MARINE ENGINES. 517

in the passage at the side of the port, the exhaust port is madebroad in the direction of the travel of the valve.

Length of ports for double-ported valves. — The steam ports inthe cylinder are divided by a centre rib about 1 inch broad, whilethe exhaust port is left free. The combined length of the steamports is found by dividing the diameter of the cylinder by 17, towhich add the breadth of the centre rib.

Steam way or opening by valve, lap, &c. —The full opening ofthe steam way by valve is found by multiplying the area of thecylinder in square inches by the speed of the piston in feet perminute, dividing the product by the constant 10,000; the resultdivided by 2 gives the openings for double-ported valves, or fouropenings formed with the central ribs. The width of the openingis of course determined by the length of the ports, and it is an all-important consideration, as it regulates the supply of the steam intothe cylinder, and consequently the number of strokes or revolutionsof the cranked shaft; and from it the correct lap of the valve isobtained for cutting off the steam at that part of the stroke which isarranged for giving the greatest indicated measure of power in con-junction with the steam pressure that is determined on. The lap ofthe valve depends on what part of the stroke of the piston the cut-offcommences, and is explained at p. 122; the length depends on whichdass of valve is adopted. The length of doubled-ported valves isconsiderable, but the benefit of having a short stroke is obvious, asthe eccentrics are not nearly so large, and all the minor details arecorrespondingly reduced. The length of the valve is simply thesum of the ports and bridges or bars on the cylinder face, to whichmust be added the two outside laps.

Steam pipe on valve casing and exhaztst pipe .-—The steam-pipebranch on the valve casing is about one-twentieth of the area ofthe cylinder; the main steam pipe is two-thirds of the two com-bined. The exhaust pipe into the condenser is one-twelfth of thearea of the cylinder.

Relief valve for cylinder .—The diameter of the relief valve isfound by dividing the diameter of the cylinder by 12. Smallercocks are sometimes fitted on the same valve ehest, and worked bylevers, rods, and handles from the starting platform.

Thickness of cylinder .—For ordinary steam pressure ranging upto 30 lbs. per square inch in the boiler, the thickness of a cylinder25 inches in diameter is inch, and for every 5 inches of additional