ATMOMETRY AND THE ATMOMETER 25 



has doubtless been largely due to the fact that the general period 

 of observation employed involved such slight alterations in the 

 effective surrounding conditions that the crude methods of meas- 

 urement failed to detect the variations emphasized above. It is 

 of course possible, also, that certain different changes of the ex- 

 ternal conditions might be found, which might happen to affect 

 the rates of water loss from different forms of atmometer in the 

 same way and to the same extent, but it is highly improbable 

 that such changes are frequently to be encountered in the diurnal 

 or annual march of atmospheric conditions anywhere in the world. 

 Where only slightly refined measurements are made, and where 

 the change in external conditions or the difference between the 

 instruments lies within certain limits of nature and magnitude, 

 it may frequently occur that the rates of water loss from one form 

 of instrument may apparently agree in their fluctuations with 

 those from another form. A closer study usually shows such 

 agreement to be only apparent. 



The writer's own experience in this connection is of illustra- 

 tive value here. When the porous cup was first employed in 

 the study of plant transpiration (summer of 1904, at the Desert 

 Laboratory; see Pub. 50 of the Carnegie Institution, already 

 cited), the earlier literature of evaporation was not available 

 and the now obvious mistake was made of supposing that the 

 readings from the cup might be reduced to depth units as lost 

 from a free water surface, without regard to alterations in ex- 

 ternal conditions. The data presented on page 28 of the publica- 

 tion just mentioned show clearly, however, just the sort of fluc- 

 tuation in reduction coefficient, for different hours, that would 

 be expected from the discussion just presented. Below the table 

 of calibration data occurs the statement, "The fluctuations in the 

 ratio are probably in large part due to the failure of slight air 

 currents to accelerate evaporation from the dish as much as they 



Operation of the porous cup atmometer. Plant World. 13 : 111-118, 1910. 



Many other instances of similar sorts are cited in: Livingston, Grace J., An 

 anotated bibliography of evaporation. Mo. Weather Rev. 36: 181-186, 301-306, 

 375-381, 190S; 37:68-72, 103-109, 157-160, 193-199, 248-252, 1909. This was re- 

 printed, repaged, in pamphlet form, Washington, 1909. Reference may be 

 made to this bibliography for papers on evaporation antedating 1908. 



