52 BURTON EDWARD LIVINGSTON 



should be given the same kind of exposure as have the surfaces . 

 upon which the action of evaporating power is to be studied. 



Aside from this general principle, however, there are various 

 advantages and disadvantages connected with the use of each 

 form of atmometer so far devised. A few considerations in this 

 connection may find place here. 



The Open Pan or Tank 



One of the main difficulties encountered in operating free 

 water surfaces of considerable extent arises from the secondary 

 wind effect. Increased wind velocity increases the evaporation 

 rate, but at the same time increases the extent and alters the 

 form of the free water surface, throwing it into ripples or waves. 

 Thus the effect of rapid air movement cannot ever be studied with 

 free liquid surfaces; the fluctuation of the condition to be studied 

 causes marked alteration of the instrument itself. Furthermore, 

 with high wind there is great danger of mechanical removal of 

 water from the pan, by spray and splashing. 



To prevent splashing it is essential to allow a portion of the 

 lateral walls of the container to project above the water sur- 

 face, and these projecting walls introduce variations in the 

 effects produced by external changes. With a high wind the 

 effect of the lateral protection may be negligible, with but slight 

 air currents this effect is much more marked. In general, it is 

 of course relatively more pronounced for small than large pans. 



Another serious difficulty in tank operation arises from the 

 fact that the depth of the water surface below the margin of the 

 tank and the amount of water beneath the surface can be main- 

 tained only approximately constant, this usually by spasmodic 

 additions. Here evaporation itself alters the nature of the ex- 

 posed surface by changing the height of the projecting rim and 

 the amount of water present. Also, the addition of water in 

 times of rain is always a source of serious trouble, further com- 

 plicated by the fact that raindrops frequently cause splashing 

 and formation of spray. 



Besides the addition of water by rain and its mechanical 

 removal by rain impact and by wind action, there is frequently 



