i8 
Dr. Wollaston's Lecture 
conceive also a species of accumulated force residing in a 
moving body, which is capable of resisting pressure during a 
time that is proportional to its momentum or quantitas motus. 
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 appel- 
lation as momental force ; for it is to be apprehended that for 
want of this distinction many writers themselves, and it is 
certain that many readers of diquisitions on this subject have 
confounded and compared together vis motrix, momentum, and 
vis mechanica: quantities, that are all of them totally dissi- 
milar, and bear no more comparison 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 
quantitas 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 force to bodies of equal or unequal magnitude have 
been so distinctly treated of by Smeaton, in a series of most 
direct experiments,* that it would be a needless waste of time 
to reconsider them in this place. So also, on the contrary, the 
quantities of extended mechanic effect producible by bodies 
moving with different quantities of impetus have been as 
clearly traced by the same accurate experimentalist.-f 
But there is one view, in which the comparative forces of 
* Phil v Trans. Vol. LXVI. 450. f Vol. LXXII, 337. 
