PHYSICAL AND LITERARY. 69 



perty, as to preferve itfelf in its original 

 degree of motion where there is no ob- 

 ftrudion, and yet to yield to the fmalleft 

 refi fiance or oppofing force. But the 'uis 

 inftta is plainly not of that nature. The 

 body not only preferves itfelf in motion 

 where there is no refinance, but exerts a 

 certain force againil every thing that re- 

 fifts its progrefs. And it is with a view 

 to this force exerted in motion, as well 

 as with a view to the refinance exerted at 

 reft, that Sir Ifaac Newton, in his third 

 definition, defcribes the 'uis inftta to be 

 " that power in matter, by which every 

 " body, as much as in it lies, perfeveres 

 ** in its prefent flate of reft or motion." 

 The force, however, exerted in motion, 

 is eflentially different from that exerted 

 at reft. The latter is mere refiftance ; 

 the former is. an impulfive force, by which 

 the body endeavours, quantum in fe ejl, to 

 overcome every obllrudion to its motion. 

 The force of refiftance is above afcertain- 

 ed, and a rule laid down for eflimatin"- 

 it, about which all philofophers are a- 

 greed. They differ widely about the im- 

 pulfive 



