280 M. L. Cordier, Examination of recent Experiments 



centigrade division. This division is also that which I have used 

 in all parts of this memoir. 



With these explanations, I now proceed to the examination 

 of the experiments which have been made on the temperature of 

 the air contained in mines. 



1. Temperature of the Air in Mines. — The experiments on 

 the temperature of the air of mines would be unobjectionable, 

 and we would have reason to suppose that they give the exact 

 temperature of the zone of rock in which they have been made, 

 had their circumstances been similar to those of the caves of 

 the Observatory of Paris, that is to say, had they been made in 

 excavations of small extent, and especially of little height, situ- 

 ated in the original rock, defended by a sufficient closure 

 from all foreign influence, such as the passage of workmen, 

 the access of water, the introduction of external air, and shut up 

 for a length of time sufficient to allow the original temperature of 

 the walls to be completely re-established. But none of these ob- 

 servations have been made in such favourable circumstances. 



To appreciate the various kinds and degrees of inaccuracy to 

 which they have all been subjected, we shall first consider what 

 might take place in a mine, which we shall suppose of some ex- 

 tent, composed of several stages, free of filtrations, and which has 

 been kept hermetically shut since the period at which it was aban- 

 doned. The air in each stage would assume the temperature of 

 the surrounding rock. This air, upon the hypothesis which we 

 maintain of a heat increasing in the earth as the depth increases, 

 would continually circulate from the lower to the upper stages, 

 and vice versa, on account of tlie differences of specific gravity, 

 arising from the inequality of the heat which would take place at 

 each level. These continual motions would be the more lively, 

 the wider and less sinuous the subterranean canals were, and the 

 greater the number of their communications. In the opposite 

 case, the displacement of the air would be produced slowly, es- 

 pecially at the most remote extremities of each stage ; and it 

 would happen, that, towards these extremities, the temperature 

 of the air would not differ much from that of the surrounding 

 rock. In this case, and still more in the former, the temperature 

 of the air would never exactly represent in any point the tempera- 

 ture of the rock in contact with it. 3 



