138 
THE FAIRY-LAND OF SCIENCE. 
and so persons 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 anything, 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 instantane- 
ously that they make one sound. This is why you 
can often hear better at the far end of a church when 
you stand against a screen or a wall, that when you 
are halfway up the building nearer to the speaker, 
because near the wall the reflected waves strike strong- 
ly on your ear and make the sound louder. 
Sometimes, when the sound comes from a great 
explosion, these reflected waves are so strong that they 
are able to break glass. In the explosion of gun- 
powder in St. John's Wood, many houses in the back 
streets had their windows broken ; for the sound-waves 
bounded off at angles from the walls and struck back 
upon them. 
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 and 
" Ha " from the wall, and here you have an echo, 
" Ha, ha." In order for this to happen in ordinary 
