Popular Science Monthly/Volume 17/July 1880/The Interior of the Earth I
|THE INTERIOR OF THE EARTH.|
By R. RADAU.
THE additions that are being continually made to our knowledge of the composition and physical condition of the most distant heavenly bodies may well prompt one to ask why we are still so poorly informed concerning the constitution of the planet which the Creator has assigned to us for a dwelling-place. Mines and wells have barely scratched the solid crust that conceals the mysteries of the earth's depths. Our vague and uncertain ideas regarding the condition of the interior of the earth are based on analogies and inductions from facts observed on its surface or in the heavens. Very little light do we get on this subject from direct experiment. The bowels of the earth are not, indeed, easily accessible. Whatever the poet may say, the descensus Averni is not easily made; the domain of the stars is not thus hidden from us. For about two centuries large sums have been expended in the construction of gigantic telescopes with which to sound the depths of space; but no attempt, as a purely scientific undertaking, has been made to fathom the secrets of the underground world. The object of the numerous mines in different parts of the world has been simply the discovery of mineral riches, and the depths they have reached barely exceed, even in a few rare instances, a thousand metres; i. e., hardly the six-thousandth of the earth's radius—corresponding, on a globe thirteen metres (about forty-two feet) in diameter, to a puncture one millimetre (about four one-hundredths of an inch) in depth.
Notwithstanding this paucity of positive data, it will not be interesting to review the state of our knowledge on this obscure subject, and to show on what sides the question is accessible to science.
The form of the planets is itself an index, to a certain point, of the mode of their origin and their actual condition. These slightly flattened globes that wheel about the sun have been subject to the same laws that shape the drop of water and the grain of shot. It is impossible not to believe that they are specimens on a vast scale of the equilibrated form assumed by free fluid masses through the action of internal forces which assemble and unite their molecules. All these spheroids have been or still are liquid drops that have become flattened by reason of their rotary motion. Newton was led to infer the flattening of the poles from the idea that the earth had originally been in a liquid state, as the centrifugal force due to rotation tends to swell the equatorial at the expense of the polar regions. By the operation of the same force that impels a stone when swung in a sling to free itself, and that causes grindstones to burst when turned too rapidly, the particles of a revolving sphere tend to fly from the axis of rotation, and this centrifugal force, nil at the poles, increases as the equator is approached, and there attains its maximum intensity. The effect of this is to diminish weight, substances being a little less heavy at the equator than at the poles.
Imagine the earth completely liquid: the equatorial portion, driven by centrifugal force, will be elevated while the poles will be depressed. To better comprehend this, let us imagine a siphon, the two arms of which, joined at the center, issue, one at one of the poles and the other at the equator. The two liquid columns therein can remain in equilibrium, as the globe revolves, only on condition that the equatorial column, which is exposed to the action of centrifugal force, be longer than the polar column, which has lost nothing of its weight from this cause. The sphere becomes a flattened spheroid. This change of form can be demonstrated by turning rapidly on its vertical axis a sphere of clay or of flexible steel circles, as used in illustration of physics. As the pliant mass solidifies more or less completely, this flattened form is preserved.
That there is a discrepancy between weight at the equator and at the poles, more marked as we approach or recede from one or the other, may be shown by noting, by the tension of a spring, the weight of the same mass under different latitudes; but a more positive means of ascertaining this fact is furnished by the oscillations of the pendulum, which are retarded as the force of the earth's attraction diminishes. The astronomer Richer, having been sent. to Cayenne in 1672 to observe the planet Mars, remarked that a timepiece regulated at Paris lost ten and a half minutes daily at Cayenne. It was this circumstance, at first inexplicable, that led Newton to suspect that the earth was a flattened spheroid.
It will be evident that an exact knowledge of the figure of the earth has an important bearing on any hypothesis of the internal constitution of our planet. Geodesy, that science that may be called surveying on a grand scale, and which takes for its bases of measurement at once the earth and the heavens, has not yet completed its work. Since the labors of the Abbot Picard, to whom we owe the first measurement of a meridional degree, and the celebrated voyages of Bouguer and La Condamine to Peru, and of Maupertuis to Lapland, which confirmed the supposition of the flattening of the earth, there have been many other immense labors of a similar kind all over the world. The Société Géodetique Internationale, organized some years ago, is occupied in compiling and perfecting the results of these researches, and in deducing therefrom a provisionally definite result. We know with certainty that the form of the earth is not greatly different from that of a perfect sphere, for the flattening ascertained by geodetic measurements is, in round numbers, equal to 300, from which it follows that the equatorial radius does not exceed the polar by more than twenty-two kilometres (a little less than fourteen miles). This number, which represents the amount of the equatorial swelling, is equal to four and a half times the height of Mont Blanc, but, on a ball thirteen metres in diameter, the twenty-two kilometres in question would make an inequality of only two centimetres (about three fourths of an inch), and this would be totally imperceptible to the eye. The natural inequalities of the earth's surface are comparatively insignificant; the Alps and Himalayas, on a ball thirteen metres in diameter, would be represented by projections of a few millimetres only, and the greatest ocean-depths would not exceed one centimetre.
The question of the true figure of the earth is one of the most difficult of problems. From the time of Newton it had been held that the earth was a revolving ellipsoid—in other words, that the meridians were ellipses, and the equator and all the parallels true circles; and it only remained to determine the ellipticity of these meridians, all being supposed alike. It is now twenty years since Captain Clark's calculations, based on the uniformity of the great triangulations made up to that period in various parts of the world, led to the conclusion that the equator itself has an elliptic form, and that, consequently, the meridians are ellipses unequally flattened. According to Clark, the equatorial flattening is 3270, or about one tenth of the average flattening of the meridians. This depression, amounting to two kilometres, occurs under the meridian passing, in the east, through the Sun da Archipelago, and in the west through the Isthmus of Panama, while the enlargement occurs under the meridian of Vienna, crossing central Europe and Africa. Thus, according to the calculations, the world is an ellipsoid with three unequal axes. This supposition can be made to harmonize with the hypothesis of the primitive fluidity of the earth, the form in question being one of those assumed by free liquids in rotation. It was found, however, that Clark's calculations were considerably affected by certain anomalies probably existing in some of the geodetic calculations employed, and it seems that a majority of those competent to judge in these matters endorse the theory of a revolving ellipsoid.
By the term "figure of the earth" is understood the geometrical form of an ideal surface coinciding with the mean level of the sea, and prolonged in thought beneath the continents. In fact, geodetic calculations are always reduced to the sea-level, the altitudes of the stations being first determined from levels based on the nearest coast-line. The great difficulty is to accurately determine this level for a given station. For a long time it was supposed that the surface of the open sea was a horizontal; in other words, that it was parallel to the surface of liquids in repose, and perpendicular to the direction of the plummet line. But this definition is insufficient, as may easily be shown. The apparent vertical indicated by the plummet-line or determined at the level of the sea, is simply the direction of weight, which may be materially affected by local attractions due to an irregular distribution of the masses composing the soil. The vicinity of a mountain will deflect the plummet to a considerable degree, and a subterranean cavity may cause a deflection in the opposite way.
Let us now imagine the continents divided by a network of canals that connect all the seas, thus making of them one continuous sheet of water, as it were. Setting aside, for the purpose of the illustration, the oscillations caused by the tides, this sheet of water, assumed to be immovable, which represents the mean level of the sea, will exhibit elevations and depressions attributable to the local influences that deflect the plummet-line. The attraction of the continents causes a notable elevation of the sea-level along the coast, and a proportionate lowering of the mid-ocean. This influence of continents was described by M. Saigey in 1842, who gave as the probable height of the sea on the coasts of Europe thirty-six metres. Seven years later Mr. Stokes, the celebrated English physicist, attacked the question, bringing to bear upon it all the resources of mathematical analysis; and Philipp Fischer, in 1868, estimated that the disturbance of level due to the attraction of continents might amount to nine hundred metres. The mean level of the sea is, therefore, in all probability, an irregularly undulated surface, and the ideal or geometrical surface of the earth a regular spheroid, deviating but little from this average level, the accidental irregularity of which is in some way equalized.
The triangulations by which the terrestrial arcs are measured define the dimensions and configuration of this spheroid by the comparison of distances measured on the earth with the corresponding angular amplitude ascertained from the astronomical latitudes and longitudes of the stations. The most delicate part of the operations consists in ascertaining the local attractions that cause the deviations of the plummet. This difficulty is felt particularly in the Russian and Indian triangulations. While Colonel Chodsko found in the Caucasus a deflection of fifty-four seconds, and Schweitzer, in an open plain in the environs of Moscow, deflections of eight and nine seconds, the Himalayan chain appears to have had but an insignificant influence in place of the considerable one which the theory required—as if these mountains were composed of less dense rocks than the soil of the plain.
The operations referred to serve to indicate the form of the earth by the angles which the verticals of a series of stations—i. e., the direction of weight—make with the earth's axis. Another mode consists in measuring at numerous points the degree of the weight, and from this the distance to the center of the earth, the rate of oscillation of the pendulum being also noted. These oscillations are accelerated as the attractive power of the earth increases—that is, as the center is approached. We have seen that Richer remarked these variations of the pendulum in his voyage to Cayenne, and that Newton furnished the explanation of the phenomenon. At the commencement of the present century Biot, Sabine, Kater, Lütke, Foster, and others, made numerous experiments of this nature which have furnished a valuable verification of the results of geodesy, properly so called. But it must not be forgotten that the degree of weight may be changed by the same causes that change its direction. A local accumulation of very dense rocks may increase the terrestrial attraction, and light ones may diminish it. The de-leveling of the ocean of which we have spoken, by which the waters near continents are elevated while the mass of the ocean at large is lowered, results in making an ocean-valley, as it were, from which the islands, that are thus nearer the earth's center than the continents, project. This will explain the increased rate of oscillation of the pendulum observed in many islands, which is otherwise inexplicable.
The perturbations to which the direction as well as the degree of weight is subject have enabled us to determine the earth's mean density. The principle of the method is easily comprehended. Let us suppose that the deflection of the plummet has been measured near an isolated mountain whose volume and weight it is possible to estimate with some degree of precision. The amount of the deflection will furnish a means of calculating the relation of the mass of the mountain to that of the earth, and, the two masses being known, their relative densities can then be determined. The oscillations of the plummet at the summit and at the foot of the mountain afford the basis for a similar calculation. On carrying the plummet to the top some oscillations per day will be lost, the distance from the earth's center being increased; but the mountain's own attraction in part offsets the decrease in weight attributable to altitude, and herein we have the means of comparing its mass with that of the earth.
These methods were not neglected by Bouguer in his voyage to Peru. Aided by La Condamine, he observed the variation in the plummet-line due to the influence of Chimborazo, and he noted the rate of the pendulum's movement on the volcanic mountain of Pichincha (which is equal in height to Mont Blanc), and at the sea-level. Unfortunately, the imperfection of his instruments, the rigor of the climate, and the violence of the winds, prevented the two French astronomers from attaining great precision in these observations. The effects they had expected to see confirmed were found to be much less marked than they had anticipated, and Bouguer therefore believed that the volcanic mountains of Peru were hollow and internally simply huge caverns. A repetition of his experiments, with the care required in researches of such a delicate nature, would determine the question whether the unsatisfactory character of his results was due to errors of observation, or if it was a case similar to that of the Himalayan chain.
Bouguer's method was employed successfully in 1774 by the celebrated English astronomer Maskelyne. He chose for his experiments Mount Schiehallion in Scotland. This mountain is wholly isolated. Its geologic constitution is known, and its form is not very irregular; the calculations were thus simplified. By observations of the stars that passed near his zenith Maskelyne first determined the latitudes of two stations, one to the south and the other to the north of the mountain—the distance between them being 1,330 metres. The difference in the two astronomical latitudes was found to be 43″ instead of 54″. 6, as shown by the measured distance. The excess of 11″.6 represented the sum of the attractive force exerted by Schiehallion on its opposite sides. It then remained to ascertain the volume of the mountain, its exact configuration, density, and total weight, and by the aid of these factors to determine the theoretic sum of the attraction it exerted on the plummet of the two stations. The geologist Hutton was intrusted with this work, which occupied him three years. The result of Lis calculations was, that the deviation observed would be explained by supposing the mean density of the mountain to be to that of the earth as 5 is to 9. Hutton first adopted for the density of Schiehallion the number 2·5—about the density of quartzose sandstone; according to this the mean density of the globe was 4·5. He afterward modified these figures, taking 3·0 for the density of the mountain and 5·4 for that of the earth. The geological study of this mountain, undertaken subsequently by Playfair and Lord Webb-Seymour, showed the density of its component rocks to be intermediate between these two estimates, from which it would appear that the earth's density is 4·7.
These experiments were not supplemented by observations of the pendulum's oscillations; it is true the mountain's slight elevation—one thousand metres—did not promise a very marked effect. An experiment of this kind was made in 1821 by the astronomer Carlini, on Mont Cenis, which showed the earth's density to be in the vicinity of the number given by Maskelyne. In 1854 Airy performed an analogous experiment at the bottom of the Harton coal-mine. At a depth of 1,220 feet it was demonstrated that the seconds pendulum advanced in speed two and a quarter seconds per day, and from this it was concluded that the mean density of the globe is to that of the surface as 2·63 to 1, and, taking the density at the surface to be 2·3, that of the globe is 6·1. M. Saigey endeavored to find the density of the globe by the deflection of the plummetdine due to a whole continent's attraction, calculating the theoretic deviation from the vertical at Evaux, a central point of France, and one of the stations of the meridian of Paris. According to Puissant's calculation, there exists between the astronomical and geodetical latitudes of Evaux a difference of about 7″, which would indicate that the attraction of the southern part of France, i. e., to the south of the latitude of Evaux, exceeds that of the northern portion. Now, with a good orographic chart the average elevation of the ground from about Evaux to the Pyrenees, the Alps, and to the neighboring seas can be calculated, and with these data the effect of all the partial attractions that affect the plummet-line at Evaux. M. Saigey has shown that, to account for the discrepancy pointed out by Puissant (who supposes the attraction of the globe to be about 30,000 times greater than that of all France above Evaux), the mean density of the earth must be to that of France alone as 1·7 is to unity. Taking 2·5 for the density of the ground, as compared with water, it gives 4·25 as the density of the globe.
The researches of Maskelyne, above referred to, may be reduced to a closet experiment: one can weigh the earth in his own room! This was first done by the illustrious Cavendish. This, the youngest, son of the Duke of Devonshire, who sacrificed his hopes of fortune to his love of science, commenced his career in poverty. "His parents," M. Biot tells us, "seeing that he was good for nothing, treated him with indifference, and gradually became estranged from him. He made amends by becoming one of the first chemists of his time, and, when he had acquired celebrity, one of his uncles, who had been a general abroad, returned at a happy moment to leave him an inheritance of three hundred thousand francs rental. He also left him at his death a fortune of thirty million francs. Cavendish was thus the most wealthy of all the learned, and probably the most learned of all the wealthy."
Cavendish had received from Hyde-Wollaston an apparatus which he in turn had obtained as a bequest from John Michell, and which was designed to determine the weight of the earth by the attraction exerted by two large globes of lead on two small balls suspended from the ends of a movable lever. There was certainly something novel and bizarre in this idea of attempting to observe the attraction of a ball of lead, which we are accustomed to consider an inert mass—in trying to demonstrate by sight its infinitesimal share in universal gravitation. It was accomplished, nevertheless. Cavendish improved Michell's apparatus by applying to it the principle of the famous torsion balance of Coulomb—the torsion of a wire opposed as a moderate force to the attraction exerted on a lever carried by the wire. His experiments were communicated to the Royal Society of London in 1798. The mode of making the observations is easily described. A horizontal lever of fir-wood was suspended to a metallic wire dependent from the ceiling of a closed chamber. At its two extremities were two small balls and two blades of ivory, marked with divisions. All the movements of the lever were observed from without through lunettes fixed in the walls of the chamber, and directed toward these divisions. Finally, two large globes of lead, each weighing 158 kilogrammes and sustained by a screw-gauge, could be moved toward or from the balls at will, by mechanism worked from the outside. Now, whenever they approached the small balls the latter were seen to obey the attraction of the globes of lead; they were displaced, and oscillated around a new point of equilibrium where the reaction of the torsion wire counterbalanced the attraction of the globes. From these experiments and the ascertained strength of the attraction of the globes in relation to their weight, it is easy to estimate the relation of the mass of the globes to that of the earth, and thence the density of the earth. Cavendish thus found the earth's density to be 5·48, that of water being unity.
Cavendish's experiments were repeated by F. Reich, at Freiberg, in two trials in 1837 and 1849, and also at London in 1842, by Francis Baily, under the auspices of the Astronomical Society. Reich's figures differed but little from Maskelyne's (5·44 to 5·58). Baily's experiment gave a little larger figure (5·67). Baily improved upon the apparatus of Cavendish in several ways: he changed the size and material of the small balls, using balls of platinum, lead, brass, zinc, glass, and ivory. The figure he settled upon was the average of over two thousand tests; still, it is not wholly reliable, his results being affected by certain errors the cause of which was for a long time unknown. The question was of an importance that warranted a reexamination of the data with all the resources of modern science. Two French physicists, A. Cornu and J. Bailie, have recently accomplished this work. Their experiments, commenced in 1870, have been the subject of various interesting communications to the Academy of Sciences. Their apparatus are deposited in the vaults of the Polytechnic School. They are much smaller than those of Cavendish and Baily, for, as Messrs. Cornu and Bailie have remarked, there is an advantage gained in these experiments by reducing the dimensions of the apparatus. The attracting mass, formed of mercury contained in two hollow spheres of bronze, 0·12 metre in diameter, weigh twelve kilogrammes. By transpiration the mercury can be made to pass from one sphere to the other, thus doubling the effect of the attraction, and this change is effected without shock or disturbance. The lever of the torsion-balance is a little tube of aluminium, 0·50 metre in length, carrying at each end a ball of copper weighing one hundred and nine grammes. A flat mirror fixed in the middle reflects the divisions of a horizontal scale five or six metres distant, and the slightest movement of the lever is thus revealed by a displacement of the scale divisions. The time of a double oscillation of the lever is about seven minutes. The phases of these oscillations are registered by electricity. A great merit of these researches consists in the opportunity they afford for a thorough study of all the causes of perturbation that can introduce error into such experiments. The definite result can be accepted with confidence. The figure thus far obtained is 5·56. It may be added that Messrs. Cornu and Bailie have discovered the cause of the too large number given by Baily. In correcting the errors of system in his experiments, it is probable that a slightly different number will be obtained—5·55. To sum up, the earth's mean density thus appears to be five and a half times that of water, and the density at the surface is less than half that of the interior, or about 2·5. Consequently there must be in the interior heavy masses whose excess of density compensates for the lack thereof in the rocks at the surface. This need not be surprising, for the heavy pressure sustained by the deeper strata must naturally increase the density. But what is the law governing this increase of density from surface to center? Legendre formulated a simple law, adopted also by Laplace, according to which the surface density is 2·5, at the middle of the radius 8·5, and at the center 11·3, the mean being taken as 5·5. A different law, to which M. Edouard Roche arrived by theoretic considerations, gives a surface density of 2·1, a mid-radius density of 8·5, and 10·6 at the center. This agreement of results deduced from three different hypotheses shows that the decision of the question is narrowed to small dimensions. Adopting M. Roche's conclusions as the most probable, it can be said that the mean density of the earth is about double that of its surface, and that the density of the center is double that of its mid-radius. The central strata or masses have a density approximating to that of lead.
The fact of an elevated temperature in the depths of the earth can no longer be doubted, though the law according to which the heat increases as we descend below the surface is still far from being perfectly understood. As early as the seventeenth century Father Kircher mentions the subterranean heat that was felt at the bottom of mines. Boerhaave and Boyle also make mention of observations concerning the heat existing in the center of the earth. Still, it was not until 1740, nearly a century and a half after the invention of the thermometer, that a serious attempt was made to measure this heat. This was first done by Gensanne, director of the lead-mines of Giromagny (Vosges), who lowered a thermometer to a depth exceeding four hundred metres, and proved that the temperature increases at the rate of one degree to nineteen metres. Toward the end of the century Horace de Saussure, desiring to ascertain whether the earth's proper heat had any effect on the melting of glaciers, made a similar experiment in the salt-mines of Bex, and found the rate of increase to be 1° to 37 metres. Many similar experiments have since been made; it will suffice to cite the most important.
Cordier, in his celebrated "Essay on the Temperature of the Interior of the Earth," read at the Academy of Sciences in the year 1827, compiled the results of his predecessors' researches in this matter and those obtained by himself in certain mines. In the mines of Carmeaux (Tarn) he found an increase of 1° to 36 metres, 1° to 19 metres in the mines of Littry (Calvados), and 1° to 15 metres at Decize (Nièvre). The average of his compilations is 1° to 25 metres. From these investigations he concluded that at a depth of some hundreds of kilometres the heat must be 100° of Wedgwood's pyrometer—sufficient to fuse lava.
To arrive at trustworthy results, it is not enough to observe merely the temperature of the air at the bottom of a mine, or that of the water that penetrates the adits, but the thermometers should be placed in cavities made in the natural rock, and left there a sufficient length of time to allow them to acquire the temperature of the surrounding medium. The currents in the air of mines lower the normal temperature, particularly by producing an evaporation of the moisture in the rock, and it thus happens in some mines that the temperature of the air is lower than that of the surface-air, as is the case in the Maestricht quarries. The heat due to the presence of workmen modifies the effect of this in a measure. It is estimated that in a gallery 4,650 metres long, and two metres high by one wide, the temperature will be raised 1° by ten men, each furnished with his lamp. As regards the water found in the adits, it is evident that they will not indicate the mine's true temperature unless they remain in it for a considerable time, for the water infiltrated from the surface, or coming from springs at certain depths, may be either warmer or colder than the rocks through which they percolate. The most reliable way, therefore, is to place the thermometers in cavities of the rocks in the mines, and also in the angles of the cuttings, where the rock is newly hewed, and still uncooled by contact with the air. Cordier pierced the rock for this purpose to a depth of 0·65 of a metre. Reich, who made a large number of observations in the mines of Erzgebirge, bored to the depth of a metre, using thermometers constructed for the purpose, with long stems projecting from the orifices in the rock, which were then packed with sand. These investigations were continued from 1830 to 1832, in twenty different mines, scattered over many square leagues. The thermometers were ranged as far as was practicable in a vertical line, at depths varying from 20 to 350 metres, the markings being taken twice or thrice weekly. From these observations it was found that the depth corresponding to an increase of 1° Cent, was 42 metres. In the Ural mines in Siberia, Kupper showed that a far more rapid rate of increase existed-1° to 20 metres—while in the mines of Prussia the rate was found to be much less rapid-1° to 57 metres, according to Gerhard. In certain isolated cases a far greater divergence is seen. It, moreover, appears to be established that the heat increases more rapidly in coal-mines than in metal mines, and in copper than in tin mines, and in the metalliferous rocks generally more rapidly than in the schists, while in granite the increase is more gradual than in any of the preceding. These differences are no doubt due to the greater facility with which certain earths conduct heat, and perhaps to chemical phenomena of which they are the seat.
It must also be said that in many cases the rate of increase, far from being uniform, appears to slacken as the greater depths are reached. Thus, according to Fox, the observations made in the Cornwall and Devonshire mines show a difference of 1° Cent, to 15 metres, down to a depth of about 100 metres, and 1° to 41 metres at a depth of 350 metres. This decrease is also very marked in the famous Tcherguine pit in Yakutsk, which is in completely frozen soil. Commenced in 1848, at the expense of a merchant named Fedor Tcherguine, who expected to find water at a depth of 10 metres, this pit was sunk in three years to a depth of 35 metres, still in frozen ground, and the work would have been abandoned if, happily for science, Admiral Wrangel, on a voyage to Yakutsk, had not represented to the proprietor the interest the undertaking would have in its bearing on the physics of the globe. It was therefore excavated for six years more, reaching a depth of 116 metres. Even there the earth was still frozen, and the work was finally abandoned in 1837, and the pit was carefully covered. In 1844 Middendorf visited it, and made a series of thermometric observations, according to which the mean temperature was found to be, at a depth of two metres, 11·2; at 60 metres, 4·8°; and at the bottom, 116 metres, 3°. It was thus seen that, whereas the rate of increase from the surface to 60 metres in depth was 6·4°, for the remainder of the depth, 56 metres, it was only 1·8°.
The experiments made in artesian wells have given analogous results—that is, wholly irregular as regards the rate of increase of temperature. The mean of 27 observations in Vienna is, according to Spasky, 1° to 20 metres. The very accurate experiments of Magnus, in 1831 at Rudersdorf, near Berlin, on the occasion of the boring of an artesian well, yielded the same result, but at Pregny, near Geneva, Messrs. Rieve and Marcet found the depth corresponding to an increase of 1 Cent, to be 32 metres. The well was sunk 220 metres. This figure represents sufficiently exactly the average rate of increase of temperature resulting from thermometric soundings made in artesian wells. Walferdin found an increase of 1° to 31 metres in the artesian wells of the Military School at Paris, in that at St. André (Eure) and in the well of Grenelle; and many others have given figures comprised between 30 and 35 metres for the difference of level representing a difference of 1° in temperature. The temperature of the water of the Grenelle well, 548 metres deep, and of the Passy well, 570 metres deep, is 28°, while the mean temperature of Paris is 10·6. These waters, therefore, receive from the deep strata an addition to their temperature of a little more than 17°; i. e., a little more than 1° to each 32 metres of depth. The much deeper borings of Musalweek, near Minden in Prussia, 700 metres, and of Mondorf in the grand duchy of Luxemburg, 730 metres, show a difference of 1° to 30 or 31 metres.
From a comparison of the temperatures observed by Walferdin near Creuzot, at the bottom of a boring 816 metres deep, and in a neighboring well 554 metres deep, it also appears that at these depths the heat increases more rapidly than at the surface. But wells situated very near each other may give widely varying results. Thus at Naples, according to M. Mallet, in two very deep artesian wells, distant from each other 1,600 metres, the depths corresponding to 1° of additional heat were 45 and 109 metres respectively.
The observations of M. Mohr, in 1876, in a well 4,000 feet deep, pierced through a salt-rock at Speremberg, near Berlin, led this physicist to believe that the rate of progression sensibly slackens as we descend below the surface—a conclusion agreeing with Fox's deductions from observations in the English coal-mines. M. Mohr remarked that from 700 feet, where the glass marked 19·6° Cent., to 3,300 feet, where it marked 46°, the difference in temperature corresponding to a difference of 100 feet, diminished in a regular ratio, so that, continuing the sounding, beyond 5,000 feet only a barely perceptible increase could be observed. But M. A. Boué, who warmly contested M. Mohr's conclusions, has observed with reason that percolated water will frequently lower the temperature of these deep beds, and this would explain the diminution observed by M. Mohr.
In this class of researches thermometers à déversement are used, the reservoirs of which overflow as the temperature rises; the mercury remaining in the ball shows the maximum attained. Walferdin's registering thermometer and the geothermometer of Magnus are constructed on this principle. Thermometers à minima, of a different construction, are used to determine the temperature of the ocean-depths, which are generally colder than the water at the surface. The many soundings made by the English scientific expeditions established beyond a doubt the fact that the temperature at the bottom of the sea is often but little above zero. This would be explained by supposing the colder water to be carried to the bottom by its specific gravity, the water warmed and dilated by the sun's heat remaining at the surface. The bed of the ocean at large, where the normal temperature is not affected by warm currents, such as the Gulf Stream, may be said to be covered with water at the freezing-point. The water at the bottom of fresh-water lakes is less cold because the maximum density of fresh water is 4°. It results from this that the portions possessing this temperature are carried to the bottom, while the colder or warmer portions rise to the surface. Thus, that portion of the earth's shell that is covered by water remains at a relatively low temperature, in consequence of the stratification resulting from the varying densities of the liquid, but, if it were possible to carry on in the bed of the sea such investigations as have been made on land, an increase of temperature, such as has been proved to exist in the frozen soil of Siberia, would doubtless be found.
The increase in heat as we descend is generally admitted to average 1° in 30 metres. If this rate is constant it is clear that, at a depth of 2,700 metres, the temperature must equal that of boiling water; and that, at a depth of 50 kilometres, the heat must exceed 1,600°, a point at which iron and the greater part of the rocks would melt. This is the principal ground for the argument of those who maintain that the earth's crust is not more than 40 to 50 kilometres thick—or, relatively to its size, of the thickness of an egg-shell compared with the egg. Certain it is that the increase of heat with the depth, confirmed by so many observers, perforce gives a warrant to the idea of a subterranean fire possessing an inconceivable degree of heat; but the question is, At what depth from the surface does this fire exist?
The thermometric observations thus far made are insufficient to decide this question. Among the mines that have reached a great depth may be mentioned those of Kitzbühel in the Tyrol (900 metres); Kutteuberg in Bohemia (1,200 metres); Mouille-Louge (920 metres); and Speremberg (1,260 metres). Why may not borings be made at the bottom of some of these very deep mines, by means of which the bowels of the earth can be still further penetrated?
It is also desirable that the natural cavities in the earth should be utilized for scientific investigation. The accounts contained in the old books that relate to this matter are unfortunately filled with exaggerations, and the lack of recent evidence prevents our extracting from them the portion of truth they perhaps contain. Pontoppidan, in his "Natural History of Norway," describes a cavity in the vicinity of Frederickshall, in which the duration of the fall of a stone appeared to be two minutes. Assuming, says Arago, that the stone fell clear, without hitting and being retarded by projections in the walls of the cavity, the total depth indicated by its two minutes' fall would be over 4,000 metres, exceeding by 800 metres the height of the highest mountain in the Pyrenees. But it would appear that the noise of the stone's falling was heard for two minutes—that it consequently rolled and bounded from point to point; and modern travelers have nothing further to say of the famous Frederickshall hole. Another account, of the legendary cavern of Dolsteen, in the Island of Herroe, Norway, is likewise doubtful. According to a belief among the inhabitants, this cavern extended to and under Scotland. It is told that, in 1750, two priests ventured far into it and heard the rumbling of the sea above them. Coming to the brink of a precipice, they threw over a large stone, which was heard to fall a minute after. Without, however, attaching importance to accounts from such unreliable sources, it may still be admitted that natural cavities exist which might be made use of in exploring the deeper strata of the earth's crust. M. Babinet, who cherished the idea of forming a society for digging a deep hole for such purposes, thought the question had its industrial side which ought not to be lost sight of. "This is no longer," he somewhere says, "the time of Voltaire, who so bitterly berated Maupertuis, whom he described as having wished to pierce the earth that we might see our antipodes by leaning over the edge of the well of this antagonist of the irascible king of literature. Nobody would today deny the possibility of sinking the shafts of mines to a depth of several thousand metres, when we have such choice of ground, dimensions, and, above all, time. Let us suppose that we have reached a depth of four kilometres only, and cleared a suitable space. If men can not support the heat, machines, which are not so delicate, can. We see ourselves in possession of a vast space, the walls of which are of the temperatures of our ovens and stoves. Conducting thereto a stream of water, it issues hotter than boiling water, and is a veritable mine of heat, as truly so as are the precious coal-mines of England and Belgium." It is a fact that the heat of the springs of Chaudes-Aigues, which reaches 80°, is made use of by the inhabitants for purposes of cooking, heating their houses, washing, etc. By conduits of wood, in all the streets of the village, reservoirs on the ground-floor of each house are supplied, and these serve the purpose of heating-stoves in cold weather, fires and chimneys being dispensed with. In summer, the inflow is stopped by little sluice-gates at the inlet of each supply-pipe, the water then flowing to the brook at the border of the village. M. Berthier, the chemist, has estimated that the heat furnished daily by these springs equals that produced by the combustion of more than four and a half tons of coal, sufficient to comfortably warm the houses and even the streets.
Now that the rapid exhaustion of coal-mines forces man to seek the precious combustible at increasingly greater depths, it interests us to know the extreme limit of accessible depth. The report of the English commission of inquiry contains very complete data on this point. The sole cause, says the report, that can place a limit on the practicable depth of mines is the elevation of temperature. In the English mines the temperature is of marked uniformity to the depth of about 15 metres, viz., 10° Cent. From that depth the heat increases at the rate of 1° to 37 metres, so that at a kilometre of depth it attains blood-heat. This heat hinders the mining operations by heating the air that is artificially made to circulate in the mines, and rendering its effect insignificant. Regarding the question of the highest temperature man can work under without danger to health, the evidence gathered by the commission exhibits some extraordinary cases, but such of them as were verified were found to be exaggerated. Competent authorities are, however, united in maintaining that steady labor is impossible, in humid air, at a temperature approaching 37°. Heat is better supported in dry air. Now, as the deepest mines are generally the least humid, we ought, with the powerful means of ventilation available in these days, to certainly reach a depth of 1,200 metres. We may perhaps go even deeper, thanks to the system of atmospheric shafts recently introduced at Epinac by a French engineer, M. Z. Blanchet. By means of a pneumatic tube the cars are propelled and a thorough ventilation is secured at the same time. By developing this system, very deep deposits can doubtless be reached.
[To be continued.]
- Translated from the "Revue des Deux Mondes," by Guy B. Seely.
- The length of a metre is about three feet three inches.
- The length of a kilometre is about five eighths of a mile.
- M. Saigey has shown that, by selecting from Bouguer's observations those which appear to have been made under favorable conditions, and by calculating the force of the attractions in a more exact manner, the density of the earth is found to accord with Maskelyne's estimate of it.
- The considerable difference between this number and that furnished by Maskelyne's observations induced Hutton, then advanced in age, to examine anew Cavendish's experiments. "I could not," he says, "rely on these results without repeating the entire computation. Still, after a long life spent in abstract researches, being now eighty, and overwhelmed with infirmities, I feel that I may be pardoned for shrinking from the task. But I should have no rest were I not myself to undertake the work." Hutton discovered many small errors in calculation, and he found 5·31 to be the measure of the earth's density.
- In the later experiments the number of the spheres was doubled.
- "Mundus Subterraneus," 1664, vol. ii.
- Only those observations made below twenty metres from the surface, where the temperature does not vary with the seasons, were taken into account.
- Elisée Reclus, "La Terre," vol. i, p. 239.
- See in the "Revue" for October 1, 1876, an article on the "Coal Production of England and France."