THE MOLTEN MASS WITHIN. 43 



dering all that we have yet learned on the subject, 

 it must he far below the greatest depths that 

 have yet been reached by our boring-rods. It 

 may therefore be assumed with all likelihood of 

 truth, that, so far downwards as the condition of 

 the ground remains nearly the same, the rise of 

 temperature with the increase of depth proceeds 

 according to the same law that has been already 

 established for the upper strata. 



If the increase of heat is taken to be one 

 degree (Centigrade) for every hundred feet of 

 depth, we should find, at a depth of ten thousand 

 feet, a temperature at which water would boil. At 

 one hundred and twenty thousand feet below the 

 surface, about twenty-three miles, and only -y-^ of 

 the length of the earth'' s radius, there must be a 

 heat of 1200° Centrigrade (nearly 2200° Fahr.) 

 at which cast-iron melts and basalt runs like water. 



Although we cannot give to these calculations 

 any greater value than that of being somewhere 

 near the truth, yet it is evident enough that the 

 cooled and hardened crust of our earth can form 

 but a small portion of its bulk, and that a vastly 

 greater mass of it must be still perhaps not 

 entirely in a molten liquid state, but at least in 

 one of glowing heat. This belief, which is so well 

 founded, as to be now accepted by all geologists, 

 gives us the key to the explanation of hot springs, 

 of earthquakes, and of volcanoes. 



