SUBTERRANEAN CHEMICAL FORCES. 407 



central fire, it is difficult to comprehend why the 

 tropical mountains should be from five to six 

 times higher than those of the polar regions, 

 why volcanos are confined chiefly to the vicinity 

 of the ocean, why they should discharge enor- 

 mous quantities of steam saturated with muriatic 

 salts, why they should continue in action at 

 intervals for unknown periods of time, and then 

 become extinct, why volcanic islands are more 

 numerous in the tropical than higher latitudes, 

 and why hot springs should not be equally dis- 

 tributed all over the earth. 



The facts collected by M. Cordier and others, 

 proving that the temperature of the earth in- 

 creases as we descend below the surface, have 

 been regarded by many geologists, as conclu- 

 sive evidence that earthquakes and volcanos are 

 caused by a central fire. It is true, that in 

 many parts of the world, the temperature of 

 rocky strata, and of the water that issues from 

 them, is higher at the bottom of deep mines than 

 at the surface, varying greatly however in dif- 

 ferent places. Mr. Fox found that in the Dal- 

 coath copper mine, in Cornwall, at the depth of 

 two hundred and thirty fathoms, a thermometer 

 placed in a hole in the rock, stood at 76 F., and 

 at 82 in water, ten fathoms deeper ; the mean 

 annual temperature of the surface being 50. 

 But the same gentleman states that about two 

 million gallons of water are daily pumped from 



