270 Prof. Whewell’s Demonstration ie 
_ “ Thus it appears that the assertion, that inertia is universally propor- 
tional to weight, is equivalent to the law of motion, that the velocity is 
as the pressure. The conception of inertia which, as we have 
a the fundamental conception is, that the velocity impressed is in- 
versely proportional to the inertia,) connects the two propositions so as 
to make them identical. 
“‘ Hence our argument with regard to the universal gravity of matter 
brings us to the above law of motion, and is proved by Newton’s expe- 
riments in the same sense in which that law of motion is so proved. 
‘“* Perhaps some persons might conceive that the identity of weight 
and inertia is obvious at once; for both are merely resistance to mo- 
_ tion ;— inertia, resistance to all motion. _ change of motion)—weight, 
resistance to motion upwards. > 
_ “ But there isa difference in these two kinds of resistance to motion. 
Inertia is instantaneous, weight is continuous resistance. Any momen- 
tary impulse which acts upon a free body overcomes its inertia, for it 
es its motion: and this change once effected, the inertia opposes 
any return to the former condition, as well as any additional change. 
The inertia is thus overcome by a momentary force. But the weight 
can only be overcome by a continuous force like itself. If an impulse 
act in opposition to the weight, it may fora moment neutralize or over- 
come the weight; but if it be not continued, the weight resumes its 
effect, and restores the condition which existed before the Rhian 
acted. 
‘** But weight not only produces rest, when it is resisted, but poi 
when it is not resisted. Weight is measured by the reaction which would 
balance it; but when unbalanced, it produces motion, and the velocity 
of this motion increases constantly. Now what determines the velocity 
thus produced in a given time, or its rate of increase? What deter- 
mines it to have one magnitude rather than another? To this we must 
evidently reply, the inertia. When weight produces motion, the inertia 
is the reaction which makes the motion determinate. The accumulated 
motion produced by the action of unbalanced weight is as determinate 
a condition as the equilibrium produced by balanced weight. In both 
cases the condition of the body acted on is determined by the opposition 
“ the action and reaction. 
_ Hence inertia is the reaction which opposes the weights when un- 
wieied But by the conception of action and reaction, (as m 
bene aanereiees a. they are measured by each éthet:: and 
