16 A GENERAL VIEW OF THE SUBJECT.
[CHAP. L
Indicator, which is illustrated in Fig. 11. This instrument com- bines the two functions of measuring the pressure, or getting the length of the ordinate HG, and of locating this ordinate on the diagram. For the first, there is the piston 8 to receive the pressure from the engine-cylinder, and the spring to measure this pressure by its varying compression; together with the pencil-mechanism, made up of pieces 13, 14, 15, and 16, which magnifies a small piston FIG. 10. The Steam Diagram, movement to an indication of convenient size. The spring is proportioned so as to give to the pressure-ordinate a scale of a cer- tain number of pounds per square inch to the inch of pencil-rise. The pencil, at 23, draws the diagram on a slip of paper carried by the drum 24. This drum is driven, through a cord which pulls against its spring 31, by a special mechanism connected to the cross-head, and so designed as to give a reduced copy of the motion of the engine-piston. The atmosphere-line MN is drawn with steam shut off from the indicator and the pressure of the air acting freely on both sides of the piston; the ordinates of the diagram are measured up (or down, in a condensing engine) from this refer- ence-line. (e) THE ACTION OF THE STEAM.-Returning now to Fig. 10, and running over the salient points of the working of the steam in the engine, we note first that the admission is made up of two parts, the rise of pressure from F to A, and the filling of the cylinder back of the piston as the latter advances; this operation being carried out, along the line AB, to cut-off at B. The clearance-space, which is being filled while the pressure rises along FA, consists of the small space left between the cylinder-head and the piston when
