er, and the more so if the stream is strong. Now if I hold the jet
close to the sheet the smooth column of liquid will press the sheet
steadily, and it will remain quiet; but if I gradually take the jet
further away from the sheet, then any waists that may have been formed
in the liquid column, which grow as they travel, will make their
existence perfectly evident. When a wide part of the column strikes the
sheet it will be depressed rather more than usual, and when a narrow
part follows, the depression will be less. In other words, any very
slight vibration imparted to the jet will be magnified by the growth of
waists, and the sheet of india-rubber will reproduce the vibration, but
on a magnified scale. Now if you remember that sound consists of
vibrations, then you will understand that a jet is a machine for
magnifying sound. To show that this is the case I am now directing the
jet on to the sheet, and you can hear nothing; but I shall hold a piece
of wood against the nozzle, and now, if on the whole the jet tends to
break up at any one rate rather than at any other, or if the wood or the
sheet of rubber will vibrate at any rate most easily, then the first few
vibrations which correspond to this rate will be imparted to the wood,
which will impress them upon the nozzle and so upon the cylinder of
liquid, where they will become magnified; the result is that the jet
immediately begins to sing of its own accord, giving out a loud note
(Fig. 47).

I will now remove the piece of wood. On placing against the nozzle an
ordinary lever watch, the jolt which is imparted to the case at every
tick, though it is so small that you cannot detect it, jolts the nozzle
also, and thus causes a neck to form in the jet of water which will grow
as it travels, and so produce a loud tick, audible in every part of
this large room (Fig. 48). Now I want you to notice how the vibration is
magnified by the action I have described. I now hold the nozzle close to
the rubber sheet, and you can hear nothing. As I gradually raise it a
faint echo is produced, which gradually gets louder and louder, until at
last it is more like a hammer striking an anvil than the tick of a
watch.

[Illustration: Fig. 47.]

[Illustration: Fig. 48.]

I shall now change this watch for another which, thanks to a friend, I
am able to use. This watch is a repeater, that is, if you press upon a
nob it will strike, first the hour, then the quarters, and then the
minutes.