136 
THE FAIRY-LAND OF SCIENCE. 
change, these atoms will bound back, and the mem- 
brane will recover itself again, but only to receive a 
second blow as the atoms are driven forward again, 
and so the membrane will be driven in and out till the 
air has settled down. 
This you see is quite different to the waves of light 
which moves in crests and hollows. Indeed, it is not 
what we usually understand by a wave at all, but a 
set of crowdings and partings of the atoms of air 
which follow each other rapidly across the air. A 
crowding of atoms is called a condensation, and a part- 
ing is called a rarefaction, and when we speak of the 
length of a wave of sound, we mean the distance be- 
FIG. 34. 
tween two condensations, a a, Fig. 34; or between 
two rarefactions, b b. 
. Although each atom of air moves a very little way 
forward and then back, yet, as a long row of atoms 
may be crowded together before they begin to part, a 
wave is often very long. When a man talks in an 
ordinary bass voice, he makes sound-waves from 8 to 
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 
