92 ISOTHERMIC LINES. 



as uniformly existing at 80, while at the poles it is- 

 below the freezing point of water ; and as far as observa- 

 tions have been made, the subterranean temperatures 

 bear a close relation to the thermic condition of the 

 climate of the surface. The circulation 6"f water through 

 faults or fissures in the strata is, without doubt, one 

 means of carrying heat downwards much quicker than 

 it would be conducted by the rocks themselves. It is 

 not, however, found that the quantity of water increases 

 with the depth. In the mines of Cornwall, unless where 

 the ground is very loose, miners find that, after about 

 150 fathoms (900 feet), the quantity of water rapidly 

 diminishes. That water must ascend from very much 

 greater depths is certain, from the high temperatures at 

 which many springs flow out at the surface. In the 

 United Mines in Cornwall, water rises from one part of 

 the lode at 90 ; and one of the levels in these workings 

 is so hot that, notwithstanding a stream of cold water is 

 purposely brought into it to reduce the temperature, the 

 miners work nearly naked, and will bathe in water at 

 80 to cool themselves. At the bottom of Tresavean 

 Mine, in the same county, about 320 fathoms from the 

 surface, the temperature is 100. 



One cause of the great heat of many of our deep 

 mines, which appears to have been entirely lost sight of, 

 is the chemical action going on upon large masses of 

 pyritic matter in their vicinity. The heat, which is so 

 oppressive in the United Mines, is, without doubt, due 

 to the decomposition of immense quantities of the sul- 

 phurets of iron and copper known to be in this condition 

 at a short distance from these mineral works. 



The heat which man is enabled to measure beneath the 

 earth's surface, appears to be alone due to the conducting 

 powers of the rocks themselves ; it has been observed 

 that the line of equal temperature follows, as nearly as 

 possible, the elevations and depressions which prevail 

 upon the surface, and the diminishing rate of increase 





