Rollo's Philosophy. [Air]. Abbott Jacob

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      "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.

      "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, when they do, men cannot paddle, or row boats upon them. A current that should run at the rate of four or five miles an hour, would be very hard to row against. But the air is seldom in a state of less motion than that. It is very often moving at the rate of fifteen or twenty miles an hour; sometimes sixty. So, you see, there is a double reason why men cannot fly in the air, as well as paddle on the water."

      "If we were only light enough," said Rollo, "to float in the air, then we could fly."

      "We could paddle about in it, when it was calm," replied his father, "but that would not be flying."

      "Is there anything light enough to float in the air?" said Rollo.

      "No," said his father, "I don't think of any visible substance that is."

      "What do you mean by visible substance?" said Nathan.

      "Why, anything that you can see," replied his father. "There are some other kinds of air, which are lighter than common air, but there is nothing else, so far as I know."

      "Why, father, there are clouds. They float, and they are visible, I am sure."

      "Yes," said his father. "There is some mystery about the floating of clouds. I don't fully understand it. Clouds are formed of small globules or little balls of water; and water, I should think, whatever the size of the little drops might be, would be heavier than air. And yet they seem to float. If they are large, like rain drops, they fall quickly to the ground. If they are small, like mist, they fall slowly. That I should expect. If they are finer still, like vapor or fog, I should think that they would fall still more slowly; but still I should suppose that they would descend. But they do not appear to descend; they seem to float, nearly at rest; though perhaps all the clouds we see, may be slowly descending all the time, while we do not perceive it."

      "The smoke goes up from the top of the chimney," said Rollo.

      "Yes," said his father, "there is no difficulty about that. The vapor from a fire is carried up by the warm air, no doubt. Air swells when it is heated, and so becomes lighter, and rises; and the hot air from the top of the chimney carries the vapor up with it, no doubt. After it rises a little way, and becomes cool, it ceases to ascend, but floats away horizontally. Perhaps it begins to descend when it gets cool, though very slowly; and perhaps all clouds are really descending all the time, though too slowly for us to perceive the motion."

      "Only," said Rollo, "after a little time, they would get down to the ground."

      "Perhaps not," said his father; "for, when they get down nearer the earth, where it is warm, they may be gradually dissolved, and disappear, and thus never reach the earth. I should think they would descend, being composed of globules of water, which, however small, must, I should think, be heavier than air."

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