OF CENTRAL FORCES. 



51 



which shews that the common center of gravity between 

 the sun and earth is within the body of the sun ; and is 

 only the 1068? part of his semidiameter from his center 

 towards his surface. 



All globular bodies, whose parts can yield, and which 

 do not turn on their axes, must be perfect spheres, be- 

 cause all parts of their surfaces are equally attracted 

 to ward their centers. But all such globes as do not turn 

 on their axes will be oblate spheroids ; that is, their sur- 

 faces will be higher, or farther from the center, in the 

 equatorial than in the polar regions. For, as the equa- 

 torial parts must be quickest, they must have the great- 

 est centrifugal force ; and will therefore recede farthest 

 from the axis of motion. Thus, if two circular hoops 

 AB and CD, 

 made thin and 

 flexible, & cross- 

 ing one another 

 at right angles, 

 be turned round 

 their axis E F 

 by means of the 

 winch 77z, the 

 wheel n, and pi- 

 nion o, and the 

 axis be loose in 

 the pole or in- 

 tersection e, the 

 middle parts A, 

 B, C, D will 

 swell out so as 



to strike against the sides of the frame at F and G, if 

 the pole e, in sinking to the pin E, be not stopped by it 

 from sinking farther : so that the whole will appear of 

 an oval figure, the equatorial diameter being conside- 

 rably longer than the polar. That our earth is of this 



E 2 



LECT. 

 IL 



