602 



Sir William Thomson 



[Jan. 29, 



rubber tied to it all round, stretched uniformly in all directions, and 

 as tightly as could be done without special apparatus for stretching 

 it and binding it to the ring when stretched. 



Fig. 27. 



Section of the air-bubble in a level ti;be filled with water, and bent so that its axis 

 is part of a circle of large radius ; scale is represented in Fig. 28. 



Fig. 28. 



Represents a length of one centimetre for Figs. 24-27. 



I now pour in water, and we find the flexible bottom assuming 

 very much the same shape as the drop which you saw hanging from 

 my finger after it had been dipped into and removed from the vessel 

 of water (see Fig. 16). I continue to pour in more water, and the 

 form changes gradually and slowly, preserving meanwhile the general 

 form of a droj) such as is shown in Fig. 15, until, when a certain 

 quantity of water has been poured in, a sudden change takes place. 

 The sudden change corresponds to the breaking away of a real drop 

 of water from, for example, the mouth of a tea-urn, when the stop- 

 cock is so nearly closed that a very slow dropping takes place. The 

 drop in the indiarubber bag, however, docs not fall away, because 

 the tension of the indiarubber increases enormously when the india- 

 rubber is stretched. The tension of the real film at the surface of 

 a drop of water remains constant, however much the surface is 

 stretched, and therefore the drop breaks away instantly when enough 

 of water has been supplied from above to feed the drop to the 

 greatest volume that can hang from the particular size of tube which 

 is used. 



