32 ESSAYS AND OBSERVATIONS 



body is deflroyed by the refiftance of the 

 body at reft, the other half remains, with 

 which the impinging body moves, and 

 carries along with it the other body from 

 reft to motion: When the refiftance of the 

 body at reft is once deftroyed, it yields to 

 the impreiTed force without any further 

 oppofition; and, by its impenetrability 

 muft be carried along with the other. 



But, in order to form an accurate no-^ 

 tion of percuffion, two preliminary points . 

 muft be fettled. The firft is, to fettle the 

 precife nature of the adtion exerted by a 

 body at reft, when impelled by one in 

 motion. Does the body at reft return 

 flroke for ftroke, or impulfe for impulfe ? 

 Does it truly rea(fb, or does it barely re- 

 fift ? That it refifts only, and does not re- 

 a6l, will be evident from a fingle confide- 

 ration, which is, that motion is efTential 

 to impulfe, not at all to refiftance. Now, 

 if the body at reft reads, by returning 

 ftroke for ftroke ; it muft rufli upon the 

 impinging body, and carry it backwards 

 at leaft for fome fmall fpace; wdiich can- 

 not be ; becaufe, once admiilting a retro- 

 grade 



