REVOLUTION OF THE EARTH. 77 



truth of the other ; * the absorption and radiation of 

 heat being directly regulated by the colours of the sur- 

 faces upon which the sun rays fall. 



It follows, as a natural consequence of the position of 

 the sun, as it regards any particular spot on the earth at 

 a given time, that the amount of heat is constantly vary- 

 ing during the year. This variation regulates the 

 seasons. 



When it is remembered that the earth is, in the 

 winter, nearly three millions of miles nearer the sun 

 than in the summer, some explanation is required to 

 account for our suffering more cold when nearer the 

 source of heat, than when at the remotest distance. 



The earth in her path around the sun describes an 

 ellipse, the sun's place being one of its foci. In obe- 

 dience to the law, already described, of the conservation 

 of the axis of rotation, the axis of the earth constantly 

 points towards the star in the constellation of the 

 Little Bear. Recollecting this, and also the two facts, 

 that a dense solid body absorbs heat more readily than 

 a fluid one, and that radiation from the surface is con- 

 stantly going on when absorption is not taking place, 

 let us follow the earth in her orbit. 



It is the time of the vernal equinox we have equal 

 day and night therefore the periods of absorption and 

 radiation of heat are alike. But at this time of the year 

 the southern hemisphere is opposite to the sun, conse- 

 quently the degree of absorption by the wide-spread 

 oceans small. 



It is the summer solstice we have sixteen hours of 

 daylight, when the absorption of heat is going on and 

 but eight hours of night, during which heat is passing 



* Moser, On Vision, and on the Action of Light on Bodies : and 

 also On Latent Liyht : Scientific Memoirs, vol. iii. Draper, On 

 certain Spectral Appearances, and on the Discovery of Latent Light : 

 Phil. Mag., Nov. 1842. 



