52 GRA VITA TIOX. 



rest in a state of equilibrium. The centre of gravity 

 will be supported when it coincides with the point of sup- 

 port, or is in the same vertical line with it. 



111. Stable Equilibrium. ^ body supported 

 in such a way that, when slightly displaced from 

 its position of equilibrium, it tends to return 

 to that position, is said to be in stable equili- 

 brium. Such a displacement raises the centre of grav- 

 ity. Examples: a disc supported above the centre; a 

 semi-spherical oil-can; a right cone placed upon its 

 base ; a pendulum or plumb-line. The cavalry-man 

 represented in Fig. 21, is in stable equilibrium, and 

 may rock up and down, 

 balanced upon his horse's 

 hind - feet, because the 

 heavy ball brings the cen- 

 tre of gravity of the com- 

 bined mass below the 

 points of support. The 

 "balancer" (Fig. 20) af- 

 fords another example of 

 stable equilibrium. 



5. Unstable Equi- FlG 2 ~ 



librium. A body sup- 

 ported in such a way that, when slightly displaced 

 from its position of equilibrium, it tends to fall 

 further from that position, is said to be in unstable 

 equilibrium. Such a displacement lowers the centre of 

 gravity. The body will not come to rest until the centre 

 of gravity has reached the lowest possible point, when it 

 will be in stable equilibrium. Examples: A disc sup- 



