lo; 



Suiiinicr icinil ruficfi of cruijunition at Mojitfioiii 



We see that the driest winds, or those whose relative humidity is 

 small, such as the north and east winds, give a large evaporation, and 

 that the velocity and temperature of the west winds, which are a 

 little less than those of the southwest winds, does not compensate for 

 the dryness, which latter enables them to evaporate a little less than 

 the southwest winds. 



By multiplying the average daily evaporation by the number of 

 days we obtain the total evaporation from the saturated paper of the 

 Piche instrument. This exceeds the total rainfall, but we are not to 

 infer that the evaporation from ground and leaves must also neces- 

 sarily exceed the rainfall, although this is generally true for the sum- 

 mer season. 



BOSTON DATA FROM E. J. FITZGERALD. 



The evaporation of the water from leaves and from the ground 

 depends upon the temperature, wind, and humidity of the air. It is 

 a rather complex result ; if the above-mentioned elements remain con- 

 stant for any time at the surface of the mass of water the evaporation 

 fi-om that surface will be closely represented by the following formula 

 which is due to Fitzgerald, of Boston, 



E=0.0166 (P— p) (1+i W), 



where W is the velocity of the wind in miles per hour ; P the tension 

 of vapor in inches of mercury corresponding to the temperature of the 

 water; p is the tension of vapor corresponding to the dew point in the 

 free air; E is the evaporation expressed in inches of depth of water 

 evaporated per hour under atmospheric pressure between 29 and 31 

 inches of the barometer. 



The evaporaticm from leaves and soils is usually less than that from 

 water about in the proportion in which the soil approximates its 



