130 THE FAIRY-LAND OF SCIENCE. 
receive 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. 31, 
and so when any shock knocks the atoms forward, 
several of them can be crowded together before they 
push on those in front. Then, as soon as they have 
passed the shock on, they rebound and begin to 
separate again, and so swing to and fro till they 
come to rest. Meanwhile the second set will go 
through just the same movements, and will spring 
apart as soon as they have passed the shock on to a 
third set, and so you will have one set of crowded 
Fig. 31- 
atoms and one set of separated atoms alternately all 
along the line, and the same set will never be crowded 
two instants together. 
You may see an excellent example of this in a 
luggage train in a railway station, when the trucks are 
left to bump each other till they stop. You will see 
three or four trucks knock together, then they will 
pass the shock on to the four in front, while they 
themselves bound back and separate as far as their 
chains will let them : the next four trucks will do the 
same, and so a kind of wave of crowded trucks passes 
on to the end of the train, and they bump to and fro 
till the whole comes to a standstill. Try to imagine a 
