44 POPULAR LECTURES AND ADDRESSES. 



of stable equilibrium, with an unstable form inter- 

 mediate between them. Here is an experimental 

 proof of this statement. With the drop in its 

 higher stable form I cause it to vibrate so as alter- 

 nately to decrease and increase the axial length, 

 and you see that when the vibrations are such as 

 to cause the increase of length to reach a certain 

 limit there is a sudden change to the lower stable 

 form, and we may now leave the mass performing 

 small vibrations about that lower form. I now 

 increase these small vibrations, and we see that, 

 whenever, in one of the upward (increasing) vibra- 

 tions, the contraction of axial length reaches the 

 limit already referred to, there is again a sudden 

 change, which I promote by gently lifting with 

 my hands, and the mass assumes the higher stable 

 form, and we have it again performing small 

 vibrations about this form. 



The two positions of stable equilibrium, and 

 the one of unstable intermediate between them, 

 is a curious peculiarity of the hydrostatic problem 

 presented by the water supported by india-rubber 

 in the manner of the experiment. 



