the earth to be studied—a great experiment—the diamond drill—a boring upwards of a mile deep—a mechanical feat—the scientific importance of the work—increase of temperature with the depth—a special form of thermometer—taking the temperature in the boring—the level of constant temperature—the rate of increase of temperature with the depth—one degree fahrenheit for every sixty-six feet in depth—temperatures at depths above a mile—conclusions as to the heat at very great depths—the heat developed by tidal action—this will not account for the earth’s internal heat—the earth must be continually cooling—inferences from the incessant loss of heat from the earth—the earth’s surface once red-hot, or molten—the earth must have originated from a nebula—the earth’s beginning.
in the last chapter we endeavoured to ascertain what can be learned from the radiation of the sun with regard to the history of the solar system. in this chapter we shall not consider any body in the heavens, but the condition of the earth itself. we have learned something of the history of the solar system from the celestial bodies; we shall now learn something about it in another way—from the condition of our globe at depths far beneath our feet.
it will be convenient to commence by mentioning a remarkable experiment which was made a few years 123ago. though that experiment is of great scientific interest, yet it was not designed with any scientific object in view. not less than ￡10,000 was expended on the enterprise, and probably so large a sum has never been expended on a single experiment of which the sole object was to add to scientific knowledge. in the present case the immediate object in view was, of course, a commercial one. there was, it may be presumed, reasonable expectation that the great initial cost, and a handsome profit as well, would be returned as the fruits of the enterprise. whether the great experiment was successful from the money-making point of view does not now concern us, but it does concern us to know that the experiment was very successful in the sense that it incidentally afforded scientific information of the very highest value.
the experiment in question was made in germany, at schladebach, about fifteen miles from leipzig. it was undertaken in making a search for coal. some enterprising capitalists consulted the geologists as to whether coal-seams were likely to be found in this locality. they were assured that coal was there, though it must certainly be a very long way down, and consequently the pit by which alone the seams could be worked would have to be unusually deep. the capitalists were not daunted by this consideration. but, before incurring the great expense of sinking the shaft, they determined to make a preliminary search and verily the actual presence of workable seams of useful fuel. they determined to bore a hole down through the rocks deep enough to reach the coal, if it could be reached. a boring for coal was, of course, by no means a novelty; but there was an unprecedented 124degree of mechanical skill and scientific acumen shown in this memorable boring near leipzig. the result of this enterprise was to make the deepest hole which, with perhaps a single more recent exception not of so much scientific interest, has ever been pierced through the crust of the earth. this boring was merely a preliminary to the operations which would follow if the experiment were successful in discovering coal. it was accordingly only necessary to make a hole large enough to allow specimens of the strata to be brought to the surface.
the instrument employed in sinking a hole of such a phenomenal depth through solid rock is characteristic of modern enterprise. the boring tool had a cutting edge of diamonds: for no other cutting implement is at once hard enough and durable enough to advance steadily, yard by yard, through the various rocks and minerals that are met with in the descent through the earth’s crust. we might, perhaps, illustrate the actual form of the tool as follows: imagine a piece of iron pipe, about six inches in diameter, cut squarely across, with diamonds inserted round its circular end, and we have a notion of the diamond drill. if the drill be made to revolve when held vertically, with the diamonds in contact with the rocks, the cutting will commence. as the rotation is continued, the drill advances through the rocks, and a solid core of the material will occupy the hollow of the pipe. we do not now enter into any description of the many mechanical details; there are ingenious contrivances for removing the débris produced by the attrition of the rocks as the diamonds cut their way, and provision is also made for carefully raising the 125valuable core which, as it provides specimens of the different strata pierced, will show the coal, if coal is ever reached. there is, of course, an arrangement by which, as the first length of drill becomes buried, successive lengths can be added, so as to transmit the motion to the cutting edge and enable the tool to be raised when necessary; in this manner one length of solid rock after another is brought up for examination. these cores, when ranged in series, give to the miner the information he requires as to the different beds of rock through which the instrument has pierced in its descent and as to the depths of the beds. a series of cores will sometimes show astonishing variety in the material through which the drill has passed. here the tool will be seen passing through a bed of hard limestone, and then entering a bed of soft shale; now the tool bores through dense and hard masses of greenstone, anon it pierces, it may be, a stratum of white marble; and finally the explorer may hope to find his expectations realised by the arrival at the surface of a cylinder of solid coal.
the famous boring to which we are now referring, though very deep, was not large in diameter. as it descended the comparatively large tool first employed was replaced by a succession of smaller tools, so that the hole gradually tapered from the surface to the lowest point. at its greatest depth the hole was indeed hardly larger than a man’s little finger. it increased gradually all the way to the surface, where it was large enough for a man’s arm to enter it easily.
how often do we find that the success which rewards mechanical enterprise greatly transcends even the most sanguine estimate previously formed! without 126the actual experience which has been acquired, i do not think anyone could have anticipated the extraordinary facilities which the diamond drill has given in the operations of a deep boring. this hole at schladebach was, indeed, a wonderful success. it pierced deeper than any previous excavation, deeper than any well, deeper than any coal pit. from the surface of the ground, where the hole was some six inches in diameter, down to the lowest point, where it was only as large as a little finger, the vertical depth was not less than one mile and a hundred and seventeen yards.
it is worth pondering for a moment on the extraordinary mechanical feat which this represents. when the greatest depth was reached, the total length of the series of boring rods from the surface where the machinery was engaged in rotating the tool down to the cutting diamonds at the lower end where the penetration was being effected, was as long as from piccadilly circus to the top of portland place. if a hole of equal length had been bored downwards from the top of ben nevis, it would have reached the sea level and gone down 1,200 feet lower still. when the foreman in charge wished to look at the tool to see whether it was working satisfactorily, or whether any of the diamonds had got injured or displaced, it was necessary to raise that tremendous series of rods. each one of them had to be lifted, had to be uncoupled, and had to be laid aside. i need hardly say that such an operation was a very tedious one. the collective weight of the working system of rods was about twenty tons, and not less than ten hours’ hard work was required before the tool was raised from the 127bottom to the surface. we may, i believe, conclude that so much ingenuity and so much trouble was never before expended on the act of boring a hole; but the results are full of information on important problems of science.
i am not going to speak of the geological results of this exploration. there is not the least doubt that the remarkable section of the earth’s crust thus obtained is of much interest to geologists. our object in now alluding to this wonderful boring is, however, very different. its significance will be realised when we say that it gives us more full and definite information about the internal heat of the earth than had ever been obtained by any other experiment on the earth’s crust. no doubt many previous observations of the internal heat of the globe were well known to the investigators who feel an interest in these important questions; but the exceptional depth of this boring, as well as the exceptionally favourable conditions under which it was made, have rendered the information derived from it of the utmost value to science.
we ought first to record our special obligation to the german engineer, captain huyssen, who bored this wonderful hole. he was not only a highly skilful mining engineer, diligent in the pursuit of his profession, but, by the valuable scientific work he has done, he has shown himself to be one of those cultivated and thoughtful students who love to avail themselves of every opportunity of searching into nature’s secrets. our thanks are due to him for the remarkable zeal with which he utilised the exceptional opportunities for valuable scientific work that 128arose, incidentally as it were, in connection with the work committed to him.
of course, everybody knows that the temperature of the earth is found to increase gradually as greater depths are reached. the rate at which the increase takes place has been determined on many occasions. but when opportunities have arisen for taking the temperature at considerable depths below the earth’s surface, it has happened sometimes that the observations have been complicated by circumstances which deprived them of a good deal of their accuracy. if our object be to learn the law connecting the earth’s temperature with the depth below the surface, it is not sufficient to study the thermometric readings in different coal pits. throughout the workings in every pit there must be arrangements for ventilation. the cool air has to be drawn down, and thus the temperature indicated in the pit is forced below the temperature which would really be found at that depth if external sources of change of temperature were absent.
captain huyssen rightly deemed that the hole which he had pierced presented exceptional opportunities for the study of the important question of the earth’s internal temperature. precautions had, of course, to be observed. the hole, as might be expected, was filled with water, and the water would tend, if its circulation were permitted, to equalise the temperature at different depths. but the ingenious captain quickly found an efficient remedy for this source of inaccuracy. he devised an arrangement, which i must not delay to describe, by which he could place temporary plugs in the hole at any depths he might desire; he then determined the temperature 129of the water in a short length, so plugged above and below that the circulation was stopped, and accordingly the water thus confined might be relied on to indicate the temperatures of the strata which hold it.
fig. 22.—special
thermometer
for use in
deep borings.
the thermometer employed in an investigation of this sort is ingenious though extremely simple. the ordinary maximum thermometer is not found to be adapted for the purpose. the instrument (fig. 22) employed in the determination of underground temperatures is very much less complicated and at the same time much more accurate. the contrivance is indeed so worthy of notice that i do not like to pass it by without a few words. the thermometer with which the temperature of the earth is ascertained in such investigations is not like any ordinary thermometer. there is no scale of degrees attached to it or engraved upon it, as we generally find in such instruments. the instrument with which the temperature of the deep hole was measured was merely a bulb of glass with a slender capillary stem, the end of which was not closed. when it was about to be lowered to test the temperature of the rocks at the lowest point to which the drill had penetrated, the bulb and the tube were first filled with mercury to the top, and brimming over. this simple apparatus was attached to a long wire, by the aid of which it could be lowered down this deep hole. down it went till at last the thermometer reached the bottom, which, as we have explained, it could not do until more than a mile of 130wire had been paid out. the instrument was then left quietly until it presently assumed the same temperature as the rocks about it. there could be no interference by heat from other strata, as the circulation of water was prevented by the plugging already referred to. the temperature to which the thermometer had been exposed must, therefore, have been precisely the temperature corresponding in that particular locality to that particular depth below the earth’s surface.
as the thermometer descended, it passed through a succession of strata of ever-increasing temperature. consequently the mercury, which, it will be remembered, had completely filled the instrument when it was at the surface, began to expand according as it was exposed to greater temperatures. as the mercury expanded, it must, of course, flow out of the tube and be lost, because the tube had been already full. so long as the mercury was gaining in temperature, more and more of it escaped from the top of the tube, and the flow only ceased when the instrument was resting at the bottom of the hole, and the mercury became as hot as the surrounding rocks. no more mercury was then expelled, the tube, however, remaining full to the brim. after allowing a sufficient time for the temperature to settle definitely, the thermometer was raised to the surface. as it ascended through the long bore the temperature surrounding it steadily declined. with the fall in the temperature of the mercury the volume of that liquid began to shrink; but the mercury already expelled could not be recalled. when at last the instrument had safely reached the surface, after its long journey down and 131up, and when the mercury had regained the temperature of the air, the lessened quantity that remained told the tale of the changes of temperature.
it is now easy to see how, even in the absence of an engraved scale on the instrument, it is possible to determine, from the amount of mercury remaining, the temperature to which the thermometer has been subjected at the bottom of the boring. it is only necessary to place this thermometer in a basin of cold water, and then gradually increase the temperature by adding hot water. as the temperature increases the mercury will, of course, rise, and the hotter the water the more nearly will the mercury approach the top of the tube. at last, when the mercury has just reached the top of the tube, and when it is just on the point of overflowing, we may feel certain that the temperature of the water in the basin has been raised to the same temperature as that to which the instrument was subjected at the bottom of the boring. in each case the temperature is just sufficient to expand the quantity of mercury remaining in the instrument so as to make it fill precisely both bulb and stem. when this critical condition is reached, it only remains to dip a standard thermometer, furnished with the ordinary graduation, into the hot water of the basin. thus we learn the temperature of the basin, thus we learn the temperature of the mercury in the thermometer, and thus we determine the temperature at the bottom of the boring over a mile deep.
i need not specify the details of the arrangements which enabled the skilful engineer also to determine the temperature at various points of the hole intermediate 132between the top and the bottom. in fact, taking every precaution to secure accuracy, he made measurements of the temperature at a succession of points about a hundred feet distant throughout the whole depth. in each case he was careful, as i have already indicated, to plug the hole above and below the thermometer, so as to prevent the circulation of water in the vicinity of the instrument. the thermometer, therefore, recorded the temperature of the surrounding rocks without any disturbing element. fifty-eight measurements at equal distances from the surface to the greatest depths were thus obtained.
we have now to discuss the instructive results to which we have been conducted by this remarkable series of measurements. first let us notice that there is much less variation in the subterranean temperatures than in the temperatures on the earth’s surface. on the surface of the earth we are accustomed to large fluctuations of temperature. we have, of course, the diurnal fluctuations in temperature from day to night; we have also the great seasonal fluctuations between summer and winter. but below a certain depth in the ground the temperature becomes much more equable. whether the temperature on the surface be high or whether it be low, the temperature of any particular point far beneath the surface does not change to any appreciable extent. in arctic regions the surface of the earth may undergo violent seasonal changes of temperature, while at a few feet below the surface the temperature, from one end of the year to the other, may remain sensibly unaltered.
in deep and extensive caverns the temperature is sometimes found to remain practically unaffected by 133the changes in the seasons. the mammoth cave of kentucky is a notable instance. the uniformity of the temperature, as well as the mildness and dryness of the air, in those wonderful subterranean vaults is such that many years ago a project was formed to utilise the cavern as an abode for consumptive patients, for whose cure, according to the belief then prevailing, an equable temperature was above all things to be desired. houses were indeed actually built on the sandy floors of the cavern, and i believe they were for some time tenanted by consumptive patients willing to try this desperate remedy. the temperature may have been uniform and the air may have been dry, but the intolerable gloom of such a residence entirely neutralised any beneficial effects that might otherwise have accrued. the ruins of the houses still remain to testify to the failure of the experiment.
the heat received from the sun does not penetrate far into the earth’s crust, and consequently the diurnal and even the seasonal changes of the temperature at the surface produce less and less effect with every increase of the depth. all such variations of temperature are confined to within 100 feet of the surface. at the depth of about 100 feet a fixed temperature of 52° fahrenheit is reached, and this is true all over the earth. it matters not whether the surface be hot or cold, whether the latitude is tropical and the season is midsummer, whether the latitude lie in the arctic regions and the season be the awful winter of iron-bound frost and total absence of sun—in all cases we find that about 100 feet below the surface the temperature is 52°. with sufficient accuracy we may say that this 134depth expresses the limit of the penetration of the earth’s crust by sunbeams. the remarkable law according to which the temperature changes below the depth of 100 feet is wholly irrespective of the solar radiation.
the study of the internal heat of the earth may be said to begin below the level of 100 feet, and the results that were obtained in the great boring are extremely accordant. the deeper the hole, the hotter the rocks; and captain huyssen found that for each sixty-six feet in descent the temperature increased one degree fahrenheit. to illustrate the actual observations, let us take two particular cases. we have said that the hole was one mile and 117 yards deep. let us first suppose the thermometer to be lowered 117 yards and then raised, after a due observance of the precautions required to obtain an accurate result. the temperature of the rocks at the depth of 117 yards is thus ascertained. in the next observation let the thermometer be lowered from the surface to the bottom of the hole, that is to say, exactly one mile below the position which it occupied in the former experiment. the observations indicate a temperature 80° fahrenheit higher in the latter case than in the former. we have thus ascertained a most important fact. we have shown that the temperature of the crust of the earth at the depth of one mile increases about 80°. this is at the rate of one degree every sixty-six feet. i should just add, as a caution, that if we choose to say the temperature increases one degree per sixty-six feet of descent, we ought to suppose that we start from a point which is not higher than that level of 100 feet above which 135as already explained, the temperature of the rocks is more or less affected by solar heat.
we have described these particular observations in some detail because they have been conducted under conditions far more favourable to accuracy than have ever been available in any previous investigations of the same kind. but now we shall omit further reference to this particular undertaking near leipzig. it is not alone in that particular locality, not alone in germany, not alone in europe, not alone on the surface of any continent, that this statement may be made. the statement is one universally true so far as our whole earth is concerned. wherever we bore a hole through the earth’s crust, whether that hole be made in the desert of sahara or through the icebound coasts of greenland, we should find the general rule to obtain, that there is an increase of temperature of about 80° for a mile of descent. this is true in every continent, it is true in every island; and, though we cannot here go into the evidence fully, there is not the least doubt that it is true also under the floor of ocean. if beneath the bed of the atlantic a hole a mile deep were pierced, the temperature of the rocks at the bottom of that hole would, it is believed, exceed by about 80° the temperature of the rocks at the surface where the hole had its origin. we learn that at the depth of a mile the temperature of the earth must generally be 80° hotter than it is at the level of constant temperature near the surface.
it may perhaps help us to realise the significance of this statement if we think of the following illustration. let us imagine that the waters of the ocean were removed from the earth. the ocean may in places be 136five or six miles deep, but that is quite an inconsiderable quantity when compared with the diameter of the earth. the change in the size of the earth by the removal of all the water would not be greater, proportionally, than the change produced in a wet football by simply wiping it dry. let us suppose that an outer layer of the earth’s surface, a mile in thickness, was then to be peeled off. if we remember that the diameter of the earth is 8,000 miles, we shall see that this outer layer, whose removal we have supposed, does not bear to the whole extent of the earth a ratio even as great as that which the skin of a peach does to the fruit inside. but this much is certain, that if the earth were so peeled there would be a wonderful difference in its nature. for though practically of the same size as it is at present, it would be so hot that it would be impossible to live upon it.
next comes the very interesting question as to the temperature that would be found at the bottom of a hole deeper still than that we have been considering. our curiosity as to the depths of the earth extends much below the point to which captain huyssen drove down his diamond drill. the trouble and the cost of still deeper exploration of the same kind seem, however, to be actually prohibitive. to bore a hole two miles deep would certainly cost a great deal more than twice the sum which sufficed to bore a hole one mile deep. at a great depth each further foot could only be won with not less difficulty and expense than a dozen, or many dozen feet, at the surface. mining enterprise does not at present seem to contemplate actual workings at depths much over a mile, so there does not seem much chance of 137any very much deeper boring being attempted. we do not say that a hole two miles deep would be actually impossible; it may well be wished that some millionaire could be induced to try the experiment. we should greatly like to be able to lower a thermometer down to a depth of two miles through the earth’s crust.
seeing there is but little chance of our wish for such future experiments being gratified, it is consolatory to find that actual observations of this kind are not indispensable to the argument on which we are to enter. our argument can indeed be conducted a stage further, even with our present information. the indications already obtained in the hole one mile deep go a long way towards proving what the temperature of a hole still deeper would be. we have already remarked that it was part of captain huyssen’s scheme to obtain careful readings of his thermometer at intervals of 100 feet from the surface to the bottom of the hole. a study of these readings shows that the increase of 80° in a mile takes place uniformly at the rate of one degree for each sixty-six feet of depth. as the temperature increases uniformly from the surface down to the lowest point which our thermometers have reached, it would be unreasonable to suppose that the rate of increase would be found to suffer some abrupt change if it were possible to go a little deeper. as the temperature rises 80° in the first mile, and as the rate of increase is shown by the observations to be quite as large at the bottom of the hole as it is at the top, we certainly shall not make any very great mistake if we venture to assume that in the second mile the temperature would also increase to an extent 138which will not be far from 80°. this inference from the observations leads to the remarkable conclusion that at a depth of two miles the temperature of the earth must be, we will not say exactly, but at all events not very far from, 160° higher than at the level of constant temperature about 100 feet down.
as in the former case, we need not confine ourselves to any particular locality in drawing this conclusion. the arguments apply not only to the rocks underneath leipzig, but to the rocks over every part of the globe, whether on continents or islands, or even if forming the base of an ocean. no one denies that the law of increase in temperature with the depth must submit to some variation in accordance with local circumstances. in essential features it may, however, be conceded that the law is the same over all the earth. if we take 52° to be the temperature of the level 100 feet down, which limits the seasonal variations, and if we add that at two miles further down the temperature is somewhere about 160° more, we come to the conclusion that at a depth of a little over two miles the temperature of the rocks forming the earth’s crust is about 212° fahrenheit. thus we draw the important inference that if, the oceans having been removed, we were then to remove from the earth’s surface a rind two miles thick—a thickness which, it is to be observed, is only the two-thousandth part of the earth’s radius—we should transform the earth into a globe which, while it still retained appreciably the same size, would have such a temperature that even the coolest spot would be as hot as boiling water. this is indeed a remarkable result.
139and now that we have gone so far, it is impossible for us to resist making a further attempt to determine what the temperature of the earth’s crust must be if we could send a thermometer still lower. a hole one mile deep we have seen; i do not think we can hope to see a hole two miles deep, but still it may not be absolutely impracticable; but a hole of three or more miles deep we may safely regard as transcending present possibilities in engineering enterprise. are we therefore to be deprived of all information as to the condition of our earth at depths exceeding those already considered? fortunately we can learn something. we are assisted by certain laws of heat, and, though the evidence on which we believe those laws is necessarily limited to the experience of nature as it comes within our observation, yet it is impossible to refuse assent to the belief that the same laws will regulate the transmission of heat in the crust of the earth two miles, three miles, or many miles beneath our feet.
i represent, in the diagram shown in fig. 23, three consecutive beds of rock—a, b, and c—as they lie in the earth’s crust, a little more than a mile beneath our feet. i shall suppose that the bed b is the very lowest rock whose temperature was determined in the great boring. the drill has passed completely through a, it has pierced to the middle of b, but it has not entered c. the observations have shown that the temperature of the stratum b exceeds that of the stratum a, and we further note that this is a permanent condition—that is to say, b constantly remains hotter than a. from this fact alone we can learn something as regards the temperature of the 140stratum c which lies in contact with b. of course we are unable to observe the temperature of c directly, because by hypothesis the boring tool has not entered that rock. we can, however, prove, from the laws of the conduction of heat, that the temperature of c must be greater than that of b; and this appears from the following consideration.
fig. 23.—at the bottom of the
great bore.
it is plain that c must be either just the same temperature as b, or it must be hotter than b, or it must be colder than b. if c were the same temperature as b, then the law of conduction of heat tells us that no heat would flow from one of these strata to the other. the laws of heat, however, assure us that when two bodies at different temperatures are in contact the heat will flow from the hotter of these bodies into the colder, so long as the inequality of temperature is maintained. as b is hotter than a, then heat must necessarily flow from b into a, and this flow must tend to equalise the temperature in these strata, for b is losing heat while none is flowing into it from c. therefore b and a could not continue to preserve indefinitely the different temperatures which observation shows them to do. we are therefore forced to the conclusion that b and c cannot be at the same temperature.
next let us suppose that the temperature of the 141stratum b exceeded that of c. then, as a is colder than b, it appears that b would be lying between two strata each having a temperature lower than itself. but that, of course, cannot be a permanent arrangement, for the heat would then escape from b on both sides. the laws of heat, therefore, tell us that b could not possibly retain permanently a temperature above both a and c. observation, however, shows that the temperatures of a and b are persistently unequal. we are therefore obliged to reject the supposition that the temperature of c can be less than that of b.
we have thus demonstrated that the temperature of the stratum c cannot be the same as that of b. we have also demonstrated that it cannot be colder than b. we must therefore believe that c is hotter than b. this proves that the stratum immediately beneath that stratum to which the observations have extended must be hotter than it. thus, though the stratum below the bottom of the hole lies beyond the reach of our actual observation, we have, nevertheless, been able to learn something with regard to its temperature.
having established this much, we can continue the same argument further; indeed, it would seem that we can continue it indefinitely, so long as there is a succession of such strata. underneath the stratum c must lie another stratum d. but we have shown that c must be hotter than b, and precisely the same argument that has proved this will prove that d is hotter than c. underneath d comes the stratum e, and again the same argument will apply. inasmuch as d is hotter than c, it follows that e must be hotter than d. these three strata, c, d, and e, are all beyond the 142reach of the thermometer, we know nothing of their temperatures by direct observation; but none the less is the argument, which we are following strictly, applicable. thus we obtain the important result that in the crust of the earth the temperature must be always greater, the greater the depth beneath the surface.
we have seen that the rate of increase of temperature with the depth is about 80° for the first mile, and we deem it probable that the rate of increase may be maintained at about the same for the second mile. but we do not suppose that the rate of increase mile after mile will remain the same at extremely great depths. it may perhaps be presumed that there must be some increase of temperature all the way to the earth’s centre; but the rate of increase per mile may change as the centre is approached. the point of importance for our present argument is, that the temperature of the earth must increase with the depth, though the rate of increase is quite unknown to us at depths greatly beyond those which the thermometer has reached. it is easy to see that the conditions prevailing in the earth’s interior might greatly modify any conclusion we should draw from observations near the surface. our argument has been based on the laws of heat, as we find them existing in matter on the surface of the earth submitted to such ranges of different physical conditions as can be dealt with in our laboratories; but at such excessively high temperatures as may exist in the earth’s interior the properties of matter may be widely different from the properties of matter as known to us within the temperatures that we are able to produce and control. the enormous 143pressure to which matter in the interior of the earth must be subjected should also be mentioned in this connection. it is wholly impossible to produce pressures by any mechanical artifice which even distantly approach in intensity to that awful force to which matter is subjected in the earth’s interior.
it may be instructive to consider a few facts with respect to this question of pressure in the earth’s interior. a column of water 34? feet high gives, as everybody knows, a pressure of fifteen pounds on the square inch. it will be quite accurate enough for our present purpose to assume that the average density of rock is three times that of water: the pressure of ten feet of rock would therefore produce the same pressure as thirty feet of water, that is to say, fifteen pounds on the square inch. the pressure due to the superincumbent weight of a mile of rock would be more than three tons on the square inch. at the depth of ten miles beneath the earth’s surface the pressure, amounting as it does to over thirty tons on the square inch, would very nearly equal the pressure produced on the inside of a 100-ton gun when the charge of cordite has been exploded to drive the missile forth. this is indeed about as large a pressure as can well be dealt with artificially, for we know that the 100-ton gun has to be enormously strong if it is to resist this pressure. but ten miles of rock is as nothing compared with the thickness of rock that produces the pressures in the earth’s interior. even if a shell of rocks ten miles thick were removed from the surface it would alter the diameter of our globe by no more than one four-hundredth part. at the depth of about thirty miles from the surface the 144pressure in the earth’s interior would amount to some 100 tons on each square inch. with each increase in depth the pressure increases enormously, though it may not be correct to say that the pressure is proportional to the depth. a pressure of 1,000 tons on the square inch must exist at a depth which is still quite small in comparison with the radius of the earth.
we have not, and apparently cannot have, the least experimental knowledge of the properties of matter at the moment when it is subjected to pressure amounting to thousands of tons per square inch; still less can we determine the behaviour of matter at that pressure of scores of thousands of tons, to which much of the interior of the earth is at this moment subjected. professor dewar, in his memorable researches, has revealed to us the remarkable changes exhibited in the properties of matter when that matter has been cooled to a temperature which lies in the vicinity of absolute zero. we can, however, hardly hope that any experiments will give us information as to the properties of matter when heated to a temperature vastly transcending that which could ever be produced in our most powerful electric furnaces, and at the same time exposed to a pressure hundreds of times, or indeed we may say thousands of times, greater than any pressure that has ever been produced artificially by the action of the most violent explosive with which the discoveries of chemistry have made us acquainted.
fig. 24.—three consecutive shells of the earth’s crust.
we really do not know how far the laws of heat, which have been employed in showing that the temperature must increase as the depth increases, can be 145considered as valid under the extreme condition to which matter is subjected in the deep interior of our globe. the laws may be profoundly modified. it suffices, fortunately for our present argument, to say that, so far as observations have been possible, the temperature does gradually increase with the depth, and that this increase must still continue from stratum to stratum as greater depths are reached, unless it should be found that by the excessive exaltation of temperature and the vast intensity of 146pressure certain properties of matter become so transformed as to render the laws of heat, as we know them, inapplicable.
in subsequent chapters we shall have some further points to consider with respect to the interior of the earth and its physical characteristics, which are, however, not necessary for our present argument. what we now desire to prove can be deduced from the demonstrated fact that the earth’s temperature does steadily increase from the level of constant temperature, 100 feet below the surface, down to the greatest depth to which thermometers have ever been lowered. we may presume that the same law holds at very much greater depths, even if it does not hold all the way to the centre.
to make our argument clear, let us think of three different strata of rock. this time, however, we shall suppose them to cover the whole earth, and we shall consider them to lie within the first mile from the surface; they will thus be well within the region explored by observation (fig. 24). we shall also regard them as shells of uniform thickness, and it will be convenient to think of them as being so very thin that we may consider any one of the shells called a to have practically a uniform temperature. the next shell b immediately inside a will have a slightly greater temperature, and be also regarded as uniform, and the shell immediately inside that again will have a temperature greater still. we shall call the innermost of the three shells c, and c is hotter than the next outer shell b, while b is hotter than a. the laws of heat tell us that as b and a are in contact, and that as b is continually hotter than a, then b must be continuously 147transmitting heat to a. in fact, b appears to be constantly endeavouring to reduce itself to the temperature of a by sharing with a the excess of temperature which it possesses. but if we consider the relation between the shell b and the hotter shell c, immediately beneath it, we see that precisely the same argument will show that b is constantly receiving heat from c. we thus see that while b is continuously discharging heat from its outside surface, it is as constantly receiving heat which enters through its inside surface. heat enters b from c, and heat passes from b into a, so that b is in fact a channel through which heat passes from c into a.
that which we have shown to take place in those three consecutive layers in the earth’s crust must also take place in every three consecutive layers. each layer is continually receiving heat from the layer below, and is as constantly communicating heat to the layer above. no doubt the rocks are very bad conductors of heat, so that the transmission of heat from layer to layer is a very slow process. but even if this flow of heat be slow, it is incessant, so that in the course of ages large quantities of heat are gradually transmitted from the earth’s interior, and ultimately reach the level of constant temperature. there is nothing, however, to impede their outward progress, so at last the heat reaches the earth’s surface.
when the surface has been reached, then another law of heat declares what must happen next. it is, of course, by conduction that the heat passes from layer to layer in its outward progress, until it ultimately gains the surface. at the surface the heat is 148then absolutely removed from the solid earth either by the convection through the air or by direct radiation into space.
i may here interrupt the argument for a moment to make quite clear a point which might perhaps otherwise offer some difficulty to the reader. when this outward flow of heat reaches the superficial layers it becomes, of course, mixed up with the heat which has been absorbed by the soil from the direct radiation of the sun, and this varies, of course, with the hour of the day and with the season of the year. the heat which steadily leaks from the interior has an effect on the rocks near the surface, which is only infinitesimal in comparison with the heat which they receive from periodic causes. we may, however, say that whatever would be the temperature of the rock, so far as the periodic causes are concerned, the actual temperature is always to some minute extent increased by reason of the heat from the earth’s interior. the argument is, perhaps, still clearer if, instead of attending to the earth’s surface, we think only of that shell, some 100 feet down, which marks the limit of the depth to which the seasonal and diurnal variations of heat extend. the argument shows how the internal heat of the earth, passing from shell to shell in the interior, reaches this layer of constant temperature, and passing through it, enters into those superficial strata of the earth which are exposed to the seasonal variations. with what befalls that heat ultimately we need not now concern ourselves; it suffices for our argument to show that there is a current of heat outward across this level. it is a current which is never 149reversed, and consequently must produce a never-ceasing drainage from the heat with which it would seem that the interior of the earth is so copiously provided.
calculations have been made to ascertain how much heat passes annually from the earth’s interior, across this surface of constant temperature, out into the superficial regions from which in due course it becomes lost by radiation. a convenient way of measuring a quantity of heat is by the amount of ice it will melt, for of course a definite quantity of heat is required to melt a definite quantity of ice. it has been estimated by professor j. d. everett, f.r.s., that the amount of internal heat escaping from our earth each year would be sufficient to melt a shell of ice one-fifth of an inch thick over the whole surface of the globe. we cannot indeed pretend that any determination of the actual loss of heat which our earth experiences could be very precise. sufficient observations have not yet been obtained, for the operation is so slow that an immense period would have to elapse before the total quantity of heat lost would be sufficient to produce effects large enough to be measured accurately. but now let us hasten to add that, for the argument as to the nebular theory with which we are at present concerned, it is not really material to know the precise rate at which heat is lost. it is absolutely certain that a perennial leakage of heat from the interior of the earth does take place. this fact, and not the amount of that leakage, is the essential point.
and this loss, which is at present going on, has been going on continually. heat from the earth has 150been lost this year and last year; it has been lost for hundreds of years and for thousands of years. not alone during the periods of human history has the earth’s heat been declining. even throughout those periods, those overwhelming periods which geology has revealed to us, has this earth of ours been slowly parting with its heat.
let us pursue this reflection to its legitimate consequence. whatever may ultimately become of that heat, it is certain that once radiated into space it is lost for ever so far as this globe is concerned. you must not imagine that the warm beams of the sun possess any power of replenishment by which they can restore to the earth the heat which it has been squandering for unlimited ages; we have already explained that the effect of the heat radiated to us from the sun is purely superficial. even amid the glories of the tropics, even in the burning heat of the desert, the vertical sun produces no appreciable effects at depths greater than this critical limit, which is about 100 feet below the surface. the rigours of an arctic winter have as little effect in reducing the temperature of the rocks at that depth as the torrid heat at the equator has in raising it. the effect in each case is nothing.
the argument which we are here employing to deduce the nebulous origin of our earth from the increase of temperature with increase in depth in the earth’s crust must be cleared from an objection. it is necessary to explain the matter fully, because it touches on a doctrine of very great interest and importance.
that a rotating body should possess a quantity 151of energy in virtue of its rotation will be familiar to anyone who has ever turned a grindstone or watched the fly-wheel of an engine. a certain amount of work has to be expended to set the heavy wheel into rotation, and when the machine is called upon to do work it will yield up energy and its motion will undergo a corresponding abatement. the heavy fly-wheel of the machine in a rolling mill contains, in virtue of its motion, enough energy to overcome the tremendous resistance of the materials submitted to it. once upon a time the earth revolved upon its axis in six hours, instead of in the twenty-four hours which it now requires. at that time the energy of the rotation must have been sixteenfold what it is at present. this consideration shows that fifteen-sixteenths of the energy that the earth originally possessed in its rotation has disappeared, and we want to know what has become of it.
we are here entering upon a matter of some difficulty. it is connected with that remarkable chapter in astronomy which describes the evolution of the earth-moon system. the moon was originally a part of the earth, for in very early times, when the earth was still in a plastic state, a separation would seem to have taken place, by which a small piece broke off to form the moon, which has been gradually revolving in an enlarging orbit until it has attained the position it now occupies. a considerable portion of the energy of the earth’s rotation has been applied to the purpose of driving the moon out to its present path, but there is a large remainder which cannot be so accounted for. it is well known that the evolution of the moon has been a remarkable consequence of 152tidal action. there are tides which sway to and fro in the waters on the earth’s surface; there are tides in any molten or viscous matter that the earth may contain, and there are even certain small tidal displacements in the solid material of our globe. tides of any kind will generate friction, and friction produces heat, and the energy of the earth’s rotation, which we have not been able to account for otherwise, has been thus transformed into heat. throughout the whole interior of the earth heat has been produced by the tidal displacement of its parts. the question therefore arises as to whether the internal heat of the earth may not receive an adequate explanation from this tidal action, which is certainly sufficient as to quantity. it is easy to calculate what the total quantity of this tidal heat may have been. we know the energy which the earth had when it rotated in six hours, and we know that it now retains no more than a sixteenth of that amount. we know also precisely how much was absorbed in the removal of the moon, and the balance can be evaluated in heat. it can be shown, and the fact is a very striking one, that the quantity of heat thus arising would be sufficient to account many times over for the internal heat of the earth. it might therefore be urged plausibly that the internal heat which we actually find has had its origin in this way. and if this were the case the argument which we are using in favour of the nebular origin of the earth, would be, of course, invalidated.
we may state the issue in a slightly different manner, as follows. heat there is undoubtedly in the earth; that heat might have come from the prim?val nebula 153as we have supposed, and as in actual fact it did come. but apparently it might have come from the tidal friction. why then are we entitled to reject the latter view, and say that the tidal friction will not explain the internal heat, and why are we compelled to fall back on the only other explanation?
lord kelvin suggested a test for deciding to which of these two sources the earth’s internal heat was to be attributed. professor g. h. darwin applied the test and decided the issue. we have dwelt upon the rate at which the heat increases with the descent, this rate being about one degree every sixty-six feet. now the distribution of the heat, if it had come from the tidal action, would be quite different from the distribution which would result from the gradual efflux of heat from the centre in the process of cooling. and, speaking quite generally, we may surmise that the heat produced by tidal friction would be distributed rather more towards the exterior of the earth than at its centre. we might therefore reasonably expect that if the internal heat of the earth arose from tidal friction it would be more uniformly distributed throughout the globe, and there would not be so great a contrast between the high temperature of the interior and the lesser temperatures near the surface as there is when the heat distribution is merely the result of cooling. it has been proved that if the internal heat had its origin from the tidal friction, the rate of increase with the depth would be totally different from what it is actually found to be. it would be necessary to go down 2,000 feet to obtain an increase of one degree, instead of only sixty-six feet, as is actually the case.
154hence we conclude that the increasing heat met with in descending through the earth’s crust is not accounted for by tidal friction; it has its origin in the other alternative, namely, from the cooling of the prim?val nebula. the heat which was undoubtedly produced by the tidal friction has gradually become blended with the heat from the other, and, as we must now say, the principal source. the facts with regard to the rate of increase with depth thus show that, whatever the tides may have done in producing internal heat, there has been another and a still more potent cause in operation. the important conclusion for our present purpose is that our argument may justly proceed without taking account of the effect of tidal friction.
we are led by these considerations to a knowledge of a great transformation in the nature of our globe which must have occurred in the course of ages. we have seen that this earth is gradually losing heat from its interior, and we have seen that this loss of heat is incessant. from the fountains of heat, still so copious, in the interior the supply is gradually dissipating. now heat is only a form of energy, and energy, like matter, cannot itself be created out of nothing. there can be no creation of heat in our earth without a corresponding expenditure of energy. if, therefore, the earth is radiating heat, then, as there is no known or, indeed, conceivable source of energy by which an equivalent can be restored, it follows that the earth must have less internal heat now than it had at any earlier period. no doubt the process of cooling is excessively slow. the earth has less internal heat at present than it had a hundred years ago, but 155i do not suppose that even in a thousand years, or perhaps in ten thousand years, there would be any appreciable decline in the quantity of heat, so far as any obvious manifestations of that heat are concerned. it is, however, certain that the earth must have been hotter, even though there are not any observations to which we can appeal to verify the statement; and as our retrospect extends further and still further through the ages we see that the globe must have been ever hotter and ever still hotter. whatever be the heat contained in our earth now, it must have contained vastly more heat ten million years ago; how otherwise could the daily leakage of heat for all those ten million years have been supplied? it follows that there must have been much more heat somewhere in our earth ten million years ago than there is at present, and the further our retrospect extends the hotter do we find the earth to have been. there was a time when the temperature of the earth’s surface must have been warmed not alone by such sunbeams as fell upon it, but by the passage of the heat from the interior.
no matter how early be the period which we consider, we find the same causes to be in operation. there was a time when, owing to the internal heat, the surface of the earth must have been as hot as boiling water. the loss of heat by radiation must then have taken place much more copiously than it does at present. the argument we are pursuing must therefore have applied with even greater force in those early days. there was a time when the materials at the surface of the earth must have been intensely heated, when they must have even been red-hot. there was a time when the earth’s surface must have 156had a temperature like that of the lava as it issues from a volcano. there must have been a time when the surface of the earth was not even solid, when indeed it was a viscid liquid, and earlier still the liquid must have been more and more incandescent. from that brilliant surface heat was vehemently radiated. each day the globe was hotter than on the succeeding day. there is no break in the argument. we have to think of this glowing globe passing through those phases through which we know that all matter will pass if only we apply to it sufficient heat. the globe assumed the liquid state from that state which is demanded by a temperature still higher, the state in which the matter is actually in the form of vapour. even the most refractory substances will take the form of vapour at a very high temperature.
thus we are conducted to a remarkable conception of the condition in which the materials now forming our solid earth must have been in the exceedingly remote past. what is now our earth must once have been a great quantity of heated vapour. it need hardly be said that in that form the volume of the earth was much larger than the volume which the earth has at present, while no doubt the mass of the earth then was even less than the mass of the earth now, by reason of the meteoric matter which has been drawn in by our globe.
but even when our earth was in this inflated state of vapour our argument can be still maintained. thus we see that the earth, or rather the cloud of vapour which was ultimately to form the earth, is ever growing larger and larger in our retrospect, ever becoming more and more rarefied; and it may well have 157been that there was a time when the materials of this earth occupied a volume thousands of times greater than they do at present.
in a previous chapter we have seen how the sun was at one time in the nebulous state, and now we have been led to a similar conclusion with regard to the earth. at that time, of course, the sun was greatly in excess of its present dimensions, and the earth was also greatly swollen. the nebula which formed our sun, and the nebula which formed our earth, were both so vast as to be confluent; they were indeed both part of the same vast nebula.
such has been the earth’s beginning so far as modern science can make it clear to us. we have at least indicated the course which events must have taken according to the laws of nature as we understand them. many of the details of the great evolution are no doubt unknown at present, and perhaps must ever remain so. that the events which we have endeavoured to describe do substantially represent the actual evolution of our system is the famous nebular theory.
