OF CENTRAL FORCES. 35 



board, keeping the same velocity with it, and having no 

 relative motion upon it, as is the case with every thing 

 that lies loose upon the plane surface of the earth, which 

 having the motion of the earth communicated to it, 

 never endeavours to remove from that place. But stop 

 the board suddenly by hand, and the ball will go on, and 

 continue to revolve upon the board, until the friction 

 thereof stops its motion : which shews, that matter being " 

 once put into motion, would continue to move for ever, 

 if it met with no resistance. In like manner, if a person 

 stands upright in a boat before it begins to move, he 

 can stand firm ; but the moment the boat sets off, he is 

 in danger of falling towards that place which the boat 

 departs from : because, as matter, he has no natural 

 propensity to move. But when he acquires the motion 

 of the boat, let it be ever so swift, if it be smooth and 

 uniform, he will stand as upright and firm as if he was 

 on the plane shore ; and if the boat strikes against any 

 obstacle, he will fall towards that obstacle ; on account 

 of the propensity he has, as matter, to keep the motion 

 which the boat has put him into. 



2. Take away this ball, and put a longer cord to it, 

 which may be put down through the hollow axis of the 

 bearer M X, and wheel G, and fix a weight to the end 

 of the cord below the machine ; which weight, if left at 

 liberty, will draw the ball from the edge of the whirling- 

 board to its center. 



Draw off the ball a little from the center, and turn the Bodies 

 winch ; then the ball will go round and round with the "behave 



board, and will gradually fly off farther and farther from a tendency 



. to fly out 



the center, and raise up the weight below the machine : of these 



which shews that all bodies revolving in circles have 

 tendency to fly off from these circles, and must have 

 some power acting upon them from the center of motion, 

 to keep them from flying off.* 3 Stop the machine, and 



Note 2a There is & very beautiful as well as simple mode of illu5- 

 D 2 



