EQUILIBEIUM. 105 



itself in the position shown at A. If placed in any 

 other position, the body returns by itself to that as- 

 sumed at first, and there will be only one other position 

 in which it will be also at rest that shown at B, which 

 is the reverse of A. But the second position of equili- 

 brium is far from being so safe as the first : the slightest 

 disturbance causes the body to swing round and to 

 assume again the first position. 



In order to show that the second position is the reverse of 

 the first, a second thread with loops at the ends is passed through 

 a, and a small weight g is suspended from both loops. The second 

 thread indicates the vertical line through a, when the pasteboard- 

 figure is at rest ; by holding thread and figure at d between thumb 

 and forefinger, a line can be drawn along the thread. This line will 

 be covered by the thread when the body is suspended in the second 

 position. 



The first position that which a freely suspended body 

 assumes if left to itself is the position of stable equili- 

 brium. The second position that in which a body may 

 be placed, but which, after the smallest displacement, it 

 changes for the first is the position of unstable equi- 

 librium. 



If other points of suspension be selected, as , c, new 

 positions of stable equilibrium (7, D, will be given to the 

 body; but, if the vertical lines through these successive 

 points be indicated on the pasteboard, it will be found 

 that they all intersect in one point, s. This point is the 

 centre of gravity of the body. It will be observed that, 

 in the positions of stable equilibrium shown at -4, 67, and 

 D, the centre of gravity is vertically below the point of 

 suspension. In the position B, and in all other possible 

 positions of unstable equilibrium, the centre of gravity 

 is vertically above the point of suspension. Two ex- 



