58 Proceedings of the Eoyal Society 
of oil have been given j none of these, however, appear to me to 
account for the facts. I have therefore ventured to place the fol- 
lowing experiments and a theory built upon them before this Society ; 
and though this theory may not appear to some to explain all the 
phenomena, yet, as it calls attention to certain effects of an oily 
film, the importance of which has not previously been taken notice 
of, I trust the subject may not be uninteresting, and may at least 
be of some little aid in helping forward a proper understanding of 
this most interesting and difficult subject. 
The wonderful effect of oil on water has suggested to many 
observers the idea that the effect was due to the oily film offering 
less resistance to the wind than the clean water surface ; that the oil 
so to speak greased the wind. The first thing therefore which 
seemed necessary to be done was to test whether there was any 
truth in this supposition ; to arrange some experiment to ascertain 
whether there was less friction between air and an. ‘oily film than 
between air and a clean film. The method adopted was to place 
some water in a circular vessel, and arrange a jet of air so that it 
should blow over the surface of the water, and cause it to take up 
a motion of rotation. Under these conditions the rate of rotation of 
the water would — if the pressure of the air jet was kept constant — 
depend on the amount of friction between the air and the water 
surface. If .the oil decreased the friction the water would be driven 
less quickly under an oily than under a clean film. 
In order to measure the amount of motion communicated by the 
air to the water, a horizontal paddle, completely submerged, was 
hung in- the middle of the vessel by means of a fine platinum wire, 
the upper end of the wire being attached to a torsion- head. A 
needle, rigidly attached to the paddle, indicated oh a circular scale 
the amount of torsion produced by the moving water, that is, showed 
the amount of energy communicated by the air jet to the water. 
The air jet was supplied from a gasometer, which could be loaded 
to different pressures. 
The following is the method adopted in working this instrument. 
The circular glass vessel was carefully washed, and filled with water 
up to a certain height, which was accurately adjusted for each 
experiment, so that the water was always exactly the same distance 
below the jet. Air at a low pressure was then blown over the water. 
