Mr . smeaton on Mechanic Power . ^ x 
will have, in that time, acquired a velocity that would 
carry it, at an uniform rate, juft double of the former; 
that is, at the rate of 64 ft. 4 in. per fecond.- Now, if in 
confequence of this equal increale of velocity, in an 
equal increafe of time, by the continuance of the fame 
impelling power, we define that to be a double quantity 
of motion, which is generated in a given quantity of 
matter, b> the adtion of the fame impelling power for 
a double time; this will be co-incident with Sir isaac 
newton’s definition abovementioned ; whereas, in trying 
experiments upon the total effedts of bodies in motion, 
it appears, that when a body is put in motion, by what- 
ever caufe, the impreffion it will make upon an uni- 
formly refilling medium, or upon uniformly yielding 
i ubftances, will be as the niafis of matter of the moving 
body, multiplied by the fquare of its velocity: the queft 
tion, theiefore, properly is, whether thole terms, the 
quantity of motion^ the momenta of bodies in motion, or 
forces of bodies in motion, which have generally been 
efteemed fynonymous, are with the moil propriety of 
language to be efteemed equal, double, or triple, when 
they have been generated by an equable impulfe, a&ing 
for an equal, double, or triple time ; or that it fiiould be 
meafured by the effedls being equal, double, or triple, in 
overcoming refinances before a body in motion can be 
flopped ? For, according as thofe terms are underftood in 
this or that way, it will neceffarily follow, that the mo- 
menta of equal bodies will be as the velocities, or as the 
fquares of the velocities refpedlively ; it being certain, 
~ Vol. LX VI# O o o that, 
