a velocity of from 3 to 10 miles per hour, and the temperature 

 ranged from 63° to 82° F. at the summit of the Mound, to 65° 

 to 83° at the base, on its south or prairie side. (See Plate V, 



fig. 1.) 



In the following table the relative humidity, and evaporation, 

 are given as before. Station I, marked a in figure, is at the 

 summit of the mound, 46 feet above the lake, station II, marked 

 5 in figure, at the middle of the slope, and stations III, marked 

 c in figure and lY, marked d in figure, on the flatter portion near 

 the base. Only the stations on the southern, or exposed, side are 

 here considered. 



RELATIVE HUMIDITY 



Hour Station I Station II Station III Station IV 



7:00 A. M. 



.70 



.76 



.75 



.71 



9 :30 A. M. 



.53 



.52 



.55 



.52 



2:15 P. M. 



.43 



.41 



.41 



.46 



3:00 p. M. 



.38 



.40 



.38 



.41 



EVAPORATION 



Piche 5.09 5.20 4.60 3.64 



Pan 109.00 cc. 99.00 cc. 137.50 cc. 129.50 cc. 



While this slope of the Twin Mounds is more exposed than 

 the stations on the north side of the lake, and while there is no 

 body of water to the south, the relative humidity is not much 

 less and the evaporation but little greater. The wind velocity 

 and temperature however were somewhat greater on the north 

 side. It is evident, however, that atmospheric conditions are 

 not materially different on the opposite sides of the lake, and that 

 both sides are dry. Vapors evidently cannot rise sufficiently 

 rapidly from the lake to materially increase the humidity of 

 the atmosphere on the north side where there are no high banks 

 or bluffs to arrest and accumulate them. 



Where high banks thus arrest vapors they need not be so high 

 that they could shelter a forest, but only sufficient to start one. 

 The trees themselves then offer a certain amount of mutual pro- 

 tection, as may be clearly seen along the outer edges of the 

 groves of this region where the outliers and border trees are 

 stunted, and their leaves are often curled and withered on hot 

 summer days, while the trees in the groves are more vigorous 



19 



