30 



OF CENTRAL FORCES. 



ends of an iron bar strong enough to hold them. So, 

 when the earth is at e, the sun will be at E ; when the 

 earth is at T, the sun will be at F; and when the earth 

 is at g, the sun will be at G, &c. 



Now, let us take in the moon q (at the top of the 

 figure) and suppose the earth to have no progressive 

 motion about the sun ; in which case, whilst the moon 

 revolves about in her orbit & IS <& ZD, the earth will 

 revolve in the circle S 13, whose center R is the com- 

 mon center of gravity of the earth and moon ; they 

 being connected by the mutual attraction between thorn 

 in the same manner as the earth and sun are. 



But the truth is, that whilst the moon revolves about 

 the earth, the earth is in motion about the sun : and 

 now, the moon will cause the earth to describe an 



