of Edinhurgli, Session 1882-83. 
71 
an expansion of the film where the drop enters. There is, however, 
another way in which these rain drops act. In falling on the sur- 
face, the drops mix up the shallow film current — which makes the 
ripple — with the water underneath, and thus destroy it. Much of 
the calming effect of rain is, however, no doubt due to the falling of 
the wind, which generally accompanies rain. 
Small floating bodies act on the surface of water in different ways. 
Professor James Thomson, in a paper containing his own and Sir 
William Thomson’s observations on Calm Lines on a Eippled Sea,” 
shows that these calm lines are associated with long sinuous lines of 
floating bodies, such as leaves, weeds, &c., and they attribute the 
calmness in part to the damping action of these bodies acting like 
floating breakwaters, and destroying small ripple undulations. The 
author of the paper referred to also points out that part of the 
calmness is due to oily scum or film ; he also shows how these floating 
bodies and scum may be collected in long sinuous lines by the 
surface currents produced by descending currents due to difference 
of temperature. These surface currents, coming from opposite 
directions, bring with them floating bodies and scum, which collect 
over descending areas or lines. That part of the calmness of these 
lines is due to the film at the place having a low surface tension pro- 
duced by some impurity is confirmed by my own observations made 
under somewhat different conditions. If we examine the surface of 
a canal when the wind is blowing very nearly straight along it, 
we see that on the side of the canal towards which the wind 
inclines to blow, there is a strip of smooth glassy water extending 
for about a meter from the bank. This smoothness is evidently not 
caused by the protection of the bank, but is due to impurities on the 
surface collected there by the wind, as the calm area has a well- 
marked line of demarcation, and is present when the wind is almost 
directly along the canal, the other side of the canal having no 
corresponding calm area.t The calm areas in the canal in the cases 
* “ Calm Lines on a Rippled Sea,” Philosophical Magazine, Sept. 1862 
(Fonrtli Series), vol. xxiv p, 247, Proceedings of Philosophical Society of 
Glasgow, l5tli Feb. 1882. 
t I have also observed that rapidly flowing streams when they enter still 
water often have calm areas in front of them. This calmess seems to be due to 
the rapid absorption of the surface film of the stream which here takes place, 
and any surface impurities brought by the stream get concentrated and 
become sufficiently great to reduce the surface tension. 
