62 ESSAYS ann OBSERVATIONS 
the fmalleft refiftance or oppofing force. But 
the ws infita is plainly not of this nature. 
The body not only preferves itfelf in motion 
where there is no refiftance, but exerts a cer- 
tain force againft every thing that refifts its 
progrefs. And it is with a view to this force 
exerted in motion, as well as with a view to 
‘the refiftance exerted at reft, that Sir I/aac 
Newton, in his third definition, defcribes the 
vis infita to be ** that’ power in matter, by 
“‘ which every body, as much as in it lies, 
“* perfeveres in its prefent {tate of reft or mo- 
a tion.” The force, however exerted in 
motion, is effentially different from that ex- 
erted at reft. The latter is mere refiftance ; 
the former is an impulfive force, by which 
the body endeavours, quantum in fe eft, to 
overcome every obftruction to its motion. 
The force of refiftance is above afcertained, 
and a rule laid down for eftimating it, about 
which all philofophers are agreed. They 
differ widely about the impulfive force of the 
vis infita: the Germans holding it to be as the 
fquares of the velocities ;_ the Englifh and 
French holding it to be fimply as the velocities. 
The difpute is fpun out to a great length: 
the parties, as is ufual, being more folici- 
tous 
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