76 TERRESTRIAL ADAPTATIONS. 



vapour contained in the atmosphere : the winds 

 and rains by which the equilibrium of the atmos- 

 phere is restored when it is in any degree dis- 

 turbed. The effects of light, of electricity, pro- 

 bably of other causes also, are no doubt important 

 in the economy of the vegetable world, but these 

 agencies have not been reduced by scientific 

 inquiries to such laws as to admit of their being 

 treated with the same exactness an d certain ty which 

 we can obtain in the case of those first mentioned. 

 We shall proceed to trace some of the pecu- 

 liarities in the laws of the different physical 

 agents which are in action at the earth's surface, 

 and the manner in which these peculiarities bear 

 upon the general result. 



The Laws of Heat ivith respect to the Earth. 



One of the main causes which determine the 

 temperature of each climate is the effect of the 

 sun's rays on the solid mass of the earth. The 

 laws of this operation have been recently made 

 out with considerable exactness, experimentally 

 by Leslie, theoretically by Fourrier, and by other 

 inquirers. The theoretical inquiries have required 

 the application of very complex and abstruse 

 mathematical investigations ; but the general 

 character of the operation may, perhaps, be 

 made easily intelligible. 



The earth, like all solid bodies, transmits into 

 its interior the impressions of heat which it re- 

 ceives at the surface ; and throws off the super- 



