1886.] 



Capillary Attraction, 



503 



I now put this siphon into action, gradually drawing off some of 

 the water, and we find the drop gradually diminishes until a sudden 

 change again occurs and it assumes the form we observed (Fig. 16) 

 when I first poured in the water. I instantly stop the action of the 

 siphon, and we now find that the great drop has two possible forms 

 of stable equilibrium, with an unstable form intermediate 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 alternately 

 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) vibrations, the contrac- 

 tion 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 indiarubber in the 

 manner of the experiment. 



Fig. 29. 



I have here a simple arrangement of apparatus (Figs. 29 and 30) 

 by which, with proper optical aids, such as a cathetometcr and a 



