ur arms to
do that?"

Rollo and Nathan looked very much interested in what their father was
saying, but they both admitted that they could not climb up such ladders
as those.

"The air," added their father, "gives way continually under the bird's
wing; and yet they have to pull themselves up by it. And this is very
hard. They must either have very large wings, and prodigious strength to
use them, so as to pull upon the air with very hard and heavy strokes,
or else, if they have small wings, they must have strength to strike
very quick and often with them.

"The wings of sparrows move so quick, that you cannot count the strokes;
and those of humming-birds, which are smaller still, so fast that you
cannot see them. They make a hum."

"I could make my wings go so fast," said Nathan; and he began to imitate
the flapping of the wings of a bird, with his arms, as rapidly and
forcibly as he could.

"So can I," said Rollo; and he made the same motions. "That is as fast
as crows' wings move, when they are flying."

"Yes," said his father, "crows move their _wings_ as fast as that,
whereas you only move hands and arms. If you had great wings, as long,
in proportion, as the crows, you could not move them so fast."

"How large would they be?" said Rollo.

"O, I don't know,--perhaps as big as the top of the dining-table."

"O father," said Rollo, "I don't think they would be as big as that. The
crow's wings are not longer than his body, and so mine would not be
longer than my body."

"Perhaps you never saw a crow's body," said his father. "His feathers
and his tail, which are very light, swell out his body, and make it
appear much larger than it really is. I presume his wings, when they are
spread, are twice or three times as long as his body. If you had wings
in proportion, it would be with the utmost difficulty that you could use
them at all. You certainly could not strike the air with them fast
enough to pull yourself up by them."

"I did not think that the birds pulled themselves up by the air," said
Nathan. "I did not know that the air was anything _real_."

"O yes; it is something real," said his father.

"I've seen birds fly without moving their wings at all," said Rollo.

"Yes," said his father, "and so have I seen a stone."

"A stone!" repeated Rollo.

"What, a stone fly?" said Nathan.

"Yes," replied his father; "did you never see a stone fly through the
air, without any wings at all?"

"Why, yes,"