107 



p. 235.) He finds that with the toiuperature of the air 84° F. and a 

 relative humidity 50 per cent tlie evaporation varies with the velocity 

 of the wind at the surface of the moist disk as in the following para- 

 graph : 



INFLUENCE OF THE WIND ON EVAPORATION. 



At 5 miles an hour the evaporation is 2.2 times that in a calm; at 10 

 miles, 3.8 times; at 15 miles, 4.9 times; at 20 miles, 5.7 times; at 25 

 miles, 6.1 times; at 30 miles, 6.3 times. 



The observations of the Piche instruments, as exposed in Signal 

 Service shelters at 18 different stations, gave the results in the table 

 following. (See Monthly Weather Review, September, 1888, p. 236.) 

 The readings on the scale of the Piche instrument have been con- 

 verted into depths of water that would be evaporated from a free 

 surface of water within the same instrument shelter during the 

 respective months by multiplying them by the constant coefficient 

 1.33, so that the evaporations here given in inches of depth of water 

 correspond entirely to the ordinary methods of measuring rainfall. 



Evaporation, in incites, ohserved with Piche instruments irithin the Signal Slervice 

 thermometer shelters in 1888. 



July. 



Septem- 

 ber. 



Boston 



New York 



Washington . . 



"Buffalo 



Cincinnati 



Memphis 



New Orleans . 



Chicago 



St. Louis 



Keeler 



Yuma 



El Paso 



Dodge City . . 

 San Antonio. 



Denver 



St. Vincent - 



Helena 



Boise City.. 



Incites. 

 5.16 

 4.49 

 4.64 



Inches. 

 5.87 

 5.36 

 5.27 



Inches. 

 5.28 

 4.14 

 4.22 



6.22 

 5.33 



5.59 

 6.18 

 11.66 

 13.86 

 13.91 

 7.80 

 2.76 

 7.01 

 9.42 

 5.63 

 4.88 

 5.83 



6.93 

 5.24 

 9.38 

 5.52 



5.79 

 12.76 

 13.63 



5.36 

 4.57 

 7.96 

 6.97 

 4.41 

 12.69 

 12.88 

 11.54 

 6.22 

 5.36 

 5.44 

 8.55 

 5.97 

 7.80 



Inches. 

 2.68 

 2.88 

 2.52 

 3.70 

 5.33 

 3.86 

 3.70 

 5.79 

 4.61 

 10.95 

 10.36 

 10.00 

 6.07 



5.94 

 6.86 



" In October at Boise City the evaporation was 7.60 inches. 



Profesor Russell has also devised the following very satisfactory 

 formula connecting the total daily evaporation in inches with the 

 meteorological elements on which it depends, viz, the vapor tensions, 

 pu: for mean wet bulb and p,i for mean dew-point temperatures, (b) 

 barometric pressure, b}' means of which he has been able to compute 



