HIGH AND LOW BOILERS.

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is a common practice for small boilers, and forthose of larger power also where height isavailable. The shape of the ends or sides isdue to the beam of the vessel and the lengthof the fire grate. The bottom portion near theback end is generally raised angularly to con-nect with the same, it being more availablefor evaporation when raised, than when flat,or on a level with the bottom of the fire box.The front portion of the shell—above thelevel of the tubes—generally projects, both forincrease of water and steam spaces. Thebottoms of the fire boxes are generally insidethe shell; it must be added, however, in someinstances, the bottoms of the boxes and shellare on the same level, in fact, the base of theformer commencing at the bridge; and thisarrangement reduces the height of the boiler,minus the water space, a matter of greatimportance sometimes—it will thus be under-stood that the word “ high,” is only in thiscase a technical term.

The next class of boilers is the low kind,and their origin may be traced from thelollowing causes :—When shallow rivers haveto be navigated, a distribution of weightsuperficially is imperative, and when a cleardeck space is requisite, any raised portionreduces the area accordingly. For warpurposes, also, it is necessary that themachinery should be below the water line, toprevent disablement from shot or shell,because a steam vessel, with non-effectivemachinery, becomes less navigable than theonce famous sailing crafts, and thus protectionis essential.

The arrangements of the class of boilersnow under notice, are perhaps more varied

than those of any other type. This may besaid to be due to the requisition of increase ofpower with a reduced height and breadth.The exterior forms of the shells of the boilersin question are cylindrical, flat, and angularsides, all in requisition for certain arrange-ments and localities.

With the cylindrical shell, the internalarrangement in some cases is thus: the firebox—either in one, two, or more compart-ments—extends direct from the front of theboiler to the combustion chamber, the latterand the water space completing the entirelength of the boiler. The tubes are dispersedat*the sides and top of the fire boxes, passingto the front of the boiler, unto which isconnected the smoke box. By this it will beunderstood that, the commencement of com-bustion and the termination of its use, are atthe same end of the boiler, or similar inarrangement to the ordinary high boilersalready described.

The next arrangement worthy of comment,is that of the direct acting kind, i.e., wherethe fire box, combustion chamber, tubes, andsmoke box are in a direct line with each other,as that of a locomotive boiler. The internalarrangements of the flat and angular shellsare much alike, the shape at the sides beingsuited to the beam of the vessel. The otherexamples most universal are as follows :—The fire boxes are at the end of the boilernearest the engines. The combustion chamberis in front of the fire grate for the entire widthof the fire boxes, and extending at the outerside—or that nearest the hull—for the entirelength of the shell. The tubes are secured atright angles with the fire boxes beyond the