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.