THE VOICES OF NATURE. ^3 
the poker once more against the fender. You will 
hear the sound quite as loudly and clearly as you did 
before, but this time the drum of your ear has not 
been agitated. How, then, has the sound been pro- 
duced? In this case, the quivering movement has 
passed through your teeth into the bones of your head, 
and from them into the nerves, and so produced sound 
in your brain. And now, as a final experiment, fasten 
the string to the mantelpiece, and hit it again against 
the fender. How much feebler the sound is this time, 
and how much sooner it stops! Yet still it reaches 
you, for the movement has come this time across the 
air to the drum of your ear. 
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 in- 
visible, 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 rest- 
ing 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 flown off and hit the bell, so that you 
