532 HISTORY OF THERMOTICS. 



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 dif- 

 ferent latitudes. This inequality is obviously con- 

 nected also with a difference of thermotical con- 

 dition. Places near the poles are colder, on the 

 whole, than places near the equator. It was a 

 natural object of curiosity to determine the law of 

 this variation. 



Such a determination, however, involves many 

 difficulties, and the settlement of several prelimi- 

 nary points. How is the temperature of any place 

 to be estimated ? and if we reply, by its mean tem- 

 perature, how are we to learn this mean? The 

 answers to such questions require very multiplied 

 observations, exact instruments, and judicious gene- 

 ralizations ; and cannot be given here. But certain 

 first approximations 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 an- 

 nual 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, as 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 distribu- 

 tion of land and water, and on other causes. M. de 



