Moving Force. 127 
are not quite so obvious as, at first sight, they 
appear to be. 
Before we enter upon the examination of 
these particular cases, it may be proper to 
observe, in addition to what has been already 
noticed ; that, in respect to the general ques- 
tion, or in respect to the existence even of any 
question at all on this subject, some of the best 
recent authorities are the most difficult to be 
reconciled with each other. 
Few authors, in our language, on the prin- 
ciples of mechanics, have been more generally 
read and referred to than Emerson. From 
the great analytical skill of this author, one 
would have expected something decisive on 
the long pending question concerning the 
measure of moving force; but he seems to take 
for granted, that the measure is the mass into 
the velocity or the momentum, for he scarcel y 
condescends to mention the other, and after a 
few observations, dismisses it in the following 
laconic manner :—“ It seems to be a neces- 
sary property of the vis viva, that the resist- 
ance is uniform. But there are infinite cases 
where this does not happen; and in such cases, 
this law of the vis viva must fail. And since it 
fails in so many cases, and is so obscure in 
itself, it ought to be weeded out, and not to 
pass for a principle in mechanics.”’* 
* Emerson’s Principles of Mechanics, p. 20. 
