Arabella B. Buckley.

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small changes, but by these we learn how greater ones have been
brought about, and how we owe the outline of all our beautiful
scenery, with its hills and valleys, its mountains and plains, its
cliffs and caverns, its quiet nooks and its grand rugged
precipices, to the work of the "Two great sculptors, Water and
Ice."



Week 16

Lecture VI

THE VOICES OF NATURE AND HOW WE HEAR THEM

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.



Week 17

If you can picture to yourself these waves spreading out in all
directions, you will easily see why sound grows fainter at the
distance. Just close round my hands when I clap them, there is
a small quantity of air, and so the shock I give it is very
violent, but as the sound-waves spread on all sides they have
more and more air to move, and so the air-atoms are shaken less
violently and strike with less force on your ear.

If we can prevent the sound-wave from spreading, then the sound
is not weakened. The Frenchman Biot found that a low whisper
could be heard distinctly for a distance of half a mile through a
tube, because the waves could not spread beyond the small column
of air. But unless you speak into a small space of some kind,
you cannot prevent the waves going out from you in all
directions.

Try and imagine that you see these waves spreading all round me
now and hitting on your ears as they pass, then on the ears of
those behind you, and on and on in widening globes till they
reach the wall. What will happen when they get there? If the
wall were thin, as a wooden partition is, they would shake it,
and it again would shake the air on the other side, and so anyone
in the next room would have the sound of my voice brought to
their ear.

But something more will happen. In any case the sound-waves
hitting against the wall will bound back from it just as a ball
bounds back when thrown against anything, and so another set of
sound-waves reflected from the wall will come back across the
room. If these waves come to your ear so quickly that they mix
with direct waves, they help to make the sound louder in this
room than you would in the open air, for the "Ha" from my mouth
and a second "Ha" from the wall come to your ear so
instantaneously that they make one sound. This is why you can
often hear better at the far end of a church when you stand
against a screen or a wall, then when you are half-way up the
building nearer to the speaker, because near the wall the
reflected waves strike strongly on your ear and make the sound
louder.

Sometimes, when the sound comes from a great explosion, these
reflected waves are so strong that they are able to break glass.
In the explosion of gunpowder in St. John's Wood, many houses in
the back streets had their windows broken; for the sound-waves
bounded off at angles from the walls and struck back upon them.

Now suppose the wall were so far behind you that the reflected
sound-waves only hit upon your ear after those coming straight
from me had died away; then you would hear the sound twice, "Ha"
from me and "Ha" from the wall, and here you have an echo, "Ha,
ha." In order for this to happen in ordinary air, you must be
standing at least 56 feet away from the point from which the
waves are reflected, for then the second blow will come one-tenth
of a second after the first one, and that is long enough for you
to feel them separately.* Miss C. A. Martineau tells a story of
a dog which was terribly frightened by an echo. Thinking another
dog was barking, he ran forward to meet him, and was very much
astonished, when, as he came nearer the wall, the echo ceased. I
myself once knew a case of this kind, and my dog, when he could
find no enemy, ran back barking, till he was a certain distance
off, and then the echo of course began again. He grew so furious
at last that we had great difficulty in preventing him from
flying at a strange man who happened to be passing at the time.
(*Sound travels 1120 feet in a second, in air of ordinary
temperature, and therefore 112 feet in the tenth of a second.
Therefore the journey of 56 feet beyond you to reach the wall
and 56 feet to return, will occupy the sound-wave one-tenth of
a second and separate the two sounds.)

Sometimes, in the mountains, walls of rock rise at some distance
one behind another, and then each one will send back its echo a
little later than the rock before it, so that the "Ha" which you
give will come back as a peal of laughter. There is an echo in
Woodstock Park which repeats the word twenty times. Again
sometimes, as in the Alps, the sound-waves coming back rebound
from mountain to mountain and are driven backwards and forwards,
becoming fainter and fainter till they die away; these echoes are
very beautiful.

If you are now able to picture to yourselves one set of waves
going to the wall, and another set returning and crossing them,
you will be ready to understand something of that very difficult
question, How is it that we can hear many different sounds at one
time and tell them apart?

Have you ever watched the sea when its surface is much ruffled,
and noticed how, besides the big waves of the tide, there are
numberless smaller ripples made by the wind blowing the surface
of the water, or the oars of a boat dipping in it, or even rain-
drops falling? If you have done this you will have seen that all
these waves and ripples cross each other, and you can follow any
one ripple with you eye as it goes on its way undisturbed by the
rest. Or you may make beautiful crossing and recrossing ripples
on a pond by throwing in two stones at a little distance from
each other, and here too you can follow any one wave on to the
edge of the pond.

Now just in this way the waves of sound, in their manner of
moving, cross and recross each other. You will remember too,
that different sounds make waves of different lengths, just as
the tide makes a long wave and the rain-drops tiny ones.
Therefore each sound falls with its own peculiar wave upon your
ear, and you can listen to that particular wave just as you look
at one particular ripple, and then the sound becomes clear to
you.

All this is what is going on outside your ear, but what is
happening in your ear itself? How do these blows of the air
speak to your brain? By means of the following diagram, Fig. 33,
we will try to understand roughly our beautiful hearing
instrument, the ear.

First, I want you to notice how beautifully the outside shell, or
concha as it is called, is curbed round so that any movement of
the air coming to it from the front is caught in it and reflected
into the hole of the ear. Put your finger round your ear and
feel how the gristly part is curved towards the front of your
head. This concha makes a curve much like the curve a deaf man
makes with his hand behind his ear to catch the sound. Animals
often have to raise their ears to catch the sound well, but ours
stand always ready. When the air-waves have passed in at the
hole of your ear, they move all the air in the passage, which is
called the auditory, or hearing, canal. This canal is lined with
little hairs to keep out insects and dust, and the wax which
collects in it serves the same purpose. But is too much wax
collects, it prevents the air from playing well upon the drum,
and therefore makes you deaf. Across the end of this canal, a
membrane or skin called the tympanum is stretched, like the
parchment over the head of a drum, and it is this membrane which
moves to and fro as the air-waves strike on it. A violent box on
the ear will sometimes break this delicate membrane, or injure
it, and therefore it is very wrong to hit a person violently on
the ear.

On the other side of this membrane, inside the ear, there is air,
which fills the whole of the inner chamber and the tube, which
runs down into the throat behind the nose, and is called the
Eustachian tube after the man who discovered it. This tube is
closed at the end by a valve which opens and shuts. If you
breathe out strongly, and then shut your mouth and swallow, you
will hear a little "click" in your ear. This is because in
swallowing you draw the air out of the Eustachian tube and so
draw in the membrane, which clicks as it goes back again. But
unless you do this the tube and the whole chamber cavity behind
the membrane remains full of air.

Now, as this membrane is driven to and fro by the sound-waves, it
naturally shakes the air in the cavity behind it, and it also
sets moving three most curious little bones. The first of the
bones is fastened to the middle of the drumhead so that it moves
to and fro every time this membrane quivers. The head of this
bone fits into a hole in the next bone, the anvil, and is
fastened to it by muscles, so as to drag it along with it; but,
the muscles being elastic, it can draw back a little from the
anvil, and so give it a blow each time it comes back. This anvil
is in its turn very firmly fixed to the little bone, shaped like
a stirrup, which you see at the end of the chain.

This stirrup rests upon a curious body which looks in the diagram
like a snail-shell with tubes coming out of it. This body, which
is called the labyrinth, is made of bone, but it has two little
windows in it, one covered only by a membrane, while the other
has the head of the stirrup resting upon it.

Now, with a little attention you will understand that when the
air in the canal shakes the drumhead to and fro, this membrane
must drag with it the hammer, the anvil, and the stirrup. Each
time the drum goes in, the hammer will hit the anvil, and drive
the stirrup against the little window; every time it goes out it
will draw the hammer, the anvil, and the stirrup out again, ready
for another blow. Thus the stirrup is always playing upon this
little window. Meanwhile, inside the bony labyrinth there is a
fluid like water, and along the little passages are very fine
hairs, which wave to and fro like reeds; and whenever the stirrup
hits at the little window, the fluid moves these hairs to and
fro, and they irritate the ends of a nerve, and this nerve
carries the message to your brain. There are also some curious
little stones called otoliths, lying in some parts of this fluid,
and they, by their rolling to and fro, probably keep up the
motion and prolong the sound.

You must not imagine we have explained here the many intricacies
which occur in the ear; I can only hope to give you a rough idea
of it, so that you may picture to yourselves the air-waves moving
backwards and forward in the canal of your ear, then the tympanum
vibrating to and fro, the hammer hitting the anvil, the stirrup
knocking at the little window, the fluid waving the fine hairs
and rolling the tiny stones, the ends of the nerve quivering, and
then (how we know not) the brain hearing the message.

Is not this wonderful, going on as it does at every sound you
hear? And yet his is not all, for inside that curled part of the
labyrinth, which looks like a snail-shell and is called the
cochlea, there is a most wonderful apparatus of more than three
thousand fine stretched filaments or threads, and these act like
the strings of a harp, and make you hear different tones. If you
go near to a harp or a piano, and sing any particular note very
loudly, you will hear this note sounding in the instrument,
because you will set just that particular string quivering, which
gives the note you sang. The air-waves set going by your voice
touch that string, because it can quiver in time with them, while
none of the other strings can do so. Now, just in the same way
the tiny instrument of three thousand strings in your ear, which
is called Corti's organ, vibrates to the air-waves, one thread to
one set of waves, and another to another, and according to the
fibre that quivers, will be the sound you hear. Here then at
last, we see how nature speaks to us. All the movements going on
outside, however violent and varied they may be, cannot of
themselves make sound. But here, in the little space behind the
drum of our ear, the air-waves are sorted and sent on to our
brain, where they speak to us as sound.



Week 18

But why then do we not hear all sounds as music? Why are some
mere noise, and others clear musical notes? This depends
entirely upon whether the sound-waves come quickly and regularly,
or by an irregular succession of shocks. For example, when a
load of stones is being shot out of a cart, you hear only a long,
continuous noise, because the stones fall irregularly, some
quicker, some slower, here a number together, and there two or
three stragglers by themselves; each of these different shocks
comes to your ear and makes a confused, noisy sound. But if you
run a stick very quickly along a paling, you will hear a sound
very like a musical not. This is because the rods of the paling
are all at equal distances one from another, and so the shocks
fall quickly one after another at regular intervals upon your
ear. Any quick and regular succession of sounds makes a note,
even though it may be an ugly one. The squeak of a slate pencil
along a slate, and the shriek of a railway whistle are not
pleasant, but they are real notes which you could copy on a
violin.

I have here a simple apparatus which I have had made to show you
that rapid and regular shocks produce a natural musical note.
This wheel (Fig. 34) is milled at the edge like a shilling, and
when I turn it rapidly so that it strikes against the edge of the
card fixed behind it, the notches strike in rapid succession, and


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