A.—­It is clear that if the pencil was moved up instantaneously to the top of its stroke, and was also moved down instantaneously to the bottom of its stroke, and if it remained without fluctuation while at the top and bottom, the figure described by the pencil would be a perfect rectangle, of which the vertical height would represent the total pressure of the steam and vacuum, and therefore the total pressure urging the piston of the engine.  But in practice the pencil will neither rise nor fall instantaneously, nor will it remain at a uniform height throughout the stroke.  If the steam be worked expansively the pressure will begin to fall so soon as the steam is cut off; and at the end of the stroke, when the steam comes to be discharged, the subsidence of pressure will not be instantaneous, but will occupy an appreciable time.  It is clear, therefore, that in no engine can the diagram described by an indicator be a complete rectangle; but the more nearly it approaches to a rectangle, the larger will be the power produced at every stroke with any given pressure, and the area of the space included within the diagram will in every case accurately represent the power exerted by the engine during that stroke.

237. Q.—­And how is this area ascertained?

A.—­It may be ascertained in various ways; but the usual mode is to take the vertical height of the diagram at a number of equidistant points on a base line, and then to take the mean of these several heights as representative of the mean pressure actually urging the piston.  Now if you have the pressure on the piston per square inch, and if you know the number of square inches in its area, and the velocity with which it moves in feet per minute, you have obviously the dynamical effort of the engine, or, in other words, its actual power.

238. Q.—­How is the base line you have referred to obtained?

A.—­In proceeding to take an indicator diagram, the first thing to be done is to allow the barrel to make two or three reciprocations with the pencil resting against it, before opening the cock attached to the cylinder.  There will thus be traced a horizontal line, which is called the atmospheric line, and in condensing engines, a part of the diagram will be above and a part of it below this line; whereas, in high pressure engines the whole of the diagram will be above this line.  Upon this line the vertical ordinates may be set off at equal distances, or upon any base line parallel to it; but the usual course is to erect the ordinates on the atmospheric line.

239. Q.—­Will you give an example of an indicator diagram?

[Illustration:  Fig. 37]

A.—­Fig. 37 is an indicator diagram taken from a low pressure engine, and the waving line a b c, forming a sort of irregular parallelogram, is that which is described by the pencil.  The atmospheric line is represented by the line o o.  The scale at the side shows the pressure of the steam, which in this engine rose to about 9 lbs. per square inch, and the vacuum fell to 11 lbs.  The steam begins to be cut off when, about one-fourth of the stroke has been performed, and the pressure consequently falls.