PHYSICAL ASTRONOMY. 



When the point is in the hemisphere opposite to the 

 Sun, cos z becomes negative, so that the force 

 draws the particle from the Earth, in a direction 

 opposite to that which it has in the other hemi- 

 sphere. 



If m' be the mass of the Moon, a' its distance from 

 the Earth, 2' the distance of the Moon from the 

 zenith, the forces by which the Moon affects the 

 gravity of bodies on the Earth's surface^is express- 

 ed by the same formulas. 



335. The preceding force may again.be resol- 

 ved into two, one of which, in the direction 

 of the Earth's radius, and opposite to gravity, is 



_ 

 cos z , and, with that already mentioned, 



as increasing gravity, viz. p, makes the whole 



force by which the Sun's action diminishes gra- 



, Smrcosz* mr 

 vity, to be - ^ - . 



The other force, draws the particles everywhere ho- 

 rizontally toward the point that has the Sun in its 



zenith, and is equal to r sin 2 z * In the he- 



ct 



misphere 



