Pumps are machines for raising or moving water and other fluids. The commonest form, or suction pump, consists of a cylinder or "barrel," to the bottom of which is joined a pipe which dips below the surface of the liquid to be raised. A valve opening upwards is fitted at the junction of barrel and pipe, and a piston can be moved up and down in the barrel by means of a lever and handle. The piston is also provided with a valve capable of moving upwards, and the outlet for the liquid is near the top of the barrel. Imagine the piston to be moving upwards from its lowest position; atmospheric pressure keeps the piston valve closed, and a partial vacuum is obtained beneath it - hence the liquid rises in the pipe, opens the junction valve, and follows the piston upwards. After a while the piston is caused to descend; the liquid beneath it pushes its way through the piston valve, for its pressure causes the lower valve to shut, and so its retreat is cut off. The next time the piston rises, the liquid above it is carried to the outlet, where an intermittent stream is therefore obtained. If a long pipe be connected to the outlet, and the top of the barrel be provided with a stuffing-box fitting the piston-rod, the water can be lifted to any height in the pipe. This form of pump is known as the lifting pump. If, again, there be no valve in the piston, but a second pipe leading from the bottom of the barrel be provided with a valve opening into it, liquid will be forced into this pipe as the piston descends. This is known as the force-pump. An air chamber is often attached to the outlet pipe to equalise the flow of liquid. The fire-engine and the steam engine feed-pumps are always of this type. Specially modified forms of pumps are used for acids, soap, sludge, etc.