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Page:Popular Science Monthly Volume 24.djvu/549

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FIFTY YEARS OF MECHANICAL ENGINEERING.

a short time ago had been left to sailing-vessels. This steamship has some points of interest, and illustrates the most advanced ideas on steam-engineering as applied to the mercantile marine. The engines of this steamer are triple expansive, having one high-pressure, one intermediate, and one low-pressure cylinder, using steam at 125 pounds pressure, generated by boilers whose only peculiarity consists in the fact that they are capable of withstanding such a pressure. On trial these engines gave one horse-power for 1·28 pound of coal burned per hour. This would, according to the usual analogy, indicate a daily working efficiency of about 1·50 pound to the one horse-power. This steamer can carry coal for a voyage of 12,000 miles, and, with proper use of sails, could probably keep under steam for two months without coaling. The weight of the engine and boilers of 1832 was about 1,000 pounds to the horse-power; to-day it is about 300, and in some instances has been reduced to forty-five pounds to the horse power.

An English firm have recently completed a small light compound engine, which, in point of weight, eclipses anything heretofore built. This engine is made of steel and phosphor-bronze; all parts are built as light as possible, the rods and shafting and all parts possible being bored out to reduce weight. At a speed of only 300 revolutions a minute they indicate over twenty horse-power, and weigh but 105 pounds all told. This engine would give fully thirty horse-power actual at a piston-speed of 500 feet a minute. The size is three and three quarters high pressure, seven and a half low pressure, and five stroke. That thirty horse-power can be had from a proper utilization of steam and proper distribution of 105 pounds of metal is certainly most astonishing, especially so, considering that the engine is compound. A ship of 2,500 tons displacement was almost unknown fifty years ago; to-day the transatlantic steamer, the highest class of the mercantile marine, has from 8,000 to 13,500 tons displacement, and engines of 5,000 to 10,000 one horse-power. Several of the transatlantic liners have shown a mean ocean-speed of twenty miles an hour, and make the passage in less than seven days.

The present generation has grown so accustomed to the results of the progress of mechanical science that it has long ceased to wonder at its greatest works.

It may be well here to speak of the torpedo-boats which have been recently built for the English Government; they indicate the extreme limit of naval construction of this day. These little instruments of destruction are only eighty-seven feet in length, ten and a half feet in beam, forward draught eighteen inches, aft fifty-two inches, total displacement thirty-three tons. The engines are compound condensing, of the intermediate receiver type, high-pressure cylinder twelve and three fourths inches, low-pressure twenty and three fourths, stroke twelve inches, and indicated over 500 horse-power, with a gross weight