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.
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.
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.