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
produce a musical sound. We can also prove by this experiment
that the quicker the blows are, the higher the note will be. I
pull the string gently at first, and then quicker and quicker,
and you will notice that the note grows sharper and sharper, till
the movement begins to slacken, when the note goes down again.
This is because the more rapidly the air is hit, the shorter are
the waves it makes, and short waves give a high note.
Let us examine this with two tuning-forks. I strike one, and it
sounds D, the third space in the treble; I strike the other, and
it sounds G, the first leger line, five notes above the C. I
have drawn on this diagram (Fig. 35), an imaginary picture of
these two sets of waves. You see that the G fork makes three
waves, while the C fork makes only two. Why is this? Because
the prong of the G fork moves three times backwards and forwards
while the prong of the C fork only moves twice; therefore the G
fork does not crowd so many atoms together before it draws back,
and the waves are shorter. These two notes, C and G, are a fifth
of an octave apart; if we had two forks, of which one went twice
as fast as the other, making four waves while the other made two,
then that note would be an octave higher.
So we see that all the sounds we hear, - the warning noises which
keep us from harm, the beautiful musical notes with all the tunes
and harmonies that delight us, even the power of hearing the
voices of those we love, and learning from one another that which
each can tell, - all these depend upon the invisible waves of
air, even as the pleasures of light depend on the waves of ether.
It is by these sound-waves that nature speaks to us, and in all
her movements there is a reason why her boice is sharp or tender,
loud or gentle, awful or loving. Take for instance the brook we
spoke of at the beginning of the lecture. Why does it sing so
sweetly, while the wide deep river makes no noise? Because the
little brook eddies and purls round the stones, hitting them as
it passes; sometimes the water falls down a large stone, and
strikes against the water below; or sometimes it grates the
little pebbles together as they lie in its bed. Each of these
blows makes a small globe of sound-waves, which spread and spread
till they fall on your ear, and because they fall quickly and
regularly, they make a low, musical note. We might almost fancy
that the brook wished to show how joyfully it flows along,
recalling Shelley's beautiful lines:-
"Sometimes it fell
Among the moss with hollow harmony,
Dark and profound; now on the polished stones
It danced; like childhood laughing as it went."
The broad deep river, on the contrary, makes none of these
cascades and commotions. The only places against which it rubs
are the banks and the bottom; and here you can sometimes hear it
grating the particles of sand against each other if you listen
very carefully. But there is another reason why falling water
makes a sound, and often even a loud roaring noise in the
cataract and in the breaking waves of the sea. You do not only
hear the water dashing against the rocky ledges or on the beach,
you also hear the bursting of innumerable little bladders of air
which are contained in the water. As each of these bladders is
dashed on the ground, it explodes and sends sound-waves to your
ear. Listen to the sea some day when the waves are high and
stormy, and you cannot fail to be struck by the irregular bursts
of sound.
The waves, however, do not only roar as they dash on the ground;
have you never noticed how they seem to scream as they draw back
down the beach? Tennyson calls it,
"The scream of the madden'd beach dragged down by the wave;" and
it is caused by the stones grating against each other as the
waves drag them down. Dr. Tyndall tells us that it is possible
to know the size of the stones by the kind of noise they make.
If they are large, it is a confused noise, when smaller, a kind
of scream; while a gravelly beach will produce a mere hiss.
Who could be dull by the side of a brook, a waterfall, or the
sea, while he can listen for sounds like these, and picture to
himself how they are being made? You may discover a number of
other causes of sound made by water, if you once pay attention to
them.
Nor is it only water that sings to us. Listen to the wind, how
sweetly it sighs among the leaves. There we hear it, because it
rubs the leaves together, and they produce the sound-waves. But
walk against the wind some day and you can hear it whistling in
your own ear, striking against the curved cup, and then setting
up a succession of waves in the hearing canal of the ear itself.
Why should it sound in one particular tone when all kinds of
sound-waves must be surging about in the disturbed air?
This glass jar will answer our question roughly. If I strike my
tuning-fork and hold it over the jar, you cannot hear it, because
the sound is feeble, but if I fill the jar gently with water,
when the water rises to a certain point you will hear a loud
clear note, because the waves of air in the jar are exactly the
right length to answer to the note of the fork. If I now blow
across the mouth of the jar you hear the same note, showing that
a cavity of a particular length will only sound to the waves
which fit it. do you see now the reason why pan-pipes give
different sounds, or even the hole at the end of a common key
when you blow across it? Here is a subject you will find very
interesting if you will read about it, for I can only just
suggest it to you here. But now you will see that the canal of
your ear also answers only to certain waves, and so the wind
sings in your ear with a real if not a musical note.
Again, on a windy night have you not heard the wind sounding a
wild, sad note down a valley? Why do you think it sounds so much
louder and more musical here than when it is blowing across the
plain? Because air in the valley will only answer to a certain
set of waves, and, like the pan-pipe, gives a particular note as
the wind blows across it, and these waves go up and down the
valley in regular pulses, making a wild howl. You may hear the
same in the chimney, or in the keyhole; all these are waves set
up in the hole across which the wind blows. Even the music in
the shell which you hold to your ear is made by the air in the
shell pulsating to and fro. And how do you think it is set
going? By the throbbing of the veins in your own ear, which
causes the air in the shell to vibrate.
Another grand voice of nature is the thunder. People often have
a vague idea that thunder is produced by the clouds knocking
together, which is very absurd, if you remember that clouds are
but water-dust. The most probable explanation of thunder is much
more beautiful than this. You will remember from Lecture III
that heat forces the air-atoms apart. Now, when a flash of
lightning crosses the sky it suddenly expands the air all round
it as it passes, so that globe after globe of sound-waves is
formed at every point across which the lightning travels. Now
light, you remember, travels so wonderfully rapidly (192,000
miles in a second) that a flash of lightning is seen by us and is
over in a second, even when it is two or three miles long. But
sound comes slowly, taking five seconds to travel half a mile,
and so all the sound-waves at each point of the two or three
miles fall on our ear one after the other, and make the rolling
thunder. Sometimes the roll is made even longer by the echo, as
the sound-waves are reflected to and fro by the clouds on their
road; and in the mountains we know how the peals echo and re-echo
till they die away.
We might fill up far more than an hour in speaking of those
voices which come to us as nature is at work. Think of the
patter of the rain, how each drop as it hits the pavement sends
circles of sound-waves out on all sides; or the loud report which
falls on the ear of the Alpine traveller as the glacier cracks on
its way down the valley; or the mighty boom of the avalanche as
the snow slides in huge masses off the side of the lofty
mountain. Each and all of these create their sound-waves, large
or small, loud or feeble, which make their way to your ear, and
become converted into sound.
We have, however, only time now just to glance at life-sounds, of
which there are so many around us. Do you know why we hear a
buzzing, as the gnat, the bee, or the cockchafer fly past? Not
by the beating of their wings against the air, as many people
imagine, and as is really the case with humming birds, but by the
scraping of the under-part of their hard wings against the edges
of their hind legs, which are toothed like a saw. The more
rapidly their wings move the stronger the grating sound becomes,
and you will now see why in hot, thirsty weather the buzzing of
the gnat is so loud, for the more thirsty and the more eager he
becomes, the wilder his movements will be.
Some insects, like the drone-fly (Eristalis tenax), force the air
through the tiny air-passages in their sides, and as these
passages are closed by little plates, the plates vibrate to and
fro and make sound-waves. Again, what are those curious sounds
you may hear sometimes if you rest your head on a trunk in the
forest? They are made by the timber-boring beetles, which saw
the wood with their jaws and make a noise in the world, even
though they have no voice.
All these life-sounds are made by creatures which do not sing or
speak; but the sweetest sounds of all in the woods are the voices
of the birds. All voice-sounds are made by two elastic bands or
cushions, called vocal chords, stretched across the end of the
tube or windpipe through which we breathe, and as we send the air
through them we tighten or loosen them as we will, and so make
them vibrate quickly or slowly and make sound-waves of different
lengths. But if you will try some day in the woods you will find
that a bird can beat you over and over again in the length of his
note; when you are out of breath and forced to stop he will go on
with his merry trill as fresh and clear as if he had only just
begun. This is because birds can draw air into the whole of
their body, and they have a large stock laid up in the folds of
their windpipe, and besides this the air-chamber behind their
elastic bands or vocal chords has two compartments where we have
only one, and the second compartment has special muscles by which
they can open and shut it, and so prolong the trill.
Only think what a rapid succession of waves must quiver through
the air as a tiny lark agitates his little throat and pours forth
a volume of song! The next time you are in the country in the
spring, spend half an hour listening to him, and try and picture
to yourself how that little being is moving all the atmosphere
round him. Then dream for a little while about sound, what it
is, how marvellously it works outside in the world, and inside in
your ear and brain; and then, when you go back to work again, you
will hardly deny that it is well worth while to listen sometimes
to the voices of nature and ponder how it is that we hear them.