363] B. E. Livingston 165 



spiration but to measure the atmometric index of the air in 

 terms of its effect upon a standard instrument whose internal 

 conditions do not change. The internal conditions of each 

 plant or group of plants must be studied in relation to the un- 

 changing ones of the instrument. A given temperature change 

 does not affect all objects or processes alike, yet we do not con- 

 struct a new thermometer scale for each object or process with 

 which we deal. It may be well to mention in this connection 

 that atmometry should furnish climatological data applicable 

 to many fields of endeavor ; the animal ecologist requires these 

 data as much as does the plant ecologist, and irrigation en- 

 gineers and students of atmospheric hygiene and ventilation 

 all have use for atmometric measurements. 



In choosing the instrument to be used the first condition 

 to be met is that its internal conditions or characteristics 

 should not alter; they should be uninfluenced by changes 

 in the surroundings, for it is changes in the latter that we wish 

 to measure. This requirement immediately excludes all forms 

 of free water surfaces, since they alter with wind, etc. Never- 

 theless, since an open pan of water is the from of atmometer 

 employed by the U. S. Weather Bureau, since this is the sim- 

 plest form of instrument that is useful in any way, and since 

 data obtained with this pan will surely prove of much greater 

 value than no atmometric data at all, the pan of water must be 

 accepted as the crudest and most imperfect form of atmome- 

 ter. It should be added that if pans of water are used they 

 should generally be of the same form, size, etc., as the stand- 

 ard recently adopted by the U. S. Weather Bureau. If this be 

 adhered to, all pan measurements will be comparable among 

 themselves and with the Weather Bureau data, as far as this is 

 possible with that general class of instruments. 



The second requirement for an evaporating surface is that 

 it should be as sensitive to all the effective conditions of the 

 surroundings as is possible, without any alteration in its in- 

 ternal characteristics. It should therefore be a surface that 

 is freely exposed to wind action. A nearly ideal surface would 



