Twentieth century hand-book for steam engineers and electricians, with questions and answers .. . FIGURE 92. in., as the width of the spaces; the two end spacesbeing one-half of this, or T\ in. wide. The first fiveordinates, counting from A, express forward pressure,represented by the arrows. The remaining five ordi-nates, counting from B, express counter or back pres-sure, represented by the arrows pointing in theopposite direction. Measuring the pressures alongthe first five ordinates, and adding them together,gives lbs., which divided by 5 gives ^s. asthe mean forward pressure (M
Twentieth century hand-book for steam engineers and electricians, with questions and answers .. . FIGURE 92. in., as the width of the spaces; the two end spacesbeing one-half of this, or T\ in. wide. The first fiveordinates, counting from A, express forward pressure,represented by the arrows. The remaining five ordi-nates, counting from B, express counter or back pres-sure, represented by the arrows pointing in theopposite direction. Measuring the pressures alongthe first five ordinates, and adding them together,gives lbs., which divided by 5 gives ^s. asthe mean forward pressure (M. F. P.). DIAGRAM ANALYSIS 261 Then figuring up the counter pressure in the samemanner on the other five ordinates, beginning at B,. COFFIN AVERAGER OR PLANIMETER. the result is lbs. The M. E. P. therefore will ~4-25 = lbs. Obtaining the M. E. P. with the Planimeter. The areaof the diagram represents the actual work done by the 262 ENGINEERING steam acting upon the piston. In a non-condensingengine the lower or exhaust line of the diagram mustbe either coincident with or slightly above the atmos-pheric line in order to express positive work. Anydeviation of this line, either above or below the atmos-pheric line, represents counter pressure, the amount ofwhich may be ascertained by measurements with thescale, and should be deducted from the mean forwardpressure. On the other hand, the exhaust line of a diagramfrom a condensing engine falls more or less below theatmospheric line, according to the degree of vacuummaintained, and the nearer this line approaches theline of perfect vacuum, as drawn by the scale, the atmospheric line, the less will be the counterpressure, which in this case
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