XIX
303
ADIABATIC CHANGES IN MOIST AIR
to trace the behaviour of the mixture through all its stages.
But as meteorology has of necessity to deal with mixtures of
very various proportions, it would for this method require a
large number of diagrams. But we find it possible to manage
with only one diagram, if in the first place we confine ourselves
to cases in which the pressure and weight of the aqueous
vapour are small compared with those of air; and if, secondly,
we expect no greater accuracy from the results than corresponds
to a neglect of the former quantities as compared with the
latter. For if we neglect μ in comparison with λ, and e in
comparison with p, the form of the curves to be drawn is the
same for all the different absolute values of μ; so that the
same curves may be used for all the various mixtures. The
points where the various stages pass one into the other are
situated very differently for different mixtures, and hence
special means must be devised to determine these points. The
diagram is constructed in accordance with these principles.
As
In the net of coordinates the pressures are introduced as
abscissæ through a range from 300 to 800 mm. of mercury,
and temperatures as ordinates from -20° to +30° C.
will be seen, a constant increase of the coordinate does not
represent a constant increase of pressure or of temperature;
but the diagram is so drawn that equal increases of length
correspond to equal increases of the logarithms of the pressure
and of the absolute temperature. The advantage of this
arrangement is that the curves of special importance become
straight lines, in part exactly, in part approximately; and
this is of considerable importance for the accurate construction
and employment of the diagram.
When μ is neglected in comparison with A, the adiabatics
of the first stage are given by the equation
const =
c, log TAR log p.
The logarithms are natural ones throughout. With Clausius
we must put
calorie
degree C. x kilogr'
Cp = 0.2375
1
calorie
A =
=3
423-55 kilogramme-metre'