  One day Uncle George made the children a fire-balloon. He took twelve strips of tissue-paper shaped as you see in the picture. These he pasted neatly together at the edges so as to form a kind of bag with a round opening at the bottom. A ring of wire was then fixed at the bottom to keep it firm, and across the ring was stretched another piece of wire. This was to hold a dry sponge by and by. Uncle George swung the balloon till it was filled with air. He told Frank to hold it by the ring while he heated the air inside the balloon. This was done by holding the mouth of the balloon over a piece of rag which had been dipped in spirits and set on fire. Strip for Balloon. Soon Frank felt the balloon rising. He lifted it up away from the flame, while Uncle George moved the little sponge along the wire to the middle of the ring. Then he soaked the sponge with spirits and set fire to it. “Let go!” said Uncle George; and away went the balloon, soaring up towards the sky. Higher and higher it rose, moving with the wind. The children watched it until at last it seemed a mere dot in the sky, and then it went quite out of sight. “What makes the balloon rise up?” Dolly asked, as they returned to the house. For answer Uncle George took a cork and held it under a trough of water. When he let it go, the cork at once rose to the top of the water. “What makes the cork rise up?” he asked. “Because it is lighter than the water,” said Tom. “For just the same reason the balloon rises in the air,” said Uncle George. “Our balloon is only a bag filled with air.” “Then the air inside the balloon is lighter than that outside,” said Frank. “Yes,” said his uncle. “What did we do to it to make it lighter?” “We heated it,” said Tom. “And what has this taught you?” asked Uncle George. “It has taught us,” said Frank, “that if we heat air we make it lighter.” “What takes place is this,” said Uncle George. “When a small quantity of air is heated it swells out and fills a much larger space than before. It therefore becomes much lighter than the air round about it, and rises up through it. Come into the house and let us take another lesson from the fire. Strips pasted together. “We have already learned that the fire in burning uses up part of the air. After using up this part of the air, how is it that the fire does not go out?” “Because there is always a fresh supply of air coming into the grate!” said Frank. “Quite right. If we stand between the “Air is being heated!” “Very good, Tom, and what becomes of the heated air?” “It rises up the chimney, carrying the smoke with it; just as the heated air in our balloon rose up, carrying the paper bag with it,” said Frank. “That is really a clever answer, Frank. Now, can you tell me what makes this constant stream of cold air from the door to the fireplace?” “It is the cold air rushing in to take the place of the heated air that has gone up the chimney,” said Frank. “Very good indeed, Frank,” said Uncle George. “And now I am going to show you something which will prove all this very nicely.” He then took a saucer and poured some water into it. He placed a piece of lighted candle in the middle of the dish and put a lamp chimney over it. The candle burned for a few seconds and then went out. “Why does the candle go out?” he asked. “Because it has used up that part of the air which makes things burn,” said Tom. “That is right,” said Uncle George, and he began to cut a piece of stout card, shaped like the letter T, but broader every way. The upright part fitted into the top of the lamp chimney. “We are now going to give the candle flame a stream of fresh air,” he said, as he fitted the piece of card into the chimney. The candle was again lit and the chimney placed over it. This time it did not go out. It burned brightly, and the flame seemed to be blown from side to side. “That is very strange,” said Frank. Draught in Chimney Glass. Uncle George lit a piece of brown paper. “You will understand it now,” he said, as he held smoking paper near the top of the chimney. Then the boys saw a stream of smoke go “Oh, I see it now,” said Tom. “The card divides the chimney into two. The air, heated by the candle flame, rises up one side of the card, and the cold air goes down the other side to supply its place, drawing the smoke with it. The candle does not go out now, because it gets a constant stream of fresh air.” “Are the balloons, which are large enough to carry people, fire-balloons?” asked Dolly. “No, my dear,” replied Uncle George. “They are filled with a gas that is lighter—very much lighter—than the air. They rise up easily, and can carry quite a heavy load.” Questions and Exercises.
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