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The Fairy-Land of Science

by Arabella B. Buckley · English · Gutenberg #5726 · 344,170 chars · public domain (US)

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Title: The Fairy-Land of Science

Author: Arabella B. Buckley


        
Release date: May 1, 2004 [eBook #5726]
                Most recently updated: December 29, 2020

Language: English

Other information and formats: www.gutenberg.org/ebooks/5726


*** START OF THE PROJECT GUTENBERG EBOOK THE FAIRY-LAND OF SCIENCE ***











The Fairy-Land of Science

Arabella B. Buckley




TABLE OF CONTENTS

Lecture I    The Fairy-Land of Science; How to Enter It;
             How to Use It; And How to Enjoy It
Lecture II   Sunbeams, and the Work They Do
Lecture III  The Aerial Ocean in Which We Live
Lecture IV   A Drop of Water on its Travels
Lecture V    The Two Great Sculptors - Water and Ice
Lecture VI   The Voices of Nature, and How We Hear Them
Lecture VII  The Life of a Primrose
Lecture VIII The History of a Piece of Coal
Lecture IX   Bees in the Hive
Lecture X    Bees and Flowers



Week 1

LECTURE I

HOW TO ENTER IT; HOW TO USE IT; AND HOW TO ENJOY IT

I HAVE promised to introduce you today to the fairy-land of
science - a somewhat bold promise, seeing that most of you
probably look upon science as a bundle of dry facts, while fairy-
land is all that is beautiful, and full of poetry and
imagination.  But I thoroughly believe myself, and hope to prove
to you, that science is full of beautiful pictures, of real
poetry, and of wonder-working fairies; and what is more, I
promise you they shall be true fairies, whom you will love just
as much when you are old and greyheaded as when you are young;
for you will be able to call them up wherever you wander by land
or by sea, through meadow or through wood, through water or
through air; and though they themselves will always remain
invisible, yet you will see their wonderful poet at work
everywhere around you.

Let us first see for a moment what kind of tales science has to
tell, and how far they are equal to the old fairy tales we all
know so well.  Who does not remember the tale of the "Sleeping
Beauty in the Wood," and how under the spell of the angry fairy
the maiden pricked herself with the spindle and slept a hundred
years?  How the horses in the stall, the dogs in the court-yard,
the doves on the roof, the cook who was boxing the scullery boy's
ears in the kitchen, and the king and queen with all their
courtiers in the hall remained spell-bound, while a thick hedge
grew up all round the castle and all within was still as death.
But when the hundred years had passed the valiant prince came,
the thorny hedge opened before him bearing beautiful flowers; and
he, entering the castle, reached the room where the princess lay,
and with one sweet kiss raised her and all around her to life
again.

Can science bring any tale to match this?

Tell me, is there anything in this world more busy and active
than water, as it rushes along in the swift brook, or dashes over
the stones, or spouts up in the fountain, or trickles down from
the roof, or shakes itself into ripples on the surface of the
pond as the wind blows over it?  But have you never seen this
water spell-bound and motionless?  Look out of the window some
cold frosty morning in winter, at the little brook which
yesterday was flowing gently past the house, and see how still it
lies, with the stones over which it was dashing now held tightly
in its icy grasp.  Notice the wind-ripples on the pond; they have
become fixed and motionless.  Look up at the roof of the house.
There, instead of living doves merely charmed to sleep, we have
running water caught in the very act of falling and turned into
transparent icicles, decorating the eaves with a beautiful
crystal fringe.  On every tree and bush you will catch the water-
drops napping, in the form of tiny crystals; while the fountain
looks like a tree of glass with long down-hanging pointed leaves.
Even the damp of your own breath lies rigid and still on the
window-pane frozen into delicate patterns like fern-leaves of
ice.

All this water was yesterday flowing busily, or falling drop by
drop, or floating invisibly in the air; now it is all caught and
spell-bound - by whom?  By the enchantments of the frost-giant
who holds it fast in his grip and will not let it go.

But wait awhile, the deliverer is coming.  In a few weeks or
days, or it may be in a few hours, the brave sun will shine down;
the dull-grey, leaden sky will melt before his, as the hedge gave
way before the prince in the fairy tale, and when the sunbeam
gently kisses the frozen water it will be set free.  Then the
brook will flow rippling on again; the frost-drops will be shaken
down from the trees, the icicles fall from the roof, the moisture
trickle down the window-pane, and in the bright, warm sunshine
all will be alive again.

Is not this a fairy tale of nature?  and such as these it is
which science tells.

Again, who has not heard of Catskin, who came out of a hollow
tree, bringing a walnut containing three beautiful dresses - the
first glowing as the sun, the second pale and beautiful as the
moon, the third spangled like the star-lit sky, and each so fine
and delicate that all three could be packed into a walnut shell;
and each one of these tiny structures is not the mere dress but
the home of a living animal.  It is a tiny, tiny shell-palace
made of the most delicate lacework, each pattern being more
beautiful than the last; and what is more, the minute creature
that lives in it has built it out of the foam of the sea, though
he himself is nothing more than a drop of jelly.

Lastly, anyone who has read the 'Wonderful Travellers' must
recollect the man whose sight was so keen that he could hit the
eye of a fly sitting on a tree two miles away.  But tell me, can
you see gas before it is lighted, even when it is coming out of
the gas-jet close to your eyes?  Yet, if you learn to use that
wonderful instrument the spectroscope, it will enable you to tell
one kind of gas from another, even when they are both ninety-one
millions of miles away on the face of the sun; nay more, it will
read for you the nature of the different gases in the far distant
stars, billions of miles away, and actually tell you whether you
could find there any of the same metals which we have on the
earth.

We might find hundreds of such fairy tales in the domain of
science, but these three will serve as examples, and we much pass
on to make the acquaintance of the science-fairies themselves,
and see if they are as real as our old friends.

Tell me, why do you love fairy-land?  what is its charm?  Is it
not that things happen so suddenly, so mysteriously, and without
man having anything to do with it?  In fairy-land, flowers blow,
houses spring up like Aladdin's palace in a single night, and
people are carried hundreds of miles in an instant by the touch
of a fairy wand.

And then this land is not some distant country to which we can
never hope to travel.  It is here in the midst of us, only our
eyes must be opened or we cannot see it.  Ariel and Puck did not
live in some unknown region.  On the contrary, Ariel's song is

 "Where the bee sucks, there suck I;
  In a cowslip's bell I lie;
  There I couch when owls do cry.
  On the bat's back I do fly,
  After summer, merrily."

The peasant falls asleep some evening in a wood and his eyes are
opened by a fairy wand, so that he sees the little goblins and
imps dancing around him on the green sward, sitting on mushrooms,
or in the heads of the flowers, drinking out of acorn-cups,
fighting with blades of grass, and riding on grasshoppers.

So, too, the gallant knight, riding to save some poor oppressed
maiden, dashes across the foaming torrent; and just in the
middle, as he is being swept away, his eyes are opened, and he
sees fairy water-nymphs soothing his terrified horse and guiding
him gently to the opposite shore.  They are close at hand, these
sprites, to the simple peasant or the gallant knight, or to
anyone who has the gift of the fairies and can see them.  but the
man who scoffs at them, and does not believe in them or care for
them, he never sees them.  Only now and then they play him an
ugly trick, leading him into some treacherous bog and leaving him
to get out as he may.

Now, exactly all this which is true of the fairies of our
childhood is true too of the fairies of science.  There are
forces around us, and among us, which I shall ask you to allow me
to call fairies, and these are ten thousand times more wonderful,
more magical, and more beautiful in their work, than those of the
old fairy tales.  They, too, are invisible, and many people live
and die without ever seeing them or caring to see them.  These
people go about with their eyes shut, either because they will
not open them, or because no one has taught them how to see.
They fret and worry over their own little work and their own
petty troubles, and do not know how to rest and refresh
themselves, by letting the fairies open their eyes and show them
the calm sweet picture of nature.  They are like Peter Bell of
whom Wordsworth wrote:-

 "A primrose by a river's brim
  A yellow primrose was to him,
  And it was nothing more."

But we will not be like these, we will open our eyes and ask,
"What are these forces or fairies, and how can we see them?"

Just go out into the country, and sit down quietly and watch
nature at work.  Listen to the wind as it blows, look at the
clouds rolling overhead, and waves rippling on the pond at your
feet.  Hearken to the brook as it flows by, watch the flower-buds
opening one by one, and then ask yourself, "How all this is
done?"  Go out in the evening and see the dew gather drop by drop
upon the grass, or trace the delicate hoar-frost crystals which
bespangle every blade on a winter's morning.  Look at the vivid
flashes of lightening in a storm, and listen to the pealing
thunder: and then tell me, by what machinery is all this
wonderful work done?  Man does none of it, neither could he stop
it if he were to try; for it is all the work of those invisible
forces or fairies whose acquaintance I wish you to make.  Day and
night, summer and winter, storm or calm, these fairies are at
work, and we may hear them and know them, and make friends of
them if we will.

There is only one gift we must have before we can learn to know
them - we must have imagination.  I do not mean mere fancy, which
creates unreal images and impossible monsters, but imagination,
the power of making pictures or images in our mind, of that which
is, though it is invisible to us.  Most children have this
glorious gift, and love to picture to themselves all that is told
them, and to hear the same tale over and over again till they see
every bit of it as if it were real.  This is why they are sure to
love science it its tales are told them aright; and I, for one,
hope the day may never come when we may lose that childish
clearness of vision, which enables us through the temporal things
which are seen, to realize those eternal truths which are unseen.

If you have this gift of imagination come with me, and in these
lectures we will look for the invisible fairies of nature.

Watch a shower of rain.  Where do the drops come from? and why
are they round, or rather slightly oval?  In our fourth lecture
we shall se that the little particles of water of which the
raindrops are made, were held apart and invisible in the air by
heat, one of the most wonderful of our forces* or fairies, till
the cold wind passed by and chilled the air.  Then, when there
was no longer so much heat, another invisible force, cohesion,
which is always ready and waiting, seized on the tiny particles
at once, and locked them together in a drop, the closest form in
which they could lie.  Then as the drops became larger and larger
they fell into the grasp of another invisible force, gravitation,
which dragged them down to the earth, drop by drop, till they
made a shower of rain.  Pause for a moment and think.  You have
surely heard of gravitation, by which the sun holds the earth and
the planets, and keeps them moving round him in regular order?
Well, it is this same gravitation which is a t work also whenever
a shower of rain falls to the earth.  Who can say that he is not
a great invisible giant, always silently and invisibly toiling in
great things and small whether we wake or sleep?

*(I am quite aware of the danger incurred by using this word
"force", especially in the plural; and how even the most modest
little book may suffer at the hands of scientific purists by
employing it rashly.  As, however, the better term "energy" would
not serve here, I hope I may be forgiven for retaining the much-
abused term, especially as I sin in very good company.)

Now the shower is over, the sun comes out and the ground is soon
as dry as though no rain had fallen.  Tell me; what has become of
the rain-drops?  Part no doubt have sunk into the ground, and as
for the rest, why you will say the sun has dried them up.  Yes,
but how?  The sun is more than ninety-one millions of miles away;
how has he touched the rain-drops?  Have you ever heard that
invisible waves are travelling every second over the space
between the sun and us?  We shall see in the next lecture how
these waves are the sun's messengers to the earth, and how they
tear asunder the rain-drops on the ground, scattering them in
tiny particles too small for us to see, and bearing them away to
the clouds.  Here are more invisible fairies working every moment
around you, and you cannot even look out of the window without
seeing the work they are doing.

If, however, the day is cold and frosty, the water does not fall
in a shower of rain; it comes down in the shape of noiseless
snow.  Go out after such a snow-shower, on a calm day, and look
at some of the flakes which have fallen; you will see, if you
choose good specimens, that they are not mere masses of frozen
water, but that each one is a beautiful six-pointed crystal star.
How have these crystals been built up?  What power has been at
work arranging their delicate forms?  In the fourth lecture we
shall see that up in the clouds another of our invisible fairies,
which, for want of a better name, we call the "force of
crystallization," has caught hold of the tiny particles of water
before "cohesion" had made them into round drops, and there
silently but rapidly, has moulded them into those delicate
crystal starts know as "snowflakes".

And now, suppose that this snow-shower has fallen early in
February; turn aside for a moment from examining the flakes, and
clear the newly-fallen snow from off the flower-bed on the lawn.
What is this little green tip peeping up out of the ground under
the snowy covering?  It is a young snowdrop plant.  Can you tell
me why it grows? where it finds its food? what makes it spread
out its leaves and add to its stalk day by day?  What fairies are
at work here?

First there is the hidden fairy "life," and of her even our
wisest men know but little.  But they know something of her way
of working, and in Lecture VII we shall learn how the invisible
fairy sunbeams have been buy here also; how last year's snowdrop
plant caught them and stored them up in it's bulb, and how now in
the spring, as soon as warmth and moisture creep down into the
earth, these little imprisoned sun-waves begin to be active,
stirring up the matter in the bulb, and making it swell and burst
upwards till it sends out a little shoot through the surface of
the soil.  Then the sun-waves above-ground take up the work, and
form green granules in the tiny leaves, helping them to take food
out of the air, while the little rootlets below are drinking
water out of the ground.  The invisible life and invisible
sunbeams are busy here, setting actively to work another fairy,
the force of "chemical attraction," and so the little snowdrop
plant grows and blossoms, without any help from you or me.



Week 2

One picture more, and then I hope you will believe in my fairies.
From the cold garden, you run into the house, and find the fire
laid indeed in the grate, but the wood dead and the coals black,
waiting to be lighted.  You strike a match, and soon there is a
blazing fire.  Where does the heat come from?  Why do the coals
burn and give out a glowing light?  Have you not read of gnomes
buried down deep in the earth, in mines, and held fast there till
some fairy wand has released them, and allowed them to come to
earth again?  Well, thousands and millions of years ago, those
coals were plants; and like the snowdrop in the garden of to-day,
they caught the sunbeams and worked them into their leaves.  Then
the plants died and were buried deep in the earth and the
sunbeams with them; and like the gnomes they lay imprisoned till
the coals were dug out by the miners, and brought to your grate;
and just now you yourself took hold of the fairy wand which was
to release them.  You struck a match, and its atoms clashing with
atoms of oxygen in the air, set the invisible fairies "heat" and
"chemical attraction" to work, and they were soon busy within the
wood and the coals causing their atoms too to clash; and the
sunbeams, so long imprisoned, leapt into flame.  Then you spread
out your hands and cried, "Oh, how nice and warm!" and little
thought that you were warming yourself with the sunbeams of ages
and ages ago.

This is no fancy tale; it is literally true, as we shall see in
Lecture VIII, that the warmth of a coal fire could not exist if
the plants of long ago had not used the sunbeams to make their
leaves, holding them ready to give up their warmth again whenever
those crushed leaves are consumed.

Now, do you believe in, and care for, my fairy-land?  Can you see
in your imagination fairy 'Cohesion' ever ready to lock atoms
together when they draw very near to each other: or fairy
'Gravitation' dragging rain-drops down to the earth: or the fairy
of 'Crystallization' building up the snow-flakes in the clouds?
Can you picture tiny sunbeam-waves of light and heat travelling
from the sun to the earth?  Do you care to know how another
strange fairy, 'Electricity,' flings the lightning across the sky
and causes the rumbling thunder?  Would you like to learn how the
sun makes pictures of the world on which he shines, so that we
can carry about with us photographs or sun-pictures of all the
beautiful scenery of the earth?  And have you any curiosity about
'Chemical action,' which works such wonders in air, and land, and
sea?  If you have any wish to know and make friends of these
invisible forces, the next question is

How are you to enter the fairy-land of science?

There is but one way.  Like the knight or peasant in the fairy
tales, you must open you eyes.  There is no lack of objects,
everything around you will tell some history if touched with the
fairy wand of imagination.  I have often thought, when seeing
some sickly child drawn along the street, lying on its back while
other children romp and play, how much happiness might be given
to sick children at home or in hospitals, if only they were told
the stories which lie hidden in the things around them.  They
need not even move from their beds, for sunbeams can fall on them
there, and in a sunbeam there are stories enough to occupy a
month.  The fire in the grate, the lamp by the bedside, the water
in the tumbler, the fly on the ceiling above, the flower in the
vase on the table, anything, everything, has its history, and can
reveal to us nature's invisible fairies.

Only you must with to see them.  If you go through the world
looking upon everything only as so much to eat, to drink, and to
use, you will never see the fairies of science.  But if you ask
yourself why things happen, and how the great God above us has
made and governs this world of ours; If you listen to the wind,
and care to learn why it blows; if you ask the little flower why
it opens in the sunshine and closes in the storm; and if when you
find questions you cannot answer, you will take the trouble to
hunt out in books, or make experiments to solve your own
questions, then you will learn to know and love those fairies.

Mind, I do not advise you to be constantly asking questions of
other people; for often a question quickly answered is quickly
forgotten, but a difficulty really hunted down is a triumph for
ever.  For example, if you ask why the rain dries up from the
ground, most likely you will be answered, "that the sun dries
it," and you will rest satisfied with the sound of the words.
But if you hold a wet handkerchief before the fire and see the
damp rising out of it, then you have some real idea how moisture
may be drawn up by heat from the earth.

A little foreign niece of mine, only four years old, who could
scarcely speak English plainly, was standing one morning near the
bedroom window and she noticed the damp trickling down the
window-pane.  "Auntie," she said, "what for it rain inside?"  It
was quite useless to explain to her in words, how our breath had
condensed into drops of water upon the cold glass; but I wiped
the pane clear, and breathed on it several times.  When new drops
were formed, I said, "Cissy and auntie have done like this all
night in the room."  She nodded her little head and amused
herself for a long time breathing on the window-pane and watching
the tiny drops; and about a month later, when we were travelling
back to Italy, I saw her following the drops on the carriage
window with her little finger, and heard her say quietly to
herself, "Cissy and auntie made you."  Had not even this little
child some real picture in her mind of invisible water coming
from her mouth, and making drops upon the window-pane?

Then again, you must learn something of the language of science.
If you travel in a country with no knowledge of its language, you
can learn very little about it: and in the same way if you are to
go to books to find answers to your questions, you must know
something of the language they speak.  You need not learn hard
scientific names, for the best books have the fewest of these,
but you must really understand what is meant by ordinary words.

For example, how few people can really explain the difference
between a solid, such as the wood of the table; a liquid, as
water; and a gas, such as I can let off from this gas-jet by
turning the tap.  And yet any child can make a picture of this in
his mind if only it has been properly put before him.

All matter in the world is made up of minute parts or particles;
in a solid these particles are locked together so tightly that
you must tear them forcibly apart if you with to alter the shape
of the solid piece.  If I break or bend this wood I have to force
the particles to move round each other, and I have great
difficulty doing it.  But in a liquid, though the particles are
still held together, they do not cling so tightly, but are able
to roll or glide round each other, so that when you pour water
out of a cup on to a table, it loses its cuplike shape and
spreads itself out flat.  Lastly, in a gas the particles are no
longer held together at all, but they try to fly away from each
other; and unless you shut a gas in tightly and safely, it will
soon have spread all over the room.

A solid, therefore, will retain the same bulk and shape unless
you forcibly alter it; a liquid will retain the same bulk, but no
the same shape if it be left free; a gas will not retain either
the same bulk or the same shape, but will spread over as large a
space as it can find wherever it can penetrate.  Such simple
things as these you must learn from books and by experiment.

Then you must understand what is meant by chemical attraction;
and though I can explain this roughly here, you will have to make
many interesting experiments before you will really learn to know
this wonderful fairy power.  If I dissolve sugar in water, though
it disappears it still remains sugar, and does not join itself to
the water.  I have only to let the cup stand till the water
dries, and the sugar will remain at the bottom.  There has been
no chemical attraction here.

But now I will put something else in water which will call up the
fairy power.  Here is a little piece of the metal potassium, one
of the simple substances of the earth; that is to say, we cannot
split it up into other substances, wherever we find it, it is
always the same.  Now if I put this piece of potassium on the
water it does not disappear quietly like the sugar.  See how it
rolls round and round, fizzing violently with a blue flame
burning round it, and at last goes off with a pop.

What has been happening here?

You must first know that water is made of two substances,
hydrogen and oxygen, and these are not merely held together, but
are joined to completely that they have lost themselves and have
become water; and each atom of water is made of two atoms of
hydrogen and one of oxygen.

Now the metal potassium is devotedly fond of oxygen, and the
moment I threw it on the water it called the fairy "chemical
attraction' to help it, and dragged the atoms of oxygen out of
the water and joined them to itself.  In doing this it also
caught part of the hydrogen, but only half, and so the rest was
left out in the cold.  No, not in the cold! for the potassium and
oxygen made such a great heat in clashing together that the rest
of the hydrogen became very hot indeed, and sprang into the air
to find some other companion to make up for what it had lost.
Here it found some free oxygen floating about, and it seized upon
it so violently, that they made a burning flame, while the
potassium with its newly found oxygen and hydrogen sank down
quietly into the water as potash.  And so you see we have got
quite a new substance potash in the basin; made with a great deal
of fuss by chemical attraction drawing different atoms together.

When you can really picture this power to yourself it will help
you very much to understand what you read and observe about
nature.

Next, as plants grow around you on every side, and are of so much
importance in the world, you must also learn something of the
names of the different parts of a flower, so that you may
understand those books which explain how a plant grows and lives
and forms its seeds.  You must also know the common names of the
parts of an animal, and of your own body, so that you may be
interested in understanding the use of the different organs; how
you breathe, and how your blood flows; how one animal walks,
another flies, and another swims.  Then you must learn something
of the various parts of the world, so that you may know what is
meant by a river, a plain, a valley, or a delta.  All these
things are not difficult, you can learn them pleasantly from
simple books on physics, chemistry, botany, physiology, and
physical geography; and when you understand a few plain
scientific terms, then all by yourself, if you will open your
eyes and ears, you may wander happily in the fairy-land of
science.  Then wherever you go you will find

 "Tongues in trees, books in the running brooks
  Sermons in stones, and good in everything."

And now we come to the last part of our subject.  When you have
reached and entered the gates of science, how are you to use and
enjoy this new and beautiful land?

This is a very important question for you may make a twofold use
of it.  If you are only ambitious to shine in the world, you may
use it chiefly to get prizes, to be at the top of your class, or
to pass in examinations; but if you also enjoy discovering its
secrets, and desire to learn more and more of nature and to revel
in dreams of its beauty, then you will study science for its own
sake as well.  Now it is a good thing to win prizes and be at the
top of your class, for it shows that you are industrious; it is a
good thing to pass well in examinations , for it show that you
are accurate; but if you study science for this reason only, do
not complain if you find it full, and dry, and hard to master.
You may learn a great deal that is useful, and nature will answer
you truthfully if you ask you questions accurately, but she will
give you dry facts, just such as you ask for.  If you do not love
her for herself she will never take you to her heart.

This is the reason why so many complain that science is dry and
uninteresting.  They forget that though it is necessary to learn
accurately, for so only we can arrive at truth, it is equally
necessary to love knowledge and make it lovely to those who
learn, and to do this we must get at the spirit which lies under
the facts.  What child which loves its mother's face is content
to know only that she has brown eyes, a straight nose, a small
mouth, and hair arranged in such and such a manner?  No, it knows
that its mother has the sweetest smile of any woman living; that
her eyes are loving, her kiss is sweet, and that when she looks
grave, then something is wrong which must be put right.  And it
is in this way that those who wish to enjoy the fairy-land of
science must love nature.

It is well to know that when a piece of potassium is thrown on
water the change which takes place is expressed by the formula K +
H2O = KHO + H. But it is better still to have a mental picture of
the tiny atoms clasping each other, and mingling so as to make a
new substance, and to feel how wonderful are the many changing
forms of nature. It is useful to be able to classify a flower and
to know that the buttercup belongs to the Family Ranunculaceae,
with petals free and definite, stamens hypogynous and indefinite,
pistil apocarpous. But it is far sweeter to learn about the life
of the little plant, to understand why its peculiar flower is
useful to it, and how it feeds itself, and makes its seed. No one
can love dry facts; we must clothe them with real meaning and love
the truths they tell, if we wish to enjoy science.

Let us take an example to show this.  I have here a branch of
white coral, a beautiful, delicate piece of nature's work.  We
will begin by copying a description of it from one of those
class-books which suppose children to learn words like parrots,
and to repeat them with just as little understanding.

"Coral is formed by an animal belonging to the kingdom of
Radiates, sub-kingdom Polypes.  The soft body of the animal is
attached to a support, the mouth opening upwards in a row of
tentacles.  The coral is secreted in the body of the polyp out of
the carbonate of lime in the sea.  Thus the coral animalcule
rears its polypidom or rocky structure in warm latitudes, and
constructs reefs or barriers round islands.  It is limited in
rage of depth from 25 to 30 fathoms.  Chemically considered,
coral is carbonate of like; physiologically, it is the skeleton
of an animal; geographically, it is characteristic of warm
latitudes, especially of the Pacific Ocean."  This description is
correct, and even fairly complete, if you know enough of the
subject to understand it.  But tell me, does it lead you to love
my piece of coral?  Have you any picture in your mind of the
coral animal, its home, or its manner of working?

But now, instead of trying to master this dry, hard passage, take
Mr. Huxley's penny lecture on 'Coral and Coral Reefs,' and with
the piece of coral in your hand try really to learn its history.
You will then be able to picture to yourself the coral animal as
a kind of sea-anemone, something like those which you have often
seen, like red, blue, or green flowers, putting out feelers in
sea-water on our coasts, and drawing in the tiny sea-animals to
digest them in that bag of fluid which serves the sea-anemone as
a stomach.  You will learn how this curious jelly animal can
split itself in two, and so form two polyps, or send a bud out of
its side and so grow up into a kind of "tree or bush of polyps,"
or how it can hatch little eggs inside it and throw out young
ones from its mouth, provided with little hairs, by means of
which they swim to new resting-places.  You will learn the
difference between the animal which builds up the red coral as
its skeleton, and the group of animals which build up the white;
and you will look with new interest on our piece of white coral,
as you read that each of those little sups on its stem with
delicate divisions like the spokes of a wheel has been the home
of a separate polyp, and that from the sea-water each little
jelly animal has drunk in carbonate of lime as you drink in sugar
dissolved in water, and then has used it grain by grain to build
that delicate cup and add to the coral tree.

We cannot stop to examine all about coral now, we are only
learning how to learn, but surely our specimen is already
beginning to grow interesting; and when you have followed it out
into the great Pacific Ocean, where the wild waves dash
restlessly against the coral trees, and have seen these tiny
drops of jelly conquering the sea and building huge walls of
stone against the rough breakers, you will hardly rest till you
know all their history.  Look at that curious circular island in
the picture, covered with palm trees; it has a large smooth lake
in the middle, and the bottom of this lake is covered with blue,
red, and green jelly animals, spreading out their feelers in the
water and looking like beautiful flowers, and all round the
outside of the island similar animals are to be seen washed by
the sea waves.  Such islands as this have been build entirely by
the coral animals, and the history of the way in which the reefs
have sunk gradually down, as the tiny creatures added to them
inch by inch, is as fascinating as the story of the building of
any fairy palace in the days of old.  Read all this, and then if
you have no coral of your own to examine, go to the British
Museum and see the beautiful specimens in the glass cases there,
and think that they have been built up under the rolling surf by
the tiny jelly animals; and then coral will become a real living
thing to you, and you will love the thoughts it awakens.

But people often ask, what is the use of learning all this?  If
you do not feel by this time how delightful it is to fill your
mind with beautiful pictures of nature, perhaps it would be
useless to say more.  But in this age of ours, when restlessness
and love of excitement pervade so many lives, is it nothing to be
taken out of ourselves and made to look at the wonders of nature
going on around us?  Do you never feel tired and "out of sorts,"
and want to creep away from your companions, because they are
merry and you are not?  Then is the time to read about the
starts, and how quietly they keep their course from age to age;
or to visit some little flower, and ask what story it has to
tell; or to watch the clouds, and try to imagine how the winds
drive them across the sky.  No person is so independent as he who
can find interest in a bare rock, a drop of water, the foam of
the sea, the spider on the wall, the flower underfoot or the
starts overhead.  And these interests are open to everyone who
enters the fairy-land of science.

Moreover, we learn from this study to see that there is a law and
purpose in everything in the Universe, and it makes us patient
when we recognize the quiet noiseless working of nature all
around us.  Study light, and learn how all colour, beauty, and
life depend on the sun's rays; note the winds and currents of the
air, regular even in their apparent irregularity, as they carry
heat and moisture all over the world.  Watch the water flowing in
deep quiet streams, or forming the vast ocean; and then reflect
that every drop is guided by invisible forces working according
to fixed laws.  See plants springing up under the sunlight, learn
the secrets of plant life, and how their scents and colours
attract the insects.  Read how insects cannot live without
plants, nor plants without the flitting butterfly or the busy
bee.  Realize that all this is worked by fixed laws, and that out
of it (even if sometimes in suffering and pain) springs the
wonderful universe around us.  And then say, can you fear for
your own little life, even though it may have its troubles?  Can
you help feeling a part of this guided and governed nature? or
doubt that the power which fixed the laws of the stars and of the
tiniest drop of water - that made the plant draw power from the
sun, the tine coral animal its food from the dashing waves; that
adapted the flower to the insect and the insect to the flower -
is also moulding your life as part of the great machinery of the
universe, so that you have only to work, and to wait, and to
love?

We are all groping dimly for the Unseen Power, but no one who
loves nature and studies it can ever feel alone or unloved in the
world.  Facts, as mere facts, are dry and barren, but nature is
full of life and love, and her calm unswerving rule is tending to
some great though hidden purpose.  You may call this Unseen Power
what you will - may lean on it in loving, trusting faith, or bend
in reverent and silent awe; but even the little child who lives
with nature and gazes on her with open eye, must rise in some
sense or other through nature to nature's God.



Week 3

Lecture II Sunbeams and How They Work

Who does not love the sunbeams, and feel brighter and merrier as
he watches them playing on the wall, sparkling like diamonds on
the ripples of the sea, or making bows of coloured light on the
waterfall?  Is not the sunbeam so dear to us that it has become a
household word for all that is merry and gay? and when we want to
describe the dearest, busiest little sprite amongst us, who wakes
a smile on all faces wherever she goes, do we not call her the
"sunbeam of the house"?

And yet how little even the wisest among us know about the nature
and work of these bright messengers of the sun as they dart
across space!

Did you ever wake quite early in the morning, when it was pitch-
dark and you could see nothing, not even your own hand; and then
lie watching as time went on till the light came gradually
creeping in at the window?  If you have done this you will have
noticed that you can at first only just distinguish the dim
outline of the furniture; then you can tell the difference
between the white cloth on the table and the dark wardrobe beside
it; then by degrees all the smaller details, the handles of the
drawer, the pattern on the wall, and the different colours of all
the objects in the room become clearer and clearer till at last
you see all distinctly in broad daylight.

What has been happening here? and why have the things in the room
become visible by such slow degrees?  We say that the sun is
rising, but we know very well that it is not the sun which moves,
but that our earth has been turning slowly round, and bringing
the little spot on which we live face to face with the great
fiery ball, so that his beams can fall upon us.

Take a small globe, and stick a piece of black plaster over
England, then let a lighted lamp represent the sun, and turn the
globe slowly, so that the spot creeps round from the dark side
away from the lamp, until it catches, first the rays which pass
along the side of the globe, then the more direct rays, and at
last stands fully in the blaze of the light.  Just this was
happening to our spot of the world as you lay in bed and saw the
light appear; and we have to learn today what those beams are
which fall upon us and what they do for us.

First we must learn something about the sun itself, since it is
the starting-place of all the sunbeams.  If the sun were a dark
mass instead of a fiery one we should have none of these bright
cheering messengers, and though we were turned face to face with
him every day we should remain in one cold eternal night.  Now
you will remember we mentioned in the last lecture that it is
heat which shakes apart the little atoms of water and makes them
gloat up in the air to fall again as rain; and that if the day is
cold they fall as snow, and all the water is turned into ice.
But if the sun were altogether dark, think how bitterly cold it
would be; far colder than the most wintry weather ever known,
because in the bitterest night some warmth comes out of the
earth, where it has been stored from the sunlight which fell
during the day.  But if we never received any warmth at all, no
water would ever rise up into the sky, no rain ever fall, no
rivers flow, and consequently no plants could grow and no animals
live.  All water would be in the form of snow and ice, and the
earth would be one great frozen mass with nothing moving upon it.

So you see it becomes very interesting for us to learn what the
sun is, and how he sends us his beams.  How far away from us do
you think he is?  On a fine summer's day when we can see him
clearly, it looks as if we had only to get into a balloon and
reach him as he sits in the sky, and yet we know roughly that he
is more than ninety-one millions of miles distant from our earth.

These figures are so enormous that you cannot really grasp them.
But imagine yourself in an express train, travelling at the
tremendous rate of sixty miles an hour and never stopping.  At
that rate, if you wished to arrive at the sun today you would
have been obliged to start 171 years ago.  That is, you must have
set off in the early part of the reign of Queen Anne, and you
must have gone on, never, never resting, through the reigns of
George I, George ii, and the long reign of George III, then
through those of George IV, William IV, and Victoria, whirling on
day and night at express speed, and at last, today, you would
have reached the sun!

And when you arrived there, how large do you think you would find
him to be?  Anaxagoras, a learned Greek, was laughed at by all
his fellow Greeks because he said that the sun was as large as
the Peloponne-sus, that is about the size of Middlesex.  How
astonished they would have been if they could have known that not
only is he bigger than the whole of Greece, but more than a
million times bigger than the whole world!

Our world itself is a very large place, so large that our own
country looks only like a tiny speck upon it, and an express
train would take nearly a month to travel round it.  Yet even our
whole globe is nothing in size compared to the sun, for it only
measures 8000 miles across, while the sun measures more the
852,000.

Imagine for a moment that you could cut the sun and the earth
each in half as you would cut an apple; then if you were to lay
the flat side of the half-earth on the flat side of the half sun
it would take 106 such earths to stretch across the face of the
sun.  One of these 106 round spots on the diagram represents the
size which our earth would look if placed on the sun; and they
are so tiny compared to him that they look only like a string of
minute beads stretched across his face.  Only think, then, how
many of these minute dots would be required to fill the whole of
the inside of Fig. 4, if it were a globe.

One of the best ways to form an idea of the whole size of the sun
is to imagine it to be hollow, like an air-ball, and then see how
many earths it would take to fill it.  You would hardly believe
that it would take one million, three hundred and thirty-one
thousand globes the size of our world squeezed together.  Just
think, if a huge giant could travel all over the universe and
gather worlds, all as big as ours, and were to make first a heap
of merely ten such worlds, how huge it would be!  Then he must
have a hundred such heaps of ten to make a thousand world; and
then he must collect again a thousand times that thousand to make
a million, and when he had stuffed them all into the sun-ball he
would still have only filled three-quarters of it!

After hearing this you will not be astonished that such a monster
should give out an enormous quantity of light and heat; so
enormous that it is almost impossible to form any idea of it.
Sir John Herschel has, indeed, tried to picture it for us.  He
found that a ball of lime with a flame of oxygen and hydrogen
playing round it (such as we use in magic lanterns and call oxy-
hydrogen light) becomes so violently hot that it gives the most
brilliant artificial light we can get - such that you cannot put
your eye near it without injury.  Yet if you wanted to have a
light as strong as that of our sun, it would not be enough to
make such a lime-ball as big as the sun is.  No, you must make it
as big as 146 suns, or more than 146,000,000 times as big as our
earth, in order to get the right amount of light.  Then you would
have a tolerably good artificial sun; for we know that the body
of the sun gives out an intense white light, just as the lime-
ball does, and that , like it, it has an atmosphere of glowing
gases round it.

But perhaps we get the best idea of the mighty heat and light of
the sun by remembering how few of the rays which dart out on all
sides from this fiery ball can reach our tiny globe, and yet how
powerful they are.  Look at the globe of a lamp in the middle of
the room, and see how its light pours out on all sides and into
every corner; then take a grain of mustard-seed, which will very
well represent the comparative size of our earth, and hold it up
at a distance from the lamp.  How very few of all those rays
which are filling the room fall on the little mustard-seed, and
just so few does our earth catch of the rays which dart out from
the sun.  And yet this small quantity (1/2000-millionth part of
the whole) does nearly all the work of our world.  (These and the
preceding numerical statements will be found worked out in Sir J.
Herschel's 'Familiar Lectures on Scientific Subjects,' 1868, from
which many of the facts in the first part of the lecture are
taken.)

In order to see how powerful the sun's rays are, you have only to
take a magnifying glass and gather them to a point on a piece of
brown paper, for they will set the paper alight.  Sir John
Herschel tells us that at the Cape of Good Hope the heat was even
so great that he cooked a beefsteak and roasted some eggs by
merely putting them in the sun, in a box with a glass lid!
Indeed, just as we should all be frozen to death if the sun were
sold, so we should all be burnt up with intolerable heat if his
fierce rays fell with all their might upon us.  But we have an
invisible veil protecting us, made - of what do you think?  Of
those tiny particles of water which the sunbeams draw up and
scatter in the air, and which, as we shall see in Lecture IV, cut
off part of the intense heat and make the air cool and pleasant
for us.



Week 4

We have now learnt something of the distance, the size, the
light, and the heat of the sun - the great source of the
sunbeams.  But we are as yet no nearer the answer to the
question, What is a sunbeam? how does the sun touch our earth?

Now suppose I with to touch you from this platform where I stand,
I can do it in two ways.  Firstly, I can throw something at you
and hit you - in this case a thing will have passed across the
space from me to you.  Or, secondly, if I could make a violent
movement so as to shake the floor of the room, you would feel a
quivering motion; and so I should touch you across the whole
distance of the room.  But in this case no thing would have
passed from me to you but a movement or wave, which passed along
the boards of the floor.  Again, if I speak to you, how does the
sound reach you ear?  Not by anything being thrown from my mouth
to your ear, but by the motion of the air.  When I speak I
agitate the air near my mouth, and that makes a wave in the air
beyond, and that one, another, and another (as we shall see more
fully in Lecture VI) till the last wave hits the drum of your
ear.

Thus we see there are two ways of touching anything at a
distance; 1st, by throwing some thing at it and hitting it; 2nd,
by sending a movement of wave across to it, as in the case of the
quivering boards and the air.

Now the great natural philosopher Newton thought that the sun
touched us in the first of these ways, and that sunbeams were
made of very minute atoms of matter thrown out by the sun, and
making a perpetual cannonade on our eyes.  It is easy to
understand that this would make us see light and feel heat, just
as a blow in the eye makes us see starts, or on the body makes it
feel hot: and for a long time this explanation was supposed to be
the true one.  But we know now that there are many facts which
cannot be explained on this theory, though we cannot go into them
here.  What we will do, is to try and understand what now seems
to be the true explanation of the sunbeam.

About the same time that Newton wrote, a Dutchman, named
Huyghens, suggested that light comes from the sun in tiny waves,
travelling across space much in the same way as ripples travel
across a pond.  The only difficulty was to explain in what
substance these waves could be travelling: not through water, for
we know that there is no water in space - nor through air, for
the air stops at a comparatively short distance from our earth.
There must then be something filling all space between us and the
sun, finer than either water or air.

And now I must ask you to use all you imagination, for I want you
to picture to yourselves something quite as invisible as the
Emperor's new clothes in Andersen's fairy-tale, only with this
difference, that our invisible something is very active; and
though we can neither see it nor touch it we know it by its
effects.  You must imagine a fine substance filling all space
between us and the sun and the starts.  A substance so very
delicate and subtle, that not only is it invisible, but it can
pass through solid bodies such as glass, ice, or even wood or
brick walls.  This substance we call "ether."  I cannot give you
here the reasons why we must assume that it is throughout all
space; you must take this on the word of such men as Sir John
Herschel or Professor Clerk-Maxwell, until you can study the
question for yourselves.

Now if you can imagine this ether filling every corner of space,
so that it is everywhere and passes through everything, ask
yourselves, what must happen when a great commotion is going on
in one of the large bodies which float in it?  When the atoms of
the gases round the sun are clashing violently together to make
all its light and heat, do you not think they must shake this
ether all around them?  And then, since the ether stretches on
all sides from the sun to our earth and all other planets, must
not this quivering travel to us, just as the quivering of the
boards would from me to you?  Take a basin of water to represent
the ether, and take a piece of potassium like that which we used
in our last lecture, and hold it with a pair of nippers in the
middle of the water.  You will see that as the potassium hisses
and the flame burns round it, they will make waves which will
travel all over the water to the edge of the basin,, and you can
imagine how in the same way waves travel over the ether from the
sun to us.

Straight away from the sun on all sides, never stopping, never
resting, but chasing after each other with marvellous quickness,
these tiny waves travel out into space by night and by day.  When
our spot of the earth where England lies is turned away from them
and they cannot touch us, then it is night for us, but directly
England is turned so as to face the sun, then they strike on the
land, and the water, and warm it; or upon our eyes, making the
nerves quiver so that we see light.  Look up at the sun and
picture to yourself that instead of one great blow from a fist
causing you to see starts for a moment, millions of tiny blows
from these sun-waves are striking every instant on you eye; then
you will easily understand that his would cause you to see a
constant blaze of light.

But when the sun is away, if the night is clear we have light
from the starts.  Do these then too make waves all across the
enormous distance between them and us?  Certainly they do, for
they too are suns like our own, only they are so far off that the
waves they send are more feeble, and so we only notice them when
the sun's stronger waves are away.

But perhaps you will ask, if no one has ever seen these waves not
the ether in which they are made, what right have we to say they
are there?  Strange as it may seem, though we cannot see them we
have measured them and know how large they are, and how many can
go into an inch of space.  For as these tiny waves are running on
straight forward through the room, if we put something in their
way, they will have to run round it; and if you let in a very
narrow ray of light through a shutter and put an upright wire in
the sunbeam, you actually make the waves run round the wire just
as water runs round a post in a river; and they meet behind the
wire, just as the water meets in a V shape behind the post.  Now
when they meet, they run up against each other, and here it is we
catch them.  Fir if they meet comfortably, both rising up in a
good wave, they run on together and make a bright line of light;
but if they meet higgledy-piggledy, one up and the other down,
all in confusion, they stop each other, and then there is no
light but a line of darkness.  And so behind your piece of wire
you can catch the waves on a piece of paper, and you will find
they make dark and light lines one side by side with the other,
and by means of these bands it is possible to find out how large
the waves must be.  This question is too difficult for us to work
it out here, but you can see that large waves will make broader
light and dark bands than small ones will, and that in this way
the size of the waves may be measured.

And now how large do you think they turn out to be?  so very,
very tiny that about fifty thousand waves are contained in a
single inch of space!  I have drawn on the board the length of an
inch, and now I will measure the same space in the air between my
finger and thumb.  Within this space at this moment there are
fifty thousand tiny waves moving up and down.  I promised you we
would find in science things as wonderful as in fairy tales.  Are
not these tiny invisible messengers coming incessantly from the
sun as wonderful as any fairies? and still more so when, as we
shall see presently, they are doing nearly all the work of our
world.

We must next try to realize how fast these waves travel.  You
will remember that an express train would take 171 years to reach
us from the sun; and even a cannon-ball would take from ten to
thirteen years to come that distance.  Well, these tiny waves
take only seven minutes and a half to come the whole 91 millions
of miles.  The waves which are hitting your eye at this moment
are caused by a movement which began at the sun only 7 1/2
minutes ago.  And remember, this movement is going on
incessantly, and these waves are always following one after the
other so rapidly that they keep up a perpetual cannonade upon the
pupil of your eye.  So fast do they come that about 608 billion
waves enter your eye in one single second.*  I do not ask you to
remember these figures; I only ask you to try and picture to
yourselves these infinitely tiny and active invisible messengers
from the sun, and to acknowledge that light is a fairy thing.
(*Light travels at the rate of 190,000 miles, or 12,165,120,000
inches in a second.  Taking the average number of wave-lengths in
an inch at 50,000, then 12,165,120,000 X 50,000 =
608,256,000,000,000.)

But we do not yet know all about our sunbeams.  See, I have here
a piece of glass with three sides, called a prism.  If I put it
in the sunlight which is streaming through the window, what
happens?  Look! on the table there is a line of beautiful
colours.  I can make it long or short, as I turn the prism, but
the colours always remain arranged in the same way.  Here at my
left hand is the red, beyond it orange, then yellow, green, blue,
indigo or deep blue, and violet, shading one into the other all
along the line.  We have all seen these colours dancing on the
wall when the sun has been shining brightly on the cut-glass
pendants of the chandelier, and you may see them still more
distinctly if you let a ray of light into a darkened room, and
pass it through the prism as in the diagram (Fig. 7).  What are
these colours?  Do they come from the glass?  No; for you will
remember to have seen them in the rainbow, and in the soap-
bubble, and even in a drop of dew or the scum on the top of a
pond.  This beautiful coloured line is only our sunbeam again,
which has been split up into many colours by passing through the
glass, as it is in the rain-drops of the rainbow and the bubbles
of the scum of the pond.



Week 5

Till now we have talked of the sunbeam as if it were made of only
one set of waves of different sizes, all travelling along
together from the sun.  These various waves have been measured,
and we know that the waves which make up red light are larger and
more lazy than those which make violet light, so that there are
only thirty-nine thousand red waves in an inch, while there are
fifty-seven thousand violet waves in the same space.

How is it then, that if all these different waves making
different colours, hit on our eye, they do not always make us see
coloured light?  Because, unless they are interfered with, they
all travel along together, and you know that all colours, mixed
together in proper proportion, make white.

I have here a round piece of cardboard, painted with the seven
colours in succession several times over.  When it is still you
can distinguish them all apart, but when I whirl it quickly round
- see! - the cardboard looks quite white, because we see them all
so instantaneously that they are mingled together.  In the same
way light looks white to you, because all the different coloured
waves strike on your eye at once.  You can easily make on of
these card for yourselves only the white will always look dirty,
because you cannot get the colours pure.

Now, when the light passes through the three-sided glass or
prism, the waves are spread out, and the slow, heavy, red waves
lag behind and remain at the lower end R of the coloured line on
the wall (Fig. 7), while the rapid little violet waves are bent
more out of their road and run to V at the farther end of the
line; and the orange, yellow, green, blue, and indigo arrange
themselves between, according to the size of their waves.

And now you are very likely eager to ask why the quick waves
should make us see one colour, and the slow waves another.  This
is a very difficult question, for we have a great deal still to
learn about the effect of light on the eye.  But you can easily
imagine that colour is to our eye much the same as music is to
our ear.  You know we can distinguish different notes when the
air-waves play slowly or quickly upon the drum of the ear (as we
shall see in Lecture VI) and somewhat in the same way the tiny
waves of the ether play on the retina or curtain at the back of
our eye, and make the nerves carry different messages to the
brain: and the colour we see depends upon the number of waves
which play upon the retina in a second.

Do you think we have now rightly answered the question - What is
a sunbeam?  We have seen that it is really a succession of tiny
rapid waves, travelling from the sun to us across the invisible
substance we call "ether", and keeping up a constant cannonade
upon everything which comes in their way.  We have also seen
that, tiny as these waves are, they can still vary in size, so
that one single sunbeam is made up of myriads of different-sized
waves, which travel all together and make us see white light;
unless for some reason they are scattered apart, so that we see
them separately as red, green, blue, or yellow.  How they are
scattered, and many other secrets of the sun-waves, we cannot
stop to consider not, but must pass on to ask -

What work do the sunbeams do for us?

They do two things - they give us light and heat.  It is by means
of them alone that we see anything.  When the room was dark you
could not distinguish the table, the chairs, or even the walls of
the room.  Why?  Because they had no light-waves to send to your
eye.  But as the sunbeams began to pour in at the window, the
waves played upon the things in the room, and when they hit them
they bounded off them back to your eye, as a wave of the sea
bounds back from a rock and strikes against a passing boat.
Then, when they fell upon your eye, they entered it and excited
the retina and the nerves, and the image of the chair or the
table was carried to your brain.  Look around at all the things
in this room.  Is it not strange to think that each one of them
is sending these invisible messengers straight to your eye as you
look at it; and that you see me, and distinguish me from the
table, entirely by the kind of waves we each send to you?

Some substances send back hardly any waves of light, but let them
all pass through them, and thus we cannot see them.  A pane of
clear glass, for instance, lets nearly all the light-waves pass
through it, and therefore you often cannot see that the glass is
there, because no light-messengers come back to you from it.
Thus people have sometimes walked up against a glass door and
broken it, not seeing it was there.  Those substances are
transparent which, for some reason unknown to us, allow the ether
waves to pass through them without shaking the atoms of which the
substance is made.  In clear glass, for example, all the light-
waves pass through without affecting the substance of the glass;
while in a white wall the larger part of the rays are reflected
back to your eye, and those which pass into the wall, by giving
motion to its atoms lose their own vibrations.

Into polished shining metal the waves hardly enter at all, but
are thrown back from the surface; and so a steel knife or a
silver spoon are very bright, and are clearly seen.  Quicksilver
is put at the back of looking-glasses because it reflects so many
waves.  It not only sends back those which come from the sun, but
those, too, which come from your face.  So, when you see yourself
in a looking-glass, the sun-waves have first played on your face
and bounded off from it to the looking-glass; then, when they
strike the looking-glass, they are thrown back again on to the
retina of your eye, and you see your own face by means of the
very waves you threw off from it an instant before.

But the reflected light-waves do more for us than this.  They not
only make us see things, but they make us see them in different
colours.  What, you will ask, is this too the work of the
sunbeams?  Certainly; for if the colour we see depends on the
size of the waves which come back to us, then we must see things
coloured differently according to the waves they send back.  For
instance, imagine a sunbeam playing on a leaf: part of its waves
bound straight back from it to our eye and make us see the
surface of the leaf, but the rest go right into the leaf itself,
and there some of them are used up and kept prisoners.  The red,
orange, yellow, blue, and violet waves are all useful to the
leaf, and it does not let them go again.  But it cannot absorb
the green waves, and so it throws them back, and they travel to
your eye and make you see a green colour.  So when you say a leaf
is green, you mean that the leaf does not want the green waves of
the sunbeam, but sends them back to you.  In the same way the
scarlet geranium rejects the red waves; this table sends back
brown waves; a white tablecloth sends back nearly the whole of
the waves, and a black coat scarcely any.  This is why, when
there is very little light in the room, you can see a white
tablecloth while you would not be able to distinguish a black
object, because the few faint rays that are there, are all sent
back to you from a white surface.

Is it not curious to think that there is really no such thing as
colour in the leaf, the table, the coat, or the geranium flower,
but we see them of different colours because, for some reason,
they send back only certain coloured waves to our eye?

Wherever you look, then, and whatever you see, all the beautiful
tints, colours, lights, and shades around you are the work of the
tiny sun-waves.

Again, light does a great deal of work when it falls upon plants.
Those rays of light which are caught by the leaf are by no means
idle; we shall see in Lecture VII that the leaf uses them to
digest its food and make the sap on which the plant feeds.



Week 6

We all know that a plant becomes pale and sickly if it has not
sunlight, and the reason is, that without these light-waves it
cannot get food out of the air, nor make the sap and juices which
it needs.  When you look at plants and trees growing in the
beautiful meadows; at the fields of corn, and at the lovely
landscape, you are looking on the work of the tiny waves of
light, which never rest all through the day in helping to give
life to every green thing that grows.

So far we have spoken only of light; but hold your hand in the
sun and feel the heat of the sunbeams, and then consider if the
waves of heat do not do work also.  There are many waves in a
sunbeam which move too slowly to make us see light when they hit
our eye, but we can feel them as heat, though we cannot see them
as light.  The simplest way of feeling heat-waves is to hold a
warm iron near your face.  You know that no light comes from it,
yet you can feel the heat-waves beating violently against your
face and scorching it.  Now there are many of these dark heat-
rays in a sunbeam, and it is they which do most of the work in
the world.

In the first place, as they come quivering to the earth, it is
they which shake the water-drops apart, so that these are carried
up in the air, as we shall see in the next lecture.  And then
remember, it is these drops, falling again as rain, which make
the rivers and all the moving water on the earth.  So also it is
the heat-waves which make the air hot and light, and so cause it
to rise and make winds and air-currents, and these again give
rise to ocean-currents.  It is these dark rays, again, which
strike upon the land and give it the warmth which enables plants
to grow.  It is they also which keep up the warmth in our own
bodies, both by coming to us directly from the sun, and also in a
very roundabout way through plants.  You will remember that
plants use up rays of light and heat in growing; then either we
eat the plants, or animals eat the plants and we eat the animals;
and when we digest the food, that heat comes back in our bodies,
which the plants first took from the sunbeam.  Breathe upon your
hand, and feel how hot your breath is; well, that heat which you
feel, was once in a sunbeam, and has travelled from it through
the food you have eaten, and has now been at work keeping up the
heat of your body.

But there is still another way in which these plants may give out
the heat-waves they have imprisoned.  You will remember how we
learnt in the first lecture that coal is made of plants, and that
the heat they give out is the heat these plants once took in.
Think how much work is done by burning coals.  Not only are our
houses warmed by coal fires and lighted by coal gas, but our
steam-engines and machinery work entirely by water which has been
turned into steam by the heat of coal and coke fire; and our
steamboats travel all over the world by means of the same power.
In the same way the oil of our lamps comes either from olives,
which grow on trees; or from coal and the remains of plants and
animals in the earth.  Even our tallow candles are made of mutton
fat, and sheep eat grass; as so, turn which way we will, we find
that the light and heat on our earth, whether it comes from
fires, or candles, or lamps, or gas, and whether it moves
machinery, or drives a train, or propels a ship, is equally the
work of the invisible waves of ether coming from the sun, which
make what we call a sunbeam.

Lastly, there are still some hidden waves which we have not yet
mentioned, which are not useful to us either as light or heat,
and yet they are not idle.

Before I began this lecture, I put a piece of paper, which had
been dipped in nitrate of silver, under a piece of glass; and
between it and the glass I put a piece of lace.  Look what the
sun has been doing while I have been speaking.  It has been
breaking up the nitrate of silver on the paper and turning it
into a deep brown substance; only where the threads of the lace
were, and the sun could not touch the nitrate of silver, there
the paper has remained light-coloured, and by this means I have a
beautiful impression of the lace on the paper.  I will now dip
the impression into water in which some hyposulphite of soda is
dissolved, and this will "fix" the picture, that is, prevent the
sun acting upon it any more; then the picture will remain
distinct, and I can pass it round to you all.  Here, again,
invisible waves have been at work, and this time neither as light
nor as heat, but as chemical agents, and it is these waves which
give us all our beautiful photographs.  In any toyshop you can
buy this prepared paper, and set the chemical waves at work to
make pictures.  Only you must remember to fix it in the solution
afterwards, otherwise the chemical rays will go on working after
you have taken the lace away, and all the paper will become brown
and your picture will disappear.

And now, tell me, may we not honestly say, that the invisible
waves which make our sunbeams, are wonderful fairy messengers as
they travel eternally and unceasingly across space, never
resting, never tiring in doing the work of our world?  Little as
we have been able to learn about them in one short hour, do they
not seem to you worth studying and worth thinking about, as we
look at the beautiful results of their work?  The ancient Greeks
worshipped the sun, and condemned to death one of their greatest
philosophers, named Anaxagoras, because he denied that it was a
god.  We can scarcely wonder at this when we see what the sun
does for our world; but we know that it is a huge globe made of
gases and fiery matter and not a god.  We are grateful for the
sun instead of to him, and surely we shall look at him with new
interest, now that we can picture his tiny messengers, the
sunbeams, flitting over all space, falling upon our earth, giving
us light to see with, and beautiful colours to enjoy, warming the
air and the earth, making the refreshing rain, and, in a word,
filling the world with life and gladness.



Week 7

LECTURE III The Aerial Ocean in Which We Live

Did you ever sit on the bank of a river in some quiet spot where
the water was deep and clear, and watch the fishes swimming
lazily along?  When I was a child this was one of my favourite
occupations in the summertime on the banks of the Thames, and
there was one question which often puzzled me greatly, as I
watched the minnows and gudgeon gliding along through the water.
Why should fishes live in something and be often buffeted about
by waves and currents, while I and others lived on the top of the
earth and not in anything?  I do not remember ever asking anyone
about this; and if I had, in those days people did not pay much
attention to children's questions, and probably nobody would have
told me, what I now tell you, that we do live in something quite
as real and often quite as rough and stormy as the water in which
the fishes swim.  The something in which we live is air, and the
reason that we do not perceive it, is that we are in it, and that
it is a gas, and invisible to us; while we are above the water in
which the fishes live, and it is a liquid which our eyes can
perceive.

But let us suppose for a moment that a being, whose eyes were so
made that he could see gases as we see liquids, was looking down
from a distance upon our earth.  He would see an ocean of air, or
aerial ocean, all round the globe, with birds floating about in
it, and people walking along the bottom, just as we see fish
gliding along the bottom of a river.  It is true, he would never
see even the birds come near to the surface, for the highest-
flying bird, the condor, never soars more than five miles from
the ground, and our atmosphere, as we shall see, is at least 100
miles high.  So he would call us all deep-air creatures, just as
we talk of deep-sea animals; and if we can imagine that he fished
in this air-ocean, and could pull one of us out of it into space,
he would find that we should gasp and die just as fishes do when
pulled out of the water.

He would also observe very curious things going on in our air-
ocean; he would see large streams and currents of air, which we
call winds, and which would appear to him as ocean-currents do to
us, while near down to the earth he would see thick mists forming
and then disappearing again, and these would be our clouds.  From
them he would see rain, hail and snow falling to the earth, and
from time to time bright flashes would shoot across the air-
ocean, which would be our lightning.  Nay even the brilliant
rainbow, the northern aurora borealis, and the falling stars,
which seem to us so high up in space, would be seen by him near
to our earth, and all within the aerial ocean.

But as we know of no such being living in space, who can tell us
what takes place in our invisible air, and we cannot see it
ourselves, we must try by experiments to see it with our
imagination, though we cannot with our eyes.

First, then, can we discover what air is?  At one time it was
thought that it was a simple gas and could not be separated into
more than one kind.  But we are now going to make an experiment
by which it has been shown that air is made of two gases mingled
together, and that one of these gases, called oxygen, is used up
when anything burns, while the other nitrogen is not used, and
only serves to dilute the minute atoms of oxygen.  I have here a
glass bell-jar, with a cork fixed tightly in the neck, and I
place the jar over a pan of water, while on the water floats a
plate with a small piece of phosphorus upon it.  You will see
that by putting the bell-jar over the water, I have shut in a
certain quantity of air, and my object now is to use up the
oxygen out of this air and leave only nitrogen behind.  To do
this I must light the piece of phosphorus, for you will remember
it is in burning that oxygen is used up.  I will take the cork
out, light the phosphorus, and cork up the jar again.  See! as
the phosphorus burns white fumes fill the jar.  These fumes are
phosphoric acid which is a substance made of phosphorous and the
oxygen of the air together.

Now, phosphoric acid melts in water just as sugar does, and in a
few minutes these fumes will disappear.  They are beginning to
melt already, and the water from the pan is rising up in the
bell-jar.  Why is this?  Consider for a moment what we have done.
First, the jar was full of air, that is, of mixed oxygen and
nitrogen; then the phosphorus used up the oxygen making white
fumes; afterwards, the water sucked up these fumes; and so, in
the jar now nitrogen is the only gas left, and the water has
risen up to fill all the rest of the space that was once taken up
with oxygen.

We can easily prove that there is no oxygen now in the jar.  I
take out the cork and let a lighted taper down into the gas.  If
there were any oxygen the taper would burn, but you see it goes
out directly proving that all the oxygen has been used up by the
phosphorous.  When this experiment is made very accurately, we
find that for every pint of oxygen in air there are four pints of
nitrogen, so that the active oxygen-atoms are scattered about,
floating in the sleepy, inactive nitrogen.

It is these oxygen-atoms which we use up when we breathe.  If I
had put a mouse under the bell-jar, instead of the phosphorus,
the water would have risen just the same, because the mouse would
have breathed in the oxygen and used it up in its body, joining
it to carbon and making a bad gas, carbonic acid, which would
also melt in the water, and when all the oxygen was used, the
mouse would have died.

Do you see now how foolish it is to live in rooms that are
closely shut up, or to hide your head under the bedclothes when
you sleep?  You use up all the oxygen-atoms, and then there are
none left for you to breathe; and besides this, you send out of
your mouth bad fumes, though you cannot see them, and these, when
you breathe them in again, poison you and make you ill.

Perhaps you will say, If oxygen is so useful, why is not the air
made entirely of it?  But think for a moment.  If there was such
an immense quantity of oxygen, how fearfully fast everything
would burn!  Our bodies would soon rise above fever heat from the
quantity of oxygen we should take in, and all fires and lights
would burn furiously.  In fact, a flame once lighted would spread
so rapidly that no power on earth could stop it, and everything
would be destroyed.  So the lazy nitrogen is very useful in
keeping the oxygen-atoms apart; and we have time, even when a
fire is very large and powerful, to put it out before it has
drawn in more and more oxygen from the surrounding air.  Often,
if you can shut a fire into a closed space, as in a closely-shut
room or the hold of a ship, it will go out, because it has used
up all the oxygen in the air.

So, you see, we shall be right in picturing this invisible air
all around us as a mixture of two gases.  But when we examine
ordinary air very carefully, we find small quantities of other
gases in it, besides oxygen and nitrogen.  First, there is
carbonic acid gas.  This is the bad gas which we give out of our
mouths after we have burnt up the oxygen with the carbon of our
bodies inside our lungs; and this carbonic acid is also given out
from everything that burns.  If only animals lived in the world,
this gas would soon poison the air; but plants get hold of it,
and in the sunshine they break it up again, as we shall see in
Lecture VII, and use up the carbon, throwing the oxygen back into
the air for us to use.  Secondly, there are very small quantities
of ammonia, or the gas which almost chokes you in smelling-salts,
and which, when liquid is commonly called "spirits of hartshorn."
This ammonia is useful to plants, as we shall see by and by.
Lastly, there is a great deal of water in the air, floating about
as invisible vapour or water-dust, and this we shall speak of in
the next lecture.  Still, all these gases and vapours in the
atmosphere are in very small quantities, and the bulk of the air
is composed of oxygen and nitrogen.

Having now learned what air is, the next question which presents
itself is, Why does it stay round our earth?  You will remember
we saw in the first lecture, that all the little atoms of a gas
are trying to fly away from each other, so that if I turn on this
gas-jet the atoms soon leave it, and reach you at the farther end
of the room, and you can smell the gas.  Why, then, do not all
the atoms of oxygen and nitrogen fly away from our earth into
space, and leave us without any air?

Ah!  here you must look for another of our invisible forces.
Have you forgotten our giant force, "gravitation," which draws
things together from a distance?  This force draws together the
earth and the atoms of oxygen and nitrogen; and as the earth is
very big and heavy, and the atoms of air are light and easily
moved, they are drawn down to the earth and held there by
gravitation.  But for all that, the atmosphere does not leave off
trying to fly away; it is always pressing upwards and outwards
with all its might, while the earth is doing its best to hold it
down.

The effect of this is, that near the earth, where the pull
downward is very strong, the air-atoms are drawn very closely
together, because gravitation gets the best of the struggle.  But
as we get farther and farther from the earth, the pull downward
becomes weaker, and then the air-atoms spring farther apart, and
the air becomes thinner.  Suppose that the lines in this diagram
represent layers of air.  Near the earth we have to represent
them as lying closely together, but as they recede from the earth
they are also farther apart.

But the chief reason why the air is thicker or denser nearer the
earth, is because the upper layers press it down.  If you have a
heap of papers lying one on the top of the other, you know that
those at the bottom of the heap will be more closely pressed
together than those above, and just the same is the case with the
atoms of the air.  Only there is this difference, if the papers
have lain for some time, when you take the top ones off, the
under ones remain close together.  But it is not so with the air,
because air is elastic, and the atoms are always trying to fly
apart, so that directly you take away the pressure they spring up
again as far as they can.



Week 8

I have here an ordinary pop-gun.  If I push the cork in very
tight, and then force the piston slowly inwards, I can compress
the air a good deal.  Now I am forcing the atoms nearer and
nearer together, but at last they rebel so strongly against being
more crowded that the cork cannot resist their pressure.  Out it
flies, and the atoms spread themselves out comfortably again in
the air all around them.  Now, just as I pressed the air together
in the pop-gun, so the atmosphere high up above the earth presses
on the air below and keeps the atoms closely packed together.
And in this case the atoms cannot force back the air above them
as they did the cork in the pop-gun; they are obliged to submit
to be pressed together.

Even a short distance from the earth, however, at the top of a
high mountain, the air becomes lighter, because it has less
weight of atmosphere above it, and people who go up in balloons
often have great difficulty in breathing, because the air is so
thin and light.  In 1804 a Frenchman, named Gay-Lussac, went up
four miles and a half in a balloon, and brought down some air;
and he found that it was much less heavy than the same quantity
of air taken close down to the earth, showing that it was much
thinner, or rarer, as it is called;* and when, in 1862, Mr.
Glaisher and Mr. Coxwell went up five miles and a half, Mr.
Glaisher's veins began to swell, and his head grew dizzy, and he
fainted.  The air was too thin for him to breathe enough in at a
time, and it did not press heavily enough on the drums of his
ears and the veins of his body.  He would have died if Mr.
Coxwell had not quickly let off some of the gas in the balloon,
so that it sank down into denser air. (*100 cubic inches near the
earth weighed 31 grains, while the same quantity taken at four
and a half miles up in the air weighed only 12 grains, or two-
fifths of the weight.)

And now comes another very interesting question.  If the air gets
less and less dense as it is farther from the earth, where does
it stop altogether?  We cannot go up to find out, because we
should die long before we reached the limit; and for a long time
we had to guess about how high the atmosphere probably was, and
it was generally supposed not to be more than fifty miles.  But
lately, some curious bodies, which we should have never suspected
would be useful to us in this way, have let us into the secret of
the height of the atmosphere.  These bodies are the meteors, or
falling stars.

Most people, at one time or another, have seen what looks like a
star shoot right across the sky, and disappear.  On a clear
starlight night you may often see one or more of these bright
lights flash through the air; for one falls on an average in
every twenty minutes, and on the nights of August 9th and
November 13th there are numbers in one part of the sky.  These
bodies are not really stars; they are simply stones or lumps of
metal flying through the air, and taking fire by clashing against
the atoms of oxygen in it.  There are great numbers of these
masses moving round and round the sun, and when our earth comes
across their path, as it does especially in August and November,
they dash with such tremendous force through the atmosphere that
they grow white-hot, and give out light, and then disappear,
melted into vapour.  Every now and then one falls to the earth
before it is all melted away, and thus we learn that these stones
contain tin, iron, sulphur, phosphorus, and other substances.

It is while these bodies are burning that they look to us like
falling stars, and when we see them we know that hey must be
dashing against our atmosphere.  Now if two people stand a
certain known distance, say fifty miles, apart on the earth and
observe these meteors and the direction in which they each see
them fall, they can calculate (by means of the angle between the
two directions) how high they are above them when they first see
them, and at that moment they must have struck against the
atmosphere, and even travelled some way through it, to become
white-hot.  In this way we have learnt that meteors burst into
light at least 100 miles above the surface of the earth, and so
the atmosphere must be more than 100 miles high.

Our next question is as to the weight of our aerial ocean.  You
will easily understand that all this air weighing down upon the
earth must be very heavy, even though it grows lighter as it
ascends.  The atmosphere does, in fact, weigh down upon land at
the level of the sea as much as if a 15-pound weight were put
upon every square inch of land.  This little piece of linen
paper, which I am holding up, measures exactly a square inch, and
as it lies on the table, it is bearing a weight of 15 lbs. on its
surface.  But how, then, comes it that I can lift it so easily?
Why am I not conscious of the weight?

To understand this you must give all your attention, for it is
important and at first not very easy to grasp.  you must
remember, in the first place, that the air is heavy because it is
attracted to the earth, and in the second place, that since air
is elastic all the atoms of it are pushing upwards against this
gravitation.  And so, at any point in air, as for instance the
place where the paper now is as I hold it up, I feel no pressure
because exactly as much as gravitation is pulling the air down,
so much elasticity is resisting and pushing it up.  So the
pressure is equal upwards, downwards, and on all sides, and I can
move the paper with equal ease any way.

Even if I lay the paper on the table this is still true, because
there is always some air under it.  If, however, I could get the
air quite away from one side of the paper, then the pressure on
the other side would show itself.  I can do this by simply
wetting the paper and letting it fall on the table, and the water
will prevent any air from getting under it.  Now see! if I try to
lift it by the thread in the middle, I have great difficulty,
because the whole 15 pounds' weight of the atmosphere is pressing
it down.  A still better way of making the experiment is with a
piece of leather, such as the boys often amuse themselves with in
the streets.  This piece of leather has been well soaked.  I drop
it on the floor and see! it requires all my strength to pull it
up.  (In fastening the string to the leather the hole must be
very small and the know as flat as possible, and it is even well
to put a small piece of kid under the knot.  When I first made
this experiment, not having taken these precautions, it did not
succeed well, owing to air getting in through the hole.)  I now
drop it on this stone weight, and so heavily is it pressed down
upon it by the atmosphere that I can lift the weight without its
breaking away from it.

Have you ever tried to pick limpets off a rock?  If so, you know
how tight they cling.  the limpet clings to the rock just in the
same way as this leather does to the stone; the little animal
exhausts the air inside it's shell, and then it is pressed
against the rock by the whole weight of the air above.

Perhaps you will wonder how it is that if we have a weight of 15
lbs. pressing on every square inch of our bodies, it does not
crush us.  And, indeed, it amounts on the whole to a weight of
about 15 tons upon the body of a grown man.  It would crush us if
it were not that there are gases and fluids inside our bodies
which press outwards and balance the weight so that we do not
feel it at all.

This is why Mr. Glaisher's veins swelled and he grew giddy in
thin air.  The gases and fluids inside his body were pressing
outwards as much as when he was below, but the air outside did
not press so heavily, and so all the natural condition of his
body was disturbed.

I hope we now realize how heavily the air presses down upon our
earth, but it is equally necessary to understand how, being
elastic, it also presses upwards; and we can prove this by a
simple experiment.  I fill this tumbler with water, and keeping a
piece of card firmly pressed against it, I turn the whole upside-
down.  When I now take my hand away you would naturally expect
the card to fall, and the water to be spilt.  But no! the card
remains as if glued to the tumbler, kept there entirely by the
air pressing upwards against it.  (The engraver has drawn the
tumbler only half full of water.  The experiment will succeed
quite as well in this way if the tumbler be turned over quickly,
so that part of the air escapes between the tumbler and the card,
and therefore the space above the water is occupied by air less
dense than that outside.)

And now we are almost prepared to understand how we can weigh the
invisible air.  One more experiment first.  I have here what is
called a U tube, because it is shaped like a large U.  I pour
some water in it till it is about half full, and you will notice
that the water stands at the same height in both arms of the
tube, because the air presses on both surfaces alike.  Putting my
thumb on one end I tilt the tube carefully, so as to make the
water run up to the end of one arm, and then turn it back again.
But the water does not now return to its even position, it
remains up in the arm on which my thumb rests.  Why is this?
Because my thumb keeps back the air from pressing at that end,
and the whole weight of the atmosphere rests on the water at the
other end.  And so we learn that not only has the atmosphere real
weight, but we can see the effects of this weight by making it
balance a column of water or any other liquid.  In the case of
the wetted leather we felt the weight of the air, here we see its
effects.

Now when we wish to see the weight of the air we consult a
barometer, which works really just in the same way as the water
in this tube.  An ordinary upright barometer is simply a straight
tube of glass filled with mercury or quicksilver, and turned
upside-down in a small cup of mercury.  The tube is a little more
than 30 inches long, and though it is quite full of mercury
before it is turned up, yet directly it stands in the cup the
mercury falls, till there is a height of about 30 inches between
the surface of the mercury in the cup, and that of the mercury in
the tube.  As it falls it leaves an empty space above the mercury
which is called a vacuum, because it has no air in it.  Now, the
mercury is under the same conditions as the water was in the U
tube, there is no pressure upon it at the top of the tube, while
there is a pressure of 15 lbs. upon it in the bowl, and therefore
it remains held up in the tube.



Week 9

But why will it not remain more than 30 inches high in the tube?
You must remember it is only kept up in the tube at all by the
air which presses on the mercury in the cup.  And that column of
mercury now balances the pressure of the air outside, and presses
down on the mercury in the cup at its mouth just as much as the
air does on the rest.  So this cup and tube act exactly like a
pair of scales.  The air outside is the thing to be weighed at
one end as it presses on the mercury, the column answers to the
leaden weight at the other end which tells you how heavy the air
is.  Now if the bore of this tube is made an inch square, then
the 30 inches of mercury in it weigh exactly 15 lbs, and so we
know that the weight of the air is 15 lbs. upon every square
inch, but if the bore of the tube is only half a square inch, and
therefore the 30 inches of mercury only weigh 7 1/2 lbs. instead
of 15 lbs., the pressure of the atmosphere will also be halved,
because it will only act upon half a square inch of surface, and
for this reason it will make no difference to the height of the
mercury whether the tube be broad or narrow.

But now suppose the atmosphere grows lighter, as it does when it
has much damp in it.  The barometer will show this at once,
because there will be less weight on the mercury in the cup,
therefore it will not keep the mercury pushed so high up in the
tube.  In other words, the mercury in the tube will fall.

Let us suppose that one day the air is so much lighter that it
presses down only with a weight of 14 1/2 lbs. to the square inch
instead of 15 lbs.  Then the mercury would fall to 29 inches,
because each inch is equal to the weight of half a pound.  Now,
when the air is damp and very full of water-vapour it is much
lighter, and so when the barometer falls we expect rain.
Sometimes, however, other causes make the air light, and then,
although the barometer is low, no rain comes,

Again, if the air becomes heavier the mercury is pushed up above
30 to 31 inches, and in this way we are able to weigh the
invisible air-ocean all over the world, and tell when it grows
lighter or heavier.  This then, is the secret of the barometer.
We cannot speak of the thermometer today, but I should like to
warn you in passing that it has nothing to do with the weight of
the air, but only with heat, and acts in quite a different way.

And now we have been so long hunting out, testing and weighing
our aerial ocean, that scarcely any time is left us to speak of
its movements or the pleasant breezes which it makes for us in
our country walks.  Did you ever try to run races on a very windy
day?  Ah! then you feel the air strongly enough; how it beats
against your face and chest, and blows down your throat so as to
take your breath away; and what hard work it is to struggle
against it!  Stop for a moment and rest, and ask yourself, what
is the wind?  Why does it blow sometimes one way and sometimes
another, and sometimes not at all?

Wind is nothing more than air moving across the surface of the
earth, which as it passes along bends the tops of the trees,
beats against the houses, pushes the ships along by their sails,
turns the windmill, carries off the smoke from cities, whistles
through the keyhole, and moans as it rushes down the valley.
What makes the air restless? why should it not lie still all
round the earth?

It is restless because, as you will remember, its atoms are kept
pressed together near the earth by the weight of the air above,
and they take every opportunity, when they can find more room, to
spread out violently and rush into the vacant space, and this
rush we call a wind.

Imagine a great number of active schoolboys all crowded into a
room till they can scarcely move their arms and legs for the
crush, and then suppose all at once a large door is opened.  Will
they not all come tumbling out pell-mell, one over the other,
into the hall beyond, so that if you stood in their way you would
most likely be knocked down?  Well, just this happens to the air-
atoms; when they find a space before them into which they can
rush, they come on helter-skelter, with such force that you have
great difficulty in standing against them, and catch hold of
something to support you for fear you should be blown down.

But how come they to find any empty space to receive them?  To
answer this we must go back again to our little active invisible
fairies the sunbeams.  When the sun-waves come pouring down upon
the earth they pass through the air almost without heating it.
But not so with the ground; there they pass down only a short
distance and then are thrown back again.  And when these sun-
waves come quivering back they force the atoms of the air near
the earth apart and make it lighter; so that the air close to the
surface of the heated ground becomes less heavy than the air
above it, and rises just as a cork rises in water.  You know that
hot air rises in the chimney; for if you put a piece of lighted
paper on the fire it is carried up by the draught of air, often
even before it can ignite.  Now just as the hot air rises from
the fire, so it rises from the heated ground up into higher parts
of the atmosphere.  and as it rises it leaves only thin air
behind it, and this cannot resist the strong cold air whose atoms
are struggling and trying to get free, and they rush in and fill
the space.

One of the simplest examples of wind is to be found at the
seaside.  there in the daytime the land gets hot under the
sunshine, and heats the air, making it grow light and rise.
Meanwhile the sunshine on the water goes down deeper, and so does
not send back so many heat-waves into the air; consequently the
air on the top of the water is cooler and heavier, and it rushes
in from over the sea to fill up the space on the shore left by
the warm air as it rises.  This is why the seaside is so pleasant
in hot weather.  During the daytime a light sea-breeze nearly
always sets in from the sea to the land.

When night comes, however, then the land loses its heat very
quickly, because it has not stored it up and the land-air grows
cold; but the sea, which has been hoarding the sun-waves down in
its depths, now gives them up to the atmosphere above it, and the
sea-air becomes warm and rises.  For this reason it is now the
turn of the cold air from the land to spread over the sea, and
you have a land-breeze blowing off the shore.

Again, the reason why there are such steady winds, called the
trade winds, blowing towards the equator, is that the sun is very
hot at the equator, and hot air is always rising there and making
room for colder air to rush in.  We have not time to travel
farther with the moving air, though its journeys are extremely
interesting; but if, when you read about the trade and other
winds, you will always picture to yourselves warm air made light
by the heat rising up into space and cold air expanding and
rushing in to fill its place, I can promise you that you will not
find the study of aerial currents so dry as many people imagine
it to be.

We are now able to form some picture of our aerial ocean.  We can
imagine the active atoms of oxygen floating in the sluggish
nitrogen, and being used up in every candle-flame, gas-jet and
fire, and in the breath of all living beings; and coming out
again tied fast to atoms of carbon and making carbonic acid.
Then we can turn to trees and plants, and see them tearing these
two apart again, holding the carbon fast and sending the
invisible atoms of oxygen bounding back again into the air, ready
to recommence work.  We can picture all these air-atoms, whether
of oxygen or nitrogen, packed close together on the surface of
the earth, and lying gradually farther and farther apart, as they
have less weight above them, till they become so scattered that
we can only detect them as they rub against the flying meteors
which flash into light.  We can feel this great weight of air
pressing the limpet on to the rock; and we can see it pressing up
the mercury in the barometer and so enabling us to measure its
weight.  Lastly, every breath of wind that blows past us tells us
how this aerial ocean is always moving to and fro on the face of
the earth; and if we think for a moment how much bad air and bad
matter it must carry away, as it goes from crowded cities to be
purified in the country, we can see how, in even this one way
alone, it is a great blessing to us.

Yet even now we have not mentioned many of the beauties of our
atmosphere.  It is the tiny particles floating in the air which
scatter the light of the sun so that it spreads over the whole
country and into shady places.  The sun's rays always travel
straight forward; and in the moon, where there is no atmosphere,
there is no light anywhere except just where the rays fall.  But
on our earth the sun-waves hit against the myriads of particles
in the air and glide off them into the corners of the room or the
recesses of a shady lane, and so we have light spread before us
wherever we walk in the daytime, instead of those deep black
shadows which we can see through a telescope on the face of the
moon.

Again, it is electricity playing in the air-atoms which gives us
the beautiful lightning and the grand aurora borealis, and even
the twinkling of the starts is produced entirely by minute
changes in the air.  If it were not for our aerial ocean, the
stars would stare at us sternly, instead of smiling with the
pleasant twinkle-twinkle which we have all learned to love as
little children.

All these questions, however, we must leave for the present; only
I hope you will be eager to read about them wherever you can, and
open your eyes to learn their secrets.  For the present we must
be content if we can even picture this wonderful ocean of gas
spread round our earth, and some of the work it does for us.

We said in the last lecture that without the sunbeams the earth
would be cold, dark, and frost-ridden.  With sunbeams, but
without air, it would indeed have burning heat, side by side with
darkness and ice, but it could have no soft light.  our planet
might look beautiful to others, as the moon does to us, but it
could have comparatively few beauties of its own.  With the
sunbeams and the air, we see it has much to make it beautiful.
But a third worker is wanted before our planet can revel in
activity and life.  This worker is water; and in the next lecture
we shall learn something of the beauty and the usefulness of the
"drops of water" on their travels.



Week 10

LECTURE IV. A DROP OF WATER ON ITS TRAVELS

We are going to spend an hour to-day in following a drop of water
on its travels. If I dip my finger in this basin of water and
lift it up again, I bring with it a small glistening
drop out of the body of water below, and hold it before you. Tell
me, have you any idea where this drop has been? what changes it
has undergone, and what work it has been doing during all the
long ages that water has lain on the face of the earth? It is a
drop now, but it was not so before I lifted it out of the basin;
then it was part of a sheet of water, and will be so again if I
let it fall. Again, if I were to put this basin on the stove till
all the water had boiled away, where would my drop be then? Where
would it go? What forms will it take before it reappears in the
rain-cloud, the river, or the sparkling dew?

These are questions we are going to try to answer to-day; and
first, before we can in the least understand how water travels,
we must call to mind what we have learnt about the sunbeams and
the air. We must have clearly pictured in our imagination those
countless sun-waves which are for ever crossing space, and
especially those larger and slower undulations, the dark heat-
waves; for it is these, you will remember, which force the air-
atoms apart and make the air light, and it is also these which
are most busy in sending water on its travels. But not these
alone. The sun-waves might shake the water-drops as much as they
liked and turn them into invisible vapour, but they could not
carry them over the earth if it were not for the winds and
currents of that aerial ocean which bears the vapour on its
bosom, and wafts it to different regions of the world.

Let us try to understand how these two invisible workers, the
sun-waves and the air, deal with the drops of water. I
have here a kettle (Fig. 18, p. 76) boiling over a spirit-lamp,
and I want you to follow minutely what is going on in it. First,
in the flame of the lamp, atoms of the spirit drawn up from below
are clashing with the oxygen-atoms in the air. This, as you know,
causes heat-waves and light-waves to move rapidly all round the
lamp. The light-waves cannot pass through the kettle, but the
heat-waves can, and as they enter the water inside they agitate
it violently. Quicker, and still more quickly, the particles of
water near the bottom of the kettle move to and fro and are
shaken apart; and as they become light they rise through the
colder water letting another layer come down to be heated in its
turn. The motion grows more and more violent, making the water
hotter and hotter, till at last the particles of which it is
composed fly asunder, and escape as invisible vapour. If this
kettle were transparent you would not see any steam above the
water, because it is in the form of an invisible gas. But as the
steam comes out of the mouth of the kettle you see a cloud. Why
is this? Because the vapour is chilled by coming out into the
cold air, and its particles are drawn together again into tiny,
tiny drops of water, to which Dr. Tyndall has given the
suggestive name of water-dust. If you hold a plate over the steam
you can catch these tiny drops, though they will run into one
another almost as you are catching them.

The clouds you see floating in the sky are made of exactly the
same kind of water-dust as the cloud from the kettle, and I wish
to show you that this is also really the same as the invisible
steam within the kettle. I will do so by an experiment
suggested by Dr. Tyndall. Here is another spirit-lamp, which I
will hold under the cloud of steam - see! the cloud disappears!
As soon as the water-dust is heated the heat-waves scatter it
again into invisible particles, which float away into the room.
Even without the spirit-lamp, you can convince yourself that
water-vapour may be invisible; for close to the mouth of the
kettle you will see a short blank space before the cloud begins.
In this space there must be steam, but it is still so hot that
you cannot see it; and this proves that heat-waves can so shake
water apart as to carry it away invisibly right before your eyes.

Now, although we never see any water travelling from our earth up
into the skies, we know that it goes there, for it comes down
again in rain, and so it must go up invisibly. But where does the
heat come from which makes this water invisible? Not from below,
as in the case of the kettle, but from above, pouring down from
the sun. Wherever the sun-waves touch the rivers, ponds, lakes,
seas, or fields of ice and snow upon our earth, they
carry off invisible water-vapour. They dart down through the top
layers of the water, and shake the water-particles forcibly
apart; and in this case the drops fly asunder more easily and
before they are so hot, because they are not kept down by a great
weight of water above, as in the kettle, but find plenty of room
to spread themselves out in the gaps between the air-atoms of the
atmosphere.

Can you imagine these water-particles, just above any pond or
lake, rising up and getting entangled among the air-atoms? They
are very light, much lighter than the atmosphere; and so, when a
great many of them are spread about in the air which lies just
over the pond, they make it much lighter than the layer of air
above, and so help it to rise, while the heavier layer of air
comes down ready to take up more vapour.

In this way the sun-waves and the air carry off water everyday,
and all day long, from the top of lakes, rivers, pools, springs,
and seas, and even from the surface of ice and snow. Without any
fuss or noise or sign of any kind, the water of our earth is
being drawn up invisibly into the sky.

It has been calculated that in the Indian Ocean three-quarters of
an inch of water is carried off from the surface of the sea in
one day and night; so that as much as 22 feet, or a depth of
water about twice the height of an ordinary room, is silently and
invisibly lifted up from the whole surface of the ocean in one
year. It is true this is one of the hottest parts of the earth,
where the sun-waves are most active; but even in our
own country many feet of water are drawn up in the summer-time.

What, then, becomes of all this water? Let us follow it as it
struggles upwards to the sky. We see it in our imagination first
carrying layer after layer of air up with it from the sea till it
rises far above our heads and above the highest mountains. But
now, call to mind what happens to the air as it recedes from the
earth. Do you not remember that the air-atoms are always trying
to fly apart, and are only kept pressed together by the weight of
air above them? Well, so this water-laden air rises up, its
particles, no longer so much pressed together, begin to separate,
and as all work requires an expenditure of heat, the air becomes
colder, and then you know at once what must happen to the
invisible vapour, -- it will form into tiny water-drops, like the
steam from the kettle. And so, as the air rises and becomes
colder, the vapour gathers into the visible masses, and we can
see it hanging in the sky, and call it clouds. When these clouds
are highest they are about ten miles from the earth, but when
they are made of heavy drops and hang low down, they sometimes
come within a mile of the ground.

Look up at the clouds as you go home, and think that the water of
which they are made has all been drawn up invisibly through the
air. Not, however, necessarily here in London, for we have
already seen that air travels as wind all over the world, rushing
in to fill spaces made by rising air wherever they occur, and so
these clouds may be made of vapour collected in the
Mediterranean, or in the Gulf of Mexico off the coast of America,
or even, if the wind is from the north, of chilly
particles gathered from the surface of Greenland ice and snow,
and brought here by the moving currents of air. Only, of one
thing we may be sure, that they come from the water of our earth.

Sometimes, if the air is warm, these water-particles may travel a
long way without ever forming into clouds; and on a hot,
cloudless day the air is often very full of invisible vapour.
Then, if a cold wind comes sweeping along, high up in the sky,
and chills this vapour, it forms into great bodies of water-dust
clouds, and the sky is overcast. At other times clouds hang
lazily in a bright sky, and these show us that just where they
are (as in Fig. 19) the air is cold and turns the invisible
vapour rising from the ground into visible water-dust, so that
exactly in those spaces we see it as clouds. Such clouds form
often on warm, still summer's day, and they are shaped like
masses of wool, ending in a straight line below. They are not
merely hanging in the sky, they are really resting upon a tall
column of invisible vapour which stretches right up from the
earth; and that straight line under the clouds marks
the place where the air becomes cold enough to turn this
invisible vapour into visible drops of water.



Week 11

And now, suppose that while these or any other kind of clouds are
overhead, there comes along either a very cold wind, or a wind
full of vapour. As it passes through the clouds, it makes them
very full of water, for, if it chills them, it makes the water-
dust draw more closely together; or, if it brings a new load of
water-dust, the air is fuller than it can hold. In either case a
number of water-particles are set free, and our fairy force
"cohesion" seizes upon them at once and forms them into large
water-drops. Then they are much heavier than the air, and so they
can float no longer, but down they come to the earth in a shower
of rain.

There are other ways in which the air may be chilled, and rain
made to fall, as, for example, when a wind laden with moisture
strikes against the cold tops of mountains. Thus the Khasia Hills
in India which face the Bay of Bengal, chill the air which
crosses them on its way from the Indian Ocean. The wet winds are
driven up the sides of the hills, the air expands, and the vapour
is chilled, and forming into drops, falls in torrents of rain.
Sir J. Hooker tells us that as much as 500 inches of rain fell in
these hills in nine months. That is to say, if you could measure
off all the ground over which the rain fell, and spread the whole
nine months' rain over it, it would make a lake 500 inches, or
more than 40 feet deep! You will not be surprised that the
country on the other side of these hills gets hardly any rain,
for all the water has been taken out of the air before
it comes there. Again for example in England, the wind comes to
Cumberland and Westmorland over the Atlantic, full of vapour, and
as it strikes against the Pennine Hills it shakes off its watery
load; so that the lake district is the most rainy in England,
with the exception perhaps of Wales, where the high mountains
have the same effect.

In this way, from different causes, the water of which the sun
has robbed our rivers and seas, comes back to us, after it has
travelled to various parts of the world, floating on the bosom of
the air. But it does not always fall straight back into the
rivers and seas again, a large part of it falls on the land, and
has to trickle down slopes and into the earth, in order to get
back to its natural home, and it is often caught on its way
before it can reach the great waters.

Go to any piece of ground which is left wild and untouched you
will find it covered with grass weeds, and other plants; if you
dig up a small plot you will find innumerable tiny roots creeping
through the ground in every direction. Each of these roots has a
sponge-like mouth by which the plant takes up water. Now, imagine
rain-drops falling on this plot of ground and sinking into the
earth. On every side they will find rootlets thirsting to drink
them in, and they will be sucked up as if by tiny sponges, and
drawn into the plants, and up the stems to the leaves. Here, as
we shall see in Lecture VII., they are worked up into food for
the plant, and only if the leaf has more water than it needs,
some drops may escape at the tiny openings under the
leaf, and be drawn up again by the sun-waves as invisible vapour
into the air.

Again, much of the rain falls on hard rock and stone, where it
cannot sink in, and then it lies in pools till it is shaken apart
again into vapour and carried off in the air. Nor is it idle
here, even before it is carried up to make clouds. We have to
thank this invisible vapour in the air for protecting us from the
burning heat of the sun by day and intolerable frost by night.

Let us for a moment imagine that we can see all that we know
exists between us and the sun. First, we have the fine ether
across which the sunbeams travel, beating down upon our earth
with immense force, so that in the sandy desert they are like a
burning fire. Then we have the coarser atmosphere of oxygen and
nitrogen atoms hanging in this ether, and bending the minute sun-
waves out of their direct path. But they do very little to hinder
them on their way, and this is why in very dry countries the
sun's heat is so intense. The rays beat down mercilessly, and
nothing opposes them. Lastly, in damp countries we have the
larger but still invisible particles of vapour hanging about
among the air-atoms. Now, these watery particles, although they
are very few (only about one twenty-fifth part of the whole
atmosphere), do hinder the sun-waves. For they are very greedy of
heat, and though the light-waves pass easily through them, they
catch the heat-waves and use them to help themselves to expand.
And so, when there is invisible vapour in the air, the sunbeams
come to us deprived of some of their heat-waves, and we
can remain in the sunshine without suffering from the heat.

This is how the water-vapour shields us by day, but by night it
is still more useful. During the day our earth and the air near
it have been storing up the heat which has been poured down on
them, and at night, when the sun goes down, all this heat begins
to escape again. Now, if there were no vapour in the air, this
heat would rush back into space so rapidly that the ground would
become cold and frozen even on a summer's night, and all but the
most hardy plants would die. But the vapour which formed a veil
against the sun in the day, now forms a still more powerful veil
against the escape of the heat by night. It shuts in the heat-
waves, and only allows them to make their way slowly upwards from
the earth - thus producing for us the soft, balmy nights of
summer and preventing all life being destroyed in the winter.

Perhaps you would scarcely imagine at first that it is this screen
of vapour which determines whether or not we shall have dew upon
the ground. Have you ever thought why dew forms, or what power has
been at work scattering the sparkling drops upon the grass?
Picture to yourself that it has been a very hot summer's day, and
the ground and the grass have been well warmed, and that the sun
goes down in a clear sky without any clouds. At once the heat-
waves which have been stored up in the ground, bound back into the
air, and here some are greedily absorbed by the vapour, while
others make their way slowly upwards. The grass, especially, gives
out these heat-waves very quickly, because the blades, being very
thin, are almost all surface. In consequence of this they part
with their heat more quickly than they can draw it up from the
ground, and become cold. Now the air lying just above the grass is
full of invisible vapour, and the cold of the blades, as it
touches them, chills the water- particles, and they are no longer
able to hold apart, but are drawn together into drops on the
surface of the leaves.

We can easily make artificial dew for ourselves. I have here a
bottle of ice which has been kept outside the window. When I
bring it into the warm room a mist forms rapidly outside the
bottle. This mist is composed of water-drops, drawn out of the
air of the room, because the cold glass chilled the air all round
it, so that it gave up its invisible water to form dew-drops.
Just in this same way the cold blades of grass chill the air
lying above them, and steal its vapour.

But try the experiment, some night when a heavy dew is expected,
of spreading a thin piece of muslin over some part of the grass,
supporting it at the four corners with pieces of stick so that it
forms an awning. Though there may be plenty of dew on the grass
all round, yet under this awning you will find scarcely any. The
reason of this is that the muslin checks the heat-waves as they
rise from the grass, and so the grass-blades are not chilled
enough to draw together the water-drops on their surface. If you
walk out early in the summer mornings and look at the fine cobwebs
flung across the hedges, you will see plenty of drops on the
cobwebs themselves sparkling like diamonds; but underneath on the
leaves there will be none, for even the delicate cobweb has been
strong enough to shut in the heat-waves and keep the leaves warm.

Again, if you walk off the grass on to the gravel path, you find
no dew there. Why is this? Because the stones of the gravel can
draw up heat from the earth below as fast as they give it out,
and so they are never cold enough to chill the air which touches
them. On a cloudy night also you will often find little or no dew
even on the grass. The reason of this is that the clouds give
back heat to the earth, and so the grass does not become chilled
enough to draw the water-drops together on its surface. But after
a hot, dry day, when the plants are thirsty and there is little
hope of rain to refresh them, then they are able in the evening
to draw the little drops from the air and drink them in before
the rising sun comes again to carry them away.

But our rain-drop undergoes other changes more strange than
these. Till now we have been imagining it to travel only where
the temperature is moderate enough for it to remain in a liquid
state as water. But suppose that when it is drawn up into the air
it meets with such a cold blast as to bring it to the freezing
point. If it falls into this blast when it is already a drop,
then it will freeze into a hailstone, and often on a hot summer's
day we may have a severe hailstorm, because the rain-drops have
crossed a bitterly cold wind as they were falling, and have been
frozen into round drops of ice.

But if the water-vapour reaches the freezing air while it is still
an invisible gas, and before it has been drawn into a drop, then
its history is very different. The ordinary force of cohesion has
then no power over the particles to make them into watery globes,
but its place is taken by the fairy process of "crystallization,"
and they are formed into beautiful white flakes, to fall in a
snow-shower. I want you to picture this process to yourselves, for
if once you can take an interest in the wonderful power of nature
to build up crystals, you will be astonished how often you will
meet with instances of it, and what pleasure it will add to your
life.

The particles of nearly all substances, when left free and not
hurried, can build themselves into crystal forms. If you melt
salt in water and then let all the water evaporate slowly, you
will get salt-crystals;  -- beautiful cubes of transparent salt
all built on the same pattern. The same is true of sugar; and if
you will look at the spikes of an ordinary stick of sugar-candy,
such as I have here, you will see the kind of crystals which
sugar forms. You may even pick out such shapes as these
from the common crystallized brown sugar in the sugar basin, or
see them with a magnifying glass on a lump of white sugar.

But it is not only easily melted substances such as sugar and
salt which form crystals. The beautiful stalactite grottos are
all made of crystals of lime. Diamonds are crystals of carbon,
made inside the earth. Rock-crystals, which you know probably
under the name of Irish diamonds, are crystallized quartz; and
so, with slightly different colourings, are agates, opals,
jasper, onyx, cairngorms, and many other precious stones. Iron,
copper, gold, and sulphur, when melted and cooled slowly build
themselves into crystals, each of their own peculiar form, and we
see that there is here a wonderful order, such as we should never
have dreamt of, if we had not proved it. If you possess a
microscope you may watch the growth of crystals yourself by
melting some common powdered nitre in a little water till you
find that no more will melt in it. Then put a few drops of this
water on a warm glass slide and place it under the microscope. As
the drops dry you will see the long transparent needles of nitre
forming on the glass, and notice how regularly these crystals
grow, not by taking food inside like living beings, but by adding
particle to particle on the outside evenly and regularly.



Week 12

Can we form any idea why the crystals build themselves up so
systematically? Dr. Tyndall says we can, and I hope by the help
of these small bar magnets to show you how he explains it. These
little pieces of steel, which I hope you can see lying
on this white cardboard, have been rubbed along a magnet until
they have become magnets themselves, and I can attract and lift
up a needle with any one of them. But if I try to lift one bar
with another, I can only do it by bringing certain ends together.
I have tied a piece of red cotton (c, Fig. 21) round one end of
each of the magnets, and if I bring two red ends together they
will not cling together but roll apart. If, on the contrary, I
put a red end against an end where there is not cotton, then the
two bars cling together. This is because every magnet has two
poles or points which are exactly opposite in character, and to
distinguish them one is called the positive pole and the other
the negative pole. Now when I bring two red ends, that is, two
positive poles together, they drive each other away. See! the
magnet I am not holding runs away from the other. But if I bring
a red end and a black end, that is, a positive and a negative end
together, then they are attracted and cling. I will make a
triangle (A, Fig. 21) in which a black end and a red end always
come together, and you see the triangle holds together. But now if
I take off the lower bar and turn it (B, Fig. 21) so that two red
ends and two black ends come together, then this bar actually
rolls back from the others down the cardboard. If I were to break
these bars into a thousand pieces, each piece would still have two
poles, and if they were scattered about near each other in such a
way that they were quite free to move, they would arrange
themselves always so two different poles came together.

Now picture to yourselves that all the particles of those
substances which form crystals have poles like our magnets, then
you can imagine that when the heat which held them apart is
withdrawn and the particles come very near together, they will
arrange themselves according to the attraction of their poles and
so build up regular and beautiful patterns.

So, if we could travel up to the clouds where this fairy power of
crystallization is at work, we should find the particles of
water-vapour in a freezing atmosphere being built up into minute
solid crystals of snow. If you go out after a snow-shower and
search carefully, you will see that the snow-flakes are not mere
lumps of frozen water, but beautiful six-pointed crystal stars, so
white and pure that when we want to speak of anything being
spotlessly white, you say that it is "white as snow." Some of
these crystals are simply flat slabs with six sides, others are
stars with six rods or spikes springing from the centre, others
with six spikes each formed like a delicate fern. No less than a
thousand different forms of delicate crystals have been found
among snowflakes, but though there is such a great variety, yet
they are all built on the six-sided and six-pointed plan, and are
all rendered dazzlingly white by the reflection of the light from
the faces of the crystals and the tiny air-bubbles built up within
them. This, you see, is why, when the snow melts, you have only a
little dirty water in your hand; the crystals are gone and there
are no more air-bubbles held prisoners to act as looking-glasses
to the light. Hoar-frost is also made up of tiny water-crystals,
and is nothing more than frozen dew hanging on the blades of grass
and from the trees.

But how about ice? Here, you will say, is frozen water, and yet
we see no crystals, only a clear transparent mass. Here, again,
Dr. Tyndall helps us. He says (and as I have proved it true, so
may you for yourselves, if you will) that if you take a
magnifying glass, and look down on the surface of ice on a sunny
day, you will see a number of dark, six-sided stars, looking like
flattened flowers, and in the centre of each a bright spot. These
flowers, which are seen when the ice is melting, are our old
friends the crystal stars turning into water, and the
bright spot in the middle is a bubble of empty space, left
because the watery flower does not fill up as much room as the
ice of the crystal star did.

And this leads us to notice that ice always takes up more room
than water, and that this is the reason why our water-pipes burst
in severe frosts; for as the water freezes it expands with great
force, and the pipe is cracked, and then when the thaw comes on ,
and the water melts again, it pours through the crack it has
made.

It is not difficult to understand why ice should take more room;
for we know that if we were to try to arrange bricks end to end
in star-like shapes, we must leave some spaces between, and could
not pack them so closely as if they lay side by side. And so,
when this giant force of crystallization constrains the atoms of
frozen water to grow into star-like forms, the solid mass must
fill more room than the liquid water, and when the star
melts, this space reveals itself to us in the bright spot of the
centre.

We have now seen our drop of water under all its various forms of
invisible gas, visible steam, cloud, dew, hoar-frost, snow, and
ice, and we have only time shortly to see it on its travels, not
merely up and down, as hitherto, but round the world.

We must first go to the sea as the distillery, or the place from
which water is drawn up invisibly, in its purest state, into the
air; and we must go chiefly to the seas of the tropics, because
here the sun shines most directly all the year round, sending
heat-waves to shake the water-particles asunder. It has been
found by experiment that, in order to turn 1 lb. of water into
vapour, as much heat must be used as is required to melt 5 lbs.
of iron; and if you consider for a moment how difficult iron is
to melt, and how we can keep an iron poker in a hot fire and yet
it remains solid, this will help you to realize how much heat the
sun must pour down in order to carry off such a constant supply
of vapour from the tropical seas.

Now, when all this vapour is drawn up into the air, we know that
some of it will form into clouds as it gets chilled high up in
the sky, and then it will pour down again in those tremendous
floods of rain which occur in the tropics.

But the sun and air will not let it all fall down at once, and
the winds which are blowing from the equator to the poles carry
large masses of it away with them. Then, as you know, it will
depend on many things how far this vapour is carried. Some of it,
chilled by cold blasts, or by striking on cold mountain tops, as
it travels northwards, will fall in rain in Europe and Asia, while
that which travels southwards may fall in South America,
Australia, or New Zealand, or be carried over the sea to the South
Pole. Wherever it falls on the land as rain, and is not used by
plants, it will do one of two things; either it will run down in
streams and form brooks and rivers, and so at last find its way
back to the sea, or it will sink deep in the earth till it comes
upon some hard rock through which it cannot get, and then, being
hard pressed by the water coming on behind, it will rise up again
through cracks, and come to the surface as a spring. These
springs, again, feed rivers, sometimes above- ground, sometimes
for long distances under-ground; but one way or another at last
the whole drains back into the sea.

But if the vapour travels on till it reaches high mountains in
cooler lands, such as the Alps of Switzerland; or is carried to
the poles and to such countries as Greenland or the Antarctic
Continent, then it will come down as snow, forming immense snow-
fields. And here a curious change takes place in it. If you make
an ordinary snowball and work it firmly together, it becomes very
hard, and if you then press it forcibly into a mould you can turn
it into transparent ice. And in the same way the snow which falls
in Greenland and on the high mountains of Switzerland becomes
very firmly pressed together, as it slides down into the valleys.
It is like a crowd of people passing from a broad thoroughfare
into a narrow street. As the valley grows narrower and
narrower the great mass of snow in front cannot move down
quickly, while more and more is piled up by the snowfall behind,
and the crowd and crush grow denser and denser. In this way the
snow is pressed together till the air that was hidden in its
crystals, and which gave it its beautiful whiteness, is all
pressed out, and the snow-crystals themselves are squeezed into
one solid mass of pure, transparent ice.

Then we have what is called a "glacier," or river of ice, and
this solid river comes creeping down till, in Greenland, it
reaches the edge of the sea. There it is pushed over the brink of
the land, and large pieces snap off, and we have "icebergs."
These icebergs - made, remember, of the same water which was
first draw up from the tropics - float on the wide sea, and
melting in its warm currents, topple over and over* (A floating
iceberg must have about eight times as much ice under the water
as it has above, and therefore, when the lower part melts in a
warm current, the iceberg loses its balance and tilts over, so as
to rearrange itself round the centre of gravity.) till they
disappear and mix with the water, to be carried back again to the
warm ocean from which they first started. In Switzerland the
glaciers cannot reach the sea, but they move down into the
valleys till they come to a warmer region, and there the end of
the glacier melts, and flows away in a stream. The Rhone and many
other rivers are fed by the glaciers of the Alps; and as these
rivers flow into the sea, our drop of water again finds its way
back to its home.

But when it joins itself in this way to its companions, from whom
it was parted for a time, does it come back clear and transparent
as it left them? From the iceberg it does indeed return pure and
clear; for the fairy Crystallization will have no impurities, not
even salt, in her ice-crystals, and so as they melt they give back
nothing but pure water to the sea. Yet even icebergs bring down
earth and stones frozen into the bottom of the ice, and so they
feed the sea with mud.

But the drops of water in rivers are by no means as pure as when
they rose up into the sky. We shall see in the next lecture how
rivers carry down not only sand and mud all along their course,
but even solid matter such as salt, lime, iron, and flint,
dissolved in the clear water, just as sugar is dissolved, without
our being able to see it. The water, too, which has sunk down
into the earth, takes up much matter as it travels along. You all
know that the water you drink from a spring is very different
from rain-water, and you will often find a hard crust at the
bottom of kettles and in boilers, which is formed of the
carbonate of lime which is driven out of the clear water when it
is boiled. The water has become "hard" in consequence of having
picked up and dissolved the carbonate of lime on its way through
the earth, just in the same way as water would become sweet if
you poured it through a sugar-cask. You will also have heard of
iron-springs, sulphur-springs, and salt-springs, which come out
of the earth, even if you have never tasted any of them, and the
water of all these springs finds its way back at last to the
sea.

And now, can you understand why sea-water should taste
salt and bitter? Every drop of water which flows from the earth
to the sea carries something with it. Generally, there is so
little of any substance in the water that we cannot taste it, and
we call it pure water; but the purest of spring or river-water
has always some solid matter dissolved in it, and all this goes
to the sea. Now, when the sun-waves come to take the water out of
the sea again, they will have nothing but the pure water itself;
and so all these salts and carbonates and other solid substances
are left behind, and we taste them in sea-water.

Some day, when you are at the seaside, take some extra water and
set it on the hob till a great deal has simmered gently away, and
the liquid is very thick. Then take a drop of this liquid, and
examine it under a microscope. As it dries up gradually, you will
see a number of crystals forming, some square - and these will be
crystals of ordinary salt; some oblong - these will be crystals
of gypsum or alabaster; and others of various shapes. Then, when
you see how much matter from the land is contained in sea-water,
you will no longer wonder that the sea is salt; on the contrary,
you will ask, Why does it not grow salter every year?

The answer to this scarcely belongs to our history of a drop of
water, but I must just suggest it to you. In the sea are numbers
of soft-bodied animals, like the jelly animals which form the
coral, which require hard material for their shells or the solid
branches on which they live, and they are greedily watching for
these atoms of lime, of flint, or magnesia, and of other
substances brought down into the sea. It is with lime and magnesia
that the tiny chalk-builders form their beautiful shells, and the
coral animals their skeletons, while another class of builders use
the flint; and when these creatures die, their remains go to form
fresh land at the bottom of the sea; and so, though the earth is
being washed away by the rivers and springs it is being built up
again, out of the same materials, in the depths of the great
ocean.

And now we have reached the end of the travels of our drop of
water. We have seen it drawn up by the fairy "heat," invisible
into the sky; there fairy "cohesion" seized it and formed it into
water-drops and the giant, "gravitation," pulled it down again to
the earth. Or, if it rose to freezing regions, the fairy of
"crystallization" built it up into snow-crystals, again to fall
to the earth, and either to be melted back into water by heat, or
to slide down the valleys by force of gravitation, till it became
squeezed into ice. We have detected it, when invisible, forming a
veil round our earth, and keeping off the intense heat of the
sun's rays by day, or shutting it in by night. We have seen it
chilled by the blades of grass, forming sparkling dew-drops or
crystals of hoar-frost, glistening in the early morning sun; and
we have seen it in the dark underground, being drunk up greedily
by the roots of plants. We have started with it from the tropics,
and travelled over land and sea, watching it forming rivers, or
flowing underground in springs, or moving onwards to the high
mountains or the poles, and coming back again in glaciers and
icebergs. Through all this, while it is being carried
hither and thither by invisible power, we find no trace of its
becoming worn out, or likely to rest from its labours. Ever
onwards it goes, up and down, and round and round the world,
taking many forms, and performing many wonderful feats. We have
seen some of the work that it does, in refreshing the air,
feeding the plants, giving us clear, sparkling water to drink,
and carrying matter to the sea; but besides this, it does a
wonderful work in altering all the face of our earth. This work
we shall consider in the next lecture, on "The two great
Sculptors - Water and Ice."



Week 13

LECTURE V. THE TWO GREAT SCULPTORS - WATER AND ICE.

In our last lecture we saw that water can exist in three forms:--
1st, as an invisible vapour; 2nd, as liquid water; 3rd, as solid
snow and ice.

To-day we are going to take the two last of these
forms, water and ice, and speak of them as sculptors.

To understand why they deserve this name we must first consider
what the work of a sculptor is. If you go into a statuary yard
you will find there large blocks of granite, marble, and other
kinds of stone, hewn roughly into different shapes; but if you
pass into the studio, where the sculptor himself is at work you
will find beautiful statues, more or less finished;  and you will
see that out of rough blocks of stone he has been able to cut
images which look like living forms. You can even see by their
faces whether they are intended to be sad, or thoughtful, or
gay, and by their attitude whether they are writhing in pain,
or dancing with joy, or resting peacefully. How has all this
history been worked out from the shapeless stone? It has been
done by the sculptor's chisel. A piece chipped off here,  a
wrinkle cut there, a smooth surface rounded off in another place,
so as to give a gentle curve; all these touches gradually shape
the figure and mould it out of the rough stone, first into a
rude shape and afterwards, by delicate strokes, into the form of
a living being.

Now, just in the same way as the wrinkles and curves of a statue
are cut by the sculptor's chisel, so the hills and valleys, the
steep slopes and gentle curves on the face of our earth, giving
it all its beauty, and the varied landscapes we love so well,
have been cut out by water and ice passing over them. It is true
that some of the greater wrinkles of the earth, the lofty
mountains, and the high masses of land which rise above the sea ,
have been caused by earthquakes and shrinking of the
earth. We shall not speak of these to-day, but put them aside as
belonging to the rough work of the statuary yard. But when once
these large masses are put ready for water to work upon, then
all the rest of the rugged wrinkles and gentle slopes which make
the country so beautiful are due to water and ice, and for this
reason I have called them "sculptors."

Go for a walk in the country, or notice the landscape as you
travel on a railway journey. You pass by hills and through
valleys, through narrow steep gorges cut in hard rock, or
through wild ravines up the sides of which you can hardly
scramble. Then you come to grassy slopes and to smooth plains
across which you can look for miles without seeing a hill; or,
when you arrive at the seashore, you clamber into caves and
grottos, and along dark narrow passages leading from one bay to
another. All these - hills, valleys,  gorges, ravines, slopes,
plains, caves, grottos, and rocky shores - have been cut out by
the water. Day by day and year by year, while everything seems
to us to remain the same, this industrious sculptor is chipping
away,  a few grains here, a corner there, a large mass in another
place, till he gives to the country its own peculiar scenery,
just as the human sculptor gives expression to his statue.

Our work to-day will consist in trying to form some idea of the
way in which water thus carves out the surface of the earth, and
we will begin by seeing how much can be done by our old friends
the rain-drops before they become running streams.

Everyone must have noticed that whenever rain falls on soft
ground it makes small round holes in which it
collects, and then sinks into the ground, forcing its way
between the grains of earth. But you would hardly think that the
beautiful pillars in Fig. 24 have been made entirely in this way
by rain beating upon and soaking into the ground.

Where these pillars stand there was once a solid mass of clay and
stones,  into which the rain-drops crept, loosening the earthly
particles; and then when the sun dried the earth again cracks
were formed, so that the next shower loosened it still more, and
carried some of the mud down into the valley below. But here and
there large stones were buried in the clay, and where this
happened the rain could not penetrate, and the stones
became the tops of tall pillars of clay, washed into shape by the
rain beating on its sides, but escaping the general destruction
of the rest of the mud. In this way the whole valley has been
carved out into fine pillars,  some still having capping-stones,
while others have lost them, and these last will soon be washed
away. We have no such valleys of earth-pillars here in England,
but you may sometimes see tiny pillars under bridges where the
drippings have washed away the earth between the pebbles, and
such small examples which you can observe for yourselves are
quite as instructive as more important ones.

Another way in which rain changes the surface of the earth is by
sinking down through loose soil from the top of a cliff to a
depth of many feet till it comes to solid rock, and then lying
spread over a wide apace. Here it makes a kind of watery mud,
which is a very unsafe foundation for the hill of earth above
it, and so after a time the whole mass slips down and makes a
fresh piece of land at the foot of the cliff. If you have ever
been at the Isle of Wight you will have seen an undulating strip
of ground, called the Undercliff, at Ventnor and other places,
stretching all along the sea below the high cliffs. This land
was once at the top of the cliff, and came down by succession of
landslips such as we have been describing. A very great landslip
of this kind happened in the memory of living people, at Lyme
Regis, in Dorsetshire, in the year 1839.

You will easily see how in forming earth-pillars and causing
landslips rain changes the face of the country, but
these are only rare effects of water. It is when the rain
collects in brooks and forms rivers that it is most busy in
sculpturing the land. Look out some day into the road or the
garden where the ground slopes a little, and watch what happens
during a shower of rain. First the rain-drops run together in
every little hollow of the ground, then the water begins to flow
along any ruts or channels it can find, lying here and there in
pools, but always making its way gradually down the slope.
Meanwhile from other parts of the ground little rills are
coming, and these all meet in some larger ruts where the ground
is lowest,  making one great stream, which at last empties itself
into the gutter or an area, or finds its way down some grating.

Now just this, which we can watch whenever a heavy shower of rain
comes down on the road, happens also all over the world. Up in
the mountains,  where there is always a great deal of rain,
little rills gather and fall over the mountain sides, meeting in
some stream below. Then, as this stream flows on, it is fed by
many runnels of water, which come from all parts of the country,
trickling along ruts, and flowing in small brooks and rivulets
down the gentle slope of the land till they reach the big stream,
which at last is important enough to be called a river.
Sometimes this river comes to a large hollow in the land and
there the water gathers and forms a lake; but still at the lower
end of this lake out it comes again, forming a new river,  and
growing and growing by receiving fresh streams until at last it
reaches the sea.

The River Thames, which you all know, and whose course you will
find clearly described in Mr. Huxley's 'Physiography,' drains in
this way no less than one-seventh of the whole of England. All the
rain which falls in Berkshire, Oxfordshire, Middlesex,
Hertfordshire, Surrey, the north of Wiltshire and north-west of
Kent, the south of Buckinghamshire and of Gloucestershire, finds
its way into the Thames; making an area of 6160 square miles over
which every rivulet and brook trickle down to the one great river,
which bears them to the ocean. And so with every other area of
land in the world there is some one channel towards which the
ground on all sides slopes gently down, and into this channel all
the water will run, on its way to the sea.

But what has this to do with sculpture or cutting out of valleys?
If you will only take a glass of water out of any river, and let
it stand for some hours, you will soon answer this question for
yourself. For you will find that even from river water which
looks quite clear, a thin layer of mud will fall to the bottom
of the glass, and if you take the water when the river is
swollen and muddy you will get quite a thick deposit. This shows
that the brooks, the streams, and the rivers wash away the land
as they flow over it and carry it from the mountains down to the
valleys, and from the valleys away out into the sea.

But besides earthly matter, which we can see, there is much
matter dissolved in the water of rivers (as we mentioned in the
last lecture), and this we cannot see.

If you use water which comes out of a chalk country you will find
that after a time the kettle in which you have been in the habit
of boiling this water has a hard crust on its bottom and sides,
and this crust is made of chalk or carbonate of lime,
which the water took out of the rocks when it was passing
through them. Professor Bischoff has calculated that the river
Rhine carries past Bonn every year enough carbonate of lime
dissolved in its water to make 332,000 million oyster-shells,
and that if all these shells were built into a cube it would
measure 560 feet.



Week 14

Imagine to yourselves the whole of St. Paul's churchyard filled
with oyster-shells, built up in a large square till they reached
half as high again as the top of the cathedral, then you will
have some idea of the amount of chalk carried invisibly past
Bonn in the water of the Rhine every year.

Since all this matter, whether brought down as mud or dissolved,
comes from one part of the land to be carried elsewhere or out
to sea, it is clear that some gaps and hollows must be left in
the places from which it is taken. Let us see how these gaps are
made. Have you ever clambered up the mountainside,  or even up
one of those small ravines in the hillside, which have generally
a little stream trickling through them? If so, you must have
noticed the number of pebbles, large and small, lying in patches
here and there in the stream, and many pieces of broken rock,
which are often scattered along the sides of the ravine; and
how, as you climb, the path grows steeper, and the rocks become
rugged and stick out in strange shapes.

The history of this ravine will tell us a great deal about the
carving of water. Once it was nothing more than a little furrow
in the hillside down which the rain found its way in a thin
thread-like stream. But by and by, as the stream carried down
some of the earth, and the furrow grew deeper and wider, the sides
began to crumble when the sun dried up the rain which had soaked
in. Then in winter, when the sides of the hill were moist with the
autumn rains, frost came and turned the water to ice, and so made
the cracks still larger, and the swollen steam rushing down,
caught the loose pieces of rock and washed them down into its bed.
Here they were rolled over and over, and grated against each
other, and were ground away till they became rounded pebbles, such
as lie in the foreground of the picture (Fig. 25); while the grit
which was rubbed off them was carried farther down by the stream.
And so in time this became a little valley, and as the stream cut
it deeper and deeper, there was room to clamber along the sides of
it, and ferns and mosses began to cover the naked stone, and small
trees rooted themselves along the banks, and this beautiful little
nook sprang up on the hill-side entirely by the sculpturing of
water.

Shall you not feel a fresh interest in all the little valleys,
ravines, and gorges you meet with in the country, if you can
picture them being formed in this way year by year? There are
many curious differences in them which you can study for
yourselves. Some will be smooth, broad valleys and here the
rocks have been soft and easily worn, and water trickling down
the sides of the first valley has cut other channels so as to
make smaller valleys running across it. In other places there
will be narrow ravines, and here the rocks have been hard, so
that they did not wear away gradually, but broke off and fell in
blocks, leaving high cliffs on each side. In some places you
will come to a beautiful waterfall, where the water has tumbled
over a steep cliff, and then eaten its way back, just like a saw
cutting through a piece of wood.

There are two things in particular to notice in a waterfall like
this.  First, how the water and spray dash against the bottom of
the cliff down which it falls, and grind the small pebbles
against the rock. In this way the bottom of the cliff is
undermined, and so great pieces tumble down from time to time,
and keep the fall upright instead of its being sloped away at the
top, and becoming a mere steam. Secondly, you may often see
curious cup-shaped holes, called "pot-holes," in the rocks on the
sides of a waterfall, and these also are concerned in its
formation. In these holes you will generally find two or three
small pebbles, and you have here a beautiful example of how water
uses stones to grind away the face of the earth. These holes are
made entirely by the falling water eddying round and round in a
small hollow of the rock, and grinding the pebbles which it has
brought down, against the bottom and sides of this hollow, just as
you grind round a pestle in a mortar. By degrees the hole grows
deeper and deeper and though the first pebbles are probably ground
down to powder, others fall in, and so in time there is a great
hole perforated right through, helping to make the rock break and
fall away.

In this and other ways the water works its way back in a
surprising manner.  The Isle of Wight gives us some good
instances of this; Alum Bay Chine and the celebrated Blackgang
Chine have been entirely cut out by waterfalls. But the best
know and most remarkable example is the Niagara Falls, in
America.  Here, the River Niagara first wanders through a flat
country, and then reaches the great Lake Erie in a hollow of the
plain. After that, it flows gently down for about fifteen miles,
and then the slope becomes greater and it rushes on to the Falls
of Niagara. These falls are not nearly so high as many people
imagine, being only 165 feet, or about half the height of St.
Paul's Cathedral, but they are 2700 feet or nearly half-a-mile
wide, and no less than 670,000 tons of water fall
over them every minute,  making magnificent clouds of spray.

Sir Charles Lyell, when he was at Niagara, came to the conclusion
that,  taking one year with another, these falls eat back the
cliff at the rate of about one foot a year, as you can easily
imagine they would do, when you think with what force the water
must dash against the bottom of the falls.  In this way a deep
cleft has been cut right back from Queenstown for a distance of
seven miles, to the place where the falls are now. This helps us
a little to understand how very slowly and gradually
water cuts its way; for if a foot a year is about the average of
the waste of the rock,  it will have taken more than thirty-five
thousand years for that channel of seven miles to be made.

But even this chasm cut by the falls of Niagara is nothing
compared with the canyons of Colarado. Canyon is a Spanish word
for a rocky gorge, and these gorges are indeed so grand, that if
we had not seen in other places what water can do, we should
never have been able to believe that it could have cut out these
gigantic chasms. For more than three hundred miles the River
Colorado, coming down from the Rocky Mountains, has eaten its way
through a country made of granite and hard beds of limestone and
sandstone, and it has cut down straight through these rocks,
leaving walls from half-a-mile to a mile high, standing straight
up from it. The cliffs of the Great Canyon, as it is called,
stretch up for more than a mile above the river which flows in
the gorge below! Fancy yourselves for a moment in a boat on this
river, as shown in Figure 27, and looking up at these gigantic
walls of rock towering above you. Even half-way up them, a man,
if he could get there, would be so small you could not see him
without a telescope; while the opening at the top between the
two walls would seem so narrow at such an immense distance that
the sky above would have the appearance of nothing more than a
narrow streak of blue. Yet these huge chasms have not been made
by any violent breaking apart of the rocks or convulsion of an
earthquake. No, they have been gradually, silently, and steadily
cut through by the river which now glides quietly in the wider
chasms, or rushes rapidly through the narrow gorges at their feet.

"No description," says Lieutenant Ives, one of the first
explorers of this river, "can convey the idea of the varied and
majestic grandeur of this peerless waterway. Wherever the river
turns, the entire panorama changes.  Stately facades, august
cathedrals, amphitheatres, rotundas, castellated walls, and rows
of time-stained ruins, surmounted by every form of tower,
minaret, dome and spire, have been moulded from the cyclopean
masses of rock that form the mighty defile." Who will say, after
this, that water is not the grandest of all sculptors, as it
cuts through hundreds of miles of rock,  forming such magnificent
granite groups, not only unsurpassed but unequalled by any of
the works of man?

But we must not look upon water only as a cutting instrument, for
it does more than merely carve out land in one place, it also
carries it away and lays it down elsewhere; and in this it is
more like a modeller in clay, who smooths off the material from
one part of his figure to put it upon another.

Running water is not only always carrying away mud, but at the
same time laying it down here and there wherever it flows. When
a torrent brings down stones and gravel from the mountains, it
will depend on the size and weight of the pieces how long they
will be in falling through the water. If you take a handful of
gravel and throw it into a glass full of water, you will notice
that the stones in it will fall to the bottom at once, the grit
and coarse sand will take longer in sinking, and lastly, the fine
sand will be an hour or two in settling down, so that the water
becomes clear. Now, suppose that this gravel were sinking in the
water of a river. The stones would be buoyed up as long as the
river was very full and flowed very quickly, but they would drop
through sooner than the coarse sand. The coarse sand in its turn
would begin to sink as the river flowed more slowly, and would
reach the bottom while the fine sand was still borne on. Lastly,
the fine sand would sink through very, very slowly, and only
settle in comparatively still water.

From this it will happen that stones will generally lie near to
the bottom of torrents at the foot of the banks from which they
fall, while the gravel will be carried on by the stream after it
leaves the mountains. This too,  however, will be laid down when
the river comes into a more level country and runs more slowly.
Or it may be left together with the finer mud in a lake, as in
the lake of Geneva, into which the Rhone flows laden with mud
and comes out at the other end clear and pure. But if no lake
lies in the way the finer earth will still travel on, and the
river will take up more and more as it flows, till at last it
will leave this too on the plains across which it moves
sluggishly along, or will deposit it at its mouth when it joins
the sea.



Week 15

You all know the history of the Nile; how, when the rains fall
very heavily in March and April in the mountains of Abyssinia,
the river comes rushing down and brings with it a load of mud
which it spreads out over the Nile valley in Egypt. This annual
layer of mud is so thin that it takes a thousand years for it to
become 2 or 3 feet thick; but besides that which falls in the
valley a great deal is taken to the mouth of the river and there
forms new land, making what is called the "Delta" of the Nile.
Alexandria, Rosetta, and Damietta, are towns which are all built
on land made of Nile mud which was carried down ages and ages ago,
and which has now become firm and hard like the rest of the
country. You will easily remember other deltas mentioned in books,
and all these are made of the mud carried down from the land to
the sea. The delta of the Ganges and Brahmapootra in India, is
actually as large as the whole of England and Wales, (58,311
square miles.) and the River Mississippi in America drains such a
large tract of country that its delta grows, Mr. Geikie tells us,
at the rate of 86 yards in year.

All this new land laid down in Egypt, in India, in America, and
in other places, is the work of water. Even on the Thames you
may see mud-banks, as at Gravesend, which are made of earth
brought from the interior of England.  But at the mouth of the
Thames the sea washes up very strongly every tide,  and so it
carries most of the mud away and prevents a delta growing up
there. If you will look about when you are at the seaside, and
notice wherever a stream flows down into the sea, you may even
see little miniature deltas being formed there, though the sea
generally washes them away again in a few hours, unless the
place is well sheltered.

This, then, is what becomes of the earth carried down by rivers.
Either on plains, or in lakes, or in the sea, it falls down to
form new land. But what becomes of the dissolved chalk and other
substances? We have seen that a great deal of it is used by river
and sea animals to build their shells and skeletons, and some of
it is left on the surface of the ground by springs when the water
evaporates. It is this carbonate of lime which forms a hard crust
over anything upon which it may happen to be deposited, and then
these things are called "petrified."

But it is in the caves and hollows of the earth that this
dissolved matter is built up into the most beautiful forms. If
you have ever been to Buxton in Derbyshire, you will probably
have visited a cavern called Poole's Cavern, not far from there,
which when you enter it looks as if it were built up entirely of
rods of beautiful transparent white glass, hanging from the
ceiling, from the walls, or rising up from the floor. In this
cavern,  and many others like it,*(See the picture at the head of
the lecture.) water comes dripping through the roof, and as it
falls carbonate of lime forms itself into a thin, white film on
the roof, often making a complete circle,  and then, as the water
drips from it day by day, it goes on growing and growing till it
forms a long needle-shaped or tube-shaped rod, hanging like an
icicle. These rods are called stalactites, and they are so
beautiful, as their minute crystals glisten when a light is
taken into the cavern, that one of them near Tenby is called the
"Fairy Chamber." Meanwhile, the water which drips on to the
floor also leaves some carbonate of lime where it falls, and this
forms a pillar, growing up towards the roof, and often the hanging
stalactites and the rising pillars (called stalagmites) meet in
the middle and form one column. And thus we see that underground,
as well as aboveground, water moulds beautiful forms in the crust
of the earth. At Adelsberg, near Trieste, there is a magnificent
stalactite grotto made of a number of chambers one following
another, with a river flowing through them; and the famous Mammoth
Cave of Kentucky, more than ten miles long, is another example of
these wonderful limestone caverns.

But we have not yet spoken of the sea, and this surely is not
idle in altering the shape of the land. Even the waves
themselves in a storm wash against the cliffs and bring down
stones and pieces of rock on to the shore below. And they help
to make cracks and holes in the cliffs, for as they dash with
force against them they compress the air which lies in the joints
of the stone and cause it to force the rock apart, and so larger
cracks are made and the cliff is ready to crumble.

It is, however, the stones and sand and pieces of rock lying at
the foot of the cliff which are most active in wearing it away.
Have you never watched the waves breaking upon a beach in a
heavy storm? How they catch up the stones and hurl them down
again, grinding them against each other! At high tide in such a
storm these stones are thrown against the foot of the cliff,  and
each blow does something towards knocking away part of the rock,
till at last, after many storms, the cliff is undermined and large
pieces fall down. These pieces are in their turn ground down to
pebbles which serve to batter against the remaining rock.

Professor Geikie tells us that the waves beat in a storm against
the Bell Rock Lighthouse with as much force as if you dashed a
weight of 3 tons against every square inch of the rock, and
Stevenson found stones of 2 tons'  weight which had been thrown
during storms right over the ledge of the lighthouse. Think what
force there must be in waves which can lift up such a rock and
throw it, and such force as this beats upon our sea-coasts and
eats away the land.

Fig. 28 is a sketch on the shores of Arbroath which I made some
years ago.  You will not find it difficult to picture to
yourselves how the sea has eaten away these cliffs till some of
the strongest pieces which have resisted the waves stand out by
themselves in the sea. That cave in the left-hand corner ends in a
narrow dark passage from which you come out on the other side of
the rocks into another bay. Such caves as these are made chiefly
by the force of the waves and the air, bringing down pieces of
rock from under the cliff and so making a cavity, and then as the
waves roll these pieces over and over and grind them against the
sides, the hole is made larger. There are many places on the
English coast where large pieces of the road are destroyed by the
crumbling down of cliffs when they have been undermined by caverns
such as these.

Thus, you see, the whole of the beautiful scenery of the sea -
the shores,  the steep cliffs, the quiet bays, the creeks and
caverns - are all the work of the "sculptor" water; and he works
best where the rocks are hardest, for there they offer him a
good stout wall to batter, whereas in places where the ground is
soft it washes down into a gradual gentle slope, and so the
waves come flowing smoothly in and have no power to eat away the
shore.

And now, what has Ice got to do with the sculpturing of the land?
First, we must remember how much the frost does in breaking up
the ground. The farmers know this, and always plough after a
frost, because the moisture, freezing in the ground, has broken
up the clods, and done half their work for them.

But this is not the chief work of ice. You will remember how we
learnt in our last lecture that snow, when it falls on the
mountains, gradually slides down into the valleys, and is pressed
together by the gathering snow behind until it becomes moulded
into a solid river of ice (see Fig. 29, Frontispiece). In
Greenland and in Norway there are enormous ice-rivers or glaciers,
and even in Switzerland some of them are very large. The Aletsch
glacier, in the Alps, is fifteen miles long, and some are even
longer than this. They move very slowly - on an average about 20
to 27 inches in the centre, and 13 to 19 inches at the sides every
twenty-four hours, in the summer and autumn. How they move, we
cannot stop to discuss now; but if you will take a slab of thin
ice and rest it upon its two ends only, you can prove to yourself
that ice does bend, for in a few hours you will find that its own
weight has drawn it down in the centre, so as to form a curve.
This will help you to picture to yourselves how glaciers can adapt
themselves to the windings of the valley, creeping slowly onwards
until they come down to a point where the air is warm enough to
melt them, and then the ice flows away in a stream of water. It is
very curious to see the number of little rills running down the
great masses of ice at the glacier's mouth, bringing down with
them gravel, and every now and then a large stone, which falls
splashing into the stream below. If you look at the glacier in the
Frontispiece, you will see that these stones come from those long
lines of stones and boulders stretching along the sides and centre
of the glacier. It is easy to understand where the stones at the
side come from; for we have seen that damp and frost cause pieces
to break off the surface of the rocks, and it is natural that
these pieces should roll down the steep sides of the mountains on
to the glacier. But the middle row requires some explanation. Look
to the back of the picture, and you will see that this line of
stones is made of two side rows, which come from the valleys
above. Two glaciers, you see, have there joined into one, and so
made a heap of stones all along their line of junction.

These stones are being continually, though slowly, conveyed by
the glacier,  from all the mountains along its sides, down to the
place where it melts.  Here it lets them fall, and they are
gradually piled up till they form great walls of stone, which
are called moraines. Some of the moraines left by the larger
glaciers of olden time, in the country near Turin, form high
hills,  rising up even to 1500 feet.

Therefore, if ice did no more than carry these stone blocks, it
would alter the face of the country; but it does much more than
this. As the glacier moves along, it often cracks for a
considerable way across its surface, and this crack widens and
widens, until at last it becomes a great gaping chasm,  or
crevasse as it is called, so that you can look down it right to
the bottom of the glacier. Into these crevasses large blocks of
rock fall, and when the chasm is closed again as the ice presses
on, these masses are frozen firmly into the bottom of the
glacier, much in the same way as a steel cutter is fixed in the
bottom of a plane. And they do just the same kind of work; for
as the glacier slides down the valley, they scratch and grind the
rocks underneath them, rubbing themselves away, it is true, but
also scraping away the ground over which they move. In this way
the glacier becomes a cutting instrument, and carves out the
valleys deeper and deeper as it passes through them.

You may always know where a glacier has been, even if no trace of
ice remains; for you will see rocks with scratches along them
which have been cut by these stones; and even where the rocks
have not been ground away, you will find them rounded like those
in the left-hand of the Frontispiece,  showing that the glacier-
plane has been over them. These rounded rocks are called "roches
moutonnees," because at the distance they look like sheep lying
down.

You have only to look at the stream flowing from the mouth of a
glacier to see what a quantity of soil it has ground off from
the bottom of the valley;  for the water is thick, and coloured a
deep yellow by the mud it carries.  This mud soon reaches the
rivers into which the streams run; and such rivers as the Rhone
and the Rhine are thick with matter brought down from the Alps.
The Rhone leaves this mud in the Lake of Geneva, flowing out at
the other end quite clear and pure. A mile and a half of land
has been formed at the head of the lake since the time of the
Romans by the mud thus brought down from the mountains.

Thus we see that ice, like water, is always busy carving out the
surface of the earth, and sending down material to make new land
elsewhere. We know that in past ages the glaciers were much
larger than they are in our time;  for we find traces of them
over large parts of Switzerland where glaciers do not now exist,
and huge blocks which could only have been carried by ice, and
which are called "erratic blocks," some of them as big as
cottages, have been left scattered over all the northern part of
Europe. These blocks were a great puzzle to scientific men till,
in 1840, Professor Agassiz showed that they must have been brought
by ice all the way from Norway and Russia.

In those ancient days, there were even glaciers in England; for
in Cumberland and in Wales you may see their work, in scratched
and rounded rocks, and the moraines they have left. Llanberis
Pass, so famous for its beauty, is covered with ice-scratches,
and blocks are scattered all over the sides of the valley. There
is one block high up on the right-hand slope of the valley, as
you enter from the Beddgelert side, which is exactly poised upon
another block, so that it rocks to and fro. It must have been
left thus balanced when the ice melted round it. You may easily
see that these blocks were carried by ice, and not by water,
because their edges are sharp,  whereas if they had been rolled
in water, they would have been smoothed down.

We cannot here go into the history of that great Glacial Period
long ago,  when large fields of ice covered all the north of
England; but when you read it for yourselves and understand the
changes on the earth's surface which we can see being made by
ice now, then such grand scenery as the rugged valleys of Wales,
with large angular stone blocks scattered over them, will tell
you a wonderful story of the ice of bygone times.

And now we have touched lightly on the chief ways in which water
and ice carve out the surface of the earth. We have seen that
rain, rivers, springs, the waves of the sea, frost, and glaciers
all do their part in chiselling out ravines and valleys, and in
producing rugged peaks or undulating plains - here cutting through
rocks so as to form precipitous cliffs, there laying down new land
to add to the flat country - in one place grinding stones to
powder, in others piling them up in gigantic ridges. We cannot go
a step into the country without seeing the work of water around
us; every little gully and ravine tells us that the sculpture is
going on; every stream, with its burden of visible or invisible
matter, reminds us that some earth is being taken away and carried
to a new spot. In our little lives we see indeed but the very
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
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

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