of Edinhiirgli, Session 1882-83. 
65 
sliow tlie movements of the surface film, was dropped on the clean 
water and near the long narrow strip of paper. The long strip was 
now moved away from the float, when it was seen that its move- 
ment had no influence on the float, as it remained in its original 
position, and furthm, the float did not move when the strip was 
moved towards it. The long strip might he moved in any direc- 
tion, hut the small piece of paper had no tendency to move till it 
was almost touched hy it. The reason for this want of sympathy, 
so to speak, between the two pieces of paper is due to the surface 
tension heing everywhere alike, and free surface appears and dis- 
appears without any disturbance of the surface tension. 
When the same experiment was repeated on the oily water the 
result was very differenti There was now a bond of union between 
the two pieces.- If the long piece was drawn away the small piece 
followed it, if moved up to it the small piece receded from it 
These movements of the float were pointed out to be the result of 
the disturbance produced in the surface tension by the movement 
of the strip. The forward movement of the strip causes the oily 
film in front to become thicker, its tension therefore becomes less, 
and it tends to expand forwards, or rather is , drawn forwards, by the 
thinner film in front, and the free surface which is developed in the 
rear of the strip is rapidly covered by the advance of the oily film 
which brings with it the paper float. It was shown from this that 
the forward movement by the wind of isolated areas of oily film 
was not only resisted, but that their forward movement gave rise to 
forward movements of the film, both in front and in rear of them. 
Trom these experiments it will be seen that a clean water surface 
acts towards the horizontal force of the wind as if it had no surface 
tension whatever, whereas an oily film acts very much as if the 
water was covered with a thin skin of india-rubber. Or the stability 
of a clean water surface might be compared to that of a perfect 
sphere on a perfectly horizontal plane, and the stability of an oily 
surface to that of an egg on its side. 
The effect of this regulating influence of the oil on the surface 
tension uiay be illustrated in another way, which shows that the 
uniform rate of advance of the surface film produced by the oil 
prevents the formation of waves or ripples. If the long narrow 
strips of paper in the previous experiments are moved quickly over 
VOL. XII. 
E 
