On ihe Motion of Bodies affected hj Friction. 49 



extremilv of which was placed a pulley, which coukl be 

 elevated or depressed in ordir to render the st; lag which 

 connected the body and the ino'-Ing force parallel to the 

 plane. A scale accurately divided was plac-l by the side of- 

 the pulley, perpendicular to the huvizon, by the side of which 

 the moving force descended ; upon the scale was placed a 

 moveable stage, which could be adjusted to the space through 

 which the moving force descended in any given lime, which 

 time was measured by a well regidated pendulum clock vi- 

 brating seconds. Every thing being thus prepared, the fol- 

 lowing experimtrnts were made to ascertain the law of iric- 

 tion. "But let me first observe, that il friction be an uniform 

 force, the difference between it and the given force of the 

 moving power must be also uniform ; and therefore the 

 moving body nuist descend with an uniformly accelerated 

 velocity, and consequently the spaces described from the 

 beginning of the motion must be as the squares of the times, 

 just as when there was no friction, only they will be dimi- 

 nished on account of the friction. 



3. Exp. 1. A body was placed upon the horizontal plane, 

 and a moving force applied, which from repeated trials was 

 found to descend 52^ inches in 4", for by the beat of the 

 clock and the sound of the moving force when it arrived at 

 the stage, the space could be very accurately adjusted to the 

 time : the stage was then removed to that point to which 

 the moving force would descend in 3", upon supposition 

 that the spaces described by the moving power were as the 

 Squares of the times ; and the space was found to agree very 

 accurately with the time : the stage was then removed to 

 that point to which the moving force ought to descend in 

 2", upon the same supposition; and the descent was found 

 to agree exactly with the time : lastly, the stage was adjusted 

 to that point to which the moving force ouglrt to descend 

 ill 1", upon the same supposition ; and the space was ob- 

 served to agree w ith the time. Now in order to find whether 

 a difference in the time of descent could be observed, by 

 removing the stage a little above and below the positions 

 which corresponded to the above times, the experiment was 

 tried, and the descent was always found too soon in the 

 former, and too late in the latter case ; by which I was as- 

 sured that the spaces first mentioned corresponded exactly 

 to tlie times. And, for the greater certainty, each descent 

 was repeated eight or ten times ; and every caution used in 

 this experiment was also made use of in all the following. 



Exi'. 2. A second body was laid upon the horizontal piane, 

 and a moving force applied which descended 41^ Indies m 



VoL.xvir. jSo.es. D ^"' 



