"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.
"Well, then, remember the general principle, at any rate," said his father, "which is this—that we live at the bottom of a vast ocean of air, and that the lower portions of this air are pressed down by the load of all the air above; that, being so pressed, the lower air is condensed,—so that we live in the midst of air that is pressed down, and condensed, by the load of all that is above it; and that, consequently, whenever the air is taken away, even in part, from any place, as you removed some of it from the china closet, the pressure upon the air outside forces the air in through every opening it can find."