Moving Force. 181 
nitely small, than in palpably large portions 
of time. 
This compression of the spring is compre- 
hended by Mr. Smeaton under the term 
change of figure; and he has shown, by some 
well-chosen experiments, that when a non 
elastic yielding body, moving with a given 
velocity, strikes directly another equal body at 
rest, exactly half the force of the striking body 
is expended in producing change of figure.*. 
The facts exhibited in the 7th case are 
similar to those which Mr. Smeaton has de- 
scribed as the results of his experiments.— 
According to the theory, the whole force of A 
(fig. 7.) before collision, is to be found in the 
motion of A and B after collision. But if that 
be admitted, we must suppose the spring to 
have been compressed without force :—yet we 
have no more reason to suppose that the spring 
can be compressed without force, than that a 
body can be put in motion without force ; and 
the amount of the force which has been ex- 
pended in compressing the spring, is ascer- 
tained by its effects in producing motion in 
C and D; and although these balls move in 
opposite directions, it cannot be supposed that 
their motion can be produced without force. 
. Experiments on Collision—Philos, Trans. 1782, 
