英文科學(xué)讀本 第四冊·Lesson 51 The Suction-Pump

Lesson 51 The Suction-Pump

“We are going to talk about the common suction-pump this morning,” said Mr. Wilson, “and I think we shall find that our lesson on the syringe has made this next step very easy.

“Let us commence by learning the construction of the pump, and we will then find out how it raises the water. The water, you know, comes from a well deep down in the ground. The first essential, therefore, of a pump must be a long pipe to dip down into the water. This is known as the suction-pipe. Our open-glass tube standing in the water would represent such a pipe. You remember that the water rested at the same level inside and outside the pipe, because the pressure of the air on both was equal. But we managed to raise the water in the tube. How did we do it?"

We removed the air from the tube, sir, said Fred, "and then the water rose in it, because of the pressure of the air all round."

Quite right, Fred, and this is exactly what we must do with the pump, or the water can never rise. Let us now go back to the suction-pipe. Fitted closely on the upper end of it is a larger tube—the barrel. The entrance to the barrel is guarded by a little door, which works on a hinge, and opens upwards only. Such a door is called a valve; it allows the water to pass upward, but prevents it from returning. This one is known as the suction-valve. In the barrel is fitted an air-tight piston, capable of being moved up and down by a piston rod, which is worked by a handle. In the centre of the piston is another valve similar to the suction-valve, and, like it, opening upwards only. This is called the piston-valve.

Now I think we are clear as to the construction of the pump. Let us see next how it works. I told you that the whole object of this contrivance is to remove the air from the barrel and suction-pipe. When this is done the water will rise by the pressure of the air all round. Imagine that the piston has been forced down, and made to rest upon the suction-valve in the bottom of the barrel. Both valves are closed. There is no space, and consequently no air between them. The pumping will begin by raising the piston. There is air, of course, above the piston, and, as the piston rises, this air forces down its valve and closes it.

But what has been happening at the suction-valve meanwhile? The raising of the piston, with its valve closed, has made an empty space or vacuum in the barrel, and the air in the pipe, below has forced open the suction -valve, and rushed through to fill up the space. There is, therefore, less air in the suction-pipe than there was; there is not the same pressure on the water within the pipe. But the air outside is pressing on the water in the well with the same force as ever, and this outer pressure forces the water to rise some distance in the pipe. The next step in the pumping operation is to lower the piston. As it descends it presses upon the air below it in the barrel, and this air forces up the piston-valve, and escapes. But why does the air not return down the suction-pipe again? It cannot. The first result of lowering the piston, and pressing it down upon the air, is to close fast the suction-valve.

The piston and suction-valve are now close together as they were at first, and the work proceeds by raising the piston as before. This time more air, probably the whole of it, is exhausted from the suction-pipe. If so, the water rushes up through the suction-valve and fills the barrel. When we lower the piston again, it is water which forces its way through the piston-valve, and by the time the piston has reached the bottom of the barrel again, we have a continuous column of water from the pipe upwards.

The next raising of the piston closes its valve by the weight of the water above, and that water is carried up to the spout, whence it runs out in a stream.