in a row on a
table: if the first ball is tapped lightly, striking gently against ball
number 2, the latter (as well as numbers 3, 4, 5, 6, and 7) will not
apparently move at all, but ball number 8 at the other end will roll
away. The air-particles act upon each other in much this same fashion,
the difference being that when they are set in motion by a vibrating
body a complete vibration backward and forward causes a similar
_backward and forward_ movement of the particles (oscillation) instead
of simply a _forward jerk_ as in the case of the billiard balls.

Another way of describing the same process is this: the vibration of
some body produces waves in the air (cf. waves in the ocean, which carry
water forward but do not themselves move on continuously), these waves
spread out spherically (i.e. in all directions) and finally reach the
ear, where they set the ear-drum in vibration, thus sending certain
sound-stimuli to the nerves of hearing in the inner ear, and thus to the
brain.

An important thing to be noted in connection with sound-transmission is
that sound will not travel in a vacuum: some kind of a medium is
essential for its transmission. This medium may be air, water, a bar of
iron or steel, the earth, etc.

4. The _rate_ at which sound travels through the air is about 1100 feet
per second, the rapidity varying somewhat with fluctuations in
temperature and humidity. In water the rate is much higher than in air
(about four times as great) while the velocity of sound through other
mediums (as _e.g._, steel) is sometimes as much as sixteen times as
great as through air.

5. Sound, like light, may be _intensified_ by a suitable reflecting
surface directly back of the vibrating body (cf. sounding board); it may
also be reflected by some surface at a distance from its source in such
a way that at a certain point (the focus) the sound may be very clearly
heard, but at other places, even those _nearer_ the source of sound, it
can scarcely be heard at all. If there is such a surface in an
auditorium (as often occurs) there will be a certain point where
everything can be heard very easily, but in the rest of the room it may
be very difficult to understand what is being said or sung.

_Echoes_ are caused by sound-reflection, the distance of the reflecting
surface from the vibrating body determining the number of syllables that
will be echoed.

The _acoustics_ of an auditorium (_i.e._, its hearing properties) depe