"Well," replied his father, "it is just so with the air. The air all around us is pressed down by the load of all that is above us. We are, in fact, down at the bottom of a great ocean of air, and the air here is loaded down very heavy." "How heavy?" said Rollo. "O, very heavy indeed," said his father. "Why, air is pretty light," said Rollo. "Yes," replied his father, "but then the column of it is very high." "How high?" said Rollo. "Why, between thirty and forty miles. But it grows thinner and thinner towards the top; so it is not as heavy, by any means, as a column of air would be, thirty miles high, and as dense all the way up as it is here." "What makes it grow thinner and thinner towards the top?" said Rollo. "Because," said his father, "that which is near the top, has not as much load of air above it, to press it down." "And that which is _at_ the top," said Rollo, "has none above it, to press it down." "No," replied his father. "And how thin is it there?" "Nobody knows," said his father. "What, nobody at all?" said Nathan. "No, I believe not; at least I do not; and I don't know that any body does." "How do they know, then, how high it is?" said Rollo. "The philosophers have calculated in some way or other, though I don't exactly know how. I believe they have ascertained how great the pressure of the air is here at the surface of the earth, and have calculated in some way, from that, how high the air must be to produce such a pressure." "And how high must it be?" said Nathan. "Why, between thirty and forty miles," said Rollo; "father told us once." "And yet," continued his father, "water, thirty or forty feet deep, would produce as great a pressure as a column of air of thirty or forty miles. That is, the air around presses about as heavily, and would force a jet of air through a hole with about as much force, as water would, coming out at the bottom of a dam, as high as a common three-story house." These explanations were all very interesting to Rollo and to his mother; but Nathan found it rather hard to understand them all, and he began to be somewhat restless and uneasy. At length he said,-- "And now, father, haven't you almost done telling about the air?" "Why, yes," said his father; "I have told you enough for this time; only you must remember it all." "I don't think I can remember it quite all," said Nathan.