170 Vapor Tension Deficit [368 
air may be very different in two locations only a few centi- 
meters apart. The differences here encountered are much 
greater than the similar ones met with in thermometry and 
the general exposure of the instrument needs to be stated in all 
climatological studies of atmometry. The readings obtained 
are taken as averages for the time period of operation and are 
stated with reference to a shorter time unit. 
To summarize the points brought out above, every atmo- 
metric measurement should be formulated so as to include all 
the five features indicated by letters in the following state- 
ment, which is given as an illustration. The atmometric in- 
dex for location A, for the period of operation B, is found to 
be C units of water lost per time unit D from an atmometer 
of type #. Filling in the features represented by these let- 
ters, to render the illustration more concrete, we may say: 
The atmometric index for a place 1 meter above the ground in 
the center of a large field of clover in northern Ohio, for the 
period of operation from May 1 to May 10, 1916, was found 
to be 12 grams of water lost per day from a standard white 
spherical atmometer. If any of these five features is omitted 
from the statement, the meaning is rendered vague and un- 
certain. 
THE VAPOR TENSION DEFICIT AS AN INDEX OF THE 
MOISTURE CONDITION OF THE AIR 
By Burton E. Livineston 
Studies on the manner in which external conditions con- 
trol the activities of animals and plants must deal with the 
moisture conditions of the air in all cases where the organ- 
isms considered are aerially exposed. While atmospheric 
evaporating power (measured with reference to some standard 
evaporating surface) furnishes an index of the air conditions 
that influence the rate of water loss from aerially exposed 
organisms, it is frequently desirable to analyze this complex 
