168 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



round their common centre of gravity. If a body be so placed 

 on the whirling-board of the machine, that the centre of 

 gravity of the body be directly over the centre of the board, 

 and the board be put into ever so rapid a motion by the 

 winch, B, the body will turn round with the board, but will 

 not remove from the middle of it ; for, as all parts of the 

 body are in equilibrio round its centre of gravity, and the 

 centre of gravity is at rest in the centre of motion, the cen- 

 trifugal force of all parts of the body will be equal at equal 

 distances from its centre of motion, and therefore the body 

 will remain in its place. But, if the centre of gravity be 

 placed ever so little out of the centre of motion, and the 

 machine be turned swiftly round, the body will fly off to- 

 wards that side of the board on which its centre of gravity 

 lies. Thus, if the wire, C, with its little ball, B (Fig. 171), 

 be taken away from the demiglobe A, and the flat side, ef, 

 of this demi-globe be laid upon the whirling -board of the 

 machine, so that their centres may coincide ; if then the 

 board be turned ever so quick by the winch, the demi-globe 



Fig. 171. 



will remain where it was placed. But if the wire, C, be 

 screwed into the demi-globe at d, the whole becomes one 

 body, whose centre of gravity is now at or near d. Let the 

 pin, c, be fixed in the centre of the whirling-board, and the 

 deep groove, b, cut in the flat side of the demi-globe, be put 

 upon the pin, so that the pin may be in the centre of A, 

 and let the whirling- board be turned by the winch, which 

 will carry the little ball, B, with its wire, C, and the demi- 

 globe, A, all round the centre pin, c i ; and then, the centri- 

 fugal force of the little ball, B, which weighs only one ounce, 

 will be so great as to draw off the demi-globe, A, which 

 weighs two pounds, until the end of the groove, at e, strikes 

 against the pin, c, and so prevents the demi-globe, A, from 

 going any further; otherwise, the centrifugal force of B 

 would have been great enough to have carried A quite off 

 the whirling-board ; which shows, that if the sun were 

 placed in the very centre of the orbits of the planets, it could 

 not possibly remain there ; for the centrifugal forces of the 



