us, may be brought home to one in
various ways. Concussion or shaking of some kind is essential to start
these vibrations. The air is made up of its particles, and one being
moved sets up, inevitably, movements in neighboring particles on all
sides, hence vibrations travel in all directions; which explains why a
sound in the street may be heard by those in every part of the street
not too distant, and also in the upper rooms of the houses and below
in the basements. This is an important fact for the singer or speaker
to bear in mind. His purpose must be to set up vibrations that will
travel with great perfection and rapidity in all directions.

The following experiments of a simple kind will serve to convince
those who may not have given much attention to the subject that sound
is due to movements of some object, which we term the sounding body,
strictly that which starts the vibrations by its own movements or
vibrations.

If a sufficiently flexible band of metal or a stiff piece of whalebone
be fixed at one end in a vice, and then sharply pulled to one side and
suddenly let go, a sound results. The same effect is produced when a
tight cord or small rope is plucked at and then suddenly released. In
each of these cases, if actual movements are not seen, a certain haze
which seems to surround the object may be observed. The same can be
seen when a tuning-fork is set into action by a bow, a blow, etc. In
the case of the fork a graphic tracing (Fig. 36) can be readily taken
on smoked paper, thus demonstrating to the eye that vibrations exist,
that they occur with perfect regularity and with a frequency that can
be measured.

[Illustration: FIG. 36 (Tyndall). Illustrates how the vibrations of a
tuning-fork are registered on a blackened (smoked) glass. In order
that the movements of the fork shall be traced in the form of regular
curves, the surface must be kept moving at a definite regular rate.]

A similar observation can be made in the case of stringed instruments.
If pieces of paper be laid on the strings of a violin, and the bow
then drawn across them, the bits of paper will fly off owing to the
movements--_i.e._, the vibrations--of the strings.

That a force applied at one end of several objects in a line or series
causes an obvious effect at the other end, can be well illustrated in
a simple way. If a number of individuals stand one behind another in a
line, each with his hands laid firmly on the shoulders of the