RESEARCH METHODS IX STUDY OF FOREST ENVIRONMENT. 157 
somewhat according- to atmospheric conditions by regulating the 
hydrostatic pressure. With the outside walls of the cup always 
moist and yet not dripping, the rate of evaporation will of course 
be governed by atmospheric conditions. It must not be expected, 
however, that the evaporation from this instrument may be com- 
pared under a variety of conditions with that from the Piche in- 
strument, or with that from a free-water surface. While the ab- 
sorption of heat from radiant sources and conduction from the air 
will be practically the same for the water surfaces in the three 
cases mentioned, yet the further absorption beyond the first water 
surface will depend on the nature of the substance behind that 
water surface — in the one case, water; in the second, paper and 
glass; in the third, clay or some similar earthy substance. There- 
fore, the three instruments will respond quite differently to the 
stimuli of warm air and sunshine. 
For these reasons, comparative data will be of value only when 
the same instrument is used in all measurements of the comparison. 
Shire's nonabsorbent porous cup atmometer. 
It has been the experience of various investigators that the Liv- 
ingston porous cup atmometer measures the evaporating power of the 
air with a very considerable degree of accuracy during periods 
when the temperature is not recorded at or below zero centrigracle. 
In 1910, Livingston (158) described a rain-correcting atmometer. 
This atmometer, while giving great satisfaction in the hands of 
many inexperienced workers, was difficult to operate in some locali- 
ties. Thus it was found impossible to obtain continuous records in 
the dry climate of the Wasatch Mountains of the Manti National 
Forest in central Utah, principally on account of the connections 
and joints of the equipment, all of which occurred outside of the 
water reservoir. Hail storms and objects carried by strong wind 
were often so severe as to disjoint or break the more delicate equip- 
ment. In connection with this instrument, it is understood that the 
automatic mercury valves which operate to prevent the water ab- 
sorbed by the porous cup in times of rain from entering the reservoir 
are externally situated. This makes it essential to have all valves 
very tightly connected in order to prevent leakage. Much to the 
satisfaction of those who have used this instrument, Shive (165) 
has described one so modified as to be self-contained, and at the 
same time to reduce to the minimum the liability of breakage and 
the difficulty of adjustment. This was accomplished by eliminating 
jointed mercury valves and decreasing the leakability and breakage 
to a nominal degree. The arrangement of the different parts of the 
