28 



THE RELATION OF DESERT PLANTS TO 



of the evaporimeter loss to that of 1 sq. cm. of the water surface. In 

 the heading for this column a signifies the area of the dish, 118.82 

 sq. cm. The different items are for the several hour periods, from 

 8 a. m. to 7 p. m. , on July 28. 



TABLE IV. Calibration Data for Evaporimeter. 



The fluctuations in the ratio are probably in large part due to the 

 failure of slight air currents to accelerate evaporation from the dish 

 as much as they hastened that from the porcelain cylinder; at -the begin- 

 ning of the test the upper surface of the water in the dish was about 7 

 mm. from the upper edge of the lateral walls. The average ratio for 

 the whole series of observation is 99.26. Other determinations gave 

 coefficients which closely approximated this one. Therefore, in prder 

 to save computation, it may be assumed that this evaporimeter lost 

 water approximately 100 times as fast as would a centimeter of free 

 water surface in the same position. In other words, the actual loss of 

 the evaporimeter is taken to be equivalent to the loss from a water sur- 

 face of one square decimeter. 



On the instrument above described readings were taken at conven- 

 ient intervals from July 24 to August 16. Unfortunately the instru- 

 ment was not installed until after the beginning of the rainy season, 

 so that a rate of evaporation approaching the maximum for the year 

 was probably not observed. Since reliable records of evaporation are 

 exceedingly rare, and especially on account of the fact that the pres- 

 ent observations were made at a station whose atmospheric phenomena 

 are especially interesting to botanists, a table of the daily increments 

 of water loss for the above-named period is worthy of its space here. 



Table V presents the daily evaporation and also the rainfall from 

 July 25 to August 22. In the first column are given the dates; in the 

 second, the actual losses of the evaporimeter in cubic centimeters; in 

 the third, centimeters of evaporation, being the loss from a single 

 square centimeter of free water surface, derived from the evapori- 

 meter losses by means of the coefficient 100 above derived; and in the 



