BOILERS FÜR MARINE PURPOSES.

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“If we contrast this with a locomotive boiler, we find that thelatter will not weigh, complete with water and in working Order,more than 12 or 13 tons. It will have 1100 feet of heating surface,and 18 to 20 square feet of grate, and it may be depended upon todevelop 600 horse-power in a non-condensing engine.

“ The cubical space occupied by the locomotive boiler will not bemore than one-fourth of that taken up by the marine boiler, and itwill be on the whole quite as economical, if not more economical.”In referring to objections to the use of the locomotive type of boilerat sea, it is pointed out that a fair trial has not yet been made ofsuch boilers at sea for mercantile purposes, and that it has provedserviceable in torpedo boats.

Attempts are being made at present to give practical effect tothis question of decreased dead weight by reducing the diameter ofthe Shells, and giving increased draught so as to consume more fuelper foot of grate surface.

As to the question of the relative economqr of chimney draughtand forced draught, it has to be borne in mind that although powerhas to be expended in driving fans or blowers to produce a forceddraught, still in the chimney draught a large proportion of the heatof the furnace is spent to produce and keep up such draught,reaching, according to some authorities, to one-fourth of the avail-able heat of combustion.

It has been proposed to carry out forced draught by jets of steamin funnel or air in ashpit, or by fans blowing air into ashpit direct,or into the stoke-hole, the latter in this case requiring to be air-tight.

Funnel, Damper, &c. — The ordinary height of funnels forsteamships of the merchant Service is about 32 feet 6 inchesfrom the top of the steam-chest, and about 48 feet height fromthe top of the fire-bars, the area = ^th of fire-grate. Wheremore than one funnel is required the arrangement of the boilerswill determine the number and positions of the funnels. It isvery usual now in large ocean-going ships to have two funnels,and in some cases even three funnels, as the City of Rome , Livadia,and other vessels. In the first-named ship the funnels are arrangedfore-and-aft, in the Livadia athwart-ships. The plates are gener-ally arranged in four lengths, the three lengths towards the bottom9 feet each, and the top plates 5 feet 6 inches. The joints are butted,with strips inside of the chimney, and the circumferential joints are