Explain the following:
271. A cowboy whirls his lasso around and around his head before he throws it.
272. Furnaces are always placed in the basements 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 shingles 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 shine 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
Section 31. Magnets; the compass.
What makes the needle of a compass point north?
What causes the Northern Lights?
For many hundreds of years sailors have used the compass to determine directions. During all this time men have known that one point of the needle always swings toward the north if there is no iron near to pull it some other way, but until within the past century they did not know why. Now we have found the explanation in the fact that the earth is a great big magnet. The experiment which follows will help you to understand why the earth's being a magnet should make the compass needle point north and south.
Experiment 61. Lay a magnetic compass flat on the table. Notice which point swings to the north. Now hold a horseshoe magnet, points down, over the compass. Turn the magnet around and watch the compass needle; see which end of the magnet attracts the north point; hold that end of it toward the south point and note the effect. Hold the magnet, ends up, under the table directly below the compass and turn the magnet, watching the compass needle.
The earth is a magnet, and it acts just as your magnet does: one end attracts one point of the compass, and the other end attracts the other point. That ought to make it clear why the compass points north. But how is the compass made? The next experiment will show this plainly.
Experiment 62. Take a long shoestring and make a loop in one end of it. Slip the magnet through the loop and suspend it, ends down. Fasten the shoestring to the top of a doorway so that the magnet can swing easily. Steady the magnet and let it turn until it comes to a rest. Mark the end that swings to the north. Turn this end around to the south; let go and watch it. Place the magnet the other way around in the loop so that you can be sure that it is not twisting of the shoestring that makes the magnet turn in this direction.
Fig. 104. The compass needle follows the magnet.
Now stroke a needle several times along one arm of the magnet, always in the same direction, as shown in Figure 105. Hold the needle over some iron filings or touch any bit of iron or steel with it. What has the needle become? Lay it on a cardboard milk-bottle top of the flat kind, and on that float it in the middle of a glass or earthenware dish of water. Notice which end turns north. Turn this end to the south and see what happens. Hold your magnet, ends up, under the dish, and turn the magnet. What does the needle do?
