8 



superfluous. The writer is aware that the instrument does not register 

 with complete accuracy the evaporative power of the air when there 

 has been precipitation during the period for which the measurement 

 is made. During a rain, indeed, there may be a flow of water into 

 the bottle. Drizzling rain for a long period will cause more water 

 to enter the cup than will the same aggregate precipitation if due to 

 a heavy rain extending over a short period. Since the results in this 

 study have only a comparative value, the entrance of water into the 

 bottles does not vitiate the results. This fact was first pointed out 

 by Brown ('lo) and later emphasized by Fuller ('ii). In the 

 handling of the instruments for scientific accuracy, in the selection 

 of the intervals of time for reading, and in plotting the results, the 

 writer had the constant advice and suggestion of Dr. Fuller, of the 

 Department of Botany of the University of Chicago. Without his 

 help many of the difficulties would not have been so readily over- 

 come. All the instruments were standardized before being placed in 

 the respective stations, and restandardized at intervals of six to eight 

 weeks. Bv the coefficients thus obtained all the readings were re- 

 duced to a common unit. The first reading was made on July _S- 

 1913. and weeklv readings were made thereafter until October 18, 

 when the last record was made. Thus the investigation extended over 

 a period of 112 davs. Very few of the instruments were molested 

 during the entire season, and hence the regularity of the results for 

 each of the stations is rarely interrupted. These results are expressed 

 as the average daily loss for the weekly interval between the readings. 

 These results are graphically represented with the weekly intervals 

 as abscissae, and the amount of daily loss by the standard atmometer 

 in cubic centimeters as ordinates. 



INTERPRETATIOX .\XD DISCUSSIOX OF RESULTS 



I. The graphs in Figure 16 (PI. X) show that the average daily 

 rate of evaporation for the weekly intervals was almost uniformly 

 greater in the embryonic ravine than in either the narrow or broad 

 portions, and greater in the broad than in the narrow. The average 

 amounts of evaporation per day for the entire season at stations i. 

 2, and 3 were 5.44 c.c, 3.27 c.c, and 4.75 c.c. respectively; that is 

 2.17 c.c. per day greater in the embryonic ravine than in the narrow 

 portion, .69 c.c. greater in the embryonic ravine than in the broad 

 portion, and 1.48 c.c. greater in the Inroad than in the narrow portion. 

 In terms of percentage the rate of evaporation was 66.3% greater 

 in the embrvonic than in the narrow portion, 14.?^^ greater in the 

 embryonic than in the broad, and 45.2 9f greater in the broad than 



