371] B. HE. Lnwingston 173 
if the evaporating power of the air for any particular surface 
is to be studied. The condensation pressure should be deter- 
mined for the general atmosphere of the space under con- 
sideration. If air circulation were infinitely rapid, which 
means, practically, if there is a high wind, this deficit value 
should be a measure of the evaporating power for the particu- 
lar location considered. Also, if two sets of conditions are to 
be compared, in which the air circulation is the same, then 
the two atmospheric evaporating powers should be propor- 
tional to the corresponding vapor pressure deficits; for the 
other factor is then common to both sets. 
To illustrate the use of the vapor pressure deficit, let it be 
supposed that there are two rooms in which the air circulation 
is alike, and let it be required to estimate the relative values 
of the evaporating powers corresponding to the two rooms. 
The data involved and the results obtained are shown below, 
together with the two relative humidity values, as usually 
given in such comparisons. 
Air tem- Vapor PRESSURE Vapor pres- _— Relative 
perature Maximum Actual sure deficit humidity 
deg. c. mm. of Hg. mm. of Hg. mm. of Hy. per cent. 
Room 1. 20° 17.41 14.50 2.91 83 
Room 2. 25° 23.55 6.14 17.41 26 
The values used in this example have been so chosen that the 
deficit for Room 2 is 17.41 mm., just what it would be for 
Room 1 if the actual vapor pressure were taken as zero. This 
is the maximum deficit for a temperature of 20°. Neverthe- 
less, it is seen that the actual vapor pressure for Room 2 is far 
from zero. This emphasizes the point that the maximum 
evaporating power of the air increases with the temperature, 
air pressure, and circulation remaining the same. 
Such comparisons have usually been made in terms of rela- 
tive humidity, the values for which are presented in the last 
column of the tabular arrangement just given. This mathe- 
matical abstraction is the ratio of the actual to the maximum 
