CONDUCTION AND RADIATION. 533 



Humboldt has expressed these deviations 12 by his 

 map of isothermal lines, and Sir D. Brewster has 

 endeavoured to reduce them to a law by assum- 

 ing two poles of maximum cold. 



The expression which Fourier finds 13 for the 

 distribution of heat in a homogeneous sphere, is 

 not immediately comparable with Mayer's empirical 

 formula, being obtained on a certain hypothesis, 

 namely, that the equator is kept constantly at a 

 fixed temperature. But there is still a general 

 agreement; for, according to the theory, there is 

 a diminution of heat in proceeding from the equator 

 to the poles in such a case ; the heat is propagated 

 from the equator and the neighbouring parts, and 

 radiates out from the poles into the surrounding 

 space. And thus, in the case of the earth, the solar 

 heat enters in the tropical parts, and constantly 

 flows towards the polar regions, by which it is 

 emitted into the planetary spaces. 



Climate is affected by many thermotic influ- 

 ences, besides the conduction and radiation of 

 the solid mass of the earth. The atmosphere, for 

 example, produces upon terrestrial temperatures 

 effects which it is easy to see are very great ; but 

 these it is not yet in the power of calculation to 

 appreciate 14 ; and it is clear that they depend upon 

 other properties of air besides its power to transmit 



12 British Assoc. 1833. Prof. Forbes's Report on Meteorology, 

 p. 215. 



13 Fourier, Mem. Inst. toni. v. p. 173. M Id. toni. vii. p. 584. 



