Scientific American Supplement, No. 647, May 26, 1888 - Various

In a trial with a compound engine, with 130 lb. absolute pressure, the missing quantity at cut-off rose from 11.7 per cent. at 405 revolutions to 29.66 per cent. at 130 revolutions, the consumption of feed water increasing from 20.35 lb. to 23.67 lb. This saving of 14 per cent. was due solely to increase of speed. Similar trials had been made with a simple engine. In one simple trial at slow speed the missing quantity rose to 44.5 per cent. of the whole feed water.

Intended mean admission pressure (Lb.)	40	90		110		130	150		160		170
Simple, Compound, or Triple.	S.	S.	C.	S.	C.	C.	C.	T.	C.	T.	T.
Actual mean admission pressure (Lb.)	40.88	92.65	87.54	106.3	109.3	130.6	149.9	151.9	158.5	158.1	172.5
Percentage ratio of actual mean pressure, referred to low pressure piston, to theoretical mean pressure	98.2	100	91.3	100.7	94.8	94.2	94.6	84.54	95.9	85.3	85.2
Indicated horse power	16.51	31.61	28.14	33.5	33	36.31	38.59	35.69	39.55	35.56	38.45
Feed water actually used per indicatedH.P.H.— Simple (Lb.) Compound (Lb.) Triple (Lb.)	42.76	26.89	...	26	...	...	...	...	...	...	...
	...	...	34.16	...	21.37	20.35	19.45	...	19.19	...	...
	...	...	...	...	...	...	...	19.68	...	19.19	18.45
Steam required theoretically per 1 H.P.H.(Lb.)	34.67	19.24	19.86	17.9	17.65	16.25	15.23	15.16	14.87	14.9	14.36
Percentage efficiency	81.1	71.5	82.2	68.8	82.5	80	78.3	77	77.4	77.6	77.8
Percentage of feed water missing at cut off in high pressure cylinder	...	...	...	...	...	...	...	5.33	...	6.84	5.01
Ditto high pressure cylinder	...	...	5	...	9.5	11.7	15.1	14.84	17	12.06	15.33
Ditto low pressure cylinder	11.7	24.8	15.2	29.56	16.25	19.1	20.6	22.12	21.3	22.11	24.21
Percentage of feed water missing at end of stroke in low pressure cylinder	10.4	18.83	14.25	21.53	16.59	17.55	20.69	18.01	19.55	18.81	19.25

The author compared a series of compound trials, at different powers, with 130 lb. absolute pressure, and various ratios of expansion, with a series giving approximately the same powers at a constant ratio of expansion, but with varying pressures, being practically a trial of automatic expansion against throttling. Starting with 40 indicated horse power, 130 lb. absolute pressure, four expansions, and a consumption of 20.75 lb. of water, the plan of varying the expansion, as compared with throttling, showed a gain of about 7 per cent. at 30 indicated horse power, but of a very small percentage when below half power. If the engine had an ordinary slide valve, the greater friction, added to irregular motion, would probably neutralize the saving, while if the engine were one in which initial condensation assumed more usual proportions, the gain would be probably on the side of variable pressure. Even as it was, the diagrams showed that the missing quantity became enormously large as the expansion increased. Judging only by the feed water accounted for by the indicator, the automatic engine appeared greatly the more economical, but actual measurement of the feed water disproved this. The position of the automatic engine was, however, relatively more favorable when simple than when compound.

In conclusion, the author referred to a trial with a condensing engine, at 170 lb. absolute pressure, in which the feed water used was 15.1 lb., a result evidently capable of further improvement, and to an efficiency trial of a combined central valve engine and Siemens' dynamo, made for the Admiralty, at various powers. At the highest power the ratio of external electrical horse power to indicated horse power in the engine was 82.3 per cent. Taking the thermo-dynamic efficiency of the engine at 80 per cent., that of the combined apparatus would be nearly 66 per cent.

[1]

Abstract of paper read before the Institution of Civil Engineers, March 13.

RAILWAY BRIDGE AT LACHINE.

The subject of our large illustration this week is a large steel bridge carrying the Central Pacific Railway over the St. Lawrence River at Lachine, near Montreal. The main features of this really magnificent structure are the two great channel spans, each 408 feet long. It will be noticed that the design combines, in a very ingenious manner, an upper and a lower deck structure, the railway track being laid on the top of the girders forming the side spans, and on the lower flanges of the channel spans, which are crossed by continuous girders, 75 feet deep, over the central pier, and supported by brackets as shown. The upper of our two engravings shows the method of constructing the principal spans, which were built outward from the side piers, while the work on the center pier was extended on each side to meet. It was built at the works of the Dominion Bridge Company, Montreal, from the design of Mr. C. Shaler Smith, the well-known American bridge engineer.—Engineering.