60 OUTLINES OF NATURAL PHILOSOPHY. 



through the axis of motion, a plane be made to 

 pass parallel to the direction of any force, the pro- 

 duct of that force into its distance from the same 

 plane, is called the momentum of the force. 



108. Any system of bodies being given, a point 

 may be found, such, that if any plane pass through 

 it, the sum of the momenta of the bodies on one 

 side of the plane, relatively to the point found, 

 shall be equal to the sum of the momenta of the 

 bodies on the other side of the plane, relatively to 

 the same point. 



a. The point thus formed is called the centre of gravity 

 of the system. 



b. Every system of bodies, therefore, and every indivi- 

 dual body, (as a body may be regarded as a system 

 of particles or infinitely small bodies), has a centre 

 of gravity ; and that centre remains the same while 

 the bodies retain their position relatively to one an- 

 other, however the whole system may change its 

 place relatively to the horizon. 



109. In any body, or system of bodies, if the 

 centre of gravity be sustained, the whole will re- 

 main at rest, 



In other words, the body or system of bodies has no 

 tendency to angular motion round its centre of gra- 

 vity. 



110. The distance of the centre of gravity of any 

 system of bodie.8 from a given plane, is equal to 



the 



