THE VOICES OF NATURE. 133 
all round me now and hitting on your ears as they 
pass, then on the ears of those behind you, and on 
and on in widening globes till they reach the wall. 
What will happen when they get there ? If the wall 
were thin, as a wooden partition is, they would shake 
rt. and it again would shake the air on the other side, 
and so anyone 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 wopld 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, than when you 
are half-way up the building nearer to the speaker, 
because near the wall the reflected waves strike 
strongly 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 
gunpowder 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. 
