PRESSURE.
One evening, just after tea, Rollo came to his father, who was sitting
by the side of the fire, and said,--
"Father, I wish we could see the air, as we can the water, and then
perhaps we could try experiments with it."
"O, we can try experiments with the air as it is," said his father.
"Can we?" said Rollo; "I don't see how."
"We cannot see the air, it is true; but then we can see its effects, and
so we can experiment upon it."
"Well, at any rate," said Rollo, "we can't build a dam, and make it
spout through a hole, like water."
"No," said his father, "not exactly. In your dam, for instance, when it
was full, you had water on one side of the board, and no water on the
other; and then, by opening a hole in the board, the water spouted
through; but we cannot very well get air on one side of a partition, and
no air on the other; if we could, it would spout through very much as
the water did."
"Why can't we do that, sir?" said Rollo.
"Because," replied his father, "we are all surrounded and enveloped with
air. It spreads in every direction all around us, and rises many miles
above us. Whereas, in respect to water, you had one little stream before
you, which you could manage just as you pleased. If you were down at the
bottom of the sea, then the water would be all around you and above you;
and there, even if you could live there, you could not have a dam."
"No, sir," said Rollo, "the water would be everywhere."
"Yes," replied his father, "and the air is everywhere. If, however, we
could get it away from any place, as, for instance, from this room, then
bore a hole through the wall, the weight of the air outside would crowd
a portion of it through the hole, exactly as the weight of the water
above the board in your dam crowded a part through the hole in the
board."
"I wish we could try it," said Rollo.
"We _can_ try it, in substance," said his father, "in this room; or--no,
the china closet will be better."
There was a china closet, which had two doors in it. One door opened
into the parlor, where Rollo and his father were sitting. The other door
opened into the back part of the entry. Rollo's father explained how he
was going to perform the experiment, thus:--
"If we could, by any means, get all the air out of the closet for a
moment, then the pressure of the air outside would force a jet of it in
through the key-holes of the doors, and the crevices."
"And how can we get the air out?" said Rollo.
"We can't," said his father, "get it all out; but we can get a part of
it out by shutting the door quick. The door will carry with it a part of
the air that was in the closet, and then the outside air will be spouted
in, through the key-hole of the other door. Only we can't see it, as we
can the water."
"No," said Rollo; "but I can put my hand there, and feel it."
"A better way," said his father, "would be to hold a lamp opposite to
the key-hole, and see if it blows the flame."
Rollo tried the experiment, in the way his father had described. He went
into the closet with the lamp. He held the lamp opposite to the
key-hole, and pretty near to it, and then he asked Nathan to shut the
other door suddenly. Nathan, who was standing all ready by the other
door, which was about half open, put his two hands against it, and
pushed it to, with all his strength, producing a great concussion.
"O Nathan," said his father, "you need not be quite so violent as that."
"It succeeded, father, it succeeded," said Rollo.
"I'm glad it succeeded," said his father; "but Nathan need not have shut
the door with so much force."
"I wanted to drive out all the air," said Nathan.
"I'll show you how to do it," said his father.
Rollo's father accordingly arose, and came to the closet door. He
opened the door wide, and then explained to the boys, that the beginning
of the movement of the door, when it was wide open, did not drive out
any air.
"For," said he, "there is so large a space between the edge of the door
and the wall, that the air that is put in motion by the movement of the
door, can pass directly round the edge, back into the closet again. It
is only when the door is almost shut, when the edge of it comes close to
the casing all around, that the movement of the door drives the air
out."
Then he took hold of the latch of the door, and put it almost to, very
gently. He turned the latch so as to prevent its snapping against the
catch, and then pushed it suddenly into its place three or four times,
opening the door only a very little way every time.
"Now," said he, "hold the lamp at the key-hole, and watch the flame,
while I shut the door two or three times in this way."
Rollo did so, Nathan standing all the time by his side. They observed
that the flame of the lamp was driven into the room every time the door
was shut; proving that, every time a little of the air was driven out
by the door, a little puff rushed in at the key-hole.
"Let us stop up the key-hole," said Rollo, "and then it can't get in."
"Yes," said his father, "there are a great many little crevices all
around the closet, where the air can come in."
"Couldn't we stop those up too?" said Rollo.
"No," said his father, "not so as to make the closet air-tight. For, if
the crevices could all be stopped exactly, the air would come in through
the very wood itself."
"How?" said Rollo.
"Why, there are little pores in wood, that is, little channels that the
sap flowed in when the wood was growing, and the air can pass through
these."
Here Rollo's father observed that Rollo was looking very intently at the
table; and he asked him what he was doing: he said he was trying to find
some of the pores.
"You can't see them there," said his father. "St. Domingo mahogany is a
very hard and close-grained kind of wood. If it was summer, and you
could dig down and get a small piece of the root of the great elm-tree
in the yard, you could see the pores and channels there."
After some more conversation on this subject, Rollo asked his father if
he could not think of some other experiments for them to try. His father
said that he did not just then think of any experiment, but that, if
Rollo and Nathan would come and sit down by the fire, he would give them
some information on the subject. Rollo's mother said that she should
like to hear too. They accordingly waited until she was ready, and then,
when all were seated, Mr. Holiday began thus:--
"Air is in many respects much like water."
"Yes," interrupted Rollo, "just like water, only thinner, because, you
see----"
"You must not interrupt me," said his father, "unless to ask some
question, which is necessary to understand what I say. It is entirely
irregular for a pupil, instead of listening to his teacher, to
interrupt, in order to tell something that he knows himself."
Rollo's father smiled, as he said this, but Rollo looked rather ashamed.
Then his father proceeded:--
"There is one very remarkable difference between them. Water is not
compressible by force; but air is."
"What is the meaning of _compressible_?" said Nathan.
"Compressible things," said his father, "are those that can be
compressed, that is, pressed together, so as to take up less room than
they did before. Sponge is compressible. A pillow is compressible. But
iron is not compressible, and water is not compressible."
"I should think it was," said Nathan; "it is very soft."
"It is very _yielding_," replied his father, "when you press it, but it
is not pressed into any smaller space. It only moves away. If you have a
tumbler half full of water, and press a ball down into it, you could not
crowd the water into any smaller space than it occupied at first; but,
as fast as the ball went down, the water would come up around the sides
of the ball."
"But suppose," said Rollo, "that the ball was just big enough to fit the
tumbler all around; then the water could not come up."
"And then," said his father, "you could not crowd the ball down."
"Could not a _very_ strong man?" said Nathan.
"No," replied his father, "the water cannot be sensibly compressed. But
now, if the tumbler contained only air, and if a ball were to be put in
at the top, just large enough to fit the tumbler exactly, and if a
strong man were to crowd it down with all his strength, he would,
perhaps, compress the air into half the space which it occupied before."
"Perhaps the tumbler would break," said Nathan.
"Yes," replied his father, "and the tumbler will answer only for a
supposition; but for a real experiment it would be best to have a
cylinder of iron."
"What is a cylinder?" said Nathan.
"An iron vessel, shaped like a tumbler, only as large at the bottom as
it is at the top, would be a cylinder. Now, if there was a cylinder of
iron, with the inside turned perfectly true, and a brass piston fitted
to it----"
"What is a piston?" said Nathan.
"A piston," said his father, "is a sort of stopper, exactly fitted to
the inside of a cylinder, so as to slide up and down. It is made to fit
perfectly, and then it is oiled, so as to go up and down without much
friction, that is, hard rubbing. There is a sort of stem coming up from
the middle of the piston, called the piston rod, which is to draw up the
piston, and to press it down by.
"Now," continued his father, "if a strong man had a cylinder like this,
with a piston fitted to it, and a strong handle across the top of the
piston rod, perhaps he might press the air into one half the space which
it occupied before. That is, if the cylinder was full of air when he put
the piston in, perhaps he could get the piston down half way to the
bottom. Then the air would be twice as _dense_ as it was before; that
is, there would be twice as much of it in the same space as there was
before. It would be twice as compact and heavy. This is called
_condensing_ air. The philosophers have ingenious instruments for
condensing air.
"If, however, a man condenses air in this way, by crowding down a
piston, he does not begin the condensation when the piston begins to
descend. The air is condensed a great deal before he begins. All the air
around us is condensed."
"How comes it condensed?" said Rollo.
"Why, you recollect that, when you bored a hole through the board in the
bottom of your dam, the water spouted out."
"No, father," said Rollo, "we pulled the plug out; Jonas bored the
hole."
"Well," said his father, "the water spouted out."
"Yes," said Rollo.
"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?"
"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.
"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?