I think the water-jet will enable you all to hear what time it
is. Listen! one, two, three, four;... ting-tang, ting-tang;... one, two,
three, four, five, six. Six minutes after half-past four. You notice
that not only did you hear the number of strokes, but the jet faithfully
reproduced the musical notes, so that you could distinguish one note
from the others.

I can in the same way make the jet play a tune by simply making the
nozzle rest against a long stick, which is pressed upon a musical-box.
The musical-box is carefully shut up in a double box of thick felt, and
you can hardly hear anything; but the moment that the nozzle is made to
rest against the stick and the water is directed upon the india-rubber
sheet, the sound of the box is loudly heard, I hope, in every part of
the room. It is usual to describe a fountain as playing, but it is now
evident that a fountain can even play a tune. There is, however, one
peculiarity which is perfectly evident. The jet breaks up at certain
rates more easily than at others, or, in other words, it will respond to
certain sounds in preference to others. You can hear that as the
musical-box plays, certain notes are emphasized in a curious way,
producing much the same effect that follows if you lay a penny upon the
upper strings of a horizontal piano.

[Illustration: Fig. 49.]

Now, on returning to our soap-bubbles, you may remember that I stated
that the catenoid and the plane were the only figures of revolution
which had no curvature, and which therefore produced no pressure. There
are plenty of other surfaces which are apparently curved in all
directions and yet have no curvature, and which therefore produce no
pressure; but these are not figures of revolution, that is, they cannot
be obtained by simply spinning a curved line about an axis. These may be
produced in any quantity by making wire frames of various shapes and
dipping them in soap and water. On taking them out a wonderful variety
of surfaces of no curvature will be seen. One such surface is that known
as the screw-surface. To produce this it is only necessary to take a
piece of wire wound a few times in an open helix (commonly called
spiral), and to bend the two ends so as to meet a second wire passing
down the centre. The screw-surface developed by dipping this frame in
soap-water is well worth seeing (Fig. 49). It is impossible to give any
idea of the perfection of the form in a figure, but fortunately this is
an