said Rollo, "when somebody threw it."

"Very well," said his father. "If you set the stone in motion, it will
continue in motion for some time, without any wings; and so will a
bird."

"But, father, they don't throw birds," said Nathan; and he laughed aloud
at such an idea.

"Birds throw themselves," said his father; "that is, they strike their
wings upon the air, hard and quick, and thus get into very quick motion,
and then they can keep their wings still for a time, and go on, as long
as the impulse they have given them lasts. This shows what prodigious
strength they have in their wings. They can not only strike the air hard
and frequently enough to raise themselves up, and move along, but they
can do it so easily, as to get such a velocity, that they can rest their
wings for some time, and sail away through the air, only expending the
impulse they had accumulated."

Rollo and Nathan were silent. Rollo was thinking how he had seen the
swallows sailing swiftly round and round in the air, with their wings
spread out motionless by their side.

"So, you see," continued his father, "the difficulty in the way of a
boy's flying, is not the want of wings, but the want of strength to use
them. It would be very easy to make wings."

"Would it?" said Nathan.

"Yes," said his father. "At least it would not be very difficult.
Ingenious mechanics would soon find out modes of making something to
answer the purpose of wings, to strike upon the air, if there was the
necessary power to work them. The great difficulty in almost all cases
in mechanics is, in getting the power; there is very little difficulty
in applying it to any purpose it is wanted for. So, you see, next time,
Nathan, when you want to fly, you must wish, not that you had wings, but
that you were strong enough to use them."

"Well, father," said Rollo, "men are strong enough to paddle themselves
along in the water; why can't they in the air?"

"Because," said his father, "water supports them by its buoyancy, and
they have nothing to do but to move themselves along upon it. But air
cannot support them; and, of course, a great part of the effort which
they would make, would be required to keep them up. And then, besides,
the water is generally nearly at rest, but the air is generally in a
state of rapid motion."

"Why, father," said Rollo, "I'm sure water is sometimes in rapid motion.
The rivers run very swiftly, often."

"Yes," replied his father; "but then