In the so-called indirect system of lighting we do not get the direct light of the lamps at all (see Fig. 187). You have probably seen this system in use in some library, or hall, or office, or bank; perhaps in some house. You do not see the source of the light because the lamps are in a recess, or groove, near the top of the walls, and you get only

(Courtesy of the National Lamp Works.)

FIG. 187. In indirect lighting the light rays are reflected to the eye entirely from the ceiling and walls of the room; none come directly from the lamp.

the irregularly reflected light from the ceiling and the upper part of the walls.

Even if we cannot have a real indirect lighting system in the house, we can have some of its advantages. If we have a central cluster of lamps in a room, we can get much of the effect of indirect lighting if we have a large, white bowl under the cluster. The light that gets through is much more comfortable to the eyes, and a large amount of it is reflected from the ceiling. Then, too, the various kinds of mantle

and electric lamps have ground or opal glass globes or shades, which we can use to get a more diffused light in our room. If we have to study by the light of a table lamp, it should by all means have a green shade. If we have to study by a direct ceiling light, we should wear such a shade ourselves. Can you tell why the green of the shade is so agreeable to our eyes?

We must remember that much of the effect of any lighting system depends upon the color of the walls of a room, upon whether they are rough or smooth, and upon the decorations used. A well-lighted home does not just "happen"; it must be planned with intelligence.

264. Exercises. 1. Report to the class a method by which you could make illuminating gas. See Fig. 25, § 36.

2. Report to the class all the uses of glass that you can think of. 3. How is window glass cut into pieces of a certain size?

4. In case a small gasoline or kerosene stove were tipped over while lighted, what would you do? Would it be best to put water upon the fire? Would you put a rug over it? What do you think would be the effect of throwing baking soda upon the fire? Note: Baking soda gives off carbon dioxide very easily when heated.

5. If you blow out a kerosene lamp flame, you can relight it if you bring a lighted match at once above the wick. Tell why. See § 28.

6. If you wish a match to burn, do you hold its head up, or down? Why?

7. Do you think it is a good practice to heat a closed room with a kerosene lamp? Why?

8. Why does a gas "mantle" give off more light than the flame

which heats it?

9. Is it worth while to continue to use an incandescent bulb which becomes very hot when lighted? Why?

10. What kind of gas is used in your house? How much does it cost for a thousand cubic feet? How much did you use last month?

11. How many incandescent bulbs are there in your house? Do you use electricity for any other purpose than for lighting? How much electricity did you use last month? How much for each bulb? What is the cost of your electricity for each kilowatt-hour?

12. Give all the reasons you can think of for having as many windows as possible in a house.

Sunlight is needed in our houses, for health as well as

Early methods of lighting were open fires, wick lamps, and candles. Kerosene lamps had wicks and burners, as well as chimneys, to bring a constant supply of air to the burning fuel.

Gas is "natural gas

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or artificial gas." Artificial gas is made from coal, or from coal and steam.

Gas stoves and mantle lights use the principle of the Bunsen burner. Fuse boxes are used in our house circuits to prevent an overheated electric wire from setting fire to the house.

Gas and electricity are measured by meters.

We should use care with regard to the amount of light by which we work or read, and with regard to the way in which it strikes our

eyes.

Diffused light is better than direct light, and indirect lighting in our houses and buildings is most comfortable to the eye.

The color and the decorations of our rooms have a great deal to do with the lighting.

CHAPTER XXVIII

THE SOUNDS WE HEAR

265. What Causes a Sound? Hasn't someone ever startled you with this remark: "How still it is tonight "? The sounds made by insects, trains, automobiles, water, the wind through the trees, birds, horses, and the sounds made by human beings, all had suddenly stopped. These sounds come to us practically all the time; so that we grow used to hearing them. Then if they cease, as they sometimes do, we are aware of the silence. What is the nature of sound? Is it like light, possessed by certain bodies only? Where does it come from? In answer to these questions carry out the following experiments:

Hold a common table fork (or, better, a tuning fork) lightly by the handle, and strike its tines against the table. Then hold the fork in the air. What happens? You say it makes a ringing sound. Why does it give off sound? The fork was lying on the table a few minutes before, and no sound came from it. Strike the fork again, and hold it against a sheet of paper, such as a folded newspaper, while it rings. What happens? Strike it once more, and this time let it touch the edge of an empty glass. Also try it upon the surface of a glassful of water. What happens to the water while the fork is ringing? The fact is that the paper, the glass, and the water are being struck by the sounding fork and set in motion by it. See Fig. 188. The reason why a sound results after you strike the fork, is that the fork is moving rapidly to and fro. We say it is vibrating. All sound is due to the

vibration, or motion, of some material. It may be a sounding violin string, or it may be a clap of thunder; but it is some material in motion. A piano stands without a sound until some one touches a key. This key is so arranged that when it is pressed, it causes a wire to be struck, and made to vibrate. In speaking or singing we set two membranes in the throat (the vocal cords) to vibrating (see § 388). We do this by forcing air through a slit between them. These vibrations of matter cause the air to vibrate and the air vibration is carried to the ear, causing us to hear.

FIG. 188. The sounding fork is moving so rapidly our eyes cannot follow it, but the sprays of water tell the story.

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Will any substance besides air carry sound? Hold your ear close to one end of an iron steam or water pipe, and have someone scratch with a pin, or a small nail, at the other end of the pipe. Can you hear the sound? Can you hear it as well when your ear is not near the pipe? Try the same problem with a wooden pole, or a yard stick. Do iron and wood conduct sound as well as air, or better? Sound may be carried by water, the earth, and other things also, but air is the most common carrier. You can make a string telephone out of two tin cans and a long enough string and talk to a friend one or two hundred feet away. The string is passed through a small hole in the bottom of each can, and knotted to keep it from slipping out. The string must be tightly stretched, and must not

FIG. 189. If a bell is rung in a jar containing air, the sound grows fainter and fainter as the air is removed.