PHYSICAL ASTRONOMY. 309 



on any particle being as its distance from the 

 plane that separates these hemispheres. 



From this it follows, that if the Earth were a perfect 

 sphere, the solar forces acting on the opposite he- 

 mispheres, would exactly balance one another, 

 and could produce no motion in the Earth or its 

 axis. 



313. The Earth may be considered as a sphere 

 circumscribed by a spheroidal shell or meniscus, 

 thickest at the equator. The tendency of the 

 Sun's action on this meniscus, except at the 

 time of the equinoxes, is always to make it turn 

 round the intersection of the equator with the 

 ecliptic, towards the plane of this latter circle. 



For the matter of the meniscus may be regarded as 

 forming a ring round the Earth, in the plane of 

 the equator. Now, the solar force acting on the 

 part of this ring that is above the ecliptic, may at 

 every point be resolved into two 5 one of which is 

 in the plane of the equator, and the other perpen- 

 dicular to it. The result of all the latter, must be 

 a force tending to impress on the ring a motion 

 round its intersection with the ecliptic. The same 

 holds of the half of the ring that is under the 

 ecliptic. 



. Hence, if the equator had no other mo- 

 tion, it would turn round its intersection with 



the 



