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

"Mundus Subterraneus," 1664, vol. ii.

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 Teherguine, 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, 48°; and * Only those observations made below twenty metres from the surface, where the temperature does not vary with the seasons, were taken into account.

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 64°, 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 Rüdersdorf, 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 1o 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 ChaudesAigues, 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