S90 Professor P. Prevost upon the 



not capable of being accounted for by any other. The follow- 

 ing is a brief account of the state in which the discussion on this 

 subject stands at present. 



The southern winter is longer than the northern, and the sum- 

 mer proportionally shorter. But this circumstance introduces 

 no diiference in the quantities of solar irradiation, which either 

 hemisphere experiences in the course of a season of the same 

 name. It is demonstrated that, in the elliptical orbit, the differ- 

 ence of the distance from the sun exactly compensates the effect 

 of the duration of the corresponding seasons of the two hemi- 

 spheres ; the quantity of rays received by the earth is constantly 

 the same for the same number of degrees traversed upon the 

 ecliptic ; in other terms, the quantity of heat which the earth 

 receives from the sun is proportional to the true anomaly. From 

 one equinox to the other, the earth therefore receives the same 

 quantity of solar rays. Thus, for a season of the same name, 

 summer for example, each of the two hemispheres similarly si- 

 tuated with relation to the sun, receives precisely the same se- 

 mestral light, measured by 180° of the ecliptic. If, therefore, 

 one of the two hemispheres is more heated by solar irradiation 

 than the other, this cannot depend upon the quantity of rays 

 which it receives. It only remains to be seen whether it might 

 not depend upon the different manner in which the distribution 

 of this same quantity is operated. But if all this quantity be 

 considered as fixed, and in some measure stored up in the bosom 

 of the earth, it is of no consequence although there should be 

 some inequality in the form of its distribution. It may there- 

 fore be said in general, that the unequal temperature of the two 

 hemispheres does not depend immediately upon the heat which 

 they receive from the sun. 



But it is known that all the rays received in this way do not re- 

 main imprisoned in the terrestrial globe. A part of them ema- 

 nates by means of radiation, and loses itself in space. If, accord- 

 ing to the general theory of radiation, regard be had to the dif- 

 ferent distribution of solar heat in the two hemispheres of the 

 earth, if, for example, confining ourselves to the summer (the 

 influence of which is entirely predominant), we consider the ef- 

 fect which the length of the northern summer has upon the ra- 



