OF THE LAWS OF MOTION. l6l 



produce motion and the masses in which motion is produced are capa- 

 ble of addition by juxtaposition, and it is easily seen by observation 

 that such addition does not modify the motion of each mass. If a 

 certain pressure upon one brick (as its own weight) cause it to fall 

 with a certain velocity, an equal pressure on another equal brick wiU 

 cause it also to fall with the same velocity ; and these two bricks 

 being placed in contact, may be considered as one mass, which a dou- 

 ble force will cause to fall with still the same velocity. And thus 

 all bodies, whatever be their magnitude, will fall with the same velo- 

 city by the action of gravity. Those who deny this (as the Aristo- 

 telians did) must maintain, that by establishing between two bodies 

 such a contact as makes them one body, we modify the motion which 

 a certain pressure will produce in them. And when we find experi- 

 mentally (as we do find) that large bodies and small ones fall with the 

 same velocity, excluding the effects of extraneous forces, this result 

 shews that there is not, in the union of small bodies into a larger one, 

 any cause which affects the motion produced in the bodies. 



It appears, therefore, that the motive quantity of force which puts 

 a body in motion is, cceteris paribus, proportional to the mass of the 

 body ; so that for a double mass a double force is requisite, in order 

 that the velocity produced may be the same. Mass considered with 

 reference to this rule, is called Inertia. 



13. The measure of mass which is used in expressing a law of 

 motion, must be obtained in some way independent of motion, other- 

 wise the law will have no meaning. Therefore, mass measured in 

 order to be considered as Inertia must be measured by the statical 

 effects of bodies, for instance, by comparison of weights. Thus two 

 masses are equal which each balance the same weight in the same 

 manner; and a mass is double of one of them which produces the 

 same effect as the two. And we find, by universal observations, that 

 the weight of a mass is not affected by the figure or the arrange- 

 ment of parts, so long as the matter continues the same. Hence it 

 appears that the mass of bodies must be compared by comparing their 

 weights, and Inertia is proportional to weight at the same place. 



