英文科学读本 第四册·Lesson 57 The Force-Pump

Lesson 57 The Force-Pump

Did any of you see that great fire last week? asked Mr. Wilson. "Ah! I thought so; several of you saw it, and a very grand and awful sight it was. Did you ever feel curious to know how the firemen send out the water from their hose in that one continuous stream?"

Oh yes, sir, said Fred, "do tell us, please. It has often puzzled me."

Well, said Mr. Wilson, "it is the work of a pump, but this pump is like neither the suction-pump nor the lifting pump. It is called a force-pump. I will describe it to you.

In this contrivance, the suction-pipe and barrel are exactly the same as in the lifting pump, and the piston too is solid—that is, it has no valve. The great difference is that in the force-pump the pipe leading from the barrel opens into an air-tight chamber of great strength—the condensing chamber. The entrance into it is guarded by a strong valve opening upwards—that is, away from the barrel. The discharge-pipe dips down into this chamber, and passes upwards into the air, but there is no valve in it. As in the case of the lifting pump, the piston, in descending, presses down on the water in the barrel and closes the suction-valve. At the same time it drives the water through the pipe into the condensing chamber, and this is repeated with each downward stroke of the piston.

You must remember that this condensing chamber is not only air-tight, but is full of air. As the water rushes in, this air is very much compressed, and then commences a struggle between the compressed air and the water which is still being forced in. The air presses down on the water, but the water is incompressible; it cannot be compressed into smaller bulk. It cannot return by the way it came, partly because of the valve, and partly because of the body of water rushing up behind it. There is one way of escape—up the discharge-pipe; and up that pipe it rushes in a continuous stream. I have been describing to you an ordinary force-pump. The fire-engine is really a double force -pump; that is all. There is a central condensing chamber with a force-pump on each side. The pipes from both pumps open into the one condensing chamber, and the delivery-hose passes out from it. A strong hose from the bottom of each pump is attached to the water supply in the road, and, as the pumping goes on, the water is raised through these pipes into the barrels of both pumps, and passed from them into the common condensing chamber, whence it is sent out in a continuous stream through the delivery hose."