Franklin Square Song Collection - online songbook

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	56                                  FRANKLIN-SQUARE SONG COLLECTION.
	Echoes.—Try to imagine that you see the sound­waves spreading all around you, striking on your ears as they pass, then on the ears of those behind you, and on and on in widening globes till they reach the wall. What will happen when they get there? If the wall were thin, as a wooden partition is, they would shake it, and it again would shake the air on the other side, and so any one in the next room would have the sound of my voice brought to their ear. But something more will happen. In any case the sound-waves hitting against the wall will bound back from it just as a ball bounds back when thrown against any hard surface, and so another set of sound-waves reflected from the wall will come back across the room. If these waves come to your	ear so quickly that they mix with direct waves, they help to make the sound louder. For instance, if I say " Ha," you hear that sound louder in this room than you would in the open air, for the " Ha" from my mouth and a second " Ha" from the wall come to your ear so instantaneously that they make one sound. This is why you can often hear better at the far end of the church when you stand against a screen or a wall, than when you are halfway up the building nearer the speaker, because near the wall the reflected waves strike strongly on your ear and thus make the sound louder. Sometimes, when the sound comes from a great explosion, as of gunpow­der or dynamite, these reflected waves are so strong that they are able to break glass. Now, suppose the
	HE GIVETH HIS BELOVED SLEEP.		Franz Abt. T. C. Tildesley.
	wall were so far behind you that the reflected sound­waves hit upon your ear only after those coming straight from me had died away; then you would hear the sound twice, "Ha" from me and "Ha" from the wall, and here you have an echo," Ha, ha." For this to happen in ordinary air, you must be standing at least 56 feet away from the point from which the waves are reflected, as then the second blow will come one-tenth of a second after the first, and that is long enough for you to feel them sepa­rately. Miss Martineau tells a story of a dog that was terribly frightened by an echo. Thinking an­other dog was barking, he ran forward to meet him, and was very much astonished when, as he came nearer the wall, the echo ceased. I myself once knew a case of this kind, and my dog, when he	could find no enemy, ran back barking, till he was a certain distance off, and then the echo, of course, began again. He grew so furious at last that we had much difficulty in preventing him from flying at a strange man who happened to be passing at the time. Sometimes, in the mountains, walls of rock rise at some distance one behind another, and then each one will send back its echo a little later than the rock before it, so that the " Ha " which you give will come back as a peal of laughter. There is an echo in Woodstock Park which repeats the word twenty times. Again sometimes, as in the Alps, the sound-waves in coming back rebound from moun­tain to mountain and are driven backwards and for­wards, becoming fainter and fainter till they die away. These echoes are very beautiful.—Buckley.