PRECESSION OF THE EQUINOXES. 403 



Sahara, which we are apt to look upon as a useless waste, we 

 are in reality much indebted for the fertility and civilization 

 of Europe. 



M. Adhemar* has suggested a mode of accounting for the 

 cold of the glacial epoch, which, if the true one, would give 

 us means of calculating its antiquity. If the plane of the 

 equator coincided exactly with that of the ecliptic, i.e. with 

 that of the earth's orbit, then it is evident that every day 

 would be followed by a night of equal length. In conse- 

 quence, however, of the obliquity of the ecliptic, there are 

 only two days in the year when this is actually the case, 

 namely, the 20th of March and the 23rd September. Thus 

 our year is divided into four well-marked periods, " Winter" 

 begins on the 22nd December, which is the shortest clay of 

 the year, and continues until the 20th March, which is called 

 the spring equinox, because on it the day and night are of 

 equal lengths. "Spring" commences on the 20th March and 

 continues till the 21st June, during which time the days 

 continue to elongate at the expense of the night. 



From the 21st June, however, which is the first day of 

 " summer," the days begin to shorten, until, on the 23rd of 

 September, day and night are again equal, and we have the 

 autumn equinox. 



Autumn commences on the 23rd September, and the days 

 continue to diminish till the 22nd December, which is the 

 shortest day, and after which they begin to lengthen. 



At present, then, the northern hemisphere enjoys in each 

 year seven days more of spring and summer than of autumn 

 and winter; while, on the other hand, the southern hemi- 

 sphere has seven days more of autumn and winter than of 

 spring and summer. This inequality of the seasons is due to 

 the greater rapidity with which the earth moves when it is 



* Revolutions de la Mer. J. Adhemar. Paris. 



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