367] B. E. Livingston 169 



ration exceeds precipitation by a certain depth, 15 cm. in this 

 example. But this means nothing at all ; if the pan used had 

 been larger or smaller, of different shape or material, or if a 

 wet soil surface had been employed, etc., the result would have 

 been quite different, and the climatic conditions would surely 

 not have been altered by merely changing the atmometer. On 

 the other hand, if any other form or size of raingage had been 

 employed the re-suits would be sensibly the same. The amount 

 of evaporation depends largely upon the atmometer but the 

 amount of rainfall recorded is practically independent of the 

 raingage, so long as the latter is a raingage at all. It is legiti- 

 mate to state the index of rainfall in depth units, for this is 

 not seriously influenced by the internal characteristics of the 

 gage, a statement that cannot be made of the index of evapo- 

 ration, nor even of the index of atmospheric evaporating pow- 

 er. The only logical way by which atmometric and precipita- 

 tion measurements may be compared is by means of their 

 ratio, in which case one set of measurements may be in depth 

 units and the other in volume or weight units. They are not 

 commensurable in any case, so it is best not to have them even 

 appear as though they were commensurable. Other considera- 

 tions, into which I cannot go in this place,, lead unequivocally 

 to the same conclusion. 



Fortunately, there is no serious difficulty encountered in 

 the statement of the time feature of atmometric measure- 

 ments. For short periods the hour is most convenient, for 

 longer periods the day, week and year are all suitable. Since 

 months vary in length, monthly atmometric indices are un- 

 satisfactory. After the three features of the unit to be used 

 have been decided upon, it is necessary to remember that at- 

 mometric measurements, like other power measurements, al- 

 ways apply to a certain set of circumstances and to a certain 

 time period. The set of circumstances here emphasized is the 

 surroundings of the atmometer, they comprise the various 

 features of its exposure. The readings refer -to the evaporat- 

 ing power of the air only for the particular location in which 

 the instrument was operated. The evaporating power of the 



