EESEAECH METHODS IN STUDY OF FOREST ENVIRONMENT. 159 



higher than in A', the difference in the height of the two columns depending 

 upon the height of the cup above the water level in the reservoir. The mer- 

 cury in B B' is drawn into the bulb C, where water rising from' the reservoir 

 is freely allowed to pass, supplying in the usual way the water lost from the 

 surface of the cup. The mercury columns in the tube A A' and B B' remain 

 in equilibrium in the position indicated 'n the figure, so long as the water 

 loss from the surface of the cup by evaporation equals or exceeds the absorp- 

 tion from without by any part of its surface. In t ; mes of rain, when the water 

 loss from the surface of the cup by evaporation is less than the absorption from 

 without, the automatic mercury valves become reversed. The mercury col- 

 umn falls in A and rises in A' at the same time that the mercury in bulb C 

 drops into the tube below and rises in the tube B' (the height to which the 

 mercury rises in this tube depending upon the height of the cup above the- 

 water level in the reservoir), thus effectually preventing water from entering 

 the reservoir from this direction. The readings obtained give the actual evapo- 

 ration minus the error introduced by the volume change required for the op- 

 eration of the mercury valves, the value of the error thus introduced depending 

 upon the number of complete reversals of the valves. 



Standardiza t ion. 



The Livingston porous-cup atmometers, whether to be used with 

 or without the Shive nonabsorbing apparatus, should be obtained in 

 the standardized form, which insures comparability of the results 

 obtained with different instruments and by different investigators. 

 This standardization of the instruments is one of the strongest fea- 

 tures, making it possible, for the first time, for different investi- 

 gators to speak of evaporation in common terms. Since the stand- 

 ardizing can not be readily accomplished in the field, it is suffi- 

 cient to state that it is very carefully done at the Johns Hopkins 

 laboratory, where each new instrument is compared with one or 

 more tried instruments. Assuming the rate of evaporation for a cer- 

 tain time to be 100 in a standard instrument, the coefficient of an in- 

 strument which in the same period evaporated 150 units, would be 

 0.G7. In short, the coefficient given, for any instrument which has 

 been "standardized," is the amount by which all records of that 

 instrument should be multiplied, to reduce them to a standard basis. 



Computation of field results. 



In computing the Held results to a standard basis, all cups used for 

 ;i lew days or weeks are restandardized. Provided the new evapor- 

 ating coefficient is found to differ from the original, the probable 

 average coefficient during regular cycles or periods is calculated. 

 It has been shown experimentally (hat the change in the evaporation 

 coefficient takes place gradually and uniformly during (he period of 

 operation (159). Thus, assume thai a given atmometer cup having 

 an Initial coefficient of 0.62 eras exposed in a habitat for a period of 

 four weeks during which it whs read three time-: namely, ;i< the 



