MECHANICS. 65 



Of the Motion of the Centre of Gravity. 



In the preceding propositions, the centres of gravity 

 of bodies or systems, in which the parts preserve 

 the same relative position, have only been consider- 

 ed. It is evident, however, that, though the parts 

 do not preserve the same relative position, the 

 centre of gravity may be found for any given state 

 of the system. The properties common to all the 

 centres so found, are of great importance in me- 

 chanics. 



117. If any number of bodies move uniformly in 

 straight lines, their common centre of gravity will 

 either move uniformly in a straight line, or remain 

 at rest. 



NEWTONI, Principia, Lib. 1. Lemma 23. 



118. The mutual action of bodies does not change 

 the state of their centre of gravity, either as re- 

 maining at rest, or as moving uniformly in a 

 straight line. 



119. The quantity of motion in any system of bo- 

 dies estimated in a given direction, remains con- 

 stantly the same, whatever may be the mutual ac- 

 tion of these bodies on one another. 



VOL. T. E a. Both 



