the down, and lay it
gently upon the hearth, before the fire, as near as you can."

Rollo did so. He had to take his hand away very quick, for it was quite
hot there. The little tuft remained quietly upon the hearth where he
placed it.

"There," said his father, "is not that a hotter place than it was over
the lamp?"

"Yes, sir," said Rollo.

"Then, if it was heat that made it rise, why does not it rise now?"

Rollo could not tell.

"I will tell you how it was," said his father. "Heat makes air more
expansive. When air is heated, it swells; when it is cool, it shrinks
again. Now, if it swells, it becomes lighter, and so it is buoyed up by
the heavier air around it; just as wood at the bottom of the sea would
be buoyed up, and would rise to the surface of the water. Now, the heat
of the lamp heats the air that is in the glass chimney, and swells it.
This makes it lighter; and so the air around it, which is heavier,
buoys it up, and it carries up the feather with it."

"No, the down, father," said Nathan.

"Yes, the down," said his father.

"Then it seems to me, after all," said Rollo, "that it is the heat which
makes it rise."

"Yes," said his father, "it does, indirectly. It expands the air; that
makes it lighter; then the heavy air around it buoys it up, and, when it
goes up, it carries up the down. So that it is not strictly correct to
say, that the heat carries it up. The heat sets in operation a train of
causes and effects, the last of which results in carrying up the
feather.

"Now," continued his father, "there is always a stream of air going up,
wherever there is a lamp, or a fire, or heat, which heats the air in any
way. The expanded air from a fire goes up chimney. The cool and heavy
air in the room and out of doors crowds it up."

"The air out of doors?" said Rollo. "How can that crowd it up?"

"Why, it presses in through all the crevices and openings all around
the room, and crowds the light air up the chimney. All the smoke is
carried up too with it, and it comes pouring out at the top of the
chimney all the time."

"You can see that the air presses in at all these crevices," continued
Rollo's father, "by experiment."

"What experiment is it?" said Rollo; "let us try it."

"I will let Nathan try it," said his father, "and you may go with him
and see the effect. First," he continued, "you see by the smoke, that
the air really goes up the chimney; and I will show you that other air
reall