"What made it?" said his father.
"Why, the water above it was heavy, and pressed down upon it, and crowded it out through the hole."
"Yes," said his father, "and the deeper the water, the more heavily it was pressed."
"Yes, sir," said Rollo, "and the farther it spouted."
"Because it was pressed down by the load of such a high column of water."
"Yes, sir," said Rollo.
"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?"
"n.o.body knows," said his father.
"What, n.o.body 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.
"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."
"I think that is a little too much for me to remember," said Nathan.
Nathan"s father and mother laughed on hearing this, though Nathan did not know what they were laughing at. His father told him that he could not expect him to remember all; and that, to pay him for his particular attention, he would tell him a story.
So he took Nathan up in his lap, and told him a very curious story of a boy, who went about the yard with a little dog upon one of his shoulders, a cat upon the other, and a squirrel on his head. The squirrel was tame.
QUESTIONS.
Why cannot experiments be performed upon the pressure of air, as conveniently as upon the pressure of water? How did Rollo"s father contrive to remove a part of the air from the china closet? Where did they expect that the air would be forced into the closet? How were they to make this effect visible? Did the experiment succeed? Suppose the key-hole had been stopped up; where would the air have been forced in? Suppose all the crevices had been closed. Is water compressible? Is air compressible? What is the shape of a cylinder? What is a piston?
How might air be compressed by means of a cylinder and piston?
What was the general principle which Rollo"s father stated, in conclusion?
CHAPTER VI.
BALLOONING.
The next evening, Rollo and Nathan had another conversation with their father, respecting air. When they were all seated, he commenced as follows:--
"I told you yesterday, that air may be compressed by force, while water cannot be. It has another property, which is in some respects the reverse of this. It springs back into its original bulk, when the pressure is removed."
"How?" said Nathan; "I don"t exactly understand you."
"Why, you remember what I said about the experiment with the iron cylinder and a piston to fit it."
"Yes, sir," said Rollo.
"What was the experiment?" said his father.
"Why, if a man were to press the piston down hard, he could crowd the air all into the lower half of the cylinder."
"Yes," replied his father. "Now, the property I am going to tell you about this evening is this--that, if the man lets go of the piston rod, the air that is condensed into the bottom of the cylinder, will spring up, and force the piston up again. This property is called _elasticity_.
It is sometimes called the _expansive force_ of the air. For it is a force tending to expand the air, that is, to swell it out into its original dimensions. This is another great difference between air and water.