a Book _^ . 1 1. f 1 ^ THE ELEMENTS OS- PHTSIOGEAPHY. SCIEl^CE CLASSICS, M^ pSJMEigCAPq ggg IIIE^LE-CLASS SCHOOLS BY JOHN J. PEINCB, Author of ''School Management and Method," etc., etc. FOURTH EDITION. -REVISED AND ENLARGED. ;^/,,<^/4 JOHN HEYWOOD, DEA:!srsGATE, AND RiDGEFiELD, Ma:n-che£tek; AND 11, Paternoster Buildings, LONDOX. IGSl. GfBss W lMamflf%r fr«m Fat. one* Lib. A»rl 1014. PREFACE, This little work has been prepared with, especial reference to the Syllabus for the Elementary Stage of Physiography — recently issued by the Science and Art Department — but not confined to it. It is hoped that it will be found useful to all who wish to inquire into the physical features of the earth, its atmosphere, &c. Ko efi'ort has been spared to render it accurate up to the information of the present day. To the student, I would say, to obtain a thorough knowledge of the Qonfiguration of the earth's surface, the superficial con- formation of the ocean, the currents thereof, and the course, tributaries, and water-partings of rivers, nothing short of a very careful study of political and physical maps (many good ones can be procured from the publisher of this present work) will enable him to succeed. J. J. P. COJSr TENTS. physics page, (including the elements of foece). Definition — Units of Time — Space — Mass — Metric System — Velocity — Force (composition and measurement of) 7 Composition and Resolution of Forces — Parallelogram and Poly- gon of Forces 9 Matter, — CompressibiKty — Elasticity 12 Atteaction. — Gravitation — Cohesion — Chemical Af&nity — Energy produced by Heat, Electricity, &c 15 Electricity and Magnetism. — Electricity — Electrical Disturb- ances — Atmospheric Electricity — Accumulation in the Clouds — Lightning — Thunder — Magnetis m 19 Terrestrial Magnetism and Electricity. — The Earth as a Magnet — Mariner's Compass — Magnetic Elements — Mag- netic Storms — ^Aurora Borealis 22 CHEMICAL ELEMENTS. Chemical Action — Compounds — The Chemical Elements — Atomicity — Binary Compounds — Terms — Compounds Bro- ken Up — Decomposition of — Rock-forming Minerals — Com- position of the Earth's Crust 26 Water. — Composition of — Expansion of — Different States — Latent and Specific Heat 33 GEOLOGY RELATING TO THE CRUST OF THE EARTH. Rocks. — Sedimentary — Stratified — Mechanically, Organically, and Chemically Formed — Surface Soil — Metamorphic — Unstratified — Plutonic 36 Internal Heat of the Globe. — The Interior of the Earth — Phenomena and Distribution of Volcanoes — ^Hot Springs — Earthquakes — Earthquake Bands — Causes of Earthquakes and Volcanoes 46 Crust of the Earth. — Upheaval and Subsidences — Relative Age of Strata — Changes of the Earth's Surface and in the Forms of Life— The Different Systems— The Age of the Earth 53 Vi C0NTE2x'TS. ASTRONOMICAL GEOGRAPHY. page. Planets.— Form and Motions of — Sun and Moon — Day and ISTight — Seasons — Nutation — Precession of the Equinoxes — Eevolution of the Apsides 59 PHYSICAL GEOGRAPHY. The Surface of the Earth. — Definitions 67 Extent and Distribution of Land and Water. The Land : Mountains — Tablelands — Plains — Deserts 70 The Ocean: Density — Weight — Depth — Composition — Colour — Waves — Tides — Currents 84 Sea's Action upon the Crust of the Earth 93 Waters of the Land — Springs — Rivers 94 Table of River Systems — Lakes 98 The Atmosphere. — Composition — Pressure — Height of — Baro- meter — Thermometer — Temperature , 104 Winds— Trade Winds— Storms 110 Vapour, Evaporation, and Condensation. — Dew — Clouds — Rain — Rainfall — Snow — Ice — Glaciers 115 PheDomena of the Arctic and Antarctic Regions ,...,.-,r..r'r--T "^23 Climate. — Causes Affecting it — Representation of i25 Life and its Distribution. Horizontal and Vertical Distribution of Vegetation 128 Distribution of Animals — Representative 132 Distribution of Marine Life 135 Distribution of Man 135 APPENDIX. Light and Heat from the Sun 138 PARALLAx^Distances of Heavenly Bodies 140 Solar System — Planets — Satellites 142 Comets and Meteors 146 Fixed Stars 148 Spectrum Analysis 151 Latitude and Longitude 157 Map Projections and Geodetical Surveys 159 Instruments 164: The Nebular Hypothesis; or the Origin of the Earth 168 Syllabus of Physiography issued by the Science and Art Department 173 Examination Papers 174 Index 186 THE ELEMENTS OF PHYSIOGRAPHY. Physical Geography relates to the great natural features and arrangements of tlie globe regarding th.e land, water, atmosphere, and animal and vegetable life; but Physiography extends over a much greater scope, taking us into Chemistry and Geology to inquire into the nature of the materials of which the earth is composed, the origin of the different rocks, and their relative ages and history ; into Astronomy, regarding the earth as a member of the solar system; and into Physics, inquiring into the laws of gravitation, the electricity and magnetism of the earth, and into the effects of these forces. Hence we may conveniently treat of the subject under five chief heads, namely, Physics, Chemical Elements, Geology, Astronomy, and Physical Geography. PHYSICS (including the elementary principles op dynamics). Before considering the different forces which affect our planet, it is necessary that the student should possess some elementary notions of the action of force; and before we can measure its effects it is necessary that we should have some standard to compare with — hence certain units of measurement are adopted, namely, tmits of time, units of space, and units of mass. 1. Unit of Time. — The mean solar day is the unit by which time is measured for ordinary purposes, it being the mean duration of a revolution of the earth upon its axis, or the interval of time that elapses between two passages of the sun across the meridian. This is divided into twenty-four parts, called hours ; these hours into sixty parts, called minutes ; and these again into sixtieths, called seconds. The second is usuaHy employed in mechanics as the unit of time. 8 PHYSIO GEAPHT. 2. Unit of Space.— The English unit of lenc;th is the Imperial yard, which is defined to be the distance between two marks on a metallic bar, kept in the House of Commons, when its temperature is 60° F. That this standard may not be lost very accurate com- parison has been made with the pendulum, from which it has been found that a pendulum in the latitude of London will vibrate or swing from the highest point on one side to the highest point on the other side in one second of time, and will always, under the same circumstances, have a constant length of 39'1393 inches. There are also certain copies of the standard yard kept in various places. The yard is divided into thirds, called feet ; and the foot into twelfths, called inches. The statute mile is 1,760 times this ^mit, and the nautical, or sea mile, nearly 2,029 — it being the length of one mean minute of longitude at the equator. Hence it is more suited for geographical and nautical measurements. 3. Unit of Mass. — The English unit of mass is the Imperial pound, which is equal to a certain piece of platinum kept in the House of Commons, certified copies of which are kept in various places. It has also been found equal to 7,000 grains, one grain being ^^^ of a cubic inch of distilled water at a temperature of 62° F.* 4. In the Metric System of Length the base is the metre, which is defined to be the ten-millionth part of a quadrant of the earth's meridian from the pole to the equator, and equal to 39-3708 of our standard inches. This metre is divided into tenths called decimetres ; one-tenth of a decimetre is called a centimetre ; and a tenth of a centimetre a milUmetre. The centimetre, or the hundredth part of a metre, is usually called the unit of length; and the kilometre, or one thousand metres, is used for the measuring of longer distances. The unit of mass in this system is the gramme, which is defined to be equal to the mass of one cubic centimetre of distilled water at 4°C. 5. Measurement of Velocity. — Velocity is the rate at which a body moves or changes its position, and is always proportional to the force by which the body is put in motion — motion being the changing of position of any body. There are two kinds of velocity — uniform and variable — ^it being uniform when equal spaces are passed over in equal times, and variable when passed over in unequal times. Uniform velocity is measured by the space passed over in a unit of time, as one foot in one minute, or one mile in one hour, and so on, the greater the space and the shorter the time the greater is the velocity. * TMs is the pound Avoirdupois, the poiind Troy containing 5,760 grains. PHYSIOGRAPHY. 9 If the space is given, and the time of the hody passing over it, the velocity mill he equal to the space divided hy the time. Thus, if a body move 15 miles in 3 hours, its velocity is 15-^3 = 5 miles an hour. The time of a body in motion may be found by dividing the space by the velocity, and the space by multiplying the velocity by the time. Variable velocity is measured at any instant hy the space tohich would he passed oxer in a unit of time, if the hody moved during that unit of time at the same rate that it had at the instant in question. Velocity is called accelerated when it moves over a greater number of units of length in each succeeding unit of time, and retarded Tfhen it moves over a lesser number of units of length in each succeeding unit of time. If the velocity is acceleratsd uniformly the space described in the given time is equal to half the space 'rces at the same time, we must take any two of them alone and find their resultant, and then take the resultant as a new force in conjunction with the third, find their resultant, and so on, whatever be the number of forces, 10. The Eesilltant of any two forces is also always described by the thirtl side of a triangle, whose other two sides represent the forces in magnitude and direction. For instance, in Fig. 3, if we have given AC and CD, AD can be found without drawing the other two sides of the parallelogram. This is called the triangle of forces. By this method the resultant of a number of forces may in many cases be obtained more easily. When three forces act in the direction of the three sides of a triangle they will remain at rest. 11. Velocities. — The method of composition and resolution of forces is also applicable to velocities. Thus, supposing a ball moving along a smooth horizontal floor, at a rate of four feet per second, is struck at a certain point in a direction inclined at 60° to its * By Trigonometry the solution is thus : Taking P and Q to represent the forces, and R their resultant— that is, A C = P, A B = Q = C D, and AD=R, ACD = 180° - C A B = 120°. .-. R2 = P2 + Q2 - 2PQ COS. 120° = ps + Q2 + PQ, since cos. 120' = - i- CD Hence R = -v/9+ 16 + 12 = 37 = 6-0S21b. and the direction of R = ^jy ein. 120°= -4^ sin. 120° = -^-^- = — 'ji— = -5695=34* 43' = angle CAD, v'S? 2v/37 37 or angle BAD =-^ = 3JL60G95 ^ .^g^i = 25° 17' 2xV37 74 12 PHYSIOGKAPHT. original path, a velocity of 3 feet per second being communicated to it, the resulting velocity is found to be 6 feet per second in the direction A D (Fig. 3, page 10), by exactly the same consti'uction. There are the simple cases of forces and velocities requiring notice. If a body is influenced by two forces of 801b. and 601b., acting in opposite directions, its resultant force is evidently equal to 201b. in the direction of the greater force ; but if both forces act in the same direction, its resultant is the sum of the forces, or 1401b. In a similar manner resulting velocities are found. For instance, if a vessel is steaming along with a velocity of twelve miles per hour against the tide, which is moving at the rate of four miles per hour, the resulting velocity of the vessel is 12 - 4 = 8 miles per hour ; but if steaming with the tide its resultant would be 12 + 4 = 16 miles per hour. MATTER. 12. Matter is the name that has been applied to the earth, with the different substances upon it. It consists of minute particles, or molecules, which is the term now used to express the smallest portion of any substance that can exist in a separate state, but still containing groups of atoms, which cannot exist in a separate state, being indivisible. All matter is continually in motion, either as a whole or among ita particles, being acted on by certain forces, each of which has its own special properties. Thus, for instance, a stone, or other mass, unsup* ported falls to the ground, and would, if there was no resistance, fall to the centre of the earth. Or again, notice how rivers run down hill. There must be a force to cause the water to run at all, and this force is the attraction of gravitatien. There are generally three different states of matter recognised — EoUd, liquid, and gaseous. The force of cohesion being greatest in solids, less in liquids, and least in gases, (See " Cohesion," 19.) A body is said to be dense when the 'pores, or spaces between the atoms, are few, so that a large number of particles unite in a small mass ; and -porous when there are many pores, as sponge, &c. 13. Compressibility.— Compressibility is the quality of being capable of being forced into a smaller space or compass. All solids, liquids, and gases are compressible ; though solids and liquids are very little in comparison with gases, except in very few cases. The compression of water at 50° F. has been shown by Canton, in his experiments, to be about 46 millionths of its volume, when the atmosphere is in its ordinary sta,te (2 9 J inches of mercury), alcohol being 66 millionths, sea w^ater 40, and mercury 3. 14. Elasticity. — A body is said to be elastic when, after being bent in any direction, it tends to recover its shape if that force PHYSIOGRAPHY. 13 which had altered its figure is removed ; and 'perfectly elastic when the force with which it tends to recover its original form, or, as it is called, the force of restitution, is exactly equal to the com- pressing force. In some bodies this resisting force of elasticity is greater than in others — steel, caoutchouc, cork, cane, &c., being very elastic. If we take a table-knife and bend it, it will return to its original shape unless it is distorted beyond the limits of elasticity. The reason it tends to recover its former shape is owing to tho exertion of two forces, namely, attraction between the partially- separated atoms on the out?ide of the knife, and repulsion between the closely-approximated atoms of the inside of the knife. When a body is drawn or twisted out of shape it is said to be strained — strain measuring the amount of deformation. For instance, if a piece of wire, or other substance, was stretched, the strain would be the elongation, or distance it had lengthened, divided by the length of the wire or other substance. Hooke's celebrated general of elastic forces is, the strain is pro- 'portional to the stress that produces it, stress being the name given to the force or forces that tend to produce the elastic strain. Hence, after finding the strain, and dividing the stress by this, we obtain the modulus of elasticity for that particular material, or, if requned, we can reverse it and find the elongation, and so on. No kind of matter in a solid state is perfectly elastic, though many are in a high degree. To be perfect the force of restitution must equal the force of compression. Hence, the elasticity of any matter is the ratio that the force of restitution bears to the force of compression, or equals the former divided by the lattei". It has been found to be in glass, '94: ; ivory, '81 ; steel, "79 ; cork, '65 ; and brass, "41. Gases are the only states of matter that possess tho property of being perfectly elastic. ATTRACTION. 15. Gravitation. — Gravitation is the tendency of all matter in the universe towards other matter ; or, in other words, any two pieces of matter have a tendency to approach each other, though in .'small bodies, or those of only moderate size, it is too feeble to be observed under ordinary circumstances, but in other cases it is presented strongly to us. For instance, notice how a large ship will attract boats, or a teaspoon in a cup of tea the bubbles on the top, &c. The weight or heaviness of bodies is due to gravitation, called terrestial, or apparent gravity — weight being the name we give to the efi'ect of gravitation or the measure of the attraction. Newton is said to have been the first who recognised the existence of gravity, and that by the falling of an apple. This same force 13 14 PHYSIOGRAPHY. exerted on all the planets. For instance, the moon is kept in its orbit revolving round the earth by the attraction of gravitation of the earth ; the earth in its orbit round the sun by the attraction of that luminary ; the attraction being in all cases inversely as the squares of the distance of the lady from its centre of gravitation. Hence the force of attraction on bodies at the moon will be 60^ = 3,600 times less than on the surface of the earth, its distance being 60 times the earth's radius. In a similar manner any matter weighed at either of the poles would be heavier than at the equator, owing to the earth not being a perfect sphere — the equatorial diameter being 7,925 miles and the polar being 7,899 miles. This fact increases the weight at the poles by -^}j-^ over its weight at the equator. There is stUl another cause of the diminution of gravity at the equator, and that is the effect of centrifugal or, as it is sometimes called, centreward force in diminishing attraction, it being greatest at that place, and decreases as we get nearer the poles. As bodies at the equator in their daily motion move more rapidly than near the poles, owing to their radii being greater, a body taken from the poles to the equator loses through this force ^^ of its weight. Hence the total loss is about -^^ ; that is, a body of 1941b. at the poles weighs 1931b. at the equator, -g-^-g- of ita weight, or h\o7s., being lost in consequence of its greater distance from the centre of the earth, and T^, or 10 Joz., in consequence of the centrifugal force acting there. 16. The Intensity of the Attraction of Gravitation, or the Attractive Energy, not only varies with the distance of the bodies from each other but also with the mass of each body. Hence it is that the sun, the centre of the whole universe, is capable of attracting the most remote planets, though their distances are hundreds of millions of miles from that body, its mass being greater than all the other planets taken together. Hence we have the follovmig law : The force of gravitation between two bodies is 'proportional to the product of their masses, and is inversely pro- portional to the square of the distance hettveen them. 17. The Law of Terrestrial Gravity is as follows : The force of gravity is greatest at the earth's surface, and decreases v.2:)wards as the square of the distance from the centre increases, and downwards simply as the distance from the centre decreases, as, sup- posing a ball that weighed a pound on the surface of the earth to be taken do-vm. to the depth of half the radius of the earth, it is evident there would be the downward force from the earth below it and the upward force from the portion above it. Hence it would be influenced only by the difference between these opposite forces ; and as the downward attraction is t\dcQ as great as the upward, it will evidently exceed the upward force by half its original attraction, the other half being balanced by the upward force j that is, the ball PHYSIOGRAPHY. 15 v7ould only weigli one haK poirad, and if taken to the centre of the earth it would have no weight at all, as the upward and downward forces will evidently balance each other. From what has been said on gravitational attraction, it would appear that all bodies are drawn towards the earth. Then what causes balloons, smoke, steam, &c,, to rise? It is the same force, namely, gravity. When a body is lighter than the air it will rise, as the air, being more strongly attracted, will get beneath it, and, dis- placing it, cause it to rise. In a similar manner cork, wood, &c., will not sink in water, owing to the same force. 18. All bodies falling are acted upon by this force, though those of diflfereut material do not always fall through the same number of units of space as each other, or bodies of the same material but different in shape ; yet, were they let fall in a glass receiver with the air pumped out, their times and spaces would be exactly equal, showing that the difference in velocity is caused by the resistance of the air to the falling bodies, varying with their forms and dimensions. When a body falls the earth attracts it, so that it falls a certain number of feet in the first second of time, the body being then in motion with a velocity of say one unit. The earth still attracts it, and during the second second it communicates to it an additional unit ; so that in every successive second of time the attraction adds to its velocity in a similar proportion. Hence the spaces in each successive second are as the odd numbers 1, 3, 5, 7, &c. A body left free to move, and acted on directly by the force of gravitation, all resistances being excluded, will, in the latitude of Greenwich, fall through 16'09o4 feet in a second, thereby acquiring by this motion a velocity of 32'190S feet per second. This velocity is called the force of gravity, or acceleration due to gravity, and is represented by g. The space travelled over by a falHng body in one second is 16 feet 1 inch very nearly ; and the space described in t seconds = ^ gf^. If we know the time required for the fall of any body through a given space, the velocity with which it moves can easily be found, or vice versa. Formula. — Let v represent the velocity, t the time of descent, s the space described in the time t, and g as above. Then w^e have : — 8=^gt^=x^l_ =x^t ; v=^gt= ^= ^J2^s ; s.ndt='^=^= ,/?i g t g V \l g If the body is projected upwards with a velocity v, then s = vt-\ gt^ ; and i£ downwards, s = vt-\-\gt'^ . 19. Cohesion. — The attraction of gravitation causes a body when unsupported to fall to the ground ; but the attraction of cohesion 16 PHYSIOGRAPHY. causes the particles to hold, together and unite in masses. When the cohesion exceeds the other forces we have a solid ; when the forces are equal we have liquid or fluid ; and when heat predomi- nates we have gas or vapour. For instance, take a piece of ice. Its particles are held together by cohesion ; but take it near a fire and it will soon melt ; the cohesive attraction being overcome by the repulsive power of the heat, its particles or molecules are driven asunder, and the ice becomes a liquid ; and now, by applying more heat, it is soon converted into steam, which on entering the cold air becomes a watery vapour. It is gravitation which brings the particles of matter close enough together for the attraction of cohesion to be exerted upon them, as in the case of sandstone and other rocks. When gravitation has finished its work — bringing the loose grains of sand together — • cohesion commences, and firmly unites these particles into a com- pact mass of sandstone. Heat and cohesion constantly act in opposition to each other. Hence, the more a body is heated the more its particles will be separated. The two may be noticed even in the efiect that tbey produce upon our bodies. For instance, on a warm day our flesh especially the hands and feet, swells from the efiects of the heat, but on cold days contracts, owing to the cold (or absence of heat) causing the particles to cohere more closely together. It is the attraction of cohesion that causes the small watery particles which compose mist or vapour to unite together in the form of drops of water, rain being thus produced. In the manufacture of shot we have a good illustration of the parts played by gravitation and cohesion. The lead for the shot is melted at the top of a high place or tower, then a little arsenic is added to give it the exact fluidity. Afterwards it is poured through a kind of sieve, throiigh which it passes by the effect of gravity — namely, its weight — and falls to the ground. In its descent it assumes the form of & sphere through the force of cohesion acting upon it. 20. The strength of materials depends enth-ely upon the force of cohesion. The reason iron is so much stronger than wood ia the cohesion existing between the particles of iron is much greater than that existing between the particles of wood, owing to the difference in the degree of closeness of the particles, and the consequent changes in the effective action of gravitation. Wood is composed of layers, namely, one year's production of wood after another. Hence, its particles csnnob be so close together as in iron or any other metal, as their particles form a granular arrangement through beicg melted, which they are in nearly every PHYSIOGRAPHY. 17 case before being used ; but even then the closeness and fineness of the particles vary much, being closest in steel. Matter is said to be hrittle if the cohesion between Its atoms is so limited in extent as to be overcome by a slight displacement, and to be tenacious when the attraction is so great that it requires a considerable force to overcome it. 21. Chemical A£5.nity is that property hy which todies comhine and form new compounds and the power which causes them to continue in combination. It dififers from the attraction of gravity in not acting on masses, and only at insensible distances requiring bodies to be in actual contact. In this last property it resembles cohesion, or cohesive affinity, but differs from it by occurring only between the particles of dissimilar bodies. For instance, the particles in a mass of copper or sulphur are held together by cohesion, but if a particle of the copper comes in contact with a particle of sulphur they unite by the power of chemical affinity — the two particles being different — and form sulphuret of copper. In chemical compounds the proportion of the two substances is always definite. Thus when oxygen combines with hydrogen to form water, there are always eight units of the former to one unit of the latter, the molecules being united by chemical affinity, but held together by cohesion. The simplest cases of chemical affinity are those in which two bodies unite into a binary compound, being the result of single affinity, which power may be also exerted either between two elementary or two compound bodies. The force with which bodies chemically unite arises from mutual and equal affinity. Chemical combination produces some remarkable effiscts, often changing the form, colour, taste, density, and qualities — harm- less elements producing strong poisons, and strong poisons harmless compounds. Among the many agents that influence this affinity the chief are heat, light, electricity, proportion, &c. For instance, potash and sand unite under a red heat and form glass ; carbonic acid and lime are separated from marble or limestone by a red heat, &c. Light is also produced by chemical affinity. The majority of substances that give light are composed of hydro-carbon. The oxygen in the air first combines with the hydrogen, it having the greatest affinity for it. The carbon is then set free, and we have an intense light as the carbon passes from the hydrogen into the oxygen during the greatest evolution of heat caused by chemical combination. In order that chemical affinity may be thoroughly understood from the other forces of attraction we will give another example. If two particles of iron be brought in close contact they adhere close together by the force of cohesion, producing a larger mass, but still possessing properties in all respects identical with those of the par- B 18 PHYSIOGRAPHY. tides of whicli it is composed. In a similar manner particles of sulphur may be made to cohere and form a larger mass of sulphur. But if the iron and sulphur be brought into contact the effect is different, as the mass so obtained is entirely distinct in its properties from either the iron or sulphur, being perfectly homogeneous, showing no traces of either of its constituents, and cannot be again separated into its elements by merely mechanical processes. This is an example of chemical affinity or attraction. It is requisite that all these characters be taken into accoiint to distinguish this affinity or attraction from cohesive attraction, because cohesion does take place between dissimilar particles, as when copper is plated with L^ilver by means of powerful pressure ; but here the mass is not homogeneous, and the silver and copper may be at once distinguished. ENERGY. 22. Energy. — AU matter in motion possesses energy, or the power to do work. It is measured by the work it can perform. This energy cannot be destroyed, even by performing work, being only changed In form. A body may possess energy in one of two forms, namely, as kinetic energy or ^potential energy. Kinetic energy is that which is due to motion, and potential energy that which is due to what may be called a position of advan- tage, or, in other words, of an arrangement capable of yielding kinetic or actual energy, when there is nothing to stop it from so doing. Thus a moving mass — a bullet or cannon ball, for example — can do work in virtue of its nation ; a running stream by its motion works the mill, &c. Energy belonging to molecular motion, elec- tricity in motion, to heat and light, and actual chemical action, are included under the name of kinetic energy ; and energy due to absorbed heat, to electrical separation due to chemical separation, or due to being raised up to a certain position so that it ia capable of doing work in falling, &c., are included under the name of potential energy. To make plain we will give a few examples for illustration : A stone on being thrown vertically upwards has, on leaving the hand, sufficient power in store to raise it, though opposed to gravity, a certain height. Owing to the effect of gravity its velocity gra- dually becomes less and less, until its motion upwards ceases, its kinetic energy being now spent.* It now possesses the other kind of energy, viz., potential, owing to its changed position, thereby falling * Height it will rise = -^ where v = velocity in feet per second, and g = gravity =; 32-2 ; and this, multiplied by the mass = measurement of energj'. PHYSIOGRAPHT. VJ to the ground, and acquiring exactly the same energy as it had in starting ; but supposing the stone had lodged on some high 'building just as its upward motion ceased, it still would have had the same energy, in virtue of its position, but prevented from falling by tlea' building. 23. Heat Produces Energy. — When the energy lies dormant in a body it is potential, but this can easily be turned into kinetic by applying heat, electricity, magnetism, &c. For instance, take the water, in a boiler, apply heat, and we soon have visible kinetic energy, viz., steam. Heat expands most substances, and in expanding or contracting they produce kinetic energy. For example, in laying rails on railroads, space is left between each joint for expansion ; if not, this force would tear them up out of the ground. Contraction produces equally as strong a force. It is taken advantage of by wheelwrights, coopers, &c. Thus the hoop or tire of a wheel is put on hot, then suddenly cooled, whereon it contracts with great force, binding the wheel and spokes firmly together. These very forces, looked into, explain such common occurrences as a cold glass vessel breaking when hot water is poured in, or a hot one when cold water is poured in, &c. To the heated state of the interior of the earth may be attributed volcanoes and earthquakes. Also, when a body is heated it exhibits energy in diminishing and increasing the powers of electricity and magnetism. For instance, in soHds a few degrees of heat will diminish the conductivity of electricity to a great extent, but in liquids, on the other hand, increase of temperature increases the conductivity. Again, an iron bar that has been magnetised suddenly loses the whole of its magnetism at a particular temperature. ELECTRICITY AND MAGNETISM. 24. Electricity, like heat, lies dormant and concealed ia matter, giving no indications of its presence when in a latent state ; but when hberated is capable of producing the most sudden and destructive results. It may be called into action by friction, chemical action, heat, or magnetic influence. Its simplest form of energy may be seen in attraction and repulsion. Bodies charged with opposite kinds of electricity attract each other, and with similar kinds repel each other. A perfectly dry and clean piece of seaHng-wax, amber, or smooth glass, sharply rubbed with a dry woollen cloth, will, if applied at that moment, attract small pieces of paper, cork, &c., causing them to adhere for some time — the attracting body being now said to be electrified or excited. They 20 PHYSIOGRAPHY. have also the power of communicating their electrification to other bodies ; and, again, a body electrified by either of them can electrify a third. There are various ways in which light may be produced from electricity. For instance, the sparks due to the discharge of electro- static accumulations from the clouds give an intense light — the most magnificent example being lightning. The greatest known heat with which we are acquainted is produced by the agency of the electric or galvanic current, as all known substances can be melted or volatilised by it. 25. Positive and Negative. — As there appears a difierence between the kind of electricity excited by rubbing a piece of glass and that excited by rubbing a piece of wax or resin, Du Fay inferred from this that there are two kinds of electricity — the one vitreous, because especially developed on glass, and the other resinorjis, because first noticed on resinous substances. He supposed these to exist in equal quantities in all neutral bodies, and that when two bodies are rubbed one on the other they are separated. One of the bodies becomes overcharged with one of the fluids, and the othei with the other fluid — it depending on the nature of the bodies v/hich shall receive the excess of the vitreous and which shall receive the excess of the resinous. The theory that seems moat generally admitted now is that of Franklin. He is of opinion that there is but one electric fluid, which possesses an attraction for various substances in various degrees, and that every body in its natural condition is associated with a certain quantity of this fluid. When two bodies are rubbed together friction causes some of the fluid to leave one body and pass to another, whereupon one becomes overcharged or positively charged, and the other undercharged or negatively charged. Hence the terms positive and negative. 28. Electrical Disturbances. — Telegraph lines are constantly troubled by electrical disturbances, called earth currents. They are frequently so powerful as to render the use of the instrument impossible for some time. These disturbances are found to be very closely connected in some way with the perturbations of terrestrial magnetism called magnetic storms, which are also closely connected with the aurora boreaHs and sun spots. 27. Atmospheric Electricity. — The atmospheric medium by which we are surrounded contains not only combined electricity, the same as every form of matter, but also a large quantity of free and uncombined electricity, sometimes being positive and sometimes negative, but generally the opposite kind to that of the earth. Various kinds of apparatus have been contrived for the examina- tion of the electric state of the atmosph»;ie, such as poles elevated PHYSIOGRAPHY. 21 about thirty feet in the air, and being provided with a metallic point at their upper ends, and insulated at their lower ends. By the aid of these instruments it has been found that, in clear weather, signs of free positive electricity are always present in the atmosphere, it being weak before sunrise, but gradually gets stronger as the sun passes the horizon, and soon afterwards gains its greatest strength ; it then rapidly diminishes, and regains its minimum state some hours before sunset, after which it again increases, gaining its second maximum state, then decreases until the following morning.* It has also been noticed that the electricity of the atmosphere increases from July to January, then decreases, being more intense in the cold weather. 28. Accumulation of Electricity in the Clouds.— The chief sources from which the clouds appear to obtain their electricity are evaporation from the earth's surface, the chemical changes which take place on the earth's surface, together with the expansion, condensation, and variation of temperature of the atmosphere and its moisture. 29. Lightning and Thunderstorms.— When a cloud over- charged with the electric fluid approaches another which is undercharged the fluid rushes from the former into the latter. This discharge produces the vivid flash known as lightning, accompanied by the sound of thunder, which is similar to the report of a gun when discharged or fired. When successive discharges of the accumulated electricity take place it causes great disturbance in the air, thereby causing thunderstorms. The air being suddenly rarefied and dispersed, in the course of the lightning, rushes together again after the discharge has passed. In this way the air is perhaps disturbed for miles at the same time. The most dreaded of lightning is what is sometimes called the returning stroke, as the earth containing positive electricity, or being overcharged, returns its overplus to the clouds. The flash of lightning always proceeds from a positive body — that is, one which is overcharged with electric fluid. From observations made in Saxony, and Kremsmunster, in Bavaria, the conclusion has been arrived at that there is a periodicity in thunderstorms as well as in other natural phenomena. Thus, accord- ing to Von Bezold, in years when the temperature is high and the sun's surface relatively free from spots, thunderstorms are abundant. It has also been observed that the maxima of the sun spots coincide with the greatest intensity of auroral displays. It follows that both groups of phenomena, thunderstorms and auroras, to a certain sxtent, supplement each other, so that years of frequent storms correspond to those of auroras, and vice versa. (See 36.) * Becquerel Trai6, t. iv., p. 84. 22 PHYSIOGRAPHY. Thunderstorms are very beneficial. They purify the air by pro- ducing nitric acid and ozone, which dispel noxious vapours, and by agitating the air stir up fresh currents of air or breezes, thereby causing it to be more healthy and pure. 30. Thunder. — Several explanations have been put forth as to the cause of the noise that we call thunder. One is, that it owes its origin to a sudden displacement of air caused by the discharge of electricity which produces the lightning ; another is that the passage of the electric current causes or creates a vacuum, which the air rushes in to fill up, thereby producing the sound. 31. Magnetism is the name given to the peculiar property possessed by certain bodies, especially iron and its compounds, whereby, under certain conditions, they mutually attract and repel each other. There appears to be two species of magnetic power — ^the northern and the southern — which are perfectly similar in their mode of action, but directly opposite in their effects. For instance, take two magnets, and it will be found that the two north poles (see 33) always repel each other, and the two south ones likewise ; but the north pole of one magnet invariably attracts the south pole of the other, or the south pole the north. Hence between like powers there is repulsion, and between unlike attraction. If a bar of mallaable iron is placed near the poles of a magnet it will become immediately magnetic, either with or without contact. This is called inductio7\ Each pole of the magnet induces the opposite kind of polarity in that end of the iron which is nearest to it. This bar of iron has now acquired the power of inducing a similar state of magnetism on other iron near it, and also reacts upon the magnet from which it first derived its power, iuca^asing the intensity of its magnetism. It has been found that the attraction and repulsion existing between two magnetic pai'ticles are always inversely as the square of the distance, thereby agreeing mth the law observed in electricity, the force of gravity, and every other known force proceeding from a centre in right lines. Coulomb introduced the theory of two opposite magnetic fluids, viz., a boreal fluid and an austral fluid, and that the magnetic body consists of small particles. The fluids, in their activity, are separated in each such particle, but never pass out of it. The amount of magnetic action is to be calculated as the accumulation of the magnetic forces of the several particles — that is, as the statical resultant of all those forces. It is necessary to remember, in Reference to a magnetic bar, that the terms ioreal, southern, and positive, all refer to that pole which would point towards the south, and austral, northern, and negative, to that which would point towards the north. PHYSIOGRAPHY. 23 TERRESTRIAL IMAGNETISM AND ELECTRICITY. 32. ThG Earth itself may be regarded as a spherical magnet, whose north pole corresponds to the south pole of an ordinary- magnet, and its south pole to the north. The cause of the earth's magnetism may be on account of the crust being largely composed of iron and other magneticable metals ; or, perhaps, due to the- earth currents moving from east to west around the earth, as the succes- sive parts of the earth face the sun. Dr. Faraday was of opinion that the magnetism of the earth is the result of the induction* of the electric currents ; or, if a terrestrial magnet really exists, its poles are close together near the earth's centre. 33. Blariner's Compass. — A magnetic bar, balanced, by being fixed at its centre of gravity on a pivot, will, if free to move, after a few oscillations, assume a constant position, one end pointing to the north, hence called the north pole of the magnet, and the other end to the south, called the south pole. This is the principle of the mariner's compass, which guides the sailor when all other indications of this course fail him. To account for this property of the magnetic needle the earth is supposed to be or to contain an enormous magnet, the poles of which correspond very nearly to the geographical poles of the earth. According to Sir J. C. Ross the north magnetic pole is situated in lat. 70° N., long. 97° W., and the southern pole in lat. 75° S., long. 184° E. The magnetic needle does not point exactly north and south. Hence the magnetic meridian does not coincide with the geographical meridian ; or, in other words, the north-seeking pole of the magnet does not agree with the geographical north pole. The difference between the two is called the declination, or magnetic variation. It is measured by taking the angle made by two vertical planes, one passing through the earth's axis and the other through the needle. At London the north-seeking end points about 17° west of north. The magnetic needle, when suspended on an axis at its centre of gravity, does not maintain its horizontal position — its austral or northern-pointing end dipping considerably in our hemisphere, and in the southern the opposite pole incHnes. This is called the dip or inclination of the needle, the needle being called a dipping needle or inclination needle. The inclination in this latitude to the horizon being about 70°, it undergoes periodic variations, though small in comparison to the declination. Lines passing through places having the same declination are called isogonal lines, and lines connecting places where the dip is the same are called isoclinals. * Induction is the production of like effects in bodies near to one another. 24 PHYSIOGRAPHY. Observation has tauglit us that the magnetic poles change their position gradually during long intervals of time, and that they coincide with the points where the greatest degree of cold is felt or where the mean of the thermometer for the year is lowest on the surface of the earth. Besides this change, there is the daily variation in the magnetic needle, which commences about midnight or shortly afterwards. The north-seeking pole moves to the east. At about seven or a quarter past it is found to be 6' or 7' from its m.ean position. It then returns, passing the mean magnetic meridian about ten o'clock, reaching its greatest deviation — probably 3° from the mean — between one and two in the afternoon. It now starts towards the east, passing the mean at about five or six o'clock, still continuing to move slowly on till ten or eleven, when it moves so slowly as to be insensible for some time. It is believed that the sun is the principle agent in causing the variations. Professors Christie and Barlow, from experiments and observations, were led to the conclusion that the daily motion was dependent upon the relative position of the sun with respect to the magnetic meridian, and that the proximate cause was to be traced in the altered distribution of temperature. There are still other variations — namely, monthly and yearly — which may be called solsticial variation, but very little definitely is known respecting them. 34. Magnetic Elements.— In determining the state of the earth's magnetisfh at any place, and at any time, there are three things to be observed, viz., the magnetic declination, the magnetic inclination, and the total force, or intensity. The declination and inclination are explained above. The total force, or intensity, is a number which expresses the force of magnetic attraction at the place and in the direction of the dipping needle. It is properly expressed in absolute units of force. Sometimes the horizontal force is given instead of the total iuteasity. The latter is derived from the former, by multiplying it by the secant of the angle of the dip." A unit of magnetism is usually defined as follows : A magnet is said to have a unit of free magnetism when it fulfils the following conditions : Let the magnet be free to turn in a horizontal plane, and at the same time, in the same plane, let there be another magnet, precisely similar and equally magnetised, at right angles to it, and opposite its centre, at a distance of one millimeter from it, then the former magnet has one unit of magnetic force if it tries to turn round with a force equal to that which would be exerted by a force of one milligram acting at right angleia to an arm one millimeter in length. PHYSIOGRAPHY. 25 The magnetic elements for 1876 at Greenwich were as follow: Declination, or variation of compass, 19° 8' W. ; inclination, or dip of the needle, 67° 39' ; horizontal force, in British units, 3"9 ; total force (1873), 10-27. 35. Magnetic Storms. — From observations in various parts of the globe the occasional occurrences which have been termed magnetic storms have been noticed. During these storms the magnetic elements — ^namely, the declination, and the amount of its horizontal and vertical components — are subjected to violent changes, which appear, frequently at the same time, over immense tracts of the earth's surface. For instance, a disturbance which occtirred at Toronto, in Canada, was found to agree precisely in time and very nearly in amount with a simiHar disturbance registered at Greenwich. From registrations taken at Greenwich and Toronto the occurrence of aurora borealis has been found, in nearly all cases, to be accom- panied by magnetic disturbance at both places. It has also been proved that the magnetic storms are in some way or other connected with the spots of the sun. Schwabe, of Dessau, shows that the various epochs of maximum spot frequency are also those of maximum magnetic disturbance of our globe. It is a remarkable fact that when a spot is forming on the sun our magnetic needles are unusually disturbed. 36. Aurora Borealis, or Northern Light— So called because they appear in the north. They are flashes of light of various colours, sometimes taking the form of a dark segment, or of an arch or crown, and sometimes in the form of luminous streamers. They are frequently seen in the North of Europe, and of late years not unfrequently seen in this country. The cause of them is supposed to be the passage of electricity through the higher regions of the atmosphere, where it is highly rarefied. It has been calculated that its probable height is usually between seventy and eighty miles. Hence the density of the atmosphere would only be about the one hundred and fiftieth part of that at the earth's eurface. Several other reasons have been put forth at different times to show the cause of these lights, nearly all agreeing that it is within the region of our atmosphere. Heli ascribed it to the reflection of the sun and moon by the clouds of snow and needles of ice which continually float in the polar regions. Bailly ascribed it to magnetism, it being remarkable that magnetic disturbances have generally been noticed at the same time. Frankhn attributed it to electricity. Kastner was of opinion that the polar Hghts are the electricity of the earth rising periodically to the poles. Similar occurrences have been seen at the south of the Southern Hemisphere, but not so frequently. They are called aurora australis, 26 PHYSIOGRAPHT. If an oval is drawn round the North Pole, passing through Iceland, the North Cape, Gulf of Obi, Northern Siberia, the mouth of the Mackenzie Eiver, the centre of Hudson's Bay, and Nain, it includea the region where the greatest number occur, averaging about forty annually. CHEMICAL ELEMENTS. 37. ChGinical Action is the name applied to those operations, whatever they may be, by which the weight, form, solidity, taste, smell, colour, and action of substances become changed, forming new bodies, with different properties from the old. For instance, if we take some water and mix with it sul]Dhuric acid chemical action takes place, and the two cold liquids produce intense heat ; if we pour cold water on lime there arises heat from it, by the chemical action ; or if we take 56 grains of iron and 32 grains^of sulphur, chemical action takes place, the two substances forming ferrous sulphide (siilphide of iron), which differs in appearance and properties from both iron and sulphur. Chemical action assists the formation of rock masses, acting thus : First, the deposit becomes dry and contracts. It is then covered over with fresh layers, and exposed to great pressure, and also to an increased temperature, owing to the greater depths, the water carrying in chemical solutions (which arise from the water passing through the earth and coming into contact with many different minerals and substances) aU the time from one layer to the under or lower ones. Chemical action generally produces a change of temperature or a change of state, and often a change in colour. 38. Compounds. — Tk^ \s>^^ assording to which chemical sub- stances combine and form compounds are very simple. The first and chief is that a compound is perfectly homogeneous, and that its composition is fixed and invariable. From experiments we learn that if a quantity of hydrogen, for instance, is taken and tried to make combine with bromine or chlorine we must take 79"75 times its weight of bromiae, or 35*5 times its weight of chloriae, as they cannot be made to unite in any other proportion. Hence if we see any substance that can be recog- nised by its external properties as a compound of hydrogen and bromine, we are certain that its constituents are in the proportion of 1 to 7975 ; oe if the compound is of hydrogen and chlorine, those elements will be in the proportion of 1 to 35 "5. Again, suppose we take potassium and bring it in contact with the latter compound, viz., hydrogen and chlorine, it will at once PHYSIOGRAPHY. ST expel the hydrogen and unite with the chlorine, forming a compound of potassium and chlorine, taking 39 "13 parts of potassium to replace the one part of hydrogen. By similar experiments it is found that 23*05 parts of sodium satisfy the affinity of 35 '5 parts of chlorine, or 107*94 parts of silver can replace the 39*13 parts of potassium. In this way there could be obtained for each of the elements a number expressing the quantity of it that will satisfy the affinity contained in one part of hydrogen. These numbers, expressing the proportion in which the elements combine, represent also the relative weight of the atoms of which the different kinds of matter are composed. For instance, an atom of chlorine equals 35*5 times the weight of an equal volume of hydrogen. The weight of an atom of chloriQe being 35*5, and an atom of hydrogen 1. Hence the term atomic weight, the atoms being all the same size, with one or two exceptions, namely, phosphorus and arsenic, whose atoms are supposed to be half the usual size, and mercury, zinc, and cadmium, whose atoms are twice the size. An atom is the least part of an elementary body which can enter into or be expelled from a compound. 39. Chemical Elements. — When the different substances found at the surface of the earth are submitted to various methods of treatment, the majority of them can be broken up into several substances of a more simple nature. Thus a piece of flint can be separated into two substances entirely different from it in appearance and properties, wood and chalk into three, alum into four, and so on ; while others, as iron, gold, copper, sulphiu*, &c., resist all the processes to which they have as yet been subjected, and appear to consist of only one kind of matter. It has been found by submitting all the rocks, minerals, animal and vegetable substances, &c., to appropriate processes, that they contain about sixty-five substances, by the union of which all the different kinds of matter are made. These sixty-five substances are called the chemical elements. The following is a list of them, with their symbols and atomic weights, that is " the proportions in which they combine among themselves." N^o combinations can take place among the elements Ojsly in these proportions, or multiples of them : — Atomic Names. Svmbols. Weight. a Hydrogen ...H i a Chlorine Cl 35*37 a Bromine Br 79*75 a Iodine I 127 a Fluorine F 19 Potassium ...K 39*13 Sodium Xa 23 Lithiurn L 7 Caesium Cs 133 Atomic Names. Symbols. "Weight. Eubidium Rb 85*4 Silver Ag 108 Thallium Tl ......204 Oxygen .. ....0 .. ... 16 Barium .. ...Ba .. ...137 Strontium .. ....Sr . ...87-5 Calcium ...Ca .. ... 40 Indium ....In .. ...113 28 PHYSIOGRAPHY. Atomic Names, > -:.^-6ymbols. Weight. Magnesium -Mg 24-4 Zinc Zn 65 Cadmium Cd 112 Copper Cu 63-5 Mercury Hg 200 Glucinium G 9-3 Didymium D 96 Lanthanium ...La 92 Yttrium Y 61-7 a Boron B . Gold Au. . 11 .196-7 Atomic Names. Symbols. Weight. a Nitrogen N 14 a Phosphorus... P Arsenic As Antimony Sb Bismuth Bi Vanadium V 31 . 75 .122 .208 . 52-5 a Sulphur S 32 a Selenium Se 79*4 Carbon C 12 Silicon Si 28 Aluminium ..Al 27-5 Zirconium Zr Thorium ...... Th Tantalum Ta Niobium Nb, Tin Bn Titanium Ti Lead PI Platinum ...Pt Palladium Pd . 89-5 .116 .138 . 97-5 .118 . 50-4 .207 ,197-3 .106-5 Tellurium Te . Chromium Cr . Manganese ...Mn. Iron (Ferrum)..Fe . Nickel Ni . Cobalt Co . Cerium Ce . Uranium U . Tungsten W . Molybdenum . . .Mo . Rhodium Ro . Ruthenium Ru . Iridium Ir . Osmium Os . Gallium Ga , Lavoesium La . .128 . 52-5 . 55 . 56 . 58-8 . 58-8 . 92 .240 .184 . 92 .104 .104 .197 .199 The elements marked (a) are non-metallic, the remainder t)eing metallic. The most important of the eleme^nts are printed in black type. 40. Atomicity. — The elements have also diflFerent powers of combining. For instance, one atom of CI (see table) can only com- bine with one atom of H, but an atom of can combine with two, N with three, and C with four atoms of the same element H. Hence an atom of has the power of replacing, or is equivalent to, two atoms of CI, an atom of N to three, an atom of C to four, of the same element, &c. In a similar manner an atom of nitrogen can be substituted for five, tin for four, iron or cobalt, for six of any monad element — as hydrogen. Those elements whose atoms are equivalent to one atom of hydro- gen are called monads j those whose atoms can replace two atoms of hydrogen, dyads ; those whose atoms can replace three, triads ; four, tetrads ; five, pentads; and six, hexads. In the above table those in the first group are monads; second group, dyads; third group, triads; fourth group, tetrads; fifth, pentads; and sixth, hexads. PHYSIOGRAPHY. 29 The symbols annexed to each element are letters tised to denote them without writing their names in full. In most cases the initial letter of the common or of the Latin name is used. This symbol also expresses, by remembering or referring to the atomic weights of the substance, the quantity by weight of the substance entering into combination. For instance, CI not only denotes an atom of chlorine but an atom consisting of 35"5 parts by weight of that element ; N, an atom, or 14 parts by weight, of nitrogen ; and so on. More than an atom of an element is expressed by a small figure placed below the symbol on the right, as CI2, the figure denoting the number of atoms. 41. Binary Compounds are composed or formed ly the union of dements — namely, in the proportion of their atomic weights — as for instance, sodium and chlorine unite and form the compound sodic- chloride, having properties entirely different from either of the elements — there being 23 parts sodium and 35*5 parts chlorine. The symbols of compounds are formed by the juxtaposition of those elements. Thus, HCl represents one atom of hydrogen combined with one atom of chlorine, forming hydrochloric acid. It also further expresses the fact that the compound hydrochloric acid is formed of 1 part of hydrogen and 35*5 parts of chlorine. The rock-forming naiuerals of this class — viz., binary compounds — are few in number, the principal of which are rock salt and quartz. The chemical formula of rock salt is NaCl (chloride of sodium), that is, one atom of sodium combined with one atom of chlorine. It is mostly contaminated by a small quantity of extraneous sub- stances. The analysis of the Cheshire rock is as follows : Chloride of sodium, 98'32 ; of magnesium, "02 ; of calcium, '01 ; sulphate of lime, '65 ; and insoluble matter, 1. 42. Terms, &C. — ^We will now give a short explanation of a few chemical names given to compounds, &c. Acids. — An acid is a compound containing a certain quantity of hydrogen, easily replaceable by a metal when it comes in contact with it, either in the free state or as an oxide. It has also, generally, the property of changing vegetable colours to red. Bases are compounds which, by reacting on acids, yield salts. The most important are oxides of metals. When brought in contact with an acid their oxygen combines with the hydrogen of the acid to form water. Silicates. — A silicate is a combination of an acid with one single base, when it is called simple ; or the acid is united to two or more bases, being then called compound. Minerals of this character — namely, silicates — are the principal constituents of rocks. Oxides are compounds formed by the union of oxygen with other bodies. 30 PHYSIOGRAPHY. Peroxide and Protoxide. — When a substance unites with oxygen in two different proportions, that which contains the greatest quantity of oxygen is called peroxide, and that which contains a less quantity a protoxide. Suboxide. — Many metals have the power of uniting with oxygen in more than the above two proportions. In this case the com- bination which contains a less quantity of oxygen than the protoxide is called a suboxide, and the highest combination of the substance with oxygen is called a hyperoxide. Su'phides are compounds of the metals with sulphur, and form a very important class of compounds, presenting many analogies with the oxides. They are obtained either by heating the metals with sulphur in proportions, or passing a current of hydrosulphuric acid gas through a solution of salt. The sulphides of the metals of the alkalies and alkaline earths are soluble in water, but of other metals insoluble. The sulphides are a very important class of compounds, forming some of the most important ores from which the metals are extracted. Pyrites. — The name given to the sulphide of iron. Alkalies. — An alkali is a body that possesses properties the converse of an acid. It has a highly bitter taste ; changes the blue juices of vegetables to a green, or the juices of vegetables which have been changed red by an acid back again to blue. Potajsh and soda are representatives of this class. 43. Compounds broken up into Simpler Forms.— As before stated, there are about 65 elementary substances. Of these only 17 occur extensively amongst mineral compounds. They are oxygen, hydrogen, carbon, sulphur, chlorine, fluorine, silicon, boron, potassium, sodium, lithium, barium, calcium, magnesium, aluminium, manganese, and iron. These, combined in various ways, compose the greater part of the earth's crust, and of its liquid envelope. By remembering or consulting the table of the atomic weights we have the proportion in which they combine among themselves, and also their atomicity, or power of replacing, sometimes called substitution by equivalents. These two facts are of great importance. All rocks are composed of minerals, sometimes of one, when it is called a simple mineral, as limestone, consisting mainly of calcite ; or it may be made up of two or more, as granite, being then called a compound mineral. With the exception of the following minerals, which are either elements or binary compounds— namely, carbon, quartz, rock salt, fluor-spar, iron pyrites, and hasma.tite — the other rock-forming minerals are chiefly silicates, carbonates, or sulphates, the silicates being by far the most abundant, the carbonates next, and then the sulphates. There are nearly in every case accessory ingredients as well as the essential ones. The principal minerals are quartz, felspar, mica, hornblende, augite, clay, calcspar, and dolomite. PHYSIOGRAPHY. 31 44. Decomposition of Compounds.— The general method of decomposing compounds is by means of chemical affinity. Thus, suppose we have a compound, AB, which we wish to resolve into its elements, A and B, we must add to the compound a substance, C, which we know has a greater affinity for one of the elements than the other element in the compound has — that is, C miv/c have a greater affinity for A than B has, the result of which is that A and C combine, leaving B at liberty. Again, if we wish to libeitite A, we must mix with the compound AB a substance, D, which has a greater affinity for B than A has. Then B and D combine, leaving A at liberty. The chief agent in decomposing rocks is carbonic acid gas, as water charged with this gas dissolves the majority of them. For example we will take granite. Granite is composed of three minerals — quartz, felspar, and mica. Of these quartz is insoluble; but the acid will readily attack the felspar, which consists of sihcate of alumina and silicate of potash, soda, or other alkali. The acid having a greater affinity for the alkali (potash, soda, &c.) of the silicate than silicic acid, forms with that alkali a carbonate which is soluble in water. The sihcate of alumina, being unaffected by the acid, is set free as an insoluble clay [Kaoliri], this decomposition yielding carbonate of potash or of some other alkali, according to the chemical composition of the silica and felspar, which are dissolved in the water, and clay. It is in this way that rocks get broken up by natural causes, carbonic acid existing largely in the atmosphere, in most waters (which is the next chief agent in breaking up rocks, &c.), and combined with minerals in a solid state, as in marble, which consists of lime united to carbonic acid. It is easy to understand that any one constituent of a substance being decomposed, the other constituents will be freed and readily removed by running water. Gneiss may also be broken up into the same substances as granite — namely, felspar, mica, and quartz — but the minerals are arranged in more or less thin layers. Fine examples of this rock occur in the Alps. Syenite, another rock of this kind, may be broken up into felspar, hornblende, and quartz. Protogine is composed of felspar, talc, and quartz. Mica-schist consists of alternate layers of quartz and mica, the latter of which mostly preponderates. Talc-schist, is composed of layers of quartz and talc. Trachyte, composed of sanidine felspar, and a Uttle mica or hornblende. Basalt, containing basic felspar, titanio-ferrite, augite, and generally olivine. Felsfone, composed of acidic felspar and quartz. Melaphyre, composed of basic felspar, magnetite, augite, and generally chlorite. Diabase is similar to melaphyre. Greenstone or diorite consists of hornblende and felspar. Dolorite consists of augite and felspar. It will be seen that felspa/r and quartz are contained in most of 32 PHYSIOGEAPHT. the principal rocks, common quartz being the most abundant of all minerals. We will now give the chemical composition of a few of the chief minerals. Quartz is formed from pure silica (SiOg). Fdspar, a silicate of alumina and potash (AI2O3, SSiOo+KO, SSiOa), giving a percentage of silica 65'35, alumina 18'0"6, and potash 16'59. A little soda always occurs. Mica (potash) is a silicate of potash and alumina (KO, SSiOg + AlgOg, SiOg). A part of the potash may be replaced by lime and the protoxides of iron and manganese, and part of the alumina by the corresponding oxides of iron, manganese, and chromium. One analysis gives siHca 47, alumina 20, potash 14:"5, oxides of iron 15*5, oxide of manganese 1'75 per cent. Mica (lithia), or Lepidolite, is a silicate of alumina, potash, and lithia, in combination with a fluoride. Hornblende is essentially a silicate of magnesia, mixed with silicates of lime, iron, &c., the chemical composition of which varies much. The minerals of the Talc group are hydrous silicates of magnesia and alumina. Tracliyte often contains disseminated crystals of glassy felspar, hornblende, a little quartz, and mica. Composition — Silica 6 7 '09, alumina 15-64, potash 3*47, soda 5'08, lime 2*25, oxides of iron 4'59, magnesia '98, oxide of manganese "15, water, &c,, *83. Cliiikstone or Phonolite is composed of silica 56 "28, alumina 20 '55, potash 5-84, soda 9-07, oxides of iron and manganese 4'31, titanic acid 1*44, magnesia '32, lithia "05, &c. Clay consists chiefly of silica and alumina, but is sometimes mixed with lime, magnesia, &c. TABLE OF THE MOST ABUNDANT SIMPLE MINERALS. PERCENTAGE ANALYSIS OP ROCK-FORMING MINERALS. ci 1 < j S 1 i m s 1 1" Quartz (when piire) . . U Albite ..".WW 100 65-35 70-48 63-50 53-70 45-5 47 50-35 56-36 47 62-80 30-40 56-50 42-30 18-06 18-45 23-10 29-67 34-5 20 28-30 12" 1 7 •80 is'-is 13 32-40 34 21 44-20 •55 2-40 12-13 17 25-46 14 14-5 16-59 2*20 14*50 9-04 10*50 9-40 4-50 1" 15-50 14* 1-60 4-40 6 •20 1-75 1-23 5*49 1' :: 2-30 12-60 2 13-30 2-6 2-5 g"- OUgoclase ^ Labradorite .... ^ VAnorthite Mica (Potash Mica).. Mica (Lepidolite) Augite 2-6 2-6 2-7 2-9 2-9 3-3 Hornblende . , Talc . 3-2 2-e Chlorite 9:1 Antinolite S Serpentine 2-8 PHYSIOGRAPHY. 33 From the preceding table it will be seen tbat the cbief con- stituents of the rocks, &c., are silica, alumina, magnesia, oxide of iron, Hme, potash, soda, carbonic acid, and water ; and reducing these still further to their elements we find that silica is a compound formed by the union of silicon with oxygen ; alumina. by the union of aluminium with oxygen. Magnesia occurs in two states, sometimes as carbonate, in certain Hmestones, and sometimes as sulphate ; hence it is composed of either carbon and magnesium or sulphur and magnesium. Oxide of iron is oxygen and iron combined ; lime is oxygen and calcium ; potash, oxygen and potassium; soda is obtained from a compound of chlorine and sodium ; carbonic acid is carbon and oxygen combined ; water, hydrogen and oxygen. From the above description we see that the elements which enter largely into the composition of rocks are very few, namely, oxygen, silicon, aluminium, calcium, magnesium, iron, carbon, sulphur, chlorine, and sodium, being in the order of their relative abundance, oxygen being the most abundant of all known substances, constituting at least one-third of the solid mass of the globe, eight-ninths of the water, and nearly one fourth part of the atmosphere ; it also exists in most organic substances. Taking into account the composition of the water of the earth and its atmosphere, the two gases, hydrogen and nitrogen, are also of primary importance, the former forming one-ninth of all waters, and the latter four-fifths of the atmosphere. WATER: ITS COMPOSITION AND SEVERAL STATES. 45. Water is composed of two volumes of hydrogen and one ot oxygen ; or,^ by weight, one part hydrogen to eight parts oxygen, namely, ll'll per cent hydrogen and 88-88 per cent oxygen. It is the most important ("element," as the ancients called it) compound in the constitution of the globe, being, we might nearly say, everywhere. It always exists in the air in an invisible state, giving the blue appearance to the sky, and becomes visible in the form of clouds. It forms a constituent of all animal and vegetable substances, and also of the rocks and minerals which compose the crust of the earth. When pure and at ordinary temperature it is a fluid without taste or smell. In large bodies, as in seas and oceans, it has a peculiar bluish-green colour, but in small quantities appears colourless. When heated, under the ordinary pressure of the atmosphere, to the temperature of 212° F., at the level of the sea, water boils and is converted into steam. The higher we ascend the pressure of the atmosphere becomes less, water thereby boiling much sooner. Thus on the top of Mont Blanc, which is about 15,000 feet above the level of the sea, water was found to boil at 178° F., or 34° below ita C 34 PHYSIOGRAPHY. usual boiling temperature. What would be cooked at tbe sea-level might remain unchanged for hours in the boiling water at the summit of the mountain. There is one point regarding water, in its diflferent states and temperature, worthy of particular notice, and that is, its expansion and contraction follow a different law to all other bodies, with the exception of bismuth, which expand in proportion as they are heated and contract in proportion as they are cooled. If water be heated to its boiling point it will expand like other liquids, and if allowed to cool it will follow the general law, viz., contract, until it attains a temperature a little below 40° F,, at which point it attains its maximum density, that is its minimum or least volume. If the water still continues to diminish in temperature it will now begin to expand until reaching the freezing point, or 32° F., and if cooled below this point, by being kept perfectly still, it will continue to expand, and in the act of freezing a sudden and considerable expansion takes place. Its effect may be noticed on water-pipes, &c. If water followed the general law, and continuously contracted on cooling, it is evident that its weight, bulk for bulk, would get heavier and heavier ; hence, as soon as the surface of our rivers was frozen and ice formed on the top it would sink to the bottom ; then the fresh surface would in its turn freeze and another layer of ice sink ; and this would go on, even if the winter was not severe, until our rivers, ponds, and lakes, were converted into solid masses of ice, thereby causing destruction to their inhabitants. But such is not the case. It has been ordained by the Creator, in His infinite wisdom, that the water should expand, instead of contracting, below 40° F,, thereby becoming lighter than the warmer water underneath, causing it to float on the surface instead of sinking, and helping to form a covering or protection to the water below and its inhabitants. Water, as found on the earth, is seldom or ever absolutely pure, but contains more or less of various substances. Even rain water, the purest of all, contains small quantities of impurity, and that of rivers and springs much more. The water running through the ground dissolves more or less of the substances it meets with, and these sub- , stances sometimes become so abundantly taken up that the water acquires a strong taste and active medical properties. Such is the cause of the mineral springs so-called. Among the various substances found in water the chief are silica, alumina, salts of lime, mag- nesia, soda, potash, iron, manganese, atmospheric air, carbonic acid, nitrogen, &c. ; in the sea are also found iodine and bromine. Water may be either in a liquid, solid, or gaseous state. As water inits fluid state, rain, dew ; solid state, ice, snow, hail ; gaseous state, vapour, steam. These different states are all caused by different degrees of heat. (See " Cohesion," page 15 ; and for formation of rain, dew, snow, hail, &c., see page 117.) PHYSIOGRAPHY. 35 Latent and Specific Heat of Water. — Water, like all other sub- stances, has what is termed latent (hidden) heat ; that is, heat ^vhich is not perceptible to our feelings. Thus, the temperature of ice is 32° ; but if 144° of heat be communicated to it it will feel no hotter, but simply cause it to become a liquid, the 144° of heat being hidden in a latent condition in the ice. In ice there is altogether 1,116° of latent heat, 972° of heat being secreted when water is converted into steam. The specific heat or capacity for heat of a body is the quantity of heat necessary to raise it through a certain number of degrees as compared with the quantity required to raise an equal weight of water through the same number of degrees. Of all substances water possesses the greatest capacity for heat ; hence, when cooled through a certain range of temperature it parts with the greatest amount of heat. The high specific heat of water plays an important part in the economy of nature, the specific heat of water being 1, and of the air -2374, or nearly 4-2 times less than that of water; there- •fore 1 unit of water in losing 1° would warm 4 '2 units of air 1° ; but water is also 770 times as heavy as air, so that, comparing equal volumes, a cubic foot of water in losing 1° would raise 4*2 x 770, or 3,234 cubic feet of air, 1°. We see from this the great influence which the ocean must exert on the climate of a country. The heat of summer is stored up in the ocean and slowly given out during the winter. Hence one cause of the absence of extremes in an island cHmate.* GEOLOGY. EELATING TO THE EARTH'S CRUST. 46. The Crust of the Earth. — By the crust of the earth we are to understand the solid exterior as far as it is known to us by observation and inference. It is supposed that the earth was once in a state of fusion, and that having cooled by radiation, the outside cooling more than the interior caused a solid superficial layer ta be formed. This consists of a variety of solid materials, to which the general term roch is given, which term includes not only stony and compact rocks like granite, limestone, &c., but soft and loose matter, as sand, clay, and gravel. The chemical character and com- position of the chief of the rocks are given on page 32. We will now consider the physical character and features of the chief of them. The greater part of the rocks at the surface of the earth occur in * Tyndall on "Heat as a Mode of Motion," page 143. 36 PHYSIOGRAPHY. regular beds — each bed maintaining an almost uniform tluckness — appearing like piles of cloth piled upon each other. This class of rock is called stratified; but also denominated aqueous rocks, on account of being deposited from water when for the time the (Substances of which they are composed were either chemically or mechanically suspended ; and sedimentary, because they are tho accumulations of sediment carried in the sea by rivers. There is another class of rocks termed unstratified, igneous, or plutonic, being called unstratified because no traces of layers or beds can be- detected, the rock forming merely a great mass of mineral matter ; and igneous, or plutonic, because they have evidently been in a melted state through the action of very great heat. SEDIMENTARY, OR AQUEOUS ROCKS. 47. The Aqueous Rocks may be divided into three classes, according to their mode of origin : (1) Mechanically-formed rocks — those formed mechanically from the ruins of existing rocks, such as conglomerate, sand, clay, shale, &c. (2) Organically -formed rocks — those consisting of accumulations of vegetable or animal remains, such as coal, peat, &c. (3) Chemically -formed rocks — those resulting from chemical means, as rock salt, gypsum, &c. They may also be divided into three classes, according to their composition, namely, as arenaceous, or sandy ; calcareous, or hmestone ; and argillaceous, or clayey rocks. (1) Mechanically -formed Rocks are merely fragments of rocks broken up by the action of frost, snow, rain, rivers, &c., and again deposited as sedimentary strata. Arenaceous. — In this division we have shingle, gravel, con- glomerate, hreccia, sandstone, and grit. Shingle consists of pebbles of rock, not cemented together in any way, being rounded by the action of the stream or river, having their sharp angles and edges worn off. When the pebbles are smaller and mixed with sand it is called gravel, and sand when the fragments are very small. Con- glomerate, or pudding-stone, is a rock consisting of rounded pebbles cemented together, the cementing material filling the interstices and rendering the whole a hard compact rock. If the materials are angular it is termed a hreccia. The pebbles may consist of any sub- stance whatever, and the conglomerate is named, according to tho constituents, silicious, or quartzose-, granitic, calcareous, &c., though mostly consisting of quartz, or some very sihcious mineral, owing chiefly to the greater abundance of the silicious over other mineral matters that enter into the composition of the rocks. When the term conglomerate is used alone it is always understood to mean a rock PHTSIOGBAPHT, 37 composed of quartz pebbles. The cementing material may be either iron (ferruginous), sand (arenaceous), lime (calcareous), or clay (argillaceous). Sandstone is fine sand consolidated, but if the par- ticles are coarse it is called grit or gritstone. Abgillaceous, OB Clayey. — The most simple class of this rock is day, which results from the decomposition of felspathic rocks, &c., by the agency of acid waters. It is mostly found in an impure state, mixed with fine sand, flakes of mica, organic remains, &c. When pure it is found to be a hydrated silicate of alumina, being found pure only in the case of kaolin. The clayey materials, by subsequent changes, become solid rocks; thus, the agency of pressure alone having successively formed shale, slaty-shale, and clay-slate, each of which varies in texture and composition. There are a great many different varieties of clay, each receiving special names. Pipe-clay, so called from its being used in the maniifacture of tobacco pipes, is white and almost pure. It is sometimes termed potters' clay. Fire-clay contains very little iron, lime, or alkalies. It contains much silica, and often carbon ; still being able to stand great heat without melting. Bituminous-clay contains bitumen. Others, con- taining oxides of iron, by which they are variously coloured, are termed variegated. Shale is a laminated clay rock, which will split into thin plates along the original planes of deposition. When very hard, splitting into fine slabs, it is called slate, or clay-slate. A very large propor- tion of the strata comprising the coal measures is constituted by this rock.' Marl is a calcareous clay, being composed of clay and carbonate of lime or carbonate of magnesia. It effervesces with an acid, and breaks when dry into small cubical or rounded fragments. When the rock contains less than one-half of clay it ceases to be marl, being then called an argillaceous hmestone. There are many kinds of marl. When it contains much carbonate of lime it is said to be a calcareous marl ; if it contains dolomite, with carbonate of Hme, a dolomitic marl ; or with a minimum percentage of calcareous matter, an argillaceous marl ; arenaceous, with much sand ; micaceous, with mica ; shell marl, found at the bottom of old ponds, ditches, or lakes, formed from the decomposition of shells, &c. Loam is a mixture of clay and sand, not so plastic as clay, and permeable by water. Mud and silt are the materials of some form of argillaceous rock not cemented together, either clay, shale, loam, or marl, as the case may be ; resulting from the waste of these rocks by running water and other natural agents of decomposition. The accumulations are formed by the particles, which are very small, being carried by running water, and deposited where the water is quieter, as at the mouths of rivers, &c. S8 PHYSIOGRAPHY. Composition op Sueface Soil. — Soil adapted to the growth of plants consists of two principal portions — the organic and the inorganic. The former, or humus, consists of decayed remains of animal and vegetable matter, but varies much in different sois. For instance, peaty soils contain from one-half to three-fourths of their whole weight of this matter ; but generally soils do not contain more than from 3 to 8 per cent, though stiff clayey soils, containing from 10 to 12 per cent, have been noticed. The inorganic portion of the soil consists of two minor divisions — the soluble saline portion, from which saline ingredients are obtained, and the insoluble caHhy portion, which constitutes the great bulk of most soils, being seldom less than six-sevenths of the whole weight, the remaining seventh consisting of organic matter and soluble saline, in about equal portions. The constituents of this insoluble earthy portion, or the greater part of all soils, are silica, alumina, and lime. The first appearing in the form of sand ; the second (alumina), mixed with sand, aa clay ; and lime, in the form of carbonate, as limestone, chalk, &c. According to Johnson, dry ordinary soil, containing one-tenth of clay, forms a sandy soil ; if it contains from one to four tenths it is a sandy loam; from four to seven tenths, a loamy soil; from 70 to 85 per cent, a clay loam; from 85 to 95 per cent, a strong clay, fit for brickmaking ; if it contains no sand, it would be a pure agricultural clay, or pipe-clay. Very few arable lands contain more than from 30 to 35 per cent of alumina. Soils are called mao-l if they contain more than 5 per cent of carbonate of lime, and calcareous or chalky when more than 20 per cent — the soil appearing whitish, as on the south-east coast of England. The soluble saline portion is made up chiefly of common salt (chloride of sodium) , gypsum (sulphate of lime), glaubers and epsom salts (sulphate of soda and of magnesia), with shght traces of the chlorides of calcium, magnesia, and potassium, the nitrates of potash, lime, soda, ^at or imperfect coal ; losing more carbonic acid and water it becomes lignite or wood coal ; still losing the same acid and water, in addition to a compound of carbon and hydrogen called marsh gas — which is the fire-damp met with by the miners — the lignite becomes coal ; further losing more carbon and hydi'ogen it is converted into the hard glossy anthracite. In some layers of peat the actual species of moss may yet be determined. The growth appears to be still going on rapidly in some places, as, according to Leonhard, in Alt-Warmbriicher Moor, near Hanover, the turf or peat has been re-formed in fifty years, and during the last thirty years a layer from four to six feet thick has been in course of formation. The chief formations of this kind are the great bogs of Ireland, in which the roots, trunks, and branches of large trees, both pine and oak, are abundant ; iron pyrites are also in abundance, very often causing spontaneous combustion and the formation of sulphates. Turf is also found in the tropics, as, for in&tance, at San-Panco, in the Brazils ; and on the banks of the North Sea a species is formed from the accumulation of seaweed. (3) Chemically-formed Rocks (Calcareous).— The greater number of the chemically-formed rocks are composed of carbonate of lime. Through the integrating power of water containing carbonic acid gas flowing through limestone rocks, some of the stone is dis- solved, and when a portion of the gas escapes from the water the dissolved limestone is again deposited, mostly in beautiful crystals, called stalactites* that hang from the roofs and sides of caverns. The water slowly percolating through the rock, or calcareous bed dissolves its substance in its progress and appears as a drop upon the roof of the cavern, where it is suspended for a moment or so, diuing which time it loses carbonic acid gas and deposits carbonate of lime. The drop then falls to the floor of the cavern, carrying some portion of the dissolved limestone. The continuation of theso drops form, on the floor of the cavern, either as a pinnacle, termed a stalagmite, -{^ or as a stalagmitic sheet covering the floor of the cavern. In districts where there is much limestone, the cold springs as they emerge from the rocks are often so highly charged with carbonate of Kme that on reaching the open air they yield calctufi", or sinter, in the form of calcspar ; and hot springs that of aragonite. Calctuff is usually a porous friable deposit, but sometimes its layers are firm enough to be used for buildmg purposes, being very valuable from their lightness. Travertine is a similar rock, but usually more compact. A mass 30 feet thick has been formed in twenty years at * Gr. Stalasso, to drop. f Gr. Stalagma, a drop. 42 PHYSIOGRAPHY. the baths of San Fillippo. These springs have received the popular name of petrifying springs, on account of their giving a stony appearance to wood, moss, and other objects placed in them for some time, but it must be remembered that the object itself is unchanged, the carbonate of lime being deposited in a firm and solid state on them. Gypsum is a chemical deposit composed of sulphate of lime. Some deposits owe their origin to the result of the evaporation of sea water, while others are supposed to be produced from the local conversion of limestones, by the agency of gases, or by infiltration. It sometimes occurs in beds, as in the neighbourhood of Paris — hence the name plaster of Paris, viz., ground gypsum — but mostly as irregular masses, intercalated in marls, appealing as veins or strings. It is also of frequent occurrence, as isolated crystals, or aggregates of crystals, in most clays, being then called alabaster. Jtock salt is also a chemical deposit, similar in origin to gypsum, namely, resulting from the evaporation of sea water, but especially of salt lakes, when there is no outlet (the evaporation being equal to the supply). The water though completely saturated with salt is continually receiving more ; hence there must be a continual deposi- tion of salt going on at the bottom. This rock is one of the few binary compounds, being composed of chloride of sodium. It occurs in large wedge-shaped masses in some localities, and sometimes in immense beds, as in Cheshire, and at Wicliczka, in Poland, and is always accompanied by gypsum. Sulphur is found in a calcareous marl in Italy, varying in thick- ness from 4 to 31 feet. METAMORPHIC EOCKS. 48. The Metamorphic Rocks are a group of rocks that were originally sedimentary or stratified, but have undergone a change of structure, or metamorphosis, by heat or pressure, the chemical com- position remaining the same, but grouped together in different ways. This metamorphism does not always destroy the original character, but simply hardens the rock, as, for instance, sandstones hardened into hard rocks, called quartzite, limestones into marbles, clays into slates, &c. ; but in some cases it does destroy the original character, rearranging the elementary substances of sedimentary rocks, con- verting them into rocks having a mineral composition similar to the igne >us rocks from which they had originally been derived by chemical agencies, as, for instance, sandstones and clays may be converted into rocks whose composition is similar to granite, namely, quartz, felspar, and mica, and so on ; but they may be generally distinguished, as they still retain more or less of their original PHTSIOGBAPHT, 43 stratification, and tlie minerals have a tendency to arrange themselves in layers, splitting easily along those planes, thereby differing from igneous rocks, which display neither of these features. The chief metamorphic rocks are quartzite, mica-schist, gneiss, granite, crystalline, limestone, serpentine, &c. Q,uartz rock, or quartzite, is a compact and granular rock, consisting of nearly pure quartz. It is not far removed from ordinary sand- stone, the half -fused state of its component grains showing at once that it is a sandstone which has been altered by the action of heat, or of heat and water. Gneiss consists of felspar, mica, and quartz, the felspar lamellar and the mica being arranged in lines, producing a foliated or schistose structure. This rock varies much in appearance. Sometimes the Lamince preserve their parallelism for great distances ; but in other cases it is so obliterated that it cannot be determined from granite. Mica-schist consists of alternate layers of mica and quartz, the mica preponderating. It readily splits into thin scales or lamince, some varieties affording good roofing slates, such as hornblende-schist, consisting of hornblende and quartz. Chlorite-schist is a green slaty rock, in which chlorite is abundant, usually blended with quartz, though sometimes with felspar and mica. Serpentine is a compact amorphous rock, consisting chiefly of silicate of magnesia. It is usually of a green colom*, but sometimes variegated, resembHng the skin of a serpent. Granite, as before stated, is a rock composed of quartz, felspar, and mica. It is easily distinguished from other rocks by its mottled appearance. There seems to be a diversity of opinion whether this rock is of igneous origin, or simply the result of the extreme of metamorphism, though it is certain that many granites are true igneous rocks. IGNEOUS OR UNSTRATIFIED ROCKS. 49. Igneous Rocks are all those which do not come under the definition of aqueous or metamorphic, showing traces of once having been in a state of fusion, or molten by heat (" igneous " meaning: fixe). They are mostly divided intp two classes, viz., volcanic and plutonic Volcanic Rocks are those which have been ejected in a melted state from volcanoes or fissures in the earth's crust, as lavas and ashes, which are good examples of igneous rocks — lava especially, as we can see it issue from the crater of a volcano as a molten stream, being afterwards turned into a hard rock. These volcanic rocks are divided into two sections — the felspathic or trachytes, having light colours and being of low specific gravity, containing an excess of silica, but poor in earthy bases and the oxides of iron ; and tha 44 PHYSIOGRAPHY. augitic or basaltic, having dark colours with high specific gravity, containing a large percentage of earthy bases and the oxides of iron, but poor in silica. (1) Felspathic or Trachyte rocks are generally crystalline granular compounds. They are called trachyte on account of their rough texture, and are chiefly composed of acidic felspar, the specific gravity varying from 2*4 to 2"8. The grandest examples of these rocks are in Central and South America, in the chain of the Andes, of which they form the summits, the beds being sometimes from 14,000 to 18,000 feet thick, as oJBf Chimborazo and the volcano Guangua-Pichincha. Trachyte is usually of a greyish colour, and of a rough texture. It is composed of sanidiue felspar, and mostly a little mica, or hornblende. When distinct crystals of felspar exist the rock is called trachyte porphyry. There are many varieties, such as domite, earthy and friable ; hornblende trachyte, containing much hornblende in disseminated crystals ; slaty trachyte, &c. Pumicestone is very light, usually of a light colour, and containing about 70 per cent of silica. It has minute capillary and parallel pores, these pores being due to the escape of gases. Obsidian is a volcanic glass, similar to coarse bottle glass, varying in colour from brown to greenish-black and black. Clinkstone or Phonolyte, so called from its ringing sound when struck, is of variable composition, but is chiefly composed of glassy felspar with a zeoHte in variable proportions. It is of a greyish blue and other shades of colour, generally splitting into thin slabs and containing zeolite disseminated through it. Pearlstone is something similar to pitchstone, but not so glassy and more pearly. It is mostly of a greyish colour. (2) Augite or Basaltic Rocks. — These rocks contain a great quantity of augite,* which prevails over the felspar, rendering them augitic rather than felspathic. They are of high specific gravity — namely, from 2*9 to 3*7 — and range in colour from dark grey to black, the dark colour and high specific gravity being due to the presence of iron. The composition of these basalts include titano-f errite (titanic acid and iron) and ohvine (a silicate of magnesia and iron), as well as augite and felspar. The most important varieties are as follows : — Basalt is a compact, and nearly or altogether a black rock, and usually composed of basic felspar, augite, titano-ferrite, &c. This rock often occurs in columns more or less hexagonal in section, examples of which are the basaltic rocks in Fingal's Cave and the Giant's Causeway. Bolerite (deceptive) consists of labradorite and augite, with some magnetic iron. It has a crystalline and granular texture, and is of a * A green mineral, composed of silicate of lime with, magnesia and iron. PHYSIOGRAPHY. 45 black or greenish-black colour. Specific gravity, from 2 "8 5 to 3'1. The variety anamesite is a similar rock but finer grained, being intermediate in texture between basalt and dolerite. Levxitophyr, or leucite rock, of a dark grey colour, fine grained, tellular, consisting of augite and leucite, &c. Ashes are merely fragments of the foregoing rocks which have been reduced to various degrees of fineness. When they appear as cinders they are called scm-ice ; when the lava, in its journey through the air, takes of a spherical form more or less, they are bombs; when cemented together in beds or masses, tuff; or as small stones, or fragments of ejected rock, lapilli, &c. 50. Plutonic Rocks are supposed to be of igneous or aqueo- igneous origin, formed under great pressure — having been melted and afterwards cooled and crystallised, but very slowly, in the depths of the earth. They consist of crystallised silicates, with or without free quartz, and other minerals — such as iron pyrites, &c. — ^in smaller quantities. The chief of this group is granite, (1) The ordinary kind consists of orthoclase quartz and white mica, disseminated in nearly equal proportions. The felspar is lamellar and the texture mostly granular, sometimes being finely grained and at other times coarsely grained. Its colour is either greyish or reddish, depending chiefly on the colour of the felspar. There are several kinds of granite, some of which are — Syenite^ which is a hornblende granite, consisting of a felspar, quartz, and hornblende, the felspar lamellar often predominating. Pegmatic consists of lamellar orthoclase felspar and quartz, often arranged in broken lines. Eurite, being blended into a finer granular mass, though of same constitution as granite. Protogine (first produced) is of the same composition, with the exception of talc in the place of mica. Granite and granitic rocks are abundant in some parts of the British Isles, constituting the greater part of the Grampians in Scotland, and the mountains of Cumberland, Devon, and Cornwall, also the Wicklow Mountains, in Ireland, &c. (2) Those generally classed as trap-rocks — from the Swedish trappa, a stair — these rocks being supposed usually to assume a step-like form, though the name trap with the Swedish geologists simply meant any compact dark-coloured rock composed of felspar with augite. Felstone is a compact, hard, flinty-looking rock, composed of acidic felspar and quartz. When this rock contains crystals of these minerals it is termed porphyry. Diorite consists of felspars, hornblende, and sometimes mica, being very nearly of the same constitution as granite, which it resembles very much, sometimes passing into that rock by metamorphic action. It is one of the most important and widely spread of rocks. Green" 46 PHYSIOGRAPEY. Stone is a variety of this rock, in which green or dark-coloured hornblende predominates. Porphyrite consists of a matrix of basic felspar, containing felspar crystals, varying in colour from grey to dark purple. If such rocks as these contain mica as an essential it is called minette (Fr., pussy) ; if quartz, a quartz porphyry ; if augite, & basalt or dolerite. Kersantite is a minette, or micaceous diorite, consisting of mag- nesian mica, hornblende, &c. Melaphyre is composed of basic felspar, augite, magnetic iron (magnetite), and sometimes chlorite,* in a glassy base. It is of a black or dirty-green colour. Diabase is a similar rock, though differing a little in the species of the felspar. Pitchstone is a compact glassy kind of rock, somewhat like solid pitch in texture, its colour varying from nearly white to a dirty or blackish green. INTERNAL HEAT OF THE GLOBE. 51. We will now consider the observations and evidences tending to prove the internal heat of the globe and that the heat increases the deeper we descend. That the earth was in amolten state at an early period of its existence, is strongly affirmed by its spheroidal shape, as any matter in a fluid state rotating on its axis would have a tendency to fly off from the equatorial region, bulging out there, on account of its centrifugal force, and flattening or compressing it about the axis of rotation. Active volcanoes point to the existence, at some unknown depths, of enormous masses of intensely-heated matter, which in many cases is in a constant state of fusion (lava). From observations made in mines and artesian wells, in France, England, Prussia, and elsewhere, it is assumed that below a depth of about 70 feet — the stratum of variable temperature — the tempera- ture increases on an average about 1° F. for every 60 feet in depth. The stratum of variable temperature is the crust of the earth as far down as the effects of the sun prevail — namely, about 70 to 80 feet — when we reach a constant temperature. Above this line the heat varies with the seasons. In the Astley Coalpit, Dukinfield, near Manchester, the hne of constant temperature was reached at the depth of 71 feet, being continually 51° F. At the bottom of the pit — namely, 2,080 feet below this line — the temperature is constantly 75°, showing an increase of 1° F. for every 86'6 feet. Observations taken at Eosebridge Colliery, near Wigan, on the rocks themselves, give the increase during the sinking there to be 1° F. for every 54*5 feet * Chlorite is a mineral composed of silica, alumina, and magnesia, of a greenish colour. PHTSIOGRAPHT. 47 descended, the temperature at the depth of 2,418 feet being as high as 93-6° F. The water in artesian wells is warmer than the mean sur- face temperature, always increasing with the depth, the water in one at Grenelle, near Paris, whose depth is 1,800 feet, being con- stantly at 81-7° F. The density of the earth affords another argument in favour of a high internal heat. The average density of known rocks is about 2"5 times that of water, and that of the whole earth about 5'5 ; but as the density increases with the depth, so much that water would be as heavy as mercury, or more than twice the specific gravity of the whole earth, at a depth of 400 miles, it is evident that if the interior of the earth be composed of such materials as occur at the surface, they would have a higher density still than this. Hence it is inferred that they must be greatly expanded by some expansive force or other, and the only force we know of capable of producing this expansion is heat. The chief facts regarding internal heat may be briefly expressed as follows : (1) The earth has an internal :emperature which increases everywhere with the depth. (2) The :ate of increase varies in different places; but in this country ;he average increase is about 1° F. for every 60 feet in depth oelow the line of variable temperature. Chemical theories have also been put forth to account for the 3xistence of internal heat, and consequently of volcanic action. The following may be mentioned : — Lemery attributes volcanic eruptions to the spontaneous combus- tion of materials existing near the surface, as sulphur and iron, beds of coal, &c. Brieslac supposes that volcanoes may arise from the mass of petroleum collected in cavities in the earth and set on fire, the combustion arising from the presence of certain combinations of phosphorus and sulphur. To substantiate his theory he calls atten- tion to the conflagrations that occur in coalmines, being set on fire by the presence of some body which must be spontaneously com- bustible. Other theories are put forth, but these two will be sufficient to give an insight into some of the supposed causes. 52. The Nature of the Interior of the Earth.— It has long been inferred that the globe consists of a melted fluid core, enclosed in a cool and hardened rind or crust ; but this notion has recently been discarded. The principal objections that have been brought to bear against this theory are briefly as follow : (1) If the inside be liquid it must obey the sun and moon in their tide-producing action, causing corresponding undulations of the solid crust, especially at new and full moon. These certainly would have been perceptible if they existed. Sir William Thomson is of opinion that it is extremely impossible that any crust thinner than 2,000 or 48 PHYSIOGRAPHY. 2,500 miles could maintain its figure with sufficient rigidity against the tide-producing forces of the sun and moon. (2) If the volcanoes proceeded from one continuous fluid mass the lava would obey the well-known fluid law, standing at the same height in all cases, which is very far from the case, ^c. The more correct notion, and that which is most generally received now, is that the earth is a solid. In support of this theory it is supposed that the solidification of the earth commenced at its centre, and also at a later period at its surface by radiation. So that there would appear to be two zones of solidification, and between these we may imagine the space to be of honeycomb structure, containing the last remnants of the fluid in detached masses, which will account for the volcanic phenomena — those which have become extinct, probably through the fluid in the cavities becoming gradually solid, and others outbursting, perhaps due to an increased temperature brought about by some cause, such as the transference of the fluid from one part to another through this honeycomb structure. VOLCANIC PHENOMENA AND DISTRIBUTION OF VOLCANOES. 53. The Phenomena of Volcanoes are the great commotions taking place under ground, ejecting through vents volatile bodies, melted rock (called lava), with fragments of sohd rocks, as cinders and ashes, and sometimes steam and sulphuretted hydrogen. The funnel-shaped depressed central openings through which the material is emitted are called craters (cups). Volcanoes are termed active when they are really in action, and extinct when they have ceased to- be active, but may become so at any time. There are also volcanoes which occur beneath the sea. These are called submarine (under the sea), and those on land subaerial (under the air). Volcanoes mostly appear in the form of cones, to account for which two theories have been put forth, namely, the elevation theory, and the eruption theory. According to the first it is supposed that the cone is formed by the swelling-up of the level lying rocks into a bubble-shaped mass, finally bursting at the top. These are known as craters of elevation. The eruption theory, however, is the one most generally accepted — namely, that the cone is the result of the ejected materials accumulating round the crater as a centre. Eruptions. — An eruptions is, in most cases, preceded by hollow rumbling noises, like thunder, and sometimes by earthquakes, dense black smoke hanging in vast heavy masses over the mouth of the crater. The snows that have been lying at the top melt, often causing sudden and destructive torrents, as in the case of the immense bed of snow upon Cotopaxi, in the Andes, being melted ia PHYSIOGRAPHY. 49 one night (1803) ; and in 1797 the melted snow from Tunguarajagua, mixed with mud, &c., filling the valleys beneath to a depth of sis hundred feet. Flashes of flame, and enormous quantities of ashes, are projected, and often carried by the wind to an incredible distance. In the case of the eruption of Cosegunia, in the Andes, ashes fell at a distance of 1,200 miles. Red-hot stones, of great weight and size, are shot out of the craters like shells from an immense cannon, and are known as honibs. One of these, weighing ten tons, was thrown by Cotopaxi, in South America, a distance of nine miles ; the flames rose to the height of more than 1,000 yards from the crater, and the noise was heard more than 600 miles off, the ashes darkening the air for days. The lava flows down the sides of the mountains in immense streams. It is at first nearly of the consistency of honey ; hence its speed is not generally great, varying from two miles an hour to that distance in from one to ten years. The amount projected at one eruption is enormous, the greatest on record being from Skdpta Yokul, in Iceland, in 1783, when it flowed in two streams, 50 miles in one direction and 40 in the other, the breadths being 15 and 7 miles respectively, and averaging 100 feet deep. So that its immense volume might be better comprehended, it has been calculated that it would cover London with a mountain equal in height to the Peak of Tenerifie, or more than 12,000 feet high. The volcanoes of South America, and many others, generally discharge no lava, but simply ashes. 54. Distribution of Volcanoes.— The chief fact regarding the distribution of volcanoes is their nearness to the sea, all — ^with the exception of two or three in Central America, and two or three in the range of Thian-Shan, in Central Asia — ^being near to the sea. Another striking feature is the tendency to a Hnear arrangement, as, for instance, in the great chain of the Andes and Asiatic Islands. Out of 407 active and extinct volcanoes, 365 are of this desci-iption, the remaining 42 being what is termed central, or central systems, consisting of a group of volcanic vents surroimding one principal cone, as those in the Canary Islands, with the central Peak of Teneriffe. The number of active volcanoes is variously estimated by different geograpl ers, but according to Professor Ansted they are distributed as folio va : — POSITION OF VOLCANOES. PBINCIPAL CONES. ( Northern part 10 Atlantic Ocean < Central part 10 ( Southern part 3 GuK of Mexico — West India Islands 10 Mediterranean Sea and coasts 5 Bed Sea and African coast adjacent 2 D 50 PHYSIOGRAPHY. POSITION OF VOLCANOES. PEINCIPAL CONai. Indian Ocean— West side 3 Asiatic continent..... 6 Asiatic coasts and islands j Sou^l^ern part 75 j Eastern part 110 Eastern Archipelago and Pacific Ocean 16 {Northern series 45 Central series 45 Southern series 54 Antarctic Land 3 Total 396 from the above table we see that the greater number of active volcanoes belong to the islands and shores of the Pacific, forming, a3 ic were, a helt to the basin of this ocean, and being at least two-thirds of the whole number. This hand or belt commences in the New South Shetlands, in lat. 62° 55' S., where there is an active volcano j passing from there to Tierra del Fuego, in Patagonia, and then on to the Andes, there being upwards of 30 in Chili, six or seven in Bolivia and South Peru, 16 or more about Quito, in Ecuador, nearly all above 14,000 feet high, the chief of which is Cotopaxi (18,876 feet). Proceeding through Central America the line continues stili northward by the volcanoes of Mexico, where there are seven or more, passing through California, Oregon, and British Columbia, the Aleutian Islands, in which there are 23 volcanoes in a distance of 900 miles, carrying the chain across to Kamtchatka, on the Asiatic side of the Pacific, passing through the Kurile Islands, where there are 13, through the islands of Japan (24), through Formosa, the Philippines (15), to Moluccas, where it sends off a branch to the south east, through New Guinea, to New Zealand ; but the line of greatest activity continues westerly, through Java (45), to Sumatra (19), and afterwards in a north-westerly direction to Barren Island, in the Bay of Bengal. In the Indian Ocean there are a few, namely^ those in Madagascar and the Isles of Bourbon, Maiuitius, &c. Volcanic mountains, central systems, are found in the Sandwich' Islands, Marquesas, Society Islands, Friendly Islands, Feejees, Iceland (24), Azores, Canary Islands, Cape Verde Islands, Ascensiony Trinidad, Italy, Sicily, Mediterranean Sea, &c. The grandest examples of volcanic action are those in the Andes, next to which come those of Kamtchatka, Java contains more volcanoes than any area of the same size in the world. Of those in Europe the chief centres occur in the Mediterranean, namely, Vesuvius, Etna, and Stromboli, the last emitting fire aiid lava almost continuously — so much indeed that it is styled the lighthouse c£ the Mediterranean. Another of this description is Becla, Ju .PHYSIOGRiPHY. SI Iceland. To volcanic energy may be attributed the elevation and -subsidence of lines of coast, and the formation of numerous islands in different parts of the globe. Even hills of considerable size form in a short period, as, for instance, Mount Jorullo, west of the city of Mexico, which in 1759 rose out of the plain (and several square miles around it), in two days being raised 1,375 feet. Its height is now 4,265 feet. Solfataras are places where sulphur vapours escape and ncrusta- tions of sulphur form, though the proportion of this mineral small. Sometimes this gas also escapes from holes and fissures in the sides of the craters, the holes being then termed fuvieroles, or smoke vents. Jlot Springs or Geysers. — Springs of boiling water are to be found in some of the volcanic regions, the waters of which are mostly of a mineral character. In Iceland there is a group of fifty or more, called geysers (roarers), situated about 36 miles north-west 'of Mount Hecla. The two largest are the Great Geyser and the Isew Geyser, about 100 yards apart, the former being 70 feet wide at its greatest diameter and 4 feet deep, situated on the top of a mound 15 feet above the adjoining ground. In the centre is a pit 6 feet wide, and 80 feet deep perpendicular, up which the boiling water constantly ascends. At intervals of a few hours the water rises a little above the surface and then subsides, but is thrown generally once a day to a height of 60 to 80 feet, appearing as a lofty column of hot water. Sometimes these springs throw the water to a height of 200 feet, covering the country around with volumes of steam. Just previous to an eruption of this description the water was found at the bottom of the central pit to be more than 260° F., or 48° above the boiling point, though generally the temperature is from 180° to 190°. Hot springs occur at Bath and Buxton, in this country, the temperature of the water being 82° and 115° respectively. The propulsion of the water is supposed to be the sudden production of steam in subterranean chambers. Geysers are found in New Zealand and Cahfornia ; also hot springs in the Azores. "When the ejection of water is in a muddy area it forms mud volcanoes, or mud cones, examples of which occur in California, Iceland, on the Caspian, and along the northern slopes of the Himalayas into China. 55. Earthquakes. — Earthquakes and volcanoes are evidently intimately connected, as the greatest number of earthquakes occur in volcanic districts ; but still they are not confined to these districts. These earthquakes consist of commotions, more or less violent, of the gurface of the earth. There are several kinds, namely, tremulouSf vertical, horizontal, and rotatory. The tremulous is the least destruc- tive. Vertical, or perpendicular ; a mine-like explosion, acting from below upwards, a c^se of this kind occurring at Eiobamba (1797), 52 PHYSIOGRAPHT. when the bodies of the inhabitants were thrown on a bank nearly 100 feet high. Horizontal^ or undulatory, resembling the undula- tions of the waves at sea, progressing at a speed of 20 to 30 miles a minute. Rotatory is the most destructive kind and most rare, the vibrations, following several cross directions, causing a whirling movement of the earth. The earthquake of Lisbon, in 1755, and that of Calabria, in 1783, were of this character. The direction of the concussions is generally in a linear direction, as that of Guadaloupe, in 1842, which extended a distance of 3,000 miles, wAth a breadth of 60 to 70 miles ; but sometimes circular, as at Calabria (1783), when all the villages within a radius of 22 miles were destroyed, and 100,000 persons perished, and fields even were found to have changed places. The most destructive earthquake experienced in the Old World was that of Lisbon (1755), when 60,000 persons lost their lives, the shock being felt over an area more than three times the size of Europe — rocking the waters of Lake Ontario, in North America, causing the Atlantic to overflow many of the West India Islands, the waves rising 60 feet above their usual level at Cadiz, and even 8 to 10 feet on the Cornish coast. Earthquake Bands. — The regions where earthquakes occur are generally the same as the volcanic districts, the most noted in America being along the east, on the west side of the Andes. Severe shocks are also felt in the Alleghany Mountains. In Europe the chief seats are in the district of the Mediterranean, though there is an important one extending from Portugal to the Azores, Canaries, and the district of Central Asia, stretching from these places as far as Lake Baikal. The district of Iceland also includes the North of France, Great Britain, Denmark, and Scandinavia. Africa experiences vory few earthquakes, with the exception of the extreme north and f outh ; and Australia very few, those being in the west, but New Zealand frequently. Causes of Earthquakes and Volcanoes. — It is believed thai both earthquakes and volcanoes are due to the same cause, but what this cause is does not seem definitely known. The ancient philosophers were of opinion that their origin was due to some sudden explosion in the internal parts of the earth ; others supposed something in the air caused them, on account of earthquakes being preceded by a calm and serene atmosphere. The geologists of the present day are of opinion that the chief cause of volcanic eruptions and earthquakes is the expansive force of steam. The earth, as before stated, is supposed to be solid, with the exception of large cavities or lakes of molten lava. The water of the ocean and land, sinking through the crevices, constantly finds its way down, and is then converted into steam, whose enormous pressure is most probably the cause of both volcanic energy and earthquakes PHYSIOGRAPHY. 53 Mitchell, in his work on earthquakes, expressed a somewhat similar opinion, attributing them to subterranean fires, whose existence in nature (he writes) we have certain evidence of, and which are capable of producing all the appearance of these actions. If a large quantity of water should be let out upon these fires auddenly, it may produce a vapour whose quantity and elastic force may be fully sufficient for that purpose. It is believed that the seat of the disturbing force is never above thirty miles below the earth's surface. The effects of earthquakes are, the elevation and depression of great areas of land, violent oceanic movements, the opening of great fissures (as in Calabria), and the swallowing up of whole cities and even mountains. THE EARTH'S CRUST. 56. Slow Upheavals and Subsidences of the Earth's Crust. — That the level of the earth changes is evident from the fact that the greater portion of the land consists of strata composed of waste matter accumulated at the bottom of seas that have existed where the rocks are now found ; and even on our own coast, forests, &c., have been found submerged. We have evidence that slow upheavals and subsidences are constantly going on over large tracts of land. The shores of Scandinavia, on the Baltic, afibrd strong testimony of this, as in the southern extremity the land is gradually sinking beneath the sea, while at the north, in the district of the North Cape, it is rising as much as five feet in a century. These facts regarding the elevation have been drawn from the appearanco of the rocks above water, which were always formerly submerged, channels becoming shallower, and the occurrences of sea-beaches at elevations above the sea-level, which are termed raised heacheSy several of which are found on our own shore, namely, in Sussex, Devon, and Cornwall — the opposite appearances showing the depression. Another instance of slow depression is afibrded on the south-west coast of Greenland, where the shore is slowly subsiding, buildings being submerged along the coast. Facts have also been discovered lately adding to the existing evidence that there is a rise of land going on in the southern circum-polar regions. In Australia, Tasmania, and New Zealand the phenomena are remarkable. For instance, in one place certain lakes and a river disappeared, owing to the rise of the land. In another place, on the western coast of New Zealand, the high water mark oi the year 1814 is now 200 yards inland. Many other facts might be cited. According to Mr. Howorth, in his communications to the Eoyal Geographical Society, they prove that the masses of land round about the south pole are at present " areas of upheaval," and that the earth's periphery is being stretched or extended in the 54 PHTSIOGBAPHT. direction of the shortest axis. One remarkable fact to be noticed in all this area, exhibiting so many signs of rapid upheaval, is the marked absence of volcanoes, as in the entire circle there are only; the two or three in North Island, New Zealand, and those m- Tierra del Fuego. Many other instances might be mentioned bearing similar testi-: mony. The most striking proof of the depression of large tracts is, afforded by the distribution of coral reefs. These reefs are of three chief '^ij[idiS— fringing reefs, harrier reefs, and atolls. The coral polyps, or reef-builders, are unable to live in water when the depth exceeds 30 fathoms. Hence these reefs cannot commence to form, in the deep ocean, as they must have land within a few fathoms of, the surface to start on. Fringing reefs are of no great thickness, and skirt the coast at a small distance from it, as the reef around the island of Mauritius, which lies half a mile from the shore in very shaHow water. Barrier reefs are much greater reefs, occurring quite away from the shore, generally running parallel to the coast, the verti- cal thickness of the formation in some cases being quite 1,000 feet. Examples of this kind are — (1) The Great Barrier which extends in a broken line along the north-east coast of Australia, at an average distance of abo*ut 30 miles, though in some parts from .50 to 70, tho depth of water being from 30 to 00 fathoms, and otitside the reef the depth in some places exceeds 300 fathoms. (2) The reef off the west coast of New Caledonia is 400 miles in length, and distant from the shore about 10 to 16 miles, the depth on the side away from the shore exceeding 1,000 feet. Atolls are ring-shaped reefs enclosing a lagoon of still water. The outer slope of the reef is veiy steep, as in the case of the Cocos Atoll, where no bottom was found at a depth of 7,200 feet. The water in the lagoon is shallow. In the above-mentioned island it varies from three to ten fathoms in depth. From the known fact of the coral builders being unable to live below 90 or 100 feet, it is evident that the bottom on which these barriers and atolls were commenced building must have been gradually sinking for ages. According to the theory of Mr. C Darwin each atoll and barrier reef began as a fringing reef round an ordinary island. First the insects built the fringe reef close to the shore, the island slowly sinking, leaving a smaller surface and causing more space between it and the reef — the insect still building upwai'ds, forming a hart ier reef, the land continuing slowing sinking until the island has entirely disappeared beneath the waters ; at the same time the reef, continuing to grow upwards, left at the surface a ring of coral around a lake. ProhaUc Cause of the Movements of the Earth's Crust. — The general opinion is that the slow upheavals and subsidences are con- sequent on the contraction of the earth by cooling — the warm interior PHYSIOGRAPHY. 55 loses heat faster thsCn the comparatively cold exterior, and contracts more than the outer part, which tries to follow the interior but cannot, owing to its curved form, except by bulging ap in other places. In this way continents may sink, and the bottom of the sea may be raised above the level of the water. RELATIVE AGE OF STRATA. 57. Changes the Earth's Surface has Undergone, and in the Forms of Life. — That the surface of the earth is continually imdergoing changes is evident from the upheavals and subsidences that are going on continuously, and by the different strata that appear above each other. If we examine the mud and sands of our coasts and seas we find. imbedded in or resting upon them relics of many living species of animals and plants. On examining sandstones and clays we find, too, they are associated with organic relics, the. beds of coal and peat, &c,, revealing the same facts. Hence we come to the conclusion that these deposits or strata are the result of forces tending either to break up and remove, or to deposit and consolidate in new forms. Where dry land is at the present day W3 have strong evidence that it has been the bottom of the sea at some previous date. The agents causing these changes on the earth's surface are : (1) Aqueous, forming sand and mud banks, &c. (2) Igneous, causing the lavas, &c., issmng from the volcanoes and earthquakes. (3) The works of the polyps in forming reefs, and the chalk-forming animals, Foraminifera. When one stratum rests upon another we come to the conclusion that the lowe?^ bed was deposited before the icpper bed was com- menced. In this way geologists are able to arrange the strata composing the earth's crust in a series commencing from the oldest, or first formed, up to the newest, or last formed. They are also guided by the fossils appearing in them ; for if rocks of the same age or formation are examined we may find some local fossils — ^yet many are constant — occurring wherever the rocks are found ; but in passing to a newer rock there appears a complete change in the fossils, which not only proves it to be of a different formation, but that each as:e was characterised by its own peculiar fauna (animals) a.nd flora (flowers), one animal or plant after another disappearing and new species taking their places. The stratified rocks fall into three great divisions — namely, the Cainozoic, or new ; the Mesozoic, or middle ; and the Palaeozoic, or ancient group. The first division includes the Tertiary and Quar- tenary, the second (Mesozoic) the Secondary, and the last, Primary, or the oldest strata of which we have any knowledge, each division representing, as nearly as can be determined, a different arrangement of sea and land. 56 PHYSIOGRAPHY. The following table gives the names of the different formations, or systems, arranged in the order of the superposition, the oldest being at the bottom and the youngest known at the top : — III. — Catnozoic (Recent Life). Post-tertiary, or Eecent Accumulations — Alluvium, fen deposits, river gravels, &c. Tertiary — PKocene — Crag ; Miocene — ^Bovey-beds, &c. ; Eocene — Fluvio, Maine series, &c. II. — Mesozoic (Middle Life). Cretaceous, or Chalk System — Chalk, greensand, Wealden, &c. Oolitic, or Jurassic System — Oolite, Has. Triassic, or Upper New Red Sandstone. I. — PALiEozoic (Ancient Life). Permian, or Lower New Red Sandstone — Magnesian limestone series, &c. Carloniferous System — Coal measures, mountain limestone, &c. Devonian, or Old Red Sandstone. Silurian — Cambrian and Laurentian. In the last (Palgeozoic) group of formations the forms of life differ greatly from what are on the earth now, and are stni different in the middle division, but not so strange in aspect. In the first, or Cainozoic, we have those that exist at present, the oldest of which is the Eocene, containing a few species of shells that now exist ; the Miocene stUl more ; the Pliocene, containing the more recent descriptions, though a number of the mammalia that then existed are now extinct. In the SiLUKiAN Age, trildbites, a peculiar form of Crustacea, abounded, also graptolites, &c. The Old Red Sandstone is sometimes caUed the age of fishes, on account of the numerous remains which are found in this system. The fishes were mostly of the ganoid type, being cased in bony enamelled plates. Other kinds are the placoid, the skin being dotted something similar to the shark, the chief of which were the asterolepis, a fish 20 or 30 feet in length; holoptychius, another large fish, with wrinkle-like marks on its scales ; ptei^chthys, cephalaspis, osteolepsis, coccosteus, &c. The Carboi^iferous System contains coral fossils, such as syHn- gopora, litkostrotion, Sec, shells, a,s spirifera Sind productiis, archemedi' pora, euomphalus, hellerophon, goniatites, and nautilus. Of the coal measures in this system the chief fossils are plants. Among the most important are stigmaria, or roots of plants ; the sigillaria, or the stems ; and lepidodendron, so called from its scaly bark. The fronds of tree-ferns are numerous, such as pecopteris and neuropteris, figured kinds ; sphenopteris, cyclopteris, odontopteris, &c. Permian System. — The fossils of this formation are not very abundant, but those which appear are associated with a considerable change of species, many species of fauna finally disappearing. Among PHYSIOGRAPHY. 57 the cHef fossils are fenestella (Polyzoa), prodnicttts horridus, spin/era alata, &c. (Brachiopoda) ; schizodus,BaJcevelUa,pecten,&c.{ConcbJ£eTa) ; Palceoniscus, platysomus, &c. (Fishes). Triassio System. — Some of the fossils of this system are ceratites nodosus, characteristic of the shells ; estheria minuta (Crustacea), and microlestes antiquus, a little beast of prey, the earliest known mammal, something like a kangaroo. In this system reptiles became more numerous than in any earlier rocks, among which may be mentioned cheirotkerium or labyrinthodon, nothosaurus, and rhyu' cosaurus, Sec. The ammonites and belemnites make their first appear- ance also. JuBASSic OB Oolite and Lias System. — ^The lias is remarkable for its fossil reptiles. It, together with the oolite, has been caUed the age of reptiles, from the great development of those animals, both in size and number, of which the genera best known are the ichthyosaurus, plesiosaurus, and pterodactylus. The first of these was a lizard, somewhat like a fish, sometimes more than 25 feet in length ; the second had a swan-like neck, and nearly as long as the first ; and the last (pterodactylus) had wings, and could fly hke a bat. Beetles, crickets, and other insects, have been found in the lias formation ; oysters also are abundant. In the oolite ammonites and Memnites are very abundant, long-legged lizards appearing, and four species of mammals, aU pouched animals like the kangaroo, the eenera being amphitherium, phascolotherium, and stereognathus. Cretaceous System. — The cretaceous fossils, compared with the oolite, are new, not a single species of the latter occurring — the foraminifera now appearing, which resemble very closely those now living in the Atlantic and other seas. The large reptiles, ichthyo- saurus, &c., occur for the last time, in the white chalk, several new genera appearing, as the mososaurus. Birds also, about the size of a pigeon, have been found in the upper greensand. Eocene System (Tertiary.) — This period — ^namely, the tertiary — is sometimes called the age of mammals, among which may be mentioned coryphodon, somewhat like the Uving tapir, though larger ; hyracotheHum, allied to the hog ; palceotherium, or ancient beast, about as large as a horse, but Hke the tapir ; anoplotherium, with a long powerful tail, and many others. It is supposed that during this system the climate of England was tropical (like that of the East Indies at the present time), on account of the palms, crocodiles, turtles, &c., which have been found ; also that this country was joined to the Continent, the proof of which lies in the remains of large mammals making their appearance in different parts of the English deposits in the Eocine time, it being evident that these animals must have walked over. Miocene Period.— The plants and flowers of this period closely resemble the flora of the present day, being, among many others, 58 PHYSIOGRAPHY. evergreens, oaks, -fig-trees, laurels, vines, palms, beeches, and trees of the cinnamon tribe. Among the mammalia there are dino- therium cheer opotamus, and others which are now extinct ; and the elephant, hippopotamus, rhinoceros, giraffe, monkey, deer, &c., which are living species. Pliocene Period. — The remains of the following mammals, among others, have been found in the English beds, or crags, of this period : IVhale, elephant, rhinoceros, tapir, horse, bear, pig, deer, hyena ; and the genera mastodon (allied to the elephant, but with very long tusks) and hijpparion (allied to the horse), both of which are now extinct. From the crags we learn that a portion of Norfolk and Suffolk were under the sea at the commencement of this period, these formations resembling those deposits now forming in the German Ocean. More than one-fourth of the fossils of these crags belong to extinct species. PosT-T£RTiAE,T, OR PosT-PLiocENE, PERIOD. — ^Remains of animals now inhabiting this country, with others inhabiting other parts of the world, and some altogether extinct, are found in the British formations of this period. Among the mammalia are the fox, wolf, hyena, reindeer, lion, hison, hippopotamus, two kinds of elephant, horse, pig, rabbit, squirrel, &c. We also meet with proofs of man's existence at this age by the disco vei-y of some of his works, such aa flint knives, hatchets, arrow heads, &c- In Continental gravels and caves of this age man's bones have been found side by side with those of the mammoth and other extinct animals. The Age of the Earth is not yet, and doubtless never will be, known, but many conjectures and calculations have been made regarding it, the chief of which are based on calculations made upon the heat of the earth, and the time it would take to cool from its molten state. From experiments made upon the cooling of lava and certain other rocks by Bischof, Professor Helmholtz concludes that the earth could not cool from the temperature of 2,000° C. to 200° C. in less than 350,000,000 years, and scores of millions more must have elapsed to have reduced it to 94° C. — the highest heat at which it is estimated that animal and vegetable life could commence. Sir W. Thompson is of opinion that the solidification of the crust took place about 100,000,000 years ago, bringing the probable age of the earth to 500,000,000 years. Other evidence regarding the antiqmty of the globe is drawn from the effects of rivers in their action on the earth's crust. For example, it is well- known that the Falls of Niengara are now seven miles nearer Lake Erie than they have been at some previous date ; and taking six inches as the calculated rate of retrocession per annum it gives 186,000 years as the time required for the river to have performed this worL PHTSrOGRAPHT. 59 ASTEONOMICAL GEOGRAPHY. THE EARTH: ITS ASTRONOMICAL RELATIONS. It has been determined by astronomers that the earth we live on is one of a number of 'planets (wanderers) which revolve round the sun as a common centre, but at different distances and velocities. These, with their satellites, or moons, together with an unknown number of comets, constitute what is termed the solar system. There are at present known eight large or primary planets, which revolve round the sun in nearly circular orbits or paths, and 220 asteroids — small planet-like bodies, which are all situated between the orbits of Mars and Jupiter, and are supposed to be fragments of an ordinary-sized planet, that has been disrupted. All the planets between the sun and this gap are called inferior or interior planets, and those beyond superior or exterior planets. There are 18 secondary planets, or moons, revolving round their primaries ; and an unknown number of comets, which revolve round the sun, but move in no fixed direction. The primary planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and JSTeptune, their names being written in the order of their nearness to the sun. Of the secondary planets, or moons, the Earth and Neptune have one each, Jupiter and Uranus four each, and Satirrn eight. The multitude of stars that move in a mass and keep their position with regard to each other, and to the Bun, moon, and a few others, are called fixed stars. 58. Form and Motion of the Planets.— The planets are all round bodies {spheres, or more correctly spheroids, like the earth, owing to the effect of centrifugal force — that is the force which causes all matter when spinning round to have a tendency to fly off in a straight line.) They have two motions. First, their motion round the sun, and secondly, their revolutionary motion on an axis, travelling in both cases from west to east, each revolution being termed a day. The paths or orlits that they describe in their journey round the sun are elliptical — that is, the figure described is an ellipse,* though differing very httle from a circle, or having very little eccentricity. The cause of the planets moving in elliptical paths is the result o£ two forces acting on the planet at the same time, but in different directions. These forces are the centrifugal (or tangential) and the * An ellipse is a plane figure bounded by a curved line, and is sucb that if from any point in the curve, two straight lines be drawn to two certain points, the sum of these lines will always be the same. These two points are called the foci of the ellipse, and the distance *-H>m the centre of the ellipse to either of its foci is called the eccentHciti/. 60 PHYSIOGRAPHY. centripetal. The former would, if not counterbalanced by some other force, cause the earth to move right away from the sim, but this is balanced by the centripetal force, which always acts at right angles, proceeding from the attraction of the sun. This force, if also unopposed, would by the laws of gravitation (15, 16) cause the planet to move towards the sun with a continually accelerated speed. But with the two forces acting on it at the same time it obeys neither, following, according to the jparallelogram of forces (7, 8), a mean course, describing a curved path, which in all cases is one of the conic sections, its shape depending on the direction, distance, and Telocity.* In the annexed figure let E represent the earth, and s the sun ; then, supposing the earth to be moviag in a straight hue, with a velocity sufficient to carry it to A in a given time, during which the E ^ sun's attraction acts upon E with sufficient force to bring it to B, the earth will describe the diagonal of the parallelogram EACB, though it doea not pursue a straight line, as in Fig. 1, page 9, but a curve, owing to the sun's power of attraction acting continually on the earth, and thereby causing a deviation from the right line. In an exactly similar manner the earth performs the ^ff- 5. curves CH, HN, &c. To understand more correctly how it is that the line is not a right line it must be remembered that the attraction of the sun is not exerted at once, or by a single impulse, but by degrees, being constant. Hence we may suppose that the parallelogram EACB is made up of an infinite number of minute parallelograms, and that the cur ved line likewise consists of an infinite number of diagonals. Kepler's laws of elliptic motion are : (1) " That every planet moves so that the radius vector, or liae drawn from it to the sun, describes about the sun areas proportional to the times. (2) That *Let d=diameter of the circle ; a, the centrifugal force ; 6, the centripetal ; V and r 1 , their respective velocities ; then the path is a circle when v^=dxvi; an elli])se, when v^> (is greater than) dy.v ; a parabola, when v'=2 fdxvj/ an hyperbola, whenr2>2 (dxv). In every case the angular velocity of the- radius-vector must he inversely proportional to the square of the mutual distance of the two bodies. PHYSIOGRAPHY. 61 the planets all move in elliptic orbits, of which the sun occupies one of the foci. (3) That the squares of the times of the revolutions of the planets are as the cubes of their mean distances from tha sun." From the first of these ]!!fewton concluded " That the force acting on the planets is directed towards the centre of the sun ; " from (2), " That the force acting on the planets is in inverse ratio of the square of the distance of their centres from that of the sun ;" and from (3), "That the force is proportionate to the mass." (See 15, 16.) The point of the planet's path farthest from the sun is called its Aphdion, and that nearest the sun its Perihelion, the sun being continually in one of the foci. Sun and Moon. — Of the heavenly bodies, the two that concern us most, as relating to Physiography, are the sun and moon, more especially the former, the centre of our universe and of this earth's annual revolution, being the source of heat and light, and the chief agent in sustaining life. The diameter of the sun is about 852,680 miles, or 107 times the earth's diameter, being equal in hulk (not mass) to about 1,249,500 earths ; but its mass, or weight, is equal only to about 315,115 times that of this earth. Hence the materials composing the sun have only one-fourth the density or weight of those composing our planet, that is bulk for bulk, being about 1*43 times the density of water. The force of gravity at the sun's surface compared with the earth is 27 ■2.* The distance of the sun from the earth is calculated to be, by recent observations, about 91,430,000 miles ; and from the spots on the sun it has been determined that it revolves upon its axis in 25 days 7 hours 48 minutes. Some of the spots seen on the sun are enormous in size, many being recorded varying from 30,000 to 45,000 miles in diameter. The appearance of these spots is generally a very dark central space, of tolerably regular form, surrounded by a more irregular belt of semi-luminous matter. The interior is called the nucleus of the spot, and the exterior the penurribra. The spots have a maximum and a minimum frequency of about eleven years, corres- ponding with the magnetic disturbances, as a precisely similar period is known to exist in the variation of the magnetic declination, the maxima and minima agreeing exactly with those of the spots. Effects op Sun" Spots ok Climate. — This question has often been discussed, though as yet our knowledge on the point is very scanty, and it is probable many years will elapse before it can be * Gravitation proceeds from its centre. Hence, distance that bodies at the surface are removed from the centre of attraction is about 426,340 miles, and its mass, or gravity, 315,115. Hence, by the laws of gravitation (16), the gravitation of the stm, compared with that of the earth, is as the square of the radius of the sun : square of the radius of the earth : : 815,115 ; or as 426,3462 : 3^9562 -. ; 315,115 ; 27-2. 62 PHTSI0GRAPHY. answered fully. Professor Langley, of Alleghany Observatory, Pennsylvania, states, from Ms own investigations, that sun spots do exercise a direct and real influence on terrestial climates by de- creasing the mean temperature of this planet at their maximum. The decrease is very minute indeed, the whole effect being repre- sented by a change in the mean temperature of our globe in eleven years not exceeding three-tenths, and not less than one-twentieth of one degree of the centigrade thermometer. That the sun has a motion through space is now an established fact, travelling at the rate of 18,000 miles an hour, or 155,000,000 miles per year, 59. The Photosphere and Cromosphere of the Sun — Whenever the sun shines we see a kind of brilliant envelope, called the photosphere or light-giving surface, it being the shining surface of the sun. This is it from which we derive our light and heat. The faculce, or brighter portions of the sun's surface, appear to be elevated masses of luminous matter when viewed stereoscopically ; and as they remain for days suspended in the same position they are probably gaseous or vaporous. From observations witnessed daring total eclipses of the sun it has been ascertained that the sun pos- sesses an exterior gaseous envelope, of great extent, above the photo- sphere, probably extending more than 800,000 miles. During an eclipse, at the moment the last remnant of the photosphere is hidd en by the dark moon, there appears a kind of white halo at the up')er part of this gaseous envelope, or atmosphere, called the corf ma, which may be regarded as a reflection of the sun's light by hit atmosphere. The lower regions of this corona is composed of layers of a greatly heated gas, of extreme tenuity, entirely surrounding the sun. This is called the chromosphere, and is subject to the disturb- ances of the photosphere, which appears to be in a constant state of agitation, causing the chromosphere to be thrown about in huge •masses, in the shape of red flames, often to the height of 100,600 miles. There is supposed to be yet another layer of very highly heated gas, so hot, in fact, that metals such as iron continue in a state of vapour. Regarding the intensity of heat proceeding from the sun, it has been calculated that the annual heat is 2,381,000,000 times that received by us ; and that received by the earth in one year would be sufficient to melt a layer of ice 114 feet thick all over the earth's surface. The light proceeding from the sun has also been proved to be 618,000 times that of the full moon, and equal to 6,560 wax candles at a distance of one foot from the eye. The sun has been found, by the aid of the spectroscope, to be composed of similar material to the earth. Among the elements that have been ascertained may be mentioned hydrogen, iron, sodium, magnesium, manganese, calcium, chromium, barium, copper, nickel, &c. PHT3I0GEAPHT. 63 60- The Moon. — 'Thin satellite revolves round tlie earth in an elliptical orbit, the earth being one of the foci, and carried with it round the siin. It takes the moon 29 days 12 hours 44 minutes 2"87 sBConds to complete its circuit, returning to the same position with regard to the earth and sun. This period is called a lunar month, or lunation. The time of the moon's rotation is exactly the same as the time of its revolution round the earth,, and in consequence of this fact the moon always turns the same side to the earth. Its diameter is 2,160 miles, or a little more than one-fourth that of the earth ; its distance from the earth being 238,851 miles, or 60 times the earth's radius, and specific gravity '61. The surface of the moon is very diversified, as seen through the telescope, high mountains existing which throw long black shadows. From the length of these the heights of many mountains have been measured, the highest points reaching nearly 23,000 feet high. They are supposed to have been raised by volcanic agency, as most of them have large crater-like basins. The light we receive from the moon arises not from its own surface but by reflected solar light. The phases of the moon prove it to be a spherical body illumined by the sun. When in conjunction with that luminary the moon is invisible. "When moving from the sun towards the east it is first visible, it being now called the neiu moon, and appears as a crescent ; when 90° from the sun — namely, at a right angle — there appears a half moon ; as it recedes farther it is gibbous ; when in opposition it shines vdth a full face, being then called full moon. On its journey towards the east, approaching the snn, the appearances are just the reverse, first being gibbous, then halved, and lastly a crescent, after which it disappears from the superior brightness of the sun and the smallness of the iUumiued part turned towards the earth. THE EARTH— ITS FORM AND MOTIONS. 61. The Form or Shape of the Earth is nearly that of a globe or sphere, or more correctly an oblate spheroid, being flattened it the poles, probably through the effect of its centrifugal force. Among the many proofs put forth regarding the rotundity of the earth, may be mentioned : (1) A vessel sailing away from the land does not become lost from view on account of its distance, but gradually sinks out of sight ; first losing sight of her hull, next her lower or main sails, and lastly her top sails, thus showing that she is passing over a convex surface. (2) By employing a surveyor's level upon the surface of the water of a canal it will be found that at the distance of one mile the water is depressed below the level of the instrument about 8 inches, which is called the dip. This not only proves that the earth is round, but also gives the diameter of Buch a globe that will have a curvature equal to this, namely, 7,920 C4 PHYSIOGRAPHY. miles,* wMcli is not very far from the mark, the true dip being 7'9821 inches. (3) In travelling any considerable distance, either north or south, new stars gradually appear in the direction in which the person is travelling, and those behiad disappear. This is exactly what would happen if the earth was round, and under no other circumstances, hence we conclude the earth must he round. (4) The shadow cast by the earth on the moon during an eclipse is always circular, which could not be the case unless the world was round. 62. The Size of tlie Earth. — Owing to the earth being correctly a spheroid, its polar diameter is greater than its equatorial diameter. According to Airy and Bessel the true dimensions are : — Polar diameter 78991 miles. Equatorial diameter , 792o"6 „ Difference, or polar compression 26*5 „ Proportion of diameters, 298 to 299, the polar diameter being -^g shorter than the equa- torial ; hence the mean diameter is 7912'35 „ The circumference of the earth is 24,856 miles ; the area of its surface, or superficial contents, 197 million square miles ;t the volume, or soHd contents, 259,000 million cubic miles ; and weight, taking the density to be 5| times that of water, 5,852 trillion (5,852,000,000, 090,000,000,000) tons. The density of the earth, as compared with the materials at its surface, has been estimated with considerable precision : (1) By observing the attraction exercised by a mountain on a plumb-line, as compared with the earth's attraction upon it ; (2) By calculating the effect of the increased and diminished distance from the earth's centre, on the vibration of pendulums, vibrating above and below the surface ; and (3) by comparing the attraction of large balls, whose weight and density are known, upon a freely suspended bar, with the attraction of the earth upon the ball. From these experiments it has been found that the mean density of the rocks at its surface is about 2^ times that of water at the temperature of 62°F., and that of the whole mass about 5^ times (5'675) that of water. Taking the earth's density as 1, the density of the Sun is '25, Moon '63, Mercury 1*24, Venus *92, Mars "52, Jupiter '22, Saturn '12, Uranus '18, and Neptune 'l?. *By a well-known property of the circle we have: 2 (the radius - dip! + dip : length of surface measured : : length of surface measured : the dip. Let a; = number of feet in radius, then we have 2(a;-|) +§ : 1760 X 3 :: 1760 X 3 : ? : that is f x -i= (1760 X 3 x 1760 X 3) feet ; hence x = ^T^P^^-^ ITg O X 3 X Z -i'jQQ X 9=3930 miles ; .'. diam.=3960 x 2=7,920 milea 1760 X 4 ^ ^ t Surface of a sphere = square of diameter multiplied by 31416. Solid contents of a sphere = cube cf diameter multiplied by '5236. PHYSIOGRAPHY. 65 63. Motion of tlie Earth. — The earth has two motions, namely, its annual motion round the sun, in about 365^ days, or, more precisely, 365 days 5 hours 48 minutes 51 '6 seconds. This is called a year. Secondly, its diurnal, or daily motion, or rotation on its own axis, in about 24 hours, or, more correctly, 23 hours 56 minutes 4 seconds, causing day and night, and the apparent rising and setting of the sun. At the equator any spot moves at the rate of more than 1,000 miles an hour, the velocity decreasing as we travel towards the poles, Great Britain travelling about 600 miles per hour, or ten- miles per minute. Also the earth's velocity in its orbit round the 8un is 65,533 miles per hour, the distance it travels La the yearbeing, about 574 mUlion miles. 64. Day and Night. — During the earth's rotation on its axis only one-half of its surface can be exposed to the sun's rays at any- one time, this being lighted while the other half is in darkness. But the length of day and night varies according to the seasons, the chief causes of which are, first, that the orbit of the earth's revolu- tion round the sun is not a perfect circle, but an ellipse ; secondly, that the earth's axis in performing this revolution is not perpendicular, but inclined to an angle of 66° 32' to the plane of its orbit, or 23^**' (23" 28') to the equator. The diagram accompanying will assist in explaining the effects of this elliptical orbit and the incHnation of its 65. Seasons. — ^The earth is represented at four different positions in its yearly orbit, S being the sun, A representing the position at the vernal equinox. The whole hemisphere from pole to pole is= illuminated by the sun, so that during the rotation every part ol the earth will have an equal share of light and darkness, day and night E 66 PHYSIOGRAPHY. being equal. Similar at B. At any other position day and night are respectively shortened and lengthened. When at D, the Slimmer solstice for the northern hemisphere, the south pole will be in darkness, and within a cu'cle of 23J° at the north pole the sua will not set. When at C it will be just the reverse. When the part presented to the sun is at a — namely, on the 22nd of December — it is midsummer to all the southern parts of the earth and winter to all the north, and as it gradually proceeds on its journey towards 6 the northern regions gradually receive more and more heat and longer and longer days, till their midsummer comes on the 21st of June, being just tA reverse with the southern regions. Nutation. — If the axis of the earth were perpendicular, instead of being inclined, the length of the day would be always and everywhere the same, and we should have no change in the seasons. Hence, if- the angle of inclination should change it is evident it would cause a change in them ; and this does actually take place. The angle at present is 23° 28', but it gradually decreases — namely, at the rate of about 48" in a century — diminishing for an immense period, after which it will begin to increase again — that is, the axis itself revolves in a small ellipse, but does not always point exactly to the same place in the heavens, thereby causing the variation in the obliquity of the ecliptic. This motion is termed nutation,* the cause of which is the influence of the planets upon the earth. The circle described is about 2° 42' in diameter, its revolution occupying a period of about 270,000 years. Precessional Motion. — Besides the above there are other movements, the chief of which is that called the -precession of the equinoxes — that is, they precede their time. This movement is caused by the attraction of the sun and moon on the equatorial regions — namely, on the portions of the earth which the centrifugal force have caused to bulge out, the effect of which is that the equinoctialf points move westward (recede) 50 ■224" per annum, causing the equinox to occur nearly 20 minutes earlier than it otherwise would. Hence, in the course of ages summer will be where winter is now. This movement, uninjlaenced by any other motion, would cause the equinoctial points to perform the circle of the equator in a period of 25,868 years — that is, the seasons toill coincide with each part of the orbit once in that period. But this movement is influenced by another motion, the result of which is that this cycle of changes is shortened. The latter motion is termed the revolution of the apsides,^ caused by the attraction of the planets. The apsides are the points at which the earth is nearest and farthest from the sun. The line connecting these points is called the line of * Nodding. f The points where the ecliptic crosses the equator. , t A curve. PHYSIOGRAPHY. 67 tht apsides. Tliis line does not keep continually directed towards the same point in the heavens, but slowly revolves ; and the result of the combination of this revolution with the precession causes the cycle of the seasons to be performed in about 21,000 years, or 4,868 years sooner than it otherwise would. PHYSICAL GEOGEAPHY. THE SURFACE OF THE EARTH— DEFINITIONS. MATHEMATICAL DIVISIONS OF THE EARTH. The Axis of the Earth is an imaginary line passing throngh its centre, and round which it rotates daily. The North and South Poles are the extreme points of its axis. The Equator is a great circle passing round the middle of the earth at equal distances from the poles, dividing it into two equal portions, the northern half being called the Northern Hemisphere, and the southern half the Southern Hemisphere. A Hemisphere is one-half of a sphere. Hence, considering the earth as as a sphere, it means one-half of it. The Meridians are great circles passing round the earth at right angles to the equator, and cutting each other at the poles. Latitude is the distance of a place north or south of the equator. Longitude is the dis- tance east or west of any given meridian. Longitude is reckoned in this country from the WEST meridian of Greenwich, which is called the first meridian. The Tropics are two circles drawn parallel to the equator, namely, the Tropic of Cancer, 'about 23i° north of the equator WOR TH POL E EAST SOOTH FQUC Fig 7. and the Tropic of Capricorn, about 23J° south of the equator. The Polar Circles are two lines drawn round the earth parallel to the equator, namely, the Arctic Circle, nearly 23^° from the north pole, and the Antarctic Circle, nearly 234** from the south pole. •68 PHYSIOGRAPHY. The Ecliptic is a great circle cutting the equator at an angle of 23 g° at two opposite points, reaching the tropics as its extreme limit, north and south. It represents the sun's apparent path in the heavens, but in reality the path of the earth round the sun. The points where the ecliptic cuts the equator are called the Equinoctial Points or Nodes, because, when the sun is in these parts of his course, the day and night are equal. These equinoxes take place twice a year, namely, on the 21st of March, and 21st of September. The Zones are five great helts into which the earth is divided by the tropics and polar circles. (1) The Torrid Zone, between the tropics, so called on account of its great heat, through the sun being always vertical in some part of that space. (2) The spaces between the tropics and the arctic and antarctic circles on either side, are called Temjperate Zones (north and south), having a milder or tern- perate climate. (3) The spaces between the polar circles and the poles are called Frigid Zones, from their extreme cold. The breadth of each of the torrid zones is about 1622"5 miles ; of each temperate, about 2969 miles ; and of each frigid, 1622'5 miles. Hence, calculating their respective areas, we have — Sq. miles. Parts North Frigid Zone 8,132,797 .. ..Or 4\ North Temperate Zone 51,041,592 26 f Torrid Zone 78,314,115 40 VOut oi 100, nearly. South Temperate Zone 51,041,592 26 I South Frigid Zone..... 8,182,797 4/ Total .*. 196,662,893 ICO NATURAL DIVISIONS OF THE SURFACE OF THE EARTH. A Continent is a large continuous extent of land, including Beveral countries, as Europe, Asia, &c. An Island is a smaller extend of land, and entirely surrounded by water, as Great Britain, Ireland, Sicily. An Archipelago consists of several clusters or groups of islands, this name was originally applied to the Gulf of the Mediterranean, between Greece and Asia. A Peninsula is land almost surrounded by water, as England, Italy. A Cape is a head or point of land stretching out into the water, as Cape of Good Hope. Other names of Capes are. Promontory, Head, Headland, Point, Naze, Ness. An Isthmus is a narrow neck of land uniting two larger portions together, as the Isthmus of Panama, between North and South America. PHYSIOGRAPHY. 69 A Coast or Shore is tlie part of a country bordering on a sea, lake, or river. A Mountain is a portion of the land raised considerably above the surrounding portion, as Mont Blanc, Snoiodon. When under 2,000 feet above the level of the sea, they are termed hills, as Cots^uold Mills, Malvern Hills. When they form a continuous line they are called chains, or ranges, a series of which are termed a system. A Volcano is a mountain which casts forth smoke, flame, lava, ashes, &c., as Vesuvius, in Italy. A Valley is a hollow, or lowland, lying between mountains and hills ; when very narrow at the bottom, with steep sides, it is called a ravine. A Plain is a portion of country nearly flat, or level, and not raised much above the level of the sea. When a tract of thisr description lies high, it is called a 'plateau or tableland. A series of plains at dififerent levels are called terraces. Plains have received specific names in difierent parts of the world, derived from the languages of the people. Thus, in North America they are called savannas or prairies (meadows) ; pampas, llanos, and selvas, in South America ; steppes, in the south-east of Europe and the north-west of Asia. Zandes is the name given to extensive marshy or sandy tracts, covered with heath, on the coast of the Bay of Biscay. A Desert is a barren tract of country, usually consisting of sand and rocks, as the Sahara Desert. A fertile spot in the midst of a desert country, caused by the presence of water, is called an oasis. The Ocean is the continuous mass of salt water which surrounds the globe, particular parts receiving particular names, as the Pacific Ocean (peaceable). A Sea is a smaller body of salt water nearly surrounded by land, as the Mediterranean and Baltic Seas. A Gulf is a portion of the sea running into the land and having a narrow opening, as the Gulf of Mexico. A Bay is a portion of the sea running into the land, having a wider opening than a gulf, as the Bay of Bengal. A Creek is a small inlet on a low coast. In Australia and America it means a small inland river. A Channel is a body of water uniting two larger bodies of water. When it is narrow it is called a strait or sound. A Lake is generally fresh water surrounded by land, as Lake Superior; but some are salt, and when large are called seas, as the Caspian Sea. A Lagoon is a shallow lake formed on low lands by the overflow- ing of rivers or seas. A KiVER is a stream of fresh water rising in the land, draining a portion of the country, and flowing into the sea, a lake, or another 70 PHYSIOGRAPHY. river. A small stream is termed a rivulet or Irook. A river that falls into another is called a tributary, and where they meet the con- fluence ; the place where the river rises its source, and where it empties itself into the sea its mouth, but when very wide it is termed an estuary, firth, or fiord. The channel which contains its waters is the bed and the sides its hanks. The Basin of a river is that portion of country drained by the river and its tributaries. All the basins inclined to any particular sea are called a river system. A Waterparting is the elevated land which separates one river basin from another. A Delta is a tract of alluvial land deposited at the mouths of certain rivers, dividing them into two or more streams, so called from its resemblance to the Greek letter A, named delta. EXTENT AND DISTRIBUTION OF LAND AND WATER. 66. Land and Water are distributed very unequally, as only a little more than one-fourth is land, the remaining part, nearly three-fourths, being water ; or, more exactly, out of 197 millions of square miles 51^ are land and 145J water — that is about 26*2 per cent land and 73 "8 per cent water, its general distribution being as tollows : — Northern Hemisphere ... | ^^f ' H^ ) Land, 13 > milhon squaxe miles. Southern Hemisphere ... | ^^^^^;^ g^^ ^ Total 197 Of the land there is about three times as much in the Northern Hemisphere as there is in the Southern, and in the Eastern about two- and-a-half times that in the Western. Regarding the distribution in the zones it may be stated that in the North Frigid Zone about one-third is land, in the North Temperate about one-half, in the Torrid Zone one-half, and in the South Temperate one-tenth. Dividing the globe into two hemispheres, one having London* for its centre, and the other New Zealand, the former will embrace -^f of the whole land, and the latter only jV land, or nearly all water. This fact may account for the prosperity of London, being placed as it were in the very centre of the nations. 67. Divisions of tlie Water. — There is but one ocean really, but for the sake of convenience it is divided into five different parts, or basins, namely — * Tlie exact spot lies in the George's Channel, near the middle. PHYSIOGRAPHY. 71 The Atlantic Ocean, between the western coasts of the Old World and the eastern coasts of the New. The Pacific Ocean, between the eastern coasts of the Old World and the western coasts of the New. The Indian Ocean, south of Asia and east of Africa. The Arctic Ocean, lying round the North Pole. The Antarctic Ocean, lying round the South Pole. The areas of these divisions are roughly estimated as follows : — Greatest Length. Greatest Breadth. Areas. Miles. Miles. Square Miles. Pacific Ocean 9,000 12,000 72,000,«-00 A.tlantic Ocean 9,000 4,100 35,000,000 Indian Ocean 4,500 4,500 25,000,000 ^ctic Ocean 3,240 2,500 5,000,000 Antarctic Ocean ...3,266 3,266 5,000,000 68. Area and Distribution of the Land.— As before stated, there are about 51^ million square miles of land out of the total 197 million square miles area of the earth. Dividing the land into four divisions — namely, Europe, Asia (with Polynesia), and Africa, in the Eastern Hemisphere, commonly known as the Old World, and America (North and South), known as the New World — ^the respective areas are as follow : — OLD WORLD, including ISLANDS. Square Miles. Relative Size. Europe 3,500,000 1 Asia, with Polynesia ...21,500,000 6 Africa 12,000,000 3f NEW WORLD, including ISLANDS. North America 7,500,000 2f South America 7,000,000 2 Taking Australia by itself it contains about 3| milHon square miles, and the islands surrounding about 1 million ; so that Oceania contains 4 J millions of square miles. It is calculated that the area of all the islands on the globe (not including Australia) is between 2 J and 3 millions of square miles. Islands are divided generally into three classes : (1) Continental^ or seaward extensions of the continent upon whose coast they he, as the British Isles, for instance, which evidently belong to the same formation as the continent of Europe, and have, at some period, been attached to it, proofs of which are shown in the fossils of animals belonging to Europe having been found in this country ; and the only way to account for their presence here is that they must have walked over ; but since that time the land on which the German Ocean now stands has sunk to a depth of — in the deepest part — 50 fathoms, so that an elevation equal to that would again connect i 3 PHYSIOGRAPHY. them. (2) Volcanic islands, which are generally of a different structure to the continent near them. The chief of this class are the Sandwich Islands, Marquesas and Society Islands, in the Pacific, Iceland, Jan Mayzen Island, the Azores, the Canaries, Cape de Verdes, St. Helena, Ascension, Trinidad, &c. (See " Volcanoes," 53 and 54.) (3) Coral islands, or those formed by the coral polyps. (See "Organically-formed Kocks," 47 and 56.) The chief of these occur in Polynesia, the West Indies, the Eed Sea, the Indian Ocean, and the Atlantic Ocean. The largest barrier-reef, off the north-east of Australia, is more than 1,000 miles in length. Among the largest of the islands may be mentioned Greenland, containing 380,000 square miles ; Borneo, 280,000 ; New Guinea, 274,500 ; Madagasgar, 234,000 ; Sumatra, 177,000 ; Niphon, 109,000 ; and Great Britain, 83,830. 69. Configuration (Shape) of the Land.— Regarding the general aspect of the surface of the land there are two things to be considered — (1) Its horizontal outline, giving us the contour ; and (2) its vertical outline, or projile. Considering the horizontal outline of the different masses, they present many points of resemblance and certain points of contrast. (a) Though the greater bulk of land lies in the Northern Hemi- sphere, the greatest extension of the Old World is from east to west, while that in the New is from north to south. Hence the latter is subject to a greater diversity of temperature, and also of vegetable and animal life, owing to its crossing the different zones — frigid, temperate, and torrid. (6) Both masses in the Old and the New World attain their greatest dimensions from east to west, along the same parallel of latitude, namely, that of 50° north, which places much of North America, Europe, and Asia within the temperate zone, while only the narrower portions of South America, Africa, and the East India Islands lie under the intense heat of the equator. (c) Both the Old and New Worlds present a broad base towards the north, terminating along the parallel of 72°, and taper towards the south, terminating in far separated promontories. The direction of the chief peninsulas in both worlds (Old and New) is towards the south, these peninsulas being, in many cases, accompanied by an outlying island or islands, as South America by Tierra del Fuego and the Falkland Islands, Africa by Madagascar, Hindostan by Ceylon, and Australia by Tasmania. It may also be noticed at the same time that these promontories terminate in abrupt rocky precipices, which are often the termination of a mountain range. (d) In each hemisphere (Western and Eastern) a large portion is nearly entirely separated from the principal mass, Africa being aearly separated from the Old World, and the severance of Australia PHYSIOGRAPHY. 73 from Asia ; while in the New, South America is very nearly separated from Korth America. (e) The general disposition of the continents and larger islands is in the direction of their principal mountain axes, or mountain ranges. The tendency of islands is generally to arrange themselves in groups, or archipelagos. (/) The extremities of each continent (Old and New World), north and south, are nearly in the same meridian — the north-west point of Greenland being nearly in the same meridian with Cape Horn, North Cape Vvith the Cape of Good Hope, &c. 70. Coast-lines. — The shape of the coast-lines presents also some peculiar features on opposite sides of the same ocean, projections or protuberances on one side corresponding with recesses on the other. This may be noticed very strikingly with regard to the Atlantic, where the recesses in the New World seem made for the protuberances of the Old to fit exactly in. The extent of the coast-line is one of the most important features in Physical Geography, as on it depends greater diversity of climate and productions, and the facihties for navigation and commerce, from which nations derive their wealth, power, and independence. This may be regarded as the chief cause of the greatness of Europe and North America, as they have the greatest relative extent of coast-line. Africa on the contrary, has the least relative coast-line, and is the most uncivilised, the country being in a great degree shut out from the influence and enterprise of commerce, and the benefits resulting therefrom. The following table shows the relative extent of coast- line of the difierent continents : — ^ .V „- Sq. miles Sq. miles. ^-^1°' ^^--^ Europe 3,500,000 20,000 17o' Asia 17,500,000 33,000 533 Africa 12,000,000 16,500 680 North America 7,500,000 28,000 260 South America 7,000,000 16,500 420 Australia 3,500,000 7,600 460 From the above it will be seen that Europe has one mile of coast for every 170 square miles of surface, and North America one mile for every 260 square miles of surface, but Africa only one mile for every 680 square miles of surface. It has hardly an inlet where a ship can harbour in round the entire coast, but in Europe there are gulfs, inland seas, and peninsulas, the latter being in places again divided into inlets, &c. 71. Vertical Outline. — ^The surface of the land is very varied, aasuming many forms and elevations. In all the continents there is 74 PHYSIOGEAPHT. a gradual nse rrom the seashore towards certain points or ridges in the interior, which form the great loaterpartings of their respective continents. (There are one or two exceptions to this rule, namely, the region surrounding the Caspian Sea, Dead Sea, and Lake Ural.) This ridge of greatest elevation is placed more towards one side than the other, so that there are two slopes of unequal length, the long side, which is generally four or five times the length of the other, forming the slope, and the shorter, the counter-slope. The long slopes in the Old World are turned towards the north, and the short ones towards the south. But in the New World the long {or gentle) slope is turned towards the east, and the short (or rapid) one towards the west. According to Hughes, the lengths of the longer and shorter slopes are : — North Slope. South Slope. Eastern Asia 2,600 400 Western Asia 900 80 Central Europe 450 100 Africa ,.o..... 3,300 600 East Slope. West Slope. Korth America 1,600 800 Central America 2,000 300 South America 1,850 50 In all continents the elevations increase from the poles to the tropics ; and also extend in the line of the greatest length of the continents. Thus the highest point in the Old World — namely. Mount Everest — is situated near the Tropic of Cancer, and that in the New World (Aconcagua in Chili) is not far south of the Tropic of Capricorn. The effect of this law is to temper the fierceness of the heat of the tropics, giving them a variety of climate. Mean Elevation. — The mean elevation of a continent is the height above the sea level that it would he if all the hills and mountains were levelled, filling all the valleys up, so that the tuhole shoidd have an even surface. It has been estimated that the mean elevation of Europe would be 670 feet ; of Asia, 1,132 feet ; of Africa, 910 feet ; of North America, 750 feet ; of South America, 1,150 feet. MOUNTAINS. 72. It is seldom that a moTintain occurs singly. They appear mostly in ranges or chains. There are a few instances where they do, owing their origin chiefly to volcanic energy, among which may be mentioned Mount Egmont in New Zealand, and the Peak of Teneriffe in the Canary Islands. The chains or ranges generally consist of parallel ridges, the centre one being the highest. They PHTSIOGR&.PHT. 75 have generally their highest elevation near the middle, gradually drooping down into the plain towards their extremities. The lateral ridges which break off from these may again, in their turn, send o£E smaller ridges or spurs in numerous ramifications. Several chains constitute what is called a group, and several groups a system. The outlines of mountains depend chiefly on the geological struc- ture, and partly on the amount of waste and degradation to which they have been subjected. In this aanner, hills that are com- posed of hard basalts and greenstones, alternating with soft tufas or stratified rocks, assume tenaciform declivities ; and extinct volcanic hnis put on a crateriform aspect. Those chiefly composed of hard massive strata — as limestone, conglomerates, and sandstone — present a tabular appearance ; and mountains capped and flanked by crystal- line schists and quartz are serrated with peaks and pinnacles. The mountain chains often traverse immense regions, forming the boundaries of great nations living round their base. For instance, the Andes, continued by the Mexican and Rocky Mountains, extend through all the different zones and climates of the world. 73. Mountain Systems. — There are really only two great mountain systems in the world. (1) That in the New World is a continuity of a vast and extremely precipitous line of very elevated mountains running parallel with the west coast of America, and from the Arctic Ocean almost to the extremity of Patagonia — a distance of nearly 9,000 miles. Throughout the whole of this border we notice a distinct and unmistakable tendency to a system of double or triple ridges, nearly or exactly parallel, extending for hundreds of miles in succession, and resumed again and again when interrupted. (2) In the Old World there is a broad belt of mountainous country running through the land in a general direction from the East Cape, in Siberia, west-south-west across Asia to Spain and Morocco, being a distance of between 8,000 and 9,000 miles. AU mountain chains, with the exception of the African and Australian ranges, are offshoots of one or the other of these two systems. For the sake of reference the mountains have been arranged in various systems. Thus, those of Europe are arranged into the BHtannic, Iberian (or Spanish), Alpine, Carpathians, Scandinavian, Uralian systems, &c. Those of Asia into the Taurus, Kuen-lun^ Thian-shan, Altai, Himalayas, &c. Ai'rica, into the Atlas, Abyssinian, Eastern, Western, or Guinea, &c. While those of the New World are the HocJcy Mountains, the Mexican Andes, and the Cordillera of the Andes. 74. Europe. — The Britannic System consists of a number of detached chains, as the Grampians, Cheviots, Cumbrian, Hibernian, and Welsh mountains ; they are sometimes said to form the southern continuation of the Scandinavian system. The highest mountains 76 PHYSIOGRAPHY. are Ben Nevis, 4,406 feet, in Inverness- shire, and Caimtoul, in Aber- deenshire, 4,285 feet ; the highest in England and Wales is Snowdon, 3,590 feet ; and in Ireland, Carn-Tual, 3,412 feet high. The Spanish System embraces several detached mountain chains, including the Pyrenees, the Cantabrian Mountains, the Sierra Nevada, the Sierra Morena, and the sierras of the central tableland. The reason of their being called sierras is on account of their jagged and sawlike appearance, Thenamecomes from a Spanish word meaning a saw. The principal chain is the Sierra Nevada, ranging from east to west, the highest point is Mulhagen, 11,678 feet, and Maladetta, in the Pyrenees, 11,168 feet. The Sierra Morena runs parallel with the Nevada chain, but lies farther north. There are several minor chains lying between the Pyrenees and the Sierra Nevada. The French System includes all the hilly eminences in France lying to the north of the Garonne, west of the Rhone and south of the Rhine. The chief detached mountains are the Auvergne Mountains, which are a group of extinct volcanoes, the highest peak of which is Plomb de Cantal, 6,113 feet. The Alpine System embraces the whole of those extensive and lofty mountains which, from Switzerland as a centre, spread in ranges more or less persistent, which confer on Southern Europe one of its chief and peculiar features. It may also be said to form the back- bone of the continent. These ranges have many minor divisions, as the Maritime, Cottian, Graian, Pennine, Bernese, Carnic, Noric, and other Alps, which extend in a north-east direction from the shores of the Mediterranean to the tableland of Bohemia ; the Apennines, traversing the entire length of Italy, and terminating in the volcano Etna, in Sicily ; the Slavo-Hellenic ranges, lying between the shores of the Adriatic and the plains of the Danube ; and the Balkan group, in Turkey, ranging from east to west. The highest point in this system is Mont Blanc, 15,744 feet. The other chief heights are Mont Pelvoux, 14,108 feet ; Etna, in the Apennines, 10,874 feet ; Tehan-Dagh, 9,700 feet, in the Balkan mountains ; Olympus, 9,749 feet, in the Hellenic range. The Carpathian System includes all the mountains and eminences situated between the Rhine, Dneiper, and Danube, the plains of Northern Germany and Western Poland. The western portion of the Carpathian chain, near the mouth of the Danube, is called the Transylvanian range. The highest point in this range is Ruska Joyana, 9,912 feet, in the Eastern Carpathians. In the main range there are several high peaks grouped upon one very large mountain, Tatra, the highest point being 8,524 feet ; the Csalic Peak, 8,314 feet ; and the Lomnitz to more than 8,000 feet. The Scandinavian System embraces the whole of the mountains and highlands of Norway and Sweden, extending in a north-eastern direction from the Naze to the North Cape — a distance of nearl?- PHYSIOGRAPHY. 77 1,000 miles. Tliey consist of the Novrska Fjellen (Norwegian range) and the Kjoien. The latter lies to the north, being ahout 500 mUes in length, but not so generally elevated as the former, though it rises to about 6,200 feet in Sulitelma. The former is about 400 miles long, lying to the south, and containing the highest points in the group, viz., Sneehatten, in the Dovrefeld (in the middle), about 8,000 feet ; though in the southern range Skegstol-tend is said to be 8,670 feet. The Ural System or chain forms the boundary line between Europe and Asia, embracing the Ural Mountains and forming the water- parting between the extensive basins of the Volga and Obi, This range runs in a true meridianal direction for a distance of more than 1,600 miles, and consists of round-backed, plateau-shaped masses, of very moderate height, in most places not exceeding 2,000 feet, though there are one or two points a little over 5,000 feet, viz., Koujak-Ofski, 5,397 feet, and Obdorsk, 5,286 feet. 75. Asia. — The Altai, the great mountain system of the Old World, commences on the shores of Behring Strait, at the East Cape, ranging for some distance to the west. Afterwards it bends toward the south, branching into Kamtschatka. The chief range bends again to the west, running through Siberia, when it is called the Aldan Mountains, still continuing westward along the 50th parallel of latitude, passing Lake Baikal, and reaching the 84th degree of longitude. The breadth of this range in many places exceeds 800 miles, but the height is not so great in comparison to its length and breadth. The highest point is Bielukha, 12,796 feet. Partly parallel to the Altai range are three ranges, viz., (1) Thian-shany (2) Kuen-lun, and (3) the Great Himalaya range. The first two run eastward, Thian-shan near to the 42nd parallel of latitude, and Kuen-lun near to the 36th, into China. The last named forms the southern boundary of the desert of Gohi, Thian-shan lying to the north of that desert. The highest point in the Thian-shan is Khan- Tengri, 21,000 feet. In the Kuen-lun some points reach the height of 22,000 feet. In these chains are active volcanoes, some as far as 1,500 miles from the sea. The last of these three ranges — namely, the Great Himalaya — extends about 1,500 miles along the southern border of the central plateau, separating Thibet from Hindostan. The highest point is Mount Everest, reaching 29,002 feet, or more than 5| miles in height, being the highest peak in the world. There are several other very high summits in the central portion, attaining in several places the height of about 25,000 feet, and between 30 and 40 more than 23,000 feet. Kinchingunga reaches 88,156 feet, and Dhawalagiri 26,826 feet. The passes of the Himalaya are from 10,000 to 17,000 feet high. It has been noted that vegetation ascends higher on the north side than on the south side. This singular fact is supposed to arise from the reflection of the sun's 78 PHYSIOGRAPHY. The Hindoo Cooch (or Koosh) traverse the north of Afghanistan and Persia. They may be regarded as prolongations of the Himalaya. The highest peak exceeds 20,000 feet. The Taurus and Anti-Taurus ranges, which encircle the tableland of Asiatic Turkey, the highest point of which is Mount Argish (or Argons) in Armenia, 13,197 feet. In connection with the Taurus may be mentioned the Lebanon range, which attains a height of 10,050 feet in Dahr-el-Khotib ; Hermon, 9,376 feet ; and Sinai and Horeb, 7,413 and 8,593 feet respectively. The Caucasian includes the mountains of Elburz and those between the Caspian and Black Seas, whose highest points are Dema- vend, 21,500 feet ; Elburz, 18,493 ; Koschtantan, 17,096 ; Dychtan, 16,925 ; and several others exceed 16,000 feet in height. There are several smaller ranges that have not been noticed, the chief of which are the Armenian Mountains, ranging between Turkey and Persia, the highest peak being Mount Ararat, 17,112 feet. 76. Africa. — In the extreme north we have the Atlas System — between the Mediterranean seaboard and the Sahara —extending from Tripoli on the east, to the Atlantic on the west, namely, to Cape Geer. Geologically, it is connected with the systems of Southern Europe, and consists of three or four parallel ranges, gradually increasing in height from east to west. At Tripoli it is only about 2,000 feet above the level of the sea, in Tunis it is 4,500 feet, in Algeria 7,700, while in Alorocco it rises to the height of 11,400 (Mount Miltsin or Atlas) and Jebel Tedla to 13,000 feet. Several smaller ranges proceed from the main range — one branch travelling north and terminating in Cape Spartel, at the Strait of Gibraltar. The next system of importance is the Abyssinian, which is con- nected with and forms the lofty tableland of Amhara, the height of which is 8,000 feet above the level of the sea. The two principal chains — namely, Samen and Taranta — range in a northerly direction, between the upper forks of the Nile and the Eed Sea, and run along the latter' s shores as far as the lower hills of Egypt. In the Samen, or upper range, we have the highest points, namely, Eas Detchen, 15,986 feet ; Buahat, 15,000 feet ; Abba Jaret, 14,707 feet. In the Taranta, or lower range, the heights descend gradually from 9,000 to 5,000 feet, towards the Red Sea and Plains of Egypt. This system consists chiefly of granites, porphyries, syenites, and crystalline schists. The Guinea System usually embraces the Kong and Cameroon Mountains. The Kong, between the Gulf of Guinea and the Niger, generally average from 1,000 to 3,000 feet ; and the latter, on the west, stretching eastward, rise to aljove 13,000 feet in height. The hills of Cape Colony form a series of sandstone plateaux, or karoosg PHYSIOGRAPHY. 79 rising from Table Mountain, 3,816 feet, to the sutotnits of Nieuvelt and Snieuvelt Mountains, in the north of the colony, which in some cases reach 10,000 feet, as in the Compass Bay, in the Sneeuwveld or Snowy Range. This system of mountains is sometimes termed the Cape System. Polynesian System. — Not much is at present known of the moun- tain chains of Austraha. The highest points in this country are Mount Kosciusko, 6,500 feet, and Sea View, 6,000 feet. These are in the chain which extends along the eastern coast, from Torres Strait on the north, to the extreme point of Tasmania on the south. The highest points in Polynesia are the active volcanoes of Manna Kea and Manna Loa, in Hawaii, each about 14,000 feet, though in. New Zealand there are some points reaching from 10,000 to 12,000 feet. The highest mountains of the Old World are formed of granite ; and gneiss and mica-slate also form large mountain masses. NEW WORLD, OR AMERICAN SYSTEMS. 77, South America. — The mountain chains of South America may be ranged into two systems. (1) The Cordillera of the Andes; and (2) The Mountains of Brazil. The Andes extend along the western coast, from the Magellan Straits to the Caribbean Sea, in two or three parallel chains, a distance of nearly 4,500 miles, and varying in breadth from 40 to 340 miles. They may be termed the largest mountains in the world, being so lofty throughout, and differing from the Himalaya by rising from the sea. The highest points are in the Bolivian Andes, reaching in many places from 13,000 to over 21,000 feet. In Chili many summits exceed 16,000 feet ; the highest peak of the range — namely, Aconcagua — being 22,300 feet. Chimborazo is 21,424 feet. In Patagonia they do not exceed much above 6,000 feet. This system (Andes) forms one of the grandest centres of volcanoes in the world, most of its highest peaks being volcanic — Aconcagua, for instance, the highest of the range. 78. North and Central America.— The Andes also form the chief system of Central and North America, though known by a different name than in the South. Continuing from the Isthmus of Panama to the North of Mexico they are called Central Andes. The greater part of Mexico consists of magnificent tablelands, from 5,000 to 8,000 feet high. The highest peak is Popocatapetl, 17,720 feet. In North America the system is termed the Rocky Mountains, which consist chiefly of two parallel ranges, running generally in the direction of the Pacific to the Arctic Ocean, a distance of more than 5,000 miles, so that the Andes extend a total distance of 80 PHYSIOGRAPHY. nearly 10,000 miles, and in places they are 1,000 miles in breadth. The highest peaks of the Rocky Mountains are Mount Brown, 16,000 feet, Mount Hooker, 15,000 feet, and Mount Murchison, 15,000 feet. The summits of the Andes are formed of porphyry and basalt (igneous rocks). In the maritime or western range the highest peak of ITorth America is to be found, namely, Mount St. Elias, on the coast, in latitude 61°, reaching a height of 17,800 feet. On the east are the AUeghanies, or Appalachian Mountains, extending for about 1,200 miles in length, the highest point of which is Mount Wash- ington, 6,634 feet. TABLELANDS, OK PLATEAUX. 79. Tablelands are extensive upland plains, consisting of very large areas of surface, high above the level of the sea, and varied by hill and dale, lake and river. Few mountains have their bases at or near the sea level, but mostly rise on these tablelands. It is from these tablelands that many of our noblest rivers have their sources. The chief tablelands are — Asia. — It is in this continent that we have the grandest examples of tablelands and plateaux, both in extent and elevation, occupying two-fifths of the entire continent, stretching from the Mediterranean to the Pacific, being 6,000 miles in length, 2,000 miles broad at the eastern extremity, 700 to 1,000 in the middle, but narrower towards the Mediterranean. We may divide these plateaux into two great divisions, namely, (1) The Central Asia or Eastern Plateaux ; (2) The Western Plateaux, joined to the Eastern by the Hindoo Cooch. In the Central Plateaux lie the vast deserts of Gobi, Scha-mo, and Hanai (Dry Sea). The rainless desert of Gobi covers an area of 400,000 square miles, rising from 4,000 to 6,000 feet in height. To the south of these lie the plateaux of Thibet, the loftiest inhabited portion of the globe, having an elevation (between the Kuen-lun and the Himalaya) of 15,000 feet, and reaching in some points 17,000 feet, with an area of 166,000 square miles To tne south-west of the Central Plateaux lie the great tablelands of Persia, or Iran, rising from 2,300 to 3,500 feet above the sea level, and with an area of 300,000 square miles, presenting a riverless and desolate region. In succession to this plateaux extend the tablelands of Arabia and the Great Desert of Sahara, in Africa. The entire sandy and arid table- lands of Asia — namely those of the central and western plateaux — extend over 120° of longitude and 17° of latitude, or an area of 6,000,000 square miles Not belonging to either of the above divisions are the plateaux of Armenia, north-east of Turkey in Asia, 7,000 feet high, and the Deccan, in the Indian Peninsula, rising from 1,600 to 2,000 feet in Hyderabad, and 4,000 feet and upwards in Mysore. PHYSIOGEAPHY. 81 Europe. — The plateaux of tliis country are very few, and of little importance. The highest are those of Spain, ranging from 2,000 to 3,000 feet above the sea, and extending into Portugal, covericg an area of 100,000 square miles. The central part is edged or fringed by mountains on all sides, and the ranges of the Sierras Nevada, Morena, &c., rise out of it. The largest plateau in extent hes in the east of Europe, separating the low plains of Northern and Central Eussia ; its area is more than 150,000 square miles. To the south of this plateau lies the Carpathian, but not near so large. Africa. — Not much is known of the tablelands of Africa; but some elevated tracts occur in Abyssinia, extending southwards to the extremity of the continent. The great lakes, Albert Nyanza and Victoria Nyanza, are situated on this tableland. Some parts of Africa lie really below the sea level. North and South America. — In Mexico, or Central America, we find the greatest unbroken extent of tableland known, extend- ing, north and south, a distance of 1,600 miles, and a breadth of 360 miles, or more. The surface is a dead level, with the exception of where volcanic cones rise up, and ranges from 4,000 to 7,000 feet in height. In North America the chief plateau is called the Great Basin, lying between the ranges of the Sierra Nevada and the Rocky Mountains, and extending from Mexico to the Arctic Sea, about 2,000 miles, its greatest breadth being 600 miles, and mean height 5,000 feet. Another tableland, but not so large, stretches from Hudson Strait, in Labrador, to the north of Alhambra, its greatest height not exceeding 2,000 feet. In South America the principa* one is Desaguadero, lying high up among the tops of the Andes, attaining in Bolivia a height of 13,000 feet, its length being about 500 miles, and breadth from 80 to 60 miles. It is on this plain that Lake Titicaca stands, at an elevation of 13,000 feet, being one of the most elevated sheets of water. The Plateau of Quito is 200 miles long and 30 wide. The city of Quito is situated at the height of 9,540 feet, having a view of eleven snow- clad mountains (nevadoes). The only other plateau worthy of note is the Plateau of Brazil, the area of which is about 1,500,000 square miles, and the mean height about 3,000 feet. The following table gives a few of the most elevated tablelands, compiled principally by Humboldt : — Feet. Bavaria, Germany 1,660 CastiUe, Spain 2,240 Plateau of Switzerland 2,000 Victoria Nyanza, East Africa 3,300 Iran, Persia 4,500 F 82 PHYSIOGRAPHY. Feet. Armenia, South of the Black Sea 7.000 Mexico, Central America 7,483 Quito, Andes 9,600 Bolivia, Andes , 12,900 Thibet 10,000 to 15,000 Desaguadero, Andes 13,000 Ravanabradu, East Asia 15,000 LOWLAND PLAINS OR DESERTS. 80, In the Old World the principal plain is known as the Great Northern Plain, which stretches in length from the German Ocean (shores of Holland) through Prussia, Poland, Russia, and Siberia to Behring Strait, only interrupted by the transverse range of the Urals ; and in breadth from the shores of the Arctic Ocean, nearly to the base of the Carpathians, in Europe, and to the table- land of Iran (Persia) and edges of the Altai Mountains, in Asia ; and altogether it extends over 190° of longitude, including an area of more than 5,000,000 square miles, being nearly one-third of the area of Asia and Europe, The part in Europe is divided into the Germanic Plain, in the west, and the Sarmatian Plain, in the east ; while in Asia we have the steppes of Kirghis, I shim, and Baraba, in the south-west, and the Siberian Plain in the north-east. In the Germanic Section occur the low-lying polders and morasses of Holland and the sandy tracts between the rivers Elbe and Weser, which are interspersed with heaths, marshes, &c. The Sarmatian Section extends from the Baltic to the Black and Caspian Seas and the Ural Mountains, the only interruption being the Valdai Mountains, It may be said to consist in the northern division of cold, swampy, and partially-wooded flats, much of it consisting of marsh land, and large tracts covered by peat, called trunda. The middle division differs much from the northern, being mild in climate, fertile, its surface undulating, richly wooded, and well watered. This is the pleasantest part of Russia. The southern division consists of steppes and river swamps, impregnated in many places with saline matter. Towards the eastern extremity the plain assumes the character of the steppes of Kirghis, Ishim, and Baraba. S'eppes, as their name implies, are wide, treeless, monotonous deserts, covered with long coarse grass and shrubs during a brief summer, and in the winter converted into bleak wastes. These steppes are estimated to cover an area of one million square miles. Landes, or heaths, are those extensive areas of sand-drift which stretch southward from the mouth of the Garonne, on the coast of the Bay of Biscay. They are sometimes marshy, but mostly covered with d^rarf shrubs (sea pine) and heath. . PHYSIOGRAPHY. 83 Polders are flat tracts of land in Holland, reclaimed from the sea and protected by dyJces or embankments. Dunes are hillocks of drift-sands, as those which stretch along the coast of the Netherlands and North of France. The minor ones of Europe are the plains of Lombardy, watered by the Po, and those of the Middle and Lower Danube. Among the other secondary plains of the Old World may be noticed the Plain of China, occupying 200,000 square miles ; the plains of Hindostan, extending from the base of the Himalayas to the Deccan, and from the Ganges to the Indus (this plain is often inundated in its lowest parts during the rainy season); the plain of Turan (1,000,000 square miles), extending along the southern shores of Lake Aral to the Caspian Sea ; and the plains of Mesopotamia (165,000 square miles), in Western Asia, between the Euphrates and the Tigris. In Africa, the Desert of Sahara, which has been till of late considered a depressed plain, consists of an elevated plain, the mean height of which is about 1,000 feet. New "World. — Between the Rocky Mountains and the Alleghany Mountains, in North America, from the Arctic Ocean, is one very large central plain, watered at the lower part by the Mississippi and its tributaries, and containing some of the largest fresh- water lakes known. This plain may be said to extend to the most southern part of America — Tierra del Fuego — the only interruptions being in the case of the Gulf of Mexico and the Caribbean Sea. Its different portions are called prairies,* or savannas.t In the Southern Con- traent it is situated between the Andes and the Cordilleras of Brazil. The entire length of this one plain is about 9,000 miles. The Atlantic Plain lies between the Alleghanies and the Atlantic Ocean, from the Gulf of Mexico to Massachusetts. The Central Plain of South America is divided into three well- marked river plains — viz., those of the Amazon, Orinoco, and La Plata — termed respectively selvas, llanos, and pampas. The first {selvas) comprise the largest river basin of the world (1,500,000 square miles), covered by an immense forest, presenting the rankest luxuriance of forest growth, which in many places is so tangled with the under- wood, &c,, that it can only be penetrated by the river courses. The llanos, or grassy flats, occupy an area of 160,000 square miles, and form the lowest and most level tracts in the world, not varying a single foot for hundreds of mUes. In the wet season these are inundated, and a rich alluvial deposit is formed, which, after the subsidence of the water, is quickly covered with verdure, so that the natives term it the Sea of Grass; but it does not last long, as the droughts which follow soon cause it to become parched and to wither away. The pampas comprise the basins of the Parana, La * Prairie, an extensive meadow. + Savanna, a bed-sheet or meado- 84 PHYSIOGRAPHY. Plata, Uruguay, &c., and cover an area of about 880,000 squaro miles. They consist of rich alluvial soil, generally covered with tall grasses, thistles (some 10ft. high), weeds, &c., though in some places they are saline and barren. The desert terrace land of Patagonia, stretching 800 miles from Rio Colorado to the very end of the continent, is a sterile country, consisting of shingle, strewn with boulders, &c. With its fierce hurricanes, hot winds, and chilling blasts, it is one of the most desolate regions on the globe. Valleys are of several kinds. Some are valleys of erosion, having been caused by the intermittent action of running water wearing away and carrying off the fragments of rock, forming deep and narrow ravines, which gradually widen into a valley ; others, according to the theory of depression and emergences, would owe their origin to some part having been raised or other parts depressed ; and others would be caused by the mountain torrent, which receives the product of the springs, snows thawed, rains, &c. Canons are narrow channels cut out by the rivers themselves, the water occupying the bottom from side to side, an example of which is the river Colorado, rising in the Rocky Mountains. The Grand Canon of this river is 240 miles long and from 2,000 to 4,000 feet deep. THE OCEAN. The waters of the globe, as previously stated, cover about 145^ millions of square miles, which all over the earth follow the well- known law of fluids, namely, that of assuming a uniform or natural level at a nearly equal distance from the centre of the earth. Though the ocean has different names in different parts of the world, yet, in reahty, there is but one ocean. Its form and . various divisions can be best learned by inspecting a map of the world, going over it several times until quite familiar with every arm, or inland sea. The ocean is one of the greatest modifiers of the climate. (See " Climate.") 81. Density of the Oceans. — Sea water has a greater density than fresh water, varying with the amount of salts dissolved in it. A cubic foot of fresh water weighs 1,000 ounces, but the same quantity of sea water weighs 1,026 ounces, its specific gravity being called 1-026, that is, taking fresh water as the standard of comparison, or 1, The densities of the oceans areas follow : North Atlantic, 1 "02664 ; South Atlantic, 1*02976 ; the North and South Pacific respectively, 1-02548 and 1-02658 ; the Indian, 1-0263 ; Mediterranean, 1-0289 ; Bed Sea, 1-0286 ; and the Baltic, 1*0086. Sea water does not freeze so soon as fresh water (which freezes at 32°), but remains in its fluid state until the thermometer reaches PHYSIOGRAPHY. 85 281° ^-3 lience it is mucli more serviceable for man. Another point worthy of notice is that it is less vapourisable than fresh water, causing a less amount of moisture to be carried from its greater expanse to the comparitively smaller expanse of land. It has been proved by the investigation of the Challenger that animals of many orders and genera exist even at the greatest depths of the ocean, sponges, molluscs, annelids, crustaceans, &c., having been found in great numbers. 82. Depth, Pressure, and Weight of the Water.— The recent soundings of the Challenger prove that our former idea of the depth of the ocean was far in excess of the truth, the deepest cast being 4,575 fathoms, and the average depth 12,000 feet, or 2^^^- miles ; hence its cubic contents equal 145,500,000 x 2i\ = 330,681,818 cubic miles, or in round numbers 330 millions. The weight from the above data may easily be found, but to have it correct we must also take into account the pressure at its mean depth, namely, of one mile ; the weight of a column of sea water of this height is 1760 x S -f 33*8 (height of a column of water equal to the weight of the atmos- phere) =156'2 atmospheres; this weight is sufficient to compress the water at that depth about '0142,* so that its density will ba 1*04076. Thus, sea water at its surface has a density of 1"026 (pii^e water being 1), but owing to the compression, (1-'0142), or -9854 cubic feet, at the mean depth, has this density ; hence one cubic foot has a mean density of l*026-f-"9854 = l'04076, svipposing the depth to be two miles. At this density a cubic foot of sea water weighs 62-5 (the weight of a cubic foot of pure water) x 1-04076 = 65-0675lb., and a cubic mile (5280^ x 65-071b. ) = 4,275,969,078 tons ; hence, entire weight of the ocean equals this number multiplied by the number of cubic miles, viz., 330 million, which gives 1,411,070 billion tons, or about Trrrth of the whole globe. 83. Its Composition and Saltness.— (For "Composition" see 45.) The saltness is caused by the presence of soluble matter, as sodic chloride (common salt), which exists in a greater quantity than any other sahne material ; next to which in abundance come mag- nesiaf and lime, which occur as carbonates, sulphates, and chlorides ; then follow soda, potash, iron, silica, various iodides &c. Silica and carbonate of lime play an important part in nature, supplying the skeleton, or hard parts of fishes, shells, and other marine organisms, such as form the bottom of the ocean for thousands of square miles. Generally the ocean is of a uniform degree of saltness, containing about 3 J per cent of saline material. Taking its specific gravity at 2, *It has been found by experiment that for every 1,000 feet of depth, water Is compressed ^i^th of its bulk. t It is the chloride of magnesium which causes the clothes of sailors, when wetted with sea water, to have that damp, sticky feeling. 86 PHTSIOGEAPHT. wliich is not far from the mark, a cubic foot would weigh 62-5x2 = 1251b. ; hence one cubic mile weighs [5380^ x 125) = 8,214,171,428-57 tons, but the salt is equal to 3^ j)er cent of the whole cubic contents, namely, 11,573,863 cubic miles, so that the weight of salt is (8214171428x11573868) or 95,069,694,766,186,364 tons, or 95,070 billion tons nearly. The amount of salt in the ocean must ever be on the increase, as the rivers in their journeys through the land wash out such soluble substances as salt, &c., and carry them into the sea, where very nearly all rivers run. When once there it must remain, as in the process of evaporation fresh toater alone is taken, which in its turn returns with its saline substances. Colour. — Though in small quantities the waters of the ocean appear colourless, in large masses it is of various hues. For instance, in the open sea, it is of a deep blue colour, while in the shallow parts it appears green. The cause of these colours has not been satisfac- torily explained, some thinking that they are due to the dififerent degrees of salt in the water. There are a few sheets of water which take their names from their colour, as the Vermilion Sea (Gulf of California), the Yelloiu Sea, whose colour is due chiefly to the sedi- ment discharged into it by the rivers ; the Green Sea (Sargasso Sea), lied, Black, White Seas, &c., whose hues are probalDly due to the presence of solid matter, either as living organisms or as sediments. The Bottom of the Ocean is somewhat similar to the surface of the land — plains at different levels, valleys, and deep depressions, rocky ridges, sometimes rising to its surface, forming islands, or sunken reefs. It is but the submerged surface of former lands, with the exception of coral reefs and submarine volcanoes. As a rule, the ocean is shallower near land ; and where the land gradually slopes towards the ocean the waters deepen gradually ; but where the land descends precipitously the sea deepens in like manner, suddenly and abruptly. Thus, on leaving Ireland to cross the Atlantic it only sinks 6 feet per mile for the 230 miles ; after- wards it makes a descent of 1,400 fathoms in about 20 miles, from which there is a plain of nearly 1,200 miles in length. 84. Movements of the Ocean.— The movements of the ocean are of three kinds — namely, ivaves, tides, and currents. These motions arise from the influence of the winds, the attraction of the sun and moon, and from the temperature. Waves are undulations of the water without progressive motion, produced by the wind. They vary in size according to the force of the wind, from a gentle ripple, to billows 40 feet in height, though a wide expanse is requisite to produce its full effect. The highest waves known are those which occur off the Cape of Good Hope and Cape Horn, where they attain sometimes between 30 and 40 feet PHYSIOGRAPHY, 87 from trough to crest ; but the depth to which the disturbance is felt is very slight. The velocity of waves, or the rate at which they travel, depends upon the breadth and depth, as a loave of a certain ireadth cannot attain more than a certain volocity, and if the depth is less than the breadth this velocity cannot be attained. It has been calculated that a wave 1,000 feet broad, formed on water 10 feet deep, travels 12 miles an hour ; on water 100 feet deep, 36 miles an hour, or 53 feet per second ; on water 1,000 feet deep, 49 miles an hour. If the wave was 10,000 feet broad, in the depth of 10 feet its rate would be 12 miles an hour ; in 100 feet, 39 miles an hour ; in 1,000 feet, 115 miles an hour; and in 10,000 feet, 154 miles an hour. It is not the water which travels at these rates, but simply the form of the water, which rises up and down. This motion is well imitated by shaking the ends of a stretched rope, giving rise to a succession of waves, or also in the shaking of carpets, when the two ends held in the hands remain fixed, while loaves are propagated from one end to the other. The waves coming near the shore are interfered with in their rising and falling by the water causing the foot of the wave to be held back, and the head to curve forward, and Ireah with great force — the momemtum in some cases being that great that large masses of stone or concrete, even weighing as much as 50 tons, are torn down from piers and breakwaters. The effective pressure of these breakers has been estimated as high as 6,0001b. per square foot. The Tides are occasioned chiefly by the attraction of the moon upon the earth, but assisted partly by the sun. Considering the earth as a solid, rigid body, the moon's attraction acts upon its centre ; but the waters of the ocean directly below the moon experience and obey a greater attraction owing to being nearer, thereby causing an immense flat wave to be heaped up below the moon. But, at the same time, the centre of the earth is attracted more than the waters on the other side of the earth, causing a similar wave to be heaped up there also ; hence it is evident that it is the difference of th« moon's attraction upon the waters on opposite sides of the globe, vertically below her, that causes the two tides. To make it plainer, let M in the annexed figure represent the moon, and E the earth — then the waters at A, being nearer to the moon than the centre of the earth 0, are attracted with greater force than the earth at (see "Gravitation," 15 and 16), and, being free to move, heap them- selves in a wave directly imder the moon, the water flowing towards this place ; but, at the same time, the moon's attrac- tion on the centre of '"^'^ Pi^ g 88 PHYSIOGRAPHY. the earth, 0, is much greater than the water at B, owing to the same cause ; hence the earth approaches toward the moon, leaving the waters behind forming a heap there, so that instead of only one tide every 24 hours and 50 minutes we have two, or one every 12 hours and 25 minutes, both occurring at the same time but on opposite sides of the globe, the 12 hours, &c., being taken up by the earth in its revolution to the place where the opposite tide took place. Spring and Neap Tides. — When the sun and moon are on the same side of the earth together they evidently act in conjunction. This occurs at new and full moon, causing higher or spring tides. But when the moon is at right angles (90°) from the sun, when she is in her first and last quarters, or half moon, his attraction, being exerted at right angles, counteracts the attraction of the moon, causing lower or neap tides — the proportion of spring to neap tides being as 69 to 31, or nearly as 7 to 3. The Tidal Wave. — The earth, by constantly revolving, causes every part to be offered in succession to the attracting influence, so that the rising waters are drawn along in an immense tidal wave around the globe ; and had the surface of the earth been entirely covered with water the tidal wave would have been regular and continuous in its journey from east to west ; but such is not the case, owing to the many interruptions of the land causing it to be deflected into various courses. The tidal wave may be regarded as receiving its first impulse in the Southern Ocean, where the greatest uninterrupted expanse of water occurs. From here it is carried northward into the Indian, Atlantic, and Pacific Oceans, where it unites with the minor tide waves generated in these oceans. It there subdivides, flows, rises, &c., according to the depth of water and the obstructions of coasts and islands. Its velonty varies much. For instance, it crosses the Indian Ocean in six hours, entering the Atlantic, travelling through it at the rate of from 500 to 700 miles an hour till it reaches the West of Ireland, reaching the Orkneys and Bergen at the same time, and travelling now as one branch through the North Sea it reaches Aberdeen in thirty-seven hours, and London twelve hours later, or in forty-nine hours from the time it left its antipodes. By noting the times at which the same high water reaches different parts of the coast a series of lines connecting these points may be laid down so as to indicate the course of the tidal wave with great precision. These series of lines are termed co-tidal lines. The height of the tidal wave in the Pacific seldom exceeds two or at the most three feet ; in the Indian and Atlantic Oceans it reaches eight or nine feet ; but in bays and gulfs, opening broadly to its course and narrowing toward the interior, as the Bristol Channel, the Bay PHYSIOGRAPHY 89 of Biscay, Bay of Fundy, &c., it may rise from thirty to seventy feet. When the seas terminate in river estuaries, the tide, being converged, rushes up the river with great force and speed. It is then called a bore, examples of which occur in the Severn, where a bore rises nine feet high ; in the Amazon, thirteen feet ; Hooghly, twenty to twenty-five feet ; Tsien-tang, thirty feet. But, on the other hand, in inland seas and gulfs, the openings of which are narrow, and lie transversely to the course of the tidal wave, little or no tides are experienced, as they are not of sufl&cient area to form any perceptible one of their own, examples of which are the Mediterranean, Baltic, &c. 85. Currents, their Causes, &C. — Currents are movements in the ocean, like great rivers, transporting the waters from one region to another. These currents, like the winds, are arranged as constant, periodical, and variable. The constant depend chiefly upon the unequal temperatures and densities in the waters of the ocean, the rotation of the earth, and the trade winds. The heat of the sun at the tropics heats and expands the water there to a considerable depth ; at the same time the cold at the poles renders the water heavy and dense, causing it to sink and flow below the warm water lying near the equator, while the lighter water of these regions flows over towards the poles to restore equilibrium. These currents do not flow exactly north and south, but are deflected in like manner to the trade winds, the polar currents tending to the west and the equatorial to the east (see 104), forming, the same as in the air, four great currents. The periodical currents are caused by the tides, monsoons, sea and land breezes within the tropics, &c. They are most common in the Indian Ocean. The variable currents are those produced by local peculiarities in the tides, winds, melting of ice in the polar regions, and other such causes. Drift currents, due to the long- continued agency of the wind, afi*ect only a very trifling depth. Deep sea currents, as their name indicates, penetrate to great depths, namely, hundreds of fathoms below the surface. The currents of each ocean will be described in their proper place, under each ocean. (For situation and extent of the ocean, see 67.) The Atlantic Ocean. — ^We will notice this one first, not on account of its size but of its great importance to us. This ocean is calculated to drain more than 19,000,000 square miles of land. It is also distinguished from its fewness of islands but numerous gulfs and arms. Its average depth, as ascertained by the Challenger, is about 2,500 fathoms, or 15,000 feet, its greatest known depth being 3,916 fathoms, or 23,500 feet. In the northern part, near the middle, ifc is a flat plain, running north and south, the average depth of this plain being about 1,800 fathoms, though at each side, and south of the equator, it is, on an average, about 2,800 fathoms. Round the 90 PHYSIOGRAPHY. Britisli Isles the portions of this ocean are not very deep, no part of the German Ocean exceeding 70 fathoms ; and the deepest between I Fig. 9.— Section of Equatorial Atlantic. tSfRMlrffA Fig. 10.— Section of the North, Atlantic. Dover and Calais does not exceed 30 fathoms. The temperature of the upper layer, of about 100 fathoms deep, may be said to average between 75° and 80° in summer, ranging down to about 55° in winter. Below this depth th effect of the sun is not felt, sinking from 55° at 100 fathoms to about 35° at 2,000 fathoms. The S.W. Atlantic is still colder, the bottom water averaging from 31° to 33^°. The principal branches and inlets belonging to this ocean are the North Sea, Baltic Sea, Irish Sea, the English Channel, the Bay of Biscay, the Mediterranean, &c., on the east side ; Hudson Bay, Gulfs of Mexico and St. Lawrence, the Caribbean Sea, &c., on the west. (For its chief affluents see "Table of Rivers," page 99.) Among its currents the principal and better known are the Equatm^al, the i PHTSIOGEAPHT. 91 Gulf Stream, the Arctic, tlie Guinea, qmS. th.Q Brazil. The first, aa its name implies, occurs chiefly in the region of the equator, flowing along the western coast of Africa, and crossing the ocean towards the American continent. After traversing a little more than half way it divides into two branches, one going northward by the coast of Guiana into the Caribbean Sea, the other, the weaker part, travelling southward along the shores of Brazil. This equatorial current, from the coasts of Africa to the Caribbean Sea, is nearly 4,000 miles in length, and varies in breadth from 150 to 450 miles. Its velocity ranges from 18 to 30 miles a day at the surface, its rate decreasing lower down till at the bottom it never exceeds 12 miles. The Gulf Stream is by far the most important current of this ocean. It leavea the Gulf of Mexico, and travels through the Strait of Florida at a meati velocity of 46 miles a day, its average temperature being a little over 8C°. Flowing northward, almost parallel with th3 American coast, at latitude 40° north it turns to the east, touching the south banks of Newfoundland, and proceeds .across the Atlantic to the Azores, where it divides into two large branches, one going north-east and onward till it reaches the Arctic Ocean, and the other Bouth-east to the west coast of Africa. The entire length of this vast river, from the Gulf of Mexico to the Azores, is 3,000 mUes, and its greatest width about 112 miles. Its velocity gradually diminishes from 46 miles a day at starting to 18 at the Newfound- land banks, stUl getting weaker as it crosses the Atlantic and also decreasing in temperature, gradually diflusing its heat all around. The water of this stream is Salter than that of the common sea water, of a more bluish colour, and possesses very little affinity for the ordinary water ; even so that in many places you may see with the naked eye where, on either sides, it touches the neighboiu-ing water. It is owing to this stream that many places may enjoy a summer climate all the year round, as on the coast of France. England enjoys a temperate climate owing to the same cause ; while other places in a lower latitude experience much more severe winters, &c. (See "Climate" 116.) The Arctic current of cold water flows along the east coast of Greenland ; being met by another south of Cape Farewell from Davis Strait they flow on southward towards the Caribbean Sea. It divides on meeting the GuK Stream, part flowing to the Caribbean Sea, entering as an under current, the othfc^ ^jart, journeying south- west, forming the United States counter current. It is this current of cold water which replaces the warm water sent through the Gulf Stream, and also modifies the climate of all the United States coast- line, &c. The Pacific Oceak. — It is in this ocean that the greatest depth has been found in the late expedition, viz., north of Papua, in lat. 11° 23' N., long. 143° 16' E., the depth being 4,575 fathoms, or 93 PHYSIOGRAPHY. 27,450 feet, about 5-|- miles. The average depth of the South Pacific is about 2,000 fathoms, and that of the North Pacific about 3,000 fathoms, increasing gradually from south to north. The surface temperature of this ocean may be taken in summer at about 80° F, ; at a depth of 80 fathoms near Tahiti it is 77° R, but 500 fathoms deep the temperature is uniformly about 44° P. ; at a greater depth than this it falls gradually to 36°, and sometimes even to 32°. The chief branches and arms are : Behring Sea, the Gulf of Cali- fornia, and Bay of Panama on the east, and Sea of Japan, Yellow Sea, China Sea, &c,, on the western side. This ocean contains a vast number of islands, both of volcanic and coral formation. A glance at a map will be of more service than a written description of their situation. The chief current is the equatorial, which originates in the Antarctic drift current, flowing north-east till near the coast of Chili, where it divides, sending one branch round by Cape Horn, and forming the current of that name. The other part travels north- ward, forming the Peruvian current, which is so remarkable on account of its cold stream along the hot coast of Africa. On the Peruvian coast, in lat. 18° S., it has a temperature of 14° below that of the neighbouring ocean, and even at lat. 8° S., where it branches off to the west, joining the great equatorial current of the Pacific, its temperature is between 9° and 10° colder than the other water. The chief current now continues until it forms, northward of the Philippine Islands, the Japan current, or what we may term the Gulf Stream of the Pacific — it now performing similar duties to the one of the Atlantic. Travelling nearly to the Aleutian Isles, it sweeps round and returns to the equatorial current. The southern part of the equatorial current journeys along southwards to the coast of Australia under the name of New South Wales current, and then wends its way to the Antarctic Ocean. The Indian Ocean. — The greatest depth hitherto known in this ocean was 2,340 fathoms, or 14,040 feet, lying to the east of Ceylon, long, 85° ; but from recent occasional soundings the depth of more than 20,000 feet has been ascertained in the south-west portion of ithis sea. The principal branches and arms of this ocean are, the Arabian Sea, Persian Gulf, Gulf of Oman, Gulf of Aden, Eed Sea, Mosambique Channel, &c., on the west ; the Bay of Bengal, and the Great Australian Bight, &c., on the east. The equatorial current in this ocean travels westward from the Indian Archipelago and Aus- tralia to the east coast of Africa, where, divided by the island of Madagascar, one branch flowing round the north of the island, forming the Mozambique current, the other southward towards the Cape of Good Hope, they again unite, forming the Cape current, part of which flows northward into the Atlantic, while the chief part is deflected to the east by the Agulhas Bank, and flows to AustraKa. This current is of much importance to vessels on their PHYSIOGRAPHY. 93 journey to Australia, travelling as it does at the rate of about 4| miles per hour. The Arctic Ocean. — This ocean does not appear near so deej^ generally as those previously mentioned, especially in the higher latitudes. The temperature reaches in the lower parts, as low as 72"^ helow zero, but in the summer about 32° at the surface. The chief branches are the White Sea, the Gulfs of Kara, Obi, Yenisei and Behring Strait, in the Old World ; and Melville Sound, Barrow Strait, Baffin Bay, &c., in the New World. The comparatively warm Gulf Stream flowing into this ocean causes the colder water to flow off as the Arctic current. (See " Atlantic Ocean," 89.) The Antarctic Ocean. — This ocean has not been traversed as far as lat. 80° on account of its being much colder than in the Arctic Ocean ; but the part that is known is much deeper than the water at its antipodes, the greatest depth being 1,975 fathoms, or 11,850 feet. The temperature of the water is lower here than in any other ocean. The surface water in February, in latitude 5J° S., and longi- tude 79° 50' E., was about 28J° F. in the pack ice ; but a short distance from it the temperature was found to be 32° F., gradually sinking as the depth increased, till, at 40 fathoms deep, it was 29°, this tempera- ture remaining constant for 260 fathoms deeper, when it began to rise, reaching 32° or 33°. In this ocean several valuable ocean cur- rents have their origin. One of the chief is the Antarctic drift (which is described under the name of Equatorial current), in the Pacific. 88. Action of the Sea on the Earth's Crust.— The sea coasts are subject to the erosive and destroying movements of the ocean, especially breakers, which are aided in their action by the tides. These breakers, which are simply the wind waves (which see) formed on the surface of the sea during storms, acquire immense force, and break with violence on the rocky shores. One thing that assists marine denudation much, is the fact of large stones or any matter having from one-third to one-half of their weight balanced by the buoyancy of the water. Hence the breakers have only about half the work to do to remove the stones, &c., when in the water. The work of the sea on the coast is not that of carving or cutting out, but simply that of levelling. The rate at which the sea wastes the land depends on its nature, and also on the alterations of the rocks of the coast, aa well as on the force and direction of the currents. Thus on the east coast of this country, where the rocks are principally sands, clays, &c., the destruction is very great. On the coast of Yorkshire the waste is from one to four yards per annum. Again on the south coast we see several cHffs, headlands, &c., still standing contending against the action of the sea, while the soft sandstones have given way much more rapidly. Yet still, though 94 PHYSIOGrvAPHY. these cliifs, &c., are composed of much harder rocts, they cannot hold their ground, but slowly, yet surely, are compelled to retreat before the action of the waves, which scoo^J them out near the ordinary sea level, causing the weight of the overhanging portion to outbalance the cohesion of the rocks. The force of breakers in 1829 (November) washed about like pebbles blocks of limestone and granite, from 3 to 5 tons in weight each, near the breakwater at Plymouth, carrying 300 tons of them a distance of 200 feet. Another, 7 tons in weight, was washed 150 feet up the incUned plane of the breakwater. Striking examples of the sea's action may be noticed in the many rocks which stand out in the sea detached from the main mass of land, but which have evidently formed part of it, such as the Needles, off the Isle of Wight, and the Drongs of Shet- land. To the sea's action may be attributed the piling up of shingle beaches. The shingle is projected on the land beyond the reach of the retiring waves, forming in many places beaches several feet in height. Inland Seas. — The Mediterranean Sea, between Europe and Africa, has an area of about 950,000 square miles ; mean depth 1,200 fathoms, varying from 300 to 500 at the Strait of Gibraltar to 2,000 fathoms in the east. The average temperature is 60° to 70°. The Black Sea, including the Sea of Azov, has an area of 185,000 square miles ; depth about 50 fathoms. The Caspian Sea may be regarded as the greatest salt water lake in the world. It is situated 81 feet below the sea level ; its area is about 177,000 square miles, and the depth varies from 8 fathoms in the north to 450 in the south ; the mean temperature is about 56°. The Red Sea has an area of 175,000 square miles ; the average depth is 200 fathoms ; temperature, 96° to 106° F. The Baltic Sea has an area of about 162,000 square miles ; average depth about 800 feet ; average surface temperature 35° in the north and 45° in the south. WATERS OF THE LAND. 87. Springs, Rivers, Lakes, Sec— Springs are principally of three kinds — land, artesian, and mineral. The former occur in places where the bed of rock is pervious to water, being underlaid by a bed that is impervious, the rain sinking through the top layer, forming pools on the underlying impervious substratum, and when a well or a pond is dry the water collects in it. These depend almost entirely on the rainfall, and are also hable to be tainted with matter from the sewers, &c. The deep-seated are just the reverse of these, being but little influenced by summer droughts or winter rains, flowing steadily at aU times. Perennial spi'ings are those which flow year after year without any signs of abatement. PHYSIOGRAPHY. 95 Artesian, or Transtratic, Springs result from one permeable stratum lying between two impermeable strata, the water piercing into the rock where the layer is at or near the surface. It then ruTis through it in the direction of the slope of the layer until it has reached the surface in some other place. It will perhaps be better understood from the following diagram, where H B represents the permeable bed. Kg. 11. C and D the two impermeable beds which it lies between after leaving H ; then, the water soaking into the earth at H, wiU soak down as low as possible, namely to the bed D, and afterwards gradually flow through the pervious bed H B, imtil it reaches B, when it rushes out as a spring. It is this class of springs which gives rise to artesian wells, as, for instance, suppose, as at 0, the top layer be pierced down to the water-bearing strata, it is evident the water will rise at 0. Mineral or Deep-seated Springs usually contain gases, salts, minerals, acids, bitumen, organic matter, &c., examples of which occur at Cheltenham, Epsom, &c., where they are saline, containing salts. In Italy and France they are calcareous, containing much lime. In the latter country there are more than 900. They are termed chalybeate when iron is present, as at Tunbridge Wells, sulphurous when containing sulphur, and carbonated when containing carbonic acid. Germany and Spain have large numbers of these springs — about 600 in the former country and 400 in the latter. (See " Geysers and Hot Springs," 54.) 88. Rivers are of great importance in nature by carrying off the surplus water into the ocean, and also by giving rise to the most fertile parts of the coimtry They are also of importance in a com- mercial respect, and the benefits arising therefrom may be noticed in such places as London, Liverpool, &c. The basin of a river is the' whole tract drained by the river and its tributaries, the area of which may be found by drawing a line connecting the source of the river with the sources of all its tributaries. The elevated ridge which separates one river basin from another is termed the 2oatershed, or 96 PHTSIOGRAPHT. water parting J as it is now called, tlie former word being now used to denote the sides of the hills sloping from the ridge towards a river. To have a good knowledge of all the chief slopes, &c., the student should consult a good map of the Physical Features of the World — or better, one of each country. The area of the basin of each river may be traced on the map, as before mentioned. Eivers have their sources in springs, snow melting on the tops of mountains, glaciers, lakes at the base of mountains, &c. The importance of a river depends chiefly on the permanence of its volume, depth, velocity, nature of channel, accessible entrance, &c. The volume depends chiefly on the extent of country drained by its affluents, which in temperate zones is generally lessened in summer and increased in winter ; but, on the whole, the supply is pretty equable. In tropical countries, where the rain falls and snow melts at regular seasons, the rivers flow the country periodically. The velocity depends mainly on the slope and the nature of its channel, according to whether it is straight, deep, &c., or the reverse, and also upon the height of its source. The average slope is about 2 feet in a mile, or 1 in 2640. When it exceeds 1 in 250 it is unnavigable. A greater slope forms rapids, and a perpendicular descent a cascade or cataract. The most remarkable waterfalls are : — Total Height. Oreo Falls, at Monte Kosa (Alps) 2,400 feet. Gavarnie, in the Pyrenees 1,400 „ Staubach, Switzerland 1,000 „ Maaneloan, Norway 940 „ Victoria Falls, on the Zambesi, Africa 100 „ Murchison Falls, on the Nile, Africa ^ 120 „ Niagara Falls, on the River Niagara 160 „ Missouri (Great Falls) 75 „ Eiakan-f OS, near Christiania 900 „ The erosive and transporting powers of a river depend nearly entirely on the rapidity of the currents — those, for instance, which run down the mountain sides, having a great slope and a swift current, will cut out deep gorges and ravines at the bottom. It has been calculated that a velocity of 3 inches per second will lift up fine clay ; that 6 inches will hft fine sand ; 8 inches, course sand ; and 12 inches, gravel ; while a velocity of 24 inches per second will roll along rounded pebbles an inch in diameter ; and at three feet per second, angular stones of the size of an egg. A fine example of the erosive powers of rivers is exemplified in the river Niagara, where it is proved that the waters at the falls have cut back their passage about seven miles, forming a gorge 200 to 300 feet in height, and 600 to 1,200 feet in width, the average rate of recession being about 1 foot per annum. PHYSIOGKAPHY. 97 The amount of matter transported to the sea and other places by rivers is simply astounding. The slow rivers deposit a considerable portion in their course, as the Amazon, Ganges, &c., but the short and rapid ones carry it forward. The river Rhine, for example, carries past Bohn about 400 tons in one hour, or between 3 and 4 million tons in a year ; and the Ganges, during the 122 days of niiny season, caiTies 339,413,760 tons past Ghazepoor, 500 miles from the sea — large islands having been made in its channel even during a man's lifetime. The principal rock-constituents carried in suspension by rivers are quartz, or some very siliceous mineral, and, in solution, common salt (sodic chloride), sulphates of lime and magnesia, carbonate of soda, and carbonate of lime. The substances held in solution still remain so, causing the saltness of the ocean (see 83), while those in suspen- sion are deposited. 89. Deltas. — The detritus transported by the rivers at their mouths has a tendency, especially in tideless or nearly tideless seas, to form more or less extensive flat plains at the point where the waters have lost their transporting powers, mostly commencing at the centre of the river's mouth, forming- an island, which gradually widens and extends till a triangular space is formed by the deposit, the apex being directed up the river. The deltas of the Nile, Danube, Volga, Rhone, and Po are examples of those formed at the mouth of tideless rivers ; but one of the most remarkable is the delta of the Ganges, the base of the triangle being 200 miles, and its side 220 miles. The delta of the Mississippi covers an area of 15,000 square miles, but is divided into innumerable lakes, marshes, &c. It has been calculated that this river carries down to its mouth 28,188,803,892 cubic feet of sediment per annum, or one cubic mile in less than 5| years. 90. Eiver Systems.— The rivers of the world are all classed into four great systems. (1) Arctic; (2) Atlantic; (3) Pacific; (4) Indian. Thus, the Danube, Dneiper. &c., may be said to com- pose the Black Sea System ; but the Black Sea is only an arm of the Mediterranean, which in its turn is only an inlet of the Atlantic; hence the Danube, Dneiper, &c., belong to the Atlantic. There are a few, however, not in communication with the ocean, losing them- selves in sandy deserts, &c. These are termed Continental Systems, the most noted of which is that which empties itself in the Aral or Caspian Sea. It is estimated that the Atlantic drains 19,050,000 square miles ; the Arctic, 7,500,000 ; the Pacific, 8,660,000 ; the Indian, 6,300,000 ; and Contmental, 10,673,000. G 98 PnrSIOGRAPHT. si oa >> •^5 i! S^ P to o o B PL, fl ro North 1:S Europe. * /■ Asia. America. | o « c b,'-*- o ?^'§ c ^ § c ~ § - g g 0- 0- "5 S^ ct so ;h fi.S 1 3 1 !ds «9 o , o 000 1 ^^ * o 1 o 000 fcj:3 «5 00 'i* CO CO t- 1 g^ o ' (N c^ m" r-T ►^ : : : : 3 f •>: » c § & ^ ^ 1 g J g K^ ^ § « w s OS c !>| fl d f^ -2 .2 _t •s "o ^ +^ Ti ti -^ ^ f 7: ^ 2 ^ 1^2 ^ < < cq < -!l W H <1 g -V : : . ) K ,_, ! lit H c O i 1 1 3 1 c3 1 i 1 i 1 1 1 s § § M 'J 1 « -2 k; a i S S ^ 1 t3 5 1 < 1 1 1 m Moi Rocky Nortlie Rocky : U \ \ ^ : f-. « > _, .SI s G i :s s 1 I 5 S ^ i2 fi Ph 1 ;h >^ U i M § !>. >. • rH t, • • -tJ JJ • ; •C -S : & I Tcr Ter rica F3 I t-, >. 2 c3 rt g 6 : P5 « .^ ] ? 1 1 1 1 1 3 c 1 1 i 3 S '1 'i -i I :: I :; 2 ^ 2 ^ 2 r^ p I P 5 P H 5 2 a 2 y 2 in K M w PHISIOGEAPHY. 99 §3 pqS II Europe bo—" §1 ^ ^ ,a o ^ o fe PQ !2; I I 3 53 3 -jj -ij ^j :5 <1 <{ ^ 100 PHYSIOGRAPHY. (SS «.2 Asia. North America. 1 3 o o o o o o o •S o o o o o o <=> <= ti^zi o r- Ti Jt~ OO S^ 0. QQ o K J E^ a ' ^ ^ j? 1 1 O O s 1 > t It 4 4 = 3 >■ 6 >■ ^ ) C 5 Cq P< o c : . 1 1 ei != , 1 ? S o 1' ^ . c S e5 ^ E- e ^ ^ « P3 ^ > f i £ > c e3 m i ^ it & P " £ a 1 1 « 1 5 c < 1 1 IS 1 1 1 g i •s ■jj •S- o o c ■ 1 1 < J i ^ 1 1 ;! i si II PHYSIOGRAPHr. Asia. 101 102 PHYSIOGRAPHY; «3 1^ Europe. Asia. PHYSIOGRAPHY. 103 91. Lakes are large collections of water formed in the depressions of the earth's surface. They are generally classed into four or five kinds, but belonging to two natural divisions, viz., (1) Those which once formed part of the sea but have been cut off by the elevation of a portion of the sea bed, as the Caspian and Ai*al Seas ; and (2) Depressions in the surface of the land, which receive a portion of the drainage, as the great lakes of Ainerica, and most others, (a) Lakes that both receive and discharge rivers, which are in most cases only expansions of the stream. Their water is always fresh, examples of which are the great American and African lakes. These lakes are sometimes called lakes of transmission. (6) Lakes which receive no streams but discharge water by their own, the supply being kept up by springs. They are mostly small and fresh. The sotxrce of the Volga is a lake of this description in the Valdai Hills. They are termed lakes of emission, (c) Those that receive water but have no visible outlet, the waste by evaporation being supposed to be equal to the supply. The lakes of this description are generally large, and either salt or brackish, such as the Caspian, Aral, and Dead Seas, Lakes Balkash, Van, &c. {d) Those which neither receive rivers nor have any visible outlets. They are generally of small size, and occupy craters of extinct volcanoes, as Lake Albano, near Rome. Distribution and Areas of Lakes. — Lakes have a peculiar tendency to occur in groups. For instance, in North America there is the series consisting of Lakes Superior, 32,000 square miles ; Huron, 24,000 ; Michigan, 20,000 ; Erie, 9,600 ; Ontario, 6,300 ; the Great Bear, 19,000 ; Great Slave, 12,000 ; Winnipeg, 9,000 ; Winnipegosis, 3,000'; Athabasca, 3,000 ; Manitoba, 2,100 ; Deer, 2,400 ; Wollaston, 1,900. In Asia they occur as three distinct groups. Many of the lakes in this country are salt or brackish, the cluef of which are the Caspian (inland sea) ; Aral, 26,000 square miles ; Balkash, 7,000 ; Urumiah, 1,800 (26-24 percent saline matter) ; Van, 1,600 ; Tongri- nor, 1,800 ; Koko-nor, 1,500 ; Lob, 1,300 ; Dead Sea, &c. Of the fresh water lakes the principal are Baikal (Holy Sea of the Russians) in Siberian Tartary, 14,800 square miles ; Tong-Ting, 2,000 ; Booka- nor, in Thibet, 1,000 ; Zaizan, 1,000, &c. In Africa the principal axe the Victoria and Albert Nyanza, Tanganyika, Dembea (1,400). In Europe the principal lakes occur as a group in Finland and Russia. They are Ladoga, 6,633 square miles ; Onega, 3,280 ; Saima, 2,000 ; Peipns, 1,250 ; Enara, 1,200— which all occur in a circumscribed area. Others occur in Sweden, namely, Wener, 2,130 square miles ; Wetter, 840 ; Maelar, 760, &c. Small lakes occur abundantly in the mountain regions, and are remarkable for their lovely scenery, examples of which are our own — namely, those in Scotland, the north of England, the south of Ireland— and the much greater ones of the Alps, The chief of the Alpine lakes are 104 PHYSIOGRAPHY. Geneva, 240 square miles ; Constance, 228 ; Garda, 182 : Maggiore 150. Neufchatel, 114 square miles ; Lucerne, 98 square miles ; Zuricli, 74 square miles, &c. Those in the British Isles are Loughs Neagh, 150 square miles ; Corrib, 63 square miles ; Erne, 56 square miles, &c., in Ireland. Loch Lomond, 40 square miles, &c., in Scotland. Windermere, 10 square miles, &c., in the Cumbrian Hills. The most elevated lake is Sir-i-kol, 15,600 feet, in Central Asia ; Tuz-Gul, in Asia Minor, the saltest known, containing 32 per cent of saline matter ; Urumiah, 26 '24 per cent ; the Dead Sea, 24 per cent ; and Lake Elton, situated in the Steppe, 70 miles E. of the Volga. The uses of lakes are, (1) Acting as regulators to rivers, and pre- venting the too rapid flow of the waters during excessive raiufaUs ; (2) Acting as settling ponds for the water to purify in ; (3) Serving to temper the climate, especially when occuriug in the central part of the continent. THE ATMOSPHERE. 92. Compositon of the Atmosphere. — The envelope of gaseous matter which surrounds our earth is called the atmosphere''' or atmospheric air. It is a mixture (not a chemical compound, each gas still possessing its own peculiar property) of oxygen and nitrogen, contaminated with a very small, but variable, proportion of carbonic acid and water in the state of vapour. It consists by weight of one atom of oxygen to two of nitrogen, and by volume of one part oxygen to four nitrogen, nearly ; or, more exactly, its percentage is 79'5 nitrogen (N), 20 oxygen (0), "45 aqueous vapour (HgO), and '05 carbonic acid (COo). Though nitrogen forms by far the greater pa,rt of the air, oxygen is the most important, as without it no animal could live or fire burn, for it is this gas that purifies the blood, keeps \ip its natural warmth, and is the supporter of flame. Nitrogen may be termed the agent by which oxygen is held in check by diluting it. The air in its diluted state forms the food upon which the vitality of both animals and plants is sustained ; but, while it is inhaled by both in breathing, animals exhale carbonic sicid, and carbonic acid is inhaled and assimilated by plants, which in their turn exhale oxygen, thereby keeping up the equilibrium or balance. Though, as above stated, the air is not a chemical compound, but simply a mixture, yet it has much permanence of character, its two chief elements being found nearly in the same proportion in all cHmates and altitudes. * AtTMS, vapour. Sjphaira, a sphere. PHYSIOGRAPHY. 1C5 93. Pressure of the Atmosphere.— Air, like other perfect gas, exerts an expansive force directly proportional to its density. This force is measured by means of the barometer, the pressure per square inch being equal in weight to a column of mercury supported in a barometric tube, the area of a section of which is one square inch, the mean height of which at the level of the sea is 30 inches, and its weight about 14|lb. (147304) avoirdupois, which is, there- fore, the mean pressure of the atmosphere per square inch. The pressure and density of the air are regulated by the following law : At the same temperature the elastic forces, or pressure, of two portions of air are in direct proportion to the densities, or in inverse propor- tion to the spaces occupied by these portions. The atmosphere is much denser near the level of the sea than at some distance above it, owing to its being confined to the surface of the earth by gravitation ; or, in other words, being pressed down by the weight of that which is above. The density rapidly decreases the higher we ascend. For instance, at an elevation of three miles it is one-half the density it is at the earth's surface ; at six miles it is one-fourth, at nine miles one-eighth, at twelve miles one-sixteenth, and so on, the density decreasing in geometrical 'progression as the height increases in arithmetical progression. The pressure of the atmosphere is exerted on all substances, both internally and externally. The body of a man of ordinary stature has an area of about 2,000 cubic inches, hence the pressure on the body will be 147304 x 2,000 = 29,460-81b. But how is it, it may be asked, that this immense weight does not crush the man ? The answer is that the air within the body and its pores counterbalances the weight of the external air. On every square mile the pressure, or weight, is 26,345,088 tons ; so that the total pressure on the earth's surface is 5,189,982,336 million tons. 94. Height of the Atmosphere.— From the law given above, regarding the density of the air the higher we ascend, it is evident that the greater part of the atmosphere is always within 32 miles of the surface of the earth, but how far it really extends is extremely tincertain, though it may with safety be afiirmed to reach the height of 45 miles, where it is about 25,000 times rarer than at the level of the sea. Even at the height of &ix miles the air is so rare that man can hardly breathe. Though the greater part of the atmosphere is always within 30 miles or so from the surface of the earth, yet it is very probable that even at the height of 250 miles a very rare atmosphere exists. Not only does the pressure decrease the higher we ascend, but also the temperature, though not at such a constant ra+e. The atmosphere being the medium throvigh which the sun's heat is con- veyed to and from the earth, the lower and denser strata or layers 106 PHYSIOGRAPHY. absorb the greatest amount, and are necessarily the warmer. In ascending mountains the decrease of temperature averages about 1° F. for every 300 feet. The averages, according to Mr. Glaisher, in his balloon ascents made in this country, when the sky was partially clear, are as under : — Number of feet Elation to feet. TSSSn'" of r F. to 1,000 140 1,000 „ 2,000 190 2,000 „ 3,000 220 3,000 „ 4,000 290 4,000 „ 5.000 370 5,000 „ 10,000 370 10,000 „ 20,000 470 20,000 „ 30,000 820 95. Weight of Air. — That air has weight may be easily proved in the following manner : Take a vessel measuring 10 inches in length, breadth, and height, and provided with a stop-cock. Let its weight be accurately determined. Exhaust the air from it by means of an air pump, turning the stop-cock immediately ; then, on weighing again, it will be found to have lost 310 grains, which is evidently the weight of the air contained in the vessel — namely, 10^ = 1,000 cubic inches ; or 1 cubic foot of dry air, at 60° F., weighs about 535 grains. 96. The Barometer is an instrument to measure the weight or pressure of the atmosphere, and thereby to indicate the variations of the weather. It consists of a cylinder or tube, from which the air has been exhausted, closed at one end, the open end being inserted in a cup of mercury, on which the ]pressure of the atmosphere is exerted, forcing the mercury up the tube (there being no pressure there) until it reaches the height of 29*94 inches, when the weight of the column of mercury equals the pressure or weight of the atmosphere on the surface outside the tube. Supposing water to be used instead of mercury, the column would be about 33-8 feet, or 13'568 times the height of the mercury, through the latter being that many times heavier than water. As the air varies in weight or pressure it is evident that it will influence the mercury in the tube, which will rise and fall in exact proportion with the pressure. The column of mercury in the barometer undergoes several regular variations in the course of the day, which are termed liorary 'variations. According to Humboldt, the maximum elevation at the equator takes place about nine o'clock in the morning, after which hour it becomes less until about four or half-past in the afternoon, attaining its minimum. It again ascends until eleven at night, PHYSIOGRAPHY. 107 when it attains its second maximum, once more descends till four, reaching its second minimum, and then starts on its journey back until nine. It has also been noticed that the elevation of the mercury at noon corresponds almost exactly with the mean diurnal height. These variations are due to the action of the sun's rays upon the air and vapour atmosphere. The mean pressure of the atmosphere is also subject to an annual oscillation, the amount of which, except for some particular places, has not yet been ascertained. GENERAL RULES PROGNOSTICATING CHANGES IN THE WEATHER BY MEANS OP THE BAROMETER. 97. The following rules, which are without doubt the best yet given by any authority, are those of Patrick. (1) The rising of the mercury presages, in general, fair weather, and its falling the contrary, as rain, snow, high winds, and storms. (2) In very hot weather the falling of the mercury indicates thunder. (3) In winter the rising presages frost ; and in frosty weather, if the mercury falls three or four divisions (tenth of an inch), there will certainly follow a thaw ; but in a continual frost, if the mercury rises there wiU be snow. (4) When foul weather happens soon after the falling of the mercury, expect but little of it ; and, on the other hand, little fair weather may be expected when it becomes quickly fair after the rising of the mercury. (5) In foul weather, when the mercury rises much and high, and so continues for two or three days before the foul weather has gone away, then a continuance of fair weather may be expected to follow. (6) In fair weather, when the mercury falls much and low, and thus continues for two or three days before the rain comes, then a great deal of wet may be expected to follow. (7) The unsettled motion, or frequent rising and falling, of the mercury denotes changeable weather. (8) The chords on the scale are not so strictly to be observed as the rising and falling of the mercxu-y, for if it stands at much rain and then rises to changeable it presages fair weather, though not to continue so long as though the mercury had risen higher ; and so, on the contrary, if the mercury stands at fair and then falls to changeable it presages foul weather, though not so much as if it had sunk lower. 98. The Barometer is also useful for measuring the heights of mountains or the height of the country above the sea level, &c., though for this purpose very accurate instruments are required, such as Roughton's portable ones. As we ascend from the level siu-face of the earth the column of atmosphere pressing on the mercury becomes lighter by the removal of the subjacent stratum, hence the fluid falls in the tube of the barometer. For ordinary purposes the following very simple rule by .Sir John Leslie is 108 PHTSIOGRAPHY. sufficiently accurate : Mark the height of the mercury in the barometer at the bottom of the height to be measured, and also at the top ; then, as the sum of the heights of the mercury at the bottom and top stations is to their difference so is 52,000 to the height to be measured in feet. Another rough rule is — Multiply the difference of the logarithms of the barometric heights by 1,000, and the difference of the levels will be obtained in fathoms. Generally for every inch that the mercury falls we may reckon 992 feet. The reverse to finding the heights of mountains, &c. — namely, estimating the depth of pits — may be found with equal facility by the aid of the barometer. For instance, at a depth of 15,000 feet it stood at 32 '28 inches, while one at the surface stood at 30'518. 99. The Atmosphere in Relation with Light.— The atmosphere is the medium by which the sun's heat and light are conveyed to this earth. Each ray proceeding from the sun consists of two distinct parts — the one producing light, the other heat. Heat and light are alike indispensable to plants and animals, and are so reflected (turned back) and diffused by the atmosphere as to become most available to vegetable and animal life. White light is a combination of red, orange, yellow, green, blue, indigo, and violet rays. Vapour, whether in the atmosphere or sea,, absorhs all the coloured rays except blue ; hence the colour of the ocean and sky, the sky always appearing of a much darker colour than the tops of high mountains. Not only is light absorbed by the atmosphere, but reflected, refracted, and diffused or dispersed. As soon as the rays meet the earth's atmosphere a portion is reflected, the remaining portion being refracted or bent on entering the air. Some of the portions, in their journey to the earth, are absorbed, while others are dispersed^ thereby causing each ray to illuminate a much greater space than if there had been no atmosphere to have disturbed its course, so that valleys and surfaces not directly exposed to the sun are lighted. If the rays fall vertically, only eight out of every ten reach the earth's surface. The greater the angle of inclination the greater the number absorbed. When the sun is rising or setting we can gaze on it with ease, owing to this ; as, for instance, when it is horizontal, only five rays out of every 10,000 reach the eye of the observer. It is to the refraction of the sun that we owe that dim light called twilight, as when the sun is from 15° to 20° heloio the horizon the rays strike the atmosphere, or clouds, and are bent doion towards the earth, producing a little light. Within the tropics the sun sets more perpendicularly, hence, speedily. In higher latitudes it sets more obliquely, taking a longer time to reach 20°, and thereby causing the longer twilight of those regions. 100. Rainbows.— The semi-circular band or arc, composed of dif- ferent colours, appearing upon the clouds during the occurrence of rain PHYSIOGRAPHY. 103 in sunshiue, whicli we term a rainbow, is caused by the refraction and reflection of the solar rays in the drops of falling rain. It can be only witnessed when the sun is in a certain altitude above the horizon — namely, 42° 30' — and when the rain is falling between the observer and the part of the sky opposite the sun. The same pheoomenon may be witnessed in the case of the spray of cascades and loaterfalls. There are sometimes two hoivs to be seen, namely, when the light is intense, or being sufficiently low in the sky, a second is formed on the outside of the first or primary one, by the solar ray entering near the bottom of the drops. The rays undergo two refractions and two reflections in passing through the drop. The colours in the second or secondary one always appear fainter than in the first. The Mirage. — The unusual elevation of islands, ships, &c., above the surface of the sea, is due to refraction, and occurs when the atmosphere is warmer than the surface of the sea. 101. Temperature is the actual state of a body at any moment, determined by a comparison of its magnitude with the heat to which it is exposed. A change in temperature is a change in magni- tude which the body suffers in the heat to which it is exposed. The intensity of heat is measured by an instrument ttrmedi a. thermometer.* This consists of a glass tube, hermetically sealed at one end, with a bulb at the other containing mercury, which was introduced at a certain temperature through the open end, then heated to drive off all the air, and afterwards sealed that no air could get in. This tube is generally fixed to a hard piece of wood or ivory, on which a scale is engraved. This scale has been previously obtained by immersing the thermometer in an upright position in melted snow or pounded ice, for about half-an-hour, until the mercury has ceased to fall, the height of which is then marked on the tube. This is the freezing point. The boiling point is obtained by placing the thermometer vertically in the steam of pure water until the mercury has ceased to rise, when a mark is instantly made. The distance between these two marks is then divided either into 212, 100, or 80 divisions, according v,'hether it is to be a Fahrenheit, Centigrade, or Reaumur, The reason mercury is chiefly used in the ther- mometer is because of its nearly uniform expansibility under a considerable range of temperature. When very low temperatures are investigated coloured alcohol is used, it being more able to resist congelation than mercury or any other known fluid. When the temperature of bodies is raised they, with few excep- tions, increase in bulk, this increase arising from the repulsive power of heat. Gaseous bodies expand equally for equal increments of temperature. Thus 1,000 parts of air at the freezing point (32° F.) are increased to 1,365 at the boiling point (212° K.) Mer- * Thermo, heat, Mecron, a measura. 110 PHYSIOGRAPHY. cury in the same range increases from 1 to 1"0019, water to 1-0046 and alcohol to 1 •0011. It is this expansion and contraction of the mercury or alcohol in the thermometers which measures the increase and decrease in temperature. There are three kinds of graduation. That of Fahrenheit takes 0° (zero), 32° below freezing point, the boiling point of water being 212°. In the Centigrade the freezing point is 0°, and the boiling point of water ] 00°. In Reaumur they are respectively 0° and 80°, In each below 0° is counted with the minus sign. Rules. — To convert the degrees of Fahrenheit into those of Reaumur : Multiply the niimher of degrees, less 32, ly 4, and divide by 9. To convert the degrees of Reaumur into those of Fahrenheit : Multiply the given temperature hy 9, divide by 4, and add 32. To convert the degrees of Fahrenheit into their Centigrade equivalent : Multiply the number of degrees, less 32, by 5, and divide by 9. To convert Centigrade degrees into those of Fahrenheit : Midtiply by 9, divide hy 5, and add 32. 102. Heat. — As the atmosphere interferes with the light- producing rays, so it does with the rays which produce heat. Though the air itself is transparent to heat, the moisture which it contains is not, being opaque to it, preventing a large quantity of it from passing through to the earth. For it must be remembered, that whatever the temperature of the air, it is constantly receiviug moisture from the surface of the land and water, thereby causing what we might nearly term a second atmosphere. (See "Vapour.") The heat that falls on the surface of the earth is partly absorbed and partly radiated into the atmosphere. The air through which the heat passes is not sensibly heated by the passing of the rays of heat, but by conduction — namely, the warmth of the heated earth is commu- nicated to the air, the air in its turn communicates heat to the over- lying strata of air, and so on. Hence each layer of the atmosphere must be cooler than the underlying one, as it would be impossible for the earth to make the nearest layer of air as hot as itself, and for the nearest layer to make the next stratum of the same heat as itself, &c. (For "Average Decrease of Temperature" see 94.) 103. Winds, or Air in Motion.— Wind is air in motion. As there are currents in the ocean, so there are currents in the atmos- phere, the cause of which is the unequal distribution of pressure in the atmosphere, owing to the unequal distribution of heat and vapour. When any portion of the air is heated it expands, and loses its specific gravity, thereby causing it to ascend, whereupon a current of colder air rushes in to supply the vacancy and to restore equilibrium. In this way winds are produced. One of the most notable characteristics of winds is their velocity, which varies from a few miles to more than a hundred miles an hour — that is, from the gentlest zephyr to the most violent hurricane. When it is moving at the speed of 7 PHYSIOGRAPHY. Ill miles an hour it is called a gentle air ; of 14, a ligJit breeze; of 21, a good sailing ireeze ; of 41, a gale; of 61, a, heavy storm; of 82, a tempest; of 92, a humcane ; and of 100, a violent hurricane, with a force sufficient to blow down buildings, tear up trees, &c. The following table will give an idea of the relation between the* velocity, force, and character of the wind more minutely : — Velocity in mUes Force in lb. Avoid.* Common name, per hour, per square foot. 1 -005 Breath of air. 5 -123 Gentle air. 10 -496 Brisk wind. 15 I'll Light breeze. 20 ,. 1-98 Brisk breeze. 30 4-5 High wind. 35 6 Gale. 40 7-9 Strong gale, 50 12-5 Storm. 60 17'75 Great storm. 80 31 '5 Hurricane or tempest. 100 49"5 Violent hurricane. Winds are classified into three classes, viz., constant, periodical, and variable ; but, in whatever class or character they occur, they are important agents in the modification and production of climate. The most remarkable of the constant or permanent air currents ars^ the trade winds and the polar winds. 104. The Trade Winds, so called from their influence on the trade and commerce of the world, are those which prevail within the tropics, extending from the parallel of about 30° north and south nearly to the equator, forming two zones of perpetual winds, the one in the Northern Hemisphere blowing from the TioHh-east, and that in the Southern Hemisphere from the south-east. This zone, being the highest in temperature, causes the heated or rarified air to ascend and flow off as upper currents, travelling towards the poles, whilst the colder air from the temperate zones rushes in as under cun^ents to supply its place. If it were not for the earth's rotation on its axis this colder air would come exactly from the north and Bouth ; but owing to the earth's revolution from west to east, and that places near the equator move at a much more rapid rate than those in the temperate or arctic regions, the air current cannot acquire all at once the velocity of that part of the earth over which it is advancing, hence it is necessarily left somewhat in the rear ; and as it is struck by the objects in that zone with a certain force it is deflected in a westerly direction, thereby becoming a north-east wind in the Northern Hemisphere and a south-east wind in the Southern. 112 PHYSIOGKAPZr. This deflection is caused by the two motions which influence the air, namely, (1) a northerly or southerly motion, caused by its ten- dency to rush to the equator to supply the place vacated' by the heated air, and (2) an easterly motion resulting from the earth's rotation. But (Art. 7) the air will not obey either motions or forces, taking an intermediate course, namely, in the direction of the diagonal of a parallelogram, the sides of which represent the mag- nitude and direction of the two forces. In the Pacific the north-east trade wind may be said to range between the 9th and 25th degrees of north latitude, and the south- east one ranges between the 10th and 21st of south latitude. In the Atlantic the former is comprised between the 30th and 8th degrees of north latitude, and the latter within the 3rd of north and the 2Sth of south latitude. These limits, however, vary, advancing with the sun. Thus at the summer solstice the region of the winds is entirely carried north of the equator, and at the winter solstice ifc is carried considerably south, but not entirely passing the equator. The reason it goes farther north than south is owing to the greater quantity of land in the Northern Hemisphere. Of the two the trade wind in the Southern Hemisphere is the stronger and more constant. Their regular rate is from 10 to 20 miles per hour, but on approaching the continents their courses are interrupted by the unequal heating of the land and water surfaces. Hence within these coast areas, instead of currents being perennial, they assume a periodical character, and as they approach the equator of tempera- ture their currents begin to abate, thereby producing the region or 'belt of calms, as they are termed. There are also other belts of calms, each formed where the winds cross, as the belt of calms of Cancer (sometimes called the horse latitudes) and the belt of calms of Capricorn, when the trades and anti-trades, as these westerly winds are called, interchange. 105. Periodical Winds. — The most important of this descrip- tion are — (1) The monsoons, which are modifications of the trade winds (or trades as they are called), being due to the presence of vast masses of territory. They are termed monsoons, or season winds, because they change their course with the seasons, blowing from one part of the earth for one-half of the year, and from the opposite part for the other half. From April to October they blow from the south-west, and from October to April from the north-east. They prevail chiefly in the Indian Ocean, extending to the north and east of Australia into about 14° west longitude. When the sun is north of the equator the large continents of India and China are heated to a very great extent, heating the surrounding air, which rises, and the south-east trade rushes in to fill the vacant space ; bub PHYSIOGRAPHY. 113 owing to the rotation of the earth and other local causes, it is deflected, becoming the south-west, south, south-east, or east monsoon on different parts of the coast. Similarly, when the sun journeys on to the Southern Hemisphere, the wind follows, and causes what is called the south-east monsoon, blowing from October to April, though this is really the ordinary north-east trade wind. At the changes from one to the other in April or October — a period known among mariners as the hreakirvg up of the monsoons — furious storms of wind, rain, and thunder occur, owing to the two opposite air currents contending for the mastery. There are other parts in which the monsoons occur, as on the West Coast of Africa, the coast of Brazil, the coast of America from California to about 45° south latitude, &c. (2) Land and Sea Breezes prevail on almost every seaboard, but more particularly in the tropics, where they occur regularly. The land breeze sets in dnring the night, and the sea breeze during the day. They owe their origin to the unequal temperature of sea and land by night and day. Thus, during the night the land loses its heat by radiation more rapidly than the sea ; hence the cool air from the land flows to the sea, to take the place of the warmer air and thus forms the land breeze. In the daytime just the opposite occurs, as the cool air flows from the sea towards the land and creates the sea breeze. To many islands they are of great import- ance, as by their influence they are kept cool and inhabitable. 106. Variable Winds. — With the exception of the winds above enumerated there are many which are very variable, still they are obedient to law and law-directed forces ; but these forces being so comphcated they are not, comparatively speaking, so well under- stood. There are two winds in the Northern Hemisphere which may be considered the prevailing winds — namely, that of the north- east, being the cold polar one on its journey to the pqiiator, and that of the south-west, being the warm equatorial currents hurrying to the poles ; and similar in the Southern Hemisphere. All other winds are local, depending on local circumstances, a few of which we will just briefly mention. Hot Winds. — The chief of these are those that originate in the Great Desert of Sahara, and they go by several names, but are generally called the simoom.* In Turkey they are called samiel; in Egypt, khansin (fifty) ; in Italy, sirocco ; in Spain, the solano ; and in Guinea and Senegambia, harmattan. The simoom is an intensely hot, suffocating wind, laden with fine particles of sand, often causing destruction to the whole caravan of men and animals. The only way to escape its effects is to lie prostrate on the ground, with the face buried in the sand, till the violence ♦Arabic, hot, ;piisonous. 114 PHYSIOGRAPHY. of the blast is passed. The fohn is the name given to the hot winds that occasionally blow over Switzerland. There are other winds just the reverse of these — namely, cold piercing ones — such as the puna, pampero, lora, mistral, &c. The puna is so called from originating in the upland of Puna, sweeping over the Plateau of Peru for about one-third of the year. The pampero is a violent west wind, which passes over the pampas of Buenos Ayres. • The iora, blowing north-east from the Alps, in Istria and Dalmatia, is at times very violent indeed, overturning both men and horses at the plough. The mistral is a violent north- west wind, blowing down the Gulf of Lyons, acd felt chiefly in the south-east of France. The etesian* winds are those which prevail very much in early summer all over Europe. 107. Storms are sudden and violent commotions of the atmo- sphere. All great storms are found to partake of a circular motion, though the diameter or whirl is often hundreds of miles in extent. Those needed to be mentioned here are cyclones, typhoons, tornadoes, hurricanes, and whirlwinds. Cyclone is the name applied by navigators to those rotatory hurricanes which most frequently occur between the equator and the tropics, and near the calms of Cancer and Capricorn. They sweep round and round with a progressive motion, describing a curve, rotating in both hemispheres in a contrary direction to the sun. In different regions they are known as tornadoes, whirlwinds, typhoons, and hurricanes, being called hurricanes or tornadoes in the West Indies, where they are most frequent about the time of the equinoxes, and also in the Indian Ocean, extending from Madagascar nearly to Australia, and from 4° to 35° south latitude, occurring here at the change of the monsoons. In the Chinese region they occur about once in three years, generally from June to November, extending from the Ganges to Japan, and from latitude 5° N. to 25° jST. They are here called typhoons. The tornadoes of the West Indies and the Indian Ocean are generally accompanied with thimder and lightning, and sometimes showers of hail. It is worthy of note that in the tropical climates the barometer indicates the approach of a hurricane, the mercury in the tube being depressed or agitated in an extraordinary manner for some time before any signs of a storm appear in the horizon ; also during the first half of the storm the barometer falls, and during the last half it rises. This is owing to the density of the air increasing from the centre to the circumference of the storm, so that when a hurricane passed diametrically over a district the pressure would decrease, but gradually increase during the last half of the storm. The direction the centre of the storm lies may be always found by •Greek, er^atos, annual. PHYSIOGRAPHY. 115 standing with, the hack to the -wind, then in the Northern Hemisphere the centre is towards the left hand, and in the Southern Hemisj)here to the right hand. This rule is of importance to mariners, showing them how to steer so as to get out of the cyclone. (For " Thunder," and "Magnetic Storms," see 30, 31, 35.) VAPOUR, EVAPORATION, AND CONDENSATION. 108. Evaporation is the conversion of a fluid into vapour. It is produced by the solar heat, which raises water into the atmosphere in an invisible form. Familiar examples of this may be noticed in the drying of wet clothes which are hung out in some open place, and watered streets beginning to dry nearly as soon as watered, &c. At all temperatures water evaporates, even if it is ice. The rate at which evaporation takes place depends upon (1) the temperature, the higher the temperature the greater the evaporation ; (2) the amount of vapour in the atmosphere, as a certain quantity of air can only receive a certain quantity of vapour, after which it is said to be saturated, or to have reached the point of saturation ; though it must be remembered that the higher the temperature the more vapour will the air be able to contain before reaching this point of satura- tion ; hence the amount of vapour in the air depends solely on its temperature. From this we see that the evaporation is greatest in the torrid zone, it being estimated that on an average 16 feet depth of water is raised annually from the surface of the sea in these parts. 109. Condensation. — Dew. — As heat evaporates water and causes it to ascend as vapour, cold, or a decrease in temperature, causes it to condense, forming dew, clouds, rain, hail, snow, &c. Thus, after sunset the earth and air lose the heat they have received from the sun during the day by radiation into space ; but the earth, being a good radiator, parts with it more rapidly, thereby causing the moisture in the air to be condensed on the earth's cool surface, forming dew. The temperature at tvhich dew begins to he deposited is termed the dew-point, and it varies according to the saturation of the air. It may be noticed that some substances have dew on them while others remain dry. This is owing to the latter being bad radiators and the former good ones. Dew is never deposited in dull cloudy weather. The conditions favourable for its formation are an unclouded sky and a calm night preceded by a warm dry day. On a windy night the radiation would be disturbed, and thereby cause the moisture to be evaporated as soon as formed. Tropical countries favour these conditions, hence the deposition of dew is most copious there, compensating in a great measure for the absence of rain in these parts. In the British Isles dew is heaviest in spring and autumn. 116 PHYSIOGRAPHY. Hoar Frost is simply dew in its frozen state. The chilling effect of radiation reaches but a short distance above the ground, as at two or three inches above the ground it is only about one-half, and at six feet only about one-twentieth. Generally the body itself is about 4° below the temperature of the air just above it. Fog and Mist. — Fogs result from currents of moist air coming in contact with the colder surface of the earth, the moisture being condensed into the visible form of fog or mist appearing near the surface of the earth. Mountain sides, river valleys, sea coasts, and cold countries favour the formation of fogs, owing to the unequal temperature of the contiguous lands and waters. According to Kaemtz, fogs may be expected frequently where the soU is moist and hot and the air moist and cold. Those occurring in Newfoundland are occasioned by the warm waters of the Gulf Stream being con- densed by the cold air of the arctic current. 110. Clouds are masses of aqueous vapour in a partially con- densed state, differing from fogs in being condensed at a greater elevation, though they are not so high as they appear, as a traveller, , for instance, on a mountain may often see clouds floating beneath him. The average height of the clouds is between one and two miles ; streaky-curling ones, like hair, are often five or six miles high, the nearest to the earth being those highly electrified. In this covmtry the height is from 2,000 to 6,500 feet, with a thickness of from 2,000 to 3,000 feet. The speed at which clouds move is much greater than appears, owing to their height, as they often move at the rate of from 70 to 100 miles an hour. Classification. — Clouds are grouped into seven classes — there original, and four compound forms arising from combinations of the others. They are : — Original. — (1) Cirrus, or curl cloud. (2) Cumulus, or summer cloud. (3) Stratus, or fall cloud. Compound. — (4) Cirro-cumulus. (5) Cirro-stratus. (6) Cumulo- stratus. (7) Nimbus, or rain cloud. (1) Cirrus clouds appear like fibres, loose hair, or thin streaks. They are the most elevated of all, being not less than three, and often reaching six, and even ten miles in height. They have a tendency to arrange themselves in parallel or divergent bands. At the equator these extend from north to south, but in higher latitudes, as in this country, they stretch from north-east to south-west, varying from that to north-west and south-east. They are supposed to consist of vapour below the freezing point, as minute ice crystals, or pure snow flakes. (2) Cum.ulus clouds appear in great masses, like volumes of smoke. They are formed after sunrise, gradually increasing and ascending higher as the day advances, disappearing towards evening. If they PHYSIOGRAPHY. 117 increase in size at sunset a thunderstorm may be expected in the night. (3) Stratus (or FaM Cloud) is a kind of heavy layer of mist or vapour, especially prevailing on a summer evening, rising at sunset or nightfall in low damp places, and vanishing at the approach of day. This is the nearest to the earth of all the clouds, and is closely allied to fogs and dew, being formed by the vapours rising from the earth as it cools by night. The cirro-cmnulus, cirro-stratus, and cumulo-stratus, as their names signify, are merely combinations of the original three. The Nimbus (or rain cloud) is of a greyish appearance, with fringed edges. It is formed from any of the others, except the cirrus, and is always low down, mostly between 1,000 and 4,000 feet from the earth. 111. Kain is vapour condensed in the air and precipitated to th? earth in showers, the condensation being caused by a considerable diminution in temperature. As long as a cloud remains where the temperature is sufficiently high it is capable of containing its mois- ture, but should it get carried by the wind into a cooler region it is unable to do so. Condensation then sets in, and the several vesicles of vapour uniting cause the weight to become too great to be supported by the air. Hence the drop thus formed falls to the groimd. Rainfall. — By the time the north-east and south-east trades meet, producing the equatorial calms, the air is heavily laden with vapour. Having travelled in each hemisphere over a very large space of the ocean, it now ascends, expanding and.- becoming cooler, part of the vapour thus condensed coming down as rain. It is thus that we have a region of constant rain at these calms. The nearer we approach the poles the less rain descends. The quantity falling in any cotmtry depends on its nearness to the ocean or other large bodies of water, upon the temperature, upon the seasons, and upon the direction of the prevailing winds. The coasts of a country, or those coimtries where the winds blow chiefly from the sea, receive a greater proportion of rain than the interior. Chains of hills also affect the air much by coming in the way of the winds and causing them to descend. Hence the amount of rainfall depends in a great measure upon whether the country is moimtainous or not, and also on the direction of the mountains. Thus, for instance, the most remarkable rainfall in the world occurs at Cherrapoonjee, in the Khasyah Mountains, its annual average being 499' 3 inches, though in some years its rainfall has exceeded 600 inches. This extraordinary downfall is attributed to the abruptness of the mountains which face the Bay of Bengal Another good example is furnished even in our own country, where the warm winds of the Atlantic meet the hills and mountains of the west side, especially in the Cumbrian Hills, where — 118 PHYSIOGRAPHT. namely, at Scathwaite — the average is 145*1 inches, the west side entirely averaging 45 inches, and the east coast only 27 inches. A similar example is to be found on the west coast of Ireland. The mean annual average for Great Britain is 34 inches, and England a little over 30 inches. The reason that the east side does not receive so much rain as the west is self evident, namely, that the mountains on the latter side condense the warm moist winds of the Atlantic, partly preventing their passage over, rain coming down in torrents, and the air which does reach the other side is thus comparatively dry. In Guiana, Brazil, and North and West India, the average rainfall exceeds 300 inches. When it is stated, as above, that the mean annual rainfall of a place is 30 inches, it implies that the amount of rain that falls in the course of a year would on an average cover the ground to a depth of that number of inches (30), supposing the ground perfectly level and the water neither to sink into the earth or evaporate. The instru- ment used for the measurement of rainfalls is called a rain gauge. SPECIMEN TABLE OF AVERAGE ANNUAL EAINFALLS OF A FEW PLACES. Names. Inches. CheiTapoonjee 499"3 Vera Cruz (Mexico) 278 Akyab (Arracan Coast) 204 Andes (Patagonia) more than 200 Scathwaite (English Lake district) 1 45 '1 Glencroe 1277 Bombay 76*2 Calcutta 66 Keswick (Cumberland) 63 Cahirciveen (Ireland) 59'4 Kendal 58 Madras 56-3 Glasgow 43-3 Manchester 35*5 Cork 35-5 Lisbon 27'5 London 24*2 Paris 199 Generally speaking the greatest rainfall is in the tropics, and gradually decreases as we journey towards the poles. At the tropics its annual average is about 95 inches, but in the frigid zone it is only about 15. As the rain depends much on the wind it seems to follow much the same arrangement, namely, periodical in the tropics, variable in the higher latitudes, and abnormal in certain districts, when it occurs either in excess or is altogether absent. In the tropica the rainy season commences at the changing of the monsoons (p. 113). PHYSIOGRAPHY. 119 In general terms, more rain falls from April till October than in the other months, especially in the northern half of the torrid zone, where the wet season occurs at this time, the dry season commencing in October and continuing until April. In the southern half this order is reversed, the dry taking the place of the wet. Rainless Districts. — There are some places where little or no rain ever falls, the principal of which are the Sahara Desert, the great deserts of Arabia, Persia, Mongoha, including Gobi, Thibet, &c., and North Mexico and west of Peru, in America, forming as it were one continuous area, varying in tread th from the 15th to the 47th parallel, and in length from 16° W. to 118° E. longitude. The least rainfall in the world of which we have record is at Suez, being 1"3 inch. The area of these rainless regions is about 5^ million square miles. 112. Snow. — "When the temperature of the air is below the freezing point — namely, 32° F. — the condensed vapour must be in the form of particles of ice ; hence when they fall to the earth we have snoio instead of rain, and these frozen particles uniting together form flakes, the size of which depends upon the amount of moisture and the extent of the prevailing low temperature. Sleet is formed by the flakes in their descent encountering warm strata of air. Snow is generally composed of crystals in the form of six pointed or angled stars of 60°. As many as 1,000 different kinds have been noticed. Snow Flakes magnified. 120 PHYISOGRAPHY. Snow-line. — At places within the tropics at the sea level, and for 15° or 20° beyond in either hemisphere, snow never falls, the reason of which is obvious, only falls during winter and at considerable elevation ; but in the polar regions, and at extreme heights in all latitudes, it becomes constant. This limit is termed the snow-line, the height of which varies not only with the latitude, which descends constantly as we travel towards the poles, but also with the situation as regards exposure to the sun and rain-bearing winds, the degree of humidity of the climate, and other causes. The following diagram gives a general idea of equator, down to the level of the Also in the higher latitudes it ikoOD (J, 10.20 20 LO 50 60 10 ZO qOt Fig. 12. its gradual descent from 16,000 feet at the at the poles. But it must be understood that it does not always follow this rule, as for instance, on the south side of the Himalayas the snow-line is about 4,000 feet higher than that on the north side, owing chiefly to the great dryness of the enormous tablelands of Central Asia, as they increase the radiation of the solar heat, hence the evaporation, and also to the moisture conveyed to the south side by the warm winds of the Indian Ocean. The highest point to which the snow-Une reaches is about 18° degrees south of the equator, namely, in the Andes of Bolivia, where it exceeds 20,000 feet. HEIGHT OF SNOW-LINE IN DIFFERENT LATITUDES : — North Latitude. Spitzbergen 78° ^North Cape 71 Suhtelma (Norway) 67° Oonalaska 53^ Altai 50° Alps 46° Caucasus 43° Eocky Mountains 43° Pyrenees 42f° Etna 371° Himalayas (North) 29° Himalayas (South) 28° Purace (Andes) 21° Height in feet. 2,400 3,850 3,500 7,030 8,890 11,060 12,470 9,000 9,750 19,000 15,000 15,380 PHYSIOGRAPHY. 121 South Height _ Latitude. in feet. Andesof Quito ^ 0° ... 15,705 Kilimanjaro 4° ... 17,000 Andes of Bolivia 16° ... 17,700 Andes of Bolivia 18° ... 20,060 Straits of Magellan 63^° ... 3,540 South Georgia 54^°... From the above it will be seen that the greatest change in the snow-line takes place between 30° and 60°. Hail is supposed to be formed in the higher regions of the atmo- sphere. It consists of snow coated with ice frozen to it in its descent to the earth. It may briefly be defined as frozen rain. It occiu's in all latitudes and at all seasons. It appears to be con- nected in some way with the electricity of the atmosphere, thunder- storms often occurring while hail is falling. Hailstones are usually pear-shaped, and small in size, but have been known as large as hen's eggs. Hailstorms occur mostly in summer and in the day- time, and most frequently near mountains, seldom occurring in low- land plains within the tropics, but common at several thousand feet of elevation. Ice is frozen water, or water that has been crystallised by the atmospheric temperature being below or at 32° ; for salt water 4^° lower. On close examination it will be found to consist of six- rayed stars, like the characters of snow, and to be of pure water. (For "Expansion," &c., see 45.) Ice has, through some circumstance or other, been formed at the bottoms of rivers and ponds — called ground ice. Occurrences of this description have been noticed in the Thames, and also in the rivers of Siberia. Avalanches are accumulations of snow, or snow and ice, which frequently roU with great violence from lofty mountains — as the Alps for instance — ^into the valleys or plains below, carrying destruc- tion and ruin. There are three descriptions. (1) The wind or dust avalanches, that is, fresh-fallen snow carried down into the valleys in the form of dust. They are very light — ^hence not so destructive. (2) Mountain, snoio, hail, or thunder avalanches. These fall by their own weight, carrying with them the ground on which they lie, together with trees, rocks, &c., and mostly occur in spring. (3) Earth avalanches, or landslips, when they occur, are by far the most destructive. The earth, having been weakened by much con- tinuous rain, slides down into the valleys with everything upon it, houses, trees, and even entire forests. 113. Glaciers are enormous masses of ice, formed on mountains above the snow-line, which creep down into warmer regions, where they melt and disappear, giving rise to streams, as, for instance, the Rhine and Rhone from the glaciers of the Alps. In mountain gorges 122 PSYSIOGRAPHT. they descend as ice streams, the ice being partly plastic, though in appearance rigid. It differs also from ordinary soUd ice, being of a blue-veined structure, due to great pressure. These streams of ice travel at the rate of from 16 inches a day in winter to about 30 in summer, and follow the same laws as rivers of water — the velocity being greatest in the centre. The glacier, in its journey over rocks, &c., often causes great cracks or clefts of great depth, called crevasses, and tears away the loose rock and debris, which it carries with it, depositing these moraines,* as they are termed, when the glacier melts away at the snow-line, beginning to flow then as streams of water. Glaciers are most abundant in the Alps, Himalayas, Norway, New Zealand, and continually in arctic and antarctic regions. The most extensive occur in Greenland, some of which are 45 miles broad and from 350 to 500 feet in height. Large ones also occur in the Himalayas, the Rocky Mountains, Iceland, Spitzbergen, &c. ; but the most remarkable are those of the Alps, where over a thou- sand distinct glaciers occur. Some of the chief are Mer-de-Glace, with an area of 18 square miles, and the Glacier-de-la-Brenva, both in the Mont Blanc group ; the Zermatt Glacier, in the Monte Rosa group ; and the Great Aletsh Glacier, with an area of 34 square miles, in the Oberland group. 114. The Action of Rain, Springs, Rivers, Snow, and Glaciers on the Earth's Crust. — The action of these agents upon the crust of the earth is termed sub-aerial'f denudation. Rain. — Part of the rain which falls on the surface runs off in brooks and rivers. Another part percolates through the ground until it meets a less pervious rock, where it forms a reservoii-, or escapes through the sides of a hill, &c., forming springs. (See 87.) It is to the effect of rain that landslips may be attributed. The water, soaking through the ground, accumulates on a bed of clay, or some such stratum, and loosens the cohesion between it and the upper beds, when, if the strata are favourably placed, it may happen that the upper layers will slide over the slippery clay in which the strata are inclined. The rainwater which flows over the surface continually sweeps with it the minute particles of sand, rock, &c., which the action of the weather has loosened. Rivers. — Streams in their course through hilly regions wear channels for themselves, and carry along with them the displaced materials. (See " Deltas.") They have generally a tendency to wind about, in many cases a great number of convolutions occurring in the space of a mile or so. The reason of these tortuous courses may * These moraines are called lateral when they Hue the sides of the glacier ; terminal when they are deposited in heaps at its extremity ; and medial when two glaciers join, causing the inside moraine of each to unite. t Sub, under. Aer, the air. PHYSIOGRAPHY. 123 be attributed to tlie operation of the natural law, that a mass of matter in motion has a tendency to move in a straight line. (Also see 88.) Frost. — TVlien water gets into tlie joints and crevasses of a rock, and then becomes frozen, it expands, forcing apart the blocks and particles, so that as soon as the thaw comes the loosened par- ticles and pieces fall asunder. Snoio also is at times a disintegrator of rocks — namely, when accumulated on mountains and sliding down as avalanches. Glaciers, in their motion over rocks, exert great erosive powers, and (not like water, dividing on meeting an obstacle) push everything before them that gets in their way. By this means all loose stones in the bed of the glacier are torn up, making it a smooth undulatit g surface, with scratches or furrows running parallel with the stream of ice, caused by angular fragments of rock, which have fallen through crevices from the surface of the glacier, being dxagged along with the weight of the ice above them. 115. Phenomena of the Arctic and Antarctic Regions. The Arctic Regions are the high latitudes surrounding the North Pole, and the Antcuctic the regions surrounding the South Pole. As they are the farthest from the equator they are the coldest regions of the world, being inaccessible to man beyond the 84th parallel of latitude. The highest point ever reached is 83° 20' 30", and that by the arctic explorers of 1876, under Captain Nares, namely Commander (now Captain) Markham and Lieutenant (now Com- mander) Parr, with 15 men under them. The Alert, one of the vessels under Captain Nares, also reached a higher latitude than had ever previously been reached by a vessel, namely, lat. 82° 24'. The temperature during the winter months is so low that mercury is of no use in the thermometers to measure the cold, as it becomes frozen, remaining so for more than a month at a time. Spirit thermometers were used, showing a temperature of — 74°, or 106° below the freezing point, in March, though the thermometer may register, during the summer monthes, a temperature nearly equal to the mean of the tropics. The seas of the polar regions are closed during the greater part of the year, being only open for a few months in summer. The extent to which the Arctic Ocean is frozen is not known. At a degree of latitude varying with the season of the year, ships are barred from going northward by a barrier of frozen ice. The outer edge of this barrier gets spUt and broken off into vast mountains of ice during the summer, commencing at the latter end of April, drifting towards the south. The largest of these ice mountains are formed from the glaciers of these regions, which spread almost over the entire surface, gliding on as rivers of ice till they reach the sea shore, and there, losing their support, the front parts break off, and float away as icebergs (ice mountains). Some of these bergs are several miles in circumference, and from 50 to 250 feet above the 124 PHYSIOGRAPHY. surface ; hence the entire thickness of the greatest are 250 X 9 = 2,250 feet, the specific gravity of ice being "9 and water 1. In the Atlantic icebergs from the arctic regions have been carried by the polar current as far as the 44th parallel of latitude, and from the antarctic they have reached the Cape of Good Hope. The ice that forms on the surface of the sea is called field ice. It forms in the winter and breaks up in the summer. A small field is termed an icefloe, and one much broken up forms a pack, or pacJc ice. Bed Snow in the Arctic Regions. — Red snow is a phenomenon which is often observed in the polar regions. Captain Ross dis- covered on the shore of Baffin's Bay a range of cliffs extending for more than eight miles covered with a brilliant red snow, in some places 12 feet deep. The cause of this appearance has been found to be due to the presence among the snow of a very minute plant, which Sir William Hooker named Palmella nivalis. This snow has also occasionally been met with in the Alps and in Scotland. Day and Night in these Regions. — As we approach the poles the inequahty between the daj'S and nights becomes greater and greater, until at the poles themselves a day of six months alternates with a night of the same duration. The most distant parallels that the sun describes north and south of the equator are 66^° from the latter, and 23^° from the poles. Hence when the sun is in the tropics all the polar circle in that hemisphere will be within the illumination of the sun, as it will be with 90° of that luminary. During the same time the other polar circle will be in darkness. Therefore during the year they have one day of exactly 24 hours and one night of similar length. The change from short days to long ones, and vice versa, in these regions takes but a very short time. We may fancy that during the six months that the sun is absent there would be total darkness, but such is not the case, owing chiefly to the reflection of the sun, namely, tvMight, which in these regions lasts for months — at the north pole from September 22nd to November 12th, and from January 25th to March 20th. Besides the twilight there are the stars, and also the snow on the ground, which mitigate the darkness, the moon also appearing every 14 days. Very few animals or birds inhabit these parts. A few musk oxen, hares, and ptarmigans reach as far as latitude 82°. One or two geological facts regarding the arctic regions have been brought to Hght by the late expedition, bearing on the changes in the climate of those regions. Miocene beds, including a thick seam of coal, were found to exist as far north as latitude 81° 44'. The shales and limestone of the same formation contain abundant examples of the flora (flowers) of that epoch, thereby proving that at a comparatively recent geological period there existed a temperate climate within 500 miles of the north pole ; and according to Mr. ( PHYSIOGRAPHY. 125 C. Markham excellent coal was found in latitude 82"*, and impressions of leaves, &c., were brought back, showing that luxuriant forests had grown within 450 miles of the pole. Wood has also been dis- covered in the now frozen regions, with the harh on, having evidently- grown where found, thereby showing that there must have been great and rapid changes of climate in a comparatively short period in the polar region ; and the only way we can account for this is a change in the inclination of the earth's axis. The Antarctic Ocean has not been explored so high in latitude as the Arctic, the highest latitude reached being 78° 15', in 1841, by- Sir James Ross. It is supposed that nearly- the entire area, embraced by the antarctic circle, is occupied by land, covered con- tinually with snow. Two volcanoes exist, if not more, in these regions — Mount Erebus, 12,400 feet above the sea level, and Mount Terror, about 9,000 feet, the former is in a state of constant activity, and the latter extinct. The cold is more severe than in the arctic region. For instance, at lat. 64° S., Captain Nares found the temperature of the atmosphere to be 65' below freezing point (31"5°) in February, which is lower than the temperature of the arctic region 10° nearer the pole. CLIMATE, AND CAUSES AFFECTING IT. 116. Under the term " Climate " we speak of the general weather conditions of any district, as mild, or severe. Climate depends on numerous circumstances, the chief of which are — (1) Latitude. (2) Altitude. (3) Nearness to the sea. (4) Distribution of land and water. (5) Mountains. (6) The pre- vailing winds and ocean currents, &c. The priaciple cause affecting climate is the latitude, so that we may say generally that the climate of a place is warmest the nearer it is to the equator, or that its temperature diminishes in proportion as its latitude is greater, or more correctly in proportion to the square of the cosine of its lati- tude. The second principal cause is the altitude. (See " Snow-line," and "Height and Pressure of the Atmosphere," 112, 94.) Thirdly, its nearness to the sea, depending upon the unequal reception and radiation of heat by land and water. The heat falling on the land is partly absorbed and conducted downwards into the soil, and partly radiated into the atmosphere. The greater part of that con- ducted in the earth (which never sinks more than 70 feet, and for practical pm-poses may be regarded as within a few feet of the surface) is given off by night, but the remainder accumulates day after day all throiTgh the summer, keeping it in store to return to the atmo- sphere in winter. On the other hand, the rays of heat falHng on the water are much more readily absorbed, and also radiate more slowly. Hence, there will be a much greater store of heat accumulated in the waters of the ocean during summer than in the earth, so that it 126 PnYSIOGRAPHT. will have a great deal more heat to return in the winter than the earth. It is owing to this that islands and seaboards have more equable climate than those in the inside of a continent. For instance, contrast the climate in winter of any place in England with that of any place in the same latitude in Russia ; or, again, the comparatively cool summers and mild winters with the interior of Germany, which experiences excessively hot summers and cold winters. Such places as the British Isles, New Zealand, &c., are said to possess an insular climate, and the interior of Germany, Russia, &c., continental. The prevailing winds and ocean currents also exercise great influence on the climate, as they may be either cold or warm, &c., especially in the case of the Gulf Stream, and the arctic and antarctic currents. The former, carrying warmth and moisture, lessens the winter climate of the west side of Europe, while the arctic current brings cold, tempering the summer climate of the eastern part of North America. The direction of mountain chains and the cultiva- tion of a country also influence the climate. Representation of Climate. — From the above considerations it will be seen that the parallels of latitude do not represent the belts of corresponding climates. So that places having the same summer temperature may readily be known, long seiies of observations have been taken, and lines of equal heat, called Isotheral Lines, have been drawn, showing these places at a glance. In a similar manner, lines are drawn through places having the same winter temperature, these are called Isocheimal Lines. Lines drawn through places having the same mean annual temperature, are called Isothermal Lines. (See map.) From the map it will be seen that the line of greatest heat does not coincide with the equator but hes north of it, from about 150° west longitude, across both hemispheres, falling below it in the island of Borneo. The average temperature of the line ia about 84° F., being hottest near the Red Sea. Lines drawn through places having the same mean barometic pressure are term|d isobaric lines. CHmates are generally grouped into three classes. (1) Those in which the temperature of summer is but little in excess of that of winter, called insular chmates, as in this country. The mean difler- ence between summer and winter here is only about 20°. (2) Those in which the difference between summer and winter is strongly marked, owing to their distance from the sea. These are termed continental. Central Russia and Germany, in the same latitude as England, have a range of 86°. (3) Those in which the difference is very great. For instance, in Siberia the difference amounts to 106°, the summer temperature being 62'2°, and the winter- 43'S°. These are termed extreme chmates. Also, regarding the distribution of the climate, there is one fact not to be lost sight of, and that is — the Northern Hemisphere is about 3|° warmer than the Southern one, owing to the much greater extent of sea to the south. MAP OF ISOTHERMAL LINES. 127 128 PHYSIOGRAPHY. LIFE AND ITS DISTRIBUTION. 117. Under the term "Life" is embraced all that appertains! to the vegetable and animal kingdoms, subjects which really belong to Biology* (Botany and Zoology). The part to be noticed chiefly, in the elements of Physiography, is their dependence on climate and other conditions regarding their distribution. In speaking of animals and plants we group together those forms which possess much in common as genera. These we split up into species, and, for the sake of brevity, the term flora is used to designate the plant life of a region or epoch, smA fauna to designate its animal life. The area within which a given plant prevails is called its habitat, or area of distribution. Plants are divided into two classes, namely, cryptogams, or non-flowering plants, embracing ferns, mosses, lichens, seaweed, fungi, &c. ; and phenogaras, or flowering plants, as the pine, chestnut, maple, spruce, &c. The number of species known is about 130,000, six-sevenths being flowering plants and the remaining one-seventh non-flowering. It is calculated that there are upwards of 220,000 species existing on the earth. 118. Distribution of Plants according to Climate.— The surface of the globe has been divided into eight zones, bounded by isothermal lines, or mean temperature. (1) The equatorial zone, or region of palms and bananas, bounded by the isotherms of 79"3° F. on each side of the equator. This zone contains the greatest variety of species and the most luxuriant, the principal of which are the palms, banyans, breadfruit trees, bamboos, orchids, arborescent grasses, &c. (2) The tropical zone of tree-ferns and figs, between the isotherms of 79'3° F. and 72*5° F. each side of the equator. Besides the ferns and figs, &c., many equatorial plants are found, as well as cofiee, cotton, pineapples, sugarcanes, cinnamon, logwood, indigo. (3) The sub-tropical zone of laurels and myrtles, between 72'5° F. and 68° F. (4) ^}ie ivarm temperate zone, or region of evergreens, hetween. 68° F. and 54"5°F., also includes oaks, figs, chestnuts, olives, oranges, the vine, pomegranates, and many other sub- tropical forms ; of grain the chief is wheat. (5) The cool temperate zone of deciduousf trees, between 54:'5'^ F. and 41° F. This zone includes all English trees and plants, and beyond it the cultivation of wheat does not extend in the Northern Hemisphere. (6) The sub-arctic zone oj coniferous trees {pine, larch, spruce, juniper, p •t = m^v^ JO QOBjjng ve if^IABJQ JO 80JO^ c, 13 S <= oq 1 J^ •T = qc^reg jo JO i£:;tsaaa: c oc 7^