RESEARCH METHODS IN" STUDY OF FOREST ENVIRONMENT. 155 



cipitation, in accordance with the measurement in a standard rain 

 gauge nearby. The evaporating pan is filled to a depth of 8 inches 

 at the outset, and refilled to this amount whenever the water has 

 receded an inch. The water is occasionally freshened by a complete 

 change. 



NONFREE WATER SURFACE. 



There are three instruments which have been sufficiently used in 

 this country in recent years to warrant discussion. Each of these 

 three exemplifies a different technical idea. 



Piche evaporimeter. 



The Piche evaporimeter, as modified by the Weather Bureau, was 

 used considerably 10 years ago and has been described by Russell 

 (161). It consists of a graduated glass tube as a reservoir for the 

 water and a filter paper held over the open end of this tube by 

 means of a horizontal glass plate, a spring, and a pressure screw. It 

 is commonly equipped with a 10-centimeter (1-inch) glass plate and 

 a 9-centimeter filter paper under ordinary conditions, or a 5^-centi- 

 meter paper of the same make when evaporation is likely, between 

 observations, to exceed the capacity of the tube, about 10 cubic centi- 

 meters. The larger paper exposes 60.91 square centimeters and the 

 smaller 21.06 centimeters. Therefore, quantities evaporated from the 

 smaller papers should be multiplied by 2.891 to make them approxi- 

 mately comparable with the others. 



Distilled water should be used in evaporimeters, both because of 

 the effect of soluble 'substances and to keep the instruments clean and 

 free acting. A nonfreezing solution of 25 per cent denatured alcohol 

 and 75 per cent distilled water has sometimes been used in cold 

 weather; but the value of records obtained under such conditions is 

 questionable, because at times the evaporation is almost wholly from 

 the alcohol, and the ratio between alcohol and water or ice would, 

 of course, depend very largely on the temperature. For this reason 

 the instrument can not properly be considered for freezing weather. 



The regulation of pressure on the glass plate is a somewhat com- 

 plicating and bothersome factor. In dry weather the pressure must 

 be made light to feed the paper sufficiently, and in damp weather it 

 mu8( be quite firm to prevent o\ erflowing on to the glass, if not actual 

 dripping. 



Evaporimeters of this kind may best be suspended on wires, hav- 

 ing hooks at. their lower ends, so that the, instruments may be readily 

 taken down for filling. In filling, a long 50-cubic centimeter pipette 

 is found most convenient, making it possible to keep the outside of the 



