Paper Atmometers for Studies in Evaporation. 283 



with the foliage of a plant. (3) The soft paper circle fails to 

 maintain its horizontal position when openly exposed; slight air 

 movement alters the exposition from time to time, and the 

 wind often throws the instrument completely out of operation. 

 The use of a thicker paper involves a larger amount of con- 

 tained water. (4) The plane evaporating surface is unsuited to 

 the purposesand needs of sunshine records with the radio-atmo- 

 meter; the evaporating surface of the latter instrument 

 mufet be placed so as to receive sunshine ever at the same angle 

 throughout the day, and the only way to meet this condition 

 with a disk is to mount it upon a heliostat, which is manifestly 

 impracticable. 



I have obviated the first two difficulties essentially by in- 

 verting the whole instrument, placing the reservoir below and 

 the paper disk above. This is, in piinciple, the modification of 

 the Piche Atmometer described by Cantoni. f The lower end 

 of a burette (see Fig. 2) is attached by rubber tube to a vertical 

 glass tube (about 6mm. in diameter with smooth end) which 

 may be at any convenient distance. A second reservoir for 

 filling the burette (usually a bottle with siphon and pinch-cock) 

 is also attached, either by means of a T-tube or simply to a side 

 opening. The burette is filled with water, the glass tube is 

 lowered till water over-flows at its upper end, and the center of 

 a disk of filter paper is firmlv appressed to the latter. Maintain- 

 ing some pressure with the finger upon the center of the paper, 

 to prevent air entering the tube, the latter is now carefully 

 raised until the paper is some distance (10 to 30 cm.) above the 

 level to be taken as zero on the burette. The finger is then re- 

 moved and the instrument is in operation. A suitable plate and 

 clamp, similar to that used on the Piche tube, renders the paper 

 less liable to be displaced. The water level in the burette should 

 not be allowed to fall more than a few centimeters; the water 

 seal where the paper is appressed will not bear high pressure. 

 A freer movement of water is obtained if one or more 

 little paper circles be placed upon the center of a large circle, 

 over the end of the tube. The evaporating disk may be several 



■{"Cantoni, G. — Sulle Condizioni di Forma e di Esposizione pui Opportune per gli Evapori- 

 metri. Rend. r. inst. Lomb, 12 (Ser. 2): 941-946, 1879. Abstract in Zeitscher. Oest Ges. 

 Met.. 16: 39-40, 1881. 



