CLIMATE AND PLANT GROWTH. 



17 



wind movement was obtained at the two upper stations. The read- 

 ings of all instruments were recorded daily at 8.30 a. m. and 4.30 p. m. 

 Because of the fact that certain important weather factors may be 

 measured by various instruments, it is possible to get a number of 

 ^different sets of values for the climatic factor, depending on the kind 

 of instrument used. Where it is desired to compare physiological 

 activities of plants with weather factors for short intervals, such as 

 a few days, a single day, or a fractional part thereof, the kind of 



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AUGUST 1916 



White sphere 



Free water surface 



Average daily re/af/ve air humidity 



Fig. 5. — Evaporation from white sphere and from free water surface compared with 

 average daily relative air humidity. 



instrument used is often a matter of important consideration. The 

 bearing which the choice of instruments has on the results obtained 

 from the measurement of two of the climatic factors, evaporation 

 and sunshine, is described in detail on pp. 18-24. 



AIR TEMPERATURE. 



The temperature was measured automatically by carefully adjusted 

 thermographs calibrated with standardized maximum and minimum 



56866°— 18— Bull. 700 2 



