PHYSICAL ann LITERARY. 87 
and the activity before defcribed. This is 
conceived to be an action in a fecondary 
 fenfe. It is only a confequence of the firft 
: 
action. A motion once produced continues, 
till it be deftroyed by an equal and oppofite 
motion, or an action capable of having pro- 
duced it. . 
THE effect produced by an agent is a cer- 
tain quantity of motion or force, which, like 
other effects, continues without any tenden- 
cy to perifh of itfelf. This quantity of mo- 
tion, is always proportional to the real fpace 
defcribed by the whole quantity of matter ; 
and that fpace having length, breadth, and 
thicknefs, is meafured by multiplying the 
quantity of matter, by the length of {pace 
defcribed by every particle, that is, by the 
velocity. If a body in motion ftrikes direct- 
ly upon another at reft, the two move on as 
one body after the ftroke. They cannot move 
together with the fame velocity that the fin- 
gle body had before the ftroke, for. then there 
__ would be an increafe of quantity of motion. 
_ And whence fhould that proceed? fuch an 
5 appearance as that, would difcover an aétivi- 
ty in bodies indeed. Upon the fuppofition 
iA 
of perfect inativity, the quantity of motion 
muft 
