ATMOMETRY AND THE ATMOMETER 27 



earlier work) to a value much lower than that exhibited in 1909, 

 and suggested the carrying out of a series of experiments bearing 

 upon the effect of temperature upon the value of this coefficient 

 of the standard cup, as compared to the standard pan then em- 

 ployed. These tests clearly showed that, as the temperature 

 rises, the coefficient of the standard cup, referred to the standard 

 pan, decreases, and it proved to be possible, by manipulating 

 the temperature, to give this coefficient almost any desired 

 value, within a broad range. It thus became clear that a rise in 

 temperature produces a relatively greater increase in evaporation 

 from the cup than from the pan. It may be suggested that the 

 higher the temperatire of the air the more pronounced should 

 be the convection currents about the cup, while the correspond- 

 ing alteration in convection currents over the free water surfac'e 

 is probably but slight, owing to the horizontal position of the 

 latter. These comparisons of cups and pans were carried out, as 

 far as possible, without breeze, so that the effect of differences 

 in air convection should have been pronounced. 



Whatever may be the physical explanation of the findings 

 of these experiments, they made it very obvious that it is im- 

 possible to reduce the rate of loss from a porous cup to units 

 of depth from a water surface, and this put an end to cup stand- 

 ardization with the pan as a basis. The coefficients of the 

 cups of 1908 were therefore recalculated, to the older basis of 

 standard cups, and the fact of this return to the original method 

 was stated in a later paper. 4 The reasons for this change have 

 never been presented until now. 



This whole matter of the influence of the atmometer itself 

 upon the evaporation rate per unit of evaporating surface is 

 quite clear from the fundamental principle that the rate of water 

 loss depends upon the conditions of the entire surroundings, not 

 merely upon the evaporating power of the air. The special con- 

 ditions due to the nature of the instrument accompany the in- 

 strument from place to place and affect the evaporation rate by 

 variously modifying the influence of the surrounding conditions 



4 Livingston, B. E., A rotating table for standardizing porous cup atmomcters. 

 Plant World 15: 157-162, 19)2. 



