LoDDER. — On Compound Engines, 145 



rods being lengtliened to enable both pistons of liigli and low pressure cylinders 

 to be fixed on one rod, while the same pair of eccentrics were arranged to work 

 the valves of the upper and lower cylinders of each combined engine. The 

 steam from the upper cylinders exhausts into the valve chest of the lower 

 cylinders, exerting its remaining pressure in them. It then escapes into the 

 surface condenser, whence it is conveyed back again to the boiler in the 

 shape of fresh water at a temperature of about 135^ Fahr. 



Both of the old air-pumps are brought into use, one as a circulating 

 pump to force the water through the tubes of the condenser, the other to 

 operate in the usual way. By this system two separate compound engines are 

 made, using the same condenser. 



The high-pressure cylinders are steam-jacketed, as also are the covers of 

 the lower cylinders and the exhaust pipes leading from the upper cylinders to 

 the lower ones. There is also an interheater placed in the lower steam chest 

 between the slide valves to assist in keeping up the tension of the steam. 

 The supply of steam for the jackets is taken from the superheater at a 

 temperature probably of 350° Fahr. 



The surface condenser is cylindiical, and contains 735 brass tiibes, four 

 feet long and tive-eighths of an inch outside diameter, giving a cooling surface 

 of 465 '5 square feet, the tubes being fixed into brass tube plates with screwed 

 glands and indiarubber washers. 



The boiler is 7 ft. 3 in. in diameter by 9 ft. long, having two furnaces 2 ft. 

 2 in. by 6 ft. There is a siqjerheater with the uptake passing through it, and 

 the total heating surface, including the superheater, is 502-56 square feet. 



These combined engines are of 38*8 hoise-power, by Watt's rule, and 45 

 nominal horse-power by the Admiralty rule ; the ratio of cylinder areas is as 

 6 to 1 nearly, all four cylinders cutting off at three-quarter sti'oke, so that the 

 steam is expanded about eight times. 



On the trial trip the boiler pressure was 80 lbs. per square inch, and the 

 diagrams taken by Mr. Stewart, Government Inspector of steamers, showed an 

 initial pressure of 72 lbs. per inch ; mean pressure 61-75 lbs., and the terminal 

 pressure, 37-5 lbs. ; average number of revolutions per minute 80, indicating 

 58 horse-power for the upper cylinders. 



The effective pressure in the lower cylinders was only 7-6 lbs., indicating 

 42 horse-power, making a total of 100 indicated horse-power for the combined 

 engines, with a consumption of 376 lbs. per hour, or 3-76 lbs. per indicated 

 horse-power per hour. 



The diagrams also showed that the steam in the lower cylindei's is under 

 atmospheric pressure, hence the smallness of the power in them as compared 

 with the power given out in the upper cylinders. There ought to have been 

 at least from 5 to 6 lbs. above the atmosphere in the lower cylinders. 



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