436

MODERN STEAM PRACTICE.

for this purpose. A small hole is bored at each end, in com-munication with the open space between the ring and the piston,and the mere forcing of the water causes a pressure inside, whichpresses the packing ring outwards against the barrel of the pump.When one ring is used, a small india-rubber ball valve openinginwards, placed at both ends, will tend to make the action moreperfect; as the piston is going forward the front valve would openand the back one would shut, and vice versa; thus there would beno escape through the piston, but probably this is not required, asa very small hole suffices, and the escape is but little feit; and iftwo rings were fitted in recesses, one hole suffices at each end.Thin spring rings of brass have been used with advantage; in thiscase the piston is made quite solid at the ends, with three recessesturned out on the rubbing surface for the reception of the rings,which are truly turned avery little larger than theinterior diameter of thepump; they are then sawnthrough at one part, ex-panded over the piston,and sprung into the re-cesses; in this way thereis a slight spring in therings, which keeps themwell up to the surface of 2the barrel. Wood packingwith lignum-vitae has beenoften successfully used ;the piston is cast as before,and the space for thepacking is filled up entirely with curved blocks, which are madeto overlap one another at the joinings, and then the ring is turnedto the exact diameter of the barrel, the body of the piston being atrifle less in diameter. This arrangement is what may be termed asolid piston, and is one which can only be adopted with a materialthat expands in water; lignum-vitae is well fitted for the purpose.Solid brass pistons without any packing have been tried, but didnot succeed, as they soon failed to afford a perfect vacuum. Hollowplunger pistons have, however, been arranged, both internally andexternally. In the former plan the plunger has a central packing

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w7y// r //////////"//////////sy-Fig. 317.—Air-pump Plunger and Gland.

. Plunger, b, Stuffing box and gland. C, Rod.