To calculate the horse-power of an engine from diagrams, each diagram should be marked off, as shown, by ten lines perpendicular to the atmospheric line A L. The extremities of the diagram are marked on the line AL and the distance between divided into twenty equal parts, perpendicular lines being erected at the first division, third division, fifth division, and so on. The diagram cuts each of these lines in two points, and the distance between these points should be measured to obtain the effective pressure shown by the card at that line. This, however, is not the effective pressure on the piston at that point in the stroke; to obtain this the two cards, front and back, must be superposed, and the back pressure shown on one deducted from the forward pressure shown on the other. This, however, has no effect in the mean pressure as obtained below. The pressure as obtained from the diagram depends on the spring used. On cards with which a 1/40 spring is used a length of l in. shows a pressure of 401b. per square inch; so that a length of 1; in. on the diagram would indicate a pressure of 1 1/2 x 40 =65 lb. per square inch. Owing to reduction, the actual scale of the illustrations is 1/80 or l in. = 80 1b. per square inch.

Measured in this way, the pressures are, commencing from the left in Fig. 1, 68, 80, 60, 50, 40,32 1/2, 25 18 1/2, 15, and 101b. per square inch, and, in Fig. 2, 101, 15, 20 25, 30, 35, 45, 55 1/2 , 77 1/2 , and 72 1/2 . The mean of each of these is their sum divided by ten. Thus the meanpressure shown by Fig. 1 is 399/10 = 39.9 1b. per square inch, and by Fig. 2 is 386/10 = 38.6 lb. per square inch. The mean pressure during the two strokes may therefore be taken at (39.9+38.6)/2 = 39.25 lb. per square inch. The horse-power may now be determined.

Determining Power of Engine from Indicator Diagrams.