DEPARTMENT OF BOTANICAL RESEARCH. 85 



very frequent adjustment. Though the radio-atmometer has been 

 operated with cyhndrical cups, it was early recognized that the instru- 

 ment could never be fully satisfactory for general field-work without 

 spherical cups, both white and black. With these spheres the instru- 

 ment should at once become of great value to climatologists as well 

 as to biologists. 



Atmometric Units, by B. E. Livingston and H. C. Sampson. 



Another feature of atmometry requiring attention as the subject has 

 advanced is the relation of various units by which evaporation may be 

 measured. Although it seems to have been clearly and repeatedly 

 shown in the literature that the rate of water-loss from any evaporating 

 surface is as much a function of the nature of the atmometer as it is 

 of the surrounding conditions, there is, nevertheless, a pronounced 

 misapprehension on the part of many workers in this field, who appear 

 to regard depth units of water-loss from "a, free water-surface" as 

 somehow the standard units in terms of which all other measurements 

 should be expressed. From elementary physics it can be readily 

 established that the ratio of water-loss from one atmometer to that 

 from any other in the same surroundings can be constant throughout 

 varying external conditions (such as those of wind, temperature, etc.) 

 only when the two water-surfaces are exactly similar in all respects. 

 If the surfaces differ but slightly, or if the external conditions (affecting 

 both instruments alike) vary within only a narrow range, constancy of 

 the ratio is of course more or less closelj^ approximated. It follows that 

 evaporation readings obtained by means of the porous-cup atmometer 

 can never be expressed in terms of water-loss from a free surface, 

 excepting for a certain, special evaporation vessel under a certain, definite 

 set of surrounding conditions. 



Mr. Homer C. Sampson, assistant in botany in the School of Educa- 

 tion of the University of Chicago, has carried out, during the summer 

 of 1914, a considerable series of evaporation measurements, under 

 widely varying external conditions and with a variety of different sizes 

 of evaporator pans and of porous clay cups, obtaining illustrative data 

 bearing upon this important and little appreciated principle. In order 

 that evaporation records may be comparable, they must all be obtained 

 by means of similar instruments, but what particular form of instru- 

 ment should be used must be determined mainly by convenience and 

 by the kind of exposure of the natural evaporating surfaces with which 

 the study deals. Thus, the porous-cup atmometer is better suited to 

 plant-transpiration studies than is an instrument employing an open 

 pan of water, because the evaporating surface of the cup is more like 

 plant surf aces than is a free water surface, and also because the exposure 

 of the cup is more like that of plants. 



