"Now," continued his father, "there is always a stream of air going up, wherever there is a lamp, or a fire, or heat, which heats the air in any way. The expanded air from a fire goes up chimney. The cool and heavy air in the room and out of doors crowds it up."
"The air out of doors?" said Rollo. "How can that crowd it up?"
"Why, it presses in through all the crevices and openings all around the room, and crowds the light air up the chimney. All the smoke is carried up too with it, and it comes pouring out at the top of the chimney all the time."
"You can see that the air presses in at all these crevices," continued Rollo's father, "by experiment."
"What experiment is it?" said Rollo; "let us try it."
"I will let Nathan try it," said his father, "and you may go with him and see the effect. First," he continued, "you see by the smoke, that the air really goes up the chimney; and I will show you that other air really crowds into the space, from other parts of the room."
So he took a lamp from the table,—not the study lamp; it was a common lamp,—and held it at various places in the opening of the fireplace, by the jambs and near the upper part; and Rollo and Nathan saw that the flame, in all cases, was turned in towards the chimney.
"Yes," said Rollo, "I see it is drawn in."
"No," said his father; "strictly speaking, it is not drawn in; it is pressed in, by the cool and heavy air of the room."
"I thought," said Rollo's mother, "that the chimney drew the air from the room into it."