songs from the book of knowledge: echo

listen to echo

Nearly all ancient peoples had poetic stories about the echo.
According to the Greeks, Echo was a mountain nymph who pined away for the love of the youth Narcissus until there was nothing left of her but her whispering voice, and this she could only use to repeat the last word of others.
When you called out “Hello!”, echo answered softly “Hello.”
And when you cried “Who are you?”, you heard only her mocking answer: “You.”
No one really knew what an echo was until wise men discovered that sound travels in waves, just as water and light travel. If a wave of water is stopped by a cliff, it is thrown back into the sea. So if a sound wave is stopped by a cliff, it is thrown back to our ears. The sound very seldom comes back just as it was made. It is usually broken into an airy shattered echo just as a wave is thrown back in spray. And it really seems as if some mocking sprite calls back from a fairy grotto in the rocks.
Sometimes the echo repeats the sound several times. This is caused by successive rebounds from several different objects, placed at varying distances from the observer. If you stand about a hundred feet from the reflecting surface, you hear only the final syllable of what you call. If you take your stand farther back, more and more syllables can be heard.
Sir Isaac Newton used the echo in a corridor at Trinity College, Cambridge to measure the speed at which sound travels. Standing at one end of the corridor he started a group of sound waves by stamping his foot. These waves were thrown back by the wall at the far end of the corridor. He timed the interval between stamping his foot and hearing the echo, he knew the distance to the wall and back, and from these factors, he calculated a speed for sound which was within a few feet of a second of the speed which modern science has determined.

Note: The words in all Songs from the Book of Knowledge are excerpts taken from a 1939 Compton’s Pictured Encyclopedia and Fact-Index.

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