CHAPTER III. 



OF THE EQUILIBRIUM OF A SYSTEM OF 

 BODIES. 



§. 13. [IntroductioTi], Conditions of the equilibrium 

 of two systems of points, meeting each other, with veloci- 

 ties directly contrary. Definition of the quantity of mo- 

 tion, and of similar moveable points. P. 36. 



[296. Definition. " 266,'' A moveable 

 body is to be imagined as a point, composed 

 of single points or particles equally moveable, 

 which, as they differ in number, constitute the 

 proportionally different mass or bulk of the 

 body. 



297. Definition. " 267.'' A reciprocal 

 action between two bodies is an action which 

 affects the single particles of both equally, 

 increasing or diminishing their distance. 



298. Definition. " 268.'' The centre 

 of inertia of two bodies is that point, in the 

 right line joining them, which divides it reci- 

 procally in the ratio of their magnitudes. 



