On the Force of Percussion. 933 
serts that the guantiias motiis of two or more bodies ésti- 
mated in any given direction is not altered by their action 
upon each other. The demonstration begins thus: 
‘© Etenim actio eique contraria reactio cequales sunt per 
legem tertiam, ideoque per legem secundam equales in mo- 
tibus efficient mutationes versus contrarias partes.” Now, if 
he had considered the third law as implying equality of more 
than mere moving forces, there could have been no occasion 
to refer to the second law, with a view thence to deduce the 
equality of momenta produced. 
* Some authors, however, have interpreted the third law 
differently, and accordingly have expressed a difficulty in 
comprchending the simple illustration given by Newton. 
When they say that action is equal to reaction, they mean 
hot only that the instantaneous intensity of the moving 
forces, er pressures opposed to each other, are necessarily 
equal, but conceive also a species of accumulated force re- 
siding in 2 moving body, which is capable of resisting pres- 
sure during a time that is proportional to Its momentum or 
quantitas motis. 
If it be of any real utility to give the name of force to 
this complex idea of vis motrix extended through time, as 
well as that of momentum to its effects when unresisted, it 
would be requisite to distinguish this force always by some 
such appellation as momental force; for it is to be appre- 
hended that for want of this distinction many writers them- 
selves, and it is certain that many readers of disquisitions 
on this subject, have confounded and compared together 
vis molrix, momentum, and vis mechanica: quantities that 
are all of them totally dissimilar, and bear no more compari- 
son to each other, than lines to surfaces, or surfaces to solids. 
In practical mechanics, however, it is at least very rarely 
that the momentum of bodies is in any degree an object of 
_ consideration ; the strength of, machinery being in every case 
to be adapted to the quantitas motrix, and the extent and 
value of the effect to.be produced depending upon the quan- 
titas mechanica of the force applied, or, in other words, to 
the space through which a given vis motrix is exerted, 
The comparative velocities given by different quantities of 
mechanic 
