"Through the upper part of the door," said his mother. "Suppose the entry were full of water, and the parlor full of air, and the door was shut, and the door and the walls were water-tight. Now, if you were to open the door, you see that the water, being heavier, would flow in, through the lower part of the door-way, into the parlor, and the air from the parlor would flow out, through the upper part of the door-way, into the entry. The water would settle down in the entry, until it was level in both rooms, and then the lower parts of both rooms would be filled with water, and the upper parts with air."
"Yes, mother," said Rollo.
"And it is just so with warm and cold air. If the parlor is filled with warm air, made so by the fire, and the entry with cold air, and you open the door, then the cold air, being heavier, will sink down, and spread over the floor of both rooms; and the warm air, being light, will spread around over the upper parts of both rooms; and this will make a current of air, in at the bottom of the door-way, and out at the top.
"Now," continued his mother, "let me recapitulate what I have taught you."
"What do you mean by recapitulating it?" said Nathan.
"Why, tell you the substance of it, so that you can write it down easier."
"O, I can write it now," said Rollo; "I remember it all."
"Can you remember it, Nathan?" said his mother.
"Perhaps I can remember some of it," said Nathan.
So Rollo and Nathan went out into another room, where Rollo kept his desk, and they remained there half an hour. When they returned, they brought their mother two papers.