64 
Proceedings of the Royal Society 
paper was put across tlie middle of each vessel. For the purpose of 
moving these papers over the surface of the water two threads were 
previously attached to each of them near their ends. Taking first 
the vessel with the clean surfaced water, the paper strip was by 
means of the threads moved into its position right across the middle 
of the vessel. It was shown that the paper strip could he moved 
backwards and forwards over the surface of the water, with the very 
slightest expenditure of energy, the film not in the slightest 
degree resisting the movement, the free siirface or film in front 
contracting quickly and pulling as strongly as the new film which 
was developed in the rear. It was further shown that after the 
paper strip had been moved backwards or forwards into any position, 
it remained where it was put.* 
Turning to the oily surfaced water, it was shown that the move- 
ment of the paper strip on its surface was resisted, the threads by 
which it was moved becoming tightened when it was dragged from 
its original position ; and further, that when the tension was dis- 
continued the paper returned to. the place from which it had been 
moved. It did not matter whether it was moved backwards or 
forwards, it always returned to its original position, so long as no 
surface film was allowed to pass round the ends and get to the 
other side of the paper. The tension on the threads in these ex- 
periments represents the resistance which an oily film offers to the 
forward movement by the wind of isolated areas of surface film. 
This resistance is not, however, the most important effect of the 
oil. It is, so to speak, the secondary effects that are most powerful 
in checking the formation of waves. On oily surfaced water the 
forward moving area reduces the tension in front, and causes the 
film there to advance on account of the greater tension in front ; it 
also increases the tension behind, and so causes the film in the rear 
to be drawn forwards, and the irregular film advance which takes 
place in clean water is converted into a uniform and regular advance 
by the oil. This was illustrated by the arrangements used in the 
previous experiments. A small piece of paper to act as a float to 
^ This was found to be an excellent test of the cleanness of a surface film. 
If there is the slightest impurity on the surface it gets collected in front of 
the strip, and the paper cannot he moved quite to the end of the vessel without 
meeting with resistance. 
