420 The Ohio Naturalist. [Vol. IX, No. 3, 



At Reno this vapor blanket seemed to have a depth of 40 feet 

 over the city reservoir, but it will vary with the size of the sheet 

 of water and the climate in which it is located. He states that 

 in dry climates it will overspread the water laterally from 300 

 feet to one-fourth mile, according to the size of the sheet of 

 water. In a moist climate it will be deeper and more extensive. 



He has determined the value of the wind velocity constant 

 and has developed the value of the water vapor in different parts 

 of the reservoir. He calls this vapor value the "diffusion coeffi- 

 cient" and it, in connection with the height above the water and 

 distance from the edge of the reservoir, suggests a logarithmic or 

 geometrical law for the dift'usion. 



In the arid regions of the West it seems probable that this 

 vapor blanket conserves about three-eighths of the water that 

 would otherwise be lost by evaporation. He states that this rule 

 may not hold true in other climates and that other observations 

 should be made elsewhere. 



He has determined that if the water evaporated between 

 7:30 A. M. and 10:30 a. m. at Reno during the summer time be 

 multiplied by 8 it will closely represent the evaporation for the 

 24 hours of the day. 



Professor Bigelow has suggested the following formula for 

 trial instead of those based on Dalton's law, because it has 

 worked well in the Reno investigations. 



A full discussion of the Reno observations is made in the 

 Monthly Weather Review for February, 1908. 



Bigelow's Formula for Evaporation per Hour. 



E= Cf(h)e, ^(1+Aw) 



Cf (h) is a variable function of the evaporation, changing 

 with the height above the water surface and the distance from 

 the center in a horizontal direction. It includes the diffusion 

 and mixing process. It has been worked out at Reno in centi- 

 meters, and the values will be given upon application to the 

 Washington office of the Bureau. 



de 



-zrp, is the rate of change of the vapor pressure with the change 



db 



of the temperature of the water at the surface. It represents 

 the Clayperon formula for the volume of vapor derived from the 

 unit volume of water at the temperature S. It can be found 

 from Table 43, Smithsonian Meteorological Tables, 1907. A 

 table of these values from to 29° C. has been worked out and 

 can be obtained in a pamphlet of instructions for evaporation 

 observations issued by the Bureau. 



e^ is the vapor pressure at the dew point temperature of the 

 air. A is the wind effect constant, 0.0175. w is the wind veloc- 

 ity in kilometers per hour as read from the metric anemometer. 



