350 STEAM ENGINE of about 15 knots or 17 statute miles an hour. The steam pressure was 25 Ibs. These engines are horizontal, and have steam cylinders 120 in. in diameter, and 4 ft. stroke of piston ; the Sistons weigh 8 tons each. The surface con- ensers contain 16,500 sq. ft. of condensing surface, the tubes being ^ in. diameter, and 6 ft. long. The propelling power is a two-bladed Griffith screw, 23 ft. diameter, 26 ft. mean pitch, expanding 5 ft. The valves are moved by a link motion, of which the reversing gear is worked by a small steam reversing engine, which weighs about 350 tons ; the boiler weighs nearly as much more. The cost of engines and boiler was 66,500. The City of Peking, a screw steamer built for the Pa- cific mail company, is a vessel of 5,000 tons. There are two pairs of compound engines, having cylinders of 51 and 88 in. diameter, and 4 ft. stroke of piston. The crank shafts are 18 in. in diameter. Steam is carried at 60 Ibs., and is expanded nine times. The boilers are ten in number, cylindrical in form and with cylindrical flues ; they are 13 ft. in diam- eter, 10 ft. long, with shells of iron ^-f in. thick, and have 520 ft. of grate surface, 16,500 sq. ft. of heating surface, and 1,600 sq. ft. of superheating surface. The smoke funnels, or stacks, are 8 ft. in diameter and 70 ft. high. Steam Pres- sure and Engine Power. The steam in the engine exerts a varying pressure from the beginning to the end of the stroke, and these pressures may be determined experi- mentally by the use of the steam engine indicator. The best form now in general use is the Richards indicator, fig. 17. A miniature steam cyl- inder, A, has within it a closely fitted piston, which by exceedingly nice construction is made to work perfectly steam-tight with- out friction or leakage. Its rod B is attached' to the parallel motion C D E F, which carries a pencil at the middle of F in a perfectly ver- tical line. To the upper side of this piston and to the cap V of the cylinder is screwed a helical steel spring, of such strength that, re- sisting the steam pressure beneath the piston, it causes the pencil to rise and fall, as pressures vary, through distances which are proportional to the changes of pressure. A scale, G, on the barrel II II, indicates the pressures per square inch which correspond with the position of the pencil at any instant. The barrel II H is con- nected by means of the string I with some part of the engine having a motion coincident in FIG. 17. Richards Indicator. time with that of the steam piston, but of sncb extent that at each stroke of the engine the barrel H will be turned about three fourths of a revolution only. A piece of paper or thin card is wrapped upon this barrel, its end being secured by the springs W, and upon this paper the indicator card or diagram is automatical- ly made by the pencil. The instrument is attached to the steam cylinder by the cock N, which i$ screwed at O into the cylinder in such a position that steam can at all times enter it, and so that the pressure in the engine and in the indicator shall be the same. The instru- ment is secured to the cock by the use of the nut with its double screw threads R S, one of which being finer than the other, the cone T may be forced into U very firmly, and per- fectly steam-tight. An indicator should be attached to each end of the cylinder, and dia- grams taken simultaneously if possible. The instrument being thus attached and steam admitted, after a few moments' working has thoroughly heated the cylinder, the steam is shut off from the indicator, and the pencil is, with its support J K, swung aromd, until it touches the paper. As the barrel revolves, the pencil makes a horizontal line, which is called the atmospheric line or line of atmospheric pressure. The reading of the barometer will then give the distance of the vacuum line, or the line of absolutely no pressure, below this line of reference. Steam is again admitted, and the pencil, rising and falling as the steam pressure changes in the cylinder, while the paper is moved laterally with a motion pre- cisely similar to that of the piston, a diagram is made, usually resembling 5 c d e f a in fig. 18, taken from the work of Mr. Charles T. Porter on the indicator. Steam from the boiler is supplied to the engine at the commencement of the stroke nearly at boiler pressure, and follows the piston at that pressure until at 6 the steam passage is gradually contracted, and finally closed by the steam valve. The steam thus confined within the cylinder expands as ! .9 C
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"F fc fc s Kj I 5! ^ ^ .15 Fro. 18. Diagram of Indicator. the piston moves forward, diminishing in pres- sure until it arrives at d, where the exhaust valve gradually opens communication with the
