RESEARCH METHODS IX STUDY OF FOEEST ENVIRONMENT. 161 



difficulty is the sensitiveness of the instrument to frost ; the slightest 

 freezing of the water in the connecting tubes drives the mercury out 

 of the valves and generally bursts the tubes, which are the most 

 expensive part of the instrument. Even when the air temperature 

 is not below 32° F., but the wet-bulb temperature is below freezing, 

 ice accumulates on the cup in large quantities. In short, the instru- 

 ment is worthless when the temperatures approach freezing. 



Other disadvantages are the necessity for frequent restandard- 

 izing, and the occasional failure of the cup to draw moisture from 

 the resorvoir as fast as evaporated. This permits the entry of air, 

 and reduces the evaporating surface, entirely vitiating the results 

 until the cup is refilled. 



Forest Service evaporimeter. 



To fill the need, in forest studies, for a substantial, essentially in- 

 destructible evaporimeter, operating as well in winter as in summer 

 (since it can not be conceded that biological activity ceases with the 

 first occurrence of frost), Bates (151) has designed a metallic, "inner 

 cell : ' evaporimeter. In addition to these desired practical qualities, 

 it was conceived that an evaporimeter might more closely resemble 

 the leaf, and thereby might show a closer correlation with plant 

 transpiration, by having the evaporating body somewhat protected 

 from air currents. Evaporation takes place in the leaf, not largely 

 on its surface ; and, while transpiration is accelerated by wind move- 

 ment, the latter can not have the direct action upon the water in the 

 leaf and its formation into vapor that it has upon a fully exposed 

 surface. 



The correctness of this principle has been demonstrated by com- 

 parative tests of evaporation and transpiration under a variety of 

 conditions. The evaporimeter may be briefly described as follows : 



The tank or reservoir has a capacity of about 450 cubic centimeters, 

 sufficient for a week's operation under extreme conditions. It is 

 seamless and is not ordinarily injured by freezing. It is protected 

 by an outer shell of polished metal, which insulates it both against 

 temperature changes and against direct radiation. 



Out of tin- tank rises a stem a few inches long and one-half inch 

 in diameter, carrying the feed-wick, which is a piece of linen rolled 

 into cylindrical form with the threads " drawn " at one end. At the 

 top of the stem this wick is flattened out to make a contact with the 

 evaporating wick. The evaporating wick is a flat, circular piece of 

 linen having an area of LOO square centimeters. It rests upon a 

 perforated metal dish of the same size, the perforations aggregating 

 : ,n area of 5 square centimeters, and designed to simulate the stomata 

 of the under side of a leaf. All vapor formed must escape through 

 m 22 11 



