nd Waves.*--If some sonorous body, as a bell, be struck, it
is given a quivering, or vibratory, motion. This is not confined to the
bell, but is imparted to the air and other substances with which the bell
comes in contact. These take up the movements and pass them to objects
more remote, and they in turn give them to others, until a very
considerable distance is reached. Such progressive vibrations are known as
waves, and, since they act as stimuli to the organs of hearing, they are
called _sound waves_. Sound waves _always originate in vibrating
bodies_.(116) They are transmitted chiefly _by the air_, which, because of
its lightness, elasticity, and abundance, readily takes up the vibrations
and spreads them in all directions (Fig. 148).

While these vibratory movements of the atmosphere are correctly classified
as waves, they bear little resemblance to the waves on water. Instead of
being made of crests and troughs, as are the water waves, the sound waves
consist of alternating successions of slightly condensed and rarefied
layers of air. Then, while the general movement of the water waves is that
of ever widening circles _over a surface_, the sound waves spread as
enlarging spherical shells _through_ the air. In sound waves, as in all
other waves, however, it is only the form of the wave that moves forward.
The individual particles of air that make up the wave simply vibrate back
and forth.

[Fig. 148]

Fig. 148--Diagram illustrating the spreading of sound waves through air.

*How Sound Waves act as Stimuli.*--Any sound wave represents a small but
definite amount of energy, this being a part of the original force that
acted on the vibrating body to set it in motion. The hammer, for instance,
in striking a bell imparts to it a measurable quantity of energy, which
the bell in turn imparts to the air. This energy is in the sound waves and
is communicated to the bodies against which they strike.(117) Though the
force exerted by most sound waves is, indeed, very slight, it is
sufficient to enable them to act as stimuli to the nervous system.

*How Sounds Differ.*--Three distinct effects are produced by sound waves
upon the nerves of hearing, and through them upon the mind. These are
known as _pitch, intensity_, and _quality_, and they are dependent upon
the vibrations of the sound-producing bodies.

_Pitch_, which has reference to the height, or degree of sharpness, of
tones, is