Principles of progrejfive and rotatory Motion . 569 
fequence of the force acting at d, begins to revolve 
about c, and confequently if immediately after the be- 
ginning of the motion a force were applied at d equal 
to it, and in a contrary diredtion, the motion of the 
body would be deftroyed, it is evident, that the efficacy 
of the body revolving about c, to turn the body about 
d, fhould any obftacle be oppofed to its motion at that 
point, mult be equal to nothing ; for were it not, the 
body, when flopped at d, would ftill have a rotatory 
motion about that point, and confequently two equal 
and oppofite forces applied at d would not deftroy each 
others effedts, which would be abfurd. Now the force 
of a particle p, in the diredtion pw, being pxpc, its 
efficacy to turn the body about the point d is p x pc x 
d w ; but by fim. triang. d w : v>b : : ac : pc, mv = 
HlLfd , and confequently the efficacy to turn the body 
about T>—px D-b x ac —p x ca x dc-c& =pxcaxvc—pxpc z i 
hence the fum of all the p x ca x dc — the fum of all 
4 J 
■1 , 2 1 r , "i _ fum of all the p x PC 2 
the px = o, and confequently cd = 
therefore d is the center of percuffion, the point of fuf- 
penfion being at c. 
Cor. From this and the preceding Propofition it ap- 
pears, that every thing which was proved in Prop. v. vi. 
vii. holds here alfo in the cafe of the adtion of one body 
on another. 
PROP. 
