98 ESSAYS AND OBSERVATIONS 



an adlion in a fecondary fenfe. It is on- 

 ly a confequence of the firft adlion. A 

 motion once produced continues, till it 

 be deftroyed by an equal and oppofite 

 motion, or an adtion capable of having 

 produced it. 



The effedl produced by an agent is a 

 certain quantity of motion or force, 

 which, like other effecls, continues with- 

 out any tendency to perifli of itfelf. This 

 quantity of motion is always proportio- 

 nal to the real fpace described by the whole 

 quantity of matter ; and that fpace ha- 

 ving length, breadth, and thicknefs, is 

 m.eafured by multiplying the quantity of 

 miatter by the length of fpace defcribed 

 by every particle, that is, by the velocity. 

 If a body in motion flrikes diredlly upon 

 another at reft, the two move on as one 

 body after the ftroke. They cannot move 

 together with the fame velocity that the 

 iingle body had before the ftroke, for. 

 then there would be an increafe of quan- 

 tity of motion. And whence fliould that 

 proceed ? Such an appearance as that 

 would difcover an adivity in bodies in- 

 deed. 



