261 



Chamberland-Pasteiir filter candle, Type B, of unglazed porcelain, attached by 

 rubber tubing to a burette, the whole being completely filled with water. This 

 atmometer was also attached to the potometer stand. For absolute measure- 

 ments it must be calibrated in terms of the loss from a free water service. This 

 was done on return to Adelaide by exposing in the same place the atmometer 

 and a Petri dish filled with water. The latter vessel was weighed each time a 

 reading was taken on the atmometer. From these data it was found that the 

 candle surface was equivalent to 100 sq. cms. of free water surface. The 

 evaporating power is measured by the loss of water per sq. cm. of free water 

 surface. 



Fig. 1 gives the curves for the various climatic factors. The quantities of 

 the latter are plotted as ordinates and time in hours along the abscissa. As 

 regards temperature, it will be seen that this did not vary greatly throughout 

 the time of experimentation, but fell gradually from 2 p.m. The evaporating 

 power was fairly high throughout the day, but fell rapidly at night. The 

 evaporating power is the product of several factors — relative humidity, wind, 

 sunlight, and temperature. It will be seen that the evaporation curve does not 

 coincide exactly with the curve for the relative humidity during the day period, 

 though the coincidence is exact during the night. This is due to the high wind 

 which commenced at 9 a.m. and did not fall until 5 p.m. The wind, as before 

 mentioned, has a high velocity, and appears to be the dominant external factor in 

 transpiration. At 3 p.m. it was blowing with very strong gusts, and the curve 

 maximum shows its effects on evaporation. The relative humidity calls for little 

 comment. The high value during the night common in this region should be 

 noted. It is due to the low temperatures prevailing. The minimum night 

 temperature recorded was 38° F. 



fio 



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Fig. 1. 



lO II 1-3. '»»/</rj/,j(/7 



Curves of external factors influencing transpiration. E, Evaporating Power ; 

 T, Temperature ; H, Relative Humidity. 



