180 



form of bars or flat rods, m, to be laid on A ; there is also 

 a circular frame, H, to be fixed to the scale at any proper 

 height, in such manner that A will pass centrally through it ; 

 when A passes through this frame, it leaves the bar by 

 which it had been accelerated on the circular part of the 

 frame. After the moving force, m, has been intercepted at 

 the end of the given space or time, there will be no force 

 operating upon any part of the system to accelerate or re- 

 tard its motion, and consequently the instant m is removed, 

 A will proceed uniformly with the velocity it had acquired 

 that instant, and the velocity being uniform, will be mea- 

 sured by the space described in any convenient number of 

 seconds. 



It may here be necessary to observe, that Mr. At wood 

 has clearly shown, that the weight of the line can have no 

 sensible effect on the experiments, for the inequality of the 

 motion occasioned by it does not amount to more than 

 -(fo^Tz f a second, a quantity too small to be distinguished 

 by the senses. 



The resistance of the air does not affect these experi- 

 ments; for, as the greatest velocity communicated in these 

 experiments does not exceed 26 inches in a second, and the 

 pieces A and B being only about If inch in diameter, the 

 resistance of the air can never increase the time of the de- 

 scent in so great a proportion as 240 to 241, and will be 

 therefore insensible in experiment. 



The effects of friction are almost wholly removed by the 

 axis of the wheel, abed, acting on the four friction wheels, 

 e, f, g, h. If the weights, A and B, be balanced in perfect 

 equilibrio, and the whole mass consist of 63 m, a weight of 

 two grains, added to A or B, will communicate motion to 

 the whole, which shows how inconsiderable the friction is ; 

 in some cases, however, particularly in experiments on re- 

 tarded motion, the effects of friction become sensible, but 

 may be very readily and exactly removed by adding rather 

 less than two grains to the descending weight ; the weight 

 should be always less than what is sufficient to put the 

 whole in motion. 



The space which bodies describe in one second, by falling 

 freely from rest, is 193 inches; but in the ensuing experi- 

 ments, the space is taken at 192 inches, which will be pro- 

 ductive of no error, in order to avoid fractions, which would 

 render the use of the instrument less easy and intelligible. 



