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.
"Now, as all the air around us," continued Rollo's father, "is pressed
down very heavily, and is condensed a great deal, it is all the time
endeavoring to expand; and it would expand, were it not that the great
burden of the air above it keeps it condensed. But water is not
compressed, and has no tendency to expand. The water of Rollo's dam, for
instance, had all the weight of the atmosphere resting upon it, but it
did not compress it at all, and so it did not tend to expand.
"And now," said his father, "I cannot perform any experiment, to show
you that air tends strongly to expand or swell out into a great space,
while water does not; but I can make a supposition, which will
illustrate it. Suppose we had a large, but very thin, glass bottle,
filled with water, and put down upon the floor in the middle of this
room. Suppose, also, that we had another bottle, of the same size and
shape, filled with air, and we put that down upon the floor by the side
of the other; both bottles being stopped very tight. Now, if we could by
any means suddenly take away all the air from the room, so that there
should be nothing around the bottles, then the bottle of water would
remain just as it is, for the glass would have nothing to support but
the weight of the water, and it would be strong enough for that. But the
bottle of air would fly all to pieces; for that would not rest quietly,
like the water, satisfied with the space which it already has, and only
pressing with its own weight upon the sides of the glass; but it would
immediately expand with so much force as to break the thin glass all to
pieces."
"Would it!" exclaimed Rollo and Nathan together. "And would it make a
loud noise?"
"Yes," replied their father, "I presume it would make a loud explosion;
that is, if the air in the room around it could by any means be all at
once and suddenly removed.
"And so you must remember," he continued, "that there are two very
remarkable differences between air and water. Air may be condensed by
pressure, and, as it exists all around us, is greatly condensed by the
pressure of the air above, and it may be compressed more. And air is
expansive, while water is not. Whenever the pressure upon it is removed,
it suddenly expands, or spreads out in all directions."
"O dear me!" said Nathan, with a sigh.
"What is the matter?" said his father.
"Why, I can't understand it very well."
"Can't you?" said his father. "Well, I must admit that you are rather
too young to study pneumatics."
"Pneumatics?" repeated Rollo.
"Yes," said his father; "that is the name of this science."
[Illustration: "Then it sailed slowly away."--Page 85.]
"What, the science of air?" said Rollo.
"Yes," said his father, "the science which treats of air, and of all
other compressible and expansive fluids. But let me think. I must try to
tell you something which Nathan can understand and be interested in. If
I had a very light feather, I could let him perform an experiment."
"Would a little down do?" said Rollo's mother.
"Yes," replied his father, "that would be better than a feather."
Mrs. Holiday then went and brought a little down, and handed it to
Rollo's father. Now, there was a lamp upon the table, of a peculiar
kind, called a study lamp. It had a glass tube, called a chimney, around
the wick, and consequently around the flame itself, being round, like a
ring.
Rollo's father told Nathan to hold the down over the top of this glass
chimney, and then to let it go.
Nathan did so. The little tuft of down was wafted up into the air, quite
high above the lamp, and then it sailed slowly away, and fell down upon
the table.
"I know what makes it rise," said Rollo. "It is the heat. The heat makes
it rise."
"Do you think so?" said his father. "Then take the down, and lay it
gently upon the hearth, before the fire, as near as you can."
Rollo did so. He had to take his hand away very quick, for it was quite
hot there. The little tuft remained quietly upon the hearth where he
placed it.
"There," said his father, "is not that a hotter place than it was over
the lamp?"
"Yes, sir," said Rollo.
"Then, if it was heat that made it rise, why does not it rise now?"
Rollo could not tell.
"I will tell you how it was," said his father. "Heat makes air more
expansive. When air is heated, it swells; when it is cool, it shrinks
again. Now, if it swells, it becomes lighter, and so it is buoyed up by
the heavier air around it; just as wood at the bottom of the sea would
be buoyed up, and would rise to the surface of the water. Now, the heat
of the lamp heats the air that is in the glass chimney, and swells it.
This makes it lighter; and so the air around it, which is heavier,
buoys it up, and it carries up the feather with it."
"No, the down, father," said Nathan.
"Yes, the down," said his father.
"Then it seems to me, after all," said Rollo, "that it is the heat which
makes it rise."
"Yes," said his father, "it does, indirectly. It expands the air; that
makes it lighter; then the heavy air around it buoys it up, and, when it
goes up, it carries up the down. So that it is not strictly correct to
say, that the heat carries it up. The heat sets in operation a train of
causes and effects, the last of which results in carrying up the
feather.
"Now," continued his father, "there is always a stream of air going up,
wherever there is a lamp, or a fire, or heat, which heats the air in any
way. The expanded air from a fire goes up chimney. The cool and heavy
air in the room and out of doors crowds it up."
"The air out of doors?" said Rollo. "How can that crowd it up?"
"Why, it presses in through all the crevices and openings all around
the room, and crowds the light air up the chimney. All the smoke is
carried up too with it, and it comes pouring out at the top of the
chimney all the time."
"You can see that the air presses in at all these crevices," continued
Rollo's father, "by experiment."
"What experiment is it?" said Rollo; "let us try it."
"I will let Nathan try it," said his father, "and you may go with him
and see the effect. First," he continued, "you see by the smoke, that
the air really goes up the chimney; and I will show you that other air
really crowds into the space, from other parts of the room."
So he took a lamp from the table,--not the study lamp; it was a common
lamp,--and held it at various places in the opening of the fireplace, by
the jambs and near the upper part; and Rollo and Nathan saw that the
flame, in all cases, was turned in towards the chimney.
"Yes," said Rollo, "I see it is drawn in."
"No," said his father; "strictly speaking, it is not _drawn_ in; it is
pressed in, by the cool and heavy air of the room."
"I thought," said Rollo's mother, "that the chimney _drew_ the air from
the room into it."
"That is what is generally said," replied Mr. Holiday, "but it is not
strictly true. The common idea is, that the hot air rises in the
chimney, and so draws the air from the room to supply its place; but
this is not so. In the first place, nothing can rise unless it is forced
up. The lightest things have some weight, and would, if left to
themselves, fall. The hottest and lightest air in a chimney would fall
to the earth, if there was no cooler and heavier air around it, to force
it to rise;--just as the lightest cork, which would rise very quick from
the bottom of the sea, would fall back again very quick, if the water
was not there.
"Remember, then, Nathan and Rollo, that, when a fire is built in a
fireplace, so as to warm the air in the chimney, it makes this air not
so heavy; and then the cool air all around it in the room and out of
doors, presses in, and crowds under the light air, and makes it
ascend."
"But, father," said Nathan, "you said I might perform an experiment."
"Very well, I am ready now. Take the lamp, and carry it around the room,
and hold it opposite any little opening you can find."
"I can't find any little openings," said Nathan.
"O yes," said his father; "the key-hole of the door is a little opening,
and there is a narrow crevice all around the door; and you will find
little crevices around the windows. Now, hold the lamp opposite any of
these, and you will see that the air presses in."
So Nathan went with the lamp, Rollo following him, and held the lamp
opposite to the key-hole, and the crevices around the door and windows;
only, when he came to the window, his father told him to be very careful
not to set the curtain on fire.
Rollo wanted Nathan to let him try it once; and so Nathan gave him the
lamp. He said he meant to make a crevice; and so he pushed up the window
a very little way, and held the lamp opposite to the opening. The air
pressed the flame in towards the room, in all cases.
"People commonly say, that it is _drawn_ in," said his father, "but that
is not strictly correct; it is really _pressed_ in. There is no power of
attraction, in the air that is in the room, to draw in the air that is
out of doors, through the crevices; but the air that is out of doors, is
so heavy, that it presses in, and crowds the warm and light air up the
chimney.
"And now," said his father, "I cannot tell you anything more this
evening; but, if you remember this, I will give you some further
instruction another time."
"Well, sir," said Nathan, "only I wish you would tell me a little story,
as you did last evening. Have not I been still?"
His father had noticed, that he had been very still and attentive, but
did not think before, that it was in expectation of being rewarded with
a story.
"Well," said his father, "I will tell you a story, or give you a little
advice. How should you like a little advice?"
"Well, father, a little advice; just which you please."
"I advise you, then,--let me see,--what shall I advise you?--No, on the
whole, I will tell you a story. Once there was a man, and he was a
philosopher. He understood all that I have been explaining to you about
the air being light when it was hot. So he got some very thin paper, and
made a large paper bag. He cut the paper very curiously, and pasted it
together at the edges in such a way, that the bag, when it was done, was
round, like a ball; and it had a round opening at the bottom of it. In
fact, it was a large paper ball."
"How large was it?" said Nathan.
"It was so large, that, when it was swelled out full, it would have been
higher than your head."
"O, what a large ball!" said Nathan. "But what was it for?"
"Why, the man thought, as hot air is lighter than cool air, and floats
up, that perhaps, if he could fill his paper ball with hot air, it would
go up too."
"And did it?" said Nathan.
"Yes," said his father. "He filled it with hot air; and the hot air was
so light, that it rose up and carried the paper ball with it."
"How did he get the hot air into it?" said Rollo.
"Why, he held it over a little fire, with the mouth down. Then the hot
air from the fire went up into the ball, and swelled it out full."
"How high did it go?" said Nathan.
"O, it soared away," said his father, "away up into the air, very high;
until at length it got cool, and then it came down."
"I should like to see such a ball as that," said Nathan.
"Such a ball as that is called a _balloon_," said his father.
"I wish I could see a balloon," said Nathan.
QUESTIONS.
What is the important difference between air and water, which
was explained in the last chapter, and mentioned in this? Does
the air tend to expand again after it is compressed? What is
this property of the air called? Is the air around us already
condensed, or is it in its natural state? What causes it to be
condensed? Suppose a thin glass vessel were to be filled with
air, and another with water, and the air suddenly removed from
the room around them; what would be the effects? What effect
does heat have upon the expansibility of air? How may this
effect be made to appear over a lamp? In a chimney? What was the
story which Rollo's father told Nathan?