i8 
The Ohio Naturalist. 
[Vol. X, No. 2, 
not an easy task to find a uniform and constant relationship for 
each one of the primary factors. It must be remembered, there- 
fore, that the following statements from a summary of the sub- 
ject in the “Monthly Weather Review’’ of 1907 and 1908, (8) 
hold true only when all other things are considered equal. 
If the rainfall is uniformly distributed throughout the year, 
the evaporation will increase proportionately. 
A heavy winter, and a light summer rainfall will, together 
show a small annual evaporation, and conversely. 
Evaporation varies nearly inversely as the atmospheric 
pressure, or nearly directly as the altitude. 
The rate of evaporation is nearly proportionate to the differ- 
ence of temperature as indicated by the wet, and dry bulb 
thermometers. 
As to temperature, it is found that the capacity of atmos- 
pheric air for moisture is approximately doubled with everv 
increase in atmospheric temperature of 20° F. 
Wind velocity has a marked influence upon evaporation 
being nearly in a direct ratio with it. 
In the light of these observations, meteorologists have 
attempted to find the relationship existing between the various 
modifying factors and evaporation, and have succeeded in 
working out formulas by means of which the evaporation from 
watersheds and water surfaces can be approximated. 
It should be stated that there are many obstacles to contend 
wfith in devising proper methods for measuring evaporation. It 
is almost impossible, in field work, to place the instrument under 
normal standard conditions, and also to eliminate the error 
caused by rainfall. It has been pointed out, moreover, that the 
evaporation from a large water surface cannot be calculated 
correctly from the evaporation of a small tank for the reason that 
air, moving over a water surface, absorbs moisture, and its 
capacity to hold water becomes gradually less. 
This difficulty may be partially overcome by measuring the 
evaporation at numerous points on the water surface, plotting the 
results and drawdng isothymes. By a summation of the evapo- 
ration over the areas between the isothvmes, the evaporation 
of the whole area can be calculated with comparative accuracy. 
New and more improved instruments have been devised, and 
are now being employed by the Weather Bureau (10) in connec- 
tion with the w f ork upon evaporation from lakes and reservoirs. 
In phvsiological work, it has been considered preferable to 
obtain the evaporation readings directly by such instruments as 
are available, rather than to depend upon formulas, which are 
necessarily somewhat inaccurate, The German Forest Service 
(9) has used a small zinc receptacle with a wooden roof, which 
allows the free access of air, but excludes rainfall. Within 
recent years the porous cup atmometer, which will be described 
later, has come into use. 
