284 The Plant WorIvD. 



centimeters in diameter; I find the smallest size of cut filters 

 suitable for rather intense evaporation, for lower rates larger 

 papers may be used. Care must be exercised that the evapora- 

 tion rate never exceeds the possible rate of supply by lateral 

 diffusion past the wall of the tube. When such is the case (the 

 same consideration applies to the Piche instrument), the mar- 

 gin of the paper becomes diy and the readings have no value. 

 Such an arrangement operates a day or two in the greenhouse 

 without attention, but is highly unsuited to w^ork in the open. 

 It will be observed that this form of atmometer resembles the 

 porous-cup form, excepting that the cup is replaced by the paper 

 disk. For radio-atmometry with this device, the black cut fil- 

 ters now prepared by Schleicher and Schiill are admirable. In 

 this form, which may be called the Cantoni-Piche atmometer, 

 the spasmodic variation in hydrostatic pressure and the diffi- 

 culty of separating the reservoir and scale fioni the evaporating 

 surface have both been removed. A wire support foi the edge 

 of the soft paper circle obviates the troubles arising from air 

 currents, but this,with the clamp, which is also necessaiy in the 

 open, renders the device cumbersome and difificult of manipu- 

 lation. 



The fourth objection mentioned above cannot leadily be 

 overcome so bng as a disk is used for the evapoiating surface, 

 but is completely obviated b\ the cylindrical poious-cup. It 

 was tbeiefore highly desirable to combine the good 

 pointj of the Cantoni-Piche apparatus with that of a 

 cylindrical evaporating surface. This has at length been ac- 

 complished, through what may be termed the paper cylinder 

 atmometer. 



The evaporating surface is furnished by one of Schleicher 

 and Schiill's extraction thimbles. These are made of pure filter 

 paper, pressed into the form of a test tube, and may be obtained 

 from dealers in chemical supplies. I have used the size 19 x 90 

 mm., S & S Extractions Hulsen No. 603. Originally, the thimble 

 was merely inverted over the end of the filled glass tube and 

 pressed firmly into place by means of the finger, in 

 a manner quite paralell to that of placing the paper disk of the 

 Cantoni-Piche form. This operation flattened the rounded end 



