"That is what is generally said," replied Mr. Holiday, "but it is not strictly true. The common idea is, that the hot air rises in the chimney, and so draws the air from the room to supply its place; but this is not so. In the first place, nothing can rise unless it is forced up. The lightest things have some weight, and would, if left to themselves, fall. The hottest and lightest air in a chimney would fall to the earth, if there was no cooler and heavier air around it, to force it to rise;—just as the lightest cork, which would rise very quick from the bottom of the sea, would fall back again very quick, if the water was not there.
"Remember, then, Nathan and Rollo, that, when a fire is built in a fireplace, so as to warm the air in the chimney, it makes this air not so heavy; and then the cool air all around it in the room and out of doors, presses in, and crowds under the light air, and makes it ascend."
"But, father," said Nathan, "you said I might perform an experiment."
"Very well, I am ready now. Take the lamp, and carry it around the room, and hold it opposite any little opening you can find."
"I can't find any little openings," said Nathan.
"O yes," said his father; "the key-hole of the door is a little opening, and there is a narrow crevice all around the door; and you will find little crevices around the windows. Now, hold the lamp opposite any of these, and you will see that the air presses in."
So Nathan went with the lamp, Rollo following him, and held the lamp opposite to the key-hole, and the crevices around the door and windows; only, when he came to the window, his father told him to be very careful not to set the curtain on fire.
Rollo wanted Nathan to let him try it once; and so Nathan gave him the lamp. He said he meant to make a crevice; and so he pushed up the window a very little way, and held the lamp opposite to the opening. The air pressed the flame in towards the room, in all cases.
"People commonly say, that it is drawn in," said his father, "but that is not strictly correct; it is really pressed in. There is no power of attraction, in the air that is in the room, to draw in the air that is out of doors, through the crevices; but the air that is out of doors, is so heavy, that it presses in, and crowds the warm and light air up the chimney.
"And now," said his father, "I cannot tell you anything more this evening; but, if you remember this, I will give you some further instruction another time."