_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 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.