. A manual of marine engineering: comprising the design, construction, and working of marine machinery. piston perminute as tlie multiplier,—the length oj stroke in feet x ninnber of strokesper minute should be substituted. Mean Pressure.—The mean pressure is usually obtained by dividing theindicator-diagram by a number of equidistant ordinates perpendicular tothe atmospheric line, and so placed that the distance of the first and lastfrom the extreme limits of the diagram is half the distance between twoconsecutive ones; the sum of their lengths, intercepted by the diagram,divided by their num
. A manual of marine engineering: comprising the design, construction, and working of marine machinery. piston perminute as tlie multiplier,—the length oj stroke in feet x ninnber of strokesper minute should be substituted. Mean Pressure.—The mean pressure is usually obtained by dividing theindicator-diagram by a number of equidistant ordinates perpendicular tothe atmospheric line, and so placed that the distance of the first and lastfrom the extreme limits of the diagram is half the distance between twoconsecutive ones; the sum of their lengths, intercepted by the diagram,divided by their number, gives the mean length, and this, referred to thescale on which the diagram was drawn, will give the mean pressure. Toillustrate this:—tig. 16 is an indicator-diagram whose length, AX, is, say,5 inches, and taken with a spring requiring a pressure of 30 lbs. per squareinch to compress it 1 inch; so that if M L is 2 inches, it represents apressure of 60 lbs. ; and if B L is 2^ inches, it signifies that, at the pointL, the pressure on the piston was 2^ x 30 lbs., or 75 lbs. per square inch. B C D E Fig. 16.—Indicator-Diagrani. above the line A X, which, in this case, shall be the line of no pressure, andhence is 75 lbs. absolute, or 60 lbs. above the atmospheric pressure. Now,for convenience of division, let theie be 10 ordinates enclosing 9 spaces—since there is to be a half space at each end, there will be in all equal to10 spaces—so that the distance between the ordinates is 5 inches -r 10, orhalf an inch. Measure off A B = ;|^ inch ; B C, C D, D E, kc, each = \ inch,and at B, C, D, E, &c., draw perpendicular lines, cutting the diagram at M L, 0 N, &c., Y Z. Then (M L + 0 N -I- &c -f YZ)-r 10 = a; inches, and x x. 30 is the mean pressure of the diagram. This diagram is from one side of the piston only, and, when one only isobtainable, it is sometimes assumed to represent both, and the meanpressure thus obtained used to calculate the power; but it seldom happens
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