THE VOICES OF NATURE. 131 
movement like this going on in the line of air-atoms, 
Fig. 31, the drum of your ear being at the end B. 
Those which are crowded together at that end will 
hit -on the drum of your ear and drive the membrane 
which covers it inVards ; then instantly the wave will 
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 forwards 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 move in crests and hollows. Indeed, it is not 
Fig. 32- 
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 
parting is called a rarefaction, and when we speak of 
the length of a wave of sound, we mean the distance 
between two condensations, a a Fig. 32, or between 
two rarefactions, b b. 
Although each atom of air moves a very little way 
forwards 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 
K 2 
