C A PILLAR Y A TTRA CTION. 43 



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 stopcock is so 

 nearly closed that a very slow dropping takes 

 place. The drop in the india-rubber bag, how- 

 ever, does not fall away, because the tension 

 of sheet india-rubber increases enormously when 

 it 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. 



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, 

 p. 39, above) 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 



