Moving Force. 179 
bodies meet and destroy each other’s motion, 
their quantities of motion, and their respective 
forces, must therefore be equal.—Dr. Reid has 
given a better enunciation of this proposition. 
He says, “ If two bodies meet directly with a 
shock, which mutually destroys their motion, 
without producing any other sensible effect, it 
may be fairly concluded that they meet with 
equal force.”* Now this is a fair reference to 
experiment, and, in the case under considera- 
tion, we certainly have a measurable, “ sensible 
effect” in the compression of the spring, which 
cannot be produced without force. But al- 
though the ends of the spring meet at bh, 
(fig. 6.) it is still held by many that that effect 
is produced equally by A and B. If the forces 
of A and B are really equal, we should have 
the same effect produced when we substitute 
for B another ball equal in weight and velocity 
to A. But the same effect cannot be produced 
by that means; and if the real effects be ex- 
amined, we shall always find that the spring is 
less compressed (as measured by the pressure 
into the space) by A than by B in the ratio 
of 1 to 2. 
It is true the common centre of gravity of 
* Essay on Quantity--Philos. Trans. 1748, p. 515. 
— £2 
