28 BURTON EDWARD LIVINGSTON 



at each locality. In other terms, the exposure of the evaporating 

 surface to the remainder of the instrument forms an important and 

 usually not negligible part of the general exposure. Thus, to say 

 that atmospheric evaporating powers are to be comparably meas- 

 ured only by giving the evaporating surfaces similar exposures, 

 implies that the entire nature of every instrument should be as 

 nearly alike as possible; what may be called the internal or 

 instrumental exposure of the active surfaces must be the same if 

 the external conditions are to register comparable effects. The 

 mutual influences of the various conditions within and without 

 the instrument, and the summed influence exerted by all these 

 conditions acting together, are far too complex to admit of cor- 

 rection being applied to bring the readings from different kinds 

 of atmometers into a comparable state. Such corrections may 

 become possible in time, through refined physical studies, but 

 numerous attempts simply to deduce the evaporation rate of 

 one kind of instrument from various measured conditions of the 

 surroundings have not furnished results that are at all encour- 

 aging in this direction. The literature contains many experi- 

 mentally derived formulas for this sort of reduction, but none 

 seem to have obtained a completeness or precision adequate to 

 render them available excepting for the roughest sort of approx- 

 imations. 



Summarizing the points thus far brought out, hazarding a 

 repetition for the sake of emphasis, the evaporating power of 

 the air — that is, its power to remove water vapor from a surface 

 of liquid or solid water — can never be directly measured only 

 with reference to some standard evaporating surface. For the 

 preparation of duplicate evaporating surfaces it is necessary, 

 not only that the areas of these be the same, but that their forms 

 or shapes be also the same. Furthermore, all of the immediate 

 surroundings of these water surfaces excepting the conditions of 

 the air, the effectiveness of which are to be studied — must also be 

 alike. The surroundings include the magnitude and form of the 

 water mass behind the exposed surface, the sort of container which 

 holds the water mass in place, etc. In brief, if the evaporating 

 power of the air is to be measured at different places, or for 



