THE PEAZREES 195 



valley on the east side are exceptionally •svell-snited for such ob- 

 servations as the exposure of the faces of the bluffs is extreme, 

 and as the ridges are often vei^- sharp so that the transition from 

 prairie to forest is ver\' rapid. For that reason a portion of this 

 bluff' region was selected for the observations on the relative rate 

 of evaporation and its relation to the native flora. 



Evaporation. 



In order that all the conditions affecting evaporation might 

 be properly taken into account a series of meteorological observa- 

 tions was undertaken at Missouri Valley in the summer of 1908. 

 This work included observations on evaporation, temperature and 

 relative humidity, velocity* and direction of wind, clearness of 

 sky. barometric pressure, and in a general way on rain-fall. 



Evaporation. An effort was made to employ three kinds of 

 evaporimeters. An open tin pan. one foot in diameter, and with 

 upright sides, was buried to the rim at each- of the four stations 

 described below. 



At two of the stations Piche evaporimeters. graduated to hun- 

 dredths of a cubic inch, were suspended eight inches above the 

 surface of the soil on iron rods driven into the ground. These 

 evaporimeters were frecpently tested and worked together per- 

 fectly. 



An effort was also made to employ porous eup-evaporimeters 

 at these two stations.^- but owing to various accidents they failed 

 several times. 



Temperature anel Relative Humidity. For this purpose a 

 Marvin Sling Psychrometer with the Fahrenheit scale (the U. S. 

 standard'! was employed, rain-water being used for the wet-bulb. 

 Readings were also taken from two Centigrade psychrometers 

 as an additional check. 



Witicl Telocity. The velocity of the wind was measured at one 

 of the stations (number 1) by a Green's cup anemometer 

 which was set up at this station throughout each of the obser- 

 vation days. A small mill anemometer (air-meter) was also em- 

 ployed for short periods at various intervals at all the stations. 



" These were kindlv furnished for the purpose bv Dr. Forrest Shreve of 

 the Came^e Desert Botanical Laboratorv. 



