THE SURFACE OF THE EARTH. 



directly will the solar rays fall upon the place ; and the longer 

 the day is, the longer will be the interval during which the 

 thermal influence of the sun is exerted, and the shorter will be 

 the interval during which its presence will be withdrawn. 



159. For all these reasons, therefore, on the 21st of June, when 

 the northern hemisphere is most inclined towards the sun, and 

 the southern most inclined from it, the thermal influence of the sun 

 will be greater in the northern, and less in the southern hemi- 

 sphere, than at any time from the 21st of March to the 21st of June. 



During this interval the northern hemisphere is gradually 

 more and more inclined towards the sun, and therefore the length 

 of the day is continually increasing, as well as the altitude to 

 which the sun rises at noon. These two circumstances combine 

 in gradually increasing the thermal influence of the sun from the 

 21st of March to the 21st of June. 



The same circumstances will show that during the same interval, 

 the thermal influence of the sun in the southern hemisphere is 

 gradually diminished ; the days being there constantly shorter, 

 and the altitude to which the sun rises at noon constantly less. 



After the 21st of June the northern hemisphere is gradually less 

 and less inclined towards the sun, and the southern less and less 

 inclined from it, until at length on the 21st of September, the day 

 of the autumnal equinox, the earth resumes the position, fig. 10, 

 with relation to the sun which it had on the 21st of March, the 

 equator E being then, as before, directed exactly towards the sun. 



160. The two hemispheres therefore, as in March, being equally 

 exposed to the sun, receive from it the same thermal influence, 

 and the parallels of latitude being all bisected by the circle 

 which bounds the enlightened hemisphere, the days and nights 

 are equal at all parts of the earth. 



Since the altitude to which the sun rises, and the length of the 

 days at equal intervals before and after the 21st of June, are 

 the same, and therefore the thermal influence of the sun also the 

 same, it might be inferred that the temperature of the weather 

 would likewise be the same ; and if this inference were just, it 

 would follow that the season from the 21st of March to the 21st of 

 June, would be similar in all its thermal characters to the season 

 from the 21st of June to the 21st of September, except that the 

 succession of temperatures would be developed in a contrary 

 order. Thus it would be expected that the temperature of the 

 weather ten or twenty days after the 21st of June would be 

 identical with its temperature ten or twenty days before the 21st 

 of June. 



161. But it is notorious that the thermal phenomena are not at 

 all in accordance with this ; the season from the 21st of March to the 



168 



