64 
METEOROLOGY OF 1 897. 
EVAPORATION, 
§67. Table lo gives the evaporation from our standard 
evaporation tank. This is sunk in the soil level with the 
surface. Observations with the hook gage are made twice 
per day from April to September, gage reading to one 
thousandths of a foot. At the same time the temperature 
of the water surface is observed twice per day at the maxi- 
mum and minimum temperatures. During September and 
October the reading is made once daily. During the winter- 
months ice forms and the observation is made at the begin- 
ning of each month. 
It is found that the evaporation runs from one to two 
inches per month during the winter. The evaporation dur- 
ing the night is practically as rapid as during the day. 
§68. An attempt was made to obtain a formula from 
the observations of 1889, and with enough success to compute 
the evaporation in 1890 from May to October with a differ- 
ence of less than half inch. The formula was as follows : 
E =.39 (T-t)(H-.02\V). 
E represents the evaporation in inches in 24 hours. 
T is the vapor tension corresponding to the temperature of the surface of 
the wat^r. 
t is the vapor tension corrt^sponding to the temperature of the dew point 
. at that time. 
\V represents the number of miles of wind in the 24 hours. 
Observations have since been made to obtain a more 
perfect formula, but are not yet reduced. 
