GRAVITATION.
If I should ask you why things in the air fall to the ground, you would
probably say it is because it is downward, and every thing must come
down that is not held up in some way. But what is down, and what is up?
This I will explain to you.
[Sidenote: How it is known that the earth is round.]
The earth, as perhaps you know, is as round as an orange, and people
can travel around it just as you can pass your finger around over the
orange. This, indeed, was one of the ways in which it was found to be
round. Another proof of its being round is this: As you see a ship go
out to sea, if you watch it for a long time, after a while the body of
the ship will go out of sight, and you will see nothing but the sails,
and then the sails will gradually go out of sight also. What does this
prove? Why, that the water is not flat, as it appears to be to us, but
that it makes a part of the rounded surface of the earth. This figure
will make this plain to you. The eye that is represented sees the whole
ship at _b_; but when it gets as far as _a_, the eye can see only the
streamer at the top of the mast.
[Illustration]
The reason that we do not see that the earth is round is that we are
so small and the earth is so large. We see that a globe is round, but
it probably seems flat to any little fly that lights upon it, just as
the earth does to us.
[Sidenote: What is up and what is down.]
You can see, then, that as the earth is round, what is down to people
on the other side of the earth is up to us. If a boy there throw up a
ball at the same time that you throw up one here, the two balls fall
toward each other when they come to the ground.
[Sidenote: Figure illustrating this.]
What we call down, then, is simply toward the ground, or, rather, it
is toward the middle of the earth, for we say down in a well or down
in the ground. Indeed, if any thing could keep on in the same line in
which it falls, it would go right to the centre of the earth. If the
ball which you throw up and the ball thrown up by a boy on the other
side of the earth should keep on in the ground in the same direction
that they fall, they would meet exactly at the earth’s centre. This is
represented in this figure. The circle represents the round earth. The
lines drawn from the two falling balls to the middle of the circle show
how they would come together at the centre of the earth if they could
keep on, instead of being stopped when they reach the ground. And all
the things that are falling any where on the earth are going toward the
same point.
[Illustration]
Now why is this? What is it that makes things in the air come to the
ground when they are not held up? They do not come down of themselves.
They are drawn down. The earth attracts or draws them. How it does
this we do not know. We can not see how it is done, but the earth does
it as really as if we could see it put up a hand and pull things down.
[Sidenote: Attraction not a thing that we can see.]
There are other kinds of attraction that operate in a way that we can
not see nor understand. There is the attraction of magnetism. If, as I
have told you in the last chapter, you bring a magnet toward a piece of
iron or steel, for example a needle, when you get it quite near, all
at once the needle will go to the magnet and stick to it. You can not
see any thing between the magnet and the needle to draw the needle to
it. You only know that the needle is drawn or attracted, but you do not
know how this is done.
It is just so with this attraction which the earth has for all things,
drawing them to it. You can not see any thing any more than you can in
the case of the magnet and the needle, but the attraction is as real as
if you could see it. You can see what it does, as you can see what is
done by the attraction of magnetism.
This attraction is called the attraction of gravitation. It is stronger
with some things than it is with others. When any thing is drawn
strongly to the earth, we say that it is very heavy; but when a thing
is not strongly attracted, we say that it is light. When you take
hold of a stone to raise it up, you have this attraction of the earth
acting against you. This is pulling the stone down while your muscles
are trying to raise it. If the stone is very large, the earth attracts
it so strongly that the force of your muscles can not overcome the
attraction. If the earth would only stop pulling upon the stone, you
could raise it easily enough.
[Sidenote: Attraction the cause of weight.]
You see, then, what weight is. It is the pressure made by a thing as
the earth draws or attracts it to itself. The stronger this attraction
is, the greater is the pressure--that is, the weight. If you lay a
foot-ball upon your foot, you scarcely feel the pressure of it; but
if you lay a stone of the same size upon your foot, it presses very
hard. The reason is, that the stone is drawn toward the earth much more
strongly than the foot-ball. The foot-ball is drawn lightly, and so
presses a little; but the stone is drawn much, and so presses a great
deal. Your foot, being between the stone and the earth, is pressed by
the stone as the earth draws it to itself. It is just as you would be
pressed if you were between me and some one that I was drawing toward
me.
The reason that the stone is attracted more strongly, or has more
weight, than the foot-ball is, that there is more substance to it--that
is, the particles in it are closer together. So lead or iron is heavier
than wood, because the wood is much more porous: you can see pores and
spaces in it, while you can not in the lead and iron. You remember
what I told you about the hot-air balloon. This has not as much weight
as it would have if it were full of cold air. The reason is, that the
particles of cold air are closer together than the particles of hot
air; for, you know, heat swells air--that is, it puts its particles
farther apart.
If you drop a bag of feathers, it falls to the ground because the earth
attracts it. If, now, you drop a stone upon this bag, it sinks down
in the midst of it, because the earth attracts it much more strongly
than it does the loose feathers. It is for the same reason that a stone
sinks in water. The earth attracts the stone more than it does the
water.
[Sidenote: Why light things rise in the air and in the water.]
Wood will not sink in water as the stone does, for it is not drawn down
to the earth as hard as the water is; but wood will fall through air to
the ground, because the wood is attracted by the earth more strongly
than the air is. If you put a block of wood down in the water, and
then let it go, it rises to the surface. Why is this? It is because
the water, being more strongly drawn down by the earth than the wood,
pushes the wood up out of the way. It is for the same reason that
the balloon filled with hot air or with light gas rises. It is not
attracted to the earth as much as the cool air around it is, and so it
is pushed up out of the way.
Every thing, you see, then, is attracted by the earth. The air itself
is kept close to the earth by this attraction. It makes a sea, as we
may say, all around the earth about forty-five miles deep. Beyond that
there is no air except around some of the other worlds that we see
far off in the sky. Now the air would fly off and spread every where
among the stars if the earth did not attract it and thus keep it around
itself. The air moves about freely like the water, but it can not fly
away from the earth any more than the water can. The earth keeps both
its air and water all to itself by attraction.
[Sidenote: Every thing tries to get as close to the earth as possible.]
Every thing gets as close to the earth as it can, because every thing
is attracted by the earth. There is nothing that is of itself disposed
to go up, but every thing, even the air, is pressing down, the heaviest
always getting the lowest if it can, and there is sometimes a sort of
strife as to which shall be lowest. When a stone is put upon a heap
of feathers, the earth pulls upon it so much harder than it does on
the feathers that the stone presses to get through them to the earth;
but as it can not thrust them out of the way, it crushes them down in
the struggle to get below them. The struggle is a different one with
the stone in water. The water clings to the earth, but it is easily
pushed away by the stone as it tries to get below the water. Even in
the going up of a balloon you can see the same struggle. It would stay
down if it could. It goes up, as I have before told you, simply because
the cold air about it, being more strongly attracted by the earth than
the balloon is, tries to get below the balloon. If the cold air could
be taken away, the balloon would stay down, for the same reason that
a block of wood would remain in the bottom of a bowl if there were no
water in it. The block, attracted by the earth, will stay as near the
earth as it can. The water pushes it up because it is attracted by the
earth more than the block is, and for the same reason the air pushes up
the balloon.
_Questions._--What is the common idea about the falling of things to
the ground? What is one of the proofs that the earth is round? What
is another proof? Why can not we see that the earth is round? What is
meant by down and up? Tell what is represented by the figure. What is
it that makes things fall to the ground? Give the comparison about the
attraction of magnetism. What is said about the earth’s attracting
some things more strongly than others? What is weight? Explain by
telling about the foot-ball and the stone. Why is the stone attracted
more strongly than the foot-ball? Why are lead and iron heavier than
wood? Why is a hot-air balloon lighter than the air around it? Tell
about the feathers and the stone. Why will not wood sink in water as
stone does? Give the comparison between the block of wood and the
balloon. What is said about the earth’s attracting the air? Is there
any thing that does not press down? Which always gets the lowest if
it can? Tell about the stone put on the feathers and dropped in the
water. Give the comparison between the balloon and the block of wood
in a bowl.