MECHANICS 45 



remain in equilibrium by being suspended i.e., by having 

 the weight at their centres of gravity supported from 

 above instead of from below. 



If a body be supported in such a manner that on 

 its equilibrium being disturbed, it tends to regain it 

 (as in an oscillating pendulum or a detached wooden 

 ball loaded with lead at one place) it is said to be in 

 stable equilibrium. In the opposite case (as when a pole 

 is balanced at one end), the equilibrium is unstable, 

 because when disturbed, it tends to fall further away 

 from, instead of regaining, the position in which the ver- 

 tical line from its centre of gravity falls within its base. 



The centre of gravity is not necessarily within the 

 solid body itself which has to be supported, but may be 

 in its vicinity, as in the case of a ring, or any hollow 

 vessel. Thus it is that a variety of posturing tricks can 

 be performed in tight-rope dancing. The dancer carries 

 a long pole, the weight of which transfers the centre of 

 gravity to the middle of the pole within the grasp of 

 his hands so that he has it under his control. Similarly, in 

 balancing rods on head or hand, the performer's art 

 consists in keeping, by means of constant movement, 

 the base of the rod under the centre of gravity. A 

 number of ingenious toys are also constructed on the 

 principle of an external position for the centre of 

 gravity. Thus the figure of a prancing horse may 

 be made to rock backwards and forwards, resting, 

 near the edge of a table, on its hind feet only, in an 

 apparently impossible position, by means of a leaden 

 weight at the end of a curved wire, the other end of 

 such wire being fixed to the belly of the horse, so that 

 the centre of gravity of the whole structure is thrown 

 behind and below the prancing figure. Thus, although 

 its position looks most insecure, its equilibrium is really 



