There was in the china closet a small piece of furniture, very convenient about a house, called a step-ladder. It consisted of two wooden steps, and was made and kept there to stand upon, in order to reach the high shelves. Rollo brought out the step-ladder, and placed it in the door-way, and then ascended it. From the top he could reach nearly to the top of the door; but then, as his tuft was at the end of the thread, it hung down, of course, some little distance below his head. "Why, mother," said Rollo, "it goes _out_." "Yes," repeated Nathan, "it goes out." In fact, Rollo found that the tuft, instead of swinging into the room, was carried out towards the entry. "You have found out, then," said his mother, "where the hot air of the room goes to, to make room for the cold air, that comes in from the entry." "Yes, out into the entry," said Rollo. "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. Their mother opened the largest paper, and read as follows:--