Moving Force. 211 
common centre of gravity is moved. But in 
either way the different quantity of that sum 
of motion, in these two positions of the rod, 
equally makes for our author’s assertion. Of 
which perhaps there is yet a more striking 
proof in the prodigious generation of motion 
by the collision of elastic bodies in certain 
arrangements, vid. Huygens De motu corpo- 
rum ex percussione.” But this is obviously 
an oversight of the learned editor; for, if he had 
bestowed a little more consideration on the 
case as it is distinctly stated hy the illustrious 
author, he would not, we must presume, have 
given a commentary so much at variance 
with the text—When A is perpendicular 
over B, B is at rest, and A only is in motion 
with the velocity 2v. The whole quantity of 
motion, when the balls are in that position, is 
therefore expressed in the usual way by 
AX2v. But when AB is in a horizontal 
position, the common centre of gravity of A 
and B is moving horizontally with the velocity 
v, and each ball is moving round that centre 
with the same velocity v. The sum of the 
motions, when in that position, must therefore 
be A+B.v+A.v+B.v ; and I think, it 
cannot admit of a doubt that Sir Isaac New- 
ton understood the case in that light, But 
although the motion is exhibited in such vari- 
pd2 + 
