ou must write down what I tell you."

"But I can't write," said Nathan.

"No," said his mother, "but you can tell Rollo what you would wish to
say, and he will write it for you."

"Why, mother," said Rollo, "I don't think that that will be very good
play."

"No," replied his mother, "I don't give it to you for play. It will be
quite hard work. I hope you will take hold of it energetically, and do
it well.

"First," said she, "I am going to perform some experiments for you,
before I tell you what I want you to write."

By this time, she had cut off the downy part of several feathers, and
had laid them together in a little heap. Then she took a fine thread,
and tied this little tuft of down to the end of it. Then she took up the
thread by the other end, and handed it to Rollo.

"There, Rollo," said she. "Now, do you remember what your father told
you, the other day, about the effect of heat upon air?"

"It makes it light," said Rollo.

"And why does it make it light?" asked his mother.

"Why, I don't exactly recollect," said Rollo.

"Because it swells it; it makes it expand; so that the same quantity of
air spreads over a greater space; and this makes it lighter, But cool
or cold air is heavier, because it is more condensed.

"Now, wherever there is heat," continued his mother, "the air is made
lighter, and the cool and heavy air around presses in under it, and
buoys it up. This produces currents of air. You recollect, don't you,
that your father explained all this to you the other day?"

"Yes," said Rollo, "I remember it."

"Well," said his mother, "now you and Nathan may take this little tuft,
and carry it about to various places, and hold it up by its thread, and
it will show you the way the air is moving; and then you may come to me,
and I will explain to you why it moves that way."

"Well," said Rollo, "come, Nathan, let us go. First we will hold it at
the key-hole of the door."

Rollo held the end of the thread up opposite to the door, in such a way,
that the tuft was exactly before the key-hole. The tuft was at once
blown out into the room.

"O, see, Nathan, how it blows out. The air is coming in through the
key-hole."

"Yes," said his mother; "when there is a fire in the room, and none in
the entry, then the cold air in the entry runs down through the
key-hole into the room."

"It don't run down, mother," said Rollo; "it blows right in straight."

"Perhaps I ought to have said it spouts i