204 NATUEAL HISTOKY BULLETIN 



sun, and that both are necessary to cause maximum evaporation ; 

 that in rough territory these surfaces are chiefly southwesterly 

 and hence exposed as stated, and this at the time of the day when 

 the relative humidity of the air is the lowest; that the effect of 

 wind upon evaporation is best brought out and is greatest when 

 the temperature is sufficiently high to produce rapid evaporation ; 

 that evaporation varies with the direction of the wind and the po- 

 sition of protective barriers such as ridges ; and finally that upon 

 all the areas exposed to maximum evaporation a prairie flora, 

 largely xerophytic, is developed, while the mesophytes of the 

 grove and forest develop in our territory only in places shel- 

 tered from the chief evaporating agencies. 



Similar observations which were made during the same period 

 at Council Bluffs and Omaha confirm these conclusions. A sta- 

 tion was established at Council Bluffs, Iowa, (on the east side 

 of the Missouri valley) on a dry prairie ridge northeast of the 

 Ninth Avenue entrance to Fairmount Park, at an altitude of 

 about 140 feet above the bottoms. This station was fully exposed 

 to the south, southwest, west and northwest. 



Another station was located at the same elevation near the in- 

 tersection of Woolworth Avenue and Sixth Street in Omaha, 

 Nebraska. This station was located on the rounded western 

 bluffs on a slope looking northeast, and was well exposed to the 

 north, northeast and southeast. This slope was formerly tim- 

 bered but is now almost bare. This station was in charge of Mr. 

 Lumir Buresh of the Omaha High School whose patient care and 

 perseverance made the double observations possible, the writer 

 taking the east bluff. 



On the 17th of August the observations on the west side were 

 made at another station located in a clearing on the timbered 

 bluffs above Florence, Nebraska, all the readings for that date 

 being made at this station. The Florence station was also about 

 140 feet above the river valley. All the other west side obser- 

 vations were made at the "Woolworth Avenue station. 



The work was conducted in the same manner as at Missouri 

 Valley, excepting that no evaporating pans were used. 



