Part 25 (1/2)

_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 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 p.r.o.ngs, tap one p.r.o.ng lightly on the desk, and listen. Move the slide somewhat toward the end of the p.r.o.ngs, and repeat. Is a higher or a lower sound produced as the slide shortens the length of the p.r.o.ngs?

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 gla.s.s half full of water, and strike it. Now pour half the water out, and strike the gla.s.s 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.