parlor would flow out, through the upper part of the door-way,
into the entry. The water would settle down in the entry, until it was
level in both rooms, and then the lower parts of both rooms would be
filled with water, and the upper parts with air."

"Yes, mother," said Rollo.

"And it is just so with warm and cold air. If the parlor is filled with
warm air, made so by the fire, and the entry with cold air, and you open
the door, then the cold air, being heavier, will sink down, and spread
over the floor of both rooms; and the warm air, being light, will spread
around over the upper parts of both rooms; and this will make a current
of air, in at the bottom of the door-way, and out at the top.

"Now," continued his mother, "let me recapitulate what I have taught
you."

"What do you mean by _recapitulating_ it?" said Nathan.

"Why, tell you the substance of it, so that you can write it down
easier."

"O, I can write it now," said Rollo; "I remember it all."

"Can you remember it, Nathan?" said his mother.

"Perhaps I can remember some of it," said Nathan.

So Rollo and Nathan went out into another room, where Rollo kept his
desk, and they remained there half an hour. When they returned, they
brought their mother two papers.

Their mother opened the largest paper, and read as follows:--

"We took a tuft of down, tied to a thread, and held it in the
cracks and places that the air came in at, to see which way it
went. We held it at the window, and it blew _in_ very strong. At
the bottom of the door, it blew _in_ very strong too; but at the
top, it blew _out_, into the entry. So, when the entry is full
of cold air, and this room full of warm, the cold air will press
in and drive out some of the warm air, into the entry.

ROLLO."

The other paper was also in Rollo's handwriting, and was as follows:--

"If the entry was full of water, and the parlor full of air, and
the walls were water-tight, and you were to open the door
between the two rooms, the water would flow into the parlor down
below, and the air would flow into the entry up above. We tried
it with a tuft.

NATHAN."

QUESTIONS.

Why were Rollo and Nathan at first glad to see the rain? What
did their mother say to herself on the second day, when she
observed their weary and listless appearance? What did she at
first direct them to do? How did she prepare th