Why should ashes or charcoal be put around the bottom of the rod? Because they are good conductors and will help to conduct the lightning into the earth. How large should the lightning-rod be? If made of iron, the lightning-rod should be about three-fourths of an inch in diameter. Why is glass put around the rod where it is joined to the building? The glass is a non-conductor, and prevents the lightning from leaving the rod. Why is it not safe to stand near a fire during a thunder-storm? Because the fire is a good conductor, and sometimes draws the electricity into the room. What part of the room is the safest place? The middle of a room is thought to be the safest place. Is a feather-bed a safe place? Yes; because the feathers are a non-conductor, and will not attract the lightning. Is dry air a non-conductor of electricity? Yes; dry air is a non-conductor of electricity. When are thunder-storms most frequent? Thunder-storms are more frequent in summer than in winter, and more so in the afternoon than in the morning. By what are thunder-storms usually attended? They are usually attended by some change in the direction of the wind. Where do thunder-storms generally prevail? They generally prevail in the lower regions of the air, near the earth. What is the rapidity of lightning? The rapidity of lightning is probably not less than 250,000 miles in a second, and the flash does not exceed the millionth part of a second in duration. Magnetism. Of what does magnetism treat? _Magnetism_ treats of the property of magnets. What is a magnet? A _magnet_ is a piece of iron ore having the power of attracting iron and steel. How many kinds of magnets are there? There are two kinds of magnets: natural magnets, and artificial magnets. What other name is given to a natural magnet? It is called a loadstone. Where are natural magnets or loadstones found? They are dug out of the earth, and are found most abundantly in Norway and Sweden. What is an artificial magnet? An artificial magnet is one made by magnetizing a bar of steel. How may a bar of steel be magnetized? A bar of steel, when rubbed on a natural magnet, becomes magnetized. When the bar of steel is bent in the shape of a horse-shoe, what is it called? It is then called a horse-shoe magnet, from its shape. Where does a magnetic bar always point, when it is made to move easily on a pivot? One end of it always points to the north, and the other end to the south. What are the ends of the magnet called? The north end is called the positive, and the south end is called the negative. What is there curious about the magnet? The positive end of one magnet always attracts the negative end of every other magnet; but if the two positive or the two negative ends are brought together, they will repel each other. Why are artificial magnets made of steel?