HANGING IN SPACE
If you are holding something in your hand and you let it go, what
happens? It falls to the ground, of course. Now, why should it do so?
You will say: 'How could it do anything else?' But that is only because
you are hampered by custom. Try to shake yourself free, and think, Why
should it go down instead of up or any other way? The first man who was
clever enough to find some sort of an answer to this question was the
great philosopher Sir Isaac Newton, though he was not quite the first to
be puzzled by it. After years of study he discovered that every thing
attracts every other thing in proportion to their masses (which is what
you know as weight) and their distance from each other. In more
scientific language, we should say every _body_ instead of every
_thing_, for the word body does not only mean a living body, but every
lump or mass of matter in the universe. The earth is a body in this
sense, and so is the table or anything else you could name. Now as the
earth is immeasurably heavier than anything that is on it, it pulls
everything toward itself with such force that the little pulls of other
things upon each other are not noticed. The earth draws us all toward
it. It is holding us down to it every minute of the day. If we want to
move we have to exert another force in order to overcome this attraction
of the earth, so we exert our own muscles and lift first one foot and
then the other away from the earth, and the effort we make in doing this
tires us. All the while you are walking or running you are exercising
force to lift your feet away from the ground. The pull of the earth is
called gravitation. Just remember that, while we go on to something else
which is almost as astonishing.
We know that nothing here on earth continues to move for ever;
everything has to be kept going. Anything left to itself has a tendency
to stop. Why is this? This is because here in the world there is
something that fights against the moving thing and tries to stop it,
whether it be sent along the ground or thrown up in the air. You know
what friction is, of course. If you rub your hands along any rough
substance you will quickly feel it, but on a smooth substance you feel
it less. That is why if you send a stone spinning along a carpet or a
rough road it stops comparatively soon, whereas if you use the same
amount of force and send it along a sheet of ice it goes on moving much
longer. This kind of resistance, which we call friction, is one of the
causes which is at work to bring things to a standstill; and another
cause is the resistance of the air, which is friction in another form.
It may be a perfectly still day, yet if you are bicycling you are
breaking through the air all the time, just as you would be through
water in swimming, only the resistance of the air is less than that of
water. As the friction or the resistance of the air, or both combined,
gradually lessens the pace of the stone you sent off with such force,
the gravitation of the earth begins to be felt. When the stone first
started the force you gave to it was enough to overcome the gravitation
force, but as the stone moves more slowly the earth-pull asserts itself,
and the stone drops down to the ground and lies still upon the surface.
Now, if there were no friction, and therefore no resistance, there would
be no reason why anything once set moving should not go on moving for
ever. The force you give to any object you throw is enough to overcome
gravitation; and it is only when the first force has been diminished by
friction that the earth asserts its authority and pulls the moving
object toward it. If it were possible to get outside the air and out of
reach of the pull of the earth, we might fling a ball off into space,
and it would go on in a straight line until something pulled it to
itself by the force of gravity.
Gravitation affects everything connected with the earth; even our air is
held to the earth by gravitation. It grows thinner and thinner as we get
further away from the earth. At the top of a high mountain the air is so
thin that men have difficulty in breathing, and at a certain height they
could not breathe at all. As they cannot breathe in very fine air, it is
impossible for them to tell by personal experiment exactly where the air
ends; but they have tried to find out in other ways, and though
different men have come to different conclusions on the subject, it is
safe to say that at about two hundred miles above the earth there is
nothing that could be called air. Thus we can now picture our spinning
earth clothed in a garment of air that clings closely about her, and
grows thinner and thinner until it melts away altogether, for there is
no air in space.
[Illustration: THE EARTH AND MOON HANGING IN SPACE]
Now in the beginning God made the world, and set it off by a first
impulse. We know nothing about the details, though further on you shall
hear what is generally supposed to have taken place; we only know that,
at some remote age, this world, probably very different from what it is
now, together with the other planets, was sent spinning off into space
on its age-long journey. These planets were not sent off at random, but
must have had some particular connection with each other and with the
sun, for they all belong to one system or family, and act and react on
each other. Now, if they had been at rest and not in movement, they
would have fallen right into the sun, drawn by the force of gravitation;
then they would have been burned up, and there would have been an end of
them. But the first force had imparted to them the impulse to go on in a
straight line, so when the sun pulled the result was a movement between
the two: the planets did not continue to move in a straight line,
neither did they fall on to the sun, but they went on a course between
the two--that is, a circle--for the sun never let them get right away
from him, but compelled them to move in circles round him. There is a
very common instance of this kind of thing which we can see, or perhaps
feel, every day. If you try to sit still on a bicycle you tumble off,
because the earth pulls you down to itself; but if, by using the force
of your own muscles, you give the bicycle a forward movement this
resists the earth-pull, and the result is the bicycle runs along the
ground. It does not get right away from the earth, not even two or three
feet above ground; it is held to the earth, but still it goes forward
and does not fall over, for the movement is made up of the earth-pull,
which holds it to the ground, and the forward movement, which propels it
along. Then again, as another instance, if you tie a ball to a string
and whirl it round you, so long as you keep on whirling it will not fall
to the ground, but the moment you stop down it drops, for there is
nothing to fight against the pull of gravitation. Thus we can picture
the earth and all the planets as if they were swinging round the sun,
held by invisible strings. It is the combination of two forces that
keeps them in their places--the first force and the sun's pull. It is
very wonderful to think of. Here we are swinging in space on a ball that
seems only large to us because we are so much smaller ourselves; there
is nothing above or below it but space, yet it travels on day by day and
year by year, held by invisible forces that the brain of man has
discovered and measured.
Of course, every planet gives a pull at every other planet too, but
these pulls are so small compared with that of the sun that we need not
at present notice them. Then we come to another point. We said that
every body pulled every other body in proportion to their weights and
their distance. Now, gravity acts much more strongly when things are
near together than when they are far away from each other; so that if a
smaller body is near to another somewhat larger than itself, it is
pulled by it much more strongly than by a very much larger one at a
considerably greater distance. We have an instance of this in the case
of the earth and moon: as the earth responds to the pull of the sun, so
the moon responds to the pull of the earth. The moon is so comparatively
near to the earth that the earth-pull forces her to keep on going round
and round, instead of leaving her free to circle round the sun by
herself; and yet if you think of it the moon does go round the sun too.
Recall that game we had when the sun was in the middle, and the two
smaller girls, representing the earth and moon, went round it. The
moon-child turned round the earth-child, but all the while the
earth-child was going round the sun, so that in a year's time the moon
had been all round the sun too, only not in a straight line. The moon is
something like a dog who keeps on dancing round and round you when you
go for a walk. He does go for the walk too, but he does much more than
that in the same time. Thus we have further completed our idea of our
world. We see it now hanging in space, with no visible support, held in
its place by two mighty forces; spinning on year after year, attended by
its satellite the moon, while we run, and walk, and cry, and laugh, and
play about on its surface--little atoms who, except for the brain that
God has given them, would never even have known that they are
continually moving on through endless space.