Principles of progrejjive and rotatory Motion. 563 
had ailed at their common center of gravity, find there- 
fore their common center of gravity, and conceive a 
force equivalent to them all to he communicated to that 
point, and the Problem is reduced to the cafe of the 
fir ft Propofition. If any of the forces had ailed on the 
oppciite fide of the lever, fuch forces muft have been 
con fide red as negative. 
If there be any number of bodies placed on the lever, 
and a fingle force ails at d, it will appear from the fame 
principles that the point c, about which they begin to re- 
volve, will be the point of fufpenfion to the center of per- 
cuffion D ; and the fame conclufion will be obtained, if the 
bodies be not fituatedin a ftraightline. As a direil invefti- 
gation, however, is always to be preferred to conclufion s 
drawn from induilion, it may be thought proper, before 
we apply any of the foregoing principles to the cafe of the 
ailion of bodies upon each other by impail, ta fhew 
how fuch a direil inveftigation to determine the point 
about which a body, having a motion communicated to 
it, begins to revolve, may be obtained ; previous to 
which, however, fome further confiderations are ne- 
cefiary. 
4 E 
Vol. LXX. 
PROP. 
