XIX
307
ADIABATIC CHANGES IN MOIST AIR
constant.
―
=
For within the limits of the diagram it varie
only from 253 to 303; so that if we give it the constant
value T, 273, the error in h will hardly ever exceed of
its total value. If we choose to neglect this error, we get h
= const RT, log p, and may at once introduce the heights
as well as the pressures as abscissæ. Everywhere indeed
equal increases of length of the abscissa will correspond to
equal increases of height. The scale of heights is marked at
the foot of the diagram; its zero is placed at the pressure
760 mm, because this is usually regarded as the normal
pressure at sea-level.
C. In order to explain the use of the diagram by an
example, let us consider the following concrete problem. We
are given at sea-level a quantity of air, whose pressure is
750 mm., temperature 27° C., and relative humidity 50 per
Required to find what states this air passes through as
it is transferred without loss or gain of heat to higher strata
of the atmosphere and thus to lower pressures; and also at
what heights approximately above sea-level the various states
are reached.
cent.
First, we look out on the diagram the point which cor-
responds to the initial state. It is the intersection of the
horizontal isothermal 27 and the vertical isobar 750.
We
observe that it lies almost exactly on the dotted line 22.
This means that each kilogramme of our air would contain
22.0 grammes of water-vapour when saturated. But as its
relative humidity is only 50 per cent it contains only 11.0
grammes per kilogramme. This we note for further use.
Again we follow the isobar 750 down to the scale of heights
at the foot of the diagram and read off 100 metres. Thus
the zero of the scale of heights lies 100 m. beneath the sea-
level chosen by us as our starting-point; and we must subtract
100 m. from all actual readings of the scale of heights in
order to get heights above sea-level. If now we raise up our
mass of air, the series of states traversed by it is first given by
the line of system a which passes through the initial state.'
There is no such line actually drawn, so we must interpolate
1 The letters a, B, y, which denote the systems, are given at the edge of the
diagram, enclosed by small circles. One line of the system which the letter
denotes is continued right up to it. The changes of state of our example are
marked by a special line of dots and dashes in the diagram.