62 . 
Proceedings of the Royal Soeiety 
to have been. When a little of the paraffin oil was put on the 
surface of the water it made it much quieter than the body of oil. 
When we pour a little oil on a wind-driven surface of water, the 
effect is so marvellous, the smoothing of the waves is so instan- 
taneous, that the imagination is carried away, and we at once 
attribute to the oil some wonderful property, though^we may not 
understand what that property is. When, however, * we examine 
its action and consider what is taking place, we see that it is not 
doing, it is preventing something which previously took place, and 
we are driven to consider what it has prevented. In fact, we are 
driven to examine how the wind gives rise to waves. It will 
therefore be necessary for us, before saying anything about the effect 
of oil, to examine how wind acts on the surface of water, and gives 
rise to waves; we shall afterwards better understand the effect 
of oil. 
The upper surface of water, where it is in contact with the air, is 
covered with a film having a well-known and definite tension. 
The tension of this film is always in a state of equilibrium, and the 
slightest stress acting on a part of it causes the surface to move 
in the direction of the strain. When a part of the film moves, new 
film or free surface is formed in the rear of the moving area, and 
covers the water which was previously covered by the displaced 
film, while the film in front of the moving film is diminished in 
area, part of it being absorbed by the water. No work is required 
to develop this new filing as the work done hy the contracting part of 
the film in front is equal to the tuorh spent in developing the new 
film in rear of the moving area. The result of this is, that the 
slightest stress acting on the surface of the water determines a move- 
ment of that surface, its motion being in no way checked by the 
surface tension. 
Suppose, now, the wind to blow over the surface of a sheet of 
water, which, for the present, we will suppose to be quite motionless, 
and for clearness let us suppose further that the wind strikes the 
water at only one place. The result is that the surface film where 
the air strikes is blown forwards, and .in sliding over the water it 
produces waves, one set of waves being produced in front of the 
moving film, and another set produced by the moving film being 
raised in the act of being driven over the water underneath. Now if, 
