The Sim and his real and apparent Motions. 273 



winter. The heat of summer depends chiefly 

 on three other causes. 



1st. In summer the solar rays strike less 

 obliquely upon the earth than in winter ; and 

 it may be demonstrated on the principles of 

 mechanics, that a body which acts perpendi- 

 cularly upon another acts with all its force; 

 whereas if it acts obliquely, its force is less in 

 proportion to the degree of the obliquity. The 

 rays of light follow the same laws as other 

 bodies, and consequently their action might be 

 measured by the sine of their angle of incidence. 

 There is no necessity for a diagram to explain 

 what is now laid down, since it is obvious that 

 as the equator divides the earth into two equal 

 parts, when the sun is on this, that is, the north 

 side of it (as he is in summer) his rays must 

 strike more vertically, or more in the perpen- 

 dicular line, than when he is in the southern 

 tropic. 2d. In summer also, the rays falling 

 more vertically, have less of atmosphere to pass, 

 and that atmosphere is usually less clouded. 

 3d. In summer the sun continues a longer time 

 above the horizon than below it; and conse- 

 quently there is time for the earth to accumulate 

 a greater portion of heat than in the days of ' 

 winter. 



Since the sun is further from us in summer 

 than in winter, it follows that the inhabitants 

 of the opposite (the southern) hemisphere must 

 have (all other circumstances equal) more heat 



