146 BULLETIN" 105&, U. S. DEPARTMENT OF AGRICULTURE. 



If only one psychrometer reading a day is feasible, both the wet 

 and the dry bulb reading may be entered in the first columns of the 

 " Humidity, Wind, and Evaporation " form, and the relative humid- 

 ity, vapor pressure, and saturation deficit calculated therefrom may 

 be set opposite. When several readings are made each day, it is 

 suggested that the calculated vapor pressures be recorded on the 

 form for "Hourly (Air, Soil, or Actinograph) Temperatures" for 

 their appropriate hours, and that only the mean vapor pressure be 

 recorded on the " Humidity, Wind, and Evaporation " form. 



The relative humidity, vapor pressure, and saturation deficit 

 should be averaged by decades and months. The means by months, 

 the year, and the growing season, should be shown on the annual 

 " Summary " form. 



Instruments. 



Psychrometer, sling, standard Weather Bureau pattern; aluminum 

 backs ; polished hardwood handles ; double-length connections ; 2 



• glass tubes, exposed, mercurial thermometers, 9 inches long; 

 stem-graduated and figure on glass for each 10 degrees ; Fahren- 

 heit or centigrade $3. 00-$6. 00 



Whirling apparatus, stationary, complete (without thermometers)- 18.50 



Cog psychrometer. Thermometers about 4J inches long, reading 



—5° to 50° C, No. 1230 , 4.50 



Hygrograph (or self -registering hygrometer) complete with a 



year's supply blank forms. No. 58-B, pen and ink — 80. 00 



WIND MOVEMENT. 



Wind movement may be effective upon plants both directly and 

 indirectly: that is, through mechanical breakage, windfall, etc., and 

 through its influence upon evaporation and transpiration. 



While mechanical injury to trees by wind seems to be a less im- 

 portant factor in American forests than in those of Europe, judged 

 by the literature on the subject, the problem of windfalls is one of 

 ever-growing importance as forestry is extended and thought is given 

 to the conservation of that portion of the stand which is not now 

 merchantable or is needed as a guarantee of future reproduction. A 

 recent article by Weidman (150) and several other articles that 

 might be cited have shown the importance of the problem and the 

 desirability of a great many more wind records than are now avail- 

 able for our forest regions, if the problem is to be scientifically solved. 

 Perhaps this is a far cry from ecology. Yet a disturbance in the 

 forest which is capable of starting a new succession is certainly of 

 some ecological significance, at least after it has occurred. 



Wind movement has without doubt a very marked effect on evapo- 

 ration; and. in addition, the moving air may be either a source of 

 heat or a means of dissipating the heat of sunlight, as suggested by 



