RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 151 



Anemometers, wind vanes, and supports — Continued. 



Combined wind vane and anemometer support, 20 feet high, adapted for 

 use with quadruple register; complete with 6-foot vane, electrical 

 contacts, etc., but without anemometer f 105. 50 



Wind vane, 4 feet, on 7-foot support, with direction arms, gilt letters, 



and anemometer support, but without anemometer 35. 00 



"Wind vane and support, as above, but without anemometer or anemom- 

 eter support arm 27. 50 



Support for anemometer alone, without vane, direction arms, or anemom- 

 eter 15. 00 



EVAPORATION. 



Xo ecological study can be considered comprehensive which does 

 not take into account the desiccating power or " evaporation stress" 

 of the atmosphere. While the water supply of the soil has been con- 

 sidered as the condition most directly determining the character of 

 vegetation on a given site, and its rate of growth, almost equal 

 attention must be given to the matter of the dissemination through 

 transpiration of the moisture winch reaches the plant. The evapora- 

 tion rate in different habitats will perhaps be found to show greater 

 variation than any other condition. It is especially valuable when 

 measured directly, because it gives the integrated effect of wind, 

 humidity, air temperature and sunshine — an integration which can 

 not be accomplished by any artificial means. While it is not to be 

 expected that any instrument will integrate the effect of these differ- 

 ent stimuli to evaporation in a manner corresponding to their com- 

 bined effect on the plant, yet this is the object to which the greatest 

 efforts have been bent and which has to some extent been attained. 



The study of the evaporation factor may be made directly, of 

 course, by observing the transpiration of plants in the field. While 

 very desirable and not very difficult, this is perhaps less satisfactory 

 than the instrumental method in reducing the conditions of the en- 

 vironment to physical terms. Since this discussion is mainly con- 

 cerned with instrumental procedure in forest investigations, the 

 instrumental method, even though less desirable than direct measure- 

 ments of transpiration, will be considered first. 



Objects and Nature of Evaporation Measurements. 



There may be two rather distinct objects in measuring evaporation 

 rates, although in ecology there is only one. Climatologists, irriga- 

 tion engineers, etc., may desire to know, for local conditions and for 

 general comparative purposes, how much capacity the atmosphere 

 possesses day by day and year by year to take up moisture when 

 offered moisture, as freely as possible, from the surface of a body 

 of water. Obviously this does not measure the capacity of the 

 atmosphere, which could better be determined by humidity observa- 



