MOTION, MECHANICS, ETC. 163 



or silk cord, b, is fast- Fi s- 169. 



ened at one end ; and 

 having made a loop 

 on the other end of 

 the cord, put the loop 

 over a pin fixed in the 

 centre of the board, 

 d. Then turning the 

 winch, B, to give the board a whirling motion, you will see 

 that the ball does not immediately begin to move with the 

 board, but, on account of its inactivity, it endeavors to con- 

 tinue in the state of rest which it was in before. Continue 

 turning until the board communicates an equal degree of 

 motion with its own to the ball, and then turning on, you 

 will perceive that the ball will remain upon one part of the 

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

 lative motion upon it, as is the case with everything that 

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

 the motion of the earth communicated to it, never endeavors 

 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 shows, that matter being once put in motion, will 

 continue to move for ever, if it meet 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 as firm as if he were on the plane 

 shore ; and if the boat strike 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 centre. 



Bodies moving in orbits have a tendency to fly out of these 

 crbits. Draw off the ball a little- from the centre, and turn 



