RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 159 



A 7 , the difference in the height of the two columns depending upon the height of 

 the cup above the water level in the reservoir. The mercury in B W is drawn into 

 the bulb 0, 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 in the 

 figure, so long as the water loss from the surface of the cup by evaporation equals or 

 exceeds the absorption from without by any part of its surface. In times of rain, 

 when the water loss from the surface of the cup by evaporation is less than the absorp- 

 tion from without, the automatic mercury valves become reversed. The mercury 

 column 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 reser- 

 voir), thus effectually preventing water from entering the reservoir from this direction. 

 The readings obtained give the actual evaporation minus the error introduced by 

 the volume change required for the operation of the mercury valves, the value of the 

 error thus introduced depending upon the number of complete reversals of the valves. 



Standardization . 



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 

 features, 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 sufficient 

 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 

 0.67. 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 field results to a standard basis, all cups used for 

 a few days or weeks are restandardized. Provided the new evapo- 

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

 average coefficient during regular cycles or periods is calculated. 

 It has been shown experimentally that the change in the evaporation 

 coefficient takes place gradually and uniformly during the period of 

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

 an initial coefficient of 0.62 was exposed in a habitat for a period of 

 four weeks during which it was read three times; namely, at the 



