270 Pvof. WhewelVs Demonstration 



" 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 (of which, as we have 

 said, the fundamental concepti(|n 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 (or change of motion) — weight, 

 resistance to motion upwards. 



" But there is a difference in these two kinds of resistance to motion. 

 Inertia is instantaneous, weigtit is continuous resistance. Any momen- 

 taiy impulse which acts upon a free body overcomes its inertia, for it 

 changes 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 for a moment neutralize or over- 

 come the weight; but if it be not continued, the weight resuines its 

 effect, and restores the condition which existed before the impulse 

 acted. 



" But weight not only produces rest, when it is resisted, but motion, 

 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 

 of the action and reaction. 



" Hence inertia is the reaction which opposes the weight, when un- 

 balanced. But by the conception of action and reaction, (as mutually 

 determining and determined,) they are measured by each other : and 

 hence the inertia is necessarily proportional to the weight. 



" But when we have readied this conclusion, the original objection 

 may be again urged against it. It may be said, that there must be some 



