IRRIGATION AND METEOROLOGY. 
9 
ATMOSPHERIC EVAPORATION. 
The record of the amount of evaporation from an exposed 
water surface has been continued since September, 1886. Since 
March 1st, of the present year, the record has included the loss 
for each day and hour of the day, an automatic register tracing 
the descent of the water surface by a pencil line on graduated 
paper. The register has thus not only served as an evaporometer, 
but as a rain guage as well. This continuous record, taken in 
connection with the meteorological observations of the A ollege, 
has afforded an opportunity for studying the atmospheric influences 
which most affect the rate of evaporation, and. as very little lias 
been written on this subject, the principal causes will be stated: 
The amount of vapor in the atmosphere. 
TheT temperature of the air and water surface. 
The pressure of the atmosphere. 
Velocity of the wind. 
Extent of water surface. 
The diminished pressure of the atmosphere, at Fort Collins be¬ 
tween one-fifth and one-sixth less than at sea level, tends to render 
the rate of evaporation excessive, as does the small amount of mois¬ 
ture in the air. Western winds increase evaporation by diminish¬ 
ing the amount of vapor in the air. 
The box employed for holding the water was three feet square 
and two and one-half feet deep, sunk in the ground in an exposed 
place and kept filled with water to within six inches of the top. 
The small area of its surface would tend to make the rate of 
evaporation greater than from a reservoir, but this was compen¬ 
sated by the obstruction to the full effect of the wind. 
Laying aside the disturbing actions of winds, the rate of 
evaporation depends on the difference between the force of vap@r 
in the air and the maximum force of vapor for the temperature of 
the water surface, or it will be sufficiently accurate to sav that 
other things being equal, the rate of evaporation bears a direct- 
ratio to the difference between the temperature of the water sur¬ 
face and the dew point. When farthest apart, evaporation will be 
most rapid, and when together, it will cease. 
The greatest recorded evaporation for twenty-four hours, was 
from noon, March 31st, to noon, April 1st, and was .4 inches. It 
