STATIONARY ENGINES.

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packing-ring System; but when the piston is cast solid, and indeedfor all pistons, it is preferable to cut a screw on the rod at thesmall end of the cone, which is fitted with a nut for pressing thepiston firmly on the cone; and to prevent the piston turning roundin the cylinder, as it may do in course of time, a small short key islet into the rod, having a corresponding part cut in the piston forits reception.

The crosshead and gudgeon for the connecting rod is of wroughtiron, suited for single or forked ends as may be desired; the hole inthe crosshead for the piston rod is bored out slightly tapered, therod being turned to suit, and secured with a cotter passing throughthem both. Holes should be drilled at the small end of the cotterfor passing a split pin through, to keep it from shaking loose. Theholes for the gudgeon in the jaws of the crosshead are bored quiteparallel, and the gudgeon, being accurately turned, is driven throughtightly, and secured with a key. The gudgeon can be of a lessdiameter at the ends for taking the guide blocks, and of sufficientlength at one end for fitting the eye of the feed-pump plunger to it.

The motion bars for guiding the crosshead in a direct line withthe piston rod are of cast iron. The bottom bars are generally castalong with the bed plate, but they sometimes form separate Castings,which require to be fitted to the bed plate; while the top bars aregenerally made—so that the gear can be adjusted—with thin Stripsof metal between them and the bottom bars, which can be reducedin thickness as the guide blocks wear. In this arrangement the topbars are secured to the bottom ones with bolts at the ends, the samebolts securing the bottom bars to the bed plate; but in some casesthe bottom and top motion bars are cast in one piece, and fittedand bolted down on the bed plate. These guiding bars must beaccurately planed, and also the guide blocks, which are cast in hardbrass. Sometimes cast-iron blocks are adopted, in which case therubbing surfaces are filled in with white metal, recesses beingleft in the casting for that purpose; plain cast-iron blocks, however,answer very well, when lubrication is properly attended to—thatbeing a most important point in all rubbing surfaces. Oil cupsshould be cast on the top motion bars, and fitted with proper coversto exclude grit, with siphon pipe and wick to supply the oil dropby drop.

The connecting rod is of wrought iron, turned from end to end,with oblong pieces at the ends accurately planed, and fitted with