114 The Plant World. 



c^ -c 



for the third week by: E-i?(c+3 ), etc., where n is the total 



n 

 number of weeks and the numerical coefficient of the fraction 

 (3, above) is the serial number of the particular week considered. 



Standardization. — The first method of standardization used 

 was to allow the cups to be tested to operate near a standard 

 cup, reading all daily for several days, and to calculate the 

 average coefficient of correction for each cup as referred to the 

 standard. This method is objectionable for two reasons. First, 

 the standard cup changes with use and this change can not be 

 directly detected nor determined. Secondly, where several 

 observers wish to standardize to the same standard, their 

 standards must previously have been compared. This neces- 

 sitates a central station for standardizations. 



To obviate these difficulties a method has been devised by 

 which every cup is standardized to a free water surface. Since 

 the rate of evaporation from such a surface varies, not only with 

 its area, but also with the nature of the vessel and with the vol- 

 ume of water contained, it is necessary to fix upon some arbitrary 

 standard. The apparatus used during 1909 is constructed as 

 follows: The basal portion of a Petri dish, 1.5 cm. high and 

 9.25 cm. in diameter inside,is placed on the rectangular base of a 

 small iron stand of the usual form, a piece of asbestos board, 10 

 cm. square and 5 mm. thick, being inserted between dish and 

 metal. Above this, its neck resting in the dish, is clamped an 

 inverted cylindrical mixing bottle (250 cc), previously filled with 

 distilled water. The bottle is then raised till the water stands 

 just 3 millimeters deep in the dish. It will remain at approxi- 

 mately this depth till the bottle is emptied. The outside 

 diameter of the bottle neck where it meets the water surface is 

 3 cm. The total area of the dish being 67.20 sq. cm. and that of 

 the bottle neck 7.10 sq. cm., the area of water actually exposed 

 is 60.10 sq. cm. The bottle needs filling every few days, this 

 being done by removing from stand. Care must be had that the 

 bottle is properly replaced. Evaporation takes place from the 

 exposed surface and is measured by weighing the entire apparatus 

 at intervals of 24 hours. A "Harvard trip" balance has been 

 used for this, weighings being made to 0.2 gram. Four such 



