"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," 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.