146 HISTORY OF THEKMOTICS. 



tlic general circumstances of the earth, and are perfectly in accordance 

 with the principles on which Fourier's theory rests. 



2. Climate. The terra climate, which means inclination, was 

 applied by the ancients to denote that inclination of the axis of the 

 terrestrial sphere from which result the inequalities of days in different 

 latitudes. This inequality is obviously connected also with a difference 

 of thermotical condition. Places near the poles are colder, on the 

 whole, than places near the equator. It was a natural object of curi- 

 osity to determine the law of this variation. 



Such a determination, however, involves many difficulties, and the 

 settlement of several preliminary points. How is the temperature of 

 :n iv place to be estimated? and if we reply, by its mean temperature, 

 how are we to learn this mean ? The answers to such questions 

 require very multiplied observations, exact instruments, and judicious 

 generalizations ; and cannot be given here. But certain first approxi- 

 mations may be obtained without much difficulty ; for instance, the 

 mean temperature of any place may be taken to be the temperature 

 of deep springs, which, is probably identical with the temperature of 

 the soil below the reach of the annual oscillations. Proceeding on 

 such facts, Mayer found that the mean temperature of any place was 

 nearly proportional to the square of the cosine of the latitude. This, 

 ac a law of phenomena, has since been found to require considerable 

 correction ; and it appears that the mean temperature does not depend 

 on the latitude alone, but on the distribution of land and water, and 

 on other causes. M. de Ilumboldt has expressed these deviations 13 

 by his map of isothermal lines, and Sir D. Brcwster has endeavored 

 to reduce them to a law by assuming 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 neighboring 

 parts, and radiates out from the poles into the surrounding space. 

 A.nd thus, in the case of the earth, the solar heat enters in the tropical 



" British Assoc. 1833. Prof. Forbes's Report on Meteorology, p. 215. 

 13 Fourier. Mim. List. torn. v. p. 173. 



