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MODERN STEAM PRACTICE.

H.M. navy to be fätted with compound engines was the Constancebuilt by the same firm in 1863. In marine engines the Cornishprinciple has to a certain extent been carried out. Large cylindershave been adopted, but we consider that there is still great room forimprovement. Longer strokes should be aimed at, although theremust be a limit in this direction, owing to the confined space of themidship section of steam vessels; but with horizontal engines on thereturn con'necting-rod principle there is nothing to hinder the intro-duction of strokes of 4 feet 6 inches and upwards for engines placedin exceedingly fine sections. This for medium power would be agreat advantage over those engines having a stroke of 3 feet or so.A full measure of expansion with a single cylinder has often beenobtained, and with good results; but there are certain difficultiesto contend with, the chief one lying in the details, as all the partsrequire to be made of extra strength, and when 60-lb. steam persquare inch is used, with a large area of piston, the engine becomesvery heavy. This is not at all to be desired, as the permanent loadshould be kept as low as possible, more especially for high-speedhorizontal engines, there being no doubt that the wear and tear oflarge heavy pistons and their adjuncts forms a serious item of ex-pense on board ship. Trunk engines have been recommended, soas to carry up as it were the weight of the piston; and there is littledoubt that the large surface of the trunks tends to lessen the wear,but it does so at the expense of lubricants, more especially with thehigh pressure and dry steam now adopted.

To obviate the defects inherent in the single-cylinder arrange-ment, using high steam with a large measure of expansion actingon these great pistons, the Compound Engine has been adopted toa considerable extent; although the same type of engine is by nomeans a novelty, as, for manufacturing purposes requiring greatregularity of motion, beam engines fitted with high and low pressurecylinders have long been used. The motion is very easy anduniform, more especially at the beginning and ending of the strokeof the piston, which is due to the steam expanding from the bottomof the small piston to the top of the large one, and vice versa; thusthe steam from the boiler acting on the small piston is met bya counteracting pressure, which tends to lessen the shock moreor less feit with ordinary engines. The large or low-pressure cylin-der is placed at the end of the beam, and the small or high-pressurecylinder is usually one-half of the diameter of the low-pressure one;