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



give B six times as much centrifugal force as any single 

 ounce of A has : but then, as B is only one ounce, and A 

 six ounces, the whole centrifugal force of A will exactly 

 balance the whole centrifugal force of B : and, therefore, 

 each body will detain the other so as to make it keep in its 

 circles. This shows, that the sun and the planets must all 

 move round the common centre of gravity of the whole 

 system, in order to preserve that just balance which takes 

 place among them. For, the planets being as inactive and 

 dead as the above balls, they could no more have put them- 

 selves into motion than these balls can ; nor have kept in 

 their orbits without being balanced at first with the greatest 

 degree of exactness upon their common centre of gravity, 

 by the Almighty hand that made them and put them in 

 motion. 



Perhaps it may be here asked, that since the centre of 

 gravity between these balls must be supported by the fork, 

 E, in this experiment, what prop it is that supports the 

 centre of gravity of the solar system, and consequently 

 bears the weight of all the bodies in it ; and by what is the 

 prop itself supported ? The answer is easy and plain ; for 

 the centre of gravity of our balls must be supported, be- 

 cause they gravitate towards the earth, and would therefore 

 fall to it : but, as the sun and planets gravitate only towards 

 one another, they have nothing else to fall to ; and therefore 

 have no occasion for anything to support their common 

 centre of gravity : and if they did not move round that cen- 

 tre, and consequently acquire a tendency to fly off from it 

 by their motions, their mutual attractions would soon bring 

 them together; and so the whole -would become one mass 

 in the sun : which would also be the case if their velocities 

 round the sun were not quick enough to create a centrifugal 

 force equal to the sun's attraction. 



11. Take away the fork and balls from trie whirling-board, 

 and place the frame, A B (Fig. 173), thereon, fixing its 

 centre to the centre of the whirling -board by the screw. In 



