132 THE FAIRY-LAND OF SCIENCE. 
12 feet long ; a woman's voice makes shorter waves, 
from 2 to 4 feet long, and consequently the tone is 
higher, as we shall presently explain. 
And now I hope that some one is anxious to ask 
why, when I clap my hands, anyone behind me or at 
the side, can hear it as well or nearly as well as you 
who are in front. This is because I give a shock to 
the air all round my hands, and waves go out on all 
sides, making as it were globes of crowdings and 
partings widening and widening away from the clap 
is circles widen on a pond. Thus the waves travel 
behind me, above me, and on all sides, until they hit 
the walls, the ceiling, and the floor of the room, and 
wherever you happen to be, they hit upon your ear. 
If you can picture to yourself these waves spreading 
out in all directions, you will easily see why sound 
grows fainter at the distance. Just close round my 
hands when I clap them, there is a small quantity of 
air, and so the shock I give it is very violent, but as 
the sound-waves spread on ail sides they have more 
and more air to move, and so the air-atoms are 
shaken less violently and strike with less force on 
your ear. 
If we can prevent the sound-wave from spreading, 
then the sound is not weakened. The Frenchman Biot 
found that a low whisper could be heard distinctly for 
a distance of half a mile through a tube, because the 
waves could not spread, beyond the small column of 
air. But unless you speak into a small space of some 
kind, you cannot prevent the waves going out from 
you in all directions. 
Try and imagine that you see these waves spreading 
