d waves are reflected in the same way.]

When you drop a pebble in water, the ripples spread in all directions.
In the same way, when you make a sound in the open air, the air waves
spread in all directions. But when you shout through a megaphone the
air waves are all concentrated in one direction and consequently they
are much stronger in that direction. However, while the megaphone
intensifies sound, the echoing from the sides of the megaphone makes
the sound lose some of its distinctness.

WHY IT IS HARD TO UNDERSTAND A SPEAKER IN AN EMPTY HALL. A speaker can
be heard much more easily in a room full of people than in an empty
hall. The sound does not reflect well from the soft clothes of the
audience and the uneven surfaces of their bodies, just as a rubber
ball does not bounce well in sand. So the sound does not reverberate
as in an empty hall.

_APPLICATION 43._ Explain why a carpeted room is quieter than
one with a bare floor; why you shout through your hands when
you want to be heard at a distance.

INFERENCE EXERCISE

Explain the following:

261. It is harder to walk when you shuffle your feet.

262. The air over a lamp chimney, or over a register in a
furnace-heated house, is moving upward rapidly.

263. The shooting of a gun sounds much louder within a room
than it does outdoors.

264. A drum makes a loud, clear sound when the tightened head
is struck.

265. The pull of the moon causes the ocean tides.

266. Sand is sometimes put in the bottom of vases to keep them
from falling over.

267. It is difficult to understand clearly the words of one
who is speaking in an almost empty hall.

268. The ridges in a washboard help to clean the clothes that
are rubbed over them.

269. One kind of mechanical toy has a heavy lead wheel inside.
When you start this to whirling, the toy runs for a long time.

270. If you raise your finger slightly after touching the
surface of water, the water comes up with your finger.

SECTION 30. _Pitch._

What makes the keys of a piano give different sounds?

Why does the moving of your fingers up and down on a violin
string make it play different notes?

Why is the whistle of a peanut roaster so shrill, and why is
the whistle of a boat so deep?

Did you ever notice how tiresome the whistle on a peanut roaster gets?
Well, suppose that whenever you spoke