he point of the pencil bears upon the surface of
the paper wound upon the drum A. If the drum A did not revolve, this pencil
would merely trace on the paper a vertical line; but as the drum A moves
round and back again every stroke of the engine, and as the pencil moves up
and down again every stroke of the engine, the combined movements trace
upon the paper a species of rectangle, which is called an indicator
diagram; and the nature of this diagram determines the nature of the
engine's performance.

236. _Q._--How does it do this?

_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 indic