MARINE ENGINES.

437

gland for forming the joint between the two ends of the pump; itis solid at the ends, and has a boss at the centre, bored out for thereception of the Muntz metal rod, which is secured by a screwedpart at the end with a brass nut, drawing up the plunger against

a collar formed on the rod. Inthe latter plan the plunger ismoved by a connecting rodworking directly from thecranked shaft, having a bear-c ing inside of the plunger. Forthis arrangement the bushesand glands should be madevery deep; while in others,Fig. 318.-Air.pump piunger and Giand. wften the main connecting rod

A , Plunger. B, Stuffing^andgland. c, End cover. f or t h e engine WOrks a plunger,

which in its turn moves thesteam piston by rods attached to it, it is advisable to form thebottom bush of the gland the entire length of the stroke, thussupporting and taking the thrust of the connecting rod on a largesurface. In this arrangement of air pump it is necessary to reducethe area of the plunger, by having a hollow guiding trunk at theother end, working through a suitable packing space and gland.

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Fig. 319.—Air-pump Plunger acting as a guide for the Crosshead and Piston Rods.a, Plunger, b, Stuffing box and gland. c, Hollow guide. d, Bearing. e, Rod for adjusting brasses.

All air-pump rods should be made of Muntz metal, secured tothe rod from the main piston by a cotter passing through a bossformed on the wrought-iron rod which is bored out for its reception.

The foot and head valves are formed of discs of india rubber,working on brass seatings. They are introduced in all fast-goingengines, to lessen the disagreeable sound caused by the metallicvalves, and they have materially assisted in bringing the direct-action engine to that high state of perfection which it has now