We have reached to-day the middle point of our course, and here
we will make a new start. All the wonderful histories which we
have been studying in the last five lectures have had little or
nothing to do with living creatures. The sunbeams would strike
on our earth, the air would move restlessly to and fro, the
water-drops would rise and fall, the valleys and ravines would
still be cut out by rivers , if there were no such thing as life
upon the earth. But without living things there could be none of
the beauty which these changes bring about. Without plants, the
sunbeams, the air and the water would be quite unable to clothe
the bare rocks, and without animals and man they could not
produce light, or sound, or feeling of any kind.
In the next five lectures, however, we are going to learn
something of the use living creatures make of the earth; and to-
day we will begin by studying one of the ways in which we are
affected by the changes of nature, and hear her voice.
We are all so accustomed to trust to our sight to guide us in
most of our actions, and to think of things as we see them, that
we often forget how very much we owe to sound. And yet Nature
speaks to us so much by her gentle, her touching, or her awful
sounds, that the life of a deaf person is even more hard to bear
than that of a blind one.
Have you ever amused yourself with trying how many different
sounds you can distinguish if you listen at an open window in a
busy street? You will probably be able to recognize easily the
jolting of the heavy wagon or dray, the rumble of the omnibus,
the smooth roll of the private carriage and the rattle of the
light butcher's cart; and even while you are listening for these,
the crack of the carter's whip, the cry of the costermonger at
his stall, and the voices of the passers-by will strike upon you
ear. Then if you give still more close attention you will hear
the doors open and shut along the street, the footsteps of the
passengers, the scraping of the shovel of the mud-carts; nay, if
he happen to stand near, you may even hear the jingling of the
shoeblack's pence as he plays pitch and toss upon the pavement.
If you think for a moment, does it not seem wonderful that you
should hear all these sounds so that you can recognize each one
distinctly while all the rest are going on around you?
But suppose you go into the quiet country. Surely there will be
silence there. Try some day and prove it for yourself, lie down
on the grass in a sheltered nook and listen attentively. If
there be ever so little wind stirring you will hear it rustling
gently through the trees; or even if there is not this, it will
be strange if you do not hear some wandering gnat buzzing, or
some busy bee humming as it moves from flower to flower. Then a
grasshopper will set up a chirp within a few yards of you, or, if
all living creatures are silent, a brook not far off may be
flowing along with a rippling musical sound. These and a hundred
other noises you will hear in the most quiet country spot; the
lowing of the cattle, the song of the birds, the squeak of the
field-mouse, the croak of the frog, mingling with the sound of
the woodman's axe in the distance, or the dash of some river
torrent. And beside these quiet sounds, there are still other
occasional voices of nature which speak to us from time to time.
The howling of the tempestuous wind, the roar of the sea-waves in
a storm, the crash of thunder, and the mighty noise of the
falling avalanche; such sounds as these tell us how great and
terrible nature can be.
Now, has it ever occurred to you to think what sounds is, and how
it is that we hear all these things? Strange as it may seem, if
there were no creature that could hear upon the earth, there
would be no such thing as sound, though all these movements in
nature were going on just as they are now.
Try and grasp this thoroughly, for it is difficult at first to
make people believe it. Suppose you were stone-deaf, there would
be no such thing as sound to you. A heavy hammer falling on an
anvil would indeed shake the air violently, but since this air
when it reached your ear would find a useless instrument, it
could not play upon it. and it is this play on the drum of your
ear and the nerves within it speaking to your brain which make
sound. Therefore, if all creatures on or around the earth were
without ears or nerves of hearing, there would be no instrument
on which to play, and consequently there would be no such thing
as sound. This proves that two things are needed in order that
we may hear. First, the outside movement which plays on our
hearing instrument; and, secondly, the hearing instrument itself.
First, then, let us try to understand what happens outside our
ears. Take a poker and tie a piece of string to it, and holding
the ends of the string to your ears, strike the poker against the
fender. You will hear a very loud sound, for the blow will set
all the particles of the poker quivering, and this movement will
pass right along the string to the drum of your ear and play upon
it.
Now take the string away from you ears, and hold it with your
teeth. Stop your ears tight, and strike 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 produced? In
this case, the quivering movement has passed through your teeth
into the bones of your hear, 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 drums 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 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.
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 movement like this going on in the line of air-
atoms, the drum of your ear being at the end. Those which are
crowded together at that end will hit on the drum of your ear and
drive the membrane which covers it inwards; then instantly the
wave will change, these atoms will bound back, and the membrane
will recover itself again, but only to receive a second blow as
the atoms are driven forwards again, and so the membrane will be
driven in and out till the air has settled down.
This you see is quite different to the waves of light which moves
in crests and hollows. Indeed, it is not what we usually
understand by a wave at all, but a set of crowdings and partings
of atoms of air which follow each other rapidly across the air.
A crowding of atoms is called a condensation, and a parting is
called a rarefaction, and when we speak of the length of a wave
of sound, we mean the distance between two condensations, or
between two rarefactions.
Although each atom of air moves a very little way forwards and
then back, yet, as a long row of atoms may be crowded together
before they begin to part, a wave is often very long. When a man
talks in an ordinary bass voice, he makes sound-waves from 8 to
12 feet long; a woman's voice makes shorter waves, from 2 to 4
feet long, and consequently the tone is higher, as we shall
presently explain.
And now I hope that some one is anxious to ask why, when I clap
my hands, anyone behind me or at the side, can hear it as well or
nearly as well as you who are in front. This is because I give a
shock to the air all round my hands, and waves go out on all
sides, making as it were gloves of crowdings and partings
widening and widening away from the clap as circles widen on a
pond. Thus the waves travel behind me, above me, and on all
sides, until they hit the walls, the ceiling, and the floor of
the room, and wherever you happen to be, they hit upon your ear.