178 On the Measure of 
the collision of soft bodies, has been supperted 
by authors of later date; and if it were ad- 
mitted that we have no indication of the lo’s 
ef 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 Newtoun’s opinion that 
motion may be lost, and he has given many 
familiar examples of the manner in which it is: 
fost. ‘It may be tried,” he says, “ by letting 
two equal pendulums fall against one another 
from equal heights. If the pendulums be of ° 
lead, or soft clay, they will lose 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 ques- 
tion, that it may appear strange that I should 
produce it again.—I shalt 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 
* Horsley’s Newton, vol. 4. p. 259. 
y 2 
