496

MODERN STEAM PRACTICE.

For engines of small power eccentrics have been successfully adoptedfor working air pumps lying in a horizontal position, but the planis not nearly so effective as a means of balancing the machinery asworking the pumps by the rocking lever already described, withlinks in connection with the crosshead of the piston rod at one endand the crosshead of the air pump at the other end.

The arrangements for compound engines of the vertical dass aresimilar to those for single engines of the same dass. The cylindersare supported from the top of the surface condenser on the oneside, and there are cast-iron frames on the other; in some caseswrought-iron columns are introduced. The high-pressure cylinderis fitted to the side of the low-pressure one, and connected bymeans of a rocking beam for each; thus the pistons travel inopposite directions, and the valve gear is simplified as in horizontalarrangements. Some engineers place the high-pressure cylinderon the top of the cover of the low-pressure one, with one pistonrod common to both pistons.

Theoretically and practically, direct action is correct, as thesteam from the one cylinder is immediately taken up by the other;but the complication of the parts is an evil to be avoided, andengines are now introduced having the minimum of working parts.These engines consist of one high-pressure cylinder and a low-pressure one. The cranks in some examples are at right anglesor 90°, in others the crank centres are 13 5 0 apart (and even, goingto extremes, in a line with each other), with which the crankfor the low-pressure cylinder leads, thus the piston being met bythe expanding steam from the high-pressure cylinder, it is cushionedat each end of the stroke—a point necessary in all engines havinga high rate of piston speed. When the cranks are placed at rightangles, the exhaust steam is admitted into a compartment, generallytaking the form of a casing around the cylinder, before it isexpanded into the large cylinder. This plan is necessary to reducethe back pressure on the small piston; besides, the large piston isnot in a proper position at the ending and returning of the high-pressure piston fully to utilize the steam. The large piston, too,must travel a little more before the expanding steam can beadmitted into the cylinder.

In the large vessels recently built three cylinders are commonlyadopted, two large and one smaller, the cranks being placed at 120°.See Plate of the engines of the S.S. Parisian.