"It makes it light," said Rollo.
"And why does it make it light?" asked his mother.
"Why, I don't exactly recollect," said Rollo.
"Because it swells it; it makes it expand; so that the same quantity of air spreads over a greater space; and this makes it lighter, But cool or cold air is heavier, because it is more condensed.
"Now, wherever there is heat," continued his mother, "the air is made lighter, and the cool and heavy air around presses in under it, and buoys it up. This produces currents of air. You recollect, don't you, that your father explained all this to you the other day?"
"Yes," said Rollo, "I remember it."
"Well," said his mother, "now you and Nathan may take this little tuft, and carry it about to various places, and hold it up by its thread, and it will show you the way the air is moving; and then you may come to me, and I will explain to you why it moves that way."
"Well," said Rollo, "come, Nathan, let us go. First we will hold it at the key-hole of the door."
Rollo held the end of the thread up opposite to the door, in such a way, that the tuft was exactly before the key-hole. The tuft was at once blown out into the room.
"O, see, Nathan, how it blows out. The air is coming in through the key-hole."