BOILERS FOR MARINE PURPOSES.

6l

For small power for great steam pressure, one furnace is fittedwith return tubes at the side; while other boilers have the furnacecentral with the boiler, with return tubes overhead and at each side.The lower tubes in such an arrangement should be larger; thus theflame is drawn down, as it were, and the bottom tubes by this planare kept free from soot deposit. Another arrangement has twofurnaces in each boiler, with the combustion chamber at the backof the furnaces, and then the tubes placed direct through the boiler,thus entailing a long boiler; and this plan is good when there is

Figs. 37, 38, 39.—Small High-pressure Boilers.a, Furnace. b, Combustion chamber. c, Tubes. r>, Dry uptake. e, Steam dorne.

sufficient room in the vessel. When a steam-chest can be fitted,it is preferable to do so; and it is found a great advantage in suchsmall boilers to line the combustion chamber with fire-bricks, havingapertures for the smoke to freely pass. The steam pressure usuallyadopted is from 40 to 60 lbs. per square inch, and such boilers arebest suited for river navigation, where good fresh water is obtained.

As some river boats are made very shallow, and constructed ofvery light scantling, it is desirable to have the boiler, and all themachinery, designed to spread over a large surface; and when