you had to utter your words in
exactly that pitch; that every time a car came down the street its
noise was like the whistle of the peanut roaster, only louder; that
every step you took sounded like hitting a bell of the same pitch;
that when you went to the moving-picture theater the orchestra played
only the one note; that when any one sang, his voice did not rise and
fall; in short, that all the sounds in the world were in one pitch.
That is the way it would be if different kinds of air vibrations did
not make different kinds of notes,--if there were no differences in
pitch.

PITCH DUE TO RAPIDITY OF VIBRATION. When air vibrations are slow,--far
apart,--the sound is low; when they are faster, the sound is higher;
when they are very quick indeed, the sound is very shrill and high. In
various ways, as by people talking and walking and by the running of
street cars and automobiles, all sorts of different vibrations are
started, giving us a pleasant variety of high and low and medium
pitches in the sounds of the world around us.

An experiment will show how pitch varies and how it is regulated:

EXPERIMENT 60. Move the slide of an adjustable tuning fork
well up from the end of the prongs, tap one prong lightly on
the desk, and listen. Move the slide somewhat toward the
end of the prongs, and repeat. Is a higher or a lower sound
produced as the slide shortens the length of the prongs?

Whistle a low note, then a high one. Notice what you do with
your lips; when is the opening the smaller? Sing a low note,
then a high one. When are the cords in your throat looser?
Fill a drinking glass half full of water, and strike it. Now
pour half the water out, and strike the glass again. Do you
get the higher sound when the column of water is shorter or
when it is longer? Stretch a rubber band across your thumb and
forefinger. Pick the band as you make it tighter, not making
it longer, but pulling it tighter with your other fingers.
Does it make a higher or a lower sound as you increase the
tightness? Stretch the band from your thumb to your little
finger and pick it; now put your middle finger under the band
so as to divide it in halves, and pick it again. Does a short
strand give a higher or lower pitch than a long strand?

[Illustration: FIG. 103. When the prongs of the tuning fork are made
longer or shorter, the pitch of the sound is changed.]

A