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whole atmosphere probably exceeds what would be disengaged if all the vegetable and animal matter on the earth's surface were burnt.

The other substances present in much more minute quantities are gases, vapours, and solid particles. Of these by much the most important is the vapour of water, which is always present, but in very variable amount according to temperature, ranging from about 4 to a maximum of 16 grains in 1000 grains of air. It is this vapour which condenses into dew, rain, hail, and snow. In assuming a visible form, and descending through the atmosphere, it takes up a minute quantity of air, and of the different substances which the air may contain. Being caught by the rain, and held in solution or suspension, these substances can be best examined by analysing rain-water. In this way ammonia, nitric, sulphurous, and sulphuric acids, chlorides, various salts, solid carbon, inorganic dust, and organic matter have been detected. M. J. J. Pierre found as the result of his analysis that in the neighbourhood of Caen, in France, a hectare of land receives annually from the atmosphere, by means of rain—

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Sulphate of soda.

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potash fime... magnesia.. To these ingredients must be added traces of ammonia, various salts, and organic substances, besides others still undetermined. The powerful oxidizing agent ozone is present in variable but always minute quantities in the air. The comparatively small but by no eans unimportant proportions of these various components of the atmosphere are much more liable than the more essential gases to great variations. Chloride of sodium, for instance, is, as might be expected, particularly abundant in the air bordering the sea. Nitric acid, ammonia, and sulphuric acid appear in the air of towns most conspicuously. The organic substances present in the air are sometimes living germs, such as probably often lead to the propagation of disease, and sometimes mere fine particles of dust derived from the bodies of living or dead organisms.3

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2. The Oceans.-About three-fourths of the surface of the globe (or about 144,712,000 square miles) is covered by the irregular sheet of water known as the sea. Within the last ten years much new light has been thrown upon the depths, temperatures, and biological conditions of the ocean-basins, more particularly by the "Lightning," "Porcupine," and Challenger" expeditions fitted out by the British Government. It has been ascertained that few parts of the Atlantic Ocean exceed 3000 fathoms, the deepest sounding obtained there being one taken about 100 miles north from the island of St Thomas, which gave 3875 fathoms, or rather less than 4 miles. The Atlantic appears to have an average depth in its more open parts of from 2000 to 3000 fathoms or from about 2 to 3 miles. In the Pacific Ocean the "Challenger" got soundings of 3950 and 4475

1 The quantity of aqueous vapour depends upon the temperature, warm air being able to retain more than cold air. Air at a temperainre of 10° C. is saturated when it contains 9-362 grammes of vapour

in a cubic metre of air.

Chimie Agricole, quoted by Dr Angus Smith, Air and Rain, p. 232. The air of towns is peculiarly rich in impurities, especially in manufacturing districts, where much coal is used. These impurities, however, though of serious consequence to the towns in a sanitary point of view, do not sensibly affect the general atmosphere, seeing that they are probably in great measure taken out of the air by rain, even in the districts which produce them. They possess, however, a economic bearings. Sce ou this whole subject Dr Angus Smith's work

trendy cited,

fathoms, or about 4 and rather more than 5 miles. But these appear to mark exceptionally abyssal depressions, the average depth being, as in the Atlantic, between 2000 and 3000 fathoms. We may therefore assume, as probably not far from the truth, that the average depth of the ocean is about 2500 fathoms, or nearly 3 miles.

The water of the oceans is distinguished from the ordinary terrestrial waters by a higher specific gravity, and the presence of so large a proportion of saline ingredients as to impart a strongly salt taste. The average density of seawater is about 1·026, but it varies slightly in different parts even of the same ocean. According to the recent observations of Mr J. Y. Buchanan during the "Challenger" expedition, some of the heaviest sea-water occurs in the pathway of the trade-winds of the North Atlantic, where evaporation must be comparatively rapid, a density of 102781 being registered. Where, however, large rivers enter the sea, or where there is much melting ice, the density diminishes; Mr Buchanan found among the broken ice of the Antarctic Ocean that it had sunk to 1.02418.4

The greater density of sea-water depends of course upon the salts which it contains in solution. There seems no reason to doubt that these salts are, in the main, parts of the original constitution of the sea, and thus that the sea has always been salt. It is also probable that, as in the case of the atmosphere, the composition of the ocean water has in former geological periods been very different from what it is now, and that it has acquired its present character only after many ages of slow change, and the abstraction of much mineral matter originally contained in it. There is evidence indeed among the geological formations that large quantities of lime, silica, chlorides, and sulphates have in the course of time been removed from the sea.5

But it is evident also that, whatever may have been the original composition of the oceans, they have for a vast section of geological time been constantly receiving mineral matter in solution from the land. Every spring, brook, and river removes various salts from the rocks over which it moves, and these substances, thus dissolved, eventually find their way into the sea. Consequently sea-water ought to contain more or less traceable proportions of every substance which the terrestrial waters can remove from the land, in short, of probably every element present in the outer shell of the globe, for there seems to be no constituent of this earth which may not, under certain circumstances, be held in solution in water. Moreover, unless there be some counteracting process to remove these mineral ingredients, the ocean water ought to be growing, insensibly perhaps, but still assuredly, salter, for the supply of saline matter from the land is incessant. It has been ascertained indeed, with some approach to certainty, that the salinity of the Baltic and Mediterranean is gradually increasing.

6

The average proportion of saline constituents in the water of the great oceans far from land is about three and a half parts in every hundred of water. But in enclosed seas, receiving much fresh water, it is greatly reduced, while in those where evaporation predominates it is correspondingly augmented. Thus the Baltic water contains from one

seventh to nearly a half of the ordinary proportion in ocean water, while the Mediterranean contains sometimes onesixth more than that proportion. The mineral constituents include the following average ratios of salts7:

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100-000 3.527

Total percentage of salts in sea-water..... Besides these chief ingredients, sea-water has yielded minute traces of iodine, fluorine, silica, phosphoric acid, carbonate of lime and magnesia, silver, lead, copper, arsenic. Doubtless more perfect analysis will greatly increase this list.

In addition to its salts sea-water always contains dissolved atmospheric gases. From the researches conducted during the voyage of the "Bonité" in the Atlantic and Indian Oceans it was estimated that the gases in 100 volumes of sea-water ranged from 1.85 to 3.04, .or from two to three per cent. From observations made during the "Porcupine" cruise of 1868 it was inferred that the proportion of oxygen was greatest (25.1 per cent.) in the surface water, and least (195) in the bottom water, while that of carbonic acid was least at the top (207) and greatest (27.9). at the bottom, and that the action of the waves was partially to eliminate the latter gas and to increase the amount of oxygen. More recently, however, during the voyage of the "Challenger," Mr J. Y. Buchanan ascertained that the proportion of carbonic acid was always nearly the same for similar temperatures, the amount in the Atlantic surface water, between 20° and 25° C., being 0-0466 gramme per litre, and in the surface Pacific water 0.0268. He points out the curious fact that, according to his analyses, sea-water contains sometimes at least thirty times as much carbonic acid as an equal bulk of fresh water would do, and he traces the greater power of absorption to the presence of the sulphates.

II. THE SOLID GLOBE.

1. General Considerations.-Within the atmospheric and oceanic envelopes lies the inner solid globe. Reference has already been made to the comparative density of the planet among the other members of the solar system. In all speculation about the history of the earth, the density of the whole mass of the planet as compared with waterthe standard to which the specific gravities of terrestrial bodies are referred-is a question of prime importance. Various methods have been employed for determining the earth's density. The deflexion of the plumb-line on either side of a mountain of known structure and density, the time of oscillation of the pendulum at great heights, at the sea-level, and in deep mines, the comparative force of gravitation as measured by the torsion balance-each of these processes has been tried with the following various results :

Plumb-line experiments on Schiehallien (Maskelyne and
Playfair) gave as the mean density of the earth.....
Do. on Arthur's Seat, Edinburgh, (James).....
Pendulum experiments on Mont Cenis (Carlini and Giulio)..
Do. in Harton coal-pit, Newcastle (Airy)..
Torsion balance experiments (Cavendish)..

Do.

do.

(Baily).....

4713 5-316 4'950 6.565 5.480 5-660

Though these observations are somewhat discrepant, we may feel satisfied that the globe has a mean density neither much more nor much less than 5·5; that is to say, it is five and a half times heavier than one of the same dimensions named of pure water. Now the average density of the materials which compose the accessible portions of the earth is between 2.5 and 3; so that the mean density of the whole globe is about twice as much as that of its outer part. We might therefore infer that the inside consists of

much heavier materials than the outside, and consequently that the mass of the planet must contain at least two dissimilar portions—an exterior lighter crust or rind, and an interior heavier nucleus. But the effect of pressure must necessarily increase the specific gravity of the interior as will be alluded to further on.

2. The Crust. It was formerly a prevalent belief that the exterior and interior of the globe differed from each other to such an extent that, while the outer parts were cool and solid, the vastly more enormous inner part being intensely hot was more or less completely fluid. Hence the term "crust " was applied to the external rind in the usual sense of that word. This crust was variously computed to be 10, 15, 20 or more miles in thickness. For reasons which will be afterwards given, the idea of internal liquidity has been opposed by eminent physicists and is now abandoned by most geologists. The term "crust," however, continues to be used as a convenient word to denote the cool, upper, or outer layer of the earth's mass, accessible to human observation. It is in the structure and history of this crust that the main subjects of geological investigation are contained. It will therefore be fully treated of in the following parts of this article.

There are, however, some general views as to its composition and the arrangement of its materials, which may appropriately find a place in this preliminary section. Evidently our direct acquaintance with the chemical constitution of the globe must be limited to that of the crust, though by inference we may eventually reach highly probable conclusions regarding the constitution of the interior. Chemical research has discovered that sixty-four simple or as yet indecomposable bodies, called elements, in various proportions and compounds, constitute the accessible part of the crust. Of these, however, the great majority are comparatively of rare occurrence. The crust, so far as we can examine it, is mainly built up of about sixteen elements, which may be arranged in the two following groups, the most abundant bodies being placed first in each list:— Metals.

Silicon

Oxygen

Carbon..

Sulphur......

Metalloids.

Atomic Weight.

15-96 Aluminium.

28.00 Calcium.....

11.97 Magnesium. 31 98 Potassium

35.37 Iron

Hydrogen (really a metal) 100 Sodium
Chlorine.....
Phosphorus.
Fluorine..

30.96 Manganese. 19.10 Barium

Atomic

Weight.

27-30

39.90

23-94

89-04

22-99

55-90

54-80

.136-80

By far the most abundant and important of these elements is oxygen. It forms about 23 per cent. by weight of air, 88-88 per cent. of water, and about a half of all the rocks which compose the visible portion or "crust" of the globe. Another metalloid, silicon, comes next in abundance. It is always united with oxygen, forming the mineral silica which, either alone or in combination with various metallic bases as silicates, constitutes a half of all the known mass of the globe. Of the remaining metalloids carbon and sulphur sometimes occur in the free state, but usually in combination with oxygen or some base or metal. Chlorine and fluorine are found associated with metallic bases. Hydrogen is properly a metal, and occurs chiefly in combination with oxygen as the oxide, water. Phosphorus occurs with oxygen principally in phosphate of lime.

Of the metals by far the most important in the architec ture of the exterior of the earth is aluminium. In con junction with oxygen and silicon it forms the basis of most crystalline rocks. Calcium, magnesium, potassium, and sodium, combined with oxygen, enter largely into the com position of rocks. Iron is the great colouring material ir nature, most of the yellow, brown, red, and green hues of

rocks being due to some of its combinations. The sixteen | based upon the known fact that the specific gravity of that elements mentioned in the foregoing lists form about nucleus is about double that of the crust. This has been ninety-nine parts of the earth's crust; the other elements held by some writers to prove that the interior must conconstitute only about a hundredth part, though they include sist of much heavier material, and is therefore probably gold, silver, copper, tin, lead, and the other useful metals, metallic. But in so reasoning they forget that the effect iron excepted. of pressure ought to make the density of the nucleus much higher, even if the interior consisted of matter no heavier than the crust. In fact, we might argue for the probable comparative lightness of the substance composing the nucleus. That the total density of the planet does not greatly exceed its observed amount seems only explicable on the supposition that some antagonistic force counteracts the effects of pressure. The only force we can suppose capable of so acting is heat. But how and to what extent this counterbalancing takes place is still unknown.

It is clear then that, so far as accessible to our observation, the outer portion of our planet consists mainly of metalloids, and its metallic constituents have in great part entered into combination with oxygen, so that the atmosphere contains the residue of that gas which has not yet united itself to terrestrial compounds. In a broad view ́of the arrangement of the chemical elements in the external crust, the suggestive speculation of Durocher deserves attention.1 He regarded all rocks as referable to two layers or magmas co-existing in the earth's crust the one beneath the other, according to their specific gravities. The upper or outer layer, which he termed the acid or siliceous magma, contains an excess of silica, and has a mean density of 2.65. The lower or inner layer, which he called the basic magma, has from six to eight times more of the earthy bases and iron oxides, with a mean density of 2.96. To the former he assigned the early plutonic rocks, granite, felsite, &c., with the more recent trachytes; to the latter he relegated all the heavy lavas, basalts, diorites, &c. The ratio of silica is 7 in the acid magma to 5 in the basic. Though the proportion of this acid or of the earthy and metallic bases cannot be regarded as any certain evidence of the geological date of rocks, nor of their probable depth of origin, it is nevertheless a fact that (with many important exceptions) the eruptive rocks of the older geological periods are very generally super-silicated and of lower specific gravity, while those of later time are very frequently poor in silica but rich in the earthy bases and in iron and manganese, with a consequent higher specific gravity. The latter, according to Durocher, have been forced up from a lower zone through the lighter siliceous crust.

3. The Interior or Nucleus.-Though we cannot hope ever to have direct acquaintance with more than the mere outside skin of our planet, we may be led to infer the irregular distribution of materials within the crust from the present distribution of land and water, and the observed differences in the amount of deflexion of the plumb-line near the sea and near mountain chains. The fact that the southern hemisphere is almost wholly covered with water appears explicable only on the assumption of an excess of density in the mass of that portion of the planet. The existence of such a vast sheet of water as that of the Pacific Ocean is to be accounted for, says Archdeacon Pratt, by the presence of "some excess of matter in the solid parts of the earth between the Pacific Ocean and the earth's centre, which retains the water in its place, otherwise the ocean would flow away to the other parts of the earth."2 The same writer points out that a deflexion of the plumb-line towards the sea, which has in a number of cases been observed, indicates that "the density of the crust beneath the mountains must be less than that below the plains, and still less than that below the ocean-bed."3 Apart therefore from the depressions of the earth's surface in which the oceans lie, we must regard the internal density, whether of crust or nucleus, to be somewhat irregularly arranged,-there being an excess of heavy materials in the water hemisphere and beneath the ocean-beds as compared with the continental

masses.

In our ignorance regarding the chemical constitution of the nucleus of our planet, an argument has sometimes been

1 Translated by Haughton in his Manual of Geology, 1866, p. 16. Figure of the Earth, 4th edit., p. 236.

Op. cit., p. 200 See also Herschel. Phys. Geog.; and O. Fisher, Cambridge Phil. Trans., xii., part ii.

If we regard the question from another point of view, however, the idea of a metallic nucleus seems not improbable. When the materials of the globe existed in a fluid condition, as they are usually supposed to have done, they would doubtless arrange themselves in accordance with their relative specific gravities. The denser elements would sink towards the centre, the lighter would remain outside. That this distribution has certainly taken place to some extent is evident from the structure of the envelopes and crust. It is what might be expected if the constitution of the globe resembles on a small scale the larger planetary system of which it forms a part. The existence of a metallic interior has always been inferred from the metalliferous veins which traverse the crust, and which are commonly supposed to have been filled from below.

Admitting the possibility or even probability of a metallic nucleus, in spite of the comparatively low density of the globe as a whole, we might speculate further as to the arrangement of the denser internal materials. The late Mr David, Forbes suggested that the planet might be supposed to consist of three layers of uniform densities, enclosed one within the other, the density increasing towards the centre in arithmetical progression. Allowing 2.5 as the specific gravity of the crust or outer layer, he assigned 120 or thereabouts as that of the middle layer, and supposed that the inner nucleus might possess one averaging 20-0.4 Materials do not yet exist for any. satisfactory conclusions on this subject.

In the evidence obtainable as to the former history of the earth, no fact is of more importance than the existence of a high temperature beneath the crust, which has now been placed beyond all doubt. This feature of the planet's organization is made clear by the following proofs:

(1.) Volcanoes.-In many regions of the earth's surface openings exist from which steam and hot vapours, ashes and streams of molten rock are from time to time emitted. The abundance of these openings seems inexplicable by any mere local causes, but must be regarded as indicative of a very high internal temperature. If to the still active vents of eruption we add those which have formerly been the channels of communication between the interior and the surface, there are probably few large regions of the globe where proofs of volcanic action cannot be found. "Everywhere we meet with masses of molten rock which have risen from below as if from some general reservoir.

(2.) Hot Springs.-Where volcanic eruptions have ceased, evidence of a high internal temperature is still often to be found in springs of hot water which continue for centuries to maintain their heat. Thermal springs, however, are not confined to volcanic districts. They sometimes rise even in regions many hundreds of miles distant from any active volcanic vent. The hot springs of Bath (temp. 120° Fahr.) and Buxton (temp. 82° Fahr.) in England are

A Popular Science Review, April 1868

fully 900 miles from the Icelandic volcanoes on the one side, and 1100 miles from those of Italy and Sicily on the other.

(3.) Borings, Wells, and Mines.-The influence of the seasonal changes of temperature extends downward from the surface to a depth which varies according to latitude, to the thermal conductivity of the soils and rocks, and perhaps to other causes. The cold of winter and the heat of summer may be regarded as following each other in successive waves downward, until they disappear along a limit at which the temperature remains constant. This zone of invariable temperature is commonly believed to lie somewhere between 60 and 80 feet down in temperate regions. At Yakutsk in eastern Siberia (lat. 62° N.), however, the soil is permanently frozen to a depth of about 700 feet.1 In Java, on the other hand, a constant temperature is said to be met with at a depth of only 2 or 3 feet.2

It is a remarkable fact, now verified by observation all over the world, that below the limit of the influence of ordinary seasonal changes the temperature, so far as we yet know, is nowhere found to diminish downwards. It always rises; and its rate of increment never falls much below the average. The only exceptional cases occur under circumstances not difficult of explanation. On the one hand, the neighbourhood of hot-springs, of large masses of lava, or of other manifestations of volcanic activity, may raise the subterranean temperature much above its normal condition; and this augmentation may not disappear for many thousand years after the volcanic activity has wholly ceased, since the cooling down of a subterranean mass of lava would necessarily be a very slow process. On the other hand, the spread of a thick mass of snow and ice over any considerable area of the earth's surface, and its continuance there for several thousand years, would so depress the subterranean isothermals that for many centuries afterwards there might be a fall of temperature for a certain distance downwards. At the present day, in at least the more northerly parts of the northern hemisphere, there are such evidences of a former more rigorous climate, as in the well sinking at Yakutsk already referred to. Sir William Thomson has calculated that any considerable area of the earth's surface covered for several thousand years by snow or ice, and retaining, after the disappearance of that frozen covering, an average surface temperature of 13° C., "would during 900 years show a decreasing temperature for some depth down from the surface, and 3600 years after the clearing away of the ice would still show residual effect of the ancient cold, in a half rate of augmentation of temperature downwards in the upper strata, gradually increasing to the whole normal rate, which would be sensibly reached at a depth of 600 metres." But beneath the limit to which the influence of the changes of the seasons extends, observations in most parts of the globe show that the temperature invariably rises as we penetrate towards the interior of the earth. According to present knowledge the average rate of increase amounts to 1° Fahr. for every 50 or 60 feet of descent, and this rise is found whether the boring be made at the sea-level or on elevated ground. The subjoined table gives the results of temperature observations at widely separated localties::

1 Helmersen, Brit. Assoc. Report, 1871. Junghuhn's Jara, ii. p. 771.

3 Professor Prestwich (Inaugura Lecture, 1875, p. 45) has suggested that to the more rapid refrigeration of the earth's surface during this cold period, and to the consequent depression of the subterraneous isothermal lines, the alleged present comparative quietude of the volcanic forces is to be attributed, the internal heat not having yet recovered its dominion in the outer crust.

Brit: Assoc. Reports, 1876, Sections, p. 3.

Sce" Reports of Committee on Underground Temperature," Brit. Assoc. Rev. from 1868 to 1877.

Fet

1 Fahr. for every 53-2

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Dukinfield, near Manchester (2040 ft., coal measures)
Rose Bridge, Wigan (2445 ft., coal measures)
South Balgray, Glasgow (525 ft., coal measures)
Kentish Town, London (1100 ft., London clay, chalk, gault, &c.) „
La Chapelle, Paris (660 metres, chalk, &c.)
Grenelle Well, Paris (1795-6 ft., do )......
St André, do. (263 metres, do.)
Neu Salzwerk boring, Westphalia (2281 ft.)
Mendorff bore, near Luxembourg (2394 ft.)
Bore near Geneva....

Mont Cenis tunnel (5280 ft. below summit of Mount Frejus,
metamorphic rocks)

Yakutsk, Siberia, (656 ft limestone, &c., and granite)

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(4.) Irregularities in the Downward Increment of Heat.— While these examples prove a progressive increase of temperature, they show also that this rate of increase is not strictly uniform. The more detailed observations which have been made in recent years have brought to light the important fact that considerable variations in the rate of increase take place even in the same bore. If, for instance, we examine the temperatures obtained at different depths in the Rose Bridge eolliery shaft cited in the foregoing list, we find them to read as in the following columns:— Temperature

Depth in

Depth in

Temperature (Fahr.).

Yards.

(Fahr.).

Yards.

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In drawing attention to the temperature-observations at the Rose Bridge colliery-the deepest mine in Great Britain-Professor Everett points out that, assuming the surface temperature to be 49° Fahr., in the first 558 yards the rate of rise of temperature is 1° for 577 feet; in the next 257 yards it is 1° in 48.2 feet; in the portion between 605 and 671 yards-a distance of only 198 feet-it is 1° in 33 feet; in the lowest portion of 432 feet it is 1° in 54 feet. When such irregularities occur in the same vertical shaft, it is not surprising that the average should vary so much in different places.

There can be little doubt that one main cause of these variations is to be sought in the different thermal conductivities of the rocks of the earth's crust. The first accurate measurements of the conducting powers of rocks were made by the late Professor J. D. Forbes at Edinburgh (1837-1845). He selected three sites for his thermometers, one in "trap-rock" (a porphyrite of Lower Carboniferous age), one in loose sand, and one in sandstone, each instrument being sunk to a depth of 24 French feet from the surface. He found that the wave of summer heat reached the first instrument on 4th January, the second on 25th December, and the third on 3d November, the trap-rock being by far the worst conductor, and the solid sandstone by far the best,8

The British Association has recently appointed a committee to investigate this subject in greater detail. Already some important determinations have been made by it regarding the absolute conductivity of various rocks. As a rule the lighter and more porous rocks offer the greatest

"Report of Committee on Underground Temperature," Brit. Assoc. Rep., 1873, p. 254.

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resistance to the passage of heat, while the more dense and crystalline offer the least resistance. The resistance of opaque white quartz is expressed by the number 114, that of basalt by 273, while that of cannel coal stands very much higher at 1538, or more than thirteen times that of quartz.1 It is evident also that, from the texture and structure of most rocks, the conductivity must vary in different directions through the same mass, heat being more easily conducted along than across the "grain," the bedding, and the other numerous divisional surfaces. Experiments have been made to determine these variations in a number of rocks. Thus, the conductivity in a direction transverse to the divisional planes being taken as unity, the conductivity parallel with these planes was found in a variety of magnesian schist to be 4.028. In certain slates and schistose rocks from central France the ratio varied from 1: 2.56 to 1: 3.952. Hence in such fissile rocks as slate and mica-schist heat may travel four times more easily along the lines of cleavage or foliation than across them.2

In reasoning upon the discrepancies in the rate of increase of subterranean temperatures, we must also bear in mind that certain kinds of rock are more liable than others to be charged with water, and that, in almost every boring or shaft, one or more horizons of such water-bearing rocks are met with. The effect of this interstitial water is to diminish thermal resistance. Dry red brick has its resistance lowered from 680 to 405 by being thoroughly soaked in water, its conductivity being thus increased 68 per cent. A piece of sandstone has its conductivity heightened to the extent of 8 per cent. by being wetted.3

Mr Mallet has contended that the variations in the amount of increase in subterranean temperature are too great to permit us to believe them to be due merely to differences in the transmission of the general internal heat, and that they point to local accessions of heat arising from transformation of the mechanical work of compression, which is due to the constant cooling and contraction of the globe. But it may be replied that these variations are not greater than, from the known divergences in the conductivities of rocks, they might fairly be expected to be.

(5.) Probable Condition of the Earth's Interior.-Various theories (mostly fanciful) have been propounded on this subject. There are only three which merit serious consideration. (1.) One of these supposes the planet to consist of a solid crust and a molten interior. (2.) The second holds that, with the exception of local vesicular spaces, the globe is solid and rigid to the centre. (3.) The third contends that, while the mass of the globe is solid, there lies a liquid substratum beneath the crust.

1. The arguments in favour of internal liquidity may be summed up as follows. (a.) The ascertained rise of temperature inwards from the surface is such that, at a very moderate depth, the ordinary melting point of even the most refractory substances would be reached. At 20 miles the temperature, if it increases progressively, as it does in the depths accessible to observation, must be about 1760° Fahr.; at 50 miles it must be 4600°, or far higher than the fusingpoint even of so stubborn a metal as platinum, which melts at 3080° Fahr. (b.) All over the world volcanoes exist from which steam and torrents of molten lava are from time to time erupted. Abundant as are the active volcanic vents, they form but a small proportion of the whole which have been in operation since early geological time. It has been inferred therefore that these numerous funnels of

Herschel and Lebour, Brit. Assoc. Rep., 1875, p. 59. Jannettaz, Bull. Soc. Geol. de France (April-June, 1874), tom. ii. p. 254; "Report of Committee on Thermal Conductivities of Rock," Brit. Assoc. Rep., 1875, p. 61.

Herschel and Lebour, Brit. Assoc. Rep., 1875, p. 58. "Volcanic Energy," Phil. Trans., 1875.

225

communication with the heated interior could not have existed and poured forth such a vast amount of molten rock, unless they drew their supplies from an immense internal molten nucleus. (c.) When the products of volcanic action from different and widely-separated regions are compared and analysed, they are found to exhibit a remarkable uniformity of character. Lavas from Vesuvius, from Hecla, from the Andes, from Japan, and from New Zealand present such an agreement in essential particulars as, it is contended, can only be accounted for on the supposition that they have all emanated from one vast common source.5 (d.) The abundant earthquake shocks which affect large areas of the globe are maintained to be inexplicable unless on the supposition of the existence of a thin and somewhat flexible crust. These arguments, it will be observed, are only of the nature of inferences drawn from observations of the present constitution of the globe. They are based on geological data, and have been frequently urged by geologists as supporting the only view of the nature of the earth's interior compatible with geological evidence.

2. The arguments against the internal fluidity of the earth are based on physical and astronomical considerations of the greatest importance. They may be arranged as follows:(a.) Argument from precession and nutation.-The problem of the internal condition of the globe was attacked as far back as the year 1839 by the late Mr Hopkins of Cambridge, who endeavoured to calculate how far the planetary motions of precession and nutation would be influenced by the solidity or liquidity of the earth's interior. He found that the precessional and nutational movements could not possibly be as they are if the planet consisted of a central ocean of molten rock surrounded with a crust of 20 or 30 miles in thickness, that the least possible thickness of crust consistent with the existing movements was from 800 to 1000 miles, and that the whole might even be solid to the centre, with the exception of comparatively small vesicular spaces filled with melted rock.

M. Delaunay, in a paper on The Hypothesis of the Interior Fluidity of the Globe, threw doubt on Hopkins's views, and suggested that, if the interior were a mass of sufficient viscosity, it might behave as if it were a solid, and thus the phenomenon of precession and nutation might not be affected. Sir William Thomson, who had already arrived at the conclusion that the interior of the globe must be solid, and acquiesced generally in Hopkins's conclusions, pointed out that M. Delaunay had not worked out the problem mathematically, otherwise he could not have failed to see that the hypothesis of a viscous and quasi-rigid interior "breaks down when tested by a simple calculation of the amount of tangential force required to give to any globular portion of the interior mass the precessional and nutational motions which, with other physical astronomers, he attributes to the earth as a whole."8 Sir William, in making this calculation, holds that it demonstrates the earth's crust down to depths of hundreds of kilometres to be capable of resisting such a tangential stress (amounting to nearlyth of a gramme weight per square centimetre) as would with great rapidity draw out of shape any plastic substance which could properly be termed a viscous fluid. "An angular distortion of 8" is produced in a cube of glass by a distorting stress of about ten grammes weight per square centimetre. We may therefore safely conclude that the rigidity of the earth's interior substance could not be less than a millionth of the rigidity of glass without very sensibly augmenting the lunar nineteen-yearly nutation."

5 See D. Forbes, "On the Nature of the Interior of the Earth," Popular Science Review, April 1869. Phil. Trans., 1839; Researches in Physical Geology, 1839-1842; Brit. Assoc. Rep., 1847. 7 Comptes Rendus, July 13, 1868. 9 Loc. cit., p. 258.

Nature, February 1, 1872.

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