144 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 
errors in evaporation records, or possibly for comparing conditions 
locally studied with stations for which there is no evaporation rec- 
ord, but which maintain a complete record of wind movement, sun- 
shine duration, and humidity. In such an event it may be possible 
to work out a fairly constant relation between the evaporation from 
any given type of atmometer, and a combination of these other con- 
ditions, properly integrated, in the general relation of : 
E (K) = (Wind movement plus saturation deficit) sunshine. 
The term " vapor pressure " expresses the weight per cubic foot, 
or the pressure, in centimeters of mercury, of the vapor currently 
in the atmosphere. The term " saturation deficit " expresses the 
lack of vapor pressure, or the difference between the existing vapor 
23ressure and that which the atmosphere would contain at the current 
temperature if the space were saturated with water vapor. The " dew 
point" indicates the temperature at which the existing vapor would 
condense ; or, in other words, the temperature at which the existing 
\ apor would produce a condition of saturation. It is readily seen, 
then, that the saturation deficit is the difference between saturation 
pressure for the current dry-bulb temperature, and saturation pres- 
sure for the temperature of the current dew point. The term i; rela- 
tive humidity " expresses, as a percentage, the relation between the 
existing vapor and that which might be present if the space were 
saturated at the current air temperature. 
The dew-point figure is used only incidentally in computing vapor 
pressure, saturation deficit, or relative humidity. Of the three, 
experience in a number of forest ecological studies has shown that 
the saturation deficit is by far the most useful, giving, as it does 
without further reference to temperature conditions, a direct measure 
of the capacity of the atmosphere for more vapor, and hence, in 
some degree, a measure of the rate at which evaporation will take 
place. 
The psychrometer, consisting of a pair of thermometers mounted 
on a frame in such manner as to be readily whirled in order to 
accelerate evaporation, is the common instrument for determining 
atmospheric humidity. One of the thermometers is covered with a 
layer of cloth (preferably linen), which is dipped in clean water 
before making the exposure. The evaporation of this water cools the 
thermometer, or, as the expression is, causes ;t a depression of the 
wet bulb ; " and the maximum depression which it is possible to 
produce by vigorous movement of the instrument through the air. 
taken with the current temperature, is considered to give a measure 
of the atmospheric humidity. Tables have been worked out. after 
experiment, for almost all possible combinations of air temperatures 
and wet-bulb depressions, showing the corresponding dew points 
and relative humidities. Of these the best-known in this country 
