60 
Proceedings of the Boyal Society 
in front, tliis new surface film can be developed by a lower air 
pressure in the weak oily film than it can in the stronger clean 
film. This extreme condition of matters, however, seems never to 
present itself in nature. 
Experiments such as these made in the laboratory on water films 
are in the highest degree unsatisfactory. Since I have begun them 
I have found this surface film of water to be far more delicate and 
variable than I had previously any idea of, and find it almost im- 
possible to get it in a uniform condition. If the experimenting 
vessels are not cleansed with something to destroy the last trace of 
soap used in washing off the oil, if the vessel or the water is touched 
with the fingers or exposed for any length of time to the air of the 
laboratory, changes take place in the surface tension. It therefore 
seems that more satisfactory experiments might be made on large 
sheets of water. As yet I have been unable to do anything in this 
direction except one experiment, which, on account of the smallness 
of the sheet of water, may not be of much value. 
The object of the experiment was to see if the wind communicated 
less horizontal motion to the surface water under an oily than under 
a clean film, which would be the case if the wind has a less grip or 
bite of oily than of clean filmed water. It will be observed here 
that I leave out of consideration any question as to a possible 
slipping of the oily film over the water, without dragging the water 
film underneath along with it. If there is any depth of oil, its 
upper surface cannot slip over the lower more easily than the same 
depth of water could, as oil is more viscous than water, though we 
might imagine the bounding surface might prevent the formation 
of eddies and the deepening of the motion; but we have seen from 
the experiment with the jet of air on water in a circular vessel, 
that there is no evidence of any slipping forward, as the body of 
water underneath takes up the same amount of motion whether the 
surface is oily or not. 
In the experiments made on the small sheet of water, the rate of 
motion communicated by the wind to the surface water was tested 
by means of small circular floats 9 cm. diameter and 4 cm. deep. 
The floats were submerged 2 or 3 m. and only a thin stem projected 
above the water. Two of these floats were dropped into the pond 
near that end from which the wind was blowing, and at some distance 
