TREATMENT OF STEAM.

the steam-pipe to the cylinder. In some instances three boxes havebeen fitted to the superheaters, involving four tube-plates, the tubesrunning right and left from the central box; the steam from theboiler enters this centre compartment, passes right and left throughthe tubes, and is taken away with separate steam-pipes at each end.Sometimes the superheater forms part of the boiler; and for lowboilers for marine purposes this plan has certain advantages, thetubes are arranged vertically, and are secured into tube-plates, run-ning the entire length of the smoke-box, the lower tube-plate beinga few inches above the tubes in the boiler, while the top tube-plate isplaced a few inches from the top of the shell of the boiler. The steamfrom the boiler enters the superheater through a number of smallapertures, these take a downward course; a division plate beingfitted, the steam passing this plate ascends, and is taken away bythe main steam-pipe; there are certain advantages connected withthis arrangement, though the main one simply consists in doingaway with a good deal of piping, as the steam from the boiler entersthe superheater directly. The great desideratum to attend to intubulär superheaters is provision fcr cleansing them from the sootin the smoke passages, whether arranged internally or externally;some mode of access must also be provided to the steam space forcleansing away the scale that rapidly forms.

To employ a large flat surface, giving time to the steam to bethoroughly dried, is certainly the correct principle to be studied,when superheaters require to be placed in small space, though thecomplication entailed renders many arrangements of flat flue super-heaters not at all to be commended in practice. Vertical flat flues,similar in construction to the overhead flue boiler with u-shaped end-pieces, and the plates flanged at the ends, for uniting them to the tube-plates, all of which are inclosed in a suitable casing, properly andsecurely stayed, with stay-bolts and ferrules, is an effective arrange-ment of the kind; the heated gases pass through the elongated tubes,while the steam is admitted at one end of the casing, and passesbetween the spaces left betwixt the flues, and is taken away by thesteam-pipe placed on the opposite end of the casing. This plan ofsuperheater is expensive in first cost, and complicated in its manyparts in the event of repairing it and keeping the apparatus inthorough working order. In all cases where separate superheatersare used for marine purposes, a stop-valve and pipe must be fittedfor each boiler, for shutting off, or allowing the steam free access to