lmost a necessity. To meet this requirement the
United States Forestry Service has devised the Humidity Diagram shown
in Figure 93. It differs in several respects from the hydrodeiks now
in use.

The purpose of the humidity diagram is to enable the dry-kiln operator
to determine quickly the humidity conditions and vapor pressure, as
well as the changes which take place with changes of temperature. The
diagram above is adapted to the direct solution of problems of this
character without recourse to tables or mathematical calculations.

The humidity diagram consists of two distinct sets of curves on the
same sheet. One set, the convex curves, is for the determination of
relative humidity of wet-and-dry-bulb hygrometer or psychrometer; the
other, the concave curves, is derived from the vapor pressures and
shows the amount of moisture per cubic foot at relative humidities and
temperatures when read at the dew-point. The latter curves, therefore,
are independent of all variables affecting the wet-bulb readings. They
are proportional to vapor pressures, not to density, and, therefore,
may be followed from one temperature to another with correctness. The
short dashes show the correction (increase or decrease) which is
necessary in the relative humidity, read from the convex curves, with
an increase or decrease from the normal barometric pressure of 30
inches, for which the curves have been plotted. This correction,
except for very low temperatures, is so small that it may usually be
disregarded.

The ordinates, or vertical distances, are relative humidity expressed
in per cent of saturation, from 0 per cent at the bottom to 100 per
cent at the top. The abscissae, or horizontal distances, are
temperatures in degrees Fahrenheit from 30 degrees below zero, at the
left, to 220 degrees above, at the right.

Examples of Use

The application of the humidity diagram can best be understood by
sample problems. These problems also show the wide range of conditions
to which the diagram will apply.

EXAMPLE 1. To find the relative humidity by use of
wet-and-dry-bulb hygrometer or psychrometer:

Place the instrument in a strong circulation of air, or wave
it to and fro. Read the temperature of the dry bulb and the
wet, and subtract. Find on the horizontal line the
temperature shown by the dry-bulb thermometer. Follow the
vertical line from this point till it intersect