1 8 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 converseh'. 



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 tempei^ature 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 

 with 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 ijfiinfall. 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 drawing isothymes. By a summation of the evapo- 

 ration over the areas between the isothymes, 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 work upon evaporation from lakes and reservoirs. 



In physiological 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. 



