*! 
224 _ On the Measure of 
ing quiescent pressure), and the “force of a 
body in motion,” are very ‘common expres- 
sions.—But these two descriptions of force 
are as different in kind, as lines are different 
from surfaces, or surfaces from solids; and 
they have been distinguished by various 
authors by different terms. From the follow- 
ing proposition it appears that Galileo apphed 
the same meaning to impetus which was after- 
wards applied by Huygens to ascensional 
force. “ Mobile grave descendendo acquirit 
eum impetum, qui illi ad eandem altitudinem 
reducendo sufficiat.”’ * 
Leibnitz and his followers adopted the dis- 
tinctive terms, vis mortua and vis viva. Dr. 
Wollaston prefers impetus to vis viva, but he 
sometimes uses energy in the same sense. The 
Edinburgh reviewers approve of Dr. Wollas- 
ton’s application of the term impetus; but 
they propose to apply the same meaning to 
energy which is applied by Sir Isaac Newton 
to vis impressa, namely the pressure multiplied 
into the time of its action. 
Mr. Smeaton uses the term mechanic power 
to express the product of the pressure into the 
space through which it acts, or the product 
af the mass into the square of its velocity. 
~ * Dialo, de Syst. Mund. p. 12. 
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