MARINE ENGINES.

38S

of a propeller, whether paddle, screw, or jet, in Ibs .:—“ Multiplytogether the transverse sectional area in square feet of the streamdriven astern by the propeller; the speed of that stream relativelyto the skip, in knots; the real slip, or part of that speed, which isimpressed on that stream by the propeller, also in knots; and theconstant 5 '66 for sea water and 5 ’5 for fresh water.”

HORIZONTAL DIRECT-ACTING AND RETURNCONNECTING-ROD ENGINES.

In these engines the cylinders are placed side by side, as in thelocomotive engine, and the steam valves are worked directly offthe cranked shaft by double eccentrics and link motion. Thecondensers are placed on the opposite side of the shaft for returnconnecting-rod engines; they are fitted with guides for the cross-heads; the piston rods are secured to arms forged on the crosshead,and are so arranged for the rods Crossing the cranked shaft, one aboveand the other under the shaft. The connecting rod by this arrange-ment is not in a direct line with the piston rods, but goes backwards,while the piston rods are connecting to the piston in a forward lineCrossing the main shaft of the engine.

The distance between the centres of the cylinders in these enginesis regulated by the arrangement of the air pumps and valves. Whenthe air pumps are close together on each side of the centre frame,the distance between the centres is greater than when the air pumpsand adjuncts are placed further apart, close to the outer frames;however, it is not a good plan to contract the water passages inconnection with the air pumps and condenser to gain a few inchesbetween the centres of the cylinders. Steam jackets are generallyused, cast along with the cylinder; the fronts and cylinder coversare also made double, so that the steam from the boiler freelycirculates all round the cylinder, and the full pressure is bettermaintained on the piston, a higher indicated measure being given outthan by an unjacketed cylinder. To prevent condensation in thesteam casing the outside of the cylinder should be covered withfeit, and then overlaid with lagging or narrow Strips of wood, whichare secured to wooden hoops, bolted to the strengthening ribs leftin the casting. The passages for the steam and exhaust are arranged

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