24 BURTON EDWARD LIVINGSTON 



above the soil, there can be obtained therefrom no definite in- 

 formation as to the relative intensities of evaporation in the 

 two localities, at the' same level. It may be said only that the 

 evaporating power of the air at one level in one locality bears 

 such and such a relation to the same power at another level in 

 the other locality. The exposure of an atmometer needs to be 

 specified as much as does the nature of the instrument. 



These points apply as well to other forms of atmometer as 

 to the form employing a free water surface. Different sized 

 paper disks on the Piche and Piche-Cantoni forms (with their 

 horizontal, saturated papers giving off water both above and 

 below) are also influenced in their relative rates of water loss by 

 changes in wind, temperature, etc. It is also clear that the alter- 

 ation of evaporation rate from any instrument employing paper 

 may not be at all the same as the corresponding alteration for 

 any free water surface, these alterations being both effected by 

 the same change in external conditions. Thus it is obviously 

 impossible to reduce Piche readings, for example, to terms of 

 depth from a pan of water, though this has often been attempted. 

 A coefficient for this reduction can of course be obtained for any 

 given set of external conditions, but when the conditions alter we 

 must expect the coefficient to alter also. Likewise, evaporation 

 rates from different forms or sizes of porous clay cups are dif- 

 ferently affected by the same alterations in the surroundings, and 

 it is quite impossible to obtain a coefficient by means of which 

 the readings of one form may be reduced to those of another, 

 excepting with specified surroundings. Nor can porous cup losses 

 be reduced to losses from pans or paper disks. 



Reduction of Readings 



The apparent success which has sometimes appeared to at- 

 tend attempts to derive coefficients for the reduction of evapora- 

 tion rates from one form of atmometer to those from another form 3 



3 Russell, T., Depth of evaporation in the United States. Mo. Weather Rev. 

 16:235-239, 1888. 



Livingston, B. E., The relation of desert plants to soil moisture and to evap- 

 oration. Pub. 50 of the Carnegie Institution of Washington. Washington, 1906. 



