Weight of -1Iachinery.—It is interesting to compare the weight of machinery relatively to the power developed; for this comparison has sometimes been adopted as the standard of ex cellence in design in respect of economy in the use of material. The principle. however, on which this has generally been done is open to some objections. It has been used to compare the weight directly with the indicated horse-power, and to the comparison in per horse-power. So long as the machinery thus compared is for vessels of the same class and working at about the same speed of revolution, no great fault can be found; but as speed of revolution is a great factor in the development of power, and as it is often dependent on circumstances altogether external to the engine and concerning rather the speed of the ship, the engines fitted to high-speed ships will thus generally appear to greater advantage than is their due. Leaving the condenser out of the question, the weight of an engine would lie much better referred to cylinder capacity and working-pressures, where these are materially different, than directly to the indicated power. In Table 11 are given the relative weights of nine triple-exiainsion engines, according to both modes of comparison, Nos. 1 to 6 are mercantile engines, and Nos. 7 to 9 are naval ex amples. It will be noticed that. though the twin-screw engines Nos. 5 and U are the same type of engine as the single-screw engines Nos. 1 to 4, as evidenced by their weights per cubic foot of cylinder capacity, yet their en gine-weights her indicated horse-power arc considerably lower by virtue of their higher speed of revolution. Comparing its predecessors with No. 9. which is a fair type of a naval engine, it will he seen that the engines usually fitted in the merchant service are about 44 per cent heavier per unit of cylinder capacity than this en gine. The low weight per unit of heating-'surface in Nos. 7, 8, and 9, which is about 22 per cent. less than in the mercantile examples, Nos. 1 to U, is due to careful use of material, as well as to the lighter scantlings adopted for boilers by the Admiralty.
Ecoaomy of Fuel.—Table I11 gives the performances of 28 three-stage expansion engines in ordinary work at sea. The average consumption of coal per indicated horse-power is lb. per hour. The average working pressure is 158.5 lbs. per sq. in. Comparing this work ing-pressure with 77'4 lbs. in 1881, a superior economy of 19 per cent might be expected now on account of the higher temperature; or, taking the lb. of coal per hour per indicated horse-power in 1881, the present per formance under similar conditions should be lb. per hour per indicated horse-power. In Table IV the prin cipal factors in the present performance of marine en gines are compared with those of 1681, and also with those of 1872, as indicated in the table accompanying Sir Frederick Bramwell's paper (Proc. Inst. E.. 1872). Compared on the same basis, then, it appears that the working-pressures have been increased twice in the last ten years, and nearly three times in the last nineteen.
The coal consumptions have been reduced 16.7 per cent. in the last ten years, and per cent in the last nineteen. The revolutions per minute have increased in the ratios of 100, 105, 114, and the piston speeds as 100, 124, 140. Although it is quite possible that further in vestigations may show that the present act nal consump tion of coal per indicated horse-power is understated in Table IV, yet it is hardly probable that the relative re sults will be affected thereby. The yet arils of the coal consumption have in all cases been taken in the same way and on the same basis as for Mr. :Nlarshall's paper in 1881 (Prue. /9/4. X. E.. I). so that whatever errors may affect the returns for the one year are likely to have affected those for the other. The probability of error lies in the statement of the horse-power indicated, which,when taken directly the ship's log. is usually in excess of that actually indicated continuously ; so that t he compar ison of coal consumption with power is open to objection.
The old - fashioned link - motion, though it seemed for a time likely to disappear, still holds its own, and in all probability will continue to do so. In the distribution of steam it may not he so mathe matically aceurate on paper, lint practically the effect is or can he made as good as with h the best radial valve gear. It does not give constant lend when linking up. 11111 constant lead is nil the ideal of perfect valve-set ting. A constant lead angle of the crank is more nearly what is required, for which a diminishing had in the valve with linking lip is the necessary condition. The old link-motion lends itself readily nil gracefully to any mwdilieahons which may be suggested by in the condition a Ihe forms do not. Besides t his. the link-motion admits of simple geomet rical treatment, which is generally understood even in the engine-room, and is consequently a safer arrange ment in the hands of the man found there.
Dimensions.—In the matter of the power put into individual vessels considerable strides have been made. In 1881 probably the greatest power which had been put into one vessel was in the case of the Arizona, whose machinery indicated about 6,360 horse-power. The following table gives an idea of the dimensions and power of the larger machinery in the later passenger-vessels: In war-vessels the increase has been equally marked. In 1881 the maximum power seems to have been in the Inflexible, namely, 8,485 indicated horse-power. The following will give an idea of the recent advance made.
Indicated horse-power.
Howe (Admiral class) 11.600 Italia and Lepanto. 19,000 Umberto.. ............ ............... 10,000 Blake and Illenheilll (building). 20,000 Sardegna (building). 22.800 It is thus evident that there are vessels at work to-day having about three times the maxi mum power of anv before 1881.