POP-GUNS.
[Sidenote: Operation of the pop-gun explained.]
Every boy and girl has played with a pop-gun, but did you ever think
how it works? I will tell you about this.
You know that the cork does not fly out till the rod is pushed a
considerable way down into the tube or barrel of the gun, and then it
flies out all at once with a popping noise. What makes it fly out? It
is not the rod alone, for it does not touch the cork. It is the air
that is between the rod and the cork that gives it the push that makes
it fly out, and it gives so quick a push as to make the pop.
I will explain this to you a little more particularly. When you put the
cork into the end of the gun, the barrel is full of air. Now, if the
cork were not in, as you pushed the rod the air would all go out before
it; but the cork in the end keeps all the air in. As you push the rod,
you crowd the air into a smaller space. If you push the rod half way,
then the same air that filled the whole gun has half the room that it
had before you pushed the rod. Now, when air is pressed or crowded in
this way, it tries, as we may say, to get away from the pressure. In
doing this, it presses on the cork; but the cork sticks fast in the
mouth of the gun till the pressure is enough to push it out, and when
it gives way it does it all at once, and so makes the popping sound. It
is as if the air gave the cork a sudden kick, and out it flies.
[Sidenote: Explanation of the potato and quill pop-gun.]
When I was a boy, we had no such nice pop-guns as boys now have. We had
to make them ourselves. We would sometimes make the tube or barrel part
out of elder, which, you know, has a large pith. We would sometimes
take a quill for a barrel. To this we would fit a stick as a rod. We
would then punch each end of the quill through a thin slice of raw
potato. This would, of course, leave a round piece of potato in each
end. Now, by pushing the rod quickly through the quill, the piece of
potato in the farther end would fly out with a pop, in the same way
that a cork does from the pop-guns nowadays. You see how this is done.
The air which is shut up in the quill between the two pieces of potato
is crowded into a small space when the stick is pushed in. It tries to
escape from this pressure, and so presses on the potato at the farther
end. This gives way all at once and flies out. But why must we have the
potato in both ends? It would not be necessary if the stick could be
made to fit the quill exactly; but it can not, and so there would be a
leaking of air by it if we should have the potato in only one end. The
piece of potato in the end where you put in the stick prevents this
leaking of air. It makes, in fact, a tight piston for the stick to work.
It is the springiness of the air that makes the pop-gun work. This you
can see by some experiments. Fill your pop-gun with water, and see how
different from the air it will act. The moment that you push the rod,
the cork will be pushed out without any popping, and the water will run
out. What is the reason of this? It is because you can not crowd the
water, as you do the air, into any smaller room. It moves straight
along, and pushes out the cork.
[Sidenote: Experiments with the pop-gun.]
As the water can not be crowded into any smaller space, it has no
spring. But the air can be shrunk up, as we may say, by pressure, and
it is ready to swell out again whenever it can have a fair chance to do
so; and the harder you press it, the greater is this springiness. You
can see that this is true by a little experiment that you can try with
your pop-gun. Press the cork end of the gun firmly against something,
so that the cork can not come out. Now push in the rod quickly, and
then let go of it. It will fly back, because the crowded air, by
a spring, throws it back. And the harder you push it in, the more
forcibly will it fly back.
Now, if you try the same experiment with the water in the gun, you
will find that you can not push the rod unless the gun leaks, and then
the water will come back by the piston. Why is this? It is because the
water can not be crowded into any smaller space, as the air can be. If
it could be, the water would do just as well in the pop-gun as air does.
You see, then, that it is the spring of the air that forces the cork
out of the gun; and the air has this spring because it is pent up and
crowded together, as we may say, into so small a space. It wants more
room, and pushes to get it.
The cork is shot out of the pop-gun in the same way that the ball is
shot out from the cannon. The air, pent up in a little space in the
pop-gun, does the same thing as the gas, pent up in the cannon, does.
The air wants more room, and so it kicks out the cork; and the gas, so
suddenly made out of the powder, wants more room, and so it kicks out
the ball. The gas has the same springiness that the air has.
[Sidenote: Elasticity of the air.]
It is this springiness of the air, called its elasticity, that makes
the foot-ball bound so. If the ball were filled with water instead of
air, it would not bound at all, because the water has no elasticity.
[Sidenote: Operation of the air-gun explained.]
I have told you that the more the air is pressed the greater is its
springiness. In what is called the air-gun, a great deal of air is
crowded into a very small space--much more than there is in a pop-gun;
and a bullet can therefore be fired from it with force enough to go
through a board. It is done in this way: The pressed air is shut up
tight, and all at once it is let into the barrel of the gun where the
bullet is. It throws the bullet out just in the same way that the gas
of the powder does in a common gun. This air-gun is only a curiosity.
It will never come into use, for it is quite a tedious operation to
load it with pressed air. The common gun, you know, is very easily
loaded with powder, and the gas which it turns into does the work even
better than the pressed air in the air-gun.
_Questions._--What makes the cork fly out of the pop-gun? Explain just
how the pop-gun operates. Tell how the quill pop-gun is made. Why do
we have the potato in both ends of the quill? What is said about the
springiness of the air? How would the pop-gun work if it were filled
with water? Why is this? Give the experiment with the pop-gun showing
how springy the air is. How is it when you try the same experiment
with the gun filled with water? Give the comparison between your
pop-gun and a cannon. What is said about the foot-ball? Tell about the
air-gun. Why is this not in common use?