34 Re-aftion is equal to Attion. * [Book I. 



bodies alvtays move in a curve, or orbit, which is more 

 or lei's elliptical as one of thefe forces happens to be 

 predominant. 



III. The third law is, that re-action is always equal 

 to action *. In plain terms, the refiftance of a body 

 at reft, which is acted or preffed upon, acts againft a 

 moving body with a certain degree of power, and pro- 

 duces the fame effects as would have been produced 

 by a certain degree of active force exerted in a direc- 

 tion contrary to that of the moving body. Hence it 

 follows, that any one body acting upon another actually 

 lofes as much force as it communicates, as will be evi- 

 dent, if with a fmall bullet fufpended from a firing we 

 ftrike another bullet which is at reft, or from obferv- 

 ing a ball in motion on a billiard- table ftrike another 

 which is at reft on the table ; in both which cafes the 

 ftriking body will lofe half its quantity of motion, and 

 that quantity of its motion which it lofes will be com- 

 municated to the other body f. 



This law is an effect of the vis inertia of matter, 

 and is extended to all cafes where there is a refifting 

 body. When a load is drawn by a horfe, the load re- 

 acts againft the motion of the horfe, and the progref- 

 fion of the animal is as much impeded by the load, as 

 the motion of the load is promoted by the efforts of 

 the animal. The ringer which preffes againft any fo- 

 lid body is preffed by thnt body; but in elaftic fub- 

 ftances the effect is mod apparent. 



* Pemberton's View, p 31. 

 f Kr.Scld's Inllitutes, p. 12, 15. 



