eavy weight and striking this weight
gently at regular, properly timed intervals with a small cork hammer.
Soon the pendulum, or weight, will be set swinging.

[Illustration: FIG. 175.--The hollow wooden box reenforces the sound.]

258. Borrowed Sound. Picture frames and ornaments sometimes buzz and
give forth faint murmurs when a piano or organ is played. The waves
sent out by a sounding body fall upon all surrounding objects and by
their repeated action tend to throw these bodies into vibration. If
the period of any one of the objects corresponds with the period of
the sounding body, the gentle but frequent impulses affect the object,
which responds by emitting a sound. If, however, the periods do not
correspond, the action of the sound waves is not sufficiently powerful
to throw the object into vibration, and no sound is heard. Bodies
which respond in this way are said to be sympathetic and the response
produced is called _resonance_. Seashells when held to the ear seem to
contain the roar of the sea; this is because the air within the shell
is set into sympathetic vibrations by some external tone. If the
seashell were held to the ear in an absolutely quiet room, no sound
would be heard, because there would be no external forces to set into
vibration the air within the shell.

Tuning forks do not produce strong tones unless mounted on hollow
wooden boxes (Fig. 175), whose size and shape are so adjusted that
resonance occurs and strengthens the sound. When a human being talks
or sings, the air within the mouth cavity is thrown into sympathetic
vibration and strengthens the otherwise feeble tone of the speaker.

259. Echo. If one shouts in a forest, the sound is sometimes heard a
second time a second or two later. This is because sound is reflected
when it strikes a large obstructing surface. If the sound waves
resulting from the shout meet a cliff or a mountain, they are
reflected back, and on reaching the ear produce a later sensation of
sound.

By observation it has been found that the ear cannot distinguish
sounds which are less than one tenth of a second apart; that is, if
two sounds follow each other at an interval less than one tenth of a
second, the ear recognizes not two sounds, but one. This explains why
a speaker can be heard better indoors than in the open air. In the
average building, the walls are so close that the reflected waves have
but a short distance to travel, and hence reach the ear at practi