310 OUTLINES OF NATURAL PHILOSOPHY. 



the ecliptic, till it coincided with that plane ; 

 the line of their intersection remaining all the 

 while at rest. 



The mean quantity of the solar force which is thus 

 decomposed, is r cos $, where m is the mass 



of the Sun, a the mean distance of the Sun from 

 the Earth, r the radius of the equator, and S the 

 declination of the Sun. 



The part of this force which is perpendicular to the 

 plane of the equator, and which tends to make it 

 move round the line of its nodes, is, 



r cos 3 x sin 2. 



315. As the ring which surrounds the equa- 

 tor, at the same time that it has the tendency 

 just described, revolves on an axis perpendicu- 

 lar to its plane in twenty-four hours, it will not 

 revolve on either of these axes, but on one in 

 the same plane which divides the angle between 

 them ? so that the sine of its angular distance 

 from each axis, is in the inverse ratio of the an-< 

 gular velocity round that axis. 



If the arch, round the intersection of the equator 

 and ecliptic, which the solar force acting upon the 

 ring of the mcniscusy would make the Earth de- 

 scribe 



