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.