Common Science Part 25

_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.

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?

[Ill.u.s.tration: FIG. 103. When the p.r.o.ngs of the tuning fork are made longer or shorter, the pitch of the sound is changed.]

A violinist tunes his violin by tightening the strings; the tighter they are and the thinner they are, the higher the note they give.

Some of the strings are naturally higher than others; the highest is a smaller, finer string than the lowest. When the violinist plays, he shortens the strings by holding them down with his fingers, and the shorter he makes them the higher the note. A ba.s.s drum is much larger than a high-pitched kettledrum. The pipes of an organ are long and large for the low notes, shorter and smaller for the high ones.

In general, the longer or larger the object is that vibrates, the slower the rate of vibration and consequently the lower the pitch. But the shorter or finer the object is that vibrates, the higher the rate of vibration and the higher the pitch.

All musical instruments contain devices which can be made to vibrate,--either strings or columns of air, or other things that swing to and fro and start waves in the air. And by tightening them, or making them smaller or shorter, the pitch can be made higher; that is, the number of vibrations to each second can be increased.

_APPLICATION 44._ Explain why a steamboat whistle is usually of much lower pitch than is a toy whistle; why a banjo player moves his fingers toward the drum end of the banjo when he plays high notes; why the sound made by a mosquito is higher in pitch than that made by a b.u.mblebee.

_APPLICATION 45._ A boy had a banjo given him for Christmas.

He wanted to tune it. To make a string give a higher note, should he have tightened or loosened it? Or could he have secured the same result by moving his finger up and down the string to lengthen or shorten it?

_APPLICATION 46._ A man was tuning a piano for a concert. The hall was cold, yet he knew it would be warm at the time of the concert. Should he have tuned the piano to a higher pitch than he wanted it to have on the concert night, to the exact pitch, or to a lower pitch?

INFERENCE EXERCISE

Explain the following:

271. A cowboy whirls his la.s.so around and around his head before he throws it.

272. Furnaces are always placed in the bas.e.m.e.nts of buildings, never on top floors.

273. A rather slight contraction of a muscle lifts your arm a considerable distance.

274. A player on a slide trombone changes the pitch of the notes by lengthening and shortening the tube while he blows through it.

275. Rain runs off a tar roof in droplets, while on s.h.i.+ngles it soaks in somewhat and spreads.

276. There is a sighing sound as the wind blows through the branches of trees, or through stretched wires or ropes.

277. Sometimes a very violent noise breaks the membrane in the drum of a person's ear.

278. As a street car goes faster and faster, the hum of its motor is higher and higher.

279. If a street is partly dry, the wet spots s.h.i.+ne more than the dry spots do.

280. Molten type metal, when poured into a mold, becomes hard, solid type when it cools.

CHAPTER SEVEN

MAGNETISM AND ELECTRICITY