244 
MOVING I'ORCB. 
Cases of diffi- parative forces of impact of dilFerent bodies was not examined 
culty in the i c i • 
doctrines of omeaton, and it may be worth while to show, that when 
moving force, the whole energy of a body A is employed without loss in giv- 
ing velocity to a second body B, the inipelus which B receives 
is, in all cases, equal to that of A, and the force transferred to 
B, or by it to a third body C, (if also communicated without 
loss, and duly estimated as a mechanic force,) is always equal 
to that from which it originated. 
“ As the simplest case of entire transfer, the body A may 
be supposed to act upon B in a direct line through the medium 
of a light spring, so contrived, that the spring is prevented by 
a ratchet from returning in the direction towards A, but ex- 
pands again entirely in the direction towards B, and by that 
means exerts the whole force which had been wound up by 
the action of A, in giving motion to B alone*.” 
In the explanations which I have offered of the phenomena 
■which occur in the collision of bodies, I have supposed all the 
changes of motion and of figure to be gradual, uot instauta- 
neous ; and it may be objected to these explanations, that they 
cannot be applied to cases of instantaneous impact. But I be* 
iieve it is now generally admitted, as I have already observed, 
that impact cannot be perfectly instantaneous, that some small 
but finite portion of lime must pass during the operation* j 
and if this be so, the changes of motion must occupy also 
some portion of space. Now, if we suppose that portion of 
space to be magnified by means of lenses, we cannot doubt that 
■W'e should see all the changes of figure, as well as of motion, 
distinctly in their order, the same as they actually appear when 
they are gradually produced in extended spaces, and the same 
explanations may be strictly applied to the changes which take 
place in the smallest as well as in the largest spaces. 
The 9 th case is stated merely to show, that we cannot form 
a gust estimate of tlie forces of bodies in motion, by attending 
solely to the quantity of motion of their common centre of 
1 
• ' ■ • Pliil. Trans, 1806, p. 19. 
• . 1 . ♦ See Hutton’s Dictionary, art. Force, vol, I, p. 496. 
gravity > 
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