106 THE CENTRE OF GRAVITY. EQUILIBRIUM. 



perimeuts are sufficient for determining the centre of 

 gravity : it is the point in which the vertical lines 

 through two points of suspension intersect. 



If we pierce a hole through the pasteboard at s, and 

 suspend the body by a thread passed through the hole, 

 it will be in equilibrium in any position which we choose 

 to give to the body ; and it will, if placed in any of the 

 four positions of fig. 74, not tend to change it for any 

 other. This kind of equilibrium in which a body re- 

 mains in equilibrium in all possible positions, is neutral 

 equilibrium ; and we may now extend the definition of 

 the centre of gravity thus : 



The centre of gravity of a body is a point such that, if 

 it be supported, the body will be in equilibrium in all 

 positions. 



A vertical line through the centre of gravity, as a d 

 in fig. 74, A and B, b e in (7, cf in D, may be called 

 a line of gravitation ; and a body is in equilibrium if 

 supported at any point in the line of gravitation ; and it 

 follows from what has been shown, that the equilibrium 

 is 



STABLE, if the point of support is ABOVE the centre of gravity. 



NEUTRAL AT 



UNSTABLE, BELOW 



In bodies with regular form, such as square or cir- 

 cular discs, spheres, cubes, etc., the centre of gravity is 

 in the centre, provided such bodies are homogeneous, that ( 

 is, are of uniform density. If a regular body consists ; 

 partly of wood and partly of lead, the centre of gravity 

 will recede from the centre towards the part which 

 contains the lead. The centre of gravity of a triangle, 

 or a triangular board, is at the point of intersection of 



