THE VOICES OF NA TURE. 
135 
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 sepa- 
rate 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 atoms and one set 
FIG. 33. 
of separated atoms alternately all along the line, and 
the same set will never be crowded two instants to- 
gether. 
You may see an excellent example of this in a 
baggage 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 movement like this going on in the line of air-atoms, 
Fig. 33, 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 inward; then instantly the wave will 
