Here we are back again in the land of invisible workers!  We have all been listening and hearing ever since we were babies, but have we ever made any picture to ourselves of how sound comes to us right across a room or a field, when we stand at one end and the person who calls is at the other?

Since we have studied the “aerial ocean,” we know that the air filling the space between us, though invisible, is something very real, and now all we have to do is to understand exactly how the movement crosses this air.

This we shall do most readily by means of an experiment made by Dr. Tyndall in his lectures on Sound.  I have here a number of boxwood balls resting in a wooden tray which has a bell hung at the end of it.  I am going to take the end ball and roll it sharply against the rest, and then I want you to notice carefully what happens.  See! the ball at the other end has flow off and hit the bell, so that you 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 forwards, 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 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 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 atoms and one set of separated atoms alternately all along the line, and the same set will never be crowded two instants together.