MOVING FORCK. 
23:5 
reiiiience to expetinienCj aud, in the case iwder consideration, 
we cortaiuly have a naeasurable, senwblo effect" in the com- 
pression of the spring, which cannot be prodnced without 
force. But although the ends of the spring meet at E, (fig. 6) 
it i.s still held by many that that effect is produced equally by 
A and B. If the forces of A and B are really equal, we should 
. hflve the same effect produced when we substitute for H another 
I hall equal in weight and velocity to A. But the same effect 
icannot be produced by that means, an<l if the real effects be 
I examined, we shall always find that the spring is less com- 
ij>ressed (as measured by the ptessure into the space) by A than 
Iw fi in the ratio of l to 2. 
It is true the common centre of gravity of A and B remains 
^undisturbed ; but is it necessar)' that we should confine our at- 
ttention solely to that centre of gravity ? If we find that the 
[motion of a body cannot be destroyed without producing cer- 
tain measurable effects of force, and if we find these effects to 
bbear an unvarying relation in quantity to the motion destroyed, 
[there surely can be no inconsistency in taking the amount of 
[these effects for tlie measure of the force of the moving body. 
I confess I have never been able to understand M. D'Aletn- 
bbert’s distinction between the sum and the number of the ob- 
•slacles overcome*. If the obstacles be equal to each other, 
lit can make no difference whether their sum or their number 
bbc taken as the measure of the force. If they be unequal, 
[the sum of their separate amounts must surely be the absolute 
[;quantity of resistance overcome, and the proper measure of the 
[force by wliich it is overcome. To say (hat the quantities of 
rvQsistance, during infinitely small instants of time, must be 
requal to each otlier, is assuming a most unreas onable pioitalatum . 
IThe difficulty cannot be removed by taking insensible, instead 
tjf sensible, portions of time ; for we h.ive no reason to sup- 
ppose, that the pressure into the space approaches nearer to 
wqualily in infinitely small, than in palpably large, portions ofj 
l;ime. 
• See page ISl. 
'VoL. XXXVI.— No. 168 . S This 
Cases of diffi- 
culty in the 
doctrines of 
nioviojj force. 
