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.[1] 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
- ↑ Only those observations made below twenty metres from the surface, where the temperature does not vary with the seasons, were taken into account.
