256. Shooting stars are really stones flying through space. When they come near the earth, it pulls them swiftly down through the air. Explain why they glow. 257. At night it is difficult to see out through a closed window of a room in which a lamp is lighted. 258. When there is a breeze you cannot see clear reflections in a lake. 259. Rubbing with coarse sandpaper makes rough wood smooth. 260. A bow is bent backward to make the arrow go forward. SECTION 29. _Echoes._ When you put a sea shell to your ear, how is it that you hear a roar in the shell? Why can you sometimes hear an echo and sometimes not? If it were not for the fact that sound travels rather slowly, we should have no echoes, for the sound would get back to us practically at the instant we made it. An echo is merely a sound, a series of air vibrations, bounced back to us by something at a distance. It takes time for the vibration which we start to reach the wall or cliff that bounces it back, and it takes as much more time for the returning vibration to reach our ears. So you have plenty of time to finish your shout before the sound bounces back again. The next experiment shows pretty well how the waves, or vibrations, of sound are reflected; only in the experiment we use waves of water because they go more slowly and we can watch them. EXPERIMENT 59. Fill the long laboratory sink (or the bathtub at home) half full of water and start a wave from one end. Watch it move along the side of the sink. Notice what happens when it reaches the other end. Air waves do the same thing; when they strike against a flat surface, they bounce back like a rubber ball. If you are far enough away from a flat wall or cliff, and shout, the sound (the air vibrations you start) is reflected back to you and you hear the echo. But if you are close to the walls, as in an empty room, the sound _reverberates_; it bounces back and forth from one wall to the other so rapidly that no distinct echo is heard, and there is merely a confusion of sound. [Illustration: FIG. 102. When the wave reaches the end of the sink, it is reflected back. Sound waves are reflected in the same way.] When you drop a pebble in water, the ripples spread in all directions. In the same way, when you make a sound in the open air, the air waves spread in all directions. But when you shout through a megaphone the air waves are all concentrated in one direction and consequently they are much stronger in that direction. However, while the megaphone intensifies sound, the echoing from the sides of the megaphone makes the sound lose some of its distinctness.