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 wouldnot 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.
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 islet out; and if it be a hot-air balloon, it will stay there till the heated air begins to cool.
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.
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.
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.
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 theoil, 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.
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.
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.