n," 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."

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 fini