* Miss C. A. Martineau tells a story of
a dog which 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 great difficulty in preventing him from
flying at a strange man who happened to be passing at the time.
(*Sound travels 1120 feet in a second, in air of ordinary
temperature, and therefore 112 feet in the tenth of a second.
Therefore the journey of 56 feet beyond you to reach the wall
and 56 feet to return, will occupy the sound-wave one-tenth of
a second and separate the two sounds.)
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.
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