117



118 THE TEMPERATURE-ENTROPY DIAGRAM.
The amount of condensation and reevaporation is
the result of so many factors, that to determine the
influence of each by the ordinary methods of compari-
son would require too much time and money. Hence
to aid in evolving a theory of the steam-engine which
shall account for aU heat losses and interchanges, some
convenient form of analysis must be adopted by which
the losses for any single test may be investigated.
Hirn's analysis makes possible the determination of
the net heat changes occurring between admission and
cut-off, cut-off and release, release and compression, and
compression and admission, but does not give informa-
tion as to the actual direction of heat-transference at
any moment. Fortunately, the T^-diagram offers a
graphical solution equivalent to that of Hirn's analysis,
and also makes clear the direction in which the inter-
change of heat is occurring at any point. Before
a r^-projection of an indicator-card can be made,
it will be necessary to discuss at length the different
lines of the card in order to determine exactly what
each represents.
The Admission Line of the Indicator-card. — During
admission the steam is not at a imiform temperature
and pressure. Part is still in the steam-pipe under
boiler pressure, part has passed through the valve-chest
and steam-ports, and has already entered the cylinder,
and still a third portion is in the process of transition.
