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The Ohio Journal of Science [Vol. XX, No. 3, 



This is done automatically by the shot dropping tubes and 

 chronograph designed by Transeau. A plant or atmometer is 

 placed on a pan of the balance, which is prevented from 

 vibrating in wind or by a slight jar with a mercury cup and 

 piston attached to one beam, and balanced by an equal weight 

 placed on the opposite pan. When the loss of weight causes the 

 pan to rise to a certain height an electric circuit is closed which 

 operates the shot dropper and the pen on the chronograph. 

 The pen makes a straight line- on the drum, which is revolved 

 by a clock at a known rate, except when the electric circuit is 

 closed. From these results (the number of shot and the time 

 interval between each) the rate of water loss in grams per hour 

 is calculated. This apparatus, which can be used to take six 

 or more records at one time, affords a very satisfactory method 

 of determining comparative transpiration rates. 



In obtaining the evaporation data, standardized white 

 spherical cups were used. The evaporation from blackened 

 cups was also measured by using spherical cups, covered with 

 washed carbon. The white cups, because of the method of 

 using them, were standardized with a new cup mounted on a 

 standard non-absorbing mounting, and the black cups were 

 standardized with the two white cups. 



TABLE I. 



Standardization of White Cups Without Mercury Valves With a New Cup 

 Mounted on St.andard Mounting With Mercury Valves. 



Cup 16-201— .96 (New) taken as standard. 



TABLE n. 



Standardization of Black Carbon Covered Cups With Restandardized 



White Cups. 



