"For," said he, "there is so large a space between the edge of the door and the wall, that the air that is put in motion by the movement of the door, can pass directly round the edge, back into the closet again. It is only when the door is almost shut, when the edge of it comes close to the casing all around, that the movement of the door drives the air out."
Then he took hold of the latch of the door, and put it almost to, very gently. He turned the latch so as to prevent its snapping against the catch, and then pushed it suddenly into its place three or four times, opening the door only a very little way every time.
"Now," said he, "hold the lamp at the key-hole, and watch the flame, while I shut the door two or three times in this way."
Rollo did so, Nathan standing all the time by his side. They observed that the flame of the lamp was driven into the room every time the door was shut; proving that, every time a little of the air was driven out by the door, a little puff rushed in at the key-hole.
"Let us stop up the key-hole," said Rollo, "and then it can't get in."
"Yes," said his father, "there are a great many little crevices all around the closet, where the air can come in."
"Couldn't we stop those up too?" said Rollo.
"No," said his father, "not so as to make the closet air-tight. For, if the crevices could all be stopped exactly, the air would come in through the very wood itself."
"How?" said Rollo.
"Why, there are little pores in wood, that is, little channels that the sap flowed in when the wood was growing, and the air can pass through these."