RESEARCH METHODS IN STUDY OF FOBEST 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 the 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 100 square centimeters. It rests upon a 
perforated metal disk of the same size, the perforations aggregating 
an area of 5 square centimeters, and designed to simulate the stomal a 
of the under side of a leaf. All vapor formed must escape through 
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