ATMOMETRY AND THE ATMOMETER 73 



however, hourly readings through the day may show that the 

 value of the coefficient fluctuates from hour to hour, for all hours 

 of the day are not alike in the atmospheric conditions which 

 they present. The conditions presented by one hour may accel- 

 erate water loss from one cup more than that from the other; 

 those occurring in another hour may accelerate loss from the 

 second more than from the first, etc. Similarly, the average 

 coefficient derived from operation on another day may be dif- 

 ferent from that first obtained. But if two days present the 

 same march of hourly conditions, or if they are so related that 

 the average effective hourly conditions for one are in agreement 

 with the corresponding average for the other (although it may be 

 that no hour of one day gives the same coefficient as the same or 

 any other hour of the other day), then the coefficient derived on 

 the two days should be the same. From this sort of consideration 

 it emerges that, the more detailed the work undertaken, and 

 the greater the range of external conditions dealt with, the 

 more attention should be given to having the cups exactly alike. 

 For usual purposes of work in the field, experience points to 

 the conviction that the 6-cm. and 8-cm. cups are not sufficiently 

 diverse to make it practically impossible to reduce the read- 

 ings of the shorter form to terms of loss from the longer, and 

 vice versa, it is clear that these two forms are of the same shape, 

 but the porous portion of one is 2 cm. longer than that of the 

 other. The effect of various climatic complexes in altering the 

 relation between evaporation rates from these two forms of cups 

 is, at any rate, only slight, and has not been definitely brought 

 out in standardizations carried out by night and by day in the 

 summer of southern Arizona and indoors in the Baltimore winter. 

 It is therefore safe to conclude that the corrected readings of 

 the 8-cm. cups are at least practically comparable with the cor- 

 rected readings of the 6-cm. cups formerly widely used. In 

 spite of the theoretical difficulty here encountered, it may be 

 assumed that the earlier results, obtained with 6-cm. cups, are 

 practically homogeneous and comparable with the later ones 

 obtained with the longer form. Thus, all published measure- 

 ments of the evaporating power of the air, in so far as these 



