"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."