THE VOICES OF NATURE. 139 
air, you must be standing at least 56 feet away from 
the point from which the waves are reflected, for then 
the second blow will come one-tenth of a second after 
the first one, and that is long enough for you to feel 
them separately.* 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 diffi- 
culty 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 be- 
fore it, so that the " Ha " which you give will come 
back as a peal of laughter. There is an echo in Wood- 
stock 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 backward and forward, becoming fainter 
and fainter till they die away; these echoes are very 
beautiful. 
If you are now able to picture to yourselves one set 
of waves going to the wall, and another set returning 
* Sound travels 1120 feet in a second, in air of ordinary tem- 
perature, and therefore 112 feet in the tenth of a second. There- 
fore 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. 
