THE EARTH'S INTERIOR. 225 



the idea would not bold water. A globe of solid 

 rock reimsiijg upon a ligbt walery nucleus is a 

 clearly impossible physical conception — the crust 

 would have to sink to the centre under the influ. 

 ence of gravity, exactly as a stone in the same 

 circumstances sinks to the bottom of a sea or 

 river. One niiglit as well expect to find rocky 

 islets floating on tht; sea, as continents and the 

 whole solid shell of earth floating idly upon an 

 aqueous centre. At the same time a good many 

 causes began to lead men to suspect that the 

 interior of the earth possesses a far higher tem- 

 perature than the cooled and solid surface. It is 

 known that, when we dig deej) below the level of 

 the soil, as in mines or well-boring, the thermom- 

 eter rises higher and higher in a fixed proportion 

 according to the depth to which we have pene- 

 trated. Pushing this calculation to its logical 

 conclusion, it was soon suggested that at four or 

 five miles below the surfiice the temperature must 

 rise to something like white heat ; we must imag- 

 ine the earth to possess a fiery core, surrounded 

 by a cooling, solidifying exterior. Again, the 

 existence of volcanoes, geysers, and hot springs, 

 all coming evidently from within the bowels of 

 our planet, and all apparently bearing witness to 

 a very hot and igneous origin, supported the 

 same rising theory. Once more, it was plausibly 

 argued that pressure by itself produces heat ; and 



