52 CAPILLARY RIPPLES. 
capillary ripples. Now there is a difficulty in seeing ripples like this 
intentionally made, because all surfaces of liquid owing to tremors have 
other ripples passing across them; and again they cannot be seen, 
because taking for instance ripples ~$5 of an inch long from crest to 
crest, travelling, according to the diagram, at the rate of about 56 
inches a second, about 5600 will pass before the eye every second, so 
of course the eye cannot follow them. In the case of a longer wave, so 
many will not pass before the eye in a second, but from the diagram 
you can at once get particulars of the number of ripples of any size that 
will pass the eye in a second, or the rate at which they will travel. 
Now I am going to deal this evening with ripples from about a 
tenth long to somewhere about a fiftieth part of an inch, travelling with 
a velocity of 20, 30 or 40 inches a second. The two difficulties to be 
overcome are (1) the fact that owing to tremors the surfaces of liquids are 
generally already covered with ripples, and (2) both these and those in- 
tentionally made move so quickly that the eye cannot follow them. 
Two devices have in consequence been made use of. In the first place, 
in order to prevent the first difficulty I have suspended a tray of mercury 
resting on a heavy stand from this gallows by a long loop of india rubber ; 
I take mercury as being more convenient than water, because it does not 
wet the wood tray, it is mobile, and it reflects the light well. The whole 
arrangement is so free, it will swing about so that, apparently, 
it is the last thing to be steady; but the mere fact that it is free to 
swing in all directions prevents any sharp disturbance from outside 
from producing these minute tremors which we wish to examine. The 
second thing is to take care to look at the liquid not continuously but 
as often as the ripples pass any particular point. Supposing that the 
ripples, for instance, pass a particular point at the rate of 120 times a 
second, if you could blink your eyes 120 times a second you would see 
them stationary. But that is impossible, and instead a blinking 
machine consisting of a little motor carrying a disc with holes in it is 
driven round so that the holes let the light through 120 times a second, 
so that every time a ripple comes to a particular place the hole is open 
and then the ripple moves on, but it is out of sight until the hole opens 
again when a new ripple is there, so that you only see the ripples in the 
same place, and therefore they seem stationary. If, on the other hand, 
the motor does not keep exact time, asit never will for long, then if it is 
going a little too fast or a little too slow these ripples will be always 
just a little bit behind or ahead of their former place, and the effect 
will be that the ripples will appear to creep along, and though absolutely 
invisible to continuous vision, will be not only visible but even extra- 
ordinarily conspicuous. The machinery is very simple to show this. 
Here is an electric lantern and a piece of common looking-glass to throw 
the light on to the mercury, a lens that will focus the surface of the 
mercury on the screen, and this little motor with a disc to make the 
light intermittent. The fine rod that you see coming down is a piece 
of glass fastened with soft wax on to one prong of a tuning-fork, and 
by means of a screw I can move the fork so as to bring it just in contact 
with the liquid. If the prong of the fork is touched in the lightest 
possible way with a gimlet with india rubber on the surface to give the 
