experiment and its explanation are due
to Lord Rayleigh.

[Illustration: Fig. 44.]

[Illustration: Fig. 45.]

The smoky flame, as lately shown by Mr. Bidwell, does the same thing.
The reason probably is that the dirt breaks through the air-film, just
as dust in the air will make the two fountains join as they did when
they were electrified. However, it is possible that oily matter
condensed on the water may have something to do with the effect
observed, because oil alone acts quite as well as a flame, but the
action of oil in this case, as when it smooths a stormy sea, is not by
any means so easily understood.

When I held the sealing-wax closer, the drops coalesced in the same way;
but they were then so much more electrified that they repelled one
another as similarly electrified bodies are known to do, and so the
electrical scattering was produced.

You possibly already see why the tuning-fork made the drops follow in
one line, but I shall explain. A musical note is, as is well known,
caused by a rapid vibration; the more rapid the vibration the higher is
the pitch of the note. For instance, I have a tooth-wheel which I can
turn round very rapidly if I wish. Now that it is turning slowly you can
hear the separate teeth knocking against a card that I am holding in the
other hand. I am now turning faster, and the card is giving out a note
of a low pitch. As I make the wheel turn faster and faster, the pitch of
the note gradually rises, and it would, if I could only turn fast
enough, give so high a note that we should not be able to hear it. A
tuning-fork vibrates at a certain definite rate, and therefore gives a
definite note. The fork now sounding vibrates 128 times in every second.
The nozzle, therefore, is made to vibrate, but almost imperceptibly, 128
times a second, and to impress upon the issuing cylinder of water 128
imperceptible waists every second. Now it just depends what size the jet
is, and how fast the water is issuing, whether these waists are about
four and a half diameters apart in the cylinder. If the jet is larger,
the water must pass more quickly, or under a greater pressure, for this
to be the case; if the jet is finer, a smaller speed will be sufficient.
If it should happen that the waists so made are anywhere about four
diameters apart, then even though they are so slightly developed that
if you had an exact drawing of them, you would not be able to detect the
slightest change of diameter, t