The casing is of 1/20-inch sheet iron, and in five parts.  The back end must be holed to allow A, B, and C to project 1 inch, and have a furnace-door opening, and an airway at the bottom, 5 inches wide and 1 inch deep, cut in it.  The airway may be provided with a flap, to assist in damping down the fire if too much steam is being raised.  In the front end make an inspection opening to facilitate cleaning the tubes and removing cinders, etc.

The side plates, m m, are bent as shown in Fig. 86, and bolted to a semicircular top plate, n, bent to a radius of 6 inches.  A slot, 1-1/2 inches wide and 11-1/2 inches long, must be cut in the top, n, to allow it to be passed over drum D; and there must also be a 3 or 3-1/2 inch hole for the chimney.  A plate, p, covers in D. A little plate, o, is slipped over the slot in n, and asbestos is packed in all round D. The interior of the end, side, and the top plates should be lined with sheet asbestos held on by large tin washers and screw bolts.  To protect the asbestos, movable iron sheets may be interposed on the furnace side.  These are replaced easily if burnt away.  The pieces m m are bent out at the bottom, and screwed down to a base-plate extending the whole length of the boiler.

The fire-bars fill the rectangle formed by the tubes B, El, and E2.  A plate extends from the top of E2 to the front plate of the casing, to prevent the furnace draught being “short circuited.”

Boiler Fittings.

[Illustration:  Fig. 89.-Safety valve.]

Safety Valves.—­The best all-round type is that shown in Fig. 89.  There is no danger of the setting being accidentally altered, as is very possible with a lever and sliding weight.  The valve should be set by the steam gauge.  Screw it down, and raise steam to the point at which you wish the safety valve to act, and then slacken off the regulating nuts until steam issues freely.  The lock nuts under the cross-bar should then be tightened up.  In the case of a boiler with a large heating surface, which makes steam quickly, it is important that the safety-valve should be large enough to master the steam.  If the valve is too small, the pressure may rise to a dangerous height, even with the steam coming out as fast as the valve can pass it.

[Illustration:  Fig. 90.-Steam gauge and siphon.]

Steam Gauges.—­The steam gauge should register pressures considerably higher than that to be used, so that there may be no danger of the boiler being forced unwittingly beyond the limit registered.  A siphon piece should be interposed between boiler and gauge (Fig. 90), to protect the latter from the direct action of the steam.  Water condenses in the siphon, and does not become very hot.

[Illustration:  Fig. 91.-Water gauge.]