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.
Week 19
LECTURE VII THE LIFE OF A PRIMROSE
When the dreary days of winter and the early damp
days of spring are passing away, and the warm bright
sunshine has begun to pour down upon the grassy paths
of the wood, who does not love to go out and bring
home posies of violets, and bluebells, and primroses?
We wander from one plant to another picking a flower
here and a bud there, as they nestle among the green
leaves, and we make our rooms sweet and gay with the
tender and lovely blossoms. But tell me, did
you ever stop to think, as you added flower after
flower to your nosegay, how the plants which bear
them have been building up their green leaves and their
fragile buds during the last few weeks? If you
