THE SURFACE OF THE EARTH. 



prevalence of canine madness during that period, or because at an 

 early epoch in astronomical history the Dog-star rose before the 

 sun in the morning at that season, and thus harbingered the God 

 of Day. It may even have happened that the Dog-star took its 

 name originally, from the prevalence of canine madness at that 

 season. 



164. The circumstances which explain the phenomena of summer 

 in the northern hemisphere, will also explain those of winter during 

 the same interval in the southern hemisphere, since the southern 

 hemisphere at all times is inclined from the sun, exactly as much 

 as the northern hemisphere is inclined towards it, as will be 

 apparent by reference to fig. 9. 



165. After the 21st of September (fig. 10), the day of the 

 autumnal equinox, the equator is gradually inclined towards the 

 north, and the northern hemisphere gradually inclined from the 

 sun, and this inclination constantly increases until the 21st 

 of December, when it is greatest, as shown in fig. 11. 



The solar rays, as will be apparent from the figure, then fall 

 with greatest obliquity on the northern hemisphere, and with 

 least obliquity on the southern. The parallels of latitude are un- 

 equally divided, in both hemispheres, by the circle which bounds 

 the enlightened part of the earth. In the northern hemisphere 

 the greater portions of these parallels are dark, and the lesser 

 portions enlightened, while the contrary takes place in the southern 

 hemisphere. The days are, therefore, shorter than the nights in 

 the northern, and longer in the southern hemisphere ; and the 

 sun rises only to low altitudes in the former, but to considerable 

 altitudes in the latter. In fine, all the circumstances show that, 

 in this position of the earth, the summer commences in the 

 southern, and the winter in the northern hemisphere. 



166. After the 21st of December, the inclination of the northern 

 hemisphere from the sun gradually and constantly diminishes 

 until the 21st of March, when the equator is once more presented 

 directly to the sun, which affects both hemispheres alike. 



Since the 21st of December is the shortest day, and that upon 

 which the sun rises to the least altitude, it is consequently that on 

 which its thermal influence is least, and it might therefore be 

 expected to be the coldest day, and consequently to be mid- 

 winter. It is notorious, on the contrary, that the coldest weather 

 is at a later period. This is explained upon the same principles 

 exactly, as those which show why the 21st of June is not the 

 hottest day. 



167. The decrement of heat which takes place in the atmosphere 

 owing to the length of the night, the shortness of the day, and 

 the low altitude of the sun on the 21st of December, is greater 



170 



