Should a tube be found defective, from whatever cause, it may be renewed or temporarily plugged, as both ends are accessible. All necessary repairs can be made by the ship's staff. The only tools required are a ripping chisel and an ordinary expander, the operation of which is familiar to every engineer.

The placing of the steam and water drum horizontal with its center on the water line of the boiler, provides a large body of water where it is most needed, and where changes in the volume of water carried least affect the levels in the gauge glasses.

The location of the drum at the front of the boiler renders all valves and fittings accessible and tends to shorten steam pipe connections. Main stop and safety valves, stop and feed check-valves for both main and auxiliary feeds, and water glasses, are flanged directly to nozzles provided with counterbored seats and riveted to the drum shell or heads. The longitudinal seams are butted and strapped, and have from four to six rows of rivets, as the steam pressure requires. Butt straps are curved to proper radius in a hydraulic press.

The rivet holes are drilled after the rolled plates are assembled; the butt straps are then removed from the drum plates and all burrs cleaned off. The rivets are driven by hydraulic pressure and held until cool.

The drum heads are formed in a single heat, by hydraulic pressure, to a spherical surface whose radius is equal to the diameter of the shell. The manhole is flanged in the shell plate, or drum head, with a stiffening ring of sufficient thickness to form, with the edge of the plate, a seat for

the manhole gasket one inch wide. The manhole plates are II x 15 inches, and are faced to a true oval to fit the manhole.

Surrounding the pressure parts, and firmly bolted to the foundation, is a structural iron framing to which the casing plates are fastened.

The spaces between the side tubes are filled with light fire tiles made of highly refractory fire clay. Against these are placed asbestos mill-board and magnesia block covering. On the outside, firmly holding the non-conducting materials in position, are the casing plates, which are clamped to the structural framing by butt straps. This method of fastening allows of easy removal, and on replacing makes an air-tight joint without the use of additional packing. The efficiency of the casing is demonstrated by the fact that

the hand can be held upon the side of the boiler, when steaming, without discomfort, and the stoke hold is always cool.

Hinged to the framing at the front and rear of the boiler are large doors, giving access to the hand-hole plates covering the tube ends.

Ample means are provided for blowing the soot from the exterior of the tubes. A steam lance may be inserted through small dusting doors empaneled in the side casing, as shown above, and communicating with the spaces between the rows of tubes. Each opening is covered by a shutter sliding vertically, which can be opened and shut by the lance. As the seat for this shutter is beveled, it tends on falling to

wedge itself into position, thereby making an air-tight joint. This panel is used on the 4-inch tube boilers. On the 2-inch tube boilers, this panel is embodied in a swinging door as shown on the last page.

As all cleaning of soot from the exterior of the tubes is performed from the sides, the continuous steaming of the boiler and coaling of the grates by the stokers are not in any way hindered.