20 CENTRE OF GRAVITY. 



have any weight. But it should be noticed, that 

 we are not yet sufficiently acquainted with this 

 latter class : among modern philosophers, there 

 are various opinions even as to their existence as 

 distinct species of matter, some considering the 

 phenomena occasioned by them as arising from 

 various modifications of motion only. 



CENTRE OF GRAVITY. 



The centre of gravity of a body is that point 

 about which all its parts exactly balance each 

 other. 



Hence if a body be suspended or supported by 

 this point, it will rest in any position into which it 

 is put. Also, whatever supports that point bears 

 the weight of the whole body ; and while it is sup- 

 ported, the body cannot fall. We may therefore 

 consider the whole weight of a body as centred in 

 this point. 



The common centre of gravity of two or more 

 bodies is the point about which they would equi- 

 ponderate or rest in any position. If the centres 

 of gravity of two bodies, A and B, (Plate 1. 

 fig, 3.) be connected by the right line A B, the 

 distances A C, and B C, from the common centre 

 of gravity C, are reciprocally as the weights or 

 bodies A and B ; that is, A C : B C : : B : A. 



If a line be drawn from the centre of gravity of 

 a body, perpendicular to the horizon, it is called 

 the line of direction, because it is the line that the 

 centre of gravity would describe if the body fell 

 freely. 



It is the property of this line, that while it falls 

 within the base upon which the body stands, the 

 body cannot fall ; but if it fall without the base, 



10 



