VrOVING FORCE. 
drawn in the plane of their circo'ar motion, the sum of the Cases of diffi- 
motions of the two globes, as often as the globes are in the 
° doctrines of 
right line describeJ by their common centre of gravity, will movitii; force. 
be bigger than the sum of their motions when they are in a 
line perpendicular to that line*.” On this passage we have 
the following note from Dr. Horsley. “ The contrary seems 
to be true, that the sum of the motions will be greatest when 
the rod connecting the revolving bodies is perpendicular to the 
right line along which the 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 motn 
corporura ex percussione." But this is obviously an ovcrsiglit 
of the learned editor j for, if he had bestowed a little more 
consideration on the case, as it is distinctly stated by the 
illustrious author, he would not, wo must presume, have given 
a commentary so mucli 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 A X But w’hen AB is in a horizontal position, the 
common centre of gravity of A and B is moving horizontally 
with the velocity r, and each ball is moving round that centre 
with the same velocity v. The sum of the motions, when in 
•hat position, must therefore be A + B.y-f A.t’-f-R t' : and I 
.hink it will not admit of a doubt that Sir Isaac Newton under- 
stood the case in that light. But although the motion is exhi- 
oited in such various quantities according to the positions of the 
od, it cannot be questioned that the quantity of force most 
remain the same under all positions of the rod. While the 
motion continues uniform, there certainly can be no variation 
f the force. It appears, therefore, (as I have before observed, 
'). 1/3) that Sir Isaac Newton understood, that* unequal quan- 
• ilorslev’* Newton, vol. 4. p. 258. 
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