THE PEAIEIES 203 



dry prairies on the southwesterly slopes, for these slopes are not 

 only exposed to the prevailing southwesterly winds of summer 

 but also to the "two-o'clock sun" which produces the maximum 

 temperature and the minimum humidity of the air. 



The clearness of the sky did not vary sufficiently to bring out 

 any striking results. The record for the several observation days 

 is briefly summarized as follows: 



August 13 Hazy. 



August 21 Hazy A. M.; cloudy P. M. 

 August 29 Cloudy A. M.; hazy P. M. 

 September 5 Clear but quite hazy. 

 September 12 Clear but somewhat hazy. 



The direction of the wind for the same days varied as follows ; 



August 13 Northwest. 



August 21 Mostly southwest. 



August 29 Southeast A. M.; south P. M. 



September 5 Southeast, then south, then southwest; at 3 o'clock P. M., 

 west; at 5 o'clock, southwest; at 6 o'clock, southeast; and at 7 o'clock 

 northwest. 



September 12 Southeast, then southwest, then southeast. 



The barometric pressure at station 1 varied as follows (meas- 

 ured in inches) : 



August 13 28.97 to 28.88 A slight rise and then a gradual fall. 

 August 21 28.79 to 28.75 A gradual fall. 

 August 28 28.70 to 28.75 Somewhat irregular. 



September 5 28.72 to 28.68 Somewhat irregular. 



September 12 28.96 to 28.84 Somewhat irregular. 



Relative humidity curves were not included in the figures on 

 plates XII and XIII for fear of causing confusion. These curves 

 may be easily constructed by using the relative humidities re- 

 corded in the table as ordinates and the hours of the day as ab- 

 scissas. For this purpose the lowest line in each figure should 

 be marked respectively .56, .47, .43 and .31 and each vertical space 

 should represent .06. Curves so constructed will correspond quite 

 closely with the evaporation curves, being of course their reverse. 



The conclusions reached from these observations may be briefly 

 stated as follows : That evaporation is most rapid from surfaces 

 exposed to the prevailing summer winds and to the afternoon 



