SOIL MOISTURE AND TO EVAPORATION. 27 



lain evaporating surface from time to time with a moist cloth, to 

 remove dust which was observed to accumulate thereon, especially dur- 

 ing the dust storms so frequent in the desert.* 



It was thought at the outset that the length of the water column to 

 be lifted by evaporation might influence the rate, so that an error 

 would be introduced by the gradual increase in the height of this column 

 as water was removed from the burette, but this was found by actual 

 tests not to be true. Apparently the tensile strength of the capillary 

 films in the porcelain is so great that their curvature is not appreciably 

 altered by changes of a meter or less in the height of the water column. 

 It was found, however, that if the top of the water column in the 

 burette was above the evaporating cylinder, water was slowly forced 

 out of the latter and appeared as dew upon its surface. Therefore the 

 cylinder was placed, as stated, well above the level of the top of the 

 burette. Had the height of water column appeared to exert any 

 influence upon the rate of evaporation the burette might have been 

 refilled after each reading, but thorough preliminary tests showed this 

 to be unnecessary. 



The calibration figures for the fixed evaporimeter above described 

 will now be given. For measuring the loss from a free water surface, 

 a cylindrical glass crystallizing dish of 118.82 sq. cm. cross section 

 and about 5 cm. high, filled with distilled water, was used. This was 

 suspended by means of wires from a wooden arm similar to the one sup- 

 porting the evaporimeter cylinder, projecting from the other side of 

 the same window out of which the evaporimeter was placed. The dish 

 was so arranged that its upper surface was at the same height from 

 the ground as the center of the porcelain tube, and also the same dis- 

 tance from the Laboratory wall, thus occupying a position corresponding 

 to that of the evaporimeter cylinder, but on the opposite side of the 

 window, about a meter distant. At hourly intervals this dish was 

 weighed and returned to its position, a reading of the evaporimeter 

 burette being taken at the same time. The first column of Table IV 

 presents the hourly losses from the burette, for the period from 8 a.m. 

 to 7 p.m., July 28. The second column presents the corresponding 

 losses from the crystallizing dish, while the third column gives the ratio 



*In the spring of the present year the author was able to test the feasibility of 

 obtaining automatic records on such an evaporimeter as the one above described by 

 means of the Ganong (1905) transpirimeter, manufactured by the Bausch & Lomb 

 Optical Company. A perfectly satisfactory record was obtained of the varying inter- 

 vals at which a gram of water was lost during several days. The instrument is well 

 adapted to this work, but could be greatly improved by being so arranged as to oper- 

 ate without attention for a week at a time. 



