e 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 e