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Proceedings of the Royal Soeiety 
observed were not due to floating bodies, as scarcely any were 
present. Further, an examination of water where a number of 
small floating bodies were present, showed theic action to be quite 
distinct from that of oily scum. Small floating bodies destroy the 
waves, but wind can ripple or roughen the surface of the water 
amongst them, whereas oily scum does not destroy waves, but pre- 
vents ripples or darkening of its surface. 
My observations on the appearance of the canal suggest another 
way, in addition to the one given by Professor Thomson, in which 
oily scum and small floating bodies collect near other floating bodies. 
One evident effect of floating bodies is to check the forward advance 
by the wind of the surface film of water, and the shallow surface 
current is thus converted by floating bodies into a deeper but 
slower current. The result of this is that more surface film with 
its impurities comes to the floating bodies than goes away. Surface 
impurities therefore tend to collect in the neighbourhood of these 
floating breakwaters. 
In addition to this action of small floating bodies in destroying 
waves there is another way in which small bodies, such as ice- 
spicules, &c., act, and smooth the water by preventing the formation 
of ripples. Small floating bodies prevent the formation of ripples 
or waves by deepening the current, and so, offering a resistance to 
the rapid advance of any part of the upper film of water. Weeds 
and grasses lying on the surface of the water prevent the irregular 
advance of the surface film, by the resistance they offer to the irre- 
gular advance of any part of it, and by the drag or pull they exert on 
the water behind the parts tending to move quickly. Weeds and 
grasses thus tend to promote uniform rate of advance at all parts of 
the surface, and thereby prevent the formation of ripples or waves. 
It is obvious that all oils will not have the same power to pre- 
vent the formation of waves. It is therefore desirable that we 
should have some information as to the value of. different oils or 
other substances for this purpose. The value of an oil will depend 
on a number of things, the most important of which are (1) the 
rate at which the particular oil will distribute itself over the sur- 
face of the water. This will depend greatly on the difference of 
tension between the surface of clean water and the surface of the 
water covered with the oil. The greater this difference the more 
