HEATED AIR.
Balloons are sometimes, as I have told you, filled with heated air.
This heated air is lighter than the cool air around it, and so the
balloon rises, or, rather, is pushed up. Now observe why the heated air
is lighter than the cool air. It is because the heat swells the air, or
_expands_ it, as it is commonly expressed. The heat, in expanding it,
makes it thinner, and of course it is lighter.
[Sidenote: Experiment with a bladder.]
You can see by a little experiment that heat swells or expands air. Lay
a bladder, partly filled with air, before the fire. The heat will fill
out the bladder, making it plump and hard, for it will expand the air
that is in it; and if the bladder is already filled with air before you
lay it down on the hearth, the swelling air will burst the bladder.
[Sidenote: Roasting apples.]
You remember that I told you about putting an apple under the jar of
an air-pump. If the apple is shriveled, the moment that you begin to
pump the air from around it the apple begins to swell out, because the
air in it swells or expands. In this experiment the air in the apple
expands because the pressure of the air around it is lessened by its
becoming thinner. Now the air in the apple can be made to expand in
another way--by applying heat. If you observe an apple put down to the
fire for roasting, you see that it swells. If it happens to be rather
wilted, the swelling of it will be very manifest; it will become as
plump as it would in the air-pump when the air is pumped out. This is
because the air in it is expanded by the heat. And when it sputters,
it is the expanded air that throws out some of the juice through the
broken skin.
[Sidenote: Popping of roasting chestnuts.]
[Sidenote: Why pricking them prevents it.]
You know that, if you roast chestnuts, they pop open with quite a
noise, and sometimes fly half across the room. This is owing to the
expansion of the air in the chestnut by the heat. This air is shut up
in the tight skin of the chestnut; and when it is considerably swelled
by the heat, it makes the skin give way all at once, and so produces
the popping noise. This is because of the springiness or elasticity of
the air. That I have explained before. If you prick a hole in the skin
of the chestnut before you put it down to the fire, there will be no
popping, for the air will gradually escape from this hole as fast as it
is expanded. This hole is to the chestnut what the safety-valve is to
a steam-engine. The engine will not burst while the steam can go out
by the valve, and so the chestnut, with a hole for the air to get out,
does not burst. In the case of both the apple and the chestnut, there
is steam mixed with the air. The steam comes from the moisture in the
apple and the chestnut, and this has the same springiness that air has,
and so helps to produce the effect. I shall tell you about steam in
another chapter.
Heated air always rises, for the same reason that a light gas rises. It
is pushed up by the cold air, which is heavier. In warming a room, the
cold air is constantly pushing the warmed air up, and the air is always
warmer in the upper part of the room than it is near the floor. So,
also, it is warmer in the galleries of a church than it is in the body
of the house, as you perhaps have sometimes noticed.
[Sidenote: Paper wind-mills on a stove-pipe.]
[Sidenote: The toy wood-sawyer.]
Around a stove-pipe, the motion of the heated air as it goes up is very
manifest. Light things are often seen flying up in the current of the
air about the pipe. Sometimes, for amusement, little paper wind-mills
are fastened to a stove-pipe, the heated air whirling them around
as it strikes them in going up. I have seen a very curious toy, in
which a wood-sawyer is made to work by the whirling of a little paper
wind-mill. Whenever there is a strong current of hot air, the wind-mill
turns quite rapidly, and this sets the sawyer to working his paper saw
most furiously. The little figure goes through the motions of sawing
very perfectly. It has sawed into the middle of the log, but never gets
any farther.
The stream or current of air about a stove-pipe is made by the cooler
air, which pushes up that which is warm. As fast as the air is heated
by the pipe, cooler air takes its place by pushing it up out of the
way; and then this air, coming thus near the pipe, gets heated, and is
pushed up in its turn by some more air. As this is constantly going on,
there is a constant upward current of air; and the hotter the pipe is,
the more rapid is the current, because it heats the air so quickly and
so much.
[Sidenote: Why heated air goes up.]
You know, in a house heated by a furnace, how the heated air comes up
from the registers. This air is pushed up. As soon as the air around
the furnace is heated, cool air comes in to push it up out of the way,
and then this cool air is heated and is pushed up by more cool air,
and so on. The heated air escapes from the pressure of the cool air
by going up in the large tin pipes. The cool air is always driving the
warm up, just as it is with the air about a stove-pipe.
[Sidenote: Why a great fire makes the wind rise.]
Whenever a great fire occurs, after it has continued some time, the
wind rises, as it is expressed; though perhaps it blew very gently
at first, now it blows very hard. What is the reason of this? It is
because the air just about the fire becomes much heated, and therefore
very light. The cold air all around rushes therefore toward the fire,
just as it does toward a stove or a fire-place in a room, and pushes the
light heated air up. In doing this it becomes itself heated, and is
pushed up by other cold air, and so on. In this way the air all around
the fire is set in motion toward it, and the hotter the fire the more
brisk is this motion--that is, the harder does the wind blow. I shall
tell you something about the way in which heat makes winds in another
chapter.
_Questions._--Why is heated air lighter than cool air? What experiment
shows that heat expands air? Tell about the shriveled apple. Why do
chestnuts often pop open when they are roasted? How can you prevent
their popping? Give the comparison of the safety-valve. In warming a
room, what is done to the heated air? What is said about the galleries
of a church? What is said about the air around a stove-pipe? Tell
about the paper wind-mills and the wood-sawyer. How is the current of
air about a stove-pipe made? What makes the hot air come up from the
registers of a furnace? Why does the wind rise in a great fire?