_Rotary Pumps._--A twin-chamber rotary pump admits water at the bottom of the chamber and forces it out through the top. Intermeshing cogs and rotary cams revolve outward from the center at the bottom, as shown by the arrows in Figure 118. The stream of water is divided by the cams, as it enters the supply pipe at the bottom, and half of the water is carried each way around the outsides of the double chamber. These streams of water meet at the top of the chamber, where they unite to fill the discharge pipe. These pumps operate without air-chambers and supply water in a continuous stream. They may be speeded up to throw water under high pressure for fire fighting, but for economy in ordinary use the speed is kept down to 200 revolutions, or thereabout. Rotary pumps are also made with one single water chamber cylinder. The pump head, or shaft, is placed a little off center. A double end cam moves the water. Both ends of the cam fit against the bore of the cylinder. It works loosely back and forth through a slotted opening in the pump head. As the shaft revolves the eccentric motion of the double cam changes the sizes of the water-pockets. The pockets are largest at the intake and smallest at the discharge. Rotary pumps are comparatively cheap, as regards first cost, but they are not economical of power. In places where the water-table is near the surface of the ground they will throw water in a very satisfactory manner. But they are more used in refineries and factories for special work, such as pumping oil and other heavy liquids. _Centrifugal Pumps._--The invention and improvement of modern centrifugal pumps has made the lifting of water in large quantities possible. These pumps are constructed on the turbine principle. Water is lifted in a continuous stream by a turbine wheel revolving under high speed. Water is admitted at the center and discharged at the outside of the casing. Centrifugal pumps work best at depths ranging from twenty to sixty feet. Manufacturers claim that farmers can afford to lift irrigation water sixty feet with a centrifugal pump driven by a kerosene engine.