So of all the sun's light, we get only the blue rays reflected from the little dust mirrors in our own atmosphere. A blue sky is the very nicest kind of sky for us. If there were no impurities, or dust, in our atmosphere it would be so dark we couldn't see very well. And if the impurities were of a different size or number, we would have a red or a yellow sky. Either one would dazzle our eyes. Now, when our sky is gray it is because the earth dust is coated with vapor or water dust, making clouds. Vapor is not as good a reflector as dust, so we do not get our blue light until after the vapor has condensed and fallen in rain. (See Sun, Tyndall, Spectrum.)
WHY THE SKY IS MANY COLORED AT SUNSET
You can't imagine a rainbow-colored sky!
Oh, yes, you can. Haven't you seen the brilliant colors of sunset? When the sun is above us, at noon, the light rays come straight down to us, or at a slight slant through the atmosphere. Then we get the reflection of the blue rays, only. But when the light comes to us in level lines along the surface of the earth at sunset, the rays pass through a thicker layer of smoke and dust. From more impurities of more varied sizes and greater densities, the red and yellow rays are reflected, as well as the blue. Some dust particles combine reflections, giving us green, orange and violet lights. At sunrise the same thing happens, but, for some reason, the colors of dawn are usually more delicate and transparent than at twilight.
Sometimes the amount of dust in the air is greater than at others. Volcanoes have been known to throw columns of ashes or volcanic dust into the sky as high as two miles. This dust is carried on the upper currents of air to great distances. It is even thought that it sometimes forms a belt entirely around the earth. After days and weeks all this great volume of volcanic dust settles, or is washed out of the air by rain. But while it is in the air we have a series of very fine sunsets and sunrises.
SOUND WAVES AND THE PHONOGRAPH
You remember how, in the telephone, the sound waves made by the voice are caught on an iron disc, or drumhead, and sent on to the wire. Now phone means sound and graph means write. In the phonograph the sound waves of the voice are caught by a drumhead and made to shake a needle that moves over a rolling cylinder,
