TRANSACTIONS OP THE SECTIONS. 205 



composing the circumference shell : these boilers contain the water, and the spaces 

 between them the fire. 



The feed water passes first into the spiral compartment, or No. 24, and from it 

 into the centre compartment, or No. 23, and then into each in rotation, and blows 

 oft" at the last compartment, or No. 1, thus rendering the water in No. 24 nearly pure 

 sea- water, and gradually from compartment to compartment more dense, till it blows 

 off at No. 1 at the usual density, and thus makes the average specific gravity of the 

 water less than usual. 



The twenty-two outside boilers are 24 feet long, 19 inches diameter, and yV of an 

 inch thick ; the bottom ends are conical for 3 feet, and kneed outwardly to give a 

 larger diameter of furnace, say 12 feet diameter. There is a furnace-door for every 

 alternate tube, or say, eleven furnace-doors, equally divided round the base of the 

 boiler, giving great facility to the firemen for doing their work efficiently. In firing 

 it is proposed to charge all the fresh coal round the circumference of the fire, in order 

 that the hydrogen of the coal may be consumed separately from the carbon ; and as 

 the furnace has great altitude, the combustion will be completed in vertical flames 

 from the coals, and will thus prevent the carbonic acid gas, given out from the com- 

 bustion of the carbon, coming so much in contact with and preventing the com- 

 bustion of the hydrogen, as is usual in ordinary furnaces. 



The centre compartment, or No. 23, is 30 feet long, 34 inches diameter, and f of 

 an inch thick, with 3 feet at the bottom and top, conically reduced to 18 inches 

 diameter, forming a man-hole door ; the upper end of tbis vertical tube forms a 

 reservoir for the steam of the whole twenty-four compartments, and acts as a super- 

 heating apparatus, and may be carried up the funnel to the extent necessary to 

 superheat the steam to 400 degrees ; the steam-pipe is taken from the top of this 

 boiler to the safety-valve chest, fastened on the front of the boiler low down, which 

 serves as a water-trap during the discharge from the safety-valve chest, the steam- 

 pipe to the engines being taken off the same pipe at a higher level than the escape 

 steam. The spiral compartment, or No. 24, is about 100 feet long, 34 inches 

 diameter, and § of an inch thick, made of best iron boiler plate ; the ends are conical 

 for 3 feet, formed into man-hole doors ; this spiral boiler makes four or five con- 

 volutions close round the centre one, and is bound close to the circumferential 

 boilers by hollow stay-bolts, and fastened to the centre one at each end only ; in the 

 same manner the steam and water flow through the whole boiler by these hollow 

 stay-bolts or rivets, and complete the entire circulation of water and steam ; the 

 whole of these twenty-four compartments or boilers terminate at the bottom, about 

 1 foot below the fire-grate, and are supported on six stanchions from the ash-pit 

 beneath, making a free passage for the air under the great bar ; the circumferential 

 compartments or boilers terminate at the top 6 feet above the ship's deck, and have 

 each a man-hole door forming the cover; the funnel is made conical at the bottom 

 to embrace the internal diameter of the boiler-shell and draw off the smoke in the 

 usual manner : this completes the whole boiler proper ; but in order to prevent 

 radiation of heat, a thin outer casing of iron is made (9 inches) clear of the boiler all 

 round, terminating about 7 feet from the stoke-hole floor ; and above, at the level of the 

 galley or funnel-house, this casing is lined with felt and thin wood to keep the deck 

 and the adjacent parts cool, and retain the heat. The twenty-two straight cylindrical 

 boilers or compartments are constructed in the sides by four plates 24 feet long and 

 ]6 inches broad, rolled to a 9|-inch radius curve at the iron works, leaving no plate 

 setting for the boiler maker of this description. 



The plates of boiler No. 24, or the spiral compartment, are delivered flat by the 

 iron-maker, and are bent to the spiral curve by one blow of a large spiral concave 

 block falling upon a counterpart convex one, prepared by the constructors of the 

 boiler. This operation has been found to simplify the making of this spiral cylin- 

 drical boiler to about the same amount as the straight cylindrical boilers. The 

 conical ends are bent in the same manner as the spiral plates,'and the whole work of 

 plate bending is reduced as far as possible to machine work. The products of com- 

 bustion, after leaving the furnace, have to travel spirally upwards a distance of 100 

 feet, and must of necessity be continually rotating during that time, and prevent the 

 possibility of any portion passing off without being brought frequently in contact 

 with the heating surface of the boiler; and will therefore be cooled down to the 



