BOILERS FOR MARINE PURPOSES.

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wheel is apt to jam in the pinion if the frame for it is not rigidlysecured. An outside air-casing must be carried up from the top ofthe boiler, or dry uptake if so fitted, to about 8 feet above the deck,for taking away the vitiated air from the stoke-hole, and protectingthe woodwork of the combings around the funnel from the great heatradiated from the uptake. On the deck there is another casing, so aseffectually to keep the passage round the funnel cool. There are holesall round at the tops of the casings, and likewise at the bottom, forthe thorough Ventilation of the stoke-hole. The waste steam-pipe islikewise made telescopic, the bottom part having a suitable stuffing-box, through which the top part slides stearri tight, the top parthaving a stay, with collars on the waste-pipe, the stay being attachedto the top or sliding part of the main funnel.

The smoke-doors should be hinged to flat bars, fastened verticallyto the smoke-box. The doors should have an inside plate and anouter one, kept apart from the door itself, but secured to it byferrules and rivets: the inner plate is to protect the door from thefire, and the outer one to keep the stoke-hole cool. By this arrange-ment a current of air passes freely between both the inside and theoutside plates. There are handles, fitted with a means of keeping thedoor shut, so constructed that the sneck or snib presses the door for-cibly against the hinge plates. The furnace doors are hinged so asto cover the apertures formed in the front plates, thus doing away withcast-iron frames, and are pierced with air holes, having the means ofregulating the supply of air to the furnaces. The ashpits are fittedwith dampers at front, hinged on a pin, having ratchet wheels andpawls, for regulating the supply of air underneath the bars, or, byshutting them, damping the fires. The manhole on the top of theboiler should be cut in the most convenient place, and the doorsecured on the outside with a bridge piece, strongly bolted. The sizeof the manhole is usually 18 inches by 14 inches, which is sufficientto allow a man of ordinary size to pass through for inspecting andcleaning the inside of the boiler. Smaller doors are left in thefront plate of the boiler, between the furnaces at the top, of sufficientsize to allow a boy to pass through for scaling the furnace tops. Allthe necessary doors must be fitted at the bottom of the boiler, forraking out the sludge, the bolts for securing them being made so asto draw up the door against the inside of the front plate of the boiler;one large bolt, having a cross bar over the hole, is by far the bestplan for securing them.