When water is raised to a greater height a force pump is used. Such a pump is shown in Figure 41. The pump cylinder A has two valves B and C. The valve B opens upwardly and valve C opens downwardly. The piston D is a solid member containing no valve. When the piston rises the valve C closes and valve B opens, letting water into the cylinder. On the reverse stroke the valve B closes and the descending piston forces the water out through valve C into pipe E. The height to which the water may be lifted depends upon the power applied to the piston and not upon atmospheric pressure. In a fire pump two force pumps are used which alternately deliver water into an air chamber. The air is compressed by the water that enters the chamber and exerts a steady pressure on the water, forcing it out through the fire hose in a stream that is practically steady. It is a decided disadvantage to install a force pump in a deep well because of the mechanical difficulty of operating the piston at a great depth. Often the suction and force pump principles are combined. The force pump piston and cylinder are located twenty-six and twenty-eight feet above the level of the water in the well with a pipe leading down into the water. The water is forced up into the cylinder by exhausting the air in the cylinder and pipe and then is forced out of the cylinder and up a pipe to the top of the well. This also has its disadvantages. In deep wells the pump piston must still be located and operated at considerable depth. THE AIR LIFT A much more simple pump for deep wells is the air lift in which there are no valves and no piston. The pump consists of two pipes that are let down into the water. (See Figure 42.) The larger pipe is open at the top and bottom, and the smaller pipe discharges air into the bottom of the larger one. This air being directed upward has a lifting effect, and mingling with the water in the pipe produces a sort of froth which is so much lighter than solid water that the atmospheric pressure on the water in the well is sufficient to raise the froth. The water is thoroughly aerated and to a large extent purified by this system of pumping, so that the air lift commends itself particularly for city waterworks. Sometimes a series of air lifts are used which lift water by successive stages to a greater height than would be possible with a single direct lift. [Illustration: FIG. 42.--PUMPING WATER WITH COMPRESSED AIR] EXCAVATING THROUGH QUICKSAND [Illustration: FIG. 43.--SECTIONAL VIEW OF A PNEUMATIC CAISSON]