MORE ABOUT BALLOONS.
[Illustration]
[Sidenote: A curious balloon that would not answer.]
Here is a balloon which was contrived in 1670, two hundred years ago,
by a man whose name was Lana. You would suppose, from the picture of
it, that it would go very well with its large sail for the wind to blow
it along. There are, you see, four large balls. These, made of copper,
were hollow. The air was to be pumped out of them, so that they might
be very light. Now with this balloon Lana did not expect to go up very
high, but to travel along considerably above all the houses and hills,
just in the direction in which the wind would carry him by his sail.
But his plan, though it looks well, as you see, on paper, failed. The
reason was this. If the balls were made quite thin, the air outside
would burst them in as soon as the air in them was pumped out; and if
they were made thick enough to prevent this, they were so heavy that
the balloon would not go up. From what I have told you in the chapter
on the air-pump, you will understand why the balls, when made thin,
were burst in by the outside air.
The first successful attempt at ballooning was made by Montgolfier, a
Frenchman, in 1783. His invention was that of the hot-air balloon, or
fire balloon, as it is often called. An improvement on this is to fill
the balloon with a light gas instead of hot air. It is in this kind of
balloon that persons go up, though some have gone up in the hot-air
balloon.
I have not yet told you the real cause of the rising of the balloon in
the air. Why, you will say, it is because it is so light, and light
things always rise. But what makes light things rise? That is the
question.
Light things do not go up of themselves. The birds and the insects, as
I have told you in Part II., make themselves go up by working their
wings with their muscles. But light things that have no life can not
rise of themselves. They are pushed up. And when any light thing has
got up as high as it can go, it stops merely because it can not be
pushed any higher.
[Sidenote: Balloons and other light things do not really go up, but are
pushed up.]
But how are balloons and other light things pushed up? This I will now
explain to you. The air around the balloon is heavier than the balloon
itself, which is filled with a light gas, or with air that is light
because it is heated; and so the air is trying all the time, as we may
say, to get below the balloon. In doing this, it pushes up the balloon;
and the balloon continues to be pressed upward till it comes to air
that is as light as the balloon is. If it be a gas balloon, it will
remain there till some of the gas is let out; and if it be a hot-air
balloon, it will stay there till the heated air begins to cool.
[Sidenote: Every thing gets as low as it can.]
Now, when the balloon goes down, it is because it has become heavier
than the air around it. It goes down because it tries, as we may say,
to get underneath the lighter air. In going up, the air pushed it up;
but now the balloon pushes the air up. The balloon presses the air that
is below it out of the way so as to get under it. This is what it keeps
doing all the way as it comes down.
[Sidenote: Experiment with a phial.]
I can make this clear by a comparison. Take a long phial. Before you
put any thing into it, you know it is filled with air. Pour some oil
into it. The oil is in the bottom of the phial, and the air is above
the oil. The reason is that the oil, being heavier than the air, has
gone down through it, and has pushed the air up from the bottom of the
phial and taken its place there. It has done to the air in the phial
what the falling balloon does to the air below it. Now pour a little
water in. This will do to the oil as the oil did to the air. It will go
down to the bottom, pushing the oil up above it; for water, you know,
is heavier than oil. If you pour now some quicksilver into the phial,
this heavy fluid will go down and push the water up above it.
You see, in this experiment, that what is heaviest always goes to the
lowest place, and so pushes up out of the way what is lighter. The oil
pushed up the air; then the water pushed up the oil; and then, again,
the quicksilver pushed up the water. And now you have all the four
things in the phial in their order. The heaviest, the quicksilver,
is at the bottom, and next is the water, and next the oil, and the
lightest, the air, is at the top.
[Sidenote: Another experiment with the phial.]
If you cork the phial and shake it well, you mix quicksilver, water,
oil, and air all together. Then, if you let it stand, you see a good
deal of confusion among them as they push to get their places. In
getting right again, each pushes up above it what is lighter than
itself. The struggle, as we may say, is to get the lowest place. Every
thing, no matter how light it is, stays down as low as it can till it
is pushed up.
[Sidenote: Experiment with a heavy gas.]
Now what you see with these different things in a phial is true of
different kinds of air, or gases. A heavy gas takes the lowest place,
while a lighter one goes up, or, rather, is pushed up. You remember
that I told you, in Chapter VIII., about a gas that is sometimes in the
bottom of wells, just above the water. This gas is heavier than air,
and so it stays at the bottom of the well, below the air, as the oil
in the phial lay between the lighter air above and the heavier water
below. If it were lighter than air, as the gas is with which balloons
are filled, the air would go down to the bottom of the well and push up
this gas, for the same reason that the oil in the phial pushed up the
air, and the water pushed up the oil, and the quicksilver pushed up the
water.
This gas can be poured out of a vessel very much as you would pour
water out of it. A pretty experiment with it is to pour it out upon a
lighted candle. It will flow down upon the flame and put it out. In
doing this, it pushes up the air that is around the candle.
Now you can see how the balloon is pushed up into the air. If a gas is
set loose that is lighter than air, it will be pushed up in the air in
the same way that, in the phial, air is pushed up by the oil, or the
oil by the water; and so the balloon, filled with the light gas, is
pushed up by the air. It makes no difference whether the gas is loose
or is in a light silk bag; in either case it will be pushed up. If
loose, it will be scattered about as it is pushed up; if in the bag or
balloon, it will be kept together.
[Sidenote: Comparison of the cork and the balloon.]
A cork rises in water for the same reason that a balloon rises in air.
The balloon is pushed up by the air around it because it is lighter
than the air, and so the cork is pushed up by the water because it is
lighter than the water. As you hold the cork under water, your hand
does to it what the fastenings do to the balloon: it keeps it from
being pushed up. And when the fastenings of the balloon are let go,
away it flies in the air, as the cork flies up in the water when you
let go of it.
When the cork gets to the surface of the water, it stops. It will not
go up in the air simply because it is heavier than air. But if you put
a bag full of light gas in the water and let it go, it will not stop,
like the cork, when it gets to the surface, but will keep on going up
because it is lighter than air, and so the air pushes it up just as the
water did.
_Questions._--Do light things, like balloons, rise in the air of
themselves? Tell about Lana’s balloon. Why did it not succeed? Who
invented the hot-air balloon? How many years ago was it? What kind of
balloon is used for going up into the air? What makes it rise? How is
it that the air pushes up a balloon? What makes the balloon go down?
What does it do to the air in going down? Tell about the experiment
with a long phial? How is it if you shake the phial well? What is
said about gases? Tell about the gas which is sometimes in wells.
Tell about the experiment with a candle. What becomes of a gas that
is lighter than air when it is set free? Does it make any difference
whether it is loose or is in a silk bag? Give the comparison of the
balloon and the cork.