ft. In all cases each furnace has a separate combustion-chamber, the draft from which may be controlled independently by dampers fitted in the uptakes. The stoke-holes can be worked both under natural and forced draft; for the latter purpose eight fans 5 ft. in diameter are fitted, capable of maintaining an air-pressure of 3 in. of water if required for testing the tightness of the whole of the space put under pressure, but the maximum allowed on the forced-draft trial is 11 in. of water.
pie-Expansion. Paddle- Wheel Engine.—Fig. 2 shows the engines of the Hygeia, a pad dle-wheel steamer built, on the Clyde in 1390. The llygeia is 300 ft. in length. 32 ft. beam, and 12 ft. in depth. The engines are constructed on Rankin's patent disconneetive" triple expansion principle, especially designed for high-speed river-steamers. They are of the diag onal direct-acting type, and have two high-pressure cylinders, each 28 in. in diameter, and placed behind an intermediate and a low pressure cylinder 56 and 86 in. in diameter, respect ively, with a piston-stroke of 66 in.. working tandemwise, in connection with a double-throw shaft. There is only one pair of stuffing-boxes between each pair of cylinders, and ample space has been provided for the easy removal of the intermediate and low pressure cylinder covers, which, in addition, have been fitted with man-holes and doors for examining the pis tons without disturbing the covers. All the pistons have deep cast-iron packing-rings held up to the bore of the cylinders by short spiral steel coils. The two high-pressure cylinder valves are of the piston type, the intermediate being on the trick principle, and the low-press ure is double-ported. The valve motion consists of the double-barred link type, and is actuated by one of Brown's steam and hydraulic reversing-engines, the starting lever for which, along with the lever for the throttle-valves, is brought to the main-deck platform, and constitutes a particularly simple arrangement. as. in consequence of there being two high-pressure evlin ders, no starting-valves are required, and it is impossible for the engines to stick on the centers, so that prompt handling is always as sured.
The diagonal fram ings have been made of wrought iron. strongly bolted to heavy brackets cast on the cylinders with round, solid flanges, and they are attached at the other ends by T heads to the massive main-benrino framings, which are of cast iron and box section. These arc firmly bolted to the sole-plates, which bind the whole structure rig idly together. The pis ton-rods are of forged steel, carefully fitted into the pistons and cross-heads with a good taper and shoulder, and secured by deep malleable iron nuts. The cross-heads and connecting-rods are of forged iron, and the latter are coupled to the crossheads by double jaws, and to the crank-pins by single jaws, all being fitted with phosphor-bronze bushes of extra large surface, secured by polished wrought-iron covers with strong steel bolts and nuts recessed into guard-rings having set pins. The cranks, shafting, and paddle arms are of forged iron ; the paddle wheels arc of the feathering de scription with outside rings, The llygeia on her official trial showed an average speed of 22.8 statute miles, the best run being at the rate of 23i statute miles. The absence of vibration in both hull and en gines. and the exceptionally smooth working of the latter, were notice able.
Triple-Screw Engines.—Within the last four or five years twin screw steamships have come gener ally into use, especially for trans atlantic vessels and for ships-of-war. The United States Government. however, has gone a step further, and is now building a protected cruiser with three screws. The special ad
vantage of three screws in a cruiser, adapted both for high speed when occasion requires and for slow speed for ordinary voyages, is that by stopping either one or two engines fuel can be saved to a much greater extent than it can in a single-screw steamer by slowing down the en gine. The principal dimensions of the new cruiser are as follows: Length on load-line, .10(1 ft.; beam (extreme), 5s ft,: draft (mean, normal), 23 ft.: draft (extreme. normal), 24 Cl.; dis plaeemeet fat '23 ft. mean). 7,1119 tons; eneffleient of displacement. 0185; speed (sustained).
21 knots: speed unaxi lllll 22 knots: indicated (estimated, sustained), 20.000: indicated hets:elmwer (estimated. maximum). 23.000; coal-stipply. 2.000 tons; untidier of screws, outboard screws in diameter, 13 ft. !I in. : center screw in diameter, 12 ft. There will be three sets of propelling-engines, each set being complete in all respects and placed in separate water-tight compartments. The amidship engine will be placed abaft the port and starboard engines. The amidship and starboard engines will tnrn right and the port one left handed when the vessel is going ahead. These engines will be of the vertical inverted cylinder, direct-acting, triple-expansion type, each with a high-pressure cylinder 42 in., an intermediate-pressure cylinder 50 in., and a low-pressure cylinder 92 in. in diameter—the stroke of all pistons being 42 in. It is estimated that the collective indicated horse-power of propelling, air-pump, and circulating-pump engines should be about 21,000 when the main engines are making about 129 revolutions per min. The high-pressure cylinder of the after engine will be forward and the low-pressure cylinder aft, and the high-pressure cylinder of each forward engine will be aft and the low-pressure cylinder forward. The main valves will be of the piston type, worked by Stephenson link-motions with double-bar links. There will be one piston-valve for each high-pressure cylinder, two for each intermediate-pressure cyl inder, and four for each low-pressure cylinder. The framing of the engines will consist of cast-steel inverted NT-frames at the back of each cylinder and cylindrical forged-steel columns at the front, as shown in figure. The main condenser for each engine will have a cooling sur Lee of about 9,474 sq. ft., measured on the outside of the tubes, the water passing through the tubes. Two of the propellers will be right and one left, to be made of manganese bronze or approved equivalent metal. There will be six double-ended boilers, about 15 ft. 6 in. diameter and 21 ft. 3 in. long, and two about 11 ft. 8 in. diameter and 18 ft. 8.3.- in. long for the main boilers, and two single-ended auxiliary boilers about 10 ft. diameter and 8 ft. in. long. The boilers will be of the horizontal return fire-tube type, all constructed of steel for a working pressure of 100 lbs. per sq. in. Each of the larger-sized double-ended boilers will have eight corrugated furnace-fiues. 3 ft. 3 in. internal diameter; each of the smaller double-ended boil ers will have four corrugated furnace-fines, 3 ft. 6 in. internal diameter, and each single-ended boiler will have two furnaces 2 ft. 9 in. internal diameter. The total heating surface for the main and auxiliary boilers will be about 43.272 sq. ft., measured on the outer surface of the tnhes, and the grate surface 1.285 sq. ft. The foreed-draft system will consist of one blower for each fire-room, discharging into an air-tight fire-room. The full coal-supply of 9,000 tons will give the vessel it radius of action of 20,240 knots, or 109 days steaming at 10 knots per hour.