That any one can speak into an instrument in London, and that at the same time a friend in Paris can hear his voice, and understand what he is saying, is a thing so wonderful that a person must have tried it for himself before he can quite believe it. Yet what is wonderful about it is not so much the fact of speech being communicated, as the distance across which the communication is made. Why should we not be able to cover such great distances by shouting? The reason is, that when we shout we create a wave of sound in the air. Such a wave of sound is in many respects like a wave in water. When we throw a stone into a pond, the wave starts from the place where the stone fell into the water, and travels out in circles which become wider and wider. Not only do the circles become wider, but they also grow fainter and fainter the wider they get. If the pond is very large, the wave may get so faint that when it reaches the bank it is too faint to be perceived. If, instead of a pond, we have a long channel or gutter filled with water and make a splash in it at one end, we find that the wave can travel a very great distance along the channel without growing much feebler. That is because it cannot spread out sideways. Similarly, if we speak into a long pipe or tube, our voice carries much farther than it does in the open air, again because the wave of sound cannot spread out sideways. This is the reason why it is so easy to talk through a speaking-tube. If we could send the same waves along a wire, we might succeed in carrying the sound much farther than we do even through a speaking-tube. This fact can be proved by taking two small tin cans, and stretching a wire from a hole in the bottom of one to a hole in the bottom of the other. Then, if one person speaks into one of the cans, and the other person holds the other can to his ear, and the wire is stretched between them, the second person will hear distinctly what the first person is saying. In this case the wave of sound travels along the wire, stretching it and compressing it in turn, until it reaches the other can, and throws the air into similar vibrations. Such an instrument could be called a telephone, but nowadays the word is used only for instruments which are worked by electricity or magnetism.