RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 155 
cipitation. in accordance with the measurement in a standard rain 
gauge nearby. The evaporating pan is rilled 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. 
^ONFKEE WATEE SETJFACE. 
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 Kussell 
(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 (4-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 40 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 
must be quite firm to prevent overflowing 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 
