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?
CHAPTER XII.
MORE ABOUT BALLOONS.
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.