MOTION, MECHANICS, ETC. 179 



will be 61 m ; add 1-2 m to A and 1-2 m to B, and the 

 mass moved will be 61 m r ; now place 2 m on A, and the 

 mass moved is 64 as before, whereof the force of accelera- 

 tion is equal to 1-32 part of the acceleration of gravity. 



Of t/ie space described. The body, A, descends, as you 

 perceive, in a vertical line, along the scale, C C, which is 

 about sixty-four inches long, and is graduated into inches 

 and tenths ; the scale is so adjusted as to be vertical, and 

 so placed that the descending weight may fall in the middle 

 of the stage D, fixed to receive it at the end of the descent ; 

 the beginning of the descent is estimated from O, on the 

 scale, when the bottom of A is level or even with ; the 

 descent of A is terminated, when the bottom strikes the 

 stage ; the situation or distance of the stage may be varied 

 at pleasure within the range of the sixty-four inches. 



Of the time of Motion. The time of the motion is ob- 

 served by the beats of this pendulum, E, affixed to the 

 pillar, G, and which vibrates seconds. The number of 

 seconds is shown by the index and dial-plate above. 



Many mechanical devices might be applied for letting the 

 weight, A, begin its descent at the instant of the beat of 

 the pendulum ; but it is simpler, and Mr. Atwood thinks 

 better, to let the bottom of the piece, or box, A, when even 

 with on the scale, rest on a flat rod held in the hand 

 horizontally, its extremity being coincident with ; by at- 

 tending to the beats of the pendulum, you may, with a little 

 practice, remove the rod which supports the box, at the 

 instant the pendulum beats, so that the descent of A shall 

 commence at the same instant. 



Of the velocity acquired. I have only now to show you 

 in what manner the velocity acquired by the descending 

 weight, A, at any given point of the space through which 

 it has descended, is made evident to the senses. 



The velocity of A's descent being continually accelerated, 

 will be the same in no two points of the space described ; 

 this is occasioned by the constant action of the moving 

 force ; and since the velocity of A, at any instant, is mea- 

 sured by the space which would be described by it moving 

 uniformly for a given time, with the velocity it had acquired 

 at that instant, this measure cannot be experimentally ob- 

 tained, but by removing the force which caused the accele- 

 ration of the descending body. 



To effect this, there are some weights or movino- forces in 



