LETTER III. 



OF THE HEAT WITHIN THE EAUTH. 



The life-giving heat, which we enjoy on the 

 surface of the earth, we owe chiefly, indeed almost 

 wholly, to the sun. The heat that reaches us 

 from the other heavenly bodies, is, by reason of 

 their vast distance from us, but a small, indeed 

 almost an imperceptible fraction, compared with 

 that which the sun pours upon the earth. 



But the earth possesses a store of heat of its 

 own, besides that which it derives from the sun. 

 The mighty changes of its external condition, 

 which the globe has passed through in countless 

 by-gone ages, and is now passing through on a much 

 reduced scale, seem to depend essentially on a 

 ceaseless struggle between gravitation and the heat 

 of the earth. 



The notion had already sprung up among the 

 naturalists even of antiquity, that the earth must 

 have been originally in a glowing melted state. 

 This belief, though it was often thrown aside, 

 always forced itself into notice again ; and, with 

 the present extent of our knowledge, its truth can 

 scarcely now be doubted. According to this 

 view, the surface of the earth must have passed 



D 



