232 
MOVING FOKCE. 
Cases of lUfli- enses where motion is destroyed, or where it is transferred from 
doctrines ‘of one body to another. 
moving force. It was a favourite doclrine with the Cartesians, and it was 
maintained also, though uj)on quite ditferent principles, by 
Leibnitz and John Bernoulli, that motion could not be lost ; 
for the same quantity of motion or of force, is was said, must 
be always preserved in the world, A similar doctrine, applied 
to explain the collision of soft bodies, has been supported by 
authors of later date ; and if it were admitted that we have 
no indication of the loss of force, unless motion be lost in the 
centre of gravity of the system in which the force acts, it 
might truly be said that no force can be lost. 
It has never been questioned that motion may be generated, 
accelerated, or retarded in a variety of ways, and there appears 
to be no good reason for supposing that it may not be destroyed 
as well as generated. 
It was Sir Isaac Newton’s opinion, that motion may bs lost, 
and he has given many familiar examples of the manner in 
which it is lost. “ It may be tried,” he says, by letting two 
equal pendulums form against one another from equal heights. 
If the pendulums be of lead, or soft clay, they will loose all, 
or almost all, their motion.” In the same way the motion of 
A and B (case 6th) is lost when the spring is compressed. This 
case has been so often brought forward, and so much has been 
said about it, on both sides of the question, that it may appear 
strange that I should produce it again. I shall endeavour to * 
confine my observations upon it in a small compass. 
It is very generally understood, and it has been received almost 
as an axiom, that if two 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 enuncia- 
tion of this proposition. He says, “ If two bodies meet di- 
rectly with a shock, which mutually destroys their motion, 
without producing any other sensible effect, it mtiy be fairly con- 
cluded, that they meet with equal force*,” Now this is a fair 
Essay on Qiiantity—Pliil. Trans, 1748, p. 5l5. 
reference 
