not stand up
straight; and that is what I call pretty wide shooting."

So saying, he rose from his seat, and walked away, appearing to be very
much amused. James laughed too, and even Rollo could not help smiling at
the ridiculous figure which his display of his learning made. As for
Nathan, he continued to look grave; and said he did not see that it was
any shooting at all.

After a short pause, Rollo's mother said, "So you see, children, the
cause of all the pressure, both of air and of water, and all the effects
produced by them, are the results of their gravitation towards the
earth."

"Yes," said Rollo, "I believe I understand it now."

After this, Rollo took James and Nathan out into the yard, to see if
some beans had come up, which he had been planting in a sunny corner of
the garden the day before.

QUESTIONS.

What was Mr. Holiday's apostrophe to the earth? What is the
cause of weight? Why did the boys wish that there was no
gravitation? What was the first evil consequence which their
father said would ensue, if there was no gravitation? What was
the second evil consequence? What did their mother say after the
conversation with their father was closed?

CHAPTER XI.

AIR IN MOTION.

Rollo's dam, which he had made when he was studying the philosophy of
water, and which was at first undermined by the pressure of the water,
was afterwards carried away by its momentum. Rollo learned, at that
time, that water moving rapidly had a great momentum; and about this
time he had an opportunity of learning that air, when in motion, had a
momentum too, capable of producing very powerful effects. The
circumstances of the case were as follows:--

One morning, towards the latter part of March, Jonas, being out in the
barn, observed some indications that the roof wanted repairs. It had
been strained and weakened by the heavy snows in the winter. He reported
the fact to Rollo's father, who said that he might go, the next day, and
get the carpenter to come and repair it. The carpenter lived ten miles
distant, near the shore of a long pond.

When Rollo heard of this proposed expedition, he wanted to go too; and
his father gave him permission. Jonas was going in the wagon. He told
Rollo, the evening before, that he meant to set out at six o'clock.

"But suppose it looks like a storm," said Rollo.

"Then there will be more need of going," said Jonas; "for if the
equinoctial storm