418 
The Ohio Naturalist. 
[Vol. IX, No. 3, 
The amount of evaporation from a water surface is obtained 
by two methods: 
(1) By direct measurements from properly exposed water 
surfaces. 
(2) By computation based upon the temperature of the water 
surface and the value of certain meteorological elements. 
The record of evaporation from exposed water surfaces is 
being very carefully made at the Columbus Reservoir and at the 
Cincinnati Water Works Reservoir in Ohio, and in other places 
thruout the country. 
A table is attached giving the actual evaporation in inches 
from floating tanks in reservoirs at Boston, Mass., Rochester, 
N. Y., Menasha, Wis., Grand River, Wis., Iowa City, la., Mad¬ 
ison, Wis., and Columbus, Ohio, in 1906. 
Evaporation From A Floating Tank in Inches, Year 1906. 
Stations 
May 
June 
July 
Aug. 
Sept. 
Oct. 
Chestnut Hill Reser- 
voir, Boston, Mass.... 
3.82 
5.34 
6.21 
5.97 
4.86 
3.47 
Mount Hope Reservoir, 
Rochester, N. Y. 
3.78 
5.05 
5.47 
4.96 
4.07 
2.92 
Menasha, Wis. 
1.85 
2.83 
3.63 
3.70 
2.20 
1.37 
Grand River, Wis. 
3.19 
2.74 
4.17 
3.65 
3.24 
1.77 
Madison, Wis. 
2.52 
2 04 
1 85 
Iowa City, la. 
4.93 
3.76 
2 21 
Storage Reservoir at 
Columbus, O. 
5.44 
5.42 
5.59 
6.36 
In the second method of determining the evaporation over 
large reservoirs and lakes the principles of the Dalton formula 
have been adopted. This referred to the metric system is 
E = C(e w —e d ) + C(e w —e d ) A w. 
In this E = the amount of evaporation in the unit of time, 
C = a constant of evaporation, 
e w = the vapor pressure of the water temperature, 
e d = the vapor pressure of the dew point temperature, 
A = a constant for the wind effect, 
w = the wind velocity in kilometers per hour. 
The constants will be changed for different units of time. 
In some investigations made at the Chestnut Hill Reservoir, 
near Boston in 1876 to 1887, Fitzgerald developed the following 
formulas to determine the evaporation from easily made 
observations. 
