RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 145 
are the " Psychrometric Tables " of the United States Weather 
Bureau, contained in its Bulletin 235. These have been worked out 
for barometric pressures of 30, 29. 27. and 23 inches. In accordance 
with American custom, vapor pressures are given in inches of 
mercury. Through the courtesy of the Weather Bureau, it is possi- 
ble to produce in the Appendix an additional Table of vapor pres- 
sures for a mean barometer of 21.42 inches, prepared by B. C. Kaclel 
for the special use of the Wagon Wheel Gap Experiment Station, 
at an elevation of 9,300 feet. This table will doubtless be of con- 
siderable assistance in ecological studies in the western mountains. 
The vapor pressure may also be determined very quickly and 
precisely by means of dew-point apparatus and a table of satura- 
tion pressures corresponding to various temperatures. This appa- 
ratus is, however, far less convenient for field use than the 
psychrometer. 
The ideal record of humidity is. of course, one which shows the 
atmposheric condition for even 7 hour of the day. Theoretically, this 
is obtained by the use of the hair hygrograph: but, actually, the 
instrument is of very little use. 
The atmospheric conditions are measured in terms of relative 
humidity, which fluctuates rapidly with every change in air tem- 
perature. The record must then be transposed, in conjunction with 
the continuous temperature record, into terms of absolute humidity 
and saturation deficit, before it can have much value. Furthermore, 
the hygrograph is probably the least reliable and accurate of the 
automatic instruments commonly used. 
Since the absolute humidity or vapor pressure usually does not 
change through a wide range in a short time, but shows a general 
tendency to increase as the air warms and to decrease with the 
cooling at night, it is possible to determine a fairly satisfactory mean 
humidity for any day (except of course during general disturbances) 
by means of two or three observations with the psychrometer. For 
example, the hours of 7 a. m.. 1 and 7 p. m., have been used, or 7 a. m., 
2 and 9 p. m. After hourly observations for a few days at any 
season and point, it should be possible to select one or more con- 
venient hours when, in the ordinary sequence of events, the mean 
humidity of the day may be approximately measured, either at each 
observation, or through averaging unlike valuations. As has been 
suggested, the absolute humidity varies less than the relative humid- 
ity or saturation deficit. Therefore, for calculating the mean sat- 
uration deficit for the day it is logical to arrive first at the mean 
vapor pressure, and then, after calculating the mean temperature 
for the whole period, to obtain the saturation deficit by deduction. 
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