PHYSICAL AND LITERARY. 31 



produced is eafily deducible from the 

 principles above laid down. The vis 

 infita and the vis reftftentiae are fufficienc 

 to account for all the efFedls that proceed 

 from the collifion of hard bodies which 

 have na elaflicity. If the refiflance of a 

 body be always lefs than the imprefled 

 force, the refifting body, after its refiflance 

 is overcome, mufl neceflariTy be carried 

 along with the body that impinges upoo 

 it. And, being once put in motion, its 

 own vis inftta preferves it in the fame de-^ 

 gree of motion. I here put the fimpleft 

 cafe, that of a body in motion, impin- 

 ging upon one at reft ; and, for the eafe of 

 the imagination, I fhall fuppofe thefe two 

 bodies equal. VVith refpe(5l to this eafe 

 it is afcertained by experiments, that the 

 two bodies in eontaft go on with half the 

 celerity which the impinging body had 

 before the callifion. For it is a general 

 rule in the percuffion of hard bodies, that 

 ihemometittim or force, diredled to the fame 

 point, continues the fame after eonta<5t as 

 before. There is no difficulty here: For 

 after half of the force of the impinging 



body 



