70 
Proceedings of the Royal Society 
and the dust floating on it to move away from the place where the 
air strikes, leaving a perfectly dustless surface. Repeat the experi- 
ment with the oily surface. It will now he found that however 
hard you blow you cannot drive back the oily surface, and that 
when the jet of air is sufficiently strong to cause a deep depression 
in the surface of the water, the dust particles are seen rushing at 
certain places into the depression and running out at others with a 
velocity perfectly surprising. 
This easy slipping of the surface film may also he seen when the 
water is in motion and the film at rest. Recently I observed a 
small stream flowing quickly enough to give rise to slight disturbance 
of the surface flatness, and yet the surface film was quite motionless. 
While the stream was flowing underneath, the advance of the 
surface film was checked by some weeds and grasses forming a 
floating bridge ' across the stream. If the surface had been quite 
clean this would not have taken place, but a film of weak tension 
had collected in front of the grasses, caused by impurities derived 
probably from the bed of the stream, and not from the air, as the 
stream was in the country, and far from towns and factories. The 
tension of the surface in front being less than that behind, the 
strain put on it by the under current was not great enough to cause 
it to be absorbed in front of the floating obstruction. A somewhat 
similar effect may be noticed when we stir up — not round — a cup 
of tea and then drop in a little cream. The surface film movements 
of the oily tea are seen to stop while the tea mixing with the cream 
can still be seen in active movement. The drag of the tea on the 
surface film is not sufficient to develop new free surface against the 
resistance of the increasing surface tension of the cleaner film, and all 
surface movements are stopped, save circular ones, which do not re- 
quire the development and absorption of free surface. These surface 
movements are sometimes stopped by a rigid pellicle forming on the 
surface, but they are also stopped when no such covering is present. 
It is generally recognised that rain and small floating bodies have 
an influence on the water somewhat similar to that of oil, and tend 
to calm its surface. In the case of the rain it seems possible that 
the great amount of free surface added by the drops causes the 
surface film to expand and act somewhat in the same manner as 
•oil, as we know that water dropped on the surface of water causes 
