THE AIR-PUMP.
[Illustration]
[Sidenote: Description of the air-pump.]
A great many interesting experiments about the pressure of the air can
be tried with the air-pump, which you see represented here. This I will
describe, so that you may understand how it works. At _a_, _a_, are two
pump-barrels. In them are two pistons with valves, such as there are in
common pumps, except that they are made a great deal more nicely. These
pistons are worked by the handle, _b_. The frame-work, _e e_, that
holds the pump-barrels, is made very strong and firm, so that the pumps
may work true. There is a large plate, _f_, of metal, made very even
and smooth. At _c_ is a large glass vessel, close at the top, but open
at the bottom. Its edge is made very smooth, so that it may fit well on
the smooth plate. In the middle of the plate is a hole. This opens into
a passage which leads to the bottom of the two pump-barrels.
Now you can see how the instrument works. The two pump-barrels work in
the same way that a common water-pump does. With them the air is pumped
out of the glass vessel by the passage which leads to them from the
centre of the plate. By this means most of the air may be pumped out.
If we want to let the air in after pumping it out, we loosen the screw
_g_, for from the opening here there is a passage that leads to the
hole in the centre of the plate.
[Illustration]
[Sidenote: Experiments.]
[Sidenote: India-rubber ball.]
I will mention only a few of the experiments that may be tried with the
air-pump. If you put an India-rubber bag, or a foot-ball, with but a
little air in it, under the glass jar, when you begin to pump this will
begin to swell, as represented here; and if you pump for some time,
it will swell very much. The reason is this. As you take away the air
from around the ball, the air in the ball expands. If you then turn the
screw that lets the air into the jar, the ball will become small again,
because it is pressed upon by the air that is let in.
[Sidenote: Bubbles.]
So, too, if some soap-bubbles be put under the jar, when you pump out
the air they will swell; that is, the air shut up in the bubbles will
expand, because the pressure of the air around them is lessened.
[Sidenote: Shriveled apples.]
It is amusing to see a shriveled apple under the jar of the air-pump.
After pumping a little it will swell out, and appear like a plump,
fresh apple; but let in the air again, and the apple becomes shriveled
as before. This is owing to the air that is in the apple, for there
is air in every thing. There is air in our bodies; and if the air all
about us could be lessened very much, just as it is in the jar of the
air-pump, we should swell up like a puff-ball. It is the pressure of
the air all around us that keeps us just of the size we are.
[Illustration]
[Sidenote: The degree of pressure of the air.]
The air does more in pressing than you think for. As you move about in
it, it does not seem to press upon you at all; but it really presses
upon you very hard. It presses on you with the weight of about fifteen
pounds upon every square inch--that is, a space of this size. It would
take many such spaces to cover over your hand. The air really presses
upon your hand, as you hold it out flat, with more than the weight of a
hundred pounds. You can hardly believe this, and you will want to know
how it is that you do not feel this weight or pressure of the air. I
will tell you.
[Illustration]
[Sidenote: How this is borne.]
Hold out your hand flat in the air. You know that there is air
underneath your hand as well as over it. And this air underneath
presses up just as much as that above presses down. Now this is the
reason that you do not feel the pressure. If the air underneath your
hand could be taken away, you would feel the pressure of that which is
above. You would not only feel it, but you could not bear it. This we
can prove by the air-pump. Take the jar off from the plate, and then
put upon it a small glass vessel, open at both ends, such as you see
here. Place your hand over it tightly as represented, and then let some
one work the pump. Your hand will be pressed down into the cup so hard
after a little pumping that you will be glad enough to have the pump
stopped and the air let in.
Observe what is done to your hand by the pumping. Some of the air is
taken away from beneath your hand--that is all; and, this being done,
you feel now the pressure of the air above it, because there is no
pressure below to balance it.
[Illustration]
You can show the same in another way with this glass cup. Tie a piece
of bladder or India-rubber over one end of it, and then place this over
the hole in the plate of the air-pump. As you pump out the air, the
India-rubber will be pressed down into the cup by the air above, as
represented here.
[Illustration]
[Sidenote: How the boy’s sucker operates.]
The pressure of the air is very well shown by the sucker, as it is
called, with which boys sometimes amuse themselves. This sucker is a
round piece of leather, with a string fastened to the middle of it.
The leather is moistened, and then pressed evenly upon the smooth
surface of a stone, and now the stone can be raised, as you see here,
by the string, even if it be a pretty large one. But how is it that
the leather sticks so fast to the stone? It is by the pressure of the
air upon it. When you pull on the string, you raise the middle of the
leather a little from the stone, and this makes a little space there
in which there is no air. But all the leather around by its edge is
pressed very tight upon the stone by the air outside; and it is because
no air can get between the leather and the stone that the leather holds
on to it so well. If the leather is not pressed down exactly even, or
if there be some unevenness in the stone where the leather is put upon
it, the air will get in between the leather and the stone, and the
sucker will not operate.
[Sidenote: Suckers in the feet of flies.]
[Sidenote: The sucking-fish and the shark.]
Flies and other insects, that walk along so well on the ceiling and on
smooth glass, have suckers on their feet, that work very much in the
same way that the boy’s sucker does upon the stone. Some fishes have
suckers by which they can stick to rocks or any thing else. In this
case, it is water that makes the pressure instead of air. Here is the
drawing of a fish that has a sucker, or, rather, a set of suckers,
on the upper part of its head. With this it can adhere to any thing
very firmly. A singular story is told by a traveler about one of these
sucking-fishes. He saw a shark attempt to seize it, but the fish dodged
him, and then fastened itself to the shark’s back by its suckers. It so
happened that one of the sailors had tied to the fish a stick of wood
by a short line. The shark dashed off with this fish thus fastened to
him towing the stick of wood astern. He soon stopped, and, getting hold
of the cord, jerked the fish off, and then dove at it as before. The
fish dodged him again, and got hold with its suckers a second time,
and when last seen, the shark was struggling in vain to get rid of the
troublesome fellow.
[Illustration]
_Questions._--Describe the air-pump, and tell how it works. Tell about
the experiment with the India-rubber ball, with the soap-bubbles, and
with the shriveled apple. How much is the pressure of the air on every
square inch of your body? How much is it on your whole hand? Why do
you not feel this pressure? What experiment with the air-pump makes
this plain? Give the other experiment that shows the same thing in
another way. How is the boy’s sucker made? Explain how it holds on
to the stone. How do flies and other insects walk on ceilings and on
glass? Tell about the sucking-fish.