420 
The Ohio Naturalist. 
[Vol. IX, No. 3, 
At Reno this vapor blanket seemed to have a depth of 40 feet 
over the city reservoir, but it will vary with the size of the sheet 
of water and the climate in which it is located. He states that 
in dr}' climates it will overspread the water laterally from 300 
feet to one-fourth mile, according to the size of the sheet of 
water. In a moist climate it will be deeper and more extensive. 
He has determined the value of the wind velocity constant 
and has developed the value of the water vapor in different parts 
of the reservoir. He calls this vapor value the “diffusion coeffi¬ 
cient’’ and it, in connection with the height above the water and 
distance from the edge of the reservoir, suggests a logarithmic or 
geometrical law for the diffusion. 
In the arid regions of the West it seems probable that this 
vapor blanket conserves about three-eighths of the water that 
would otherwise be lost by evaporation. He states that this rule 
may not hold true in other climates and that other observations 
should be made elsewhere. 
m He has determined that if the water evaporated between 
7:30 a. m. and 10:30 a. m. at Reno during the summer time be 
multiplied by 8 it will closely represent the evaporation for the 
24 hours of the day. 
Professor Bigelow has suggested the following formula for 
trial instead of those based on Dalton’s law, because it has 
worked well in the Reno investigations. 
A full discussion of the Reno observations is made in the 
Monthly Weather Review for February, 1908. 
Bigelow’s Formula for Evaporation per Hour. 
E = Cf(h)e d ^|(l + Aw) 
Cf (h) is a variable function of the evaporation, changing 
with the height above the water surface and the distance from 
the center in a horizontal direction. It includes the diffusion 
and mixing process. It has been worked out at Reno in centi¬ 
meters, and the values will be given upon application to the 
Washington office of the Bureau. 
— is the rate of change of the vapor pressure with the change 
dS 
of the temperature of the water at the surface. It represents 
the Clayperon formula for the volume of vapor derived from the 
unit volume of water at the temperature S. It can be found 
from Table 43, Smithsonian Meteorological Tables, 1907. A 
table of these values from 0 to 29° C. has been worked out and 
can be obtained in a pamphlet of instructions for evaporation 
observations issued by the Bureau. 
e d is the vapor pressure at the dew point temperature of the 
air. A is the wind effect constant, 0.0175. w is the wind veloc¬ 
ity in kilometers per hour as read from the metric anemometer. 
