PUMPS.
[Sidenote: Explanation of the operation of sucking.]
You know that you can suck up water or any fluid through a straw or any
other tube. Now what is it that makes the water go up through the tube
into your mouth? I will tell you. When you put the tube into your mouth
it is full of air, and so long as the air is there the water will be
kept out; but when you suck you remove the air from the tube; and as
the air goes out, the water comes in, following right on after the air.
But what makes the water come in? Does it come in of itself because
there is room made for it? No. Water can not move itself. It must be
moved by something else. It is the air pressing on the water in the
vessel you are sucking from that pushes it up into the tube. You do not
really draw up the water. You get the air out of the way in the tube,
and then the air that is all the time pressing on the water in the
vessel pushes it up into your mouth. As soon as you stop sucking, and
take your mouth from the tube, the water that is in the tube will run
down into the vessel, because it is pressed down by the air that goes
in at the top of the tube.
You know that you have to suck commonly several times before the water
will reach your mouth. If the tube is a very large one, you suck a
great many times to get all the air out of it. At first you suck out a
little of the air in the tube, and the water is pushed up to take its
place; then you suck a little more out, and more water is pushed up,
and so on till it reaches the top of the tube. Here is a boy that has
partly filled his tube, and one more suck would bring the fluid to his
mouth.
[Illustration]
[Sidenote: How pumping is like sucking.]
You can now see how we pump up water out of a well or cistern. The
water is not drawn up, but it is pushed up just as it is in the tube
when you suck. When you work the handle, you do the same thing for the
pump that your mouth does for the tube in sucking any liquid; and when
the pump has not been worked for some time, you have to move the handle
up and down several times before the water comes, just as you have to
suck several times to fill a tube of any length with water.
[Sidenote: The operation of a pump explained.]
I will show by some figures how a pump operates. In the first figure
the hand is raising the handle, as you know we always do when we begin
to pump. The raising of the handle, you see, makes the piston, as it
is called, go down in the pump. Here it is going down through air, for
the water has not as yet got up as far as the piston. Now, if this
piston were a whole solid piece of wood, it would do no good, for it
would press the air down before it. But it is not solid. It has a hole
through it, and a sort of clapper or valve on the hole. Therefore, as
the piston goes down, the air pushes up the valve, and goes up through
the hole. You see that this air is shut in between the piston and the
water; and when the piston presses down, the only way for it to get
out of the way is to press upon that little door, and go up above the
piston.
[Illustration]
[Illustration]
[Sidenote: Explanation of the pump continued.]
Well, the handle is up. The next thing is to bring it down, as
represented in this picture. As the handle goes down, the piston goes
up, as you see. You remember that I told you that, as the piston was
going down, as seen in the first figure, some of the air went up
through the hole and got above the piston. Now this air can not get
down again, for the moment that the piston begins to move up, the air,
pressing on the valve, shuts it down. Now, as the piston goes up, there
is room made below it. How is this room filled? The air that is there,
as you see, rises up to fill it, and the water follows the air.
[Illustration]
The next moving of the piston down will carry it below all the air and
down into the water; and the water will go up through the little door,
just as the air has done before it. Then the moving of the piston up
will carry this water so high as to make it run out of the mouth of the
pump, as seen in this figure.
But there is a valve in the pump that I have said nothing about as yet.
This lower valve operates in this way: As the air or the water goes up
in the pump, the valve is pushed open by it, as you see in the second
figure and in the last one; but when the piston works down, as seen in
the first figure, this valve is shut, so that all the water that gets
above it is safe, and can not go back.
What is it that makes the air and the water rise in the pump? All that
gets above the piston is lifted up by the piston, as you see. But what
makes that rise which is below the piston? It is the pressure of the
air on the water in the well or cistern. This pushes up the water as
fast as there is room made for it.
If a cistern were full of water, and were air-tight also, you could
not pump up the water from it. You must have air there to push up the
water, or it will not come up when you make room for it by working the
pump.
[Sidenote: How the tongue in sucking acts like the piston of a pump.]
You see, then, that sucking and pumping are very much alike. In the
pump the piston makes the room for the air and the water to be pushed
up into. Now, when you suck, there is a piston that operates very much
as the piston of a pump does. Your tongue is the piston. See how this
is. When you suck through a tube held in water, you move your tongue
in such a way as to make a space in the mouth, and the air in the tube
is pushed in to fill up this space; and when the air is all pushed in,
the water is pushed in after it. Both are pushed in, as I have before
told you, by the air pressing on the water in the vessel. It is just
as water is pushed up into a squirt-gun when you draw the piston. This
piston does in the gun, when you draw it, the same thing that your
tongue does in your mouth when you move it in sucking. It makes space,
and the water is pushed into the gun, as it is into the mouth, to fill
up this space. The way in which the space is made in the mouth in
sucking is this. Before you begin to suck, the tongue fills the mouth,
so as to be up against its roof; but when you suck, you move the tongue
down from the roof of the mouth, and this makes a space there; and
whatever is in the tube, whether it be air or water, is pushed in to
fill this space.
[Sidenote: The common language about sucking and pumping incorrect.]
The common language, then, which is used about sucking and pumping is
not exactly correct. When we suck or pump, it seems to us as if the
liquid was drawn up, and so we use the word draw in regard to it. So,
too, we talk about the suction or drawing power. But, as I have showed
you, the liquid is pushed up instead of being drawn. All that the
piston in a pump does is to make room. It does not draw the water into
that room, but the pressure of the air forces it in. Whenever there is
any room made, the air is always ready either to go in itself or to
force something else in.
_Questions._--Explain the operation of sucking up water through a
tube. Why does the water in the tube run down into the vessel when you
stop sucking and take your mouth away? Why is it that you commonly
have to suck several times before the water reaches your mouth? How is
pumping like sucking? What is shown by the first figure? What by the
second? What by the third? Explain the operation of the lower valve of
the pump. What makes the air and the water rise in the pump? Why would
they not rise if the cistern were full and were air-tight? Explain how
the tongue acts as a piston in sucking. Give the comparison about the
squirt-gun. What is said about the language used about sucking and
pumping?