"No," replied his mother, "I don't give it to you for play. It will be quite hard work. I hope you will take hold of it energetically, and do it well. "First," said she, "I am going to perform some experiments for you, before I tell you what I want you to write." By this time, she had cut off the downy part of several feathers, and had laid them together in a little heap. Then she took a fine thread, and tied this little tuft of down to the end of it. Then she took up the thread by the other end, and handed it to Rollo. "There, Rollo," said she. "Now, do you remember what your father told you, the other day, about the effect of heat upon air?" "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." "Yes," said his mother; "when there is a fire in the room, and none in the entry, then the cold air in the entry runs down through the key-hole into the room." "It don't run down, mother," said Rollo; "it blows right in straight." "Perhaps I ought to have said it spouts in," said his mother, "just as the water did from the hole in your dam. And, now," she continued, "come and hold the tuft near the chimney." Rollo did so; and he found that it was carried in, proving, as their father had showed them before, that the heavy, cold air, pressing into the room, crowded the warm, light air up the chimney. "Now, should you think," said their mother, "that the cold air could come in through the key-hole, as fast as it goes up the chimney?" Both Rollo and Nathan thought that it could not. "Then go all around the room," said she, "and see if you can find any other place, where it comes in. For it is plain, you see, that the light air cannot be driven up chimney any faster than cold and heavy air comes in to drive it up and take its place."