OF CENTRAL FORCES. 41 



end of the tube, where it will remain upon the lowest L ECT 

 end of the water near b ; for the heavier body, having IL 

 the greater centrifugal force, will therefore possess the 

 uppermost part of the tube ; and the lighter body will 

 keep between the heavier and the lowermost part. 



This demonstrates the absurdity of the Cartesian doc- 

 trine of the planets moving round the sun in vortexes ; 

 for, if the planet be more dense or heavy than its bulk 

 of the vortex, it will fly off therein, farther and farther 

 from the sun ; if less dense, it will come down to the 

 lowest part of the vortex, at the sun : and the whole 

 vortex itself must be surrounded with something like a 

 great wall, otherwise it would fly quite off, planets and 

 all together. But while gravity exists, there is no occa- 

 sion for such vortexes ; and when it ceases to exist, a 

 stone thrown upwards will never return to the earth 

 again. 



9. If a body be so placed on the whirling board of if one bo- 

 the machine, 8 that the center of gravity of the body d ^ moves 



n J J round ano- 



be directly over the center of the board, and the board ther, both 

 be put into ever so rapid a motion by the winch B, the m u g t e ove 

 body will turn round with the board, but will not re- roundtheir 

 move from the middle of it ; for, as all parts of the body center of 

 are in equilibria round its center of gravity, and the s^- 

 center of gravity is at rest in the center of motion, the 

 centrifugal force of all parts of the body will be equal 

 at equal distances from its center of motion, and there- 

 fore the body will remain in its place. But if the cen- 

 ter of gravity be placed ever so little out of the center 

 of motion, and the machine be turned swiftly round, the 

 body will fly off towards that side of the board on which 

 its center of gravity lies. Thus, 

 if the wire C with its little ball 

 B be taken away from the demi- 

 globe A t and the flat side e f 

 of this demi-globe be laid upon 



See eograving, page 33, 



