CAPILLARY RIPPLES. B57) 
more rapidly they would die out. And the same thing is true of all liquids ; the 
smaller the waves the more rapidly they die out. All this is capable of being 
represented by straight lines on the logarithmic chart as they follow an ex- 
ponential law, and this is one of the advantages of the logarithmic method of 
plotting. 
There is one other point in connection with ripples that I might mention, I 
can do it with the help only of the black-board and chalk. The experiment is one 
which Lord Kelvin has described, and it is one which everyone who goes out in 
smooth water in a boat is capable of observing for himself. If you are drifting 
along slowly ina boat and a fishing-line is being drawn through the smooth, 
reflecting surface of the water, then if you travel at a speed more than the 
minimum velocity at which a wave can travel in the surface of water, namely, 
about nine inches a second corresponding to the wave length of about '5-inch, a 
permanent wave pattern accompanies the line. If, however, you are travelling at 
a speed which is less than nine inches a second then exactly as a bullet, going less 
quickly than sound, goes through the air without making any waves, and leaves 
the air all round it still, so does this fishing-line cutting the surface of the water 
leave that surface without a ripple upon it, and the reflection from such surface is 
as perfect as if the fishing-line were not going through it. If you start at a speed 
considerably greater than nine inches a second, say, three or four feet a second, 
you will find a series of small ripples, too small almost to be seen, branching out 
hyperbolically, into a series of waves with which everyone is familiar. The angle 
being such that the minimum velocity of the wave, divided by the velocity of the 
boat, is the sine of half the angle; and if you slowly diminish in speed these 
ripples slowly open out, presenting a perfectly visible and beautiful phenomena. 
Further, just as they are opening out as nearly as possible into straight 
bands, z.e. just as the boat reaches the minimum velocity of water waves they 
vanish ; it is almost instantaneous, but give the slightest impulse to the boat and 
they will suddenly come into view again. And this is an interesting thing—I 
have not tried it in a boat but in the laboratory. When you have a thing moving 
through water at such a speed the ripples just vanish, if you touch the surface of 
the water with soap, ox-gall, ether, or with anything that reduces the surface 
tension to a considerable amount—for instance, with soap the surface tension is 
reduced to one-third—the minimum velocity of waves on the soapy liquid may 
still be exceeded by that of the moving point, the ripples flash out again. That 
is an experiment with which there is no difficulty whatever in succeeding. 
I think that the other points which have been touched upon are such that I can 
hardly answer. I am exceedingly gratified that the trouble which I have taken 
in presenting this matter to you this afternoon should have been appreciated. I 
have not succeeded in presenting the experiments in as finished a manner as I 
should have liked, but I have succeeded in showing you the ripples. 
THE CHAIRMAN—Ladies and gentlemen, it only remains that I should on your 
behalf thank Professor Boys very heartily for the most interesting lecture that he 
has given us, and congratulate him on the success of his experiments, which we 
know have been carried out under great difficulties, and I will express the hope 
that he will come down again at no very distant period, so that we may have an 
opportunity of hearing him again. 
