to the
motion of matter, and that a sounding body of any kind is in rapid
motion.

251. Sound is carried by Matter. In most cases sound reaches the ear
through the air; but air is not the only medium through which sound is
carried. A loud noise will startle fish, and cause them to dart away,
so we conclude that the sound must have reached them through the
water. An Indian puts his ear to the ground in order to detect distant
footsteps, because sounds too faint to be heard through the air are
comparatively clear when transmitted through the earth. A gentle
tapping at one end of a long table can be distinctly heard at the
opposite end if the ear is pressed against the table; if the ear is
removed from the wood, the sound of tapping is much fainter, showing
that wood transmits sound more readily than air. We see therefore that
sound can be transmitted to the ear by solids, liquids, or gases.

Matter of any kind can transmit sound to the ear. The following
experiments will show that matter is necessary for transmission.
Attach a small toy bell to a glass rod (Fig. 166) by means of a rubber
tube and pass the rod through one of two openings in a rubber cork.
Insert the cork in a strong flask containing a small quantity of water
and shake the bell, noting the sound produced. Then heat the flask,
allowing the water to boil briskly, and after the boiling has
continued for a few minutes remove the flame and instantly close up
the second opening by inserting a glass stopper. Now shake the flask
and note that the sound is very much fainter than at first. As the
flask was warmed, air was rapidly expelled; so that when the flask was
shaken the second time, less air was present to transmit the sound. If
the glass stopper is removed and the air is allowed to reenter the
flask, the loudness of the sound immediately increases.

[Illustration: FIG. 166.--Sound is carried by the air.]

Since the sound of the bell grows fainter as air is removed, we infer
that there would be no sound if all the air were removed from the
flask; that is to say, sound cannot be transmitted through empty space
or a vacuum. If sound is to reach our ears, it must be through the
agency of matter, such as wood, water, or air, etc.

252. How Sound is transmitted through Air. We saw in Section 250
that sound can always be traced to the motion or vibration of matter.
It is impossible to conceive of an object being set into sudden and
continued motion without dis