180 ‘On the Measure of 
A and B remains undisturbed ; but is it neces- 
sary that we should confine our attention solely 
to that centre of gravity ?—If we find that 
the motion of a body. cannot be destroyed 
without producing certain measurable effects 
of force, and if we find these effects to bear an 
unvarying relation in quantity to the motion 
destroyed, there surely can be no inconsistency 
in taking the amount of these effects for the 
measure of the force of the moving body. 
~ I confess I have never been able to under- 
stand M. D’Alembert’s distinction between the 
sum and the number of the obstacles over- 
come.* If the obstacles be equal to each 
other, it can make no difference whether their 
sum or their number be taken as the measure 
of the force. If they be unequal, the sum of 
their separate amounts must surely be the abso- 
lute quantity of resistance overcome, and the 
proper measure of the force by which it is 
overcome. ‘To say that the quantities of re- 
sistance during infinitely small instants of time 
must be equal to each other, is assuming a most 
unreasonable postulatum.—The difficulty can- 
not be removed by taking insensible, instead 
of sensible portions of time; for we have no 
reason to suppose that the pressure into the : 
space approaches nearer to equality in infi- 
* See page 131. 
