134 THE FAIRY-LAND OF SCIENCE. 
Now, suppose the wall were so far behind you that 
the reflected sound-waves only hit upon your ear after 
those coming straight from me had died away ; then 
you would hear the sound twice, " Ha " from me ami 
" Ha " from the wall, and here you have an echo, 
" Ha, ha." In order 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, 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 oft", 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 before 
it, so that the "Ha" which you give will come back 
as a peal of laitghter. There is an echo in Woodstock 
Park which repeats the word twenty times. Again 
sometimes, as in the Alps, the sound-waves in coming 
* Sound travels 1120 feet in a second, in air of ordinary temperature, 
and therefore 112 feet in the tenth of a second. Therefore the journev 
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 
.oundv 
