. A manual of marine engineering: comprising the design, construction, and working of marine machinery. effects of lead and lap will, how-ever, be shown later on. The Long and Short D Valves spoken of are slide-valves whose cross-section is shaped like that letter, the flat part bearing on the cylinder face,so as to cover the port, and the curved back fitting against elastic ormetallic packing to prevent the passage of steam past it. Fig. 83 showsthis form of valve, and it will be seen that it differs from the locomotiveslide-valve in not covering the exhaust-port at the sides, and by providin
. A manual of marine engineering: comprising the design, construction, and working of marine machinery. effects of lead and lap will, how-ever, be shown later on. The Long and Short D Valves spoken of are slide-valves whose cross-section is shaped like that letter, the flat part bearing on the cylinder face,so as to cover the port, and the curved back fitting against elastic ormetallic packing to prevent the passage of steam past it. Fig. 83 showsthis form of valve, and it will be seen that it differs from the locomotiveslide-valve in not covering the exhaust-port at the sides, and by providingfor passing of steam fiom one end to the other through itself, instead ofbeing surrounded by steam. This kind of valve is practically relieved fromthe pressure on the face due to the steam, and therefore should have lessresistance to motion than the slide-valve ; but in practice the pressure fromthe packing was excessive, and even then it was very difficult to keepsteam-tight, especially in the corners at the sides; it was consequently dis-carded in favour of the piston-valve or locomotive ^^ Fig. 83.—Long D Slide-valve. Seawards Valves.—To get a more perfect arrangement of cut-off andrelease than is possible with one valve. Seaward fitted a separate valve andports for steam and exhaust, and to avoid the large amount of clearancewhich such an arrangement would entail, he used four valves, a steam andan exhaust-valve for each end of the cylinder, and by placing the ports closeto the ends of the cylinders, reduced the passages to a minimum. Eachvalve was simply a flat plate, and worked by cams on the shaft, and how-ever early the steam-valves cut oft, the exhaust-valves always opened justat tlie end of the stroke. This arrangement, although admirable in manyways, has succumbed on account of complication of gear and multiplicityof parts; and, notwithstanding attempts to modernise it to suit the com-pound engine, it is now a thing of the past. Common or Locomotive Slid
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