BALLOONS AND BUBBLES.
[Sidenote: What makes the balloon go up?]
What is it that makes a balloon go up in the air? It is because it is
so light, you will say; but what it is made of is not as light as air
is. It will not, you know, fly off into the air before it is filled. It
is what it is filled with, then, that makes it so light.
[Sidenote: Its car.]
The balloon is filled with a gas that is much lighter than the air is
around it. This makes it so light that it flies up in the air very
rapidly, and to a great height; and if the balloon is very large, it
can carry up a person, or even two or three persons, in a sort of car
or boat attached to it, as represented on the following page.
The car is attached to the balloon in this way: A netting covers the
balloon, and the cords that hold the car are fastened to this netting.
It would not do, you know, to fasten them to the balloon itself, for
that is made of such light material that the cords would tear out with
the slightest pull upon them.
[Sidenote: How the balloon is made to come down.]
[Illustration]
[Sidenote: How the balloon is prevented from going down too fast.]
[Sidenote: The parachute.]
How do you think the person in this car manages when he wants to come
down? So long as all the light gas remains in the balloon, it will stay
up in the air. So, when he wants to come down, he lets out some of
this gas. He does this very carefully; for, if he lets out too much,
he will go down too fast. Sometimes he will go down too fast in spite
of all his care. He is prepared for this, however, in two ways. There
are sand-bags in the car, which he can throw out when the balloon is
falling too fast. This makes the car so much lighter that it commonly
relieves the difficulty; but if it does not, he can use the parachute.
This is a sort of umbrella, made very large and very strong. It is
represented here both as shut and as opened. You can see how the
resistance of the air against this when open would make him go down
much slower than he would go without his parachute.
[Illustration]
[Sidenote: The danger of going up in balloons.]
[Sidenote: A great escape.]
[Illustration]
Balloons are sometimes used in warfare, to observe battle-fields, or
send messages to and from besieged cities. They were so used in our own
war and at the recent siege of Paris. They will probably never come
into use in traveling; for, besides the expense and danger, a balloon
will always go with the wind, and you never can tell just how the wind
blows very high up in the air. It may blow there in a direction wholly
different from what it does below, close to the earth. An Englishman,
Major Money, went up in a balloon, with the wind blowing from the sea;
and he supposed that he should be carried far into the country, and
come down safely upon dry land. All was right till he had got up about
a mile. Then suddenly the balloon changed its course, and went out
toward the sea. This was because the wind up there blew in a direction
just opposite to that of the wind below. This wind took him far out to
sea, and when he came down he was nine miles from the land. He came
near being drowned. He held on to the cords of his balloon, as you see
here, for some time. After a while, a vessel came to his relief, and
took him on board. As such dangers attend going up in balloons, it is
wrong for any one to do it.
[Illustration]
[Sidenote: The hot-air balloon.]
You can fill a balloon with common air so as to make it fly up like
the gas balloon; but the air must be heated to do this. A boy can make
such a balloon very easily out of thin paper. He pastes the paper
together so as to shape it like a balloon, leaving one end open. It can
be filled with hot air by holding it over something burning, with its
open end down. It is sometimes done in another way. A sponge wet in
turpentine or alcohol is fixed under the opening of the balloon by a
little frame-work, as represented here; and if the balloon goes up with
the sponge still burning, it will stay up longer than it will if the
sponge goes out before it is let off, because the air will keep heated
longer.
It is because heated air is so much lighter than the air around it
that a balloon filled in this way goes up; but such a balloon comes
down soon. It will not keep up so long as a gas balloon will. Why is
this? It is because the heated air in the balloon becomes cooled, and
then it is no longer lighter than the air around it. The balloon itself
is heavier than air, and it goes up and stays up only when it is full
of something which is lighter than air.
[Illustration]
[Sidenote: How soap-bubbles are like balloons.]
Children often make balloons in another way. They make them of soap and
water, as you see here; for the soap-bubble that flies up in the air is
really a balloon; and how beautiful a one it is! How thin and delicate
is the covering of this ball of air! It is a sheet of nothing but soap
and water, and a touch breaks it; but it answers the purpose. It holds
the air, and away it flies.
Now what is the reason that the bubble flies up a little way and then
comes down? It flies up because the air in it is slightly heated, and
so is lighter than the air around it. It is heated or warmed air,
because it comes from the warm lungs of the person that blows the
bubble. But it soon becomes cool, and then the bubble comes down, just
as the balloon filled with hot air does when the air in it becomes
cool.
[Sidenote: Some things about bubbles that are not commonly thought of.]
There are some things to be noticed about this ballooning with soap
and water. The water must be warm, or your little balloons will not go
up. Why is this? If the water is cold, it will cool the air that comes
from your warm lungs, and so your soap and water balloon will be filled
with cold air instead of warm air. It will therefore drop to the floor
when you expect to see it go up. So, too, the bubbles will not go up so
easily and so high in a warm room as they will in cold air. The greater
the difference is in warmth between the air in the bubble and that
around it, the better it will go up.
The reason of this is plain. The cooler the air is, the heavier it is;
and the warmer the air inside of the bubble, the lighter is the bubble;
and the very light bubble goes up quickly in the heavy cold air for
the same reason that any light thing, like cork, rises very quickly in
water. Why it is that light things go up in the air and the water I
shall explain in the next chapter.
_Questions._--What is it in a balloon that makes it so light? How is
the car attached to the balloon? How does the person in the car manage
when he wants to come down? What does he do if he is coming down too
fast? What is a parachute, and of what use is it? Why will balloons
never be used for traveling? Tell about the Englishman. Tell about the
air balloon. Why will not this stay up as long as the gas balloon?
How do children often make balloons? Why does the soap-bubble go up a
little way and then come down? Why is it that the bubbles do not go up
if you use cold water? Why will they go up better in the cold air than
they will in a warm room?