DYNAMICS* 



SECT. III. 



SECOND LAW OF MOTION. 



71. JL HE action and reaction of bodies on 

 another are equal ; in other words, the changes in 

 the quantities of motion of the bodies which act, 

 and of the bodies which are acted on, are equal, and 

 in opposite directions. 



This comprehends two cases, according as the bodies 

 act merely in consequence of the motions impres- 

 sed on them, as in impulse or collision ; or in con- 

 sequence of some unknown agency, which commu- 

 nicates motion from the one to the other, though 

 they be distant, as in the cases of gravity, magne- 

 tism, &c. Both kinds of action are subject to the 

 same law : In the first, the quantity of motion 

 gained by the one body, is just equal to that lost by 

 the other in the same direction : In the second, 

 the quantity of motion gained by the one in any 

 direction, is just equal to that gained by the other 

 in the opposite direction. This last proposition is 

 only known from experience : the first may be pro- 

 ved from the nature of Body, and depends on the 

 theorem in the next article. 



VOL. I. c As* 



