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.