So Rollo and Nathan went around the room, holding their tuft at all the places they could find, where they supposed there could be openings for the cold air to press in. They found currents coming in around the windows, and by the hinges of the doors; and at length Rollo said, he meant to open the window a little way, and see if the cold air from out of doors would not press in there too. He did so, and the tuft was blown in very far, showing that the cold air from out of doors pressed in very strongly. "Now, if all these openings were to be stopped," said their mother, "then no cold air could crowd into the room; and of course the hot air could not be buoyed up into the chimney, and a great deal of the hot air and smoke would come into the room. This very often happens when houses are first built, and the rooms are very tight. "But now, Rollo," she continued, "suppose that the door was opened wide; then should not you think that _more_ cold and heavy air would press in, than could go up the chimney?" "Yes, mother, a great deal more," said Rollo. "Try it," said his mother. So Rollo opened the door, and held his tuft in the passage-way; and he found that the air was pressing in very strongly through the open space. Wherever he held it, it was blown into the room a great deal, showing that the heavy air pressed in, in a torrent. "Now, as much warm air must go out," said she, "as there is cold air coming in; but I don't believe that you and Rollo can find out where it goes out." Rollo looked all around the room, but he could not see any opening, except the chimney and the door, and the little crevices, which he had observed about the finishing of the room. He said he could not find any place. His mother then told him to hold his tuft down near the bottom of the door-way. He did so, and found that the current of air was there very strong. The tuft swung into the room very far. "Now hold it up a little higher," said his mother. Rollo obeyed, and he found that it was still pressed in, but not so hard. "Higher," said his mother. Rollo raised it as high as he could reach. The thread was of such a length, that the tuft hung about opposite to his shoulder. The tuft was still pressed in, but not nearly as far as before. "So you see," said his mother, "that the air pours in the fastest at the lowest point, where the weight and pressure of the air above it are the greatest; just as, in your dam, the water from the lowest holes spouted out the farthest." "Yes," said Rollo, "it is very much like that." "Now," continued his mother, "you see that a great deal of air comes in, and if you look up chimney, you will see that there is scarcely room for so much to go up there;--and yet just as much must go out as comes in. "Get the step-ladder," said his mother, "and stand up upon it, and so hold your tuft in the upper part of the door-way."