“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. [Illustration: 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 door and the fire we can feel this stream of air. Something else is taking place in the grate besides the burning of coal. What is it, Tom?” “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. [Illustration: 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 down one side of the card and come up the other side.