| ... | ... | ... | ... | 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. * * * * * RAILWAY BRIDGE AT LACHINE.