BOILERS FOR MARINE PURPOSES.

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over the furnaces on the return principle, while the latter havegenerally the furnaces fore and aft, with the tubes athwart ship,the tubes reaching no higher than the tops of the furnaces. Thebest arrangement for the high dass are furnaces athwart ship, withthe stoke-hole between the boilers on the centre line of the vessel—the distance apart from front to front of the boilers being io feet;this is considered ample room for the firemen. As the top of theboilers requires to be at least i foot below the water line, the ordinarysteam-chest is dispensed with, sufficient height being left betweenthe top and the water in the boiler. To give free circulation foreand aft, the uptake or dry smoke pipe is shaped thus, A, flat atthe bottom sides, but rounded at the top, to take the main funnel.This is a very good plan.

We have also seen many arrangements formed with the steam-chest over the firemen’s heads; a plan that should never be attempted,as such require an artificial blast to keep free circulation in the stoke-hole, the usual plan being a fan driven by a separate engine; butin some classes of war ships, such as “ Monitors,” even this fan isnecessary, the “free-board” in such ships being so low that in roughweather the hatches require battening down, and then Ventilationmust be kept up by mechanical appliances.

The low dass boiler is^admirably suited for finemidship sections, firingfore and aft. They areplaced closely together atthe centre line of the ves-sel, leaving only a spaceof 2 inches between thelagging, or the wood cover-ing which is placed overthe boilers to prevent ra-diation. The furnaces, saythree in number, join inone “ athwart -ship flue,”widening from the furnace

ät th.6 centre of the vessel Fig. 32.—Lo w Boilers, asarranged for the Royal Navy . Lon-gitudinal and Horizontal Sections. aa, Furnaces. b, Com-to those 3.t the sides cind bustion Chamber, c, Tubes. d, Smoke-box. e, Uptake.

then passing into the combustion chamber, which runs fore and aft,this chamber tapering from the furnaces to the extreme end. This