XIX
305
ADIABATIC CHANGES IN MOIST AIR
and are drawn according to the same scale as a and ß; but
in general they are not continuations of the curves B.
Means must now be provided for finding the points of
transition between the various stages. The dotted lines serve
They give in grammes
to determine the end of the first stage.
the greatest amount of water which one kilogramme of the
mixture can just retain as vapour in the various states,
calculated by means of the formula v = Re/R,T. Thus the
curve 25 connects together all those states in which 1 kilo-
gramme of the mixture is just saturated by 25 grammes of
steam. These curves are drawn for every gramme.
If a
mixture contain n grammes of steam in every kilogramme, we
may follow the curve of the first stage up to the dotted line
n; then we must change to the second, or the fourth stage, as
the case may be.
The boundary of the second and third stages is given by
the intersection of the corresponding adiabatic B with the
isothermal 0° C. The pressure Po, corresponding to this inter-
section, and μ, the amount of water present, determine p₁, the
pressure at which we must pass from the third to the fourth
stage. To determine p, we must use the small supplementary
diagram, which is just below the larger one. It has for
abscissæ the pressures arranged as in the large diagram, and
for ordinates the total quantity μ of water in all the stages,
in grammes per kilogramme of the mixture. The inclined
lines of the diagram are merely the curves which correspond
to equation III of the third stage, when in this equation we
regard po as constant, but p₁ and μ as variable coordinates.
These lines are not quite straight, though on the scale of the
diagram they are not to be distinguished from straight lines.
The highest point of each line corresponds to the case Pi-Po
The corresponding value of μ is not zero, but is equal to v,
the least value μ must have in order that the mixture may be
saturated at 0° and that the supplementary diagram may be
required at all. When for given values of p, and μ we require
the corresponding value of p₁, we must look out the inclined line
whose highest point has the abscissa Po, and follow it down
to the ordinate μ The pressure at which this ordinate is
reached is p₁, the pressure sought. With it the point of
transition to the fourth stage is found.
M. P.
X
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