“To this I reply, that, practically speaking, quantity of matter is 
always measured by weight, both by mechanicians’ and chemists: and 
as we have proved th is procedure is utterly insecure in all. cases, 
on the hypothesis of weightless matter, the practice rests upon a con- 
viction that the hypothesis is false. And yet the practice is universal. 
Every experimenter measures quantity of matter by the balance. No 
one has ever thought of measuring quantity of matter. by its inertia 
practically ; no one has constructed a measure of quantity of matter in 
which the matter produces its indications of quantity by its motion. 
When we have to take into account the inertia of a body, we inquire 
what its weight is, and assume this as the measure of the inertia; but 
we never take the contrary course, and ascertain the inertia first in 
order to determine by that means the weight. 
» “ But it may be asked, Is it not then true, and an important nid 
truth, that the quantity of matter is measured by the inertia? Is it not 
true, and proved by experiment, that the weight is proportional to the 
inertia ?. If this be not the result of Newton’s experiments mentioned 
above, what, it may be demanded, do they prove? 
~“'To these questions I reply: It is true that-quantity of matter is 
measured by the inertia, for it is true that inertia is as the quantity of 
matter. This truth is indeed one of the laws of motion. That weight 
is proportional to inertia is proved by experiment, as far as the laws of 
motion are so proved: and Newton’s experiments prove one of the 
laws of motion, so far as any mete can i aoe or are 
Pipi to prove them. 
“ That inertia is pipes to ae is a : lew equivalent to that 
ies which asserts, that when pressure produces motion ina given body, 
the velocity produced ina given time is as the pressure. For if the 
velocity be as the pressure, when the body is given, the velocity will 
be constant if the inertia also be as the pressure. For the inertia is 
understood to be that property of bodies to which, ceteris paribus, the 
Velocity impressed is inversely proportional. One body has twice as 
much inertia as another, if, when the same force acts upon it for the 
Same time, it acquires but half the eghelhds This is the fundamental 
ption of inertia. 
“In Newton’s pendulum experiments, the pressure producing motion 
WAS ‘eeCertain‘resolved part of the weight, and was proportional to the 
Weight. It appeared by the experiments, that whatever were the mate- 
‘Mal of which the pendulum was formed, the rate of oscillation was the 
Same; that is, the velocity acquired was the same. Hence the inertia 
of the different bodies must have been in each case as the weight ;and 
this assertion is true of all different kinds of bodies. pAb abs 
