AT THt; earth's SURFACE. 429 



sooner, but still more from the unequal action of the solar 

 rays, which is more intense towards the equator. Hence the 

 countries round the poles must have been the first that w*re 

 habitable, and this gradual cooling has caused the same tem- 

 perature to proceed successively over all the regions of the 

 globe *. Not only are these conclusions unfounded, but the 

 changes which have occurred are probably the very reverse. No 

 heat can escape from this planet, but in consequence of a high 

 temperature being kept up at its surface by the communica- 

 tion of heat from an external source. Until this be attained, it 

 must retain the heat it receives from the sun ; and this must 

 accumulate principally towards the poles. 



That the temperature at the surface has risen above the ori- 

 ginal temperature of the earth, may be inferred not only from 

 the consideration, that solar heat has been communicated to it, 

 which could not be discharged until a certain elevated tempe- 

 rature were attained, but also from this, that there is no natu- 

 ral operation actually generating cold ; there is only the pro- 

 duction of heat ; and cold prevails where this is less powerful, 

 or is counteracted. To account, therefore, for the low tempe- 

 rature at the colder regions of the earth, only two suppositions 

 can be advanced. Either the original temperature must have 

 been as low as this, or lower, and have been raised higher, 

 where it is actually superior, by the reception of the sun's rays : 

 Or it must be assumed, with Buffon and Bailly, that heat 

 ,can escape from this planet to an indefinite extent, and that it 

 is in a state of progressive refrigeration, the eflTects of which 

 are prevented from being apparent at the equatorial regions 

 hy the direct communication of solar heat, but towards the po- 

 lar circles are evident, as not being counteracted by the same 



Vol. VII. P. II. 3 H cause. 



* Letlres sur rOrigine des Sciences, p. 329. 



