DEVELOPMENT OF THE EARTH. 319 



arrived at this idea by different confirmatory experi- 

 ments and reasonings. In the first place, the observation 

 that the temperature of tlie earth's crust continually increases 

 towards the centre is in favour of this supposition. The 

 deeper we descend, the greater the warmth of the ground, 

 and in such proportion, that with every 100 feet the 

 temperature increases about one degree. At a depth of 

 six miles, therefore, a heat of loOO*^ would be attained, suffi- 

 cient to keep most of the firm substances of our earth's crust 

 in a molten, fiery, fluid state. This depth, however, is only 

 the 286th part of the whole diameter of the earth (1717 

 miles). We further know that springs which rise out of a 

 considerable depth possess a very high temperature, and 

 sometimes even throw water up to the suiface in a boiling 

 state. Lastly, very important proofs are furnished by 

 volcanic phenomena, the eruption of fiery fluid masses of 

 stone bursting through certain parts of the earth's crust. 

 All these phenomena lead us with gi'eat certainty to the im- 

 portant assumption that the firm crust of the earth forms 

 only quite a small fraction, not nearly the one-thousandth 

 part of the whole diameter of the terrestrial globe, and that 

 the rest is still for the most part in a molten or fiery 

 fluid state. 



Now if, starting with this assumption, we reflect on the 

 ancient history of the development of the globe, we are 

 logically carried back a step further, namely, to the assump- 

 tion that at an earlier date the whole earth was a fiery fluid 

 body, and that the formation of a thin, stiffened crust on the 

 surface was only a later process. Only gradually, by 

 radiating its intrinsic heat into the cold space of the universe, 

 has the surface of the glowing ball become condensed into 



