198

MODERN STEAM PRACTICE.

of the pump; they are placed directly under the ribs, and when itis required to lower the lift the yokes are loosened to let the liftdrop through. Each time there is a new length or pump put in,the bücket has to be lowered, and that it may be done without thenecessity of making a new drawing lift Connection on the main rods,the arrangement shown in Fig. 125 is introduced; a chain and hookserves to make the connection between the main rods and the pumprod, one staple only being used to steady the top of the pump rod.This arrangement affords facilities both for drawing the bücketand putting in a new pump. The pump joints are made with flatrings of wrought iron, covered with baize and dipped in tar.

“ Balance bobs are sometimes placed below the surface to take upsome of the weight of the pump rods. The connection with themain rods is usually of wood; the Vibration is given by the elasticityof the wood. In the example shown in Figs. 124, 125, the connectingrod is 15 fathoms long, and it is guided and steadied by means ofa plain turned pulley, which bears against a curved filling piecebolted on to the connecting rod. Plungers are sometimes substitutedfor balance bobs, and are, when so employed, constantly submittedto the pressure of the column of water in the pumps; they are fixedto the main rods precisely in the same way as the ordinary plunger.” 1

With the view of securing greater regularity in the motion,and of equalizing the strain on the various parts, the compoundor double-acting engine has been introduced. An example ofthis engine is shown in Fig. 126. The high-pressure cylinder is36 inches in diameter, and the low-pressure cylinder 54 inchesin diameter, both working an 8-feet stroke in the pumps. Thepiston rod of each cylinder is coupled directly to the pump rods,and from the crosshead of each piston rod run two short connect-ing rods, attached to two bell cranks; these cranks are connectedto each other at the top by the connecting rods on each side, thuscoupling the two engines and equalizing their duty. From thelongitudinal centre of one of the bell cranks the motion for thetappet rod is taken, and from the back of the other one; an arm iscast on each, with connecting rods for taking the crosshead for theair pump. The cylinders, with their covers and ends, are steam-jacketed, and securely bolted down to a bed plate resting on foun-dations of stone. The cast-iron guides for the piston-rod crossheadare bolted to the under side of the bed plate, and the bottom end is

1 The Engineer.