134 THE FAIRY-LAND OF SCIENCE. 
hear it ring. Yet the other balls remain where they 
were before. Why is this? It is because each of the 
balls, as it was knocked forward, had one in front of 
it to stop it and make it bound back again, but the 
last one was free to move on. When I threw this ball 
from my hand against the others, the one in front of 
it moved, and hitting the third ball, bounded back 
again; the third did the same to the fourth, the fourth 
FIG. 32. 
to the fifth, and so on to the end of the line. Each 
ball thus came back to its place, but it passed the 
shock on to the last ball, and the ball to the bell. If I 
now put the balls close up to the bell, and repeat the 
experiment, you still hear the sound, for the last ball 
shakes the bell as if it were a ball in front of it. 
Now imagine these balls to be atoms of air, and the 
bell your ear. If I clap my hands and so hit the air 
in front of them, each air-atom hits the next just as 
the balls did, and though it comes back to its place, 
it passes the shock on along the whole line to the 
atom touching the drum of your ear, and so you re- 
ceive a blow. But a curious thing happens in the 
air which you cannot notice in the balls. You must 
remember that air is elastic, just as if there were 
springs between the atoms as in the diagram, Fig. 33, 
and so when any shock knocks the atoms forward, 
