Beam pumping-engines are now almost invariably built with crank and fly-wheel, and very frequently are compound engines. The accompanying illustration represents an engine of the latter form. [Illustration: FIG. 105.--Double-Cylinder Pumping-Engine, 1878.] [Illustration: FIG. 106.--The Lawrence Water-Works Engine.] _A_ and _B_ are the two steam-cylinders, connected by links and parallel motion, _C D_, to the great cast-iron beam, _E F_. At the opposite end of the beam, the connecting-rod, _G_, turns a crank, _H_, and fly-wheel, _L M_, which regulates the motion of the engine and controls the length of stroke, averting all danger of accident occurring in consequence of the piston striking either cylinder-head. The beam is carried on handsomely-shaped iron columns, which, with cylinders, pump, and fly-wheel, are supported by a substantial stone foundation. The pump-rod, _I_, works a double-acting pump, _J_, and the resistance to the issuing water is rendered uniform by an air-chamber, _K_, within which the water rises and falls when pressures tend to vary greatly. A revolving shaft, _N_, driven from the fly-wheel shaft, carries cams, _O P_, which move the lifting-rods seen directly over them and the valves which they actuate. Between the steam-cylinders and the columns which carry the beams is a well, in which are placed the condenser and air-pump. Steam is carried at 60 or 80 pounds pressure, and expanded from 6 to 10 times. [Illustration: FIG. 107.--The Leavitt Pumping-Engine.]