t with the lamp. He held the lamp opposite to the
key-hole, and pretty near to it, and then he asked Nathan to shut the
other door suddenly. Nathan, who was standing all ready by the other
door, which was about half open, put his two hands against it, and
pushed it to, with all his strength, producing a great concussion.

"O Nathan," said his father, "you need not be quite so violent as that."

"It succeeded, father, it succeeded," said Rollo.

"I'm glad it succeeded," said his father; "but Nathan need not have shut
the door with so much force."

"I wanted to drive out all the air," said Nathan.

"I'll show you how to do it," said his father.

Rollo's father accordingly arose, and came to the closet door. He
opened the door wide, and then explained to the boys, that the beginning
of the movement of the door, when it was wide open, did not drive out
any air.

"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 p