210 On the Measure of 
In the {4th Case the same effects are pro- 
duced by percussion, which, in the 5th case, 
are produced by gradual pressure through 
sensible spaces; and we must either admit 
that the moving force of D (fig. 14) is greater 
than that of C, or conclude that the rotatory 
motion is produced without force. It may be 
said that there is in both cases only the same 
quantity of motion in one direction.—I must 
observe however, that Sir Isaac Newton 
understood the swm of the motions of the two 
bodies to‘include the rotatory as well as the 
progressive motion. ‘“ If two globes,” he 
says, “ joined by a slender rod, revolve about 
their common centre of gravity with an uni- 
form motion, while that centre moves on 
uniformly in a right line drawn in the plane 
of their circular motion, the sum of the mo- 
tions of the two globes, as often as the globes 
are in the right line described by their common 
centre of gravity, will 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 perpen- 
dicular to the right line, along which the 
* Horsiey’s Newton, yol. 4, p. 258. 
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