THE INTERIOR OF THE EARTH. 299 



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 pe rhaps to chemi- 

 cal 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 pro- 

 prietor the interest the undertaking Avould 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 thermo- 

 metric 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. 



