502 PHYSIOLOGICAL PHYSICS. [Chap. XLII. 



force of gravity and the force of resistance through 

 the pin are both acting through the same vertical 

 line, and if the plate be displaced, both forces act so 

 as to bring themselves again into the same vertical. 

 This position is called the position of stable equili- 

 brium, because the body, if displaced, will not de- 

 part farther from equilibrium, but will return to it. 

 Now let the plate be suspended from a pin at A' ; it 

 is plain the body is not in equilibrium at all. It is 

 under the influence of the force of gravity acting 

 downwards through G, and of the force of resistance 

 acting upwards through A' ; the plate will, conse- 

 quently, turn so as to place the centre of gravity 

 directly under the point of support A', and will then 

 rest in stable equilibrium. In the same way, if the 

 pin be at A" the body is under the influence of an up- 

 ward force at A", and a downward force at G, and 

 again it will turn till A" comes to be over G, where it 

 will rest. Let the pin be placed at A'", directly under G. 

 If G and A'" be accurately in the same vertical line, 

 the body will be in a position of equilibrium, for the 

 downward force of gravity and the upward force of 

 resistance are opposing one another along the same 

 vertical line. But let the body be displaced to either 

 side, and suppose it to be displaced, as shown in Fig. 

 212 B'C'D', it is evident that the forces no longer 

 act in the same vertical, gravity is acting through G', 

 and resistance through A"'. The result will be that 

 the plate will rapidly turn round A'", till the centre of 

 gravity is below the point of suspension, when the 

 body will be again in stable equilibrium. The body 

 is, then, in equilibrium when its point of suspension is 

 below its centre of gravity ; but it is unstable equi- 

 librium, and the body falls away from it on the 

 slightest movement. The same facts apply to a body 

 resting on a table. It is in equilibrium when the 

 vertical through the centre of gravity also passes 



