256 CAPILLARY RIPPLES. 
Mr. Boys has the talent of revealing to us the minute and invisible phenomena 
of Nature—phenomena hitherto invisible because of the swiftness of their propa- 
gation, and by reason of their minute scale. We have had the privilege of seeing 
his photographs of bullets exhibited in this room, as has already been mentioned 
by Major MacMahon, showing a bullet in full flight accompanied by waves which 
have never before been seen or been possible to be seen by the unaided eye—waves 
such as we see accompanying a ship in its progress. Mr. Boys explained to us 
his ingenious diagram on the wall by which the wave length is exhibited graphi- 
cally. I should like to ask Mr. Boys whether he thinks it is possible to 
interpolate there some curves representing the waves that we see on the surface of 
ice, because a card of such a nature might prove very useful to a skater finding 
himself with an inviting stretch of ice in front of him and which this card would 
enable him to attempt with security or the reverse (applause). 
Cotonet WatkIN, c.B., R.A.—There is one thing, Sir, that I would like to 
suggest. Professor Boys has already given us three most interesting lectures, and 
T hope that this will not be the last (applause). We have been taken into the 
region of minute time and now into the region of minute ripples, and I think that 
before, in one other lecture, the Professor -took us into the region of minute 
draughts, telling us that in the experiments of which he gave us an account, a 
draught moving at the rate of one inch a fortnight would have been fatal. It is 
refreshing to think of such fairy zephyrs suffering as we have been from the 
recent gales. I trust that when Professor Boys has completed the experiments 
at the butts on high explosives he will give us the advantage of hearing him again 
(applause). 
REPLY. 
Proressor Boys—There is no doubt that the logarithmic chart is fully 
competent to deal with the question raised by Professor Greenhill. I have not 
myself numerically gone into the question of waves on water coated with ice, but 
the ice has in consequence of its elasticity some action akin to that produced by 
the surface tension of water. The thicker the ice is the more it resists the bend- 
ing; the parts raised tend to move down and the parts bent down tend to rise, so 
that exactly in the same way ice should tend to hurry on or increase the rate of 
propagation of the wave. In addition to that the fact that ice weighs somewhat 
less than water is all to the good, so that there will be a more rapid advance of 
the waves the thicker the ice is. If, therefore, a skilful skater, when he came to 
a place where there was beautiful black ice in front of him, had time to see the 
advancing wave, to measure its wave length and to see how fast it was going, and 
then to refer to a chart and see whether such velocity corresponded with a thick- 
ness that he concluded he could traverse safely, then, of course, such a chart 
would be of real practical use. 
Meanwhile a chart of that sort is exceedingly convenient; a logarithmic chart 
is, in fact, capable of dealing with all sorts of points of great practical importance— 
enabling one to take out quantities with a considerable degree of accuracy and 
over an enormous range. And, moreover, I believe also it is competent to deal 
with another question which I have not dwelt upon this afternoon, namely, the 
rate at which waves and ripples die owt in passing over liquids of any kind what- 
ever. For instance, a large wave would travel perfectly well on treacle ; a wave 
half a mile long would travel so well on treacle that it would be the same practi- 
cally to all intents and purposes as a wave on water. On the other hand very 
minute ripples such as you have seen to-day would not travel an appreciable 
distance before they were dead, and the more viscous the liquid might be the 
