the shaft. The thickness
of the web at the crank pin centre, supposing it to be
continued thither, would be 0'022 times the square
root of the pressure on the piston in lbs. per square
inch, multiplied by the diameter of the cylinder.  The
breadth of the web of the crank of the shaft centre
should be twice the thickness, and at the pin centre 12
10*




114               A CATECHISM OF
times the thickness of the web; the length of the
large eye of the crank should be equal to the diameter
of the shaft, and of the small eye 0 0375 times the
square root of the pressure on the piston in lbs. per
square inch, multiplied by the diameter of the cylinder.
The diameter of the piston rod may be found by
multiplying the diameter of the cylinder in inches by
the square root of the pressure on the piston in lbs.
per square inch, and dividing by 50, which makes
the strain Ith of the elastic force.   The diameter
of the connecting rod at the ends, may be found by
multiplying 0'019 times the square root of the pressure
on the piston in lbs. per square inch by the diameter
of the cylinder in inches; and the diameter of the
connecting rod in the middle may be found by the
following rule: —to 0'0035 times the length of the
connecting rod in inches, add 1, and multiply the sum
by 0'019 times the square root of the pressure on the
piston in lbs. per square inch, multiplied by the diameter of the cylinder in inches.  The diameter of the
cylinder side rods at the ends may be found by multiplying 0 0129 times the square root of the pressure
on the piston in lbs. per square inch by the diameter of
the cylinder; and the diameter of the cylinder side
rods at the middle is found by the following rule:to 0-0035 times the length of the rod in inches add 1,
and multiply the sum by 0'0129 times the square root
of the pressure on the piston in lbs. per square inch,
multiplied by the diameter of the cylinder in inches;
the product is the diameter of each side rod at the centre
in inches.  The strain upon the connecting rod and
side rods is by these rules equal to -th of the elastic




THE STEAM ENGINE.                115
force.  If the length of the cross head be taken at 1'4
times the diameter of the cylinder, the dimensions of
the cross head  will be as follows: -the exterior
diameter of the eye in the cross head for the reception
of the piston rod, will be equal to the diameter of the
hole, plus 0'02827 times the cube root of the pressure
on the piston in lbs. per square inch, multiplied by
the diameter of the cylinder in inches; and the depth
of the eye will be 0'0979 times the cube root of the
pressure on the piston in lbs. per square inch, multiplied by the diameter of the cylinder in inches.  The
diameter of each cross head journal will be 0'01716
times the square root of the pressure on the piston
in lbs. per square inch, multiplied by the diameter of
the cylinder in inches -  the length of the journal
being 9ths its diameter.  The thickness of the web at
centre will be 0'0245 times the cube root of the pressure on the piston in lbs. per square inch, multiplied
by the diameter of the cylinder in inches, and the
depth of web at centre will be 0'09178 times the cube
root of the pressure on the piston in lbs. per square
inch, multiplied by the diameter of the cylinder in
inches.  The thickness of the web at journal will be
0'0122 times the square root of the pressure on the
piston in lbs. per square inch, multiplied by the diameter of the cylinder in inches, and the depth of the
web at journal will be 0'0203 times the square root of
the pressure upon the piston in lbs. per square inch,
multiplied by the diameter of the cylinder in inches.
In these rules for the cross head, the strain upon the
web is 2-.2f  times the elastic force; the strain upon
the journal in ordinary working is 2.~  times the elastic




116               A CATECHISM OF
force; and if the outer ends of the journals are the
only bearing  points, the strain is YT.T   times the
elastic force, which is very little in excess of the elastic
force. The diameter of the main centre may be found
by multiplying 0'0367 times the square of the pressure
upon the piston in lbs. per square inch, by the diameter
of the cylinder in inches, which will give the diameter
of the main centre journal when of malleable iron, and
the length of the main centre journal should be 1times its diameter; the strain upon the main centre
journal in ordinary working  will be about -  the
elastic force.   In oscillating engines the diameter of
the trunnions is regulated  by the diameter of the
pipes, and the thickness of the trunnions is about the
same as the thickness of the cylinder.  It is obvious,
from the varying proportions subsisting in the different
parts of the engine between the strain and the elastic
force, that in engines proportioned by these ruleswhich represent nevertheless the average practice of
the best constructors-some of the parts must possess
a considerable excess of strength over other parts, and
it appears expedient that this disparity  should be
diminished, which may best be done by increasing the
strength of the parts which are weakest; inasmuch as
the frequent fracture of some of the parts shows
that the dimensions at present adopted for those parts
are scarcely sufficient, unless the iron of which they
are made is of the best quality.  At the same time it
is quite certain, that engines proportioned by these
rules will work satisfactorily where good materials are
employed; but it is important to know in what parts
good materials and large dimensions are the most in



THE STEAM ENGINE.                  1 17
dispensable. The depth of gibs and cutters for attaching the piston rod to the cross head, is 0'0358 times
the cube root of the pressure of the steam on the piston
in lbs. per square inch, multiplied by the diameter of
the cylinder; and the thickness of the gibs and cutters
is 0'007 times the cube root of the pressure on the
piston in lbs. per square inch, multiplied by the diameter of the cylinder. The depth of the cutter through
the piston is 0'017 times the square root of the pressure
on the piston in lbs. per square inch, multiplied by the
diameter of the cylinder in inches; and the thickness
of the cutter through the piston is 0'007 times the
square root of the pressure on the piston in lbs. per
square inch, multiplied by the diameter of the cylinder.
The whole of the answers to these rules are given in
inches.
139. Q.-How  do you determine the strength  of
boilers?
A.-The iron of boilers, like the iron of machines
or structures, is capable of withstanding a tensile strain
of from 50,000 to 60,000 lbs. upon every square inch
of section; but it will only bear a third of this strain
without permanent derangement of structure, ard it
does not appear expedient in any boiler to let the
strain exceed 4000 lbs. upon the square inch of sectional area of metal; and 3000 lbs. on the square inch
of section, is a preferable proportion. The question
of the strength of boilers was investigated very elaborately  a few  years ago, by a Committee of the
Franklin  Institute, in  America, and  it was found
that the tenacity of boiler plate increased with the
temperature up to 550~, at which point the tenacity




118                A CATECHISM OF
began to diminish.  At 32~ the cohesive force of a
square inch of section was 56,000 lbs.; at 570~ it was
66,500 lbs; at 720~, 55,000 lbs.; at 1050~, 32,000 lbs.;
at 1240~, 22,000 lbs.; and at 1317~, 9000 lbs.  Copper follows a different law, and appears to be diminished
in strength by every addition to the temperature. At
32~ the cohesion of copper was found to be 32,800 lbs.
per square inch of section, which exceeds the cohesive
force at any higher temperature, and the square of
the diminution of strength seems to keep pace with
the cube of the increased temperature. Strips of iron
cut in the direction of the fibre were found to be about
6 per cent. stronger than when cut across the grain.
Repeated  piling and welding was found to increase
the tenacity of the iron, but the result of welding
together different kinds of iron was not found to be
favorable.  The accidental overheating of a boiler was
found to reduce the ultimate or maximum strength of
the plates from 65,000 lbs. to 45,000 lbs. per square
inch of section, and riveting the plates was found to
occasion a diminution in their strength to the extent
of one-third.  In some locomotive boilers which are
worked with a pressure of 80 lbs. upon the square
inch, the thickness of the plates is only -5ths of
an inch, while the barrel of the boiler is 39 inches
in diameter.  It will require a length of 3 2 inches
of the boiler when the plates are -Uths thick to make
up a sectional area of one square inch, and the separating force will be 39 times 3'2 multiplied by 80,
which makes the separating force 9984 lbs. sustained
by two square inches of sectional area,-one on each
side; or the strain is 4992 lbs. per square inch of




THE STEAM ENGINE.                 119
sectional area, which is a greater strain than is advisable.  The accession of strength derived from  the
boiler ends is not here taken into account; but neither
is the weakening effect counted that is caused by the
rivets, which certainly would not be less in amount.
The proper thickness for cylindrical boilers or other
cylindrical vessels, whether of cast or wrought iron,
exposed to an internal pressure, may be found  by
the following rule:-multiply 2'54 times the  internal diameter of the cylinder in inches by the greatest
pressure within the cylinder per circular inch, and
divide by the tensile force that the metal will bear
without permanent derangement of structure, which
for malleable iron is 17,800 lbs., and for cast-iron
15,300 lbs. per square inch of section; the result is
the thickness in inches.  Where  the sides of the
boiler are flat instead of being cylindrical, a sufficient
number of stays must be introduced to withstand the
pressure, and it is expedient not to let the strain
upon these stays be more than 3000 lbs. per square
inch of section, as the strength of internal stays in
boilers is generally soon diminished by corrosion. It
is expedient also that the stays should be small and
numerous rather than large and few in number, as
when large stays are employed it is difficult to keep
them  tight at the ends, and oxidation of the shell
follows from leakage at the ends of the stays. A strain
at all approaching that upon locomotive boilers would
be very unsafe in the case of marine boilers, on account
of the corrosion, both internal and external, to which
marine boilers are subject.  All boilers should  be
proved, when new, to three or four times the pressure




120               A CATECHISM O'
they are intended to bear, and they should be proved
occasionally by the hand pump when in use, to detect
any weakness which corrosion may have occasioned.
140. Q.-What is the cause of the rapid corrosion
of marine boilers?
A.-Marine boilers are corroded externally in the
region of the steam  chest by the dripping of water
from the deck; the bottom of the boiler is corroded by
the action of the bilge water, and the ash pits by the
practice of quenching the ashes with salt water. These
sources of injury, however, admit of easy remedy: the
top of the boiler may be preserved from external corrosion by covering it with felt, upon which is laid sheet
lead soldered at every joint so as to be impenetrable to
water; the ash pits may be shielded by guard plates,
which are plates fitting into the ash pits and attached
to the boiler by a few bolts, so that when worn they
may be removed and new ones substituted, whereby
any wear upon the boiler in that part will be prevented;
and there will be very little wear upon the bottom of
a boiler if it be imbedded in mastic cement laid upon
a suitable platform. The greatest part of the corrosion
of a boiler, however, takes place in the inside of the
steam chest, and the origin of this corrosion is one of
the obscurest subjects in the whole range of engineering.  It cannot be from  the chemical action of the
salt water upon the iron, for the flues and other parts
of the boiler beneath the water suffer very little from
corrosion; and in steam vessels provided with Hall's
condensers, which supply the boiler with fresh water,
scarcely any increased durability of the boiler has been
experienced. Nevertheless, marine boilers seldom last




TIlE STEAM ENGINE.               121
more than for 4 or 5 years, whereas land boilers made
of the same quality of iron often last 18 or 20 years,
and it does not appear probable that land boilers would
last a very much shorter time if salt water were used
in them. The thin film of scale spread over the parts
of a marine boiler situated beneath the water, effectually protects them  from  corrosion; and when the
other parts are completely worn out, the flues generally
remain so perfect that the hammer marks upon them
are as conspicuous as at their first formation. The
operation of the steam in corroding the interior of the
boiler is most capricious-the parts which are most
rapidly worn away in one boiler being untouched in
another; and in some cases one side of a steam chest
will be very much wasted away while the opposite side
remains uninjured.  Sometimes the iron exfoliates in
the shape of a black oxide which comes away in flakes
like the leaves of a book, while in other cases the iron
appears as if eaten away by a strong acid which had a
solvent action upon it. The application of felt to the
outside of a boiler, has in several cases been found to
accelerate sensibly its internal corrosion; boilers in
which there is a large accumulation of scale appear to
be more corroded than where there is no such deposit,
and where the funnel passes through the steam chest,
the iron of the steam chest is invariably much more
corroded than where the funnel does not pass through
it. These facts appear to indicate that the internal
corrosion of marine boilers is attributable chiefly to
the existence of surcharged steam within them, which
is steam to which an additional quantity of heat has
been communicated subsequently to its generation, so
11




122               A CATECIISM OF
that its temperature is greater than is due to its elastic
force; and on this hypothesis the observed facts relative to corrosion become easily explicable.  Felt, applied to the outside of a boiler, may accelerate its
internal corrosion by keeping the steam  in a surcharged state, when by the dispersion of a part of the
heat it would cease to be in that state. Boilers in which
there is a large accumulation of scale must have worked
with the water very salt, which necessarily produces
surcharged steam; for the temperature of steam cannot be less than that of the water from  which it is
generated, and inasmuch as the boiling point of water,
under any given pressure, rises with the saltness of the
water, the temperature of the steam  must rise with
the saltness of the water, the pressure remaining the
same; or in other words, the steam must have a higher
temperature than is due to its elastic force, or be in
the state of surcharged steam.  The circumstance of
the chimney flue passing through the steam will manifestly surcharge the steam with heat, so that all the
circumstances which are found to accelerate corrosion,
are, it appears, such as would also induce the formation of surcharged steam.  Besides, the natural effect
of surcharged steam is to oxidate the iron with which
it is in contact, as is illustrated by the familiar process
for making hydrogen gas by sending steam  through a
red-hot tube filled with pieces of iron; and although
the action of the surcharged steam in a boiler is necessarily very much weaker than where the iron is red
hot, it manifestly must have some oxidizing effect, and
the amount of corrosion produced may be very material where the action is perpetual.  Boilers with




THE STEAM ENGINE.                 123
a large extent of heating surface, or with descending
flues circulating through the cooler water in the bottom
of the boiler before ascending the chimney, will be less
corroded internally than boilers in which a large quantity of the heat passes away in the smoke; and the
corrosion of the boiler will be diminished if the interior of any flue passing through the steam be coated
with fire-brick, so as to prevent the transmission of
the heat in that situation. The best practice, however,
appears to consist in the transmission of the smoke
through a suitable orifice below the water level, so as
to supersede the necessity of carrying any flue through
the steam at all; or a column of water may be carried
round the chimney, into which as much of the feed
water may be introduced as the heat of the chimney
is capable of raising to the boiling point, as under
this limitation the presence of feed water around the
chimney in the steam chest will fail to condense the
steam.
141. Q.-Will you explain the course of procedure
in the construction and setting of wagon boilers?
A.-Most boilers are made of plates three-eighths
of an inch thick, and the rivets are from three-eighths
to three-fourths of an inch in diameter. In the bottom
and sides of a wagon boiler, the heads of the rivets,
or the ends formed before the rivets are inserted, should
be large, and placed next the fire, or on the outside;
whereas on the top of the boiler the heads should be
on the inside. The rivets should be placed about two
inches distant from centre to centre, and the centre of
the row of rivets should be about one inch from the
edge of the plate. The edges of the plate should be




124               A CATECHISM OF
truly cut, both inside and outside, and after the parts
of the boiler have been riveted together, the edges of
the plate should be set up or caulked with a blunt
chisel about a quarter of an inch thick in the point,
and struck by a hammer of about 3 or 4 lbs. weightone man holding the  caulking  tool while another
strikes. The boiler should then be filled with water
and caulked afresh in any leaky part; all the joints
above the water should next be painted with a solution
of sal ammoniac in urine, and so soon as the seams are
well rusted they should be dried with a gentle fire,
and then be painted over with a thin putty formed
of whiting and linseed oil-the heat being continued
until the putty becomes so hard that it cannot be
readily scratched with the nail, and care must be taken
neither to burn the putty nor to discontinue the fire
until it has become quite dry. In building the brickwork for the setting of the boiler, the parts upon which
the heat acts with most intensity is to be built with
clay instead of mortar; but mortar is to be used on the
outside of the work. Old bars of flat iron may be laid
under the boiler chime to prevent that part of the
boiler from being burned out, and bars of iron should
also run through the brick-work to prevent it fiom
splitting. The top of the boiler is to be covered with
brick-work laid in the best lime; and if the lime be
not of the hydraulic kind, it should be mixed with
Dutch terrass to make it impenetrable to water.  The
top of the boiler should be well plastered with this
lime, which will greatly conduce to the tightness of
the seams.  Openings into the flues must be left in
convenient situations to enable the flues to be swept




THE STEAM ENGINE.                125
out when required, and these openings may be closed
with cast-iron doors jointed with clay or mortar, which
may be easily removed when required.  Adjacent to
the chimney a slit must be left in the top of the flue
with a groove in the brick-work to enable a sliding
door or damper to be fixed in that situation, which by
being lowered into the flue will obstruct the passage
of the smoke and moderate the draught, whereby the
chimney will be prevented from drawing the flame into
it before the heat has acted sufficiently upon the boiler.
142. Q.-Are marine constructed in the same way
as land boilers?
A.  There is very  little  difference in the two
cases: the whole of the shells of marine boilers, however, should be double riveted with rivets {1th of an
inch in diameter, and  2-th inches from  centre to
centre, the weakening effect of double riveting being
much less than that of single riveting. The furnaces
above the line of bars should be of the best Lowmoor,
Bowling, or Staffordshire scrap plates, and the portion
of each furnace above the bars should consist only of
three plates, one for the top and one for each side, the
lower seam of the side plates being situated beneath
th3 level of the bars, so as not to be exposed to the
heat of the furnace.  The tube plates of tubular
boilers should be of the best Lowmoor or Bowling
iron, seven-eighths to one inch thick: the shells should
be of the best Staffordshire or Thornycroft S Crown
iron, seven-sixteenths of an inch thick.  Angle iron
should not be used in the construction of boilers, as in
the manufacture it becomes reedy, and is apt to split
up in the direction of its length: it is much the safer
11*




126              A CATECHISM OF
practice to bend the plates at the corners of the
boiler, but this must be carefully done, without introducing any more sharp bends than can be avoided,
and plates which require to be bent much should be
of Lowmoor iron.  It will usually be found expedient
to introduce a ring of angle iron around the furnace
mouths, though it is discarded in the other parts of the
boiler; but it should be used as sparingly as possible,
and any that is used should be of the best quality.
The whole of the plates of a boiler should have the
holes for the rivets punched, and the edges cut straight,
by means of self-acting machinery, in which a travelling
table carries forward the plate with an equal progression every stroke of the punch or shears; and machinery of this kind is now  extensively employed.
The practice of forcing the parts of boilers together
with violence, by means of screw-jacks, and drifts
through the holes, should not be permitted; as a great
strain may thus be thrown upon the rivets, even
when there is no steam  in the boiler.  All rivets
should be of the best Lowmoor iron.  The work
should be caulked both within and without wherever
it is accessible; but in the more confined situations
within the flues, the caulking will in many cases have
to be done with the hand or chipping instead of the
heavy hammer previously prescribed.  In the setting
of marine boilers care must be taken that no copper
bolts or nails project above the platform upon which
they rest, and also that no projecting copper bolts in
the sides of the ship touch the boiler, as the galvanic
action in such a case would probably soon wear the
points of contact into holes. The platform may con



THE STEAM ENGINE.               127
sist of 3-inch planking laid across the keelsons nailed
with iron nails, the heads of which are well punched
down, and caulked and puttied like a deck.  The surface may then be painted over with thin putty, and
fore and aft boards of half the thickness may then be
laid down and nailed securely with iron nails, having
the heads well punched down.  This platform  must
then be covered thinly and evenly with mastic cement,
and the boiler be set down upon it, and the cement
must be caulked beneath the boiler by means of wooden
caulking tools, so as completely to fill every vacuity.
Coomings of wood sloped on the top must next be set
round the boiler, and the space between the coomings
and the boiler must be caulked full of cement, and be
smoothed off on the top to the slope of the coomings, so
as to throw off any water that might be disposed to
enter between the coomings and the boiler.  Mastic
cement proper for the setting of boilers is sold in many
places ready made; Hamelin's mastic is compounded
as follows:-~To any given weight of sand or pulverized earthenware add two-thirds such given weight
of powdered Bath, Portland, or other similar stone,
and to every five hundred and sixty pounds weight of
the mixture add forty pounds weight of litharge, two
pounds of powdered glass or flint, one pound of
minium, and two pounds of gray oxide of lead; pass
the mixture through a sieve, and keep it in a powder
for use.  When wanted for use, a sufficient quantity
of the powder is mixed with some vegetable oil'upon a
board or in a trough in the manner of mortar, in the
proportion of six hundred and five pounds of the
powder to five gallons of linseed, walnut, or pink oil,




128              A CATECHISM OF
and the mixture is stirred and trodden upon until it
assumes the appearance of moistened sand, when it is
ready for use. The cement should be used on the same
day the oil is added, else it will set into a solid mass.
143. Q.-Will you state any other constructive details in connection with marine boilers which occur to
you as deserving of enumeration?
A.-It will not be possible to state them in a very
systematic manner by adopting this course, but perhaps it may be of most importance to dispatch them
summarily.  It has already been stated that furnace
bars should not much exceed 6 ft. in length, as it is
difficult to manage long furnaces; but it is a frequent
practice to make furnaces long and narrow, the consequence of which is, that it is impossible to fire them
effectually at the after end, especially upon long
voyages and in stormy weather, and air escapes into
the flues at the after end of the bars, whereby the
efficacy of the boiler is diminished. Where the bars
are very long, it will generally be found that an increased supply of steam and a diminished consumption
of coal will be the consequence of shortening them;
and the bars should always lie with a considerable in
clination, to facilitate the distribution of the fuel over
the after part of the furnace.  When there are two
lengths of bars in the furnace, it is expedient to make
the central cross-bar for bearing up the ends double,
and to leave a space between the ends of the bars so
that the ashes may fall through between them.  The
space thus left enables the bars to expand without injury on the application of heat; whereas, without some
such provision, the bars are very liable to get burned




THE STEAM ENGINE.              129
out by bending up in the centre, or at the ends-as they
must do if the elongation of the bars on the application
of heat be prevented; and this must be the effect of
permitting the spaces at the ends of the bars to be
filled up with ashes. At each end of each bed of bars
it is expedient to leave a space which the ashes cannot
fill up so as to cause the bars to jam; and care must be
taken that the heels of the bars do not come against
any of the furnace bearers, whereby the room left at
the end of the bars to permit the expansion would be
rendered of no avail.  The furnace bridges of marine
boilers are either made of fire-brick or of plate iron
containing water: in the case of water bridges, the
top part of the bridge should be made with a large
amount of slant, so as to enable the steam to escape
freely.;  but, notwithstanding  this  precaution, the
plates of water bridges are apt to. crack at the bend,
so that fire-brick bridges appear on the whole to be
preferable.  In shallow  furnaces the bridges often
come too near the furnace top to enable a man to pass
over them, and it will save expense if, in such bridges,
the upper portion is constructed of two or three fireblocks, which may be lifted off where a person requires
to enter the flues to sweep or repair them, whereby
the perpetual demolition and reconstruction of the
upper part of the bridge will be prevented.  The
furnaces are shallowest in boilers with a double tier
of furnaces one above the other; but such boilers are
now little used: the steam  of the lower furnace appears to be condensed to a certain extent in coming
into contact with the iron of the superior ash-pit, and
it is difficult, in such shallow furnaces as the plan in



130               A CATECHISM OF
volves, to  give the bars a sufficient inclination to
enable the fuel to pass on to the after end. The flue
of flue boilers generally contracts in area as it approaches the chimney; and it is a common practice to
place a hanging bridge, consisting of a plate of iron
descending a certain distance into the flue, at that
part of the flue where it enters the chimney, whereby
the stratum of hot air which occupies the highest part
of the flue is kept in protracted contact with the boiler,
and the cooler air occupying the lower part of the flue
is that which alone escapes.  The practice of introducing a hanging bridge is a beneficial one in the
case of some boilers, but is not applicable universally,
as boilers with a small calorimeter cannot be further
contracted in the flue without a diminution in their
evaporating  power.  In  tubular boilers a hanging
bridge is no; applicable; but in some cases a perforated
plate is placed against the ends of the tubes, which,
by suitable connections, is made to operate as a sliding
damper, which partially or totally closes up the end of
every tube, and, at other times, a damper constructed
in the manner of a Venetian blind is employed in the
same situation.  These varieties of damper, however,
have only yet been used in locomotive boilers, though
applicable to tubular boilers of every description.  It
is an advantage that there should be a division between the tubes pertaining to each furnace of a marine
tubular boiler, so that the smoke of each furnace may
be kept apart from the smoke of the furnace adjoining
it until the smoke of both enters the chimney, as by,this arrangement a furnace only will be rendered inoperative in cleaning the fires instead of a boiler, and




THE STEAM ENGINE.                131
the tubes belonging to one furnace may be swept if
necessary at sea without interfering injuriously with
the action of the rest: the connection between the
tubes of the inactive furnace and the chimney would
obviously require to be closed in such a case, either by
a damper or otherwise. The sides of the internal furnaces or flues in all boilers should be so constructed
that the steam may readily escape from their surfaces,
with which view it is expedient to make the bottom of
the flue somewhat wider than the top, or slightly
conical in the cross section, and  the upper plates
should always be overlapped by the plates beneath, so
that the steam cannot be retained in the overlap, but
escapes as soon as it is generated.  If the sides of the
furnace be made high and perfectly vertical, they will
speedily be buckled and cracked by the heat, as a film
of steam in such a case will remain in contact with the
iron, which will prevent the access of the water, and
the iron of the boiler will be injured.by the high temperature it must in that case acquire. To moderate
the intensity of the heat acting upon the furnace sides,
it is expedient to bring the outside fire-bars into close
contact with the sides of the furnace, so as to prevent
the entrance of air through the fire in that situation,
by which the intensity of the heat would be increased.
The tube plate nearest the furnace in tubular boilers
should also be so inclined as to facilitate the escape of
the steam; and the short bent plate or flange of the
tube plate, connecting the tube plate with the top of
the furnace, should be made with a gradual bend, as
if the bend be sudden the iron will be apt to crack or
burn away from  the concretion of salt.  Where the




132              A CATECHISM OF
furnace mouths are contracted by bending in the sides
and top of the furnace, as is the general practice, the
bends should be gradual, as salt is apt to accumulate
in the pockets made by a sudden bend, and the plates
will then burn into holes. It is very expedient that
sufficient space should be left between the furnace and
the tubes in all tubular boilers to permit a boy to go
in to clear away any scale that may have formed, and
to hold on the rivets in the event of repair being
wanted; and it is also expedient that a vertical row of
tubes should be left out opposite to each water space
to allow the ascent of the steam and the descent of the
water, as it has been found that the removal of the
tubes in that position, even in a boiler with deficient
heating surface, has increased the production of steam,
and diminished the consumption of fuel.  The tubes
should all be kept in the same vertical line, so as to
permit the introduction of an instrument to scrape
them, but they should be zig-zagged in the horizontal
line, whereby a greater strength of metal will be
obtained around the holes in the tube plates.
144. Q.-In what manner is the tubing and staying
of boilers performed?
A.-The tubes of marine boilers are generally iron
tubes, 3 inches in diameter, and between 6 and 7 ft.
long, but sometimes brass tubes of similar dimensions
are employed. When brass tubes are employed, the
use of ferules driven into the ends of the tubes appears
to be indispensable to keep them tight; but when the
tubes are of malleable iron, of the thickness of Russell's
boiler tubes, they may be made tight merely by driving
them firmly into the tube plates. The holes in the




THE STEAM ENGINE.              133
tube plate next the front of the boiler are made about
one-sixteenth of an inch larger in diameter than the
holes in the other tube plate, and the holes upon the
outer surfaces of both tube plates are slightly countersunk.  One end of each tube is enlarged about a sixteenth of an inch to fit the enlarged holes in one of the
plates; both ends may then be turned slightly in the
lathe to make them  smooth and uniform in diameter
and length, and the whole of the tubes are then to be
all driven through both tube plates from the front of
the boiler —the precaution, however, being taken to
drive them in gently at first with a light hand-hammer,
until the whole of the tubes have been inserted to an
equal depth, and then they may be driven up by degrees
with a heavy hammer, whereby any distortion of the
holes from unequal driving will be prevented. Finally,
the ends of the tubes should be riveted up so as to fill
the countersink; the tubes should be left a little longer
than the distance between the outer surfaces of the
tube plates, so that the countersink at the ends may
be filled by staving up the end of the tube rather than
by riveting it over, and the staving will be best accomplished by means of a mandril with a collar upon
It, which is driven into the tube so that the collar rests
upon the end of the tube to be riveted.  It appears
expedient in all cases where ferules are not used, that
some of the tubes should be screwed at the ends, so as
to serve as stays if the riveting at the tube ends happens
to be burned away, and also to act as abutments to the
riveted tube.  To prevent leakage through the thread
of the screw, if the plan of screwing some of the tubes
be adopted, it will be expedient to let the screwed ends
12




134              A CATECHISM OF
project about half an inch, and to put thin nuts upon
them with a little white lead interposed. If the tubes
are long, their expansion when the boiler is being
blown off will be apt to start them at the ends, unless
very securely fixed, and it is impossible to prevent
brass tubes of large diameter and proportionate length
from being started at the ends, even when secured by
ferules; but the brass tubes commonly employed are
so small as to be susceptible of sufficient compression
endways by the adhesion due to the ferules to compensate for the expansion, whereby they are prevented
from starting at the ends. In some of the early marine
boilers fitted with brass tubes, a galvanic action at the
ends of the tubes was found to take place, and the iron
of the tube plates was wasted away in consequence,
with rapidity; but further experience proved the injury to be attributable chiefly to imperfect fitting,
whereby a leakage was caused that induced oxidation:
and where the tubes were well fitted, any injurious
action at the ends of the tubes was found to cease.
When the pressure of steam within the boiler is considerable, the boiler must be very securely stayed: the
top and bottom, and also the sides of the boiler must
be stayed to one another, and it will not be sufficient
to stay the top of the boiler to the top of the furnaces,
and the bottom of the boiler to the bottom of the furnaces; for if a furnace changed its form, as it would be
likely to do in such circumstances, the stays connecting it to the top and bottom of the boiler would be of
very little utility in preventing the boiler from bursting. If the pressure of steam be 20 lbs. on the square
inch, which is a very common pressure in tubular




THE STEAM ENGINE.                135
boilers, there will be a pressure of 2880 lbs. on every
square foot of flat surface, so that if the strain upon
the stays is not to exceed 3000 lbs. oi the square
inch of section, there must be nearly a square inch of
sectional area of stay for every square foot of flat surface on the top and bottom, sides, and ends of the
boiler. This very much exceeds the proportion usually
adopted, and in scarcely any instance are boilers stayed
sufficiently to be safe when the shell is composed of
flat surfaces. The furnaces should be stayed together
with bolts of the best scrap iron 1 inch in diameter,
tapped through both plates of the water space with
thin nuts in each furnace; and it is expedient to make
the row of stays running horizontally near the level of
the bars, sufficiently low to come beneath the top of
the bars, so as to be shielded from the action of the
fire, with which view they should follow the inclination of the bars.  The row of stays between the level
of the bars and the top of the furnace should be as
near the top of the furnace as will consist with the
functions they have to perform, so as to be removed as
far as possible from the action of the heat; and to
support the furnace, top cross-bars may either be
adopted, to which the top is secured with bolts as in
the case of locomotives, or stays tapped into the furnace top with a thin nut beneath may be carried to
the top of the boiler; but very little dependance can
be put in such stays as stays for keeping down the top
of the boiler, and the top of the boiler must therefore
be stayed nearly as much as if the stays connecting it
with the furnace crowns did not exist.   The large
rivets passing through thimbles, sometimes used as




136                A CATECHISM OF
stays for water spaces or boiler shells, are objectionable; as fiom  the great amount of hammering such
rivets have to receive to form  the heads, the iron
becomes crystalline, so that the heads are liable to
come off, and indeed sometimes fly off in the act of
being formed.  If such a fracture occurs between the
boilers after they are seated in their place, or in any
position not accessible from the outside, it will in
general be necessary to empty the faulty boiler, and
repair the defect from  the inside.   The stays where
the sides of the boiler are flat and the pressure of the
steam is from 20 to 30 lbs., should be pitched about a
foot or 18 inches asunder; and in the wake of the tubes
where stays cannot be carried across to connect the
boiler sides, angle iron ribs, like the ribs of a ship,
should be riveted to the interior of the boiler, and
stays of greater strength than the rest should pass
across above and below the tubes, to which the angle
irons would communicate the strain.  The whole of
the long stays within a boiler should be firmly riveted
to the shell, as if built with and forming a part of it;
as by the common method of fixing them in by means
of cutters, the decay or accidental detachment of a pin
or cutter may endanger the safety of the boiler.
Wherever a large perforation in the shell of any circular boiler occurs, a sufficient number of stays should
be put across it to maintain the original strength; and
where stays are intercepted by the root of the funnel,
short stays in continuation of them should be placed
inside. The flues of all flue boilers diminish in their
calorimeter as they approach the chimney: some very
satisfactory boilers have been made by allowing a pro



THE STEAM ENGINE.               137
portion of 0'6 of a square foot of fire grate per nominal
horse power, and making the sectional area of the flue
at the largest part 1th of the area of fire grate, and at
the smallest part where it enters the chimney -Tth of
the area of the fire grate; but in some of the boilers
proportioned on this plan the maximum sectional area
is only 7g. or -8, according to the purposes of the
boiler.  These proportions are retained whether the
boiler is flue or tubular, and from 14 to 16 square feet
of tube surface is allowed per nominal horse power;
but such boilers although they may give abundance of
steam are generally needlessly bulky, and the method
of fixing the proportions does not appear so eligible as
that previously suggested.  In sea-going steamers the
funnel plates are usually about 9 ft. long and 13ths
thick, and where different flues or boilers have their
debouch in the same chimney, it is expedient to run
divsion plates up the chimney for a considerable distance to keep the draughts distinct.  The dampers
should not be in the chimney, but at the end of the
boiler flue, so that they may be available for use if the
funnel by accident be carried away.  The waste steam
pipe should be of the same height as the funnel, so as to
carry the waste steam clear of it, for if the waste steam
strikes the funnel it will wear the iron into holes; and
the waste steam pipe should be made at the bottom with
a faucet joint, to prevent the working of the funnel
when the vessel rolls from breaking the pipe at the neck.
There should be two hoops round the funnel for the attachment of the funnel shrouds, instead of one, so that
the funnel may not be carried overboard if one hoop
breaks, or if the funnel breaks at the upper hoop from
12*




138              A CATECHISM OF
the corrosive action of the waste steam, as sometimes
happens.  The deck over the steam  chest should be
formed of an iron plate supported by angle iron beams,
and there should be a high angle iron cooming round the
hole in the deck through which the chimney ascends,
to prevent any water upon the deck from  leaking
down upon the boiler. Around the lower part of the
funnel there should be a sheet-iron casing to prevent
any inconvenient dispersion of heat in that situation;
and another short piece of casing, of a somewhat
larger diameter and riveted to the chimney, should
descend over the first casing, so as to prevent the rain
or spray which may beat against the chimney from
being poured down within the casing upon the top of
the boiler.  The pipe for conducting away the waste
water from the top of the safety valve should lead
overboard, and not into the bilge of the ship, as inconvenience arises from the steam occasionally passing
through it, if it has its termination in the engine room.
The man-hole and mud-hole doors, unless put on from
the outside like a cylinder cover with a great number
of bolts, should be put on from the inside with cross
bars on the outside; and the bolts should be strong,
and have coarse threads and square nuts so that the
threads may not be overrun, nor the nuts become
round by the unskilful manipulations of the firemen,
by whom these doors are removed or replaced.   If
from any imperfection in the roof of a furnace or flue
a patch requires to be put upon it, it will be better to
let the patch be applied upon the upper rather than
upon the lower surface of the plate; as if applied
within the furnace a recess will be formed for the




THE STEAM ENGINE.                 139
lodgment of deposit, which will prevent the rapid
transmission of the heat in that part, and the iron will
be very liable to be again burned away.  A crack in
a plate may be closed by boring holes in the direction
of the crack, and inserting rivets with large heads, so
as to cover up the imperfection.  If the top of the
furnace be bent down, from  the boiler having been
accidentally allowed to get short of water, it may be
set up again by a screw-jack-a fire of wood having
been previously made beneath the injured plate; but
it will in general be nearly as expeditious a course toremove the plate and introduce a new one, and the
result will be more satisfactory.
145.  Q.-Is much inconvenience experienced in
marine boilers  from  saline  incrustations upon  the
flues?
A.-Incrustation in  boilers  at one time caused
much more perplexity than it does at present, as it
was supposed that in some seas it was impossible to
prevent the boilers of a steamer from becoming salted
up: but it has now  been satisfactorily ascertained
that there is very little difference in the saltness of
different seas, and that, however salt the water may
be, the boiler will be preserved from  any injurious
amount of incrustation by blowing off, as it is called,
very frequently, or by permitting a considerable portion of the super-salted water to escape at short intervals into the sea.  Sea water contains about -Ld its
weight of salt, and in the open air it boils at the temperature of 213-2~; if the proportion of salt be increased to -A-Zds of the weight of the water, the boiling
point will rise to 214'4~; with -3-ds of salt the boiling
8




140                A CATECHISM OF
point will be 215'5~; ~-ds, 216-7~;  -ds, 217 9~;
6 ds, 219~; -7ds, 22020o; 3ds, 221 4~; -ds, 222-5~;
33          33             33             33
10ds, 2237~; 311ds, 2249~; and  2ds, which is the
-_'           33                   33
point of saturation, 226~.  In  a steam  boiler the
boiling points of water containing these proportions
of salt must be higher, as the elevation of temperature
due to the pressure of the steam  has to be added to
that due to the saltness of the water: the temperature
of steam at the atmospheric pressure being 212~, its
temperature at a pressure of 15 lbs. per square inch
will be 250~; and adding to this 4-7~ as the increased
temperature due to the saltness of the water when it
contains Ads of salt, we have 254-7~ as the temperature  of the water in the boiler, when it contains
-4ds of salt and the pressure of the steam is 15 lbs.
on the square inch.   It is found by experience that
when the concentration of the salt water in a boiler is
prevented from exceeding that point at which it contains -4-ds its weight of salt, no injurious incrustation
will take place; and as sea water contains only -J-d of
its weight of salt, it is clear that it must be reduced
by evaporation to one-fourth of its bulk before it can
contain  -ds of salt;  or, in other words, a boiler
must blow out into the sea one-fourth of the water it
receives as feed, in order to prevent the water from
rising above  — ds of concentration.  Taking the latent
heat of steam at 1000~ at the temperature of 2120,
and reckoning the sum of the latent and sensible heats
as forming  a constant quantity, the latent heat of
steam at the temperature of 250~ will be 962~, and the
total heat of the steam will be 1212~ in the case of
fresh water; but as the feed water is sent into the




THE STEAM ENGINE.                141
boiler at the temperature of 100~, the accession of
heat it receives from  the fuel will be 1112~ in the
case of fresh water, or 1112~ increased by 3'98~ in the
case of containing -Ads of salt-the 3 98~ being the
4'70 increase of temperature due to the presence of
-4-ds of salt, multiplied by 0 847, the specific heat of
steam.  This makes the total accession of heat received by the steam in the boiler equal to 1115'98~,
or say 111 6, which multiplied by 3, as 3 parts of the
water are raised into steam, gives us 3348~ for the
heat in the steam, while the accession of heat received
in the boiler by the 1 part of residual brine will be
154'7~, multiplied by 0'85, the specific heat of the
brine, or 130'495~; and 3348~ divided by 130-495~ is
about -lth.  It appears, therefore, that by blowing
off the boiler to such an extent, that the saltness shall
not rise above what answers to -A-ds of salt, about
216th of the heat is blown into the sea: this is but a
small proportion, and as there will be a greater waste
of heat, if from the existence of scale upon the flues
the heat can be only imperfectly transmitted to the
water, there cannot be even an economy of fuel in
niggard blowing off, while it involves the introduction
of other evils. To save a part of the heat lost by the
operation of blowing off, the hot brine is sometimes
passed through a number of small tubes surrounded
by the feed water; but there is scarcely any gain
from the use of such apparatus, and the tubes are apt
to become choked up, whereby the safety of the boiler
may be endangered by the injurious concentration of
its contents.  Pumps, worked by the engine for the
extraction of the brine, are generally used in con



142               A CATECHISM OF
nection with the small tubes for the extraction of the
heat from the super-salted water; and if the tubes
become choked, the pumps will cease to eject the
w:-ter, while the engineer may consider them  to be
all the while in operation.   The general mode of
blowing off the boiler is to allow the water to rise
gradually for an hour or two above the lowest working level, and then to open the cock communicating
with the sea, and keep it open until the surface of the
water within the boiler has fallen several inches; but
in some cases a cock of smaller size is allowed to run
water continuously, and in other cases brine pumps
are used, as already mentioned: but in every case in
which the super-salted water is discharged from the
boiler in a continuous stream, a hydrometer or salt
gauge of some convenient construction should be applied to the boiler, so that the density of the water
may at all times be visible. Blowing off from a point
near the surface of the water is more beneficial than
blowing off from the bottom of the boiler.  Solid partides of any kind, it is well known, if introduced into
boiling water, will lower the boiling point in a slight
degree, and the steam will chiefly be generated on the
surface of the particles, and indeed will have the appearance of coming out of them: if the particles be
small, the steam generated beneath and around them
will balloon them to the surface of the water, where
the steam  will be liberated and the particles will
descend; and the impalpable particles in a marine
boiler, which by their subsidence upon the flues con.
crete into scale, are carried in the first instance to the
surface of the water, so that if they be caught there,




THE STEAM ENGINE.                143
and ejected from the boiler, the formation of scale will
be prevented.  Advantage is taken of this property
in Lamb's Scale Preventer, which is substantially a
contrivance for blowing off from  the surface of the
water that in practice is found to be very effectual;
but a float in connection with a valve at the mouth of
the discharging  pipe is there introduced, so as to
regulate  the  quantity  of water blown out by the
height of the water level, or by the extent of opening
given to the feed cock; the operation, however, of
the contrivance would be much the same if the float
were dispensed  with.   In some  boilers  sheet-iron
vessels called sediment collectors are employed, which
collect into them  the impalpable matter which in
Lamb's apparatus is ejected from the boiler at once.
One of these vessels, of about the size and shape of a
loaf of sugar, is put into each boiler, with the apex
of the cone turned  downwards into a pipe leading
overboard, for conducting the sediment away from  the
boiler.  The base of the cone stands some distance
above the water line, and in its side conical slits are
cut, so as to establish a free communication between
the water within the conical vessel and the water outside it. The particles of stony matter, which are ballooned to the surface by the steam in every other part
of the boiler, subside within the cone, where no steam
is generated, and the water is consequently tranquil;
and the deposit is discharged overboard at intervals
by means of the cock communicating with the sea.
By blowing off from  the surface of the water, the
requisite cleansing action is obtained with less waste
of heat; and where the water is muddy, the foam




144               A CATECHISM OF
upon the surface of the water is ejected from  the
boiler-thereby removing one of the chief causes of
priming.  As it is very desirable that boilers acting
on the principal of continuously blowing off a smal
stream of the super-salted water, should be provided
with a salt gauge which will give immediate notice of
any interruption of the operation, various contrivances
have been devised for this purpose, the most of which
operate on the principal of a hydrometer; but perhaps
a more satisfactory principal would be that of a differential steam gauge, which shall indicate the difference of pressure between the steam in the boiler
and the steam  of a small quantity of fresh water
enclosed in a suitable vessel, and immerged in the
water of the boiler.  If blowing off be sufficiently
practiced, the scale upon the flues will never be much
thicker than a sheet of writing paper, and no excuse
should be accepted from  engineers for permitting a
boiler to be damaged by the accumulation of calcareous deposit.  Flue boilers generally require to be
blown off once every watch, or once in the two hours;
but tubular boilers may require to be blown off once
every twenty minutes, and such an amount of blowing
off should in every case be adopted, as will effectually
prevent any injurious amount of incrustation. Even
with judicious management, however, the boilers may
sometimes require to be scaled, and the best method
of performing this operation appears to be the following:-Lay a train of shavings along the flues,
open the safety valve to prevent the existence of any
pressure within the boiler, and light the train of
shavings, which, by expanding rapidly the metal of




THE STEAM ENGINE.               145
the flues, while the scale from its imperfect conducting power can only expand slowly, will crack off
the scale; by washing down the flues with a hose the
scale will be carried to the bottom of the boiler, or
issue with the water from the mud-hole doors. This
method of scaling must be practiced only by the
engineer himself, and must not be intrusted to the
firemen, who in their ignorance might damage the
boiler by overheating the plates.  It is only where
the incrustation upon the flues is considerable, that
this method of removing it need be practiced; in
other cases the scale may be chipped off by a hatchetfaced hammer, and the flues may then be washed
down with the hose in the manner before described.
In tubular boilers a good deal of care is required to
prevent the ends of the tubes next the furnace from
becoming coated with scale. Even when the boiler is
tolerably clean in other places the scale will collect
here; and in many cases where the amount of blowing
off previously found to suffice for flue boilers has been
adopted, an incrustation five-eighths of an inch in
thickness has formed in twelve months round the
furnace ends of the tubes, and the stony husks enveloping them have actually grown together in some
parts so as totally to exclude the water.  When a
boiler gets into this state the whole of the tubes must
be pulled out, which may be done by a Spanish windlass combined with a pair of blocks; and three men
when thus provided will be able to draw out from 50
to 70 tubes per day-those tubes with the thickest and
firmest incrustations being of course the most difficult to
remove.  The act of drawing out the tubes removes
13




146               A CATECHISM OF
the incrustation; but the tubes should afterwards be
scraped  by drawing them  backwards and forwards
between two old files, fixed in a vice, in the form  of
the letter V. The ends of the tubes should then be
heated and dressed with the hammer, and plunged
while at a blood heat into a bed of sawdust to make
them cool soft, so that they may be riveted again with
facility. A few of the tubes will be so far damaged
at the ends by the act of drawing them  out, as to be
too short for reinsertion: this result might be to a
considerable  extent obviated, by setting  the  tube
plates at different angles, so that the several horizontal rows of tubes would not be originally of the
same length, and the damaged tubes of the long rows
would serve to replace the short ones: but the practice would be attended  with other inconveniences.
Muriatic  acid, or muriate of ammonia, commonly
called sal-ammoniac, introduced into a boiler, prevents
scale to a great extent; but it is liable to corrode the
boiler internally, and also to damage the engine, by
being carried over with the steam; and the use of such
intermixtures does not appear to  be  necessary, if
blov-ing off from the surface of the water is largely
practiced.  The soot which collects in the inside of
the tubes of tubular boilers is removed by means of a
brush, like a large bottle brush; and the carbonaceous
scale, which remains adhering to the interior of the
tubes, is removed by a circular scraper.  Ferules in
the tubes interfere with the action of this scraper, and
in the case of iron tubes ferules are now generally
discarded; but it will sometimes be necessary to use
ferules for iron tubes, where the tubes have beea




THE STEAM ENGINE.                147
drawn and reinserted, as it may be difficult to refix the
tubes without such an auxiliary. Tubes one-tenth of
an inch in thickness are too thin: one-eighth of an inch
is a better thickness, and such tubes will better dispense with the use of ferules, and will not so soon wear
into holes
146. Q.-Will you explain the nature and cause of
priming?
A.-Priming is a violent agitation of the water
within the boiler, in consequence of which a large
quantity of water passes off with the steam  in the
shape of froth or spray.  Such a result is injurious,
both as regards the efficacy of the engine, and the
safety of the engine and boiler; for the large volume
of hot water carried by the steam  into the condenser
impairs the vacuum, and throws a great load upon the
air pump, which diminishes the speed and available
power of the engine; and the existence of water within the cylinder, unless there be safety valves upon the
cylinder to permit its escape, will very probably cause
some part of the machinery to break, by suddenly
arresting the motion of the piston when it meets the
surface of the water,-the slide valve being closed to
the condenser before the termination of the stroke, in
all engines with lap upon the valves, so that the water
within the cylinder is prevented from escaping in that
direction.  At the same time the boiler is emptied of
its water too rapidly for the feed pump to be able to
maintain the supply, and the flues are in danger of
being burnt from a deficiency of water above them.
The causes of priming are an insufficient amount of
steam  room, an inadequate area of water level, an in



148               A CATECHISM OF
sufficient width between the flues or tubes for the
ascent of the steam and the descent of water to supply
the vacuity the steam occasions, and the use of dirty
water in the boiler.  New  boilers prime more than
old boilers; and steamers entering rivers from the sea
are more addicted to priming than if sea or river
water had alone been used in the boilers-probably
from the boiling point of salt water being higher than
that of fresh, whereby the salt water acts like so much
molten metal in raising the fresh water into steam.
Opening the safety valve suddenly may make a boiler
prime, and if the safety valve be situated near the
mouth of the steam pipe, the spray or foam thus created
may be mingled with the steam passing into the engine,
and materially diminish its effective power; but if the
safety valve be situated at a distance from the mouth
of the steam pipe, the quantity of foam or spray passing into the engine may be diminished by opening the
safety valve; and in locomotives, therefore, it is found
beneficial to have a safety valve on the barrel of the
boiler at a point remote from the steam chest, by partially opening which, any priming in that part of the
boiler adjacent to the steam chest is checked, and a
purer steam than before passes to the engine. When
a boiler primes, the engineer generally  closes the
throttle valve partially, turns off the injection water,
and opens the furnace doors, whereby the generation
of steam is checked, and a less violent ebullition in
the boiler suffices.  Where the priming arises from an
insufficient amount of steam room, it may be mitigated
by putting a higher pressure upon the boiler, and
working more expansively, or by the interposition of




THE STEAM ENGINE.                 149
a perforated plate between the boiler and the steam
chest, which breaks the ascending water and liberates
the steam.  In some cases, however, it may be necessary to set a second steam  chest on the top of the
existing one, and it will be preferable to establish a
communication with this new chamber by means of a
number of small holes, bored through the iron plate
of the boiler, rather than by a single large orifice.
Where priming  arises from  the existence of dirty
water in the boiler, the evil may be remedied by the
use of collecting vessels, or by blowing off largely
from the surface; and where it arises from an insufficient area of water level, or an insufficient width between the flues for the free ascent of the steam and
the descent of the superincumbent water, the evil may
be abated by the addition of circulating pipes in some
part of the boiler which will allow the water to descend
freely to the place from  whence the steam  rises, the
width of the water spaces being virtually increased by
restricting their function to the transmission of a current of steam and water to the surface. It is desirable,
however, to arrange the heating surface in such a way
that the feed water entering the boiler at its lowest
point is heated gradually as it ascends, until towards
the superior part of the flues it is raised gradually
into steam; and in boilers designed upon this principle,
there will be less need for any special provision to
enable currents to rise or descend.  The steam pipe
proceeding to the engine should obviously be attached
to the highest point of the steam chest, in boilers of
every construction.
13*




150               A CATECHISM OF
147. Q.-What is the chief cause of boiler explo.
sions?
A.-The chief cause of boiler explosions is, undoubtedly, too great a pressure of steam, or an insuffi.
cient strength of boiler; but many explosions have
also arisen from the flues having been suffered to become red hot.  If the safety valve of a boiler be
accidentally jammed, or if the plates or stays be much
worn by corrosion while a high pressure of steam is
nevertheless maintained, the boiler necessarily bursts;
and if from an insufficiency of water in the boiler, or
from any other cause, the flues become highly heated,
they may be forced down by the pressure of the steam,
and a partial explosion may be the result.  The worst
explosion is where the shell of the boiler bursts, but
the collapse of a furnace or flue is also very disastrous
generally to the persons in the engine room, and sometimes the shell bursts and the flues collapse at the
same time; for if the flues get red hot, and water be
thrown upon them  either by the feed pump or otherwise, the generation of steam may be too rapid for the
safety valve to permit its escape with sufficient facility,
and the shell of the boiler may in consequence be rent
asunder.  Sometimes the iron of the flues becomes
highly heated  in consequence of the improper configuration of the parts, which by retaining the steam
in contact with the metal, prevents the access of the
water: the bottoms of large flues upon which the
flame beats down, are very liable to injury from this
cause, and the iron of flues thus acted upon may be
so softened that the flues will collapse upwards with
the pressure of the steam.  The flues of boilers may




THt STEAM ENGINE.                  151
also become red hot in some parts from the attachment of scale, which from  its imperfect conducting
power will cause the iron to be unduly heated; and if
the scale be accidentally detached, a partial explosion
may occur in consequence.  It is found, however, that
a sudden disengagement of steam does not immediately
follow the contact of water with the hot metal, for
water thrown upon red hot iron is not immediately
converted into steam, but assumes the spheroidal form,
and rolls about in globules over the surface.  These
globules, however high the temperature of the metal
may be on which they are placed, never rise above
the temperature of 205~, and give off but very little
steam; but if the temperature of the metal be lowered,
the water ceases to retain the spheroidal form, and
comes into intimate contact with the metal, whereby
a rapid disengagement of steam takes place.  If water
be poured into a very hot copper flask, the flask may
be corked up, as there will be scarce any steam produced so long as the high temperature is maintained;
but so soon as the temperature is suffered to fall below
350~ or 400~, the spheroidal condition being no longer
maintainable, eteam is generated with rapidity, and
the cork will be projected from the mouth of the flask
with great force.  Oae useful precaution against the
explosion of boilers from  too great an internal pressure, consists in the application of a steam gauge to
each boiler, which will make the existence of any undue pressure in any of the boilers immediately visible;
and every boiler should have a safety valve of its own,
the passage leading to which should have no connection
with the passage leading to any of the stop valves




152              A CATECHISM OF
used to cut off the connection between the boilers; so
that the action of the safety valve may be made independent of the action of the stop valve.  In some cases
stop valves have jammed, or have been carried from
their seats into the mouth of the pipe communicating
between them, and the action of the safety valves
should be rendered independent of all such accidents.
Safety valves, themselves, sometimes stick fast from
corrosion, from the spindles becoming bent, from a
distortion of the boiler top with a high pressure, in
consequence of which the spindles become jammed in
the guides, and from  various other causes which it
would be tedious to enumerate; but the inaction of
the safety valve is at once indicated by the steam
gauge, and, when discovered, the blow-through valves
of the engine and blow-off cocks of the boiler should
at once be opened, and the fires raked out. A cone
in the ball of the waste steam pipe to send back the
water carried upwards by the steam, should never be
inserted; as in some cases this cone has become loose,
and closed up the mouth of the waste steam  pipe,
whereby the safety valves being rendered inoperative
the boiler was in danger of bursting. If the water be
carried out of the boiler so rapidly by priming that
the level of the water cannot be maintained, and the
flues or furnaces are in danger of becoming red hot,
the best plan is to open every furnace door and throw
in a few buckets full of water upon the fire, taking
care to stand sufficiently to the one side to avoid being
scalded by the rush of steam from the furnace. There
is no time to begin drawing the fires in such an emergency, and by this treatment the fires, though not




THE STEAM ENGINE.                153
altogether extinguished, will be rendered incapable of
doing harm.  If the flues be already red hot, on no
account must cold water be suffered to enter the boiler,
but the heat should be maintained in the furnaces, and
the blow-off cocks be opened, or the mud-hole doors
loosened, so as to let all the water escape; but at the
same time the pressure must be kept quite low in the
boiler, so that there will be no danger of the hot flues
collapsing with the pressure of the steam. Plugs of
fusible metal were at one time in much repute as a
precaution against explosion, the metal being so compounded that it melted with the heat of high-pressure
steam; but the device, though ingenious, has not been
found of any utility in practice. The basis of fusible
metal is mercury, and it is found that the compound
is not homogeneous, and that the mercury is forced
by the pressure of the steam out of the interstices of
the metal combined with it, leaving a porous metal
which is not easily fusible, and which is therefore unable to perform its intended function.  In locomotii es,
however, and also in some other boilers, a lead rivet
is inserted with advantage in the crown of the fire
box, which is melted out if the water becomes too low,
and thus gives notice of the danger.  All boilers in
actual use should be proved at least once a year by
forcing water into them by the hand feed-pump until
the safety valve is lifted, which should be loaded with
at least twice the working pressure for the occasion.
If a boiler will not stand this test it is not safe, and
either its strength should be increased or the working
pressure should be diminished.




154                A CATECHISM OF
148. Q.-Will you state something of the peculiarities of structure of locomotive boilers?
A.-Locomotive boilers consist of three portionsthe barrel containing the tubes, the fire box, and the
smoke box; of which the barrel, smoke box, and
external fire box are always of iron, but the internal fire box is generally made of copper, though
sometimes also it is made of iron.  The tubes are
sometimes of iron, but generally of brass fixed in by
ferules. The whole of the iron plates of a locomotive
boiler which are subjected to the pressure of steam
should be Lowmoor or Bowling plates of the best
quality; and the copper should  be coarse grained,
rather than rich or soft, and be perfectly free from
irregularities of structure and lamination.  The thickness of the plates composing the barrel of the boiler
varies generally from  five-sixteenths to three-eighths
of an inch, and the plates should run in the direction
of the circumference, so that the fibres of the iron
may be in the direction of the strain.  The diameter
of the barrel commonly varies from  3 ft. to 3 ft.
6 inches; the diameter of the rivets should be from
eleven-sixteenths to three-fourths of an  inch, and
the pitch of the rivets or distance between their centres should be from  seventeen-eighths to 2 inches.
The thickness of the plates composing the external
fire box is in general three-eighths of an inch if the
fire box is circular, and from three-eighths to one-half
inch if the fire box is square; and the thickness of the
internal fire box is in most cases seven-sixteenths if
copper, and from three-eighths to seven-sixteenths of
an inch if of iron.  Circular internal fire boxes, if




THE STEAM ENGINE.               155
made of iron, should be welded rather than riveted, as
the rivet heads are liable to be burnt away by the
action of the fire; and when the fire boxes are square
each side should consist of a single plate, turned over
at the edges with a radius of 3 inches, for the introduction of the rivets.  The space between the external and internal fire boxes forms a water space,
which must be stayed every 4- or 5 inches by means
of copper or iron stay-bolts, screwed through the
outer fire box into the metal of the inner fire box, and
securely riveted within it: iron stay-bolts are as
durable as copper, and their superior tenacity gives
them  an advantage.  The tube plates are generally
made from five-eighths to three-fourths of an inch
thick; but seven-eighths of an inch thick appears to
be preferable, as when the plate is thick the holes will
not be so liable to change their figure during the process of feruling the tubes: the distance between the
tubes should never be made less than three-fourths of
an inch, and the holes should be slightly tapered, so
as to enable the tubes to hold the tube plates together.
The ferules are for the most part made of steel at the
fire box end, and of wrought-iron at the smoke box
end, though ferules of malleable cast-iron have in
some cases been used with  advantage; malleable
cast-iron ferules are almost as easily expanded when
hammered  cold  upon a mandril, as the common
wrought-iron ones are at a working heat.  Springsteel, rolled with a feather-edge, to facilitate its conversion into ferules, is supplied by some of the steel
makers of Sheffield, and it appears expedient to make
use of steel thus prepared when steel ferules are em




156               A CATECHISM OF
ployed.  The roof of the internal fire box, whethel
flat as in Stephenson's engines, or dome-shaped as in
Bury's, requires to be stiffened with cross stay-bars;
but the bars require to be stronger and more numerous
when applied to a flat surface.  The ends of these
stay-bars rest above the vertical sides of the fire box;
and to the stay-bars thus extending across the crown,
the crown is attached at intervals by means of staybolts. There are projecting bosses upon the stay-bars
encircling the bolts at every point where a bolt goes
through, but in the other parts they are kept clear of
the fire box crown, so as to permit the access of water
to the iron; and, with the view of facilitating the ascent of the steam, the bottom of each stay-bar should
be sharpened away in those parts where it does not
touch the boiler. The internal and external fire boxes
are joined together at the bottom by a N shaped iron,
and round the fire-door they are connected by means
of a copper ring 11 in. thick, and 2 in. broad; the
inner fire box being dished sufficiently outwards at
that point, and the outer fire box sufficiently inwards,
to enable a circle of rivets three-fourths of an inch in
diameter passing through the copper ring and the two
thicknesses of iron to make a water-tight joint. To
find the proper length of bar requisite for the formation
of a hoop of any given diameter, add the thickness of
the bar to the required diameter, and the corresponding
circumference in a table of circumferences of circles is
the length of the bar. If the iron be bent edgewise,
the breadth of the bar must be added to the diameter;
for it is the thickness of the bar measured radially that
is to be taken into consideration.  In the tires of




THE STEAM ENGINE.               157
railway wheels, which have a flange on one edge, it is
necessary to add not only the thickness of the tire, but
also two-thirds of the depth of the flange; generally,
however, the tire bars are sent from  the forge so
curved, that the plain edge of the tire is concave, and
the flange edge convex, while the side which is afterwards to be bent into contact with the cylindrical
surface of the wheel is a plane. In this case the addition of the diameter of two-thirds of the depth of
the flange is unnecessary; for the curving of the
flange edge has the effect of increasing the real length
of the bar.  When the tire is thus curved, it is only
necessary to add the thickness of the hoop to the diameter, and then to find the circumference from a table;
or the same result will be obtained by multiplying
the diameter thus increased by the thickness of the
hoop by 3'1416.
149. Q.-Are locomotive boilers provided with a
steam chest?
A.-The upper portion of the external fire box is
usually formed into a steam chest, which is sometimes
dome-shaped, sometimes semi-circular, and sometimes
of a pyramidical form, and from this steam chest the
steam  is conducted away by an internal pipe to the
cylinders; but, in other cases, an independent steam
chest is set upon the barrel of the boiler, consisting of
a plate-iron cylinder, 20 inches in diameter, 2 ft. high,
and three-eighths of an inch thick, with a dome-shaped
top, and with the seam welded and the edge turned
over to form a flange of attachment to the boiler. The
pyramidical dome, of the form employed in Stephenson's locomotives, presents a considerable extent of
flat surface to the pressure of the steam, and this flat
14




158               A CATECHISM OF
surface requires to be very strongly stayed with angle
irons and tension rods; whereas the semi-globular
dome of the kind employed in Bury's engines requires
no staying whatever.  The man-hole, or entrance into
the boiler, consists of a circular or oval aperture, of
about 15 in. diameter, placed in Bury's locomotive at
the apex of the dome, and in Stephenson's upon the
front of the boiler, a few inches below the level of the
rounded part; and the cover of the man-hole in Bury's
engine contains the safety valve seats. In whatever
situation this man-hole is placed, the surfaces of the
ring encircling the hole, and of the internal part of the
door or cover, should be accurately fitted together by
scraping or grinding, so that they need only the interposition of a little red lead to make them quite tight
when screwed together.  Lead or canvas joints, if of
any considerable thickness, will not long withstand the
action of high pressure steam; and the whole of the
joints about a locomotive should be such that they require nothing more than a little paint or putty, or a
ring of wire-gauze smeared with white or red lead, to
make them  perfectly tight.  There must be a mudhole opposite the edge of each water space, if the fire
box be square, to enable the boiler to be easily cleaned
out, and these holes are most conveniently closed by
screwed plugs made slightly taper.  A cock for emptying the boiler is usually fixed at the bottom of the
fire box; and it should be so placed as to be accessible
when the engine is at work, in order that the engine
driver may blow off some water if necessary; but it
must not be in such a position as to send the water
blown off among the machinery, as it might carry sand




THE STEAM ENGINE.                 159
or grit into the bearings, to their manifest injury. To
save the steam  which is formed when the engine is
stationary, a pipe is usually fitted to the boiler, which,
on a cock being turned, conducts the steam into the
water in the tender, whereby the feed water is heated,
and less fuel is subsequently required. This method
of disposing of the surplus steam may be adopted when
the locomotive is descending inclines, or on any occasion where more steam  is produced than the engine
can consume.  The fire bars in locomotives have always been a source of trouble, as, from  the intensity
of the heat in the furnace, they become so hot as to
throw off a scale, and to bend under the weight of the
fuel. The best alleviation of these evils lies in making
the bars deep and thin: 4 inches deep by five-eighths
of an inch thick on the upper side, and three-eighths of
an inch on the under side, are found in practice to be
good dimensions. In some locomotives, a frame carrying
a number of fire bars is made so that it may be dropped
suddenly by loosening a catch; but it is found that
any such mechanism can rarely be long kept in working order, as the molten clinker, by running down between the frame and the boiler, will generally glue the
frame into its place: it is therefore found preferable
to fix the frame, and to lift up the bars by the dart
used by the stoker, when any cause requires the fire
to be withdrawn. The furnace bars of locomotives are
always made of malleable iron; and for every species
of boiler malleable iron bars are to be preferred to
bars of cast-iron, as they are more durable, and may,
from their thinness, be set closer together, whereby the
small coal or coke is saved that would otherwise fall




160               A CATECHISM OF
into the ash-pit.  The ash-box of locomotives is made
of plate-iron a quarter thick: it should not be less
than 10 in. deep, and its bottom should be about 9 in.
above the level of the rails. The chimney of a locomotive is made of plate-iron one-eighth of an inch
thick: it is usually of the same diameter as the cylinder, and must not stand more than 14 ft. high above
the level of the rails.
150. Q.-What is the best diameter for the tubes
of locomotive boilers?
A.-Bury's locomotive with 14 in. cylinders contains
92 tubes of 2 —th in. external diameter, and 10 ft. 6 in.
long; whereas Stephenson's locomotive with  15 in.
cylinders, contains 150 tubes of ]-ths external diameter, and 13 ft. 6 in. long. In Stephenson's boiler, in order
that the part of the tubes next the chimney may be
of any avail for the generation of steam, the draught
has to be very intense, which in its turn involves a
considerable expenditure of power: and it is questionable whether the increased expenditure of power upon
the blast, in Stephenson's long tubed locomotives, is
compensated by the increased generation of steam
consequent upon the extension of the heating surface.
When the tubes are small in diameter they are apt to
become partially choked with pieces of coke, but an
internal diameter of ]1  may be employed without
inconvenience, if the draught be of medium intensity.
The intensity of the draught may easily be diminished
by partially closing the damper in the chimney, and
it may be increased by contracting the orifice of the
blast. A variable blast pipe, the orifice of which may
be enlarged or contracted at pleasure, is now much
used.  There are various devices for this purpose, but




THE STEAM ENGINE.               161
the best appears to be that adopted in Stephenson's
engine, where a conical nozzle is moved up or down
within a blast pipe, which is made somewhat larger
in diameter than the base of the cone, but with a ring
projecting internally, against which the base of the
cone abuts when the nozzle is pushed up.  When the
nozzle stands at the top of the pipe the whole of the
steam has to pass through it, and the intensity of the
blast is increased by the increased velocity thus given
to the steam; whereas when the nozzle is moved downward the steam  escapes through the annular opening
left between the nozzle and the pipe, as well as through
the nozzle itself, and the intensity of the blast is diminished by the enlargement of the opening for the
escape of the steam  thus made available.  In most
locomotives the velocity of the draught is such that it
would require very long tubes to extract the heat
from  the products of combustion, if the heat were
transmitted through the metal of the tubes with only
the same facility as through the iron of ordinary flue
boilers, and if it were required at the same time that
the heat should be as thoroughly extracted.  The
Nile steamer, with engines of 110 nominal horses power
each, and with two boilers having two independent
flues in each, of such dimensions as to make each flue
equivalent to 55 nominal horses power, works at 62
per cent. above the nominal power, so that the actual
evaporative efficacy of each flue would be equivalent
to 89 actual horses power, supposing the engines to
operate without expansion; but as the mean pressure
in the cylinder is somewhat less than the initial pressure, the evaporative efficacy of each flue may be
14*




162               A CATECHISM OF
reckoned equivalent to 80 actual horses power. With
this evaporative power there is a calorimeter of 990
square inches, or 12 3 square inches per actual horse
power; whereas in Stephenson's locomotive with 150
tubes, if the evaporative power be taken at 200 cubic
feet of water in the hour, which makes the engine
equal to 200 actual horses power, and the internal
diameter of the tubes be taken at thirteen eighths of
an inch, the calorimeter per actual horse power will
only be 1-1136 square inches; or, in other words, the
calorimeter in the locomotive boiler will be 11 11 times
less than in the flue boiler for the same power, so that
the draught in the locomotive must be 11 11 times
stronger, and the ratio of the length of the tube to its
diameter 11-11 times greater than in the flue boiler,
supposing the heat to be transmitted with only the
same facility. The flue of the Nile, as stated in the
answer to Question 93, would require to be 35- in. in
diameter, if made of the cylindrical form, and 47- ft.
long: the tubes of a locomotive if -1 thin. in diameter
would only require to be 22 19 in. long with the same
velocity of draught' but as the draught is 11 11 times
faster than in a flue boiler, the tubes ought to be
246 558 inches, or about 201 ft. long according to this
proportion. In practice, however, they are one-third
less than this, which reduces the heating surface from 9
to 6 square feet per actual horse power, and this length
even is found to be inconvenient.  It is greatly preferable therefore to increase the calorimeter, and diminish the intensity of the draught.
151. Q.-You have mentioned the existence of the
steam gauge, the vacuum gauge, the salt gauge, and the




THE STEAM ENGINE.                 163
indicator; what other gauges or instruments are there for
telling the state, or regulating the power, of an engine?
A.-There is the counter for telling the number
of strokes the engine makes, and the dynamometer for
ascertaining the tractive power of steam vessels or
locomotives; then there are the gauge cocks, and glass
tubes, or floats, for telling the height of water in the
boiler; and in pumping engines there is the cataract
for regulating the speed of the engine. The counter
consists of a train of wheel work, so contrived that by
every stroke of the engine, an index hand is moved
forward a certain space, whereby the number of strokes
made by the engine in any given time is accurately
recorded. In most cases the motion is communicated
by means of a detent, attached to some reciprocating
part of the engine, to a ratchet wheel which gives
motion to the other wheels in its slow revolution: but
it is preferable to derive the motion from some revolving part of the engine by means of an endless screw,
as where the ratchet is used the detent will sometimes
fail to carry it round the proper quantity. In the
counter contrived  by Mr. Adie, an endless  screw
works into the rim  of two small wheels situated on
the same axis, but one wheel having a tooth more
than the other, whereby a differential motion is obtained; and the difference in the velocity of the two
wheels, or their motion upon one another, expresses
the number of strokes performed. The endless screw
is attached  to some revolving part of the engine
whereby a rotary motion is imparted to it; and the
wheels into which the screw works hang down from
it like a pendulum, and are kept stationary by the




164               A CATECHISM OF
action of gravity.  The dynamometer employed for
ascertaining the traction upon railways consists of
two flat springs joined together at the ends by links,
and the amount of separation of the springs at the
centre indicates, by means of a suitable hand and dial,
the force of traction.  In screw vessels the forward
thrust of the screw is measured by a dynamometer
constructed on the principle of a weighing machine,
in which a small spring pressure at the index will
balance a very great pressure where the thrust is applied; and in each case the variations of pressure
are recorded by a pencil, on a sheet of paper, carried
forward by suitable mechanism, whereby the mean
thrust is easily ascertained. The tractive force of
paddle wheel steamers is ascertained by a dynamometer fixed on shore, to which the floating vessel is
attached  by a rope.- By means of the glass tubes
affixed to the fronts of boilers the height of the water
within any of the boilers is readily ascertainable, for
the water will stand at the same height in the tube
as in the boiler, with which there is a communication
maintained both at the top and bottom of the tube by
suitable stop-cocks.  The gauge cocks are cocks penetrating the boiler at different heights, and which when
opened tell whether it is water or steam  that exists
at the level at which they are respectively inserted.
The cocks connecting the glass tube with the boiler
should always be so constructed that the tube may be
blown through with the steam, to clear it of any internal concretion that may impair its transparency;
and the construction of the sockets in which the tube
is inserted should be such, that, even when there is




THE STEAM ENGINE.               165
steam in the boiler, a broken tube may be replaced
with facility. It is unsafe to trust to the glass gauges
altogether, as a means of ascertaining the water level,
as sometimes they become choked, and it is necessary,
therefore, to have gauge cocks in addition; but if the
boiler be short of steam, and a partial vacuum be produced within it, the glass gauges become of essential
service, as the gauge cocks will not operate in such
a case, for though opened, instead of steam and water
escaping fiom them, the air will rush into the boiler.
It is expedient to carry a pipe from the lower end of
the glass tube downward into the water of the boiler,
and a pipe from the upper end upward into the steam
in the boiler, so as to prevent the water from boiling
down through the tube, as it might otherwise do, and
prevent the level of the water from being ascertainable.
The average level cf water in the boiler should be
above the centre of the tube, and the lowest of the
gauge cocks should always run water, and the highest
should always blow steam.  The float for telling the
height of water in the boiler is employed only in the
case of land boilers, and its action is like that of a
buoy floating on the surface, which, by means of a light
rod passing vertically through the boiler, shows at
what height the water stands.  The float is usually
formed of stone or iron, and is so counterbalanced as
to make its operation the same as if it were a buoy of
timbei; and it is generally put in connection with the
feed valve, so that in proportion as the float rises, the
supply of feed water is diminished.  The feed water
in land boilers is admitted from a small open cistern,
situated at the top of an upright or stand pipe set




166               A CATECHISM OF
upon the boiler, and in which there is a column of
water sufficiently high to balance the pressure of the
steam.  The water is sent up into the small cistern
by the force pump, and any that does not gain admission to the boiler through the valve in the cistern,
runs to waste by an overflow shoot.  The height of
the water in the stand pipe rises and falls with the
pressure of the steam, and a float is therefore placed
within it to operate upon the damper, which, when
the level of the water rises, from the pressure of the
steam  becoming strong, partially closes the damper,
and thus moderates the intensity of the fire.  The
cataract consists of a small pump-plunger and barrel,
set in a cistern of water, the barrel being furnished
on the one side with a valve opening inwards, through
which the water obtains admission to the pump chamber from  the cistern, and on the other by a cock
through which, if the plunger be forced down, the
water must pass out of the pump chamber.  The engine in the upward stroke of the piston, which is accomplished by the preponderance of weight at the
pump end of the beam, raises up the plunger of the
cataract by means of a small rod, the water entering
readily through the valve already referred to; and
when the engine reaches the top of the stroke, it
liberates the rod by which the plunger has been drawn
up, and the plunger then descends by gravity, forcing
out the water through the cock, the orifice of which
has previously been adjusted, and the plunger in its
descent opens the injection valve, which causes the
engine to make a stroke.  If the cock of the cataract
be shut, it is clear that the plunger cannot descend at




THE STEAM ENGINE.               167
all, and as in that case the injection valve cannot be
opened, the engine must stand still; but if the cock
be slightly opened the plunger will descend slowly,
the injection valve will slowly open, and the engine will
make a gradual stroke as it obtains the water necessary for condensation. The extent to which the cock
is open, therefore, will regulate the speed with which
the engine works, so that by the use of the cataract,
the speed of the engine may be varied to suit the
variations in the quantity of water required to be
lifted from the mine. In some cases an air cylinder,
and in other cases an oil cylinder, is employed instead
of the apparatus just described; but the principle on
which the whole of these contrivances operate is identical, and the only difference is in the detail.
152. Q.-Will you explain the course of procedure
in the erection of a pumping engine, such as is used in
Cornwall?
A.-Having fixed on the proper situation of the
pump in the pit, from  its centre measure out the
distance to the centre of the cylinder, from which set
off all the other dimensions of the house, including the
thickness of the walls, and dig out the whole of the included ground to the depth of the bottom of the cellar,
so that the bottom of the cylinder may stand on a level
with the natural ground of the place, or lower if convenient; for the less the height of the house above
the ground, the firmer it will be. The foundations of
the walls must be laid at least two feet lower than the
bottom  of the cellar, unless the foundation be firm
rock; and care must be taken to leave a small drain
into the pit quite through the lowest part of the




168               A CATECHISM OF
foundation of the lever wall, to let off any water that
may be spilt in the engine house, or may naturally
come into the cellar.  If the foundation at that depth
does not prove good, you must either go down to a
better, if in your reach, or make it good by a platform
of wood or piles, or both.  Within the house, low
walls must be built to carry the cylinder beams, so as
to leave sufficient room to come at the holding-down
bolts, and the ends of these beams must also be lodged
in the wall. The lever wall must be built in the firmest
manner, and run solid, course by course, with thin
lime mortar, care being taken that the lime has not
been long slaked. If the house be built of stone, let
the stones be large and long, and let many headers be
laid through the wall: it should also be a rule, that
every stone be laid on the broadest bed it has, and
never set on its edge.  A  course or two above the
lintel of the door that leads to the condenser, build in
the wall two parallel flat thin bars of iron equally
distant from each other, and from the outside and inside of the wall, and reaching the whole breadth of
the lever wall.  About a foot higher in the wall, lay
at every four feet of the breadth of the front, other
bars of the same kind at right angles to the former
course, and reaching quite through the thickness of the
wall; and at each front corner lay a long bar in the
middle of the side walls, and reaching quite through
the fiont wall: if these bars are 10 ft. or 12 ft. long it
will be sufficient. When the house is built up nearly
to the bottom of the opening under the great beam,
another double course of bars is to be built in, as has
been directed.  At the level of the upper cylinder




THE STEAM ENGINE.              169
beams, holes must be left in the walls for their ends,
with room to move them laterally, so that the cylinder
may be got in; and smaller holes must be left quite
through the walls for the introduction of iron bars,
which being firmly fastened to the cylinder beams at
one end, and screwed at the other or outer end, will
serve, by their going through both the front and back
walls, to bind the house more firmly together. The
spring beams or iron bars fastened to them, must reach
quite through the back wall, and be keyed or screwed
up tight; and they must be firmly fastened to the lever
wall on each side, either by iron bars, firm pieces of
wood, or long strong stones, reaching far back into the
wall.  They must also be bedded solidly, and the
residue of the opening must be built up in the firmest
manner.  If there be no water in the neighborhood
that can be employed for the purpose of condensation,
it will be necessary to make a pond, dug in the earth,
for the reception of the water delivered by the air
pump, to the end that it may be cooled and used again
for the engine.  The pond may be 3 ft. or 4 ft. deep,
and lined with turf, puddled, or otherwise made water
tight.  Throwing up the water into the air in the
form of a jet to cool it, has been found detrimental;
as the water is then charged with air which vitiates
the vacuum.  To pack the piston, take 60 commonsized white or untarred rope-yarns, and with them
plait a gasket or flat rope as close and firm as possible,
tapering for 18 in. at each end, and long enough to go
round the piston, and overlapped for that length; coil
this rope the thin way as hard as possible, and beat it
with a sledge hammer until its breadth answers the
15




170              A CATECHISM OF
place; put it in, and beat it down with a wooden drift
and a hand mallet; pour some melted tallow all around;
then pack in a layer of white oakum half an inch thick,
so that the whole packing may have the depth of 5 to
6 inches, depending on the size of the engine; finally,
screw down the junk ring.  The packing should be
beat solid, but not too hard, otherwise it will create so
great a friction as to prevent the easy going of the
engine.  Abundance of tallow should be allowed, especially at first; the quantity required will be less as
the cylinder grows smooth.  In some of the more
modern pumping engines, the piston is provided with
metallic packing, consisting for the most part of a
single ring with a tongue-piece to break the joint, and
packed behind with hemp.  The upper edge of the
metallic ring is sharpened away from the inside so as
to permit more conveniently the application of hemp
packing behind it; and the junk ring is made much
the same as if no metallic packing were employed. To
set the engine going, the steam must be raised until
the pressure in the steam pipe is at least equal to three
pounds on the square inch; and when the cylinder
jacket is fully warmed, and steam issues freely from
the jacket cock, open all the valves or regulators: the
steam will then forcibly blow out the air or water
contained in the eduction pipe; and to get rid of the
air in the cylinder, shut the steam valve after having
blown through the engine for a few minutes.   The
cold water round the condenser will condense some of
the steam contained in the eduction pipe, and its place
will be supplied by some of the air from the cylinder.
The steam valve must again be opened to blow ou




THE STEAM ENGINE.              171
that air, and the operation is to be repeated until the
air is all drawn out of the cylinder. When that is the
case shut all the valves, and observe if the vacuum
gauge shows a vacuum in the condenser. When there
is a vacuum equivalent to three inches of mercury,
open the injection a very little, and shut it again immediately; and if this produces any considerable vacuum, open the exhausting valve a very little way,
and the injection at the same time.  If the engine
does not now commence its motion, it must be blown
through again until it moves. If the engine be lightly
loaded, or if there he no water in the pumps, the
throttle valve must be kept nearly closed, and the top
and exhaustion regulators must be opened only a very
little way, else the engine will make its stroke with
violence, and perhaps do mischief.  If there is much
unbalanced weight on the pump end, the plug which
opens the steam valve must be so regulated, that the
valve will only be opened very slightly; and if, after
a few strokes, it is found that the engine goes out too
slowly, the valve may be then so adjusted as to open
wider.  The engine should always be made to work
full stroke, that is, until the catch pins be made to
come within half an inch of the springs at each end,
and the piston should stand high enough in the cylinder
when the engine is at rest, to spill over into the perpendicular steam pipe any water which may be condensed above it; for if water remain upon the piston,
it will increase the consumption of steam.  When the
engine is to be stopped, shut the injection valve and
secure it, and adjust the tappets so as to prevent the
exhausting valve from opening, and to allow the steam




172              A CATECHISM OF
valve to open and remain open; otherwise a partial vacuum  may arise in the cylinder, and it may be filled
with water from the injection or from leaks. A single
acting engine, when it is in good order, ought to be
capable of going as slow as one stroke in ten minutes, and as fast as ten strokes in one minute; and
if it does not fulfil these conditions, there is some fault
which should be ascertained and remedied.  In the
modern Cornish engines the steam is used very expansively, and a high pressure of steam is employed.
In some cases a double-cylinder engine is used, in
which the steam, after having given motion to a small
piston on the principle of a high pressure engine, passes
into a larger cylinder, where it operates on the principle
of a condensing engine; but there is no superior
effect gained by the use of two cylinders, and there is
greater complexity in the apparatus.  Instead of the
lever walls, cast-iron columns are now frequently used
for supporting the main beam; and the cylinder end
of the main beam is generally made longer than the
pump end, so as to enable the cylinder to have a long
stroke, and the piston to move quickly, without communicating such a velocity to the pump buckets as
will make them work with such a shock as to wear
themselves out quickly.  A  high pressure of steam,
too, can be employed where the stroke is long, without
involving the necessity of making the working parts of
such large dimensions as would otherwise be necessary;
for the strength of the parts of a single acting engine
will require to be much the same, whatever the length
of the stroke may be.  The pump now universally
preferred is the plunger pump, which admits of being




THE STEAM ENGINE.               173
packed or tightened while the engine is at work; but
the lowest lift of a mine is generally supplied with a
pump on the suction principle, both with the view of
enabling the lowest pipe to follow the water with
facility as the shaft is sunk deeper, and to obviate the
inconvenience of the valves of the pump being rendered inaccessible by any flooding in the mine. The
pump valves of deep mines are a perpetual source of
expense and trouble, as, from the pressure of water
upon them, it is difficult to prevent them from closing
with violence; and many expedients have been contrived to mitigate the evil, of which the valve known
as Harvey and West's valve has perhaps gained the
widest acceptation. This valve is a compromise between the equilibrium valve, of the kind employed for
admitting the steam to and from the cylinder in single
acting engines, and the common spindle valve formerly
used for that purpose; and to comprehend its action,
it is necessary that the action of the equilibrium valve
should first be understood.  This valve consists substantially of a cylinder open at both ends, and capable
of sliding upon a stationary piston fixed upon a rod
the length of the cylinder, which proceeds from the
centre of the orifice the valve is intended to close.  It
is clear, that when the cylinder is pressed down until its
edge rests upon the bottom of the box containing it, the
orifice of the pipe must be closed, as the steam can neither
escape past the edge of the cylinder nor between the
cylinder and the piston; and it is equally clear, that
as the pressure upon the cylinder is equal all around it,
and the whole of the downward pressure is maintained
by the stationary piston, the cylinder can be raised or
15*




174              A CATECHISM OF
lowered without any further exertion of force than is
necessary to overcome the friction of the piston and of
the rod by which the cylinder is raised. Instead of
the rubbing surface of a piston, however, a conical
valve-face between the cylinder and piston is employed, which is tight only when the cylinder is in its
lowest position; and there is a similar face between
the edge of the cylinder and the bottom of the box in
which it is placed.  The moving part of the valve,
too, instead of being a perfect cylinder, is bulged outwards in the middle, so as to permit the steam to escape past the stationary piston when the cylindrical
part of the valve is raised. It is clear, that if such a
valve were applied to a pump, no pressure of water
within the pump would suffice to open it, neither
would any pressure of water above the valve cause it
to shut with violence; and if an equilibrium valve,
therefore, be used as a pump valve at all, it must be
opened and shut by mechanical means.  In Harvey
and West's valves, however, the equilibrium principle
is only partially adopted; the lower face is considerably
larger in diameter than the upper face, and the difference constitutes an annulus of pressure, which will
cause the valve to open or shut with the same force as
a spindle valve of the area of the annulus.  To deaden
the shock still more effectually, the lower face of the
valve is made to strike upon end wood driven into an
annular recess in the pump bucket;. and valves thus
constructed work with very little noise or tremor; but
it is found in practice, that the use of Harvey and
West's valve, or any contrivance of a similar kind,
adds materially to the load upon the pump. In some




THE STEAM ENGINE.               175
cases canvas valves similar to those referred to in
page 68 are used for pumps with the effect of materially mitigating the shock; but they require frequent
renewal, and are of inferior eligibility in their action
to the side valve, which might be applied to pumps
without inconvenience. The centrifugal pump, however, threatens to supersede pumps of every other
kind; and if the centrifugal pump be employed, there
will be no necessity for pump valves at all. Indeed,
it appears probable, that by working a common reciprocating pump at a high speed, a continuous flow of
water might be maintained through the pipes in such
a way as to render the existence of any valves superfluous. The best form of the centrifugal pump appears
to be that in which the arms diverge from the bottom,
like the letter V.  Such pumps both draw and force:
and by arranging them in a succession of lifts in the
shaft of the mine, the water may be drawn without inconvenience from  any depth.  The introduction of
the centrifugal pump will obviously extinguish the
single acting engine, as rotative engines working at a
high speed will be the most appropriate form of engine
where the centrifugal pump is employed. The single
acting engine, indeed, is a remnant of engineering barbarism which must now be superseded by more compendious contrivances. The Cornish engines, though
rudely manufactured, are very expensive in production,
as a large engine does but little work; whereas, by
employing a smaller engine, moving with a high speed,
the dimensions may be so far diminished that the most
refined machinery may be obtained at less than the
present cost. It is a mistake to suppose that there is




176              A CATECHISM OF
any peculiar virtue in the exisiting form of Cornish
engine to make it economical in fuel, or that a less
lethargic engine would necessarily be less efficient.
The large duty of the engines in Cornwall is traceable
to the large employment of the principle of expansion,
and to a few other causes which may be made of quite
as decisive efficacy in smaller engines working with a
quicker speed; and there is therefore no argument in
the performance of the present engines against the proposed substitution.
153. Q.-What description of rotative engine do
you consider the best?
A.-The oscillating engine appears to me the best
form of rotative engine yet introduced, and I would
prefer it to any other in every case where a rotary
motion is required.  For land purposes it is simpler
and more compact than the common beam engine; for
steam vessels it appears to be preferable to the side
lever engine, as well as to any of the other forms of
direct action engines yet introduced; and for locomotives its employment seems to promise the advantage
of diminishing the mass of reciprocating material,
whereby there will be less of the sinuous or oscillating
motion, by which the safety of railway trains is sometimes endangered.  The chief varieties of direct action
engines used for steam vessels, besides the oscillating
engine, are, the Gorgon engine, with the connecting
rod reaching from the top of the piston rod to the
crank situated above it; the Steeple engine, with the
connecting rod above the cran^k; and the Annular and
Siamese engines of Maudslay.  The Gorgon engine
has the disadvantage of inconveniently raising the




THE STEAM ENGINE.               177
shaft to give room  for the stroke, whereby a large
paddle wheel becomes necessary; and with a large
wheel there must either be a great deal of slip, or the
engines must work but slowly.  The Steeple engine
has the inconvenience of protruding a large portion of
the machinery above the deck; and the Annular and
Siamese engines have peculiar complications, which
render them inferior to the oscillating engine for every
purpose.  Against the oscillating engine itself various
objections have been brought at various times:-the
cylinder, it was said, would become oval, the trunnion
bearings would be liable to heat and the trunnion
joints to leak, the strain upon the trunnions would be
apt to bend in or bend out the sides of the cylinder,
and the circumstance of the cylinder being fixed across
its centre, while the shaft requires to accommodate
itself to the working of the ship, might be the occasion
of such a strain upon the trunnions as would either
break them  or bend the piston rod.  It is a sufficient
reply to these objections to say that they are all hypothetical, and that none of them in practice have been
found to exist-to such an extent at least as to occasion any inconvenience; but it is not difficult to show
that they are altogether unsubstantial, even without a
recourse  to  the  disproofs  afforded by experience.
There is, no doubt, a tendency in oscillating engines
for the cylinder and the stuffing-box to become oval,
but after a number of years' wear it is found that the
amount of ellipticity is less than what is found to
exist in the cylinders of side-lever engines after a
similar trial.  The resistance opposed by friction to
the oscillation of the cylinder is so small, that a man




178              A CATECHISM OF
is capable of moving a large cylinder with one hand,
whereas in the side-lever engine, if the parallel motion be in the least untrue, which is, at some time or
other, an almost inevitable condition, the piston is
pushed with great force against the side of the cylinder, whereby a large amount of wear and friction
is occasioned.  The  trunnion  bearings, instead  of
being liable to heat like other journals, are kept down
to the temperature of the steam by the flow of steam
passing through them; and the trunnion packings are
not liable to leak when the packings before being introduced are squeezed in a cylindrical mould.  In
some cases a hollow, or lantern brass, about one-third,
or one-fourth, the length of the packing space, and
supplied with steam or water by a pipe, is introduced
in the middle of the packing, so that if there be any
leakage through the trunnion, it will be a leakage
of steam or water, which will not vitiate the vacuum:
but in ordinary cases this device will not be necessary,
and it is not commonly employed.  It is clear that
there can be no buckling of the sides of the cylinder
by the strain upon the trunnions, if the cylinder be
made strong enough, and in cylinders of the ordinary
thickness such an action has never been experienced;
nor is it the fact, that the intermediate shaft of steam
vessels, to which part alone the motion is communicated by the engine, requires to adapt itself to the
altering forms of the vessel, as the engine and intermediate shaft are  rigidly connected, although the
paddle shaft requires to be capable of such an adaptation.  Even if this objection  existed, however, it
could easily be met by making the crank pin of the




THE STEAM ENGINE.                179
ball and socket fashion, which would permit the position of the intermediate shaft, relatively with that of
the cylinder, to be slightly changed, without throwing
an undue strain upon any of the working parts.
154. Q.-Will you describe the structure of an oscillating engine as made by Messrs. Penn?
A.-To  do this it will be expedient to take an
engine of a given power, and then the sizes may be
given as well as an account of the configuration of the
parts: we may take for an example a pair of engines
of 21- in. diameter of cylinder, and 22 in. stroke,
rated by Messrs. Penn at 12 horses power each. The
cylinders of this oscillating engine are placed beneath
the cranks, and, as in all Messrs. Penn's smaller
engines, the piston rod is connected to the crank pin
by means of a brass cap, provided with a socket, by
means of which it is cuttered to the piston rod.
There is but one air pump, which is situated within
the condenser between the cylinders, and is wrought
by means of a crank in the intermediate shaft-this
crank being cut out of a solid piece of metal, as in the
formation of the cranked axles of locomotive engines.
The steam  enters the  cylinder through  the outer
trunnions, or the trunnions adjacent to the ship's
sid s, and enters the condenser through the two midship  trunnions-a  short three-ported  valve  being
placed on the front of the cylinder to regulate the
flow of steam to and from the cylinder in the proper
manner.  This valve is balanced by a weight upon
the other side of the cylinder, but in the most recent
engines this weight is discarded, and two valves are
used, which balance one another.  The framing con



180               A CATECHISM OF
sists of an upper and lower frame of cast-iron, bcnmd
together by eight malleable iron columns: upon the
lower frame the pillow blocks rest which carry the
cylinder trunnions, and the condenser and the bottom
frame are cast in the same piece. The upper frame
supports the paddle shaft pillow blocks; and pieces
are bolted on in continuation of the upper frame to
carry the paddle wheels, which are overhung from the
journal. The web, or base plate, of the lower frame
is 3 of an inch thick, and a cooming is carried all
round the cylinder, leaving an opening of sufficient
size to permit the necessary oscillation. The cross
section of the upper frame is that of a hollow beam
6 in. deep, and about 31 in. wide, with holes at the
sides to take out the core; and the thickness of the
metal is -1ths of an inch.  Both the upper and the
lower frame is cast in a single piece, with the exception of the continuations of the upper frame,
which support the  paddle wheels.  An  oval ring
3 in. wide is formed in the upper frame, of sufficient
size to permit the working of the air pump crank;
and from  this ring feathers run to the ends of the
cross portions of the frame which support the intermediate shaft journals.  The columns are 1- in
in diameter; they are provided with collars at tht
lower ends, which rest upon bosses in the lowei
frame, and with collars at the upper ends for supporting the upper frame; but the upper collars of two ol
the corner columns are screwed on, so as to enable
the columns to be drawn up when it is required to get
the cylinders out. The cross section of the bottom
frame is also of the form of a hollow beam, 7 in. deep,




THE STEAM ENGINE.               181
except in the region of the condenser, where it is, of
course, of a different form; the depth of the boss for
the reception of the columns is a little more than 7 in.
deep on the lower frame, and a little more thai 6 in.
deep on the upper frame; and the holes through them
are so cored out, that the columns only bear at the
upper and lower edges of the hole, instead of all
through it-a formation by which the fitting of the
columns is facilitated.  The condenser, which is cast
upon the lower frame, consists of an oval vessel 221
in. wide, by 2 ft. 41- in. long, and 1 ft. 10 in. deep;
it stands 9 in. above the upper face of the bottom
frame, the rest projecting beneath it; and it is enlarged at the sides by being carried beneath the trunnions.  The air pump, which is set in the centre of
the condenser, is 15 - in. in diameter, and has a stroke
of 11 in. The foot valve is situated in the bottom of
the air pump, and consists of a disc of brass, in which
there is a rectangular flap valve, but rounded on one
side to the circle of the pump, opening upwards, and
so balanced as to enable the valve to open with facility; and the balance weight, which is formed of
brass cast in the same piece as the valve itself, operates as a stop, by coming into contact with the disc
which constitutes the bottom of the pump when the
valve has opened sufficiently.  This disc is bolted
to the bottom of the pump by means of an internal
flange, and before it can be removed the pump must
be lifted out of its place.  The air pump baJrrel is
of brass, to which is bolted a cast-iron mouth piece,
with a port for carrying the water to the hot well;
and within the hot well the delivery valve, which
16




182               A CATECHISM OF
consists of a common flap valve, is situated.  The
mouth piece and the air pump barrel are made tight
to the condenser, and to one another, by means of
metallic joints carefully scraped to a true surface, so
that a little white or red lead interposed, makes an
air-tight joint. The air pump bucket is of brass, and
the valve of the bucket is of the common pot-lid, or
spindle kind. The injection water enters through a
single cock in front of the condenser, the jet striking
against the barrel of the air pump: the air pump rod
is maintained in its vertical position by means of
guides, the lower ends of which are bolted to the
mouth of the pump, and the upper to the oval in the
top frame, within which the air pump crank works,
and the motion is communicated from this crank to
the pump rod by means of a short connecting rod.
The lower frame is not set immediately below the top
frame, but 21 in. behind it, and the air pump and
condenser are 2- in. nearer one edge of the lower
frame than the other. The thickness of the metal of
the cylinder is 9 ths of an inch; the depth of the
belt of the cylinder is 91 in., and its greatest projection from the cylinder is 2- in.  The distance from
the lower edge of the belt to the bottom of the cylinder is 11 in., and from the upper edge of the belt to
the top flange of the cylinder is 9 in. The trunnions
are 71 in. diameter in the bearings, and 31 in. in
4                                      Y
width; and the flanges to which the glands are attached for screwing in the trunnion packings are 1 - in.
thick, and have  -ths of an inch projection.  The
width of the packing space round the trunnions is
Iths of an inch, and the diameter of the pipe pass



THE STEAM ENGINE.               183
ing through the trunnions 4A-ths, which leaves I-ths
for the thickness of the metal of the bearing. The
pipe leading to the condenser, from the cylinder, is
made somewhat bell-mouthed where it joins the condenser, and the gland for compressing the packing is
made of a larger internal diameter in every part except at the point the pipe emerges from it, where it
accurately fits the pipe, so as to enable the gland to
squeeze the packing. By this construction the gland
may be drawn back without being jammed upon the
enlarged part of the pipe, and the enlargement of the
pipe towards the condenser prevents the air pump
barrel from offering any impediment to the free egress
of the steam.  The gland is made altogether in four
pieces: the ring which presses the packing is made
distinct from the flange to which the bolts are attached which force the gland against the packing, and
both ring and flange are made in two pieces, to enable
them  to be got over the pipe.  The ring is halfchecked in the direction of its depth, and is introduced without any other support to keep the halves
together than what is afforded by the interior of the
stuffing box; and the flange is half-checked in the
direction of its thickness, so that the bolts which
press down the ring by passing through this halfchecked part also keep the segments of the flange
together. The bottom of the trunnion packing space
is contracted to the diameter of the eduction pipe, so
as to prevent the packing from being squeezed into
the jacket; but the eduction pipe does not fit quite
tight into this contracted part, but, while in close
contact on the lower side, has about 32d of an inch of




184               A CATECHISM OF
space between the top of the pipe and the cylinder,
so as to permit the trunnions to wear to that extent
without throwing a strain upon the pipe. The eduction pipe is attached to the condenser by a flange
joint, and the bolt-holes are all made somewhat oblong
in the perpendicular direction, so as to permit the
pipe to be slightly lowered, should such an operation
be rendered necessary by the wear of the trunnion
bearings; but in practice the wear of the trunnion
bearings is found to be so small, as to be almost inappreciable. It is not expedient to cast the trunnion
plummer blocks upon the lower frame, as is sometimes
done; for the cylinders, being pressed from the steam
trunnions by the steam, and drawn in the direction of
the condenser by the vacuum, have a continual tendency to approach one another; and as they wear
slightly towards midships, there would be no power of
readjustment unless the plummer blocks were movable. The flanges of the trunnions should always fit
tight against the plummer block sides, but there should
be a little play sideways at the necks of the trunnions,
so that the cylinder may be enabled to expand when
heated, without throwing an undue strain upon the
trunnion supports.  Above and below each trunnion a
feather or bracket runs from the edge of the belt between 3 in. and 4 in. along the cylinder, for the sake
of additional support; and in large engines this feather
is continued through the interior of the belt, and cruciform feathers are added for the sake of greater stiffness. The projection of the outer face of the trunnion
flange from the side of the cylinder is 61 in.; the
thickness of the flange round the mouth of the cyl.




THE STEAM ENGINE.                 185
inder is A of an inch, and its projection 13- in.; the
height of the cylinder stuffing-box above the cylinder
cover is 4  in., and its external diameter 48 in.the diameter of the piston rod being 2- in.; and the
thickness of the stuffing-box flange is 1 — in.  The
length of the valve casing is 16- in., and its projection
from  the cylinder is 3- in. at the top, 4- in. at the
centre, and 2- in. at the bottom, so that the back of
2
the valve casing is not made flat, but is formed in a
curve.  The width of the valve casing is 9 in., but
there is a portion the depth of the belt 1 in. wider,
to permit the steam  to enter from  the belt into the
casing. The valve casing is attached to the cylinder
by a metallic joint; the width of the flange of this
joint is 1 in., the thhickness of the flange on the casing
in., and the thickness of the flange on the cylinder
i-ths of an inch. The valve is of the ordinary threeported description, and both cylinder and valve faces
are of cast-iron. The projection from the cylinder of
the passage for carrying the steam  upwards  and
downwards, from  the valve to the top and bottom  of
the cylinder, is 2{ in., and its width externally 81 in.
The piston is packed with hemp, but the junk ring is
made of malleable iron, as cast-iron junk rings have
been found liable to break: there are four plugs
screwed into the  cylinder cover, which, when removed, permit a box-key to be introduced, to screw
down the piston packings.  The screws in the junk
ring are each provided with a small ratchet, cut in a
fixed washer upon the head, to prevent the screw
from  turning back; and the number of clicks given
by there ratchets, in tightening up the bolts, enables
16*




186               A CATECHISM OF
the engineer to know when they have all been tightened equally.  The cap for attaching the piston rod
to the crank pin is formed altogether of brass, which
brass serves to form  the bearing of the crank pin.
The external diameter of the socket by which this
cap is attached to the piston rod is 3 —1 in. The diameter of the crank pin is 3 in., and the length of the
crank pin bearing 37 in. The thickness of the brass
around the crank pin bearing is I in., and the upper
portion of the brass is secured to the lower portion by
means of lugs, which are of such a depth that the
perpendicular section through the centre of the bearing has a square outline measuring 7 in. in the horizontal direction, 3- in. from the centre of the pin to
the level of the top of the lugs, and 21 in. from the
centre of the pin to the level of the bottom of the
lugs.  The width of the lugs is 2 in., and the bolts
passing through them  are 14 in. in diameter.  The
bolts are tapped into the lower portion of the cap, and
are fitted very accurately by scraping where they pass
through the upper portion, so as to act as steady pins
in preventing the cover of the crank pin bearing from
being worked sideways by the alternate thrust on
each side.  The distance between the centres of the
bolts is 5 in., and in the centre of the cover, where
the lugs, continued in the form  of a web, meet one
another, an oil cup 1  in. in diameter, 1  hin. high, and
provided with an  internal pipe, is cast upon the
cover, to contain oil for the lubrication of the crank
pin bearing.  The depth of the cutter for attaching
the cap to the piston rod is I1 in., and its thickness is
three-eighths of an inch.  A similar cap attacles the




THE STEAM ENGINE.               187
air pump crank to the connecting rod by which the
air pump rod is moved; but in this instance the diameter of the bearing is 5 in., and the length of the
bearing -about 3 in.  The thickness of the brass encircling the bearing is three-fourths of an inch upon
the edge, and 1- in.- in the centre, the back being
slightly rounded; the width of the lugs is 1- in., and
the depth of the lugs is 2 in. upon the upper brass,
and 2 in. upon the lower brass, making a total depth
of 4 in. The diameter of the bolts passing through
the lugs is 1 in., and the bolts are tapped into the
lower brass, and accurately fitted into the upper one,
so as to act as steady pins, as in the previous instance.
The lower eye of the connecting rod is forked, so as
to admit the eye of the air pump rod; and the pin
which connects the two together is prolonged into a
cross head, the ends of which move in the guides.
The forked end of the connecting rod is fixed upon
the cross head by means of a feather, so that the cross
head partakes of the motion of the connecting rod,
and a cap, similar to that attached to the piston rod,
is attached to the air pump rod, for connecting it with
the cross head. The diameter of the air pump rod is
12 in.; the external diameter of the socket encircling
the rod is 2- in., and the depth of the socket 41 in.
from the centre of the cross head. The depth of the
cutter for attaching the socket to the rod is 1 in., and
its thickness 5 in.  The breadth of the lugs is 18 in.,
the depth 1- in., making a total depth of 2~ in.; and
the diameter of the bolts seven-eighths of an inch.
The diameter of the cross head at the centre is 2 in.,
the thickness of each jaw  around the bearing 1 in.,




188               A CATECHISM OF
and the breadth of each 9 in.  The diameter of the
intermediate shaft journal is 41- in., and of the paddle
shaft journal 43 in.; the length of the journal in each
case is 5 in.  The diameter of the large eye of the
crank is'7 in., and the diameter of the hole through
it is 43 in.; the diameter of the small eye of the crank
is 54 in., the diameter of the hole through it being
3 in. The depth of the large eye is 4- in., and Df the
small eye 33- in.; the breadth of the web is 4 in. at
the shaft end, and 3 in. at the pin head, and the thickness of the web is 24 in.  The width of the notch
forming the crank in the intermediate shaft for working the air pump is 3- in., and the width of each of
the arms of this crank is 315- in.; both the outer and
inner corners of the crank are chamfered away, until
the square part of the crank meets the round of the
shaft.  The method of securing the crank pins into
the crank eyes of the intermediate shaft consists in the
application of a nut to the end of each pin, where it
passes through the eye, the projecting end of the pin
being formed with a thread, upon which the nut is
screwed.  The eccentric strap is half an inch thick,
and 14 in. broad; and the flanges of the eccentric,
within which the strap works, are each three-eighths
of an inch thick.  The eccentric is put on in two
halves, joined in the diameter of largest eccentricity
by means of a single bolt passing through lugs on the
central eye, and the back balance is made in a separate piece five-eighths of an inch thick, and is attached
by means of two bolts, which also help to bind the
halves of the eccentric together. The eccentric rod
is attached to the eccentric hoops by means of two




THE STEAM ENGINE.                 189
bolts passing through lugs upon the rod, and tapped
into a square boss upon the hoop; and pieces of iron,
of a greater or less thickness, are interposed between
the surfaces in setting the valve, to make the eccentric
rod of the right length. The eccentric rod is kept in
gear by the pull of a small horizontal rod, attached to
a vertical blade-spring, and it is thrown out of gear
by means of the ordinary disengaging apparatus, which
acts in opposition to the spring, as, in cases where
the eccentric rod is not vertical, it acts in opposition
to the gravity of the rod. The paddle shaft plummer
blocks are altogether of brass, and are formed in much
the same manner as the cap of the piston rod, only
that the sole is flat, as in ordinary plummer blocks,
and is fitted between projecting lugs of the framing,
to prevent side motion.  In the bearings fitted on
this plan, however, the upper brass will generally acquire a good deal of play after some amount of wear.
The bolts are worked slack in the holes, though accurately fitted at first; and it appears expedient, therefore, either to make the bolts very large, and the
sockets through which they pass very deep, or to let
one brass fit into the other. The trunnion plummer
blocks are formed in the same manner as the shaft
plummer blocks; the nuts are kept from turning back
by means of a pinching screw  passing through a stationary washer.
155. Q.-Will you explain in detail the construction
of the valve gearing, or such parts of it as are peculiar
to the oscillating engine?
A. —The eccentric rod is attached by a pin, 1 in. in
diameter, to an open curved link with a tail projecting




190               A CATECHISM OF
upwards, and passing through an eye to guide the link
in a vertical motion.  The link is formed of iron casehardened, and is 2- in. deep at the middle, and 23 in.
deep at the ends, and 1 in. broad.  The opening in
the link, which extends nearly its entire length, is
1- in. broad, and into this opening a brass block 2 in.
long is truly fitted, there being a hole through the
block - in. diameter, for the reception of the pin of
the valve shaft lever. The valve shaft is 1  in. diameter at the end next the link or segment, and
diminishes regularly to the other end; but it assumes
the form of an octagon in its passage round the cylinder, measuring midway 1- in. deep, by about A- in.
thick; and the greatest depth of the finger for moving
the valve is about 1 in.  The depth of the lever for
moving the valve shaft is 2 in. at the broad, and 1~ in.
at the narrow end. The internal breadth of the mortise in which the valve finger moves is 1-5 in.,' and
its external depth is 14 in., which leaves three-eighths
of an inch as the thickness of metal round the hole;
and the breadth, measuring in the direction of the
hole, is 1- in. The valve rod is three-fourths of an
inch in diameter, and the mortise is connected to the
valve rod by a socket 1 in. long, and 1- in. diameter,
through which a small cutter passes.  A continuation
of the rod, eleven-sixteenths of an inch diameter,
passes upward from  the mortise, and works through
an eye, which serves the purpose of a guide.  In
addition to the guide afforded to the segment by the
ascending tail, it is guided at the ends upon the
columns of the framing by means of thin semicircular
brasses, 4 in. deep, passing round the columns, and




THE STEAM ENGINE.                191
attached to the segment by two - in. bolts at each
end, passing  through  projecting feathers upon the
brasses and segment, three-eighths of an inch in thickness.  The curvature of the segment is such as to
correspond with the are swept from the centre of the
trunnion with the distance from  the centre of the
trunnion to the centre of the valve lever pin when
the valve is at half stroke as a radius; and the operation of the segment is to prevent the valve from
being affected by the oscillation of the cylinder; but
the same action would be obtained by the employment
of a smaller eccentric with more lead. In some engines the hollow segment is not formed in a single
piece, but of two curved blades, with blocks interposed at the ends, which may be filed down a little,
to enable the sides of the slot to be brought nearer, as
the metal wears away.
156. Q.-What kind of paddle wheel is supplied
with these oscillating engines?
A.-The wheels are of the feathering kind, 9 ft.
8 in. in diameter, measuring to the edges of the floats;
and there are 10 floats upon each wheel, measuring
4 ft. 6 in. long each, and 181 in. broad. There are
two sets of arms to the wheel, which converge to a
cast-iron centre, formed like a short pipe with large
flanges, to which the arms are affixed. The diameter
of the shaft, where the centre is put on, is 41 in., the
external diameter of the pipe is 8 in., and the diameter
of the flanges is 20 in., and their thickness 1- in.
The flanges are 12 in. asunder at the outer edge, and
they partake of the converging direction of the arms.
The arms are 2 1in. broad, and half an inch thick;




192               A CATECHISM OF
the heads are made conical, and each is secured into
a recess upon the side of the flange by means of three
bolts.  The ring which connects together the arms,
runs round at a distance of 3 ft. 6 in. from the centre,
and the projecting ends of the arms are bent backwards the length of the lever which moves the floats,
and are made very wide and strong at the point where
they cross the ring, to which they are each attached
by four rivets. The feathering action of the floats is
accomplished by means of a pin fixed to the interior
of the paddle box, set 3 in. in advance of the centre
of the shaft, and in the same horizontal line. This
pin is encircled by a cast-iron collar, to which rods
are attached 1- in. diameter in the centre, proceeding
to the levers, 7 in. long, fixed on the back of the floats
in the line of the outer arms.  One of these rods,
however, is formed of nearly the same dimensions as
one of the arms of the wheel, and is called the driving
arm, as it causes the cast-iron collar to turn round
with the revolution of the wheel; and this collar, by
means of its attachments to the floats, accomplishes
the feathering action. The eccentricity in this wheel
is not sufficient to keep the floats nearly in the vertical position; but this is of less consequence, as only
one float is wholly immersed.  The diameter of the
pins upon which the float turns is 1- in., and between
the pins and the paddle ring two stud rods are set between each of the projecting ends of the arms, so as
to prevent the two sets of arms from  being forced
nearer or further apart; and thus prevent the ends
of the arms from hindering the action of the floats, by
being accidentally jammed upon the sides of the joints.




THE STEAM ENGINE.                193
Stays, crossing one another, proceed from  the inner
flange of the centre to the outer ring of the wheel,
and from  the outer flange of the centre to the inner
ring of the wheel, with the view of obtaining greater
stiffness. The floats are formed of plate iron, and the
whole of the joints and joint pins are steeled, or formed
of steel.
157. Q.-Will you give the dimensions of some
other oscillating engines?
A.-In Messrs. Penn's 50-horse power oscillating
engine, the diameter of the cylinder is 3 ft. 4 in., and
the length of the stroke 3 ft.  The thickness of the
metal of the cylinder is 1 in., and the thickness of the
cylinder bottom is 1- in., crossed with feathers, to give
it additional stiffness. The diameter of the trunnion
bearings is 1 ft. 2 in., and the breadth of the trunnion
bearings 5~ in. Messrs. Penn, in their larger engines,
generally make the area of the steam  trunnion less
than that of the eduction trunnion, in the proportion
of 32 to 37; and the diameter of the eduction trunnion is regulated by the internal diameter of the eduction pipe, which is about one-fifth of the diameter of
the cylinder. But a somewhat larger proportion than
this appears to be expedient: Messrs. Rennie make
the area of their eduction pipes, in oscillating engines,
-ld of the area of the cylinder.  In  the oscillating
engines of the Oberon, by Messrs. Rennie, the cylinder
is 61 in. diameter, and 1  in. thick above and below
the belt, but in the wake of the belt it is 1 in. thick,
which is also the thickness of metal of the belt itself.
The internal depth of the belt is 2 ft. 6 in., and its
internal breadth is 4 in.  The piston rod is 61- in. in
17




194               A CATECHISM OF
diameter, and-the total depth of the cylinder stuffing
box is 2 ft. 4 in., of which 18 in. consists of a brass
bush; this depth of bearing being employed to prevent
the stuffing box or cylinder from wearing oval.  It is
expedient, in oscillating engines, to form  the piston
with a projecting rim  round the edge above and below, and a corresponding recess in the cylinder cover
and cylinder bottom, whereby the breadth of bearing
of the solid part of the metal will be increased, and
in many engines this is now  done.  The diameter of
cylinders of the oscillating engines of the steamers
Pottinger, Ripon, and Indus, by Miller and Ravenhill,
is 76 in., and the length of the stroke is 7 ft.  The
thickness of the metal of the cylinder is 1- in.; diameter of the piston rod 84 in.; total depth of cylinder
stuffing box 3 ft.; depth of bush in stuffing box 4 in.;
the rest of the depth, with the exception of the space
for packing, being occupied with a very deep gland,
bushed with brass.  The internal diameter of the
steam  pipe is 13 in.; diameter of steam  trunnion
journal 25 in.; diameter of eduction trunnion journal
25 in.; thickness of metal of trunnions 21 in.; length
of trunnion  bearings 11 in.; projection  of cylinder
jacket, 8 in.; depth of packing  space in  trunnions
10 in.; width of packing space in trunnions, or space
round the pipes, 12 in.; diameter of crank pin 10} in.;
length of bearing of crank pin 15b in.  There are six
boilers on the tubular plan in each of these vessels;
the length of each boiler is 10 ft. 6 in., and the breadth
8 ft., and each boiler contains 62 tubes 3 in. in diameter, and 6 ft. 6 in. long, and two furnaces 6 ft. 43 in.
long, and 3 ft. 14 in. broad. In all oscillating engines




THE STEAM ENGINE.                195
of any considerable size, the cover of the connecting
brass, which attaches the crank pin to the connecting
rod, is formed of malleable iron; and the socket also,
which is cuttered to the end of the piston rod, is of
malleable iron, and is formed with a T head, through
which bolts pass up through the brass, to keep the
cover of the brass in its place. The packing of the
trunnions, after being plaited as hard as possible, and
cut to the length to form one turn round the pipe, is
dipped into boiling tallow, and is then compressed in
a mould, consisting of two concentric cylinders, with
a gland forced down into the annular space by three
to six screws in the case of large diameters, and one
central screw in the case of small diameters. Unless
the trunnion packings be well compressed, they will
be likely to leak air, and it is therefore necessary to
pay particular attention to this condition.  It is also
very important that the trunnions be accurately fitted
into their brasses by scraping, so that there may not
be the smallest amount of play left upon them; for
if any upward motion is permitted, it will be impossible to prevent the trunnion packings from leaking.
158. Q.-How do you set out the trunnions of oscillating engines, so that they shall be at right angles
with the interior of the cylinder?
A.-Having bored the cylinder, faced the flange,
and bored out the hole through which the boring bar
passes, put a piece of wood across the mouth of the
cylinder, and jam it in, and put a similar piece in the
hole through the bottom  of the cylinder.  Mark the
centre of the cylinder upon each of these pieces, and
put into the bore of each trunnion an iron plate, with




196                A CATECIISM OF
a small indentation in the middle to receive the centre
of a lathe, and adjusting screws to bring the centre
into any required position.  The cylinder must then
be set in the lathe, and hung by the centres of the
trunnions, and a straight edge must be put across the
cylinder mouth and levelled, so as to pass through
the line in which the centre of the cylinder lies.
Another similar straight edge, and similarly levelled,
must be similarly placed across the cylinder bottom,
so as to pass through the central line of the cylinder,
and the cylinder is then to be turned round in the
trunnion centres-the straight edges remaining stationary, which will at once show whether the trunnions
are in the same horizontal plane as the centre of the
cylinder, and if not, the screws of the plates in the
trunnions must be adjusted, until the central point of
the cylinder just comes to the straight edge, whichever
end of the cylinder is presented. To ascertain whether
the trunnions stand in a transverse plane, parallel to
the cylinder flange, it is only necessary to measure
down from the flange to each trunnion centre, and if
both these conditions are satisfied, the position of the
centres may be supposed to be right.  The trunnion
bearings are then turned, and are fitted into blocks of
wood, in which they run while the packing space  s
being turned out.  Where many oscillating  engines
are made, a lathe with four centres is used, which
makes the use of straight edges in setting out the
trunnions superfluous.
159. Q.-Is it a beneficial practice to make cylinders with steam jackets?
A.-In Cornwall, where great attention is paid to




THE STEAM ENGINE.                 197
economy of fuel, all the engines are made with steam
jackets, and, in some cases, a flue winds spirally round
the cylinder, for keeping the steam hot. Mr. Watt in
his early practice discarded the steam jacket for a time,
but resumed it again, as he found its discontinuance
occasioned a perceptible waste of fuel; and in modern
engines it has been found that where a jacket is used
less coal is consumed than where the use of a jacket is
rejected. The cause of this diminished effect is not of
very easy perception, for the jacket exposes a larger
radiating surface for the escape'of the heat than the
cylinder; nevertheless, the fact has been established
beyond doubt by repeated trials, that engines provided
with a jacket are more economical than engines without one.  The exterior of the cylinder, or jacket,
should be covered with several plies of felt, and then
be cased in timber, the boards, which must be very
narrow, being first dried in a stove, and then bound
round the cylinder with hoops, like the staves of a
cask.  In many of the Cornish engines the steam is let
into casings formed in the cylinder cover and cylinder
bottom, for the further economization of the heat, and
the cylinder stuffing box is made very deep, and a
lantern or hollow brass is introduced into the centre
of the packing, into which brass the steam gains admission by a pipe provided for the purpose, so that in
the event of the packing becoming leaky, it will be
steam that will be leaked into the cylinder instead of
air, which, being incondensable, would  impair the
efficiency of the engine. A lantern brass, of a similar
kind, is sometimes introduced into the stuffing-boxes
of oscillating engines, but its use there is to receive
17*




198               A CATECHISM OF
the lateral pressure of the piston rod, and thus take
any strain off the packing.
160. Q.-Can you give a rule for determining the
thickness of the cylinder and of the trunnion bearings
in oscillating engines?
A.-In low  pressure engines, whether oscilhating
or otherwise, the thickness of metal of the cylinder
should be about -1th of the diameter of the cylinder,
which, with a pressure of steam of 20 lbs. above the
atmosphere, will occasion a strain of only 400 lbs. per
square inch of section of the, metal: the thickness of
the metal of the trunnion bearing should be -i-d of
the diameter of the cylinder, and the breadth of the
bearing should be about half its diameter.  In high
pressure engines the thickness of the cylinder should
be about i —th its diameter, which, with a pressure of
steam of 80 lbs. upon the square inch, will occasion a
strain of 640 lbs. upon the square inch of section of the
metal; and the thickness of the metal of the trunnion
bearings of high pressure oscillating engines should be
Iath of the diameter of the cylinder.  It is very
doubtful whether the steam  trunnions of a high pressure oscillating engine will continue long tight if the
packing consists of hemp; and it appears preferable
to introduce a brass ring, to embrace the pipe, cut
spirally, with an overlap piece to cover the cut, and
packed behind with hemp.
161. Q.-Will you explain the various stages of
the manipulation connected with the preparation of
cylinders?
A.-In the first place the cylinder has to be cast.
The mould into which the metal is poured is built up




THE STEAM ENGINE.                 199
of bricks and loam, which is clay and sand ground together in a mill, with the addition of a little horse
dung to give it a fibrous structure and prevent cracks.
The loam board, by which the circle of the cylinder is
to be swept, is attached to an upright iron bar, at the
distance of the radius of the cylinder; and a cylindrical
shell of brick is built up, which is plastered on the inside with loam, and made quite smooth by traversing
the perpendicular loam  board round it.  A  core is
then formed in a similar manner, but so much smaller
as to leave a space between the shell and the core equal
to the thickness of the cylinder, and into this space the
melted metal is poured.  Whatever nozzles or projections are required upon the cylinder must be formed
by means of wooden patterns, which are built into the
shell, and subsequently withdrawn; but where a number of cylinders of the same kind are required, it is advisable to make these patterns of iron, which will not
be liable to warp or twist while the loam  is being
dried.  The general ambition in making cylinders is
to make them  sound and hard; but it is expedient
also to make them tough, so as to approach as nearly
as possible to the state of malleable iron. This may
be done by mixing in the furnace as many different
kinds of iron as possible; and it may be set down as a
general rule in iron founding, that the greater the
number of the kinds of metal entering into the composition of any casting, the denser and tougher it will
be.  The constituent atoms of the different kinds of
iron appear to be of different sizes, and the mixture of
different kinds maintains the toughness, while it adds
to the density and cohesive power.  Hot blast-iron




200               A CATECHISM OF
was at one time generally believed to be weaker than
cold blast-iron; but it is now questioned whether it
is not the stronger of the two.  The cohesive strength
of unmixed iron is not in proportion to its specific
gravity, and its elasticity and power to resist shocks
appears to become greater as the specific gravity becomes less.  Nos. 3 and 4 are the strongest irons.  In
most cases, iron melted in a cupola is not so strong as
when remelted in an air furnace, and when run into
green sand it is not reckoned so strong as when run into
dry sand, or loam. The quality of the fuel, and even
the state of the weather, exerts an influence in the
quality of the iron: smelting furnaces, on the cold
blast principle, have long been known to yield better
iron in winter than in summer, probably from  the
existence of less moisture in the air; and it would
probably be found to accomplish an improvement in
the quality of the iron if the blast were made to pass
through a vessel containing muriate of lime, by which
the moisture of the air would be extracted; and the
expense of such a preparation would not be consider
able, as, by subsequent evaporation, the salt might be
used over and over again for the s*me purpose.  Before the iron is cast into the mould, the interior of the
mould must be covered with finely powdered charcoal,
or blackening, as it is technically termed-and the
secret of making finely skinned castings lies in using
plenty of blackening. In loam and dry sand castings
the charcoal should be mixed with thick clay water,
and applied until it is an eighth of an inch thick, or
more; the surface should be then very carefully
smoothed, or sleeked, and if the metal has been ju



THE STEAM ENGINE.               201
diciously mixed, and the mould thoroughly dried, the
casting is sure to be a fine one.  Dry sand and loam
castings should be, as much as possible, made in boxes:
the moulds may thereby be more rapidly and more
effectually dried, and better castings will be got with a
less expense.  The next stage is the boring, and in
boring cylinders of 74 in. diameter, the boring bar must
move so as to make one revolution in about 41 minutes,
at which speed the cutters will move at the rate of
about 5 ft. per minute. In boring brass the speed must
be slower; the common rate at which the tool moves in
boring brass air pumps is about 3 ft. per minute.  If
this speed be materially exceeded the tool will be
spoiled, and the pump made taper. The speed proper
for boring a cylinder will answer for boring the brass
air pump of the same engine.  A brass air pump of
36- in. diameter requires the bar to make one turn in
about three minutes, which is also the speed proper
for a cylinder 60 in. in diameter.  To bore a brass
air pump 36- in. in diameter requires a week, an iron
one requires 48 hours, and a copper one 24 hours.
In turning a malleable iron shaft 123 in. in diameter
the shaft should make about five turns per minute,
which is equivalent to a speed in the tool of about 16
ft. per minute.  A boring mill, of which the speed
may be varied from one turn in six minutes to twentyfive turns in one minute, will be suitable for all ordinary wants that can occur in practice.  Messrs. Penn
grind their cylinders after they are bored, by laying
them on their side, and rubbing a piece of lead smeared
with emery and oil, and with a cross iron handle
like that of a rolling stone, backwards and forwards



202              A CATECHISM OF
the cylinder being gradually turned round so as to subject every part successively to the operation. The lead
by which this grinding is accomplished is cast in the
cylinder, whereby it is formed of the right curve; but
the part of the cylinder in which it is cast should be
previously heated by a hot iron, else the metal may be
cracked by the sudden heat.  In fixing a cylinder into
the boring mill great care must be taken that it is not
screwed down unequally; and indeed it will be impossible to bore a large cylinder in a horizontal mill without being oval, unless the cylinder be carefully gauged
when standing on end, and be set up by screws when
laid in the mill until it again assumes its original form.
A large cylinder will inevitably become oval if laid
upon its side; and if, while under the tension due to
its own weight, it be bored round, it will become oval
again when set upon end. If the bottom be cast in the
cylinder it will be probably found to be round at one
end and oval at the other, unless a vertical boring mill
be employed, or the precautions here suggested be
adopted.  Nor is it only in the boring of the cylinder
that it is r scessary to be careful that there is no change
of figure; for it will be impossible to face the valves
truly in the case of large cylinders, unless the cylinder
be placed on end, or internal props be introduced to
prevent the collapse due to the cylinder's weight.
162. Q.-Have you any thing further to add upon
the subject of cylinders?
A. - Nothing that may not be stated in a few
words.  Locomotive cylinders are generally made an
inch longer than the stroke, or there is half an inch of
clearance at each end of the cylinder, to permit the




THE STEAM ENGINE.               203
springs of the vehicle to act without causing the piston
to strike the top or bottom  of the cylinder.  The
thickness of metal of the cylinder ends is usually about
a third more than the thickness of the cylinder itself,
and both ends are generally made removable.  The
operation of priming is very injurious to the cylinders
and valves of locomotives, especially if the water be
sandy, as the grit carried over by the steam wears
the rubbing surfaces rapidly away.  The face of the
cylinder on which the valve works is raised a little
above the metal around it, both to facilitate the opera.
tion of facing, and with the view of enabling any
foreign substance deposited on the face to be pushed
aside by the valve into the less elevated part, where it
may lie without occasioning any further disturbance.
The valve casing is sometimes cast upon the cylinder,
and it is generally covered with a door which may be
removed to permit the inspection of the faces.  In
some valve casings the top as well as the back is removable, which admits of the valve and valve bridle
being removed with greater facility. A cock is placed
at each end of locomotive cylinders, to allow the water
to be discharged which accumulates in the cylinder
from priming or condensation; and the four cocks of
the two cylinders are usually connected together; so
that, by turning a handle, the whole are opened at
once. In Stephenson's engines, however, with variable
expansion, there is but one cock provided for this purpose, which is on the bottom of the valve chest.  In
all engines the valve casing, if made in a separate
piece from the cylinder, should be attached by means
of a metallic joint, as such a barbarism as a rust joint




204               A CATECHISM OF
in such situations is no longer permissible.  In the
case of large engines with valve casings suitable for
long slides, an expansion joint in the valve casing
should invariably be inserted; otherwise the steam, by
gaining admission to the valve casing before it can
enter the cylinder, expands the casing while the cylinder remains unaltered in its dimensions, and the
joints are damaged, and in some cases the cylinder is
cracked by the great strain thus introduced.  The
chest of the blow-through valve is very commonly
cast upon the valve casing; and in engines where the
cylinders are stationary, this is the most convenient
practice. All engines, where the valve is not of such
a construction as to leave the face when a pressure exceeding that of the steam is created in the cylinder by
priming or otherwise, should be provided with an escape valve to let out the water, and such valve should
be so constructed that the water cannot fly out with
violence over the attendants; but it should be conducted away by a suitable pipe, to a place where its
discharge can occasion no inconvenience.  The stuffing
boxes of all engines which cannot be stopped frequently to be repacked, should be made very deep:
metallic packing in the stuffing box has been used in
some engines, consisting in most instances of one or
more rings, cut, sprung, and slipped upon the piston
rod before the cross head is put on, and packed with
hemp behind.  This species of packing answers very
well when the parallel motion is true, and the piston
rod free from scratches, and it accomplishes a material
saving of tallow. In some cases a piece of sheet-brass,
packed behind with hemp, has been introduced with




THE STEAM ENGINE.                205
good effect, a flange being turned over on the under
edge of the brass to prevent it from slipping up or
down with the motion of the rod.  The sheet brass
speedily puts an excellent polish upon the rod, and
such a packing is more easily kept, and requires less
tallow than where hemp alone is employed.  In side
lever engines the attachments of the cylinder to the
diagonal stay are generally made of too small an area,
and the flanges are made too thick.  A  very thick
flange cast on any part of a cylinder endangers the
soundness of the cylinder, by inducing an unequal
contraction of the metal; and it is a preferable course
to make the flange for the attachment of the framing
thin, and the surface large-the bolts being turned
bolts and nicely fitted. If from malformation in this
part the framing works to an inconvenient extent, the
best expedient appears to be the introduction of a
number of steel tapered bolts, the holes having been
previously bored out; and if the flanges be thick
enough, square keys may also be introduced, half into
one flange and half into the other, so as to receive the
strain. If the jaw cracks or breaks away, however,
it will be best to apply a malleable iron hoop round
the cylinder to take the strain, and this will in all
cases be the preferable expedient, where, from any
peculiarities of structure, there is a difficulty in introducing bolts and keys.
163. Q.-Which is the most eligible species of piston?
A.-For large engines, pistons with a metallic packing, consisting of a single ring, with the ends mortised
into one another, and a piece of metal let in flush over
18




206               A CATECHISM OF
the joint and riveted to one end of the ring, appears to
be the best species of piston; and if the cylinder be
oscillating, it will be expedient to chamfer off the upper
edge of the ring on the inner side, and to pack it at the
back with hemp.  If the cylinder be a stationary one,
springs may be substituted for the hemp packing; but
in any case it will be expedient to make the vertical
joints of the ends of the ring run a little obliquely, so
as to prevent the joint forming a ridge in the cylinder.
For small pistons two rings may be employed, made
somewhat eccentric internally to give a greater thickness of metal in the centre of the ring; these rings
must be set one above the other in the cylinder, and the
joints, which are oblique, must be set at right angles
with one another, so as to obviate any disposition of the
rings in their expansion, to wear the cylinder oval.
The rings must first be turned a little larger than the
diameter of the cylinder, and a piece is then to be cut
out, so that when the ends are brought together the
ring will just enter within the cylinder. The ring, while
retained in a state of compression, is then to be put in
the lathe and turned very truly, and finally, it is to be
hammered on the inside with the small end of the hammer, to expand the metal, and thus increase the elasticity. The rings are then to be fitted laterally to the
piston, and to one another, by scraping-a steady pin
being fixed upon the flange of the piston, and fitting
into a corresponding hole in the lower ring, to keep the
lower ring from turning round; and a similar pin being
fixed into the top edge of the lower ring to prevent the
upper ring from turning round; but the holes into
which these pins fit must be made oblong, to enable the




THE STEAM ENGINE.               207
rings to press outward as the rubbing surfaces wear.
In most cases it will be expedient to press the packing
rings out with springs where they are not packed behind
with hemp, and the springs should be made very strong,
as the prevailing fault of springs is their weakness.
Sometimes short bent springs, set round at regular
intervals between the packing rings and body of the
piston, are employed, the centre of each spring being
secured by a steady pin or bolt screwed into the side
of the piston; but it will not signify much what kind
of spring is used, provided they have sufficient tension.
The piston rod, where it fits into the piston, should have
a good deal of taper; for if the taper be too small the
rod will be drawn through the hole, and the piston will
be split asunder. Small grooves are sometimes turned
out of the piston rod above and below the cutter hole,
and hemp is introduced in order to make the piston eye
tight.  Most piston rods are fixed to the piston by
means of a gib and cutter, but in some cases the upper
portion of the rod within the eye is screwed, and it is
fixed into the piston by means of an indented nut. This
nut is in some cases hexagonal, and in other cases the
exterior forms a portion of a cone which completely
fills a corresponding recess in the piston; but nuts
made in this way become rusted into their seat after
some time, and cannot be started again without much
difficulty.  Messrs. Miller, Ravenhill, and Co. fix in
their piston rods by means of an indented hexagonal
nut, which may be started by means of an open boxkey.  The thread of the screw is made flat upon the
one side and much slanted on the other, whereby a
greater strength is secured, without creating any dis



208               A CATECHISM OF
position to split the nut. In side lever engines it is a
judicious practice to add a nut to the top of the piston
rod, in addition to the cutter for securing the piston rod
to the cross head. In a good example of an engine thus
provided, the piston rod is 7 in. in diameter, and the
screw 5 in., the part of the rod which fits into the cross
head eye is 1 ft. 52 in. long, and tapers fiom 61 in. to
613 in. diameter.  This proportion of taper is a good
one: if the taper be less, or if a portion of the piston
rod within the cross head eye be left untapered, as is
sometimes the case, it is very difficult to detach the
parts from  one another.  When pistons are made of a
single ring, or of a succession of single rings, the
strength of each ring should be tested previously to its
introduction into the piston, by means of a lever loaded
by a heavy weight.  The old practice was to depend
chiefly upon grinding as the means of making the rings
tight upon the piston or upon one another; but scraping is now chiefly relied on. Some makers, however,
finish their steam surfaces by grinding them with powdered Turkey-stone and oil.  A  slight grinding, or
polishing, with powdered Turkey-stone and oil, appears
to be expedient in ordinary cases, and may be conveniently accomplished by setting the piston on a revolving table, and holding the ring stationary by a cross
piece of wood while the table turns round. Pieces of
wood may be interposed between the ring and the body
of the piston, to keep the ring nearly in its right position, but these pieces of wood should be fitted so loosely
as to give some side play, else the disposition would
arise to wear the flange of the piston into a groove.
Messrs. Penn's piston for oscillating engines has a single




THE STEAM ENGINE.                209
packing ring, with a tongue piece, or mortise end, made
in the manner already prescribed.  The ring is packed
behind with hemp packing, and the piece of metal
which covers the joint is a piece of thick sheet copper,
and is indented into the iron of the ring, so as to offer
no obstruction to the application of the hemp. The
ring is fitted to the piston only on the under edge: the
top edge is rounded to a point from the inside, and the
junk-ring does not bear upon it, but the junk-ring
squeezes down the hemp packing between the packing
ring and the body of the piston. The variety of pistons
employed in locomotives is very great, and sometimes
even the more complicated kinds are found to work very
satisfactorily; but, in general, those pistons which consist of a single ring and tongue piece, or of two single
rings set one above the other, so as to break joint,
are preferable to those which consist of many pieces.
In Stephenson's pistons the screws are liable to work
slack, and the springs to break. The piston rods of all
engines are now either case hardened very deeply, or are
made of steel; and in locomotive engines the diameter
of the piston rod is about one-seventh of the diameter
of the cylinder, and is formed of tilted steel. The cone
of the piston rod, by which it is attached to the piston,
is turned the reverse way to that which is adopted in
common engines, with the view of making the cutter
more accessible from the bottom of the cylinder, which
is made to come off like a door. The top of the piston
rod is secured with a cutter into a socket with jaws,
through the holes of which a cross head passes, which is
embraced between the jaws by the small end of the connecting rod, while the ends of the cross heads move in
18*




210               A CATECHISM OF
guides.  Between the piston rod clutch and the guide
blocks, the feed pump rod joins the cross head in some
engines. The guides are formed of steel plates attached
to the framing, between which work the'guide blocks,
fixed on the ends of the cross head, which have flanges
bearing against the inner edges of the guides.  Steel or
brass guides are better than iron ones. Stephenson and
Hawthorn attach their guides at one end to a cross stay,
at the other to lugs on the cylinder cover; and they
are made stronger in the middle than at the ends. Stout
guide rods of steel, encircled by stuffing boxes on the
ends of the cross head, would probably be found superior
to any other arrangement.  The stuffing boxes might
contain conical brushes cut spirally, in addition to the
packing, and a ring, cut spirally, might be sprung upon
the rod, and fixed in advance of the stuffing box, with
lateral play to wipe the rod before entering the stuffing
box, to prevent it from being scratched by the adhesion
of dust.
164. Q.-Will you explain the method of fitting
together the valve and cylinder faces?
A.-Both faces must first be planed, then filed according to the indications of a metallic straight edge,
and subsequently of a thick metallic face plate, and
finally scraped very carefully until the face plate bears
equally all over the surface. In planing any surface,
the catches which retain the surface on the planing
machine should be relaxed previously to the last cut,
to obviate distortion from  springing.  To  ascertain
whether the face plate bears equally, smear it over
with a little red ochre and oil, and move the face plate
slightly, which will fix the color upon the prominent




THE STEAM ENGINE.                211
points. This operation is to be repeated frequently,
and as the work advances, the quantity of coloring
matter is to be diminished, until finally it is spread
over the face plate in a thin film, which only dims the
brightness of the plate.  The surfaces at this stage
must be rubbed firmly together to make the points of
contact visible, and the higher points will become
slightly clouded, while the other parts are left more
or less in shade.  If too small a quantity of coloring
matter be used at first, it will be difficult to form a
just conception of the general state of the surface, as
the prominent points will alone be indicated; whereas
the use of a large quantity of coloring matter in the
latter stages, would destroy the delicacy of the test
the face plate affords. The scraping tool should be
of the best steel, and should be carefully sharpened
at short intervals on a Turkey-stone, so as to maintain
a fine edge.  A  flat file bent, and sharpened at the
end, makes an eligible scraper for the first stages;
and a three-cornered file, sharpened at all the corners,
is the best instrument for finishing the operation.  The
number of bearing points which it is desirable to establish on the surface of the work, depends on the use
to which the surface is to be applied, but whether it
is to be finished with great elaboration, or otherwise,
the bearing points should be dist:ibuted equally over
the surface.  It has already been stated, that in the
preparation of valve faces it is necessary to take into
account the strain which the metal suffers from  its
own weight; and, it may be added, that the change
of figure consequent thereupon is not instantaneous,
but becomes greater after some continuance of the




212               A CATECHISM OF
strain than it was at first; so that in gauging a cylinder to ascertain the difference of diameter when it
is placed on its side, it should have lain some days
upon its side to ensure the accuracy of the operation.
Face plates, or planometers, as they are sometimes
ternled, are supplied by most of the makers of engineering tools: every factory should be abundantly
supplied with them, and also with steel straight edges,
and there should be a master face plate, and a master
straight edge, for the sole use of testing, from time to
time, the accuracy of those in use.  In fitting the
faces of a D valve, great care must be taken that the
valve be not made conical: unless the back be exactly
parallel with the face, it will be impossible to keep
the packing from being rapidly cut away.  When the
valve is laid upon the face plate, the back must be
made quite fair along the whole length, by draw filing,
according to the indications of a straight edge; and
the distance from the face to the extreme height of
the back, must be made identical at each extremity.
Should a hole occur either in the valve, in the cylinder, or any other part where the surface requires to
be smooth, it may be plugged up with a piece of castiron, as nearly as possible of the same texture.  Bore
out the faulty part, and afterwards widen the hole
with an eccentric drill, so that it will be of the least
diameter at the mouth.  The hole may go more than
half through the iron; fit then a plug of cast-iron
roughly by filing, and hammer it into the hole, whereby the plug will become riveted in it, and its surface
may then be filed smooth.  Square pieces may be let
in after the same fashion, the hole being made dove



THE STEAM ENGINE.                 213
tailed, and the pieces thus fitted will never come out.
Brass faces are put upon valves or cylinders by means
of small brass screws, tapped into the iron with conical
necks for the retention of the brass; they are screwed
by means of a square head, which, when the screw is
in its place, is cut off and filed smooth. In some cases
the face is made of extra thickness, and a rim not so
thick runs round it, forming a step or recess for the
reception of brass rivets, the heads of which are clear
of the face. Much trouble is experienced with every
modification of valve face; but cast-iron working upon
cast-iron is, perhaps, the best combination yet introduced.  A usual practice is to pin brass faces on the
cylinder, allowing the valve to retain its cast-iron
face.  Some makers employ brass valves, and others
pin brass on the valves, leaving the cylinder with a
cast-iron face.  Speculum  metal and steel have been
tried for the cylinder faces, but only with moderate
success.  In some cases the brass gets into ruts; but
the most prevalent affection is a degradation of the
iron, owing to the action of the steam, and the face
assuming a granular appearance, something like loaf
sugar.  This action  shows itself only  at particular
spots and chiefly about the angles of the port or valve
face. At first the action is slow; but when once the
steam  has worked  a passage for itself, the cutting
away becomes very rapid, and, in a short time, it will
be  impossible  to prevent the engine from  heating
when stopped, owing to the leakage of steam  through
the valve into  the condenser.  Copper steam  pipes
seem to have some galvanic action on valve faces, and
malleable iron pipes have sometimes been substituted;




214               A CATECHISM OF
but they are speedily worn out by oxidation, and the
scales of rust which are carried on by the steam
scratch the valves and cylinders, so that the use of
copper pipes is the least evil. In some engines the
valve rod is fitted with a parallel motion, provision
being made for the attachment of its standards on the
valve cover; but more frequently guides are employed.
In those long D valves in which exhaustion is performed from below, it is expedient to cast two projections on the sole plate, to prevent the valve from
falling down inconveniently far when the valve links
are taken off. In locomotive engines, the valve universally employed is the three-ported valve as already
explained.
165. Q.-What is the most beneficial construction
of slide valve?
A.-The best construction of slide valve appears
to be that adopted by Messrs. Penn for their larger
engines, and which consists of a three-ported valve,
to the back of which a ring is applied of an area equal
to that of the exhaustion port, and which, by bearing
steam tight against the back of the casing, so that a
vacuum  may be maintained within the ring, puts the
valve in equilibrium, so that it may be moved with
an inconsiderable exercise of force. The back of the
valve casing is put on like a door, and its internal
surface is made very true by scraping.  There is a
hole through the valve so as to conduct away any
steam  which may enter within the ring by leakage,
and the ring is kept tight against the back of the
casing, by means of a ring situated beneath the bearing
ring, provided with four lugs, through which bolts




THE STEAM ENGINE.                215
pass tapped into bosses on the back of the valve, and,
by  unscrewing  these bolts, which may be done by
means of a box key which passes through holes in the
casings closed with screwed plugs, the lower ring is
raised upwards, carrying the bearing ring before it.
The rings must obviously be fitted over a boss upon
the back of the valve; and between the rings, which
are of brass, a gasket ring is interposed to compensate
by its compressibility for any irregularity of pressure,
and each of the bolts is provided with a ratchet collar
to prevent it from turning back, so that the engineer,
in tightening these bolts, will have no difficulty in
tightening them  equally, if he counts the number of
clicks made by the ratchet.  Where this species of
valve is used, it is indispensable that large escape
valves be applied to the cylinder, as a valve on this
construction is unable to leave the face.
166. Q.-Will you describe the configuration and
mode of attachment of the eccentric by which the
valve is moved?
A.-In marine engines, the eccentric is loose upon
the shaft, for the purpose of backing, and is furnished
with a back balance and catches, so that it may stand
either in the position for going ahead, or in that for
going astern.  The body of the eccentric is of castiron, and it is put on the shaft in two pieces. The
halves are put together with rebated joints to keep
them from separating laterally, and they are prevented
from sliding out by round steel pins, each ground into
both halves: square keys would probably be preferable to round pins in this arrangement, as the pins
tend to wedge the jaws of the eccentric asunder.  In




216                A CATECHISM OF
some cases the halves of the eccentric are bolted together, by means of flanges, which is, perhaps, the
preferable practice.  The eccentric hoop in  marine
and land engines is generally of brass; it is expedient
to cast an oil cup on the eccentric hoop, and where
practicable, a pan should be placed beneath the eccentric for the reception of the oil droppings. The notch
of the eccentric rod for the reception of the pin of the
valve shaft is usually steeled, to prevent inconvenient
wear; for when the sides of the notch wear, the valve
movement is not only disturbed, but it is very difficult
to throw the eccentric rod out of gear. It is found
to be preferable, however, to fit this notch with a
brass bush, for the wear is then less rapid, and it is
an easy thing to replace this bush with another when
it becomes worn.  The eccentric catches of the kind
usually employed in marine engines, sometimes break
off at the first bolt hole, and it is preferable to have
a bolt in advance of the catch face, or to have a hoop
encircling the shaft with the catches welded on it, the
hoop itself being fixed by bolts or a key.  This hoop
may either be put on before the cranks in one piece,
or afterwards in  two  pieces.  In  locomotives the
structure and attachments of the eccentric differ somewhat from the foregoing.  The body of the eccentric
here also is of cast-iron, but the eccentric hoops are
generally of wrought-iron, as brass hoops are found
liable to break. In inside cylinder engines the eccentrics are set on the axle between the cranks, and
they are put on in two pieces held together by bolts;
but in straight axle engines the eccentrics are cast in
a piece, and are secured on the shaft by means of a




THE STEAM ENGINE.                217
key.  The eccentric, when in two pieces, is retained
at its proper angle on the shaft by a pinching screw,
which is provided with a jam  nut to prevent it from
working loose.  A piece is left out of the eccentric in
casting it to allow of the screw being inserted, and
the void is afterwards filled by inserting a dove-tailed
piece of metal.  Stephenson  and  Hawthorn  leave
holes in their eccentrics on each side of the central
arm, and they apply pinching screws in each of these
holes.  The method of fixing the  eccentric to  the
shaft by a pinching screw is scarcely sufficiently substantial, and cases are perpetually occurring, when
this method of attachment is adopted, of eccentrics
shifting from their place. In the Rouen engines with
straight axles, the four eccentrics are cast in one
piece.  When the eccentric hoops are formed of malleable iron, one-half of the strap is forged with the
rod, the other half being secured to it by bolts, nuts,
and jam  nuts.  Pieces of brass are, in some cases,
pinned within the malleable iron hoop, but it appears
to be preferable to put brasses within the hoop to
encircle the eccentric, as in the case of any other
bearing.  When brass straps are used, the lugs have
generally nuts on both sides, so that the length of the
eccentric rod may be adjusted by their means to the
proper length; but it is better for the lugs of the hoops
to abut against the necks of the screws, and if any
adjustment be necessary from the wear of the straps,
washers can be interposed.  In some engines the ad.
justment is effected by screwing the valve rod, and
the cross head through which it passes has a nut on
either side of it, by which its position upon the valve
19




218               A CATECHISM OF
rod is determined.  The forks of the eccentric rod are
of steel; the length of the eccentric rod is the distance
between the centre of the crank axle, and the centre
of the valve shaft. The valve lever in locomotives is
usually longer than the eccentric lever, to increase the
travel of the valve. The pins of the eccentric lever wear
quickly; Stephenson puts a ferule of brass on these
pins, which being loose, and acting like a roller, facilitates the throwing in and out of gear, and when worn
can easily be replaced, so that there need be no material derangement of the motion of the valve from
play in this situation.  The starting  lever travels
between two iron segments, and can be fixed in any
desired position.  This is done by a small catch or
bell crank, joined to the bottom of the handle at the
end of the lever, and coming up by the side of the
handle, but pressed out from  it by a spring.  The
smaller arm of this bell crank is jointed to a bolt,
which shoots into notches, made in one of the segments between which the lever moves.  By pressing
the bell crank against the handle of the lever the bolt
is withdrawn, and the lever may be shifted to any
other point, when the spring being released, the bolt
flies into the nearest notch.
167. Q.-Will you explain the operation of expansion valves?
A.-Expansion valves have the effect of closing the
steam  passage leading to the cylinder before  the
stroke of the piston is completed, whereby the steam
shut within the cylinder is enabled to expand; and
the mechanical efficacy of a given bulk of steam is




THE STEAM ENGINEJ.              219
increased by the average pressure of the expanding
steam, multiplied by the distance through which it
has urged the piston.  The structure of expansion
valves is very various; some are slide valves, but the
expansion valve commonly used in marine engines is
of the kind used in the Cornish engines, and known
as the equilibrium valve; and it is usually worked by
a cam on the shaft.  The expansion cam  is put on
the shaft in two pieces, which are fastened to each
other by means of four bolts passing through lugs,
and is fixed to the shaft by keys.  A roller at one
end of a bell-crank lever, which is connected with the
expansion valve, presses against the cam so that the
motion of the lever will work the valve.  The roller
is kept against the cam  by a weight on a lever attached to the same shaft. If the cam were concentric
with the shaft, the lever which presses upon it would
remain stationary, and also the expansion valve; but
by the projection of the cam, the end of the lever
receives a reciprocating  motion, which is communicated to the valve. The position of this projection
determines the point in relation to the stroke at which
the valve is opened, and its circumferential length
determines the length of the time during which the
valve continues open.  The time at which the valve
should begin to open is the same under all circumstances, but the duration of its opening varies with
the amount of expansion desired. In order to obtain
this variable extent of expansion, there are several
projections made upon the cam, each of which gives
a different degree, or grade, as it is usually called, of
expansion.  These grades all begin at the same point




220               A CATECHISM OF
on the cam, but are of different lengths, so that they
would begin to move the lever at the same time, but
would differ in the time of returning it to its original
position.  The change of expansion is effected by
moving the roller on to the desired grade; which is
accomplished by slipping the lever carrying the roller
endways on the shaft or pin sustaining it.  In locomotive engines, where the use of cams is inadmissible,
other expedients are employed, of which those contrived by Stephenson and by Cabrey operate on the
principle of accomplishing the requisite variations of
expansion by altering the throw  of the slide valve.
Stephenson connects the ends of the forward and
backward eccentric rods by a link with a curved slot,
in which a pin upon the end of the valve rod works.
By moving this link so as to bring the forward eccentric rod in the same line with the valve rod, the
valve  receives the  motion due  to that eccentric;
whereas, if the backward eccentric rod is brought in a
line with the valve rod, the valve gets the motion
proper for reversing, and if the link be so placed that
the valve rod is midway between the two eccentric
rods, the valve will remain nearly stationary.  Mr.
Cabrey makes his eccentric rod terminate in a pin
which works into a straight slotted lever, furnished
with jaws similar to the jaws on the eccentric rods of
locomotives.  By raising the pin of the eccentric rod
in this slot, the travel of the valve will be varied, and
expansive action will be the result.  Both Stephenson's and Cabrey's expansion gear are inferior in efficacy to those which operate by the aid of a separate
valve.  Gonzenbach's arrangement consists of an addi



THE STEAM ENGINE.               221
tional slide valve and valve casing placed on the back
of the ordinary slide valve casing, and through this
supplementary valve the steam must first pass. This
supplementary valve is worked by a double-ended
lever, slotted at one end for the reception of a pin on
the valve link, the position of which in the slot determines the throw of the supplementary valve, and the
consequent degree of expansion. The other end of
the lever is provided with a pin, which gears into a
notch on the backward eccentric rod, when that rod
is not in gear with the steam valve. By this arrangement, while the steam  valve receives as usual the
motion of the forward eccentric, the expansion valve
receives the motion of the reversing eccentric.  In
the expansion gear of Meyer, the steam  valve is a
plate perforated by two ports, which are covered by
two blocks working upon the back of the valve, and
retained on a spindle which screws into both blocks,
but in one with a right-handed screw, and in the
other with a left-handed screw, so that by turning
the spindle the distance between the blocks may be
increased or diminished, and the degree of expansion
altered accordingly.  The amount of expansion may
also be varied by means of a movable plate, which, by
closing the port more or less, and wire-drawing the
steam, will accomplish the amount of expansion which
is desired.
168. Q.-What are the details of the air pump?
A. —The air pump bucket and valves are all of
b:ass in all modern engines, and the chamber of the
pump is lined with copper, or made wholly of brass,
whereby a single boring suffices.  When a copper
19*




222               A CATECHISM OF
lining is used, the pump is first bored out, and a bent
sheet of copper is introduced, which is made accurately to fill the place, by hammering the copper on
the inside.  Air pump rods of Muntz's metal or copper are much used.  Iron rods covered with brass
are generally wasted away where the bottom cone fits
into the bucket-eye, and if the casing be at all porous
the water will insinuate itself between the casing and
the rod and eat away the iron.  If iron rods covered
with brass be used, the brass casing should come some
distance into the bucket-eye; the cutter should be ol
brass, and a brass washer should cover the under side
of the eye, so as to defend the end of the rod from the
salt water.  Rods of Muntz's metal are probably on
the, whole to be preferred. It is a good practice to
put a nut on the top of the rod, to secure it more
firmly in the cross head eye, where that plan can be
conveniently adopted.  The part of the rod which fits
into the cross head eye, should have more taper when
made of copper or brass, than when made of iron; as
if the taper be small, the rod may get staved into the
eye, whereby its detachment will be difficult.  Metallic packing has in some instances been employed
in air pump buckets, but its success has not been
such as to lead to its further adoption. A deep solid
block of metal however, without any packing, has in
a few instances been employed with a satisfactory
result.  Where ordinary packing is employed, the
bucket should always be made with a junk ring,
whereby the packing may be easily screwed down at
any time with facility. The bucket valve is generally
of the spindle or pot-lid kind, but butterfly valves are




THE STEAM ENGINE.               223
sometimes used.  The foot and delivery valves are
for the most part of the flap or hanging kind. These
valves all make a considerable noise in working, and
are objectionable in many ways.  Valves on Belidor's
construction, which is in effect that of a throttle valve
hung off the centre, were some years ago proposed
for the delivery and foot valves; and it appears probable that their operation would be more satisfactory
than that of the valves usually employed. Some delivery valve seats are bolted into the mouth of the air
pump, whereby access to the pump bucket is rendered
difficult. If delivery valve seats be put in the mouth
of the air pump at all, the best mode of fixing them
appears to be that adopted by Messrs. Maudslay. The
top of the pump barrel is made quite fair across, and
upon this flat surface a plate containing the delivery
valve is set, there being a small ledge all round, to
keep it steady.  Between the bottom of the stuffing
box of the pump cover and the eye of the valve seat a
short pipe extends encircling the pump rod, its lower
end checked into the eye of the valve seat, and its
upper end widening out to form  the bottom  of the
stuffing box of the pump cover. Upon the top of this
pipe some screws press, which are accessible from the
top of the stuffing box gland, and the packing also
aids in keeping down the pipe, the function of which
is to retain the valve seat in its place. When the
pump bucket has to be examined the valve seat may
be slung witW  the cover, so as to come up with
the same purchase.  For the bucket valves, Messrs.
Maudslay employ two or more concentric ring valves,
with a small lift.  These valves have given a good




224               A CATECHISM OF
deal of trouble in some cases, in consequence of the
frequent fracture of the bolts which guide and confine
the rings; but this is only a fault of detail, which is
easily remedied, and the principle appears to be superior to that of any of the other air pump valves at
present in common use.
169. Q.-What are the most important details of
the construction of paddle wheels?
A.-The structure of the feathering wheel has already been described in connection with a description
of the oscillating engine; and it will be expedient now
to restrict any account of the details to the common
radial paddle, as applied to ocean steamers. The best
plan of making the paddle centres is with square eyes,
and each centre should be secured in its place by means
of eight thick keys. The shaft should be burred up
against the heads of these keys with a chisel, so as to
prevent the keys from coming back of their own accord. If the keys are wanted to be driven back, this
burr must be cut off; and if made thick, and of the
right taper, they may then be started without difficulty.
The shaft must of course be forged with square projections on it, so as to be suitable for the application
of centres with square eyes.  Messrs. Maudslay and
Co. bore out their paddle centres, and turn a seat for
them on the shaft, afterwards fixing them on the shaft
with a single key. This plan is objectionable for the
two reasons, that it is insecure when new, and when
old is irremovable.  The general practe among the
London engineers is to fix the paddle arms at the
centre to a plate by means of bolts, a projection being
placed upon the plates on each side of the arm, to




THE STEAM ENGINE.                225
prevent lateral motion; but this method is inferior in
durability to that adopted in the Clyde, in which each
arm  is fitted into a socket by means of a cutter,-a
small hole being left opposite to the end of each arm,
whereby the arm may be forced back by a drift.  A
preferable way would be to form the paddle centre out
of the arms themselves, by widening them at the head
until they touch  one another, and then applying a
boiler plate upon one side, and riveting the arms
firmly to it. If this plan be adopted, it will be expedient to swell the tops of the uncovered side at the
part nearest the centre of the shaft.  In the manufacture of this centre, the heads of the arms would
first be forged, then placed on the edges, and fitted
together on the plate. The holes would then be bored
for the rivets, temporary bolts fitted into them, and the
key-seats cut, and the ends of the arms pared in the
slotting machine.  Finally, the arms would be welded
on to the heads, and the various parts of the wheel
ri-eted together.  Some engineers join the paddle arms
to the outer ring by means of bolts; but those bolts
after a time generally become slack sideways, and a
constant working of the parts of the wheel goes on in
consequence.  Sometimes the part of the outer ring
opposite the arm is formed into a mortise, and the arms
are wedged tight in these holes by wedges driven in on
each side; but the plan is an expensive one, and not
satisfia tory, as the wedges work loose even though
riveted over at the point. The best mode of making a
secure attachment of the arms to the ring, consists in
making the arms with long T heads, and riveting the
cross piece to the outer ring with a number of rivets,




226               A CATECHISM OF
not of the largest size, which would weaken the outer
ring too much.  The best way of securing the inner
rings to the arms is by means of lugs welded on the
arms, and to which the rings are riveted.  The paddle
floats are usually made either of elm or pine: if of the
former, the common thickness for large sea-going
vessels, is about 21 inches; if of the latter, 3 inches.
The floats should have plates on both sides, else the
paddle arms will be very liable to cut into the wood,
and the iron of the arms will be very rapidly wasted.
When the floats have been fresh put on they must be
screwed up several times before they come to a bearing.
If this be not done the bolts will be sure to get slack
at sea, and all the floats on the weather side may be
washed off. It is a good plan to give the thread of the
paddle bolts a nick with a chisel, after the nut has been
screwed up, which will prevent the nut from turning
back. The floats should not be notched to allow of
their projection beyond the outer ring, as if the sides
of the notch be in contact with the outer ring, the
ring is soon eaten away in that part, and the projecting part of;he float, being unsupported, is liable to be
broken off.  It is usual to put a steel plate at each
end of the paddle shafts tightened with a key, to
prevent end-play when the vessel rolls, but the arrangement is precarious and insufficient.  Messrs. Maudslay
make their paddle shaft bearings with very large fillets
in the corner, with the view of diminishing the evil;
but it would be preferable, we conceive, to make the
bearings of the crank shafts spheroidal; and, indeed,
it would probably be an improvement, if most of the
bearings about the engine were to be made in the same




TIE STEAM ENGINE.              227
fashion.  The loose end of the crank pin should be
made not spheroidal, but consisting of a portion of a
sphere; and a brass bush might then be fitted into the
crank eye, that would completely incase the ball of the
pin, and yet permit the outer end of the paddle shaft
to fall without straining the pin, the bush being at the
same time susceptible of a slight end motion.  The
paddle shaft, where it passes through the vessel's side,
is usually surrounded by a lead stuffing box, which
will yield if the end of the shaft falls: this stuffing
box prevents leakage into the ship from the paddle
wheels; but it is expedient, as a further precaution,
to have a small tank on the ship's side immediately
beneath the stuffing box, with a pipe leading down to
the bilge to catch and conduct away any water that
may enter around the shaft. The bearing at the outer
end of the paddle shaft is sometimes supplied with
tallow, forced into a hole in the plummer block cover,
as in the case of water wheels; but for vessels intended
to perform long voyages, it is preferable to have a pipe
leading down to the oil cup above the journal from the
top of the paddle box, through which pipe oil may at
any time be supplied. The bolts for holding on the
paddle floats are made extra strong, on account of the
corrosion to which they are subject;- and the nuts
should be made large, and should be square, so that
they may be effectually tightened up, even though
their corners be worn away by corrosion.  Paddle
floats, when consisting of more than one board, should
be bolted together edgeways, by means of bolts running
through their whole breadth.
170. Q. -Will you describe the structure and




228              A CATECHISM OF
arrangement of the pumps, pipes, and cocks of locomotive and marine engines?
A. - The feed pumps of locomotives are generally
made of brass, but the plungers are sometimes made
of iron, and are generally attached to the piston cross
head, though in Stephenson's engines they are worked
by rods attached to eyes on the eccentric hoops.
There is a ball valve between the pump and the
tender, and two usually in the pipe leading from the
pump to the boiler, besides a cock close to the boiler,
by which the pump may be shut off from the boiler in
case of any accident to the valves.  The ball valves
are guided by four branches, which rise vertically,
and join together at the top in a hemispherical form.
The shocks of the ball against this cap have in some
cases broken it after one week's work, from the top of
the cage having been flat, and the branches not having
had their junction at the top properly filleted. These
valve guards are attached in different ways to the
pipes; when one occurs at the junction of two pieces
of pipe it has a flange, which, along with the flanges
of the pipes and that of the valve seat, are held together by a union joint.  It is sometimes formed with
a thread at the under end, and screwed into the pipe.
The balls are cast hollow to lessen the shock, as wll
as to save the metal.  In some cases where the feed
pump plunger has been attached to the cross head,
the piston rod has been bent by the strain; and that
must in all cases occur, if the communication between
the pump and boiler be closed when the engine is
started, and there be no escape valve for the water.
Spindle valves have in some cases been used instead




THE STEAM ENGINE.               229
of ball valves, but they are more subject to derangement; but piston valves, so contrived as to shut a
portion of water in the cage when about to close,
might be adopted with a great diminution of the
shock.  Slide valves might easily be applied, and
would probably be found preferable to any of the expedients at present in use.  It would be a material
improvement if the feed pumps were to be set in the
tender, and worked by means of a small engine, such
as that now used in steam  vessels for feeding the
boilers. The present action of the feed pumps of locomotives is precarious, as if the valves leak in the
slightest degree the steam  or boiling water from the
boiler will prevent the pumps from drawing.  It appears expedient, therefore, that the pumps should be
far from the boiler, and should be set among the feed
water, so that they will only have to force.  If the
pumps were arranged in the manner suggested, the
boiler could still be fed regularly, though the locomotive
was standing still; but it would be prudent to have
one pump still wrought in the usual way by the engine, in case of derangement of the other, or in case
the pump in the tender might freeze. The pipes connecting the tender with the pumps should allow access
to the valves and free motion to the engine and tender.
This end is attained by the use of ball and socket
joints; and, to allow some end play, one piece of the
pipe slides into the other like a telescope, and is kept
tight by means of a stuffing box.  Any pipe joint between the engine and tender must be made in this
fashion.  The feed pipe of most locomotive engines
enters the boiler near the bottom, and about the
20




230              A CATECHISM OF
middle of its length.  In Stephenson's engine thle
water is let in at the smoke box end of the boiler, a
little below the water level: by this means the heat is
more fully extracted from the escaping smoke, but the
arrangement is of questionable applicability to engines
of which the steam dome and steam pipe are at the
smoke box end, as in that case the entering cold water
would condense the steam.  In steam vessels, the feed
pump plunger is generally of brass, and the barrel of
the pump is sometimes of brass, but generally of castiron. There should be a considerable clearance between the bottom of the plunger and the bottom of the
barrel, as otherwise the bottom of the barrel may be
knocked out, should coal dust or any other foreign
substance gain admission, as it probably would do if
the injection water were drawn at any time from the
bilge of the vessel, as is usually done if the vessel
spring a leak. The valves of the feed pump in marine
engines are generally of the spindle kind, and are
most conveniently arranged in a chest, which may be
attached in any accessible position to the side of the
hot well. There are two side nozzles upon this chest,
of which the lower one leads to the pump, and the
upper one to the boiler.  The pipe leading to the
pump is a suction pipe when the plunger ascends, and
a forcing pipe when the plunger descends.  The
plunger in ascending draws the water out of the hot
well through the lowest of the valves, and, in descending, forces it through the centre valve into the
space above it, which communicates with the feed pipe.
Should the feed cock be shut so as to prevent any feed
water from passing through it, the water will raise t he




THE STEAM ENGINE.               231
topmost valve, which is loaded to a pressure considerably above the pressure of the steam, and escape into
the hot well. This arrangement is neater and less expensive than that of having a separate loaded valve on
the steam pipe, with an overflow through the ship's
side, as is the more usual practice.  To enable the
boilers to be fed in steam vessels, there is a separate
pump provided distinct from the engines, and which
may be worked by men standing on the deck by means
of appropriate handles; and this pump, in addition to
its function of replenishing the boilers, is used to wash
the decks, or as a fire engine to quench accidental fire.
For that purpose, a double acting feed pump of the
plunger kind is preferable to one which operates by a
piston.  The air vessel of the pump should be furnished
with an escape valve to prevent the pump from being
split, should it be put in connection with the engine
when the cocks in the pipe leading to the boiler are
closed, an accident which not unfrequently happens.
In this species of pump, the application of a four-way
cock enables the pump to draw from the sea, from the
boiler, or from  the bilge, and the pump can deliver
either into the boiler or upon deck. In most of the
new vessels fitted with tubular boilers, small engines
have been introduced to pump water into the boiler
when the vessel stops under steam.  Most of these
engines are furnished with a crank and fly wheel;
but as there is no strain on the fly wheel shaft, the
crank is connected to the piston rod by means of a
horizontal slot.  In some steam  vessels floats have
been introduced to regulate the feed, but their action
cannot be depended on in agitated water, if applied




232               A CATECHISM OF
after the common fashion.  Floats would probably
answer if placed in a cylinder which communicates
with the water in the boiler by means of small holes;
and a disc of metal might be attached to the end of a
rod extending beneath the water level, so as to resist
irregular movements from  the motion of the ship,
which would otherwise impair the action of the apparatus. The admission of feed water into the boiler is
sometimes  regulated  by  cocks, and  sometimes by
spindle valves raised and lowered by a screw.  Cocks
appear to us to be the preferable expedient, as they are
less liable to accident or derangement than screw
valves, and in modern steam vessels they are generally
employed.  The feed water is usually conducted from
the feed cock to a point near the bottom of the boiler by
means of an internal pipe, the object of this arrangement being to prevent the rising steam  from  being
condensed by the entering water.  By being  introduced near the bottom of the boiler, the water comes
into contact in the first place with the bottoms of the
furnaces and flues, and extracts heat from them which
could not be extracted by water of a higher temperature, whereby a saving of fuel is accomplished. In
some cases the feed water is introduced into a casing
around the chimney, from whence it descends into the
boiler. This plan appears to be an expedient one when
the boiler is short of heating surface, and more than a
usual quantity of heat ascends the chimney; but in
well-proportioned  boilers a water casing round  the
chimney is superfluous.  When a water casing is used
the boiler is usually fed by a head of water, the feed
water being forced up into a small tank, from whence




THE STEAM ENGINE.                 233
it descends into the boiler by the force of gravity,
while the surplus runs to waste, as in the feeding apparatus of land engines.  The blow-off cocks of a
boiler are generally placed some distance from  the
boiler, but it appears preferable that they should be
placed quite close to it, as there are no means of shutting off the water from the pipe between the blow-off
cock and the boiler, should fracture or leakage there
arise. Every boiler must be furnished with a blow-off
cock of its own, independently of the main blow-off
cocks on the ship's side, so that the boilers may be
blown off separately, and may be shut off fiom  one
another.  The preferable arrangement appears to be,
to cast upon each blow-off cock a bend for attaching
the cock to the bottom  of the boiler, and the plug
should stand about an inch in advance of the front of
the boiler, so that it may be removed, or re-ground,
with facility. The general arrangement of the blowoff pipes is to put a main blow-off pipe beneath the
floor plates, across the ship, at the end of the engines,
and into this pipe to lead a separate pipe, furnished
with a cock, from  each boiler.  The main blow-off
pipe, where it penetrates the ship's sides, is furnished
with a cock: and in modern steam  vessels Kingston's
valves are also used, which consist of a spindle or plate
valve, fitted to the exterior of the ship, so that, if the
internal pipe or cock breaks, the external valve will
still be operative.  Some expedient of this kind is
almost necessary, as the blow-off cocks require occasional re-grinding, and the sea cocks cannot be reground without putting the vessel into dock, except
by the use of Kingston's valves, or some equivalent
20*




234               A CATECHISM OF
expedient. All the cocks about an engine should be
provided with bottoms and stuffing boxes, and reliance
should never be placed upon a single bolt passing
through a bottom  washer for keeping the plug in its
place, in the case of any cock communicating with the
boiler; for a great strain is thrown upon that bolt if
the pressure of the steam be high, and if the plug be
made with much taper, and should the bolt break, or
the threads strip, the plug will fly out, and persons
standing near may be scalded to death.  In large
cocks, it appears the preferable plan to cast the bottoms
in; and the metal of which all the cocks about a
marine engine are made, should be of the same quality
as that used in the composition of the brasses, and
should be without lead, or other deteriorating material.
In some cases the bottoms of cocks are burnt in with
hard solder, but this method cannot be depended upon,
as the solder is softened and wasted away by the hot
salt water, and in time the bottom leaks, or is forced out.
The stuffing box of cocks should be made of adequate
depth, and the gland should be secured by means of
four strong copper bolts. The taper of blow-off cocks
is an important element in their construction; as, if
the taper be too great, the plugs will have a continual
tendency to rise, which, if the packing be slack, will
enable grit to get between the faces, while, if the taper
be too little, the plug will be liable to jam, and a few
times grinding will sink it so far through the shell that
the water-ways will no  longer correspond.  Oneeighth of an inch deviation from the perpendicular for
every inch in height, is a common angle for the side
of the cock, which corresponds with one quarter of an




THE STEAM ENGINE.             235
inch difference of diameter in an inch of height; but
perhaps a somewhat greater taper than this, or onethird of an inch difference in diameter for every inch
of height, is a preferable proportion. The bottom of
the plug must be always kept a small distance above
the bottom of the shell, and an adequate surface must
be left above and below the water-way to prevent
leakage.  Cocks formed according to these directions
will be found to operate satisfactorily in practice,
while they will occasion perpetual trouble if there be
any malformation.  Gauge cocks are generally very
inartificially made, and occasion needless annoyance.
They are rarely made with bottoms, or with stuffing
boxes, and  are  consequently, for the  most part,
adorned with stalactites of salt after a short period of
service.  The water discharged from them, too, from
the want of a proper conduit, disfigures the front of
the boiler, and adds to the corrosion in the ash pits.
It would be preferable to combine the gauge cocks appertaining to each boiler into a single upright tube,
connected suitably with the boiler, and the water
flowing from them could be directed downwards into a
funnel tube communicating with the bilge. The cocks
of the glass tubes, as well as of the gauge cocks, should
be furnished with stuffing boxes and with bottoms, unless the water enters through the bottom  of the plug,
which in gauge cocks is sometimes the case.  The
glass gauge tubes should always be fitted with a cock
at each neck communicating with the boiler, so that
the water and steam may be shut off if the tube breaks;
and the cocks should be so made as to admit of the
tubes being blown through with steam to clear them,




236               A CATECHISM OF
as in muddy water they will become so soiled that the
water cannot be seen.  The gauge cocks frequently
have pipes running up within the boiler, to the end
that a high water-level may be made consistent with
an easily accessible position of the gauge cocks themselves. With the glass tubes, however, this species of
arrangement is not possible, and the glass tubes must
always be placed in the position of tie water level.
The sea injection cocks are usually made in the same
fashion as the sea blow-off cocks, and of about the
same size.  The injection water is generally admitted
to the condenser by means of a slide valve; but a cock
appears to be preferable, as it is more easily opened,
and has not any disposition to shut of its own accord.
The sea injection pipes should be put through the
ship's sides in advance of the paddles, so that the
water drawn in may not be injuriously charged with
air. The waste water pipe passing from the hot well
through the vessel's side is provided with a stop valve,
called the discharge valve, which is usually made of
the spindle kind, so as to open when the water coming
from  the air pump presses against it.  In some cases
this valve is a sluice valve, but the hot well is then
almost sure to be split, if the engine be set on without
the valve having been opened.  The opening of the
waste water pipe should always be above the load
water line, as it will otherwise be difficult to prevent
leakage through the engine into the ship when the
vessel is lying in harbor.  Where the pipes pierce the
ship's side, they should be made tight, as follows:The hole being cut, a short piece of lead pipe, with a
broad flange at one end, should be fitted into it, the




TIE STEAM ENGINE.               237
place having been previously smeared with white lead,
and the pipe should then be beaten on the inside, until
it comes into close contact all around with the wood.
A loose flange should next be slipped over the projecting end of the lead pipe, to which it should be
soldered, and the flanges should both be nailed to the
timber with scupper nails, white lead having been
preriously spread underneath.  This method of procedure, it is clear, prevents the possibility of leakage
down through the timbers; and all, therefore, that has
to be guarded against after this precaution, is to prevent leakage into the ship.  To accomplish this object,
let the pipe which it is desired to attach be put through
the leaden hause, and let the space between the pipe
and the lead be packed with gasket and white lead, to
which a little olive oil has been added.  The pipe
must have a flange upon it to close the hole in the
ship's side; the packing must then be driven in from
the outside, and be kept in by means of a gland
secured with bolts passing through the ship's side.
If the pipe is below the water line, the gland must be
of brass; but for the waste water pipe, a cast-iron
gland will answer.  This method of securing pipes
penetrating the side, however, though the best for
wooden vessels, will, it is clear, fail to apply to iron
ones.  In the case of iron vessels, it appears to be the
best practice to attach a short iron nozzle, projecting
inwards, to the skin, for the attachment of every pipe
below the water line, as the copper or brass would
waste the iron of the skin if the attachment were made
in the usual way.  Marine boilers are now generally
supplied with stop valves, whereby one boiler may be




238                A CATECHISM OF
thrown out of use without impairing the efficacy of
the remainder.  These stop valves are usually spindle
valves of large size, and they are for the most part set
in a pipe which runs across the steam chests, connecting the several boilers together. The spindles of these
valves should project through stuffing boxes in the
covers of the valve chests, and they should be balanced
by a weighted lever, and kept in continual action by
the steam. If the valves be lifted up, and be suffered
to remain up, as is the usual practice, they will become
fixed by corrosion in that position, and it will be impossible after some time to shut them  on an emergency.  These valves should always be easily accessible from the engine room; and it ought not to be
necessary for the coal boxes to be empty to gain access
to them.  The pipes of marine engines should always
be made of copper.  Cast-iron blow-off pipes have in
some cases been employed, but they are liable to fracture, and are dangerous.  Every pipe passing through
the ship's side, and every pipe fixed at both ends, and
liable to be heated and cooled, should be furnished
with a faucet or expansive joint; and in the case of
cast-iron pipes, the part of the pipe fitting into the
faucet should be turned.  In the distribution of the
faucets of the pipes exposed to pressure, care must be
taken that they be so placed, that the parts of the pipe
cannot be forced asunder by the strain, as serious accidents have occurred from the neglect of this precaution.  In locomotives, the admission of the steam from
the boiler to the cylinders is regulated by a valve called
the regulator, which is generally placed immediately
above the internal fire box, and is connected with two




THE STEAM ENGINE.              239
copper pipes; one conducting steam from the highest
point of the dome down to it; and the other conducting the steam  that has passed through it along the
boiler to the upper part of the smoke box. Regulators
may be divided into two sorts, viz. those with sliding
valves and steam ports, and those with conical valves
and seats, of which the latter kind are the best.  The
former kind have for the most part consisted of a
circular valve and face, with radial apertures, the valve
resembling the outstretched wings of a butterfly, and
being made to revolve on its central pivot by connecting links between its outer edges, or by its central
spindle. In some of Stephenson's engines with variable expansion gear, the regulator consists of a slide
valve covering a port on the top of the valve chests.
A rod passes from this valve through the smoke box
below the boiler, and by means of a lever parallel to
the starting lever, is brought up to the engineer's
reach.  Cocks were at first used as regulators, but
were given up, as they were found liable to stick fast.
A gridiron slide valve has been used by Stephenson,
which consists of a perforated square moving upon a
face with an equal number of holes.  This plan of a
valve gives, with a small movement, a large area of
opening.  In Bury's engines a sort of conical plug is
used, which is withdrawn by turning the handle in
front of the fire box: a spiral groove of very large
pitch is made in the valve spindle, in which fits a pin
fixed to the boiler, and by turning the spindle an end
motion is given to it, which either shuts or opens the
steam passage according to the direction in which it is
turned. The best regulator would probably be a valve




240               A CATECHISM OF
of the equilibrium description, such as is used in the
Cornish engine: there would be no friction in such a
regulator, and it could be opened or shut with a small
amount of force.
171. Q.-Will you enumerate the most interesting
details which occur to you in connection with the
structure of locomotives?
A.-All locomotives are now made with the framing
which supports the machinery situated within  the
wheels; but for some years a vehement controversy
was maintained respecting the relative merits of outside
and inside framing, which has terminated however in
the universal adoption of the inside framing. A similar
diversity of opinion obtains at present as to the relative
merits of outside and inside cylinders, the outside
cylinders being so designated when placed upon the
outside of the framing, with their connecting  rods
operating upon pins in the driving wheels; while the
inside cylinders are situated within the framing, and
the connecting rods attach themselves to cranks in the
driving axle.  The chief objection to outside cylinders
is, that they occasion a sinuous motion in the engine
which is apt to send the train off the rails; but this
action may be made less perceptible or be remedied
altogether, by setting the crank pins nearly in the
same line instead of at right angles, or by placing a
weight upon one side of the wheels, the mQmentum of
which will just balance the momentum  of the piston
and its connections.  The sinuous or rocking motion of
locomotives is traceable to the arrested momentum  of
the piston and its attachments at every stroke of the
engine, and the effect of the pressure thus created will




THE STEAM ENGINE.               241
be more operative in inducing oscillation the farther it
is exerted from the central line of the engine. If both
cylinders were set at right angles in the centre of the
carriage, and the pistons were both attached to a
central crank, there would be no oscillation produced;
or the same effect would be realized by placing one
cylinder in the centre of the carriage, and two at the
sides-the pistons of the side cylinders moving simultaneously; but it is impossible to couple the piston of
an upright cylinder direct to the axle of a locomotive,
without causing the springs to work up and down
with every stroke of the engine: and the use of three
cylinders, though adopted in some of Stephenson's
later engines, involves too much complication to be a
beneficial innovation.  It is difficult, in engines intended for the narrow gauge, to get cylinders within
the framing of sufficient diameter to meet the exigencies of railway locomotion; by casting both cylinders in a piece, however, a considerable amount of
room may be made available to increase their diameters.
It is very desirable that the cylinders of locomotives
should be as large as possible, so that expansion may
be adopted  to a large extent; and with any given
speed of piston, the power of an engine either to draw
heavy loads, or achieve high velocities, will be increased
with every increase of the dimensions of the cylinder.
The framing of locomotives, to which the boiler and
machinery are attached, and which rests upon  the
springs situated above the axles, is formed generally
of malleable iron, but in some engines the side frames
consist of oak with iron plates riveted on each side.
The guard plates are in these cases generally of equal
21




242               A CATECHTSM OF
length, the frames being curved upwards to pass over
the driving axle.  Hard cast-iron blocks are riveted
between the guard plates to serve as guides for the
axle bushes. The side frames are connected across at
the ends, and cross stays are introduced beneath the
boiler to stiffen the frame sideways, and prevent the
ends of the connecting or eccentric rods from faling
down if they should be broken.  The springs are of
the ordinary carriage kind, with plates connected at
the centre, and allowed to slide on each other at their
ends.  The upper plate terminates in two eyes, through
each of which passes a pin, which also passes through
the jaws of the bridle, connected by a double-threaded
screw to another bridle, which is jointed to the framing;
the centre of the spring rests upon the axle box.
Sometimes the springs are placed between the guard
plates, and below the framing which rests upon their
extremities. One species of spring, which has gained
a considerable introduction, consists of a number of
flat steel plates, with a piece of metal or other substance
interposed between  them  at the centre, leaving the
ends standing apart.  It would be preferable, perhaps,
to make the plates of a common spring with different
curves, so that the leaves, though in contact at the
centre, would not be in contact at the ends with light
loads, but would be brought into contact gradually,
as the strain comes on: a spring would thus be obtained  that was suitable for all loads.  Behind the
locomOtive runs another carriage, called a tender, for
holding coke and water.  A  common mode of connecting the engine and tender is by means of a rigid
bar, with an eye at each end through which pins are




THE STEAM ENGINE.               243
passed.  Between the engine and tender, however,
buffers should always be interposed, as their pressure
contributes greatly to prevent oscillation and other
irregular motions of the engine.  In most engines a
bar is strongly attached to the front of the carriage on
each side, and projects perpendicularly downwards to
within a short distance of the rail, to clear away stones
or other obstructions that might occasion accidents if
the engine ran over them. The axles bear only against
the top of the axle boxes, which are generally of brass,
but a plate extends underneath the bearing, to prevent
sand from being thrown up on it.  The upper part of
the box in most engines has a reservoir of oil, which
is supplied to the journal by tubes with siphon wicks.
Stephenson uses cast-iron axle boxes with brasses,
and grease instead of oil; and the grease is fed upon
the journal by the heat of the bearing melting it,
whereby it is made to flow down through a hole in
the  brass.  Any  engines constructed with  outside
bearings have inside bearings also, which are supported by longitudinal bars, which serve also in some
cases to support the piston guides; these bearings are
sometimes made so as not to touch the shafts unless
they break.  The wheels of a locomotive are always
made of malleable iron. The driving wheels are made
large to increase the speed; the bearing wheels also
are easier on the road when large.  In the goods
engines the driving wheels are smaller than in the
passenger engines, and are generally coupled together.
Wheels are made with much variety in their constructive details; sometimes they are made with castiron naves, with the spokes and rim of wrought-iron;




244                A CATEClHISM OF
but in the best modern wheels, the nave is formed of
the ends of the spokes welded together at the centre.
When  cast-iron naves are adopted, the  spokes are
forged out of flat bars with T formed heads, and are
arranged radially in the founders' mould, the cast-iron,
when fluid, being poured among them.  The ends of
the T heads are then welded together to constitute the
periphery of the wheel or inner tire; and little wedgeform pieces are inserted where there is any deficiency
of iron. In some cases the arms are hollow, though
of wrought-iron; the tire of wrought-iron, and the
nave of cast-iron; and the spokes are turned where
they are fitted into the nave, and are secured in their
sockets by means of cutters.   Hawthorn makes his
wheels with  cast-iron  naves and wrought-iron  rims
and arms; but instead of welding the arms together,
he makes palms on their outer end, which are attached
by rivets to the rim. These rivets, however, unless very
carefully formed, are apt to work loose; and it would
probably be found an improvement if the palms were
to be slightly indented into the rim, in cases in which
the palms do not meet each other at the ends. When
the rim is turned it is ready for the tire, which is now
made of steel. The materials for wheel tires are first
swaged separately, and then welded together under the
heavy hammer at the steel works; after which they
are bent to the circle, welded, and turned to certain
gauges.  The tire is now heated to redness in a circular
furnace; during the time it is getting hot, the iron
wheel, turned to the right diameter, is bolted down
upon a face-plate or surface; the tire expands with
the heat, and when at a cherry red, it is dropped over




THE STEAM ENGINE.               245
the wheel, for which it was previously too small, and
it is also hastily bolted down to the surface plate; the
whole mass is then quickly immersed by a swing crane
in a tank of water five feet deep, and hauled up and
down till nearly cold; the tires are not afterwards
tempered.  It is not indispensable that the whole tire
should be of steel; but a dove-tail groove, turned out
of the tire at the place where it bears most on the rail,
and fitted with a band of steel, will suffice. This band
may be put in in pieces, and the expedient appears to
be the best way of repairing a worn tire; but particular care must be taken to attach these pieces very
securely to the tire by rivets, else in the rapid revolution of the wheel the steel may be thrown out by
the centrifugal force.  In aid of such attachment the
steel after being introduced is well hammered, which
expands it sideways, until it fills the dove-tail groove.
The tire is attached to the rim  with rivets having
counter-sunk heads, and the wheel is then fixed on its
axle. The tire is turned somewhat conical, to facilitate
the passage of the engine around curves,-the diameter
of the outer wheel being virtually increased by the
centrifugal force of the engine, and that of the inner
wheel being correspondingly diminished, whereby the
curve is passed without the resistance which would
otherwise arise from the inequality of the spaces passed
over by wheels of the same diameter fixed upon the
same axle.  The rails, moreover, are not set quite upright, but are slightly inclined inwards, in consequence
of which the wheels must be either conical or slightly
dished, to bear fairly upon the rails. One benefit of in-,lining the rails in this way, and coning the tires, is that
21*




246               A CATECHISM OF
the flange of the wheels is less liable to bear against
the sides of the rail, and with the same view the
flanges of all the wheels are made with large fillets in
the corners.  Wheels have been placed loose upon the
axle, but they have less stability, and are not now
much used.  Much controversial ingenuity has been
expended upon the question of the relative merits of
the four and six wheeled engines; one party maintaining that four-wheeled engines are most unsafe, and
the other that six-wheeled engines are unmechanical,
and are more likely to occasion accidents.  The fourwheeled engines, however, appear to have been charged
with faults that do not really attach to them when
properly constructed; for it by no means follows that
if the axle of a four-wheeled engine breaks, or even
altogether comes away, that the engine must fall down
or run off the line; inasmuch as, if the engine be
properly coupled with the tender, it has the tender to
sustain it. It is obvious enough, that such a connection may be made between the tender and the engine, that either the fore or hind axle of the engine
may be taken away, and yet the engine will not fall
down, but will be kept up by the support which the
tender affords; and the arguments hitherto paraded
against the four-wheeled engines are, so far as regards
the question of safety, nothing more than arguments
against the existence of the suggested connection.  It
is no doubt the fact, that locomotive engines are now
becoming too heavy to be capable of being borne
on four wheels at high speeds without injury to the
rails; but the objection of damage to the rails applies
with at least equal force to most of the six-wheeled




THE STEAM ENGINE.               247
engines hitherto constructed, as in those engines the
engineer has the power of putting nearly all the
weight upon the driving wheels; and if the rail be
wet or greasy, there is a great temptation to increase
the bite of those wheels by screwing them down more
firmly upon the rails. A greater strain is thus thrown
upon the rail than can exist in the case of any equally
heavy four-wheeled engine; and the engine is made
very unsafe, as a pitching motion will inevitably be
induced at high speeds, when an engine is thus poised
upon the central driving wheels, and there will also
be more of the rocking or sinuous motion.  Stephenson makes his driving wheels without flanges, to facilitate the passage of the engine around curves; and
if six-wheeled engines be made at all, it appears expedient to construct them  in that manner; but instead of making enormously heavy six-wheeled engines, it appears the preferable alternative to use fourwheeled engines of a moderate weight, and to apply
a sufficient number of them to a train, to enable it
to reach the required velocity.   To this arrangement
there is no doubt the objection, that the expense of
the propelling power is greater, as a small engine
requires a driver and stoker for itself as well as a
large engine; but by making the tender double, with
one engine before it and another behind it, a single
driver and a single stoker would suffice for the two
engines.  The starting handles of both engines might
be brought to the middle of the tender, so that the
engines  might be started  simultaneously, and  be
made to act in this respect like a single engine.  This
arrangement appears to me to be greatly preferable




248               A CATECHISM C F
to that of making heavy six-wheeled engines, as the
rail will be preserved from the injurious effects of
excessive weight, and there will be less loss of power
in contracting the blast pipe, when the fire and flue
surface is increased by the addition of another engine.
Tenders are now made larger than heretofore to obviate the necessity of so many coke and water stations;
they should have glass windows all round them  to
shield the engine driver, and enable him during the
worst winds and rains to keep a steady look out.
Tenders may be put on any number of wheels, so that
inconvenience is not likely to arise from their size and
weight.  The cranked axle of locomotives is always
made of wrought-iron, with two cranks forged upon it
towards the middle of its length, at a distance from
each other answerable to the distance between the
cylinders.  Bosses are made on the axle for the
wheels to be keyed upon, and bearings for the support
of the framing.  The axle is usually forged in two
pieces, which are afterwards welded together. Sometimes the pieces for the cranks are put on separately,
but the cranks so made are liable to give way.  In
engines with outside cylinders the axles are made
straight-the crank pins being inserted in the naves
of the wheels.  The bearings to which the connecting
rods are attached are made with very large fillets in
the corners, so as to strengthen the axle in that part,
and to obviate side play in the connecting rod.  In
engines which have been in use for some time, however, there is generally a good deal of end play in the
bearings of the axles themselves, and this slackness
contributes to make the oscillation of the engine more




THE STEAM ENGINE.                249
violent; but this evil may be remedied by making the
bearings spheroidal, whereby end play becomes impossible.  In every kind of locomotive it is very desirable that the length of the connecting rod should
remain invariable, in spite of the wear of the brasses;
for there is a danger of the piston striking against the
cover of the cylinder if it be shortened, as the clearance
is left as small as possible in order to economize steam.
In some engines the strap encircling the crank pin is
fixed immovably to the connecting rod by dove-tailed
keys, and a bolt passes through the keys, rod, and
strap, to prevent the dove-tail keys from working out.
The brass is tightened by a gib and cutter, which is
kept from working loose by three pinching screws and
a cross pin or cutter through the point.  The effect of
this arrangement is to lengthen the rod, but at the
cross-head end of the rod the elongation is neutralized
by making the strap loose, so that, in tightening the
brass, the rod is shortened by an amount equal to its
elongation at the crank-pin end. The tightening here
is also effected by a gib and cutter, which is kept from
working loose by two pinching screws pressing on the
side of the cutter. Both journals of the connecting
rod are furnished with oil cups, having a small tube in
the centre with siphon wicks.  The connecting rod is
a thick flat bar, with its edges rounded.
172. Q.-Will you explain in what manner the
joints of an engine are made?
A.-Rust joints are not now much used in engines
of any kind, yet it is necessary that the engineer should
be acquainted with the manner of their formation.
One ounce of sal-ammoniac in powder is mingled




250               A CATECHISM OF
with 18 ounces, or a pound, of borings of cast-iron,
and a sufficiency of water is added to wet the mixture
thoroughly, which should be done some hours before it
is wanted for use.  Some persons add about half an
onnce of flowers of brimstone to the above proportions,
and a little sludge from the grindstone trough. This
cement is caulked into the joints with a caulking iron,
about three-quarters of an inch wide, and one quarter
of an inch thick, and after the caulking is finished the
bolts of the joints may be tried to see if they cannot
be further tightened. The skin of the iron must, in
all cases, be broken where the rust joint is to be made;
and, if the place be greasy, the surface must be well
rubbed over with nitric acid, and then washed with
water, till no grease remains. The oil about engines
has a tendency to damage rust joints by recovering
the oxide.  Coppersmiths staunch the edges of their
plates and rivets by means of a cement, formed of
pounded quicklime, with serum of blood, br white of
egg; and in copper boilers such a substance may be
useful in stopping the impalpable leaks which sometimes occur, though Roman cement appears to be
nearly as effectual.
173. Q.-Will you explain the method of case-hardening the parts of engines?
A.-The most common plan  for case-hardening
consists in the insertion of the articles to be operated
upon among horn'or leather cuttings, bone dust, or
animal charcoal, in an iron box provided with a tight
lid, which is then put into a furnace for a period answerable to the depth of steel required. In some cases
the plan pursued by the gunsmith may be employed




THE STEAM ENGINE.                251
with convenience. The article is inserted in a sheetiron case amid bone dust, often not burned; the lid of
the box is tied on with wire, and the joint luted with
clay; the box is heated to redness as quickly as possible, and kept half an hour at a uniform heat: its contents are then suddenly immersed in cold water. The
more unwieldy portions of an engine may be casehardened by prussiate of potash-a salt made from
animal substances, composed of two atoms of carbon
and one of nitrogen, and which operates on the same
principle as the charcoal. The iron is heated in the
fire to a dull red heat, and the salt is either sprinkled
upon it or rubbed on in a lump, or the iron is rubbed
in the salt in powder. The iron is then returned to
the fire for a few minutes, and finally immersed in
water.  By some persons the salt is supposed to act
unequally, as if there were greasy spots upon the iron
which the salt refused to touch, and the effect under
any circumstances is exceedingly superficial; nevertheless, upon all parts not exposed to wear, a sufficient
coating of steel may be obtained by this process. In
the malleable iron work of engines scrap-iron has long
been used, and considered preferable to other kinds;
but if the parts are to.be case-hardened, as is now the
usual practice, the use of scrap-iron is to be reprehended, as it is almost sure to make the parts twist in
the case-hardening process.  In case-hardening, iron
absorbs carbon, which causes it to swell; and as some
kinds of iron have a greater capacity for carbon than
other kinds, in case-hardening they will swell more,
and any such unequal enlargement in the constituent
portions of a piece of iron will cause it to change its




252                A CATECHISM OF
figure. In some cases, case-hardening has caused such
a twisting of the parts of an engine that they could
not afterwards be fitted  together; it is preferable,
therefore, to make such parts as are to be case-hardened to any considerable depth of Lowmoor iron,
which being homogeneous will absorb carbon equally,
and will not twist.
174. Q.-What is the composition of the brass used
in the construction of engines?
A.-The brass bearings of an engine are composed
principally of copper and tin: the ordinary range of
good yellow brass that files and turns well, is about
4- to 9 oz. of zinc to the pound of copper. Brazing
solders when stated in the order of their hardness are,
-three parts copper and one part zinc (very hard),
eight parts brass and one part zinc (hard), six parts
brass, one part tin, and one part zinc (soft); a very
common solder for iron, copper, and brass, consists of
nearly equal parts of copper and zinc. Muntz's metal
consists of forty parts zinc and sixty of copper; any
proportions between the extremes of fifty parts of zinc
and fifty parts copper, and thirty-seven zinc and sixtythree copper, will roll and work at a red heat, but forty
zinc to sixty copper are the proportions preferred.
Bell metal, such as is used for large bells, consists of
41 oz. to 5 oz. of tin to the pound of copper; speculum
metal consists of from  71 oz. to 8- oz. of tin to the
pound of copper; tough brass for engine work, 1- lb.
tin, 1~ lb. zinc, and 10 lbs. copper; brass for heavy
bearings, 21- oz. tin, - oz. zinc, and I lb. copper. There
is a great difference in the length of time brasses wear,
as made by different manufacturers; but the difference




THE STEAM ENGINE.                253
arises as much from a different quantity of surface, as
from  a varying composition  of the metal.  Brasses
should always be made strong and thick, as when thin
they collapse upon the bearing, and increase the friction and the wear. Babbitt's patent lining metal for
bushes has latterly been introduced in the bushes of
locomotive axles and other machinery; it is composed
of 1 lb. of copper, 1 lb. regulus of antimony, and 10 lbs,
of tin, or other similar proportions, the presence of
tin being the only material condition. The copper is
first melted, then the antimony is added, with a small
portion of tin; charcoal being strewed over the surface of the metal in the crucible to prevent oxidation.
The bush or article to be lined having been cast with
a recess for the soft metal, is to be fitted to an iron
mould, formed of the shape and size of the bearing or
journal, allowing a little in size for the shrinkage.
Drill a hole for the reception of the soft metal, say
2 to A in. diameter, wash the parts not to be tinned
with a clay wash to prevent the adhesion of the tin,
wet the part to be tinned with alcohol, and sprinkle
fine sal-ammoniac upon it; heat the article until fumes
arise from  the ammonia, and immerse it in a kettle
of Banca tin, care being taken to prevent oxidation.
When sufficiently tinned the bush should be soaked
in water, to take off any particles of ammonia that
may remain upon it, as the ammonia would cause the
metal to blow.  Wash with pipe clay and dry; then
heat the bush  to the melting  point of tin, wipe it
clean, and pour in the metal, giving it sufficient head
as it cools; the bush should then be scoured with fine
sand to take off any dirt that may remain upon it,
22




254               A CATECHISM OF
and it is then fit for use.  This metal wears for a
longer time than ordinary gun metal, and its use is
attended with very little friction.  If the  bearing
heats, however, from the stopping of the oil hole or
otherwise, the metal will be melted out.  A  metallic
grease, containing particles of tin in the state of an impalpable powder, would probably be preferable to the
lining of metal just described.
175. Q.-Have you any information to offer relative
to the lubrication of engine bearings?
A.-A  very useful species of oil cup is now  employed in a number of steam  vessels, and which, it
is said, accomplishes a considerable saving of oil, at
the same time that it more effectually lubricates the
bearings. A ratchet wheel is fixed upon a little shaft
which passes through the side of the oil cup, and is
put into slow revolution by a pendulum  attached to
its outside, and in revolving it lifts up little buckets
of oil, and empties them down a funnel upon the centre
of the bearing. Instead of buckets a few short pieces
of wire are sometimes hung on the internal revolving
wheel, the drops of oil which adhere on rising from
the liquid being deposited upon a high part set upon
the funnel, and which, in their revolution, the hanging wires touch.  By this plan, however, the oil is
not well supplied at slow speeds, as the drops fall
before the wires are in the proper position for feeding
the journal.  Another lubricator consists of a cock or
plug inserted in the neck of the oil cup, and set in
revolution by a pendulum  and ratchet wheel, or any
other means.  There is a small cavity in one side of
the plug which is filled with oil when that side is




THE STEAM ENGINE.                255
uppermost, and delivers the oil through the bottom
pipe when it comes opposite to it.  In some cases
bearings heat from the existence of a cruciform groove
on the top brass for the distribution of the oil, the effect
of which is to leave the top of the bearings dry. In
the case of revolving journals the plan of cutting a
cruciform channel for the distribution of,he oil does
not do much damage; but in other cases, as in beam
journals, for instance, it is most injurious, and the
brasses cannot wear well wherever the plan is pursued.  The right way is to make a horizontal groove
along the brass where it meets the upper surface of
the bearing, so that the oil may be well deposited on
the highest point of the journal, leaving the force of
gravity to send it downwards.  This channel should,
of course, stop short a small distance from each flange
of the brass; otherwise the oil would run out at the ends.
176. Q.-Will you explain the operation of erecting
engines in the workshop?
A.-In beginning the erection of side lever marine
engines in the workshop, the first step is to level the
bed plate lengthways and across, and strike a line up
the centre, as near as possible in the middle, which
indent with a chisel in various places, so that it may
at any time be easily found again. Strike another line
at right angles with this, either at the cylinder or
crank centre, by raising a perpendicular in the usual
manner.  Lay the other sole plate alongside at the
right distance, and strike a line at the cylinder or
crank centre of it also, shifting either sole plate a little
endways until these two transverse lines come into
the same line, which may be ascertained by applying




256                A CATECHISM OF
a straight edge across the two sole plates. Strike the
rest of the centres across, and drive a pin into each
corner of each sole plate, which file down level, so as
to serve for points of reference at any future stage;
next, try  the cylinder, or plumb it on the inside
roughly, and see how  it is for height, in order to
ascertain whether much will be required to be chipped
off the bottom, or whether more requires to be chipped
off the one side than the other.  Chip the cylinder
bottom fair; set it in its place, plumb the cylinder
very carefully with a straight edge and silk thread,
and scribe it so as to bring the cylinder mouth to the
right height, then chip the sole plate to suit that
height.  The cylinder must then be tried on again,
and the parts filed wherever they bear hard, until the
whole surface is well fitted. Next chip the place for
the framing; set up the framing, and scribe the horizontal part of the jaw with the scriber used for the
bottom of the cylinder, the upright part being set to
suit the shaft centres, and the angular flange of
cylinder, where the stay is attached, having been previously chipped plumb and level.  The stake wedges
with which the framing is set up preparatorily to the
operation of scribing, must be set so as to support
equally the superincumbent weight, else the framing
will spring from  resting unequally, and it will be
altogether impossible to fit it well. These directions
obviously refer exclusively to the old description of
side lever engine with cast-iron framing; but there is
more art in erecting an engine of that kind with
accuracy, than in erecting one of the dire(t action
engines, where it is chiefly turned or bored surfaces




THE STEAM ENGINE.                257
that have to be dealt with.  It will be proper, however, to describe the method pursued in erecting oscillating  engines.  The columns here are of wroughtiron, and in the case of small engines there is a
template made of wood and sheet-iron, in which the
holes are set in the proper positions, by which the
upper and lower frames are adjusted; but in the case
of large engines, the holes are set off by means of trammels. The holes for the reception of the columns are
cast in the frames, and are recessed out internally: the
bosses encircling the holes are made quite level across,
and made very true with a face plate, and the pillars
which have been turned to a gauge are then inserted.
The top frame is next put on, and must bear upon
the collars of the columns so evenly, that one of the
columns will not be bound by it harder than another.
If this point be not attained, the surfaces must be
further scraped, until a perfect fit is established. The
whole of the bearings in the best oscillating engines
are fitted by means of scraping, and on no other mode
of fitting can the same reliance be placed for exactitude. In fixing the positions of the centres in side
lever engines, it appears to be the most convenient
way to begin with the main centre.  The height of
the centre of the cross head at half stroke above the
plane of the main centre, is fixed by the drawing of
the engine, which gives the distance from the centre
of cross head at half stroke to the flange of the cylinder;
and from  thence it is easy to find the perpendicular
distance from the cylinder flange to the plane of the
main centre, merely by putting a straight edge along
level, from the position of the main centre to the
22*




258               A CATECHISM OF
cylinder, and measuring from the cylinder flange down
to it, raising or lowering the straight edge until it
rests at the proper measurement. The main centre is
in that plane, and the fore and aft position is to be
found by plumbing up from  the centre line on the
sole plate. To find the paddle shaft centre, plumb up
from  the centre line marked on the edge of the sole
plate, and on this line lay off from the plane of the
main centre the length of the connecting rod, if that
length be already fixed, or otherwise the height fixed
in the drawing of the paddle shaft above the main
centre.  To fix the centre for the parallel motion
shaft when the parallel bars are connected with the
cross head, lay off upon the plane of main centre the
length of the parallel bar from  the centre of the
cylinder; deduct the length of the radius crank, and
plumb up the central line of motion shaft; lay off on
this line, measuring from the plane of main centre, the
length of the side rod; this gives the centre of parallel
motion shaft when the radius bars join the cross head,
as is the preferable practice where parallel motions
are used.  The length of the connecting rod is the
distance from  the centre of the beam  when level, or
the plane of the main centre, to the centre of the
paddle shaft.  The length of the side rods is the distance from  the centre line of the beam  when level,
to the centre of, the cross head when the piston is at
half stroke.  The length of the radius rods of the parallel motion is the distance from the point of attachment
on the cross head or side rod, when the piston is at half
stroke, to the extremity of the radius crank, when
the crank is horizontal; or in engines with the parallel




THE STEAM ENGINE.               259
motion attached to the cross head, it is the distance
from the centre of the pin of the radius crank when
horizontal to the centre of the cylinder. Having fixed
the centre of the parallel motion shaft in the manner
just described, it only remains to put the parts together when the motion is attached to the cross head;
but when the motion is attached to the side rod, the
end of the parallel bar must not move in a perpendicular line, but in an arc, the versed sine of which
bears the same ratio to that of the side lever, that the
distance from the top of the side rod to the point of
attachment bears to the total length of the side rod.
The parallel motion when put in its place should be
tested by raising and lowering the piston by means of
the crane: first, set the beams level, and shift in or
out the motion shaft plummer blocks or bearings,
until the piston rod is upright. Then move the piston
to the two extremes of its motion; if at both ends the
cross head is thrown too much out, the stud in the
beam to which the motion side rod is attached is too
far out, and must be shifted nearer to the main centre;
if at the extremities the cross head is thrown too far
in, the stud in the beam is not out far enough. If the
cross head be thrown in at the one end, and out equally
at the other, the fault is in the motion side rod, which
must be lengthened or shortened to remedy the defect.
177. Q.-Will you explain how the slide valve of
an engine is set?
A.-Place the crank in the position corresponding
to the end of the stroke, which can easily be done in
the shop with a level or plumb line; but in a steam
vessel another method becomes necessary.  Draw the
transverse centre line, answering to the centre line of




260               A CATECHISM OF
the crank shaft, on the sole plate of the engine, or on
the cylinder mouth if the engine be direct action; describe a circle of the diameter of the crank pin upon
the large eye of the crank, and mark off on either side
of the transverse centre line a distance equal to the
semidiameter of the crank pin.  From  the point thus
found, stretch a line to the edge of the circle described
)n the large eye of the crank, and bring round the
crank shaft till the crank pin touches the stretched
line; the crank may thus be set at either end of its
stroke.  When the crank is thus placed at the end of
the stroke, the valve must be adjusted so as to have the
amount of lead, or opening on the steam side, which it
is intended to give at the beginning of the stroke; the
eccentric must then be turned round upon the shaft
until the notch in the eccentric rod comes opposite the
pin on the valve lever, and falls into gear; mark upon
the shaft the situation of the eccentric, and put on the
catches in the usual way.  The same process must be
repeated for going astern, shifting round the eccentric
to the opposite side of the shaft, until the rod again
falls into gear.  In setting valves, regard must be had
to the kind of engine, the arrangement of the levers,
and the kind of valve employed. In setting the valves
of locomotives a similar method is adopted: place the
crank in the position answerable to the end of the
stroke of the piston, and draw  a straight line, representing the centre line of the cylinder, through the
centres of the crank shaft and crank pin.  From the
centre of the shaft describe a circle with the diameter
equal to the throw of the valve, another to represent
tile crank shaft, and a third circle to represent the
path of the crank pin.  From the centre of the cran



THE STEAM ENGINE.                261
shaft, draw a line perpendicular to the centre line of
the cylinder and crank shaft, and draw another perpendicular at a distance from the first equal to the
amount of the lap and the lead of the valve: the
points in which this line intersects the circle of the
eccentric are the points in which the centre of the
eccentric should be placed for the forward and reverse
motions.  When the eccentric rod is attached directly
to the valve, the radius of the eccentric, which precedes lhe crank in its revolution, forms with the crank
an obtuse angle; but when by the intervention of levers
the valve has a motion opposed to that of the eccentric
rod, the angle contained by the crank and the radius
of the eccentric must be acute, and the eccentric must
follow the crank: in other words, with a direct attachment to the valve the eccentric is set more than
one-fourth  of a revolution in advance of the crank,
and with an indirect attachment the eccentric is set
less than one-fourth of a circle behind the crank. If
the valve were without lead or lap the eccentric would
be exactly one-fourth of a circle in advance of the
crank or behind the crank, according to the nature of
the valve connection; but as the valve would thus
cover the port by the amount of the lap and lead, the
eccentric must be set forward so as to open the port
to the extent of the lead, and this is effected by the
plan just described.  In working locomotives the eccentrics sometimes shift upon the shaft, in which case
they may be easily refixed by setting the valve open
the amount of the lead, setting the crank at the end of
the stroke, and bringing round the eccentric upon the
shaft till the eccentric rod gears with the valve.  It




262                A CATECHISM OF
would often be troublesome in practice to get access
to the valve for the purpose of setting it, and this
may be dispensed with if the amount of lap on the
valve and the length of the eccentric rod be known.
To this end draw upon a board two straight lines at
right angles to one another, and from their point of
intersection as a centre describe two circles, one representing the circle of the eccentric, the other the crank
shaft; draw  a straight line parallel to  one of the
diameters, and distant from it the amount of the lap
and the lead; the points in which this parallel intersects
the circle of the eccentric are the positions of the
forward and backward eccentrics.   Through  these
points draw radial lines from the centre of the circle,
and mark the intersections of these lines with the circle
of the crank shaft; measure with a pair of compasses
the chord of the arc intercepted between either of these
intersections and the diameter which is at right angles
with the crank; and the diameters being first marked
on the shaft itself, it will follow that by transferring
with the compasses the distance found in the diagram,
and marking the point, the position of the eccentric
will be fixed without difficulty.
178. Q.-Will you explain the method of putting
engines into a steam vessel?
A.-As an illustration of this operation it may be
advisable to take the case of a side lever engine, and
the method of proceeding is as follows: —First measure across from the inside of paddle bearers to the
centre of the ship, to make sure that the central line,
running in a fore and  aft direction on the deck or
beams, usually drawn by the carpenter, is really in




THE STEAM ENGINE.                263
the centre.  Stretch a line across between the paddle
bearers in the direction of the shaft: to this line in the
centre of the ship where the fore and aft mark has
been made, apply a square with arms six or eight feet
long, and bring a line stretched perpendicularly from
the deck to the keelson, accurately to the edge of the
square; the lower point of the line where it touches
the keelson will be immediately beneath the marks
made upon the deck.  If this point does not come in
the centre of the keelson, it will be better to shift it a
little, so as to bring it to the centre, altering the
mark upon the deck correspondingly, provided either
paddle shaft will admit of this being done-one of the
paddle brackets being packed behind with wood, to
give it an additional projection from the side of the
paddle bearer.  Continue the line fore and aft upon
the keelson as nearly as can be judged in the centre of
the ship; stretch another line fore and aft through
the mark upon the deck, and look it out of winding
with the line upon the keelson.  Fix upon any two
points equally distant from  the centre, in the line
stretched transversely in the direction of the shaft;
and from those points as centres, and with any convenient radius, sweep across the fore and aft line to see
that the two are at right angles; and, if not, shift the
transverse line a little to make them so.  From  the
transverse line next let fall a line upon each outside
keelson, bringing the edge of the square to the line,
the other edge resting on the keelson.  A point will
thus be got on each outside keelson perpendicularly
beneath the transverse line running in the direction of
the shaft, and a line drawn between those two points




264               A CATECIISM OF
will be directly below the shaft.  To this line the line
of the shaft marked on the sole plate has to be brought,
care being taken, at the same time, that the right distance is preserved between the fore and aft line upon
the sole plate, and the fore and aft line upon the central
keelson.   Before any part of the machinery is put in,
the keelsons should be dubbed fair and straight, and be
looked out of winding by means of two straight edges.
The art of placing engines in a ship is more a piece of
plain common sense than any other feat in engineering,
and  every man of intelligence may easily settle a
method of procedure for himself.   Plumb lines and
spirit levels, it is obvious, cannot be employed on
board a vessel, and the problem consists in so placing
the sole plates, without these aids, that the paddle
shaft will not stand awry across the vessel, nor be carried forward beyond its place by the framing shouldering up more than was expected.  As a plumb line
cannot be used, recourse must be had to a square; and
it will signify nothing at what angle with the deck the
keelsons run, so long as the line of the shaft across
the keelsons is squared down from  the shaft centre.
The sole plates being fixed, there is no difficulty in
setting the other parts of the engine in their proper
places upon them.  The paddle wheels must be hung
from  the top of the paddle box to enable the shaft
to be rove through  them, and the cross stays between the engines should be fixed in when the vessel
is afloat.  To try whether the shafts are in a line, turn
the paddle wheels, and try if the distance between the
cranks is the same at the upper and under end, and the
two horizontal centres; if not, move the end of the
paddle shaft up or down, backwards or forwards, until




THE STEAM ENGINE.               265
the distance between the cranks at all the four centres
is the same.
179. Q.-In what manner are the engines of a
steam-vessel secured to the hull?
A.-The engines of a steamer are secured to the
hull by means of bolts called holding-down bolts; and
in most steam-vessels a good deal of trouble is caused
by these bolts, which are generally made of iron.
Sometimes they go through the bottom of the ship,
and at other times they merely go through the keelson,-a recess being made in the floor or timbers to
admit of the introduction of a nut. The iron, however, wears rapidly away in both cases, even though
the bolts are tinned; and it has been found the preferable method to make such of the bolts as pass
through the bottom, or enter the bilge, of Muntz's
metal, or of copper. In a side lever engine, four
Muntz's metal bolts may be put through the bottom
at the crank end of the framing of each engine, four
more at the main centre, and four more at the cylinder, making twelve through-bolts to each engine;
and it is more convenient to make these bolts with a
nut at each end, as in that case the bolts may be
dropped down from the inside, and the necessity is
obviated of putting the vessel on very high blocks in
the dock, in order to give room to put the bolts up
from  the bottom.  The remainder of the holdingdown bolts may be of iron, and may, by means of a
square neck, be screwed into the timber of the keelsons as wood screws-the upper part being furnished
with a nut which may be screwed down upon the sole
plate, so soon as the wood screw portion is in its
23




266                A CATECHISM OF
place.  If the cylinder be a fixed one it should be
bolted down to the sole plate by as many bolts as are
employed to attach the cylinder cover, and they should
be of copper or brass, in any situation  that is not
easily  accessible.  In  well formed  bolts, the  spiral
groove penetrates about one-twelfth of the diameter
of the cylinder round which it winds, so that the diameter of the solid cylinder which  remains is fivesixths of the diameter over the thread.  If the strain
to which iron nway be safely subjected in machinery
is one-fifteenth of its utmost strength, or 4000 lbs. on
the square inch, then 2180 lbs. may be sustained by
a screw  an inch  in diameter, at the outside of the
threads.   The strength  of  the holding-down  bolts
may easily be computed, when the elevating force of
the piston or main centre is known, but it is expedient very much to exceed this strength in practice,
on account of the elasticity of the keelsons, the liability to corrosion, and other causes.  It is difficult
to fix engines effectually which  have once begun to
work in the ship, for in time the surface of the keelsons on which the engines bear becomes worn uneven,
and  the  engines necessarily rock  upon it.  As a
general rule, the bolts attaching the engines to the
keelsons are too few and of too large a diameter: it
would  be preferable to  have smaller bolts, and a
greater number of them.  In addition  to the bolts
going through the keelsons, or the vessel's bo.tonmn,
there should be a large numlber of wood screw\s securing the sole plate to  the keelson, and  a large
number of bolts securing the various parts of the
engine to  the sole plate.  In  iron vessels, holdingdown bolts passing through the bottom  are not ex.




THE STEAM ENGINE.              267
pedient; and there the engine has merely to be secured
to the iron plate of the keelsons, which are made hollow, to admit of a more effectual attachment.
180. Q.-What are the most important of the points
which suggest themselves to you, in connection with
the management of marine engines?
A.-The attendants upon engines should prepare
themselves for any casualty that may arise, oy considering possible cases of derangement, and deciding
in what way they would act should certain accidents
occur.  The course to be pursued must have reference to particular engines, and no general rules can
therefore be given, but every marine engineer should
be prepared with the measures to be pursued in the
emergencies in which he may be called upon to act,
and where every thing may depend upon his energy
and decision. In some cases of collision, the funnel
is carried away and lost overboard, and such cases
are among the most difficult for which a remedy can
be sought.  If flame come out of the chimney when
the funnel is knocked away, so as to incur the risk of
setting the ship on fire, the uptake of the boiler must
be covered over with an iron plate, or be sufficiently
covered to prevent such injury. A temporary chimney must then be made of such materials as are on
board the ship. If there are bricks and clay or lime
on board, a square chimney may be built with them;
or if there be sheet-iron plates on board, a square
chimney may be constructed of them. In the absence
of such materials, the awning stanchions may be set
up round the chimney, and chain rove in through
among them  in the manner of wicker work, so as to




268               A CATEC1IS9M OF
make an iron wicker chimney, which may then be
plastered outside with wet ashes mixed with clay,
flour, or any other material that will give the ashes
cohesion.  War steamers should carry short spare
funnels, which may easily be set up should the ori
ginal funnel be shot away; and if a jet of steam  be
let into the chimney, a very short and small funnel
will suffice for the purpose of draught. If the crank
pin breaks, the other engine must be worked with the
one wheel.  It will sometimes happen, when there is
much lead upon the slide valve, that the single engine,
on being started, cannot be got to turn the centre,
if there be a strong opposing wind and sea; the piston going up to near the end of.the stroke, and then
coming down again without the crank being able to
turn the centre.  In such cases, it will be necessary
to turn the vessel's head sufficiently from the wind to
enable some sail to be set; and if once there is weigh
got upon the vessel the engine will begin to work
properly, and will continue to do so though the vessel
be put head to wind as before.  If the eccentric
catches, or hoops break or come off, and the damage
cannot readily be repaired, the valve may be worked
by attaching the end of the starting handle to any
convenient part of the other engine, or to some part
in connection with the connecting rod of the same
engine.  In side lever engines, with the starting bah
hanging fiom  the top of the diagonal stay, as is a
very  common  arrangement, the  valve  might be
wrought by leading a rope from  the side lever of the
other engine through blocks, so as to give a horizontal
pull to the hanging starting bar, and the bar could be




THE STEAM ENGINE.               69
brought back by a weight.  Another plan would be,
to lash a piece of wood to the cross-tail butt of the
damaged engine, so as to obtain a sufficient throw for
working the valve, and then to lead a piece of wood
or iron, from a suitable point in the piece of wood
attached to the cross-tail, to the starting handle,
whereby the valve would receive its proper motion.
If the shafts or cranks break, the engine may nevertheless be worked with moderate pressure to bring
the vessel into port; but if the crank be very bad, it
will be expedient to fit strong blocks of wood under
the ends of the side levers, or other suitable part, to
prevent the cylinder bottom  or cover from  being
knocked out, should the damaged part give way.
The same remark is applicable when flaws are discovered in any of the main parts of the engine, whether
they be malleable or cast-iron; but they must be carefully watched, so that the engines may be stopped if
the crack is extending further. Should fracture occur,
the first thing obviously to be done is to throw the
engines out of gear, and should there be much weigh
on the vessel, the steam should at once be thrown on
the reverse side of the piston, so as to counteract the
pressure of the paddle wheel.
181. Q.-What are the chief duties of the engine
driver of a locomotive?
A.-The engineer of a locomotive should constantly
be upon the foot-board of the engine, so that the regulator, the whistle, or the reversing handle may be used
instantly, if necessary; he must see that the level of
the water in the boiler is duly maintained, and that
the steam is kept at a uniform pressure.  In feeding
23*




770               A CATECHISM OF
the boilers with water, and the furnaces with fuel, a
good deal of care and some tact is necessary, as irregulaiity in the production of steam  will often occasion
priming, even though the water be maintained at a
uniform level; and an excess of water will of itself
occasion priming, while a deficiency is a source of
obvious danger.  The engine is generally furnished
with three gauge cocks, and water should always come
out of the second gauge cock and steam out of the top
one when the engine is running; but when the engine
is at rest, the water in the boiler is rather lower than
when in motion, so that when the engine is at rest,
the water will be high enough if it just reaches to the
middle gauge cock.  The boiler should be well filled
with water on approaching a station, as there is then
steam  to spare, and additional water cannot be conveniently supplied when the engine is stationary. The
furnace should be fed with small quantities of fuel at
a time, and the feed should be turned off just before a
fresh supply of fuel is introduced. The regulator may,
at the same time, be partially closed; and, if the blast
pipe be a variable one, it will be expedient to open it
widely while the fuel is being introduced, to check the
rush of air in through the furnace door, and then to
contract it very much so soon as the furnace door is
closed, in order to recover the fire quickly. The propel
thickness of coke upon the grate depends upon the
intensity of the draught: but in heavily loaded engines
it is usually kept up to the bottom of the fire door.
Care, however, must be taken that the coke does not
reach up to the bottom row of tubes so as to choke
them  up.  The fuel is usually disposed on the grate




I'IH  fSTEAM ENGINE.             271
like a vault; and if the fire box be a square one, it is
heaped high in the corners, the better to maintain the
combustion.  In starting from  a station, and also in
as ending inclined planes, the feed water is generally
shut off; and therefore, before stopping or ascending
inclined planes, the boiler should be well filled up with
water. In descending inclined planes an extra supply
of water may be introduced into the boiler, and the
fire may be fed, as there is at such times a superfluity
of steam. In descending inclined planes the regulator
must be partially closed, and it should be entirely
closed if the plane be very steep. The same precaution
should be observed in the case of curves, or rough
places on the line, and in passing over points or
crossings.  To ascertain whether the pumps are acting
well, the pet cock, which is a small cock opening into
the pump, must be turned, and if any of the valves
stick they will sometimes be induced to act again by
working with the pet cock open, or alternately open
and shut. Should the defect arise from a leakage of
steam  into the pump, which prevents the pump from
drawing, the pet cock remedies the evil by permitting
the steam to escape. Should priming occur from the
water in the boiler being dirty, a portion of it may be
blown out; and should there be much boiling down
through the glass-gauge tube, the stop cock may be
partially closed. The water should be wholly blown
out of locomotive beolers three times a week, and at
those times two mud hole doors at opposite corners of
the boiler should be opened, and the boiler be washed
internally by means of a hose.  On approaching a
station the regulator should be gradually closed, and it




272              A CATECIISM OF
should be completely shut about half a mile from the
station-if the train be a very heavy one; the train
may then be brought to rest by means of the breaks.
Too much reliance, however, molit not be put upon
the breaks, as they sometimes give way, and in frosty
weather are nearly inoperative. In cases of urgency
the steam may be thrown upon the reverse side of the
piston, but it is desirable to obviate this necessity as
far as possible. At terminal stations the steam should
be shut off earlier than at roadside stations, as a collision will take place at terminal stations if the train
overshoots the place where it ought to stop. There
should always be a good supply of water when the
engine stops; but the fire may be suffered gradually to
burn low towards the conclusion of the journey. So soon
as the engine stops it should be wiped down, and be
then carefully examined: the brasses should be tried,
to see whether they are slack or have been heating;
and, by the application of a gauge, it should be ascertained occasionally whether the wheels are square on
their axles, and whether the axles have end play,
which should be prevented. The stuffing boxes must
be tightened, and the valve gear examined, and the
eccentrics be occasionally looked at to see that they
have not shifted on their axles, though this defect will
be generally intimated by the irregular beating of the
engines.  The tubes should also be examined and
cleaned out, and the ashes emptied out of the smoke
box through the small ash door at the end.  If the
engine be a six-wheeled one, it will be liable to pitch
and oscillate if too much weight be thrown upon the
driving wheels; and where such faults are found to




THE STEAM ENGINE.               273
exist, the weight upon the driving wheels should be
diminished.  The practice of blowing off the boiler by
the steam, as is always done in marine boilers, should
not be permitted as a general rule in locomotive
boilers, when the tubes are of brass and the fire box of
copper; but when the tubes and' fire boxes are of iron,
there will not be an equal risk of injury.  Before
starting on a journey, the engine man should take a
summary glance beneath the engine-but before doing
so he ought to assure himself that no other engine is
coming up at the time.  The regulator, when the
engine is standing, should be closed and locked, and
the eccentric rod be fixed out of gear, and the tender
break screwed down; the cocks of the oil vessels
should at the same time be shut, but should all be
opened a short time before the train starts. When a
tube bursts, a wooden or iron plug must be driven into
each end of it, and if the water or steam  be rushing
out so fiercely that the exact position of the imperfection cannot be discovered, it will be advisable to
diminish the pressure by increasing the supply of feed
water. Should the leak be so great that the level ot
the water in the boiler cannot be maintained, it will
be expedient to drop the bars and quench the fire, so
as to preserve the tubes and fire box from  injury.
Should the wooden casing of the boiler catch fire, it
may be extinguished by throwing a few buckets of
water upon it, or, if the engine is at a station, it may
be brought under the water crane.  Should the piston
rod or connecting rod break, or the cutters fall out or
be clipped off-as sometimes happens to the piston
cutter when the engine is suddenly reversed upon a




274               A CATECHISM OF
heavy train-the parts should be disconnected, if the
connection cannot be restored, so as to enable one
engine to work; and of course the valve of the faulty
engine must be kept closed.  If one engine has not
power enough to enable the train to proceed with the
blast pipe full open, the engine may perhaps be able
to take on a part of the carriages, or it may run on by
itself to fetch assistance. The same course must be
pursued if any of the valve gearing becomes deranged,
and the defects cannot be rectified upon the spot. To
economize fuel, the variable expansion gear, if the
engine has one, should be adjusted to the load, and the
blast pipe should be worked with the least possible
contraction; and at stations the damper should be
closed to prevent the dissipation of heat.
182. Q.-Having now given a general outline of
the principles and details of construction involved in
the production of steam  engines, according to the
existing system, can you give any suggestion of material improvement?
A.-For all land purposes, and for some marine
purposes, engines on the high speed principle (described in pages 68 and 69) should be adopted; and
in locomotive engines much change is necessary, as the
existing locomotive engine intended for high speeds
upon the narrow gauge is a very defective machine.
It is a monstrous thing that, in the present age of
mechanical proficiency, half the engine power should
be dissipated upon the blast pipe, as in high speeds
appears to be the case; and the first indication of improvement is to obviate this loss by enlarging the
grate surface and the area of the tubes. This may be




THE STEAM ENGINE.               275
most conveniently accomplished by the introduction of
vertical tubes, inserted in short barrels, each furnished
with a chimney, and set upon a square fire-box extending the whole length of the engine; the axle of
the driving wheel may be introduced above the firebox, and between two of the barrels, whereby the
engine will be kept so near the ground as to obviate
the liability of being top-heavy.  If the two cylinders,
which may be oscillating, be set upon the top of the
fire-box, at the angle of 45~ with each other, but in the
same vertical plane, and if both piston rods be coupled
to the same crank, the shaft of which extends to each
side of the carriage, where it is provided with other
cranks operating by means of horizontal rods upon
pins in the driving wheel, the highest velocities will
be attainable without any sinuous or rocking motion;
and the whole of the machinery being above the boiler,
will be accessible while the train is running, and will
not be exposed so much to ashes and dust. With such
a construction, indeed, it would be expedient to enclose
the whole of the machinery in a carriage or house with
glass windows round it.  Every locomotive should be
furnished with efficient expansion gear of some kind or
other; and it is very desirable that the fire should be
fed by some self-acting arrangement, which will make
it unnecessary to open the furnace-door frequently.
The use of sediment collectors in locomotive boilers is
also expedient, as, if judiciously applied, they will
effectually prevent the formation of scale upon the
tubes, and will also operate as an antidote to priming,
in many cases.  The form of collector best adapted to
a locomotive boiler will depend, in a great measure, upon




276    A CATECHISM OF THE STEAM ENGINE.
the peculiar structure of the boiler; but generally any
form will answer which communicates with the water
level, and contains water within it in a tranquil state.
TABLE 1.-Page 277.
Table of Nominal Horse Power of Low Pressure
Engines.
THE table on the opposite page contains the dimensions of cylinder corresponding with any given power
in a low pressure engine. The table is constructed according t tthe rule given in page 50.
To find the nominal power of a low pressure engine,
of 20 inches diameter of cylinder, and 21 feet stroke:Find the diameter in the left-hand column, and in the
same horizontal line with it, and in the vertical column
headed'2, will be found 11'55, the number of horses
power of the engine.
The dimensions of cylinder corresponding to a given
power may be found by a converse process.




277
Table of Nominal Horse Power of Low Pressure Engines.
"5~~'~,5 S~ Length of Stroke in Feet.
I      11    2    2A    3    3         4    4       5    5j   6        7
4   -34   -391   43   -46    49    52        54    56   -58   -60    62   s65
5    53   -611   67   -72   -76   -81   -84   -88'91   -94    96  1-02
6   -76   -87   -96  1-04  1-10  1-16  1-22  1-26  1.31  1-35  1-39  1-47
7  1-04  1-19  1-31  1-41  1-50  1-58  1-65  1-72  1:78  1-84  1'89  1-99
8  1-36  1-56  1-72  1-85  1'96  2-07  2-16  2-25  2-33  2'40  2'47  2-60
9  1-72  1-97  2-17  2-34  2-49  2-62  2-74  2-84  2-95  3-041  3-13  3-30
10  2-13  2-44  2'68  2.89  3-07  3-23  3-38  3-51  3-64  3-76  3-87  4-07
11  2-57  2-95  3-24  3-49  3-77  3'91  4-15  4-25  4-40  4-54  4681 4-92
12  3.06  3'51  3-86  4-16  442  4465  4-86  5-06  5-24  5-41  5-57  5-86
13  3-60  4-12  4-53  4-88  5-19  5-46  5-64  5-94  6-15  6-35  6-53  6-88
14  4-17  4.77  525  5.66(  6-01  6-33  662  6-88  7-13  7-361 7-58  7-98
15  4-77  5-48  6-03  6-50  6-90  7-27  7-60  7-90  8-19  8-45  8-70  9-16
16  5-45  6-23  6-86  7-39  7-86  8-27  8-65  8-99  9-31  9611 9-90 1042
17  6-15  7'04  7-75  8-35  8-86  9-34  9-76 10-15 10-52 10'85, 11-17 11-76
18  6-89  7-89  8-68  9-36  9-94 10-47 10-94 11-38 11-79 12.17 12-53 13-19
19  7-68  8-79  9-68 10'42 11-17 11-66 12-19 12-68 13-13 13-561 13-96 14-69
20  8-51  9'74 10-72 11-55 12-27 12-92 13-51 14-05 14-55 15-02 15-46 1628
22'1030 11'79 12-97 13-98 14-85 15-63 16-62 17-30 17-65 18-181 18-71 19-70
24 12-26 14'03 15-44 16-63 17-67 18-61 19-45 20-23 20-95 2163! 2-227 23-44
26 14'39 16'46 1812 19-52 20-75 21-84 22-56 23-75 24-6  25-39 26-14 27-51
28 16-68 19'09 21-02 22-64 24-06 25-33 26-48 27-54 28-52 29-441 30'31 31-90
30 19'15 21 92 24-13 25'99 27-62 29-07 30-40 31-61 32-74 33.80 34-80 36-63
32 21-79 24-96 27-51 29-57 31-42 33-08 34-59 35-97 37-26 38.46 39 59 41-68
34 24-60 28'16 30-99 33-39 35-44 37-34 39-04 40-60 42-06 43.41 44 69 47-05
36 27-57 31'56 34'74 37-42 39'77 41-87 4377 45-52 4715 48.67 50-11 52-75
38 30-72 35'17 38-71 41-69 44-66 46-64 4877 50-72 52'54 54.23 55-83 58-78
40 34.04 3897 42-89 46-20 49-10 51-69 54-04 56-20 58-21 60.09 61-86 65-12
42 37-53 42-96 47.29 50-94 54-13 56-98 59-58 61-96 64-18 66.251 68-21 7178
44 41-19 47'15 51.90 55-91 59-38 62-54 66-46 68-00 70'44 72.711 74-85 78-79
46 4502 51-54 5672 61-1064-88 68-1  71-43 7            633    69 79.471 18181 86-12
48 490256-11 61-76 66-53 7070 74-42 77-82 80-94 83-83 8653 8908 9378
50 53-19 60-89 67.02 72-19 76-71 80-76 84-44 87-82 90-96 93 89 96-65 101-7
52 57.55 65-86 72-48 78-08 83-00 87-35 90-25 94-98 98-40 10155 104-5 110.0
54 62'04 7102 7817 84-20'89-48 94-20 98-49 102-4 106.1 109.5 112-7 118-7
56 6672'76.38 84-07 90-55 9623 101-30 105-9 110-1 114-1 117.8 121-2 127-6
58 71'58 81.931 90-18 97-1411032- 108-6 113-6 118-2 122-4 126.3 129.9 136-7
60 76'60 87681 96-50103-9  110-4 116-3 121-6 126-4 131-0 135.2 139-2 146-5
62 81'79.93'6210304 111-0  117-96 124-18 129-81 135-031139-86 144-37 148-6 156-7
64 87-15 99-84 110-0 118-3 125-7 132-3 138-3 143-9 149-0 153.82 158-4 1667'
66 92'68106'1 116-8 125-8  133-6 140-7 147-3 153-0 158-5 163.6 168-4 177-3
68 98-40'112-6 123-9 133-6  141-8 149-4 156-2 162-4 168-2 173.6 178-8 188-2
70 104'26'119-3 131-3 141-5  150-4 158-3 165-5 172-1 178-2 184.0 189-4 199-4
721110'3011262 139-0 149-7  159'1 1674 175-1 182-1 188-6 194-7 200'4 211-0
741116'5 133-4 146-8 158-1 167-9 176-7 185-4 192-4 199-2 205-7 211-6 223-4
761122'9 140-7 154-8 166-8  178-6 186-6 195-0 202-9 210'1 216.9 12233 235-1
781129'4 148-2 163-1 175-6  186-7 196-5 205-4 212-1 221-4 228-5 i235-2 247-6
80i136-2 155-8 171-6 184-8  196-4 206-7 216-1 224-8 232-8 240-4 247-4 260-5
821143-0 163-8 180-2 194-2  206.2 217-3 226-9 2378 244.6 2525 1260-0 273-8
84150-1 171-8 189-1 203-8 216-5 2279 238-3 247-8 256.7 265-0 1272-8 287-1
861157'4 180-1 198-2 213-6 227-0 237-8 247-4 258'2 269-1 277-8 i2860 301-0
881164-8 188-6 207-6 223-6  237-5 250-2 261-6 272-0 281-7 290-8 1299-4 315-2
0172-3 197*3 2171 2339 248-6 261-7 273-6 284-5 294'7 304-2 313-2 329-7
24




278
TABLE 2.-Page 279.
Table of Nominal Horse Power of High Pressure
Engines.
THIE table on the opposite page contains the dimensions of cylinder corresponding with any given power
in a high pressure engine. The table is constructed
according to the rule given in page 67, which is virtually the same rule as that for low pressure engines,
with the exception of the element of pressure being
taken at three times the value.
To find the nominal power of a high pressure engine
of 20 inches diameter of cylinder, and 2~ feet stroke:Find the diameter in the left-hand column, and in the
same horizontal line with it, and in the vertical column
headed 2a, will be found 34-65, the number of horses
power of the engine.
The dimensions of cylinder corresponding to a given
power may be found by the converse process from the
table.




279
Table of Nominal Horse Power of High Pressure Engines.
Length of Stroke in Feet.' 1         2 1  j 2      3     3 14 4                                   H 4  5   6
2    2    -29   -32   -35    37   -38    4'42   -44   -45    4       49
2  39    45    50   -54    -57    60   -63   -66   -68    -701   ~2    7'
3     *57   -65   -72   -78   *83   -87   -91   -95    98   1-01   1-04  1-10I
3     -78   *89   -98  1-06   1-13  1-19  1-24  1-29   1-34   1-38   1-4-2  1-49i
4    1-02  1-17  1 29   1-38   147   156   (62   1 681  1'74  1-80   186   1-95'
41 1-29   1-48 1  163  1-75   186   1'9o  2-05  2-13  2'21  228  2-33J  2-47
5    15 159    83   2(11  2-16   2-28  243  2-52  2-641  2-73   282  2-88  3-061
512  1-93  2-211  2-43  2-62   2-78  2.93  3 12  3-18' 3-30  3-42'   3 51  3-69
6   2-28 2'61  2-88  3-12   3-30  3'48  3-66  3-781 3-93  4-U5  4-17  4-41
6-  269  309  3-39  3-66   3-90  4-08  4z3  4-44' 462  477   489  5-16
7    3-12  3-57  3 93  4-23   450  443   4   4-   4 51 516   534  5-5.2  5-67  5-97
71  3-60  4-11  4-53  4-86  5'19  5'46  5'70  5-94  6 15  6-33  6-51  6-87'
8          4-08  468  5-16  5-55   5-88   61    48 6' 75  6-99  7-20  7-41  7-80i
81  4'62  5-28  5-82  6271  6-63  6t99  7321  762I 7'89  8-13  8-37  882
9            16 16   591  651  7-021  747  7-81  8-22  8-521 8-5  9-12  9-39  9901
9.  5-76  6-60  726   7-80   8-37  8-76  9-15  9-51  984   10-17 10-47 1 Ol
10   6-39   7.3)  8-'04   8t767'    21   9-69 16.14  10-53! 10-l992 11'28i 11-61 1-i21,
10   7-05   804    88   9-541 i'i4  10681 11-16  1161 12-03; 1421 14'78 13-47
11   7-71  8851  9-72  10-47  11-31  1-731 12-45  12-75  13-20  13-62 14-04  14-76
11   8'43  9,61 10-62  1 1-46  12-15! 1:-78 13'80 13-92  14-611.14-91 15-33 16 141
12   9-18 101531 11-58  12-48  13-26' 1.95 14-58' 15-18 1    172  l-231 16-71 1758
121  9-96 11'-40 12-57 13-53  14371 1515 15-84 16-47 1 7041 1758 18 12 19'08
13! 108'  1812    13-5'3 14-i4  15-57 16 -3, 16-92    82 7 18-45 19-05 19J-5  20-64
131 11-64 1332  14-i4  15-78  16i77i 1'67  16-48 1920i 19-89 20-52 21-15 22-261
14  12-51 I1  1. 1575  1698  18-03   1899 19-i6 2'i64 ^l-'9 U-081 22-74 2394
141 13-41i 15- 56 16-92  18-21  19-35 2';7 21-30 22-i4'  22'-95    70: 24-139 -5'62'
15   14-31  1644  18-09l 19-50  20-70 z1 d81 2e0) 2370''24-7 25o35 S ti. 10 27.48
16i  16l.5 18d69' 20-58 22-17 23-58 240.11 25-95 2.-97 2793     563; 29 70 31i26
17  1845 211 231   -50 2321  50   26ii58 2ov2'  2    3145  3i 6 3- -5 335;1 3518'8  20-67 23;'7 2i6-04 28-08  2982: 31.41 32-82 34-141' 5-37;36-51' 3'759 3957
19  23'04 26t371'904 31.26  3.-,1 34-98! 36-j7 38 -04' 39.i- 4u-68 4 188 44-07
20  25-53 29221 32-16 34-65  36-81 3b 761 4u-)53 42 15' 46t05 45.o66 46-38' 48-84
22  30-90 35.37, 38-91 41-94  44-55 46'8,1 49-86 51-90 52. 95 51-54 56-13 59-10
24  36-78 491 462-   3  439- 49  53-01 55-83 5a-35 tiou-61t b-5  i-4'   66-'81 7u 32
26   43-17 49-38 5436 5b'6-  62) 5 5 60'52: 67 68 7 25 7.8'b-17 78-'21 8-253
28  5u004 57271 (3-0!6 67'92  7218 75-991 79-44 8z-62 85-56 8:- 2 9uJ93i 95-70
30  57-45 65-76 7239'7797  82-86 r7-211 9-'20 94-83 9o2 lvil'i40144 44lu99 9
32  65-37 74881 8253 e8-71  9426 99-24'103-7 107-9 11'       8 11-o4 1118 7 125-0
34  73-80 8448! 9297 100-22 106l  3 110 1117-1 121-8 11 6-2  130-2 1134-0 1411
36  82'71 94'681,4-2  112-2  119-3 125-6 131-3 136-5 14.-4  1460 i150-3  158-2
38  92'1610.55 116-1 1-25-0  134-0 1369 1146-3 152-1 157-6 16i27 11675  176-3
40 12-1  116-9 112.j-6 1-28-6  147-3;155-l 1l62-      1616 17496 18u-2 185t6  195-3
42 112-6  128-9  141-8  152-8      4  71624 17-9 11787 185-9 1192-5  198'7 204-6 215-3
44 123-5 141-4 155-7  167-7  178-1 11876 1199-4 204-0 211-3'2181 1224-5 236-3
46  135-0 154-6 170-1 183-3  194-6'2046 1214-3 223'0 1230-0 238-4 1245-4  -58,3
48  147-0 168-3 185-3  199-6  212-1 223-2 1233-4 242-8 251-5 2596 1267-2 281-3
50 159-6 182-6 201-0 216-5  230-1 1242-3 253'3 263-4 12729 281-6 12~9-9 305-1
52 172-6 197-6 217-4 2342  249-0 12620 270-7 284-9 295 2 304-6!313-5 330-0
54 186-1 213-0 234-5 2526  268-4 282-6 295'4 307'2 318-3 328'5 1'381  356'1
56 200-1 229-1 252-2 -271-6  288-7  303-9 317'7 330-3 342-3 353-4 131i3-6 382-8
58 214-7 245-8 270-5 291-4  309-6'3258 340-8 354i6 367 2 378 9 1389-7 41111
60 1229-8 1263-0 289-5 311-7  331-2 348-9 1364-8 379-2 393-0 405-6 1417-6 439'5




280
TABLE 3.-Page 281.
Expanded Steam.-Mean Pressure at different Densi.
ties and Rates of Expansion.
THE tables on the opposite page are intended to facilitate the computation of the power of an engine
working expansively. The first column in each table
contains the initial pressure of the steam in pounds;
and the remaining columns contain the mean pressure
of steam throughout the stroke, with the different degrees of expansion indicated at the top of the columns,
and which express the portion of the stroke during
which the steam acts expansively. Thus, for example,
if steam be admitted to the cylinder at a pressure of 3
pounds per square inch, and be cut off within ith of the
end of the stroke, the mean pressure during the whole
stroke will be 2 96 pounds per square inch. In like
manner, if steam, at the pressure of 3 pounds per
square inch, were cut off after the piston had gone
through jth of the stroke, leaving the steam to expand
through the remaining 7-ths, the mean pressure during
the whole stroke would be 1-154 pounds per square
incn. Steam, however, of as little as 3 lbs. per square
inch, is never used in working an engine, for this would
be steam of 12 lbs. below the atmospheric pressure;
and if steam of 3 lbs. above the atmospheric pressure be
taken, this will be steam of 18 lbs. pressure upon the
square inch; the mean pressure of which throughout
the stroke, if cut off at the time the piston has gone
through Ith of the stroke, or has 7-ths of the stroke
still to perform, is 6-927 lbs. per square inch, being
nearly 8 lbs. below the atmospheric pressure.




EXPANDED STEAM.
MEAN PRESSURE AT DIFFERENT DENSITIES AND RATES OF EXPANSION.
The Columns headed 0 contain the Initial Pressures in lbs., and the remainin,' columns contain the Mean Pressure
in lbs., with different Grades of Expansszn.
Expansion by Eighths.                                Expansion by Tenths.
0~ 1   I  |  1      |        0 |W    TV ~     Tll  T  5  | _6           V   TV    I 0
3 2-96  2-89  2-75  23  222  1789 1-154  3 2 980 2-930 2-830 2-710 2-539' 2-299 1-981 1-568 0-990
4 3-95  3-85  3-67  3-38  2-96  2-386 1539  4 3-974 3913 3780 3-614 3-386 3-065 2-642 20(87 1-320
5 4-948 4-818 4-593 4-232 3-708 2-982 1-924  5 4-968 4-892 4-725 4-518 4-232 3-832 3-303 2-609 1-651
6 5-937 5-782 5-512 5-079 4450 3-579 2-300  6 5-961 5-870 5-670 5-421 5-079  4598 3-963 3-130 1'981
7 6-927] 6-746 6-431 5-925 5-241 4-175 2-694  7 6-955 6-848 6-615 6-325 5-925 5-364 4-624 3-652 2-311
8 7-917; 7-710 7-350i 6-772 5-934 4-772 3079  8 7-948 7827 7-560 7-228 6-772 6-131 5-284 4-174 6-641
9 8-906 8-673 8.268 7-618 6-675 5-368 3-463  9 8942 8-805 8-505 8-132 7-618 6-897 5-945 4-696 2-971
10 9-896 9-637 9.1871 8-465 7-417 5-965 3-848 10 9-936 9-784 9-450 9-036 8-465 7-664 6-606 5-218 3-302
11 10-88510-601 10106' 9-311 8-159 6-561 4-233 11 10-929 10762 10395 9-939 9-311 8-430 7-266 5-739 3-632
1211-87511-565 10-92510-158 8-901 7-158 4-618 12 11923 11-740 11-340 10-843 10-158 9-196 7-927 6-261 3-962
13 12 865 12-528 11-943 11-004 9-642 7-754 5003 13 12-856 12-719 12'285 11-746 10-994 9-963 8-587 6-783 4-292
1413-854 13492 12-862 11851 10-384 8-531 5388 14 13-910 13-967 13-230 12-650 11-851 10-729 9-248 7-305 4-622
15i14-844 14-456 13-781 12-697 11-126 8-947 5'773 15 14-904 14-676 14-175 13-554 12-697 11-496 9-909 7-827 4-953
16 15834 15-420 14-700 13-544 11-868 9-544 6-158 16 15-897 15-654 15-120 14-457 13-544 12-262! 10569 8-348 5-283
17|16-823 16-383 15618 14-390 12-609 10-140 6-542 17 16-891116-632 16-065 15-361 14-051 13-028'11230 8-870 5-613
18117-813 17-347 16-537 15-237 13-351 10-737 6-927 18 17-884 17-611 17-010 16-264 15-237 13-795J11890 9-392 5'944
19118-702 18-311 17-448 16-803114-093 11-333 7-312 19 18-878 18-589 17'955 17-168 16-083 14-56112-551 9-914 6-273
20119792 19-275 18-375 16-930 14-835 11-930 7-697 20 19-872 19-568 18-900 18-072 16-93() 15328!13-212 10-436 6-600
25124'740 24-093 22-968 21-162 18-543 14-912 9-621 25 24-840124460 23-625 22'590 21-162 1 9160116-515 13-040 8-255
30129-688 28-912 27-562 25-395 22-252 17-895 11 546 30 29-808 29'352 28-350 27i108 25-395 22992:19-818 15-654 9-960
3534-636 33-731 33-156 29627 25-961 20-877 13-470 35 34.776 34'244 33-075 31-626 29.627 26-824 23-121 18-263 11-557
40"39-585 38-550 36-750 33-860 29-670 23-860 15-395 40 39-744 39-136 37-800 36-144 33-860 30-656 26-224 21)872 13-208
45'44-533 43-368 41-343 38 092 33-378 26842 17-319 45 449 12 44-028 42-525 40-662 38-092 34-888 29-727 23-481 14-859
50i49-481 48-187!45-937142-325 37-067 29-825 19-243 50 49-680148-920 47-250145-180 42'325138-320133-030 26-090 16-510




282
TABLE 4.-Page 283.
TABLE 4 contains four small tables. The first gives the temperature and elastic borce of steam, as experimentally determined by
Dulong anJ Arato; the second gives tie expansion of air by heat;
the renlin;inin  tables have reference to tile use of steam expansively,.by means of lap on the valve.'I'le larger of tiese tables is intended to show the amount of cover
required oil tie steam side of the valve to cut off the steam at different parts of the stroke.'iie first column contains different lengths of
the stroke of tie valve    in ic.ies, and thle remaining columns are
headed by fractions, indicating the degree of expansion. To cut off
the steam at th fl rom the end of the stroke, tile stroke of the valve
being 17 inches: —In the column headed —, and in a line with 17. will
be found 3'47, wiiicii is the required cover in inches, the valve being
without lead. To give it - inch of lead, subtract one-half the lead
from the cover, and the remainder (3'345 inches) is the cover for the
required expansion and lead. Subtract the cover from one-half the
length of the valve stroke, and the remainder (5155 inches) is the
greatest breadth of port with that stroke.
The remaining table is to show at what parts of the stroke, under
any given arrangement of slide valve, the exhausting ports close and
open. The colunmns headed A contain the amount of cover on the
exhausting side of tle valve in parts of its stroke. In the column B,
the figures represent the distance of the piston, in terms of the length
of its stroke, from the end of its stroke, when the exlhausting port before it is shut; and C shows similarly at what part of the stroke the
exhausting port behind tile piston is opened-the steam, in both
cases, being cut off at ^d from the elnd of the stroke. In like manner,
in D E the steam is cut off at -7-ths from the end of the stroke; in
F       G   t   inKat  5th; in L        thh; inat  in LM  atth; in N Oat th; in P
Q at -rth; in R Sat mtth.  Ineach case, the left hand single column of each double column represents the distance of the piston from
the end of its stroke, when the exhausting port before it is shut; and
the right hand single column when the exhausting port behind it is
opened.
Let the stroke of the piston be 6 feet, and the slide valve cut off the
steam at id from the end of the stroke, and the cover on the exhausting side of the valve be 8th of the valve stroke. In a line with
}th, and under the columns B C, will be found 178 and'033, which.
multiplied respectively by 72 inches, will give 12'8 inches for the distance of the piston from the end of its stroke, when the exhausting
port before it is shut, and 2'38 inches when the exhausting port beVindu it is opened.




Temperature and elastic Force of    Expsion of Air bv Heat in       Amount of Cover required on the Steam Side of the Valve to
Steam in Arnnoosherea.        Degrees of Fahteriheit.    Length   ct the Steam ot' at any of (he uideri-oted parts ul' the'-roke.
Stroke 
Force.  Temp. 1 Force.  Temp.   Temp. Vol. Temp. Vol.    o< ~I
I                        -)d~~~ V~~le    I         _~Vt 7_v   1  5      I             I      I
oi Ins.   3        24    45                           1     2 I
1 2120  13 38()'660  320 10  100 1152                                                  ___________
1i  2340       14   386'940    33   1002   110  1173           24    604   648   600   547 
2              15   392860    34   1004   120  1194                  6-94   6     575   547  4-90   4        3-47  2-45
2~  263-80    16   398 480    35   1007   130  12t5                       36  594   550   5(-)  449   389  313  2-24
3    75~'20    17   403.830    36   1009   140 140    5        21     07     67   5.95   4.79   4.28:72   3(3 
31  2850       18   4O8.920    37   10 2   150  1255           29    578   540   500   456   408   3-54         9    04'q   ^ o    ^    120                                                   5 -40  5-00   4-56   4-08   3-54  25'920
4   293-70    19   413-780    38   1015   160  1275            19    5-49   5.13   4.75   4-331 3-8   3-36   274   1-04
4t  309:30    20   418-460    39   1018   170  1295             8    52    4.86  450   410   67    19  260 
5   30 7-50    21   422-960    40   10 i   180  1315           1.7   4.1  4-59   4"-25  3        ~88  347   301  20-45   1-73
51  314-240   22   427/-^80    45   1032   190  1334           ip    46Q  4.-1  1~63
6   320'360   23   431.420    50   1043   200  1364            15    4:33  405   375   342   306  )115   216
61  326-260   24   435'560    55   10)55   210  1372           14    405   378  3-50   3-19   28i   2-48'2)2  1-43
7   331-70    25   439-:340    60   1066   212  1376           13    376  351   325  296   26-5   230  -188   1-3 2
8   341-780   30   457-160    65   1077   302  1558            12    347  3.24  3(10  274  0245   212   173   1 22            G
9   350780   35   472-730    70   1089   392  1739             11    3%:2  3-10  287  262      35  203  166   117            eo
10   358780   40   486-590    75   1099   482  1919             11    318  297  275   251   224  195   158
11   366-850   45   499-140    80   1110   572  2098            i     303   283   2-62   239.i4 1186 1.51  1.07
12   374-001   50   510-60       90   1132   680  2312          10    2-89  2-70   2-50  2-28   2-04   1-77   1-44   102
~~~__  ____ ____ ____ ____ ____ ___ _9^           2-65  2-56   2-37   2-17  1-93   1-68   1-32    -
-~~ ___ - ___- ___- ______9    2-60  2-43   2-25   2-05   1-84   159   1-30 (             92
A     B      C     D      E      F     G      H     K           8    2 2-46  2-23   2-12   1-94   1-73   1-50   1-23   -86
1-8th  -178  -033  -161  -026  -143  -019  -126'012             8    2-31  2-16  2-00   1-82   1-63   1-42   1-15   -81
1-16Lh'130  -060  -118  -052  -100'040'085  -030            7    216  202   187   1-71  1-53   1-33   1-08   -76
1-32d  -113'073  -101  -066'085'051  (-069   042            7    2'02   189   175   1-60  1-43   1-24   1-01   -71
0    -092  -092  -082  -082 1067'067   055'055              61   188   1-75   1-62   1-48   1-32   1-15   -94   -66
~_______________ __________ ___________6 1-73 1-62 1-50 1-37 1 22 1-06 -86 -61
A        ~~~~~~~I gL M~~~                    51   1-58   1-48  1-37   1-25   112   -97   -79   -56
A.             N      0      P     Q      R      S          5    1-44   1-35   1-25   1-14  1-02   -88   -72   -51
1-8th  -109  -008  -093  -004   074'001  -053  -001             4-   1-30   1-21  1-12   1-03   -92   -80    65   -46
l-16th  -071  -022  -058  -015  -043  -008  -027  -002           4    1-16   1-08   1-00   -91   -82   -71'58.41
1-32'  -053  -033  -043  -'023'033  -013  -024'004            3-   1-01   -94   *87   -80   -71   -62    50'35
0    043  -043'033'033'022  -022  -011  -011              3'86'81'75   -68'61   -53   -44    30




284
Screw Steam Vessels.
TABLE showing the principal Dimensions and the Performance per Expert.
ments of the Screw Steam Vessels, "Free Trade," " Eider," " Thames," and
"Elbe," constructed by Boulton, Watt, & Co., 1847.
Trade     "Eider." 1" Thames."  " Elbe."
Length between perpendiculars. 140-0 ft.  140'0 ft.  130'0 ft.  130-0 ft.
Breadth...................   25ft. 8 in. 25 ft. 8 in. 24 ft. 2 in. 24 ft. 2 in.
Depth....................... 14 ft. 6 in. 14 ft. 6 in. 13 ft. 6 in. 13 ft. 6 il.
Tonnage..................    424          424 5 4      355        355
Immersed sectional area....... 166 sq. ft. 179 sq. ft. 136 sq. ft. 133 sq. ft.
Draft of water................    8 ft. 4 in. 8 ft. 10 m. 7 ft. 41 in.  7 ft. 2 in.
Speed in knots................              731 766  7            -   -
"    miles................  89         8-32       8'209       7-544
Diameter of screw.............] 7-9 ft.    8-3 ft.    7-3 ft.     7-3 ft.
Slip of the screw in miles....... 1 1-33    -
Pitch of screw.................  9-9 ft.    10-3 ft.    8-0 ft.   9-3 ft.
Length of screw.............       3 ft.     1  ft. 18   4 ft.     1-4 ft.
Number of blades of screw.   Three.    Two.             Two.       Two.
Number of revolutions of screw
per minute................    941          90         108        93
Diameter of cylinder...........   313 in.    313 in.    27 in.     27 in.
Length of stroke..............    3 ft.       3 ft.    2-6 ft.    2-6 ft.
Number of cylinders...........    Two.       Two.       Two.       Two.
Number of strokes per minute..   31-          30         36         31
Vacuum by the barometer....    27 in.        26 in.    241 in.    26 in.
Nominal power...............    60horses. {0 horses. 40horses. 40 horses.
Actual power......132 horses. 146 horses. 102 horses. 100 horses.
Pressure of steam in boiler.... 13bs.    13 Ibs.    12 bs.    II Ibs.
Average effective pressure on
piston.1575 lbs..17'30.bs. 1..975b..   19-9bs.
Steam cut off at -of stroke...                3          1          1s
Cubic feet of water evaporated
I per hour....................  105 ft.    105 ft.    70 ft.     70 ft.
Square feet of surface of boiler
to evaporate a cubic foot.....  5 ft      9.5 ft.    9-585 ft.  9-585 ft.
Square feet of fire bars to evaporate a cubic root........... 0'433 ft.   0'433 ft.  0-4928 ft.  0'4928 ft.
Actual horse-power produced by
the evapora ion of a cubic foot. 1-257h. p.  1'39 h. p. 1-457 h p.  1-428 h. p.
Sectional area )f tubes per cubic
foot evaporated............. -851 sq. in. 8'51 sq. in. 10 sq. in. 10 sq. in.,Multiple of the wheels.........   1   3     1 to 3       to 3     1 to 3
Diameter of wheel.............              9 ft. 4 in.   9 ft. 4in. 8 ft. 6 in. 8 ft. 6 in.
"     pinion............  3 ft. 1 in. 3 ft. 1 in. 2 ft. 10 in. 2 ft. 10 in.
Pitch.......................   3-927 in.  3-927 in.  3            -141 in.  3141 in.
Br,^earllth  ~~~~~~~ ~~~~.~~~~   2 teeth,   2 teeth,   2 teeth,   2 teeth,
Breadth...........................each 6 in. each 6 in. each 41 tn. each 41 in.' Wheel    Wheel   Wheel    Wheel
geared    geared    geared      geared
with horn- with horn- with horn- with horn
Whether teeth of wood......       beam.      beam.    beam. b       eam.
The       The        The         The
pinion of  pinion of pinion of  pinion of
iron.     iron.      iron.  iron.
L____[__e_




286
INDEX.
Actual and nominal horse pow- Centre of gravity, 13.
er, 49-52.                   Centre of gyration, 13.
Air-pump, details of, 221-224. Centre of oscillation, 14.
Air-pump, proportions of, 76.   Centre of pressure, 97.
Air-pump for high speeds, 68, Centrifugal force, 13. 15. 19.
69.                          Centrifugal pump, 175.
Atmospheric resistance to loco- Centripetal force, 13. 19.
motives, 84-87.               Chimney, proportions of, 55Beams, strength of, 108.          57.
Blowing off boilers, 140-144.   Chimney, temporary, for steamBoilers, wagon, construction and    ers, 267, 268.
setting of, 123-125.         Coal, consumption of, per horse
Boiler explosions, 150-153.       power, 43.
Boilers, incrustation of, 139-  Coal, evaporating efficacy of,
146.                            53, 54.
Boilers, locomotive, 154-162.  Cocks, pumps, and pipes, 228
Boiler, locomotive, steam  chest   -240.
of, 157.                      Combustion, 44.
Boilers, locomotive, tubes of, Condensation of steam, 3'. 38.
160, 161.                    Condenser, proportions of, 76.
Boilers, marine, construction of, Co:ndensing  engines, varieties
1z5-132.                       of, 8, 9.
Boilers, marine, corrosion of, Condensing engines, high speed,
120-123.                        68-70.
Boilers, scaling of, 144.       Connecting rod, proportions of,
Boilers, strength of, how deter-   110-114.
mined, 117-120.              Cornish boilers, 39, 40. 43, 44.
Boilers, tubing and staying of, Cornish pumping engine, erec132-136.                       tion of, 167-176.
Brass for engine work, 252-  Corrosion  of marine boilers,
254.                            cause of, 120-123.
Calorimeter, 57.                Counter, 163.
Case hardening, 250, 251.       Crank pin, diameter of, 110Cataract, 166, 167.               113.




286                         INDEX
Cylinder faces, 210-213.         Furnace bridges, 129, 130.
Cylindlers, manipulation of, 198 Furnaces, firing of, 45-47.
— 2!i2.                       Gauge cocks, 8. 164.
Cylinder passages, dimensions Gauge, steam, 7.
of, 76-79.                    Gauge, vacuum, 7.
Cylinders, steam jackets, bene- Glass gauges, 164, 165.
fit of, 196, 197.             Governor, 17-19.
Cylinder, thickness in oscilla- Gravity, centre of, 13.
ting engines, 198.            Gudgeons, strength of, 109.
Direct acting engines, varieties Gyration, centre of, 13.
of, 176.                      Harvey and West's valve, 173,
Double acting engines, 9, 10.      174.
Duty of an engine, 52-54.        Heat, latent, z.8, 29.
Dynanmometer, 164.               Heat, specific, 29, 30.
Eccentric pulley, 215-217.       Heating surface, 40-43.
Engine bearings, lubrication of, High pressure steam  engines,
254, 255.                        power of, 65-67.
Engines, erection of, in work- High speed condensing engines,
shops, 255-259.                  68, 69.
Engines, erection of, in steam  High speed engines, power of,
vessels, 262-267.                69, 70.
Expansion of gases, 283.         Horse power, actual and nomiExpansion of steam in cylinder,   nal, 25, 26. 49-51. 277. 279.
27, 28. 72-75. 281. 283.      Hydrometer, 142. 144.
Expansion valves, operation of, Incrustation of marine boilers,
218-221.                         139-146.
Explosion of boilers, cause of, Indicator, 26.
150-153.                      Iron, strength of, 108.
Feathering paddle wheel, 96. Joints of engines, 249, 250.
191, 192.                     Jucke's fire grate, 49.
Feed pump, capacity of, 36. 64, Lamb's scale preventer, 143.
65.                           Lap of the valve, 72-75. 283
Fire bars of locomotives, 159.   Latent heat, 28, 29.
Fly wheel, 70.                   Lead of the valve, 71. 283.
Fly wheel shaft, diameter of, Locomotive boilers, 154-162.
109.                          Locomotive boilers, fire bars,
Flues, dimensions of, 57-64.   159.
136, 137.                     Locomotive boilers, steam chest
Flues, incrustation of, 139-146.   of, 157.
Friction, nature of, 22-24.      Locomotive  boilers, tubes of,
Friction, amount of, 81-83.        160, 161.
Fuel, comparative evaporative Locomotives, cocks, pumps, and
power,.54.                      pipes, 228-240.
Funnel, 137. 267.                Locomotives, cost and performFurnace bars, 128, 129.            ance of, 83-88.




INDEX.                         287
Locomotives, details of, 238-  Pendulous bodies, vibration of
250.                             14-18.
Locomotives, duties of engine Piston, construction of, 205driver, 269-274.                 210.
Locomotives, wheels of, 243- Piston rod, diameter of, 110246.                             114.
Lubrication of engine bearings, Power of engines, actual and
254, 255.                       nor..nal, 49-51. 277-279.
Marine boilers, cause of corro- Power of engines, measure of,
sion, 120-123.                   25, 26.
Marine boilers, construction of, Power of high pressure engines,
125-132.                        how ascertained, 65-69.
Marine boilers, incrustation of, Pressure, centre of, 97.
139-146.                      Priminig, nature and causes of,
Marine engine, pumps, pipes,   147-149.
and cocks, 228-240.           Pump, centrifugal, 175.
Marine engine, management of, Pumping engine, Cornish erec267-269.                         tion of, 167-176.
Mechanical powers, 20-22.        Pumps, pipes, and cocks of enMomentum, 12.                      gines, 228-240.
Muntz's metal, 252.              Pump valves, 173-175.
Nominal horse power, 49-51. Rails, adhesion of wheels to, 83.
277-279.                      Rotatory engines, 8, 9.
Oscillation, centre of, 14.      Safety valve, area of, 75.
O)cillating engine, objections to, Salt gauge, 142-144.
177, 178.                     Scale Preventer, Lamb's, 143.
Oscillating engine by Penn, de- Scaling of boilers, 144, 145.
scription of, 179-192.        Screw propeller, 105-107.
Oscillating engines, dimensions Sdrew steam vessels, 284.
of, 193-195.                  Sediment Collectors, 143.
Oscillating engines, thickness of Single acting engines, 9, 10.
cylinder, 198.                Shaft-paddle, diameter of, 110.
Oscillating engines, trunnions,   112.
195, 196.                     Slide valve, 70, 71.
Oscillating  engines,  trunnion Slide valve, best construction of,
bearings, 198.                   214, 215.
O.scillating engine, valve gear- Slide valve, expansion by, 283.
in(, &c., 189-191.            Slide valve, setting of, 259Paddle wheels, details of, 224    262.
-227.                         Smoke, consumption of, 45Paddle wheels, feathering, 101.   49.
191-193.                      Specific heat, 29, 30.  ---
Paddle wheel shaft, 110-112.  Speed of engines, 67, 68.
Paddle wheels, theory of, 95-  Staying of boilers, 134-136.
105,                          Steam, properties of, 30-40.




288                        rIND1x.
Steam chest of locomotive boil- Vacuum, nature and properties
ers, 157.                       of, 5-7. 10, 11.
Steam, consumption per horse Vacuum gage, 7.
power, 26, 27.               Valve and cylinder faces, meSteam, elasticity  of, 34, 35.  -thod of fitting, 210-213.
283.                         Valves, expansion, operation of,
Steam  engine, suggestions for   218-221.
improvement of; 274-276.   Valve gearing of oscillating enSteam gauge, 7.                   gines, 189. 191.
Steam, high pressure, benefit of, Valve, Harvey and West's, 173
31-34.                          -175.
Steam jackets, benefit of, 196, Valve, lap of, 72-75. 283.
197.                         Valve, lead of, 71.
Steam  passages, dimensions of, Valve, pump, 173-175.
76-80.                       Valve, safety, area of, 75.
Steam pipe, area of, 76.        Valve, slide or sluice, 70, 71.
Steam vessel, method of putting Valve, slide, setting of, 259.
engines into, 262-265.          262.
Steam  vessels, securing of en- Velocity of falling bodies, 10gines to hull, 265-267.         12.
Strength of boilers, how deter- Vent of boilers, 57.
mined, 117-120.              Vessels, resistance of water to,
Strength of parts of steam  en-   88-94.
gine, 107-117.               Wagon boilers, construction and
Surcharged steam, 30, 35, 121.    setting of, 123-125.
Teeth of wheels, proportions of, Water, quantity requisite for
110, 111.                      boiler, 36.
Tractive force on railways, 81.  Water, quantity requisite for
Trunnions of oscillating engines,   condensation, 37, 38.
195, 196.                    Water, resistance of, 88-94.
Trunnion bearings of oscillating  Waste steam pipe, 137.
engines, 198.                Waste water pipe, 138.
Tubes, incrustation of, 145, 146. Wheels, adhesion on rails, 83.
Tubes of locomotive  boilers, Wheels of locomotives, 243160, 161.                       246.
Tubing of boilers, 132-134.    Wheels, strength of teeth, 110;
Unguents, 24, 25.                 111.