20 
Dr. Wollaston's Lecture 
that the utmost which the body B could effect in return would 
be the reproduction of A’s velocity, and restitution of its 
entire mechanic force neither increased nor diminished, ex- 
cepting by the necessary imperfection of machinery. The 
possibility of perpetual motion is consequently inconsistent 
with those principles which measure the quantity of force by 
the quantity of its extended effect, or by the square of the 
velocity which it can produce. 
In estimating the utmost effect which one body can produce 
upon another at rest, the same result is obtained by employ- 
ing impetus as ascensional force, according to Huygens ; for 
if the body A were allowed to ascend to the height due to its 
velocity, and if by any simple mechanical contrivance of a 
lever or otherwise the body B were to be raised by the 
descent of A, it is well known that the heights of ascent 
would be reciprocally as the bodies ; and consequently that 
the square of the velocity to be acquired by free descent of B 
would be in that ratio, and the quantity of mechanic force 
would be preserved as before unaltered. 
It may be of use also to consider another application of the 
same energy, and to shew more generally that the same 
quantity of total effect would be the consequence not only of 
direct action of bodies upon each other, but also of their indi- 
rect action through the medium of any mechanical advantage 
or disadvantage ; although the time of action might by that 
means be increased or decreased in any desired proportion. 
For instance, if the body supposed to be in motion w r ere to 
act by means of a lever upon a spring placed at a certain 
distance from the centre of motion, the retarding force op- 
posed to it would be inversely as the distance of the body 
