MOVING FORCE. 
259 
lines are different from surfaces, or surfaces from solids ; and Cases of diffi- 
they have been distinguished by various authors by different 
* T- .1 r 11 • ... ■ docrniies of 
terms. J* rom the following proposition it appears, that Galileo moving force. 
applied the same meaning to impetus which was afterwards 
applied by Huygens to ascensional force. “ Mobile grave de- 
cendendo acquirit euin impelum, qui illi ad candem aliitudinem 
reducendo sutficiat*.” 
Leibnitz and his followers adopted the distinctive terms, 
tis ntortua and vis viva. Dr. Wollaston prefers impetus to vir 
rii’a, but he sometimes uses energy in the same sense. T he 
Edinburgh reviewers approve of Dr. Wollaston’s application of 
the term impetus ; but they propose to apply the same meaning 
to energy, which is applied by Sir Isaac Xewton to vis impressa, 
namely, the pressure multiplied into the time of its action. 
Mr. Smeaton uses the term mechanic power to expre.ss the 
product of the pressure into the space through which it acts, 
or the product of the mass into the square of its velocity. In 
bis definition of power (which I have quoted at page 120) he 
refers only to its effects in producing motion. But we have 
seen, that he understands the same measure to be the proper 
one, whether the force be expended in producing motion or 
change of figure, and he concludes, that the effects of force 
“ cannot be so easily, distinctly, and fundamentally compared, 
as by having recourse to the common measure, viz. mechanic 
powerf.” 
If this principle be capable of such general application, it 
is desirable that it should be denoted by a distinct term, in order 
to obviate ambiguity or misapprehension. The compound 
term moving force has been commonly applied, by various au- 
thors, to signify the action of moving pressure, as distinguished 
from quiescent pressure ; and, from its general use in this ac- 
cept.ilion, I have been induced to adopt it. 
It is sometimes, indeed, used tor motive force, or the pres- 
•ure uncombined with time or with the space through which 
* Dialo. de Syst. Mond. p. 12. 
t Phil. Trans. 1776, p 473. 
it 
