326 Observations on Subterraneous Heat, 



the surface of the ground, ought to be regarded as giving the 

 real degree of heat of the surface of the country in general. 

 I see no cause which could alter the temperature naturally 

 proper for this place, which is far distant from any working 

 places : one thing is certain, that it continues the same 

 during the whole year ; and the result is precisely the 

 same as pointed out by theory. Observations 2 and 3 also 

 show that this temperature increases in proportion as wc 

 descend. The current of air in the first gallery account^ 

 for the trifling excess of heat we remark there in propor- 

 tion to the depth. 



As to the temperature of the Observations made in the 

 southern part of the mine, it is visibly influenced by an ex- 

 traneous cause ; namely, by the vitriolic water which flows 

 from the south. On digging a new pit 100 metres distant 

 from the south part of the present workings, they have cut 

 through beds of an aluminous schistus, which has a very 

 strong styptic taste. By the help of a microscope we dis- 

 cover in it a multitude of pyritous points, which, by their 

 decomposition and their action upon the schistus, have 

 probably produced a disengagement of caloric, which must 

 have heated the water passing through these beds. The 

 latternot being very deep, communicate with the atmosphere 

 by some fissures, while decompositions and disengagements 

 must have been effectuated in the interior of the earth. 



However this may be, it seems certain that it is by pass- 

 ing through these beds that the water must have acquired a 

 heat of 20°, a heat far superior to that which agrees with 

 the depth at which it is found. 



I shall also here observe, that if we ascribe this .heat to 

 the pyrites, they produce it by their action upon the schis- 

 tus. In the observations made at Poullaouen, we have seen 

 pyrites in a considerable quantity occasion no particular in- 

 crease of heat. I shall repeat here what I have said in an- 

 other place : I have seen workings of pyrites, and I have 

 not found the heat sensibly stronger than in other mines: 

 thus I am led to think that the pyrites in a mass, at beaut 

 those not radiated, produce no subterraneous heat: but those 

 which are disseminated in minute particles in a body upon 



which 



