26 



THE RELATION OF DESERT PLANTS TO 



For most of the work at the Desert Laboratory the fixed form of 

 evaporimeter was used. The porcelain cylinder was supported verti- 

 cally, with the stopper uppermost, on a wooden 

 arm reaching out from a window-casing on the 

 north side of the building. The center of the 

 cylinder was 50 cm. from the stone wall of 

 the Laboratory and 2 meters above the ground. 

 It was well under the projecting eaves of the 

 building and was thus protected to a great 

 extent from rain. The sun shone upon it for 

 a few hours in the early morning and again 

 in the late afternoon. The connecting tube, 

 partly of glass and partly of rubber, passed 

 into the building through a hole bored in the 

 window-casing. A burette of 100 cc. capacity 

 was used and stood on a support inside the 

 window, at such height that its upper end was 

 several centimeters below the base of the por- 

 celain cylinder outside. The window was kept 

 closed, excepting when momentarily opened to 

 obtain data for other experiments carried on 

 in the same place, which will be described 

 farther on. An air thermometer, graduated 

 in degrees Fahrenheit, was placed outside the 

 building near the evaporimeter and readings 

 upon it were taken whenever the burette was 



read. 



For absolute measuremements of evapora- 

 tion it is necessary to calibrate each evapori- 

 meter by exposing for some time and in the 

 same place an open vessel of water with a 

 known area of exposed surface, and weighing 

 this vessel whenever a reading is taken on the 

 evaporimeter. From data thus obtained a co- 

 efficient is easily derived by which to multiply 

 any increment of loss from the evaporimeter 

 in order to obtain the rate of evaporation for 

 the same period from any assumed standard 

 area of free water surface. After such cali- 

 bration has been accomplished, the evapori- 

 meter may be operated indefinitely, care being, 

 of course, taken never to allow air to enter the 

 cylinder. In this work the precaution was taken to wipe off the porce- 



FIG. 4. Evaporimeter, consist- 

 ing of porous clay cylinder, 

 burette, and water reservoir, 

 the latter in the form of a 

 separatory funnel. 



