t cool, and then it came down."

"I should like to see such a ball as that," said Nathan.

"Such a ball as that is called a _balloon_," said his father.

"I wish I could see a balloon," said Nathan.

QUESTIONS.

What is the important difference between air and water, which
was explained in the last chapter, and mentioned in this? Does
the air tend to expand again after it is compressed? What is
this property of the air called? Is the air around us already
condensed, or is it in its natural state? What causes it to be
condensed? Suppose a thin glass vessel were to be filled with
air, and another with water, and the air suddenly removed from
the room around them; what would be the effects? What effect
does heat have upon the expansibility of air? How may this
effect be made to appear over a lamp? In a chimney? What was the
story which Rollo's father told Nathan?

CHAPTER VII.

PHILOSOPHICAL DISCUSSION.

Some time after this, Rollo, and Nathan, and James, were playing in the
shed, one pleasant morning in the spring. They were building with sticks
of wood, which they piled in various ways, so as to make houses. They
took care not to pile the wood, in any case, higher than their
shoulders, for Jonas had told them that, if they piled the wood higher
than that, there would be danger of its falling down upon them.

After some time, Rollo went into the house a few minutes, and James and
Nathan went to the open part of the shed, and began to look out of
doors. The sun was shining pleasantly, but the ground was wet, being
covered with streams and pools of water, and melting snow-banks.

"What a pleasant day!" said James. "I wish it was dry, so that we could
go out better."

"I wish we could fly," said Nathan, "for it is very pleasant up in the
air."

"I wish we had a balloon," said James. "If we had a balloon, we could go
up in the air, easier than to fly."

"O James," said Nathan, "you could not get into a balloon if you had
one."

"Why not?" said James.

"Because," said Nathan, "it would not be big enough."

"Why, Nathan," said James, "a balloon is bigger than this house."

"O James, it is not higher than my head."

"It is," said James, "I know it is. I have read about balloons bigger
than a house."

"And my father," said Nathan, putting down his foot in a very positive
air, "my father told me himself, that a balloon was about as high as my
head.