"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." "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."