You see, you can have no sound of any kind without sound waves. You could not make a sound if there were no air. Now, when you shout, you start a series of sound waves that go out from you in every direction and they spread away from you in circles just like the rings of ripples that are caused when you drop a stone into a pool of water. You can prove this to yourself easily by having one, two, three or more of your friends stand around you in a large circle. You can place them as far away from you as your shout can be heard if you wish. When you shout, each of your friends will hear the shout at the same time, provided, of course, they are at equal distances from you. Sometimes these sound waves as they go away from you in circles strike objects that turn the waves back unbroken just as they came to them. The waves will bounce back just like a rubber ball from a wall against which it has been thrown and this is the echo. However, some things that the sound waves strike break up these waves entirely and others partially. No doubt you have sometimes noticed when you shout you hear a distinct echo and that at other times, standing in the same place, you cannot hear any echo, although you shout in the same way. This is explained by the fact that at times conditions of the air are such that no echo is produced while at other times a perfect echo results. What is a Whispering Gallery? The possibilities of an echo have to be taken into account by the architects and builders of all public buildings, such as theaters, halls and churches, where anyone is to speak or entertain others. Unless they are very careful the walls and ceilings may be so arranged that when any one sings or speaks in the room, there is such an echo that it interferes with the music or speaking. It sometimes happens also that through some peculiarity in which the walls and ceiling of a building are constructed there will be certain places in the room where an echo can be heard, even a whisper, and which cannot be heard in other parts of the room at all. This is likely to occur in rooms where there is a dome-shaped ceiling. There will be certain spots in the room hundreds of feet apart, where if you stand on one spot and another person is on another definite spot clear across the room, the tiniest whisper can be heard, while the people in between cannot hear at all. This is called a whispering gallery. Of course, loud talking would produce the same effect. A whispering gallery is a gallery with an echo which can be heard from certain positions. There are a number of famous whispering galleries of the world. In the room beneath the great dome of our Capitol at Washington is an almost perfect whispering gallery. There are quite a number of points at which you can stand and hear the whispers across the room which is more than a hundred feet. These whispering galleries come accidentally, of course. It would be difficult to deliberately construct a building in such a way as to