The Earth considered as a Planet. 85 



of time 15 degrees difference of longi- 

 tude. Consequently, if the beginning or 

 ending of a lunar eclipse be observed, 

 suppose at London, to be exactly at mid- 

 night, and in some other place at 11 at 

 night, that place is 15 degrees westward 

 from the meridian of London ; if the same 

 eclipse be observed at one in the morn- 

 ing at another place, that place is 15 de- 

 grees eastward from the said meridian. 



Of the Precession of the Equinoxes, &?c. 

 Beside the changes in the length ol the 

 days already described in the different 

 seasons, there is another sort of change, 

 not so obviously noticeable. The day 

 and night together, or, in general, 24 

 hours, form the natural or solar day ; and 

 this period of time also varies in its 

 length. The inequality of the solar day 

 is produced by two causes, either o 

 which singly would yield the effect ; 

 these are the obliquity of the ecliptic, and 

 the earth's unequal motion therein. The 

 earth's motion on its axis being uniform 

 and equal at all times of the year, its days 

 would be equal, if its orbit were a perfect 

 circle, and its axis perpendicular to the 



