Observations on Subterraneous Heat. 323 



K".B. The waters coming from the flltrations (which prin- 

 cipally take place in the upper parts of the ancient works) 

 are cold ; and as they form the greater part of those which 

 flow into the pit or well of St. Barbe, they are the v cause of 

 the little heat presented by those which exist there. 



10th, In an excavation not far distant from the bottom 

 of the well of St. Barbe (called the furnace gallery), the sides 

 of which are almost completely covered with radiated py- 

 rites, partly efflorescent, the thermometer left for more 

 than a quarter of an hour in a small hollow made in the 

 midst of the pyrites, and which contain a good deal of white 

 sulphate, — in this case the thermometer stood at, — Tempe- 

 rature 14-6°. Depth 140 m . 



1 lth, When afterwards plunged into a small hole whence 

 a very strong spring issued, it also stood at, — Temperature 

 14-6°. Depth. 1 JO m . 



Consequences. — 1st, Observations 2, 3, and 4, prove in- 

 contestably that the heat of the rock in the upper parts of the 

 mine is 12°, as the waters which indicated it filtered through 

 the rock ; and we find that this temperature does not sen- 

 sibly differ from that- pointed out by theory. If the first Ob- 

 servation gave a greater heat, it is because it was made in a 

 place through which air from without continually passes ; 

 and this air was warm, the experiments having been made 

 at the end of summer. 



2d, Observations 5 and 6 also show that the temperature 

 of the lower parts of the mine is more considerable than 

 that of the upper parts. If in deep places the air appears 

 to be warmer than the water, it is probably because it has 

 preserved a part of the heat which it had upon entering the 

 mine. I have already assigned the reason which accounts 

 for our having in Observations 7, 8, 9, a v less heat than 

 might be expected from the depth. 



3d, Experiments 10 and 11 show that there are cases in 

 which the presence of pyrites does not produce heat : the 

 heat indicated in these cases cannot depend upon that cause : 

 in the pit of St. George there is no pyrites, and the tempera- 

 ture is the same. 



Thus, if we abstract every extraordinary cause, the Ob- 

 X 2 servations 



