134 



NEW YORK STATE MUSEUM 



As regards the relation between mean annual temperatuTe and 

 evaporation, the questions raised by Mr Vermeule are very inter- 

 esting and have received considerable study from the writer ever 

 since the publication of Mr Vermeule's report in 1894. This study 

 has been specially directed toward determining whether there was 

 any way of showing by diagrams, definitely, that any such relation 

 really existed. 



Evaporation 



FitzGerald's formula for evaporation. We may consider Mr 

 FitzGerald's formula for evaporation, 1 which is 



E 



(V-*)(l + - 9 ) (30) 



60 



In this formula V=the maximum force of vapor in inches of 

 mercury corresponding to the temperature of the water; the 

 force of vapor present in the air; W=the velocity of the wind in 

 miles per hour; and E=the evaporation in inches of depth per 

 hour. It can be shown that there is going on nearly always a 

 condensation of moisture from the air upon any water surface. 

 At the same time there is going on a loss of moisture from the 

 water surface by evaporation. The intensity of both these opera- 

 tions depends upon the difference in temperature between the air 

 and any water surface with which it may be in contact. When 

 the temperature of air and water is the same, theoretically both 

 processes stop. Broadly, evaporation may be said to measure the 

 difference of these two exchanges. Wind velocity also exerts a 

 decided effect on the intensity of evaporation. 



For illustrative purposes, v, the force of vapor present in the air 

 may be computed by the following: 



1130— J v ; 



In which r=force of vapor in the air at time of observation; 

 . . Y=force of vapor in a saturated air at tempera hi re 

 of V ; 



^temperature of the air in Fahrenheit degrees, in- 

 dicated by the dry bulb ; 

 temperature of evaporation given by wet bulb; 

 7i=hight of barometer. 



'Trans. Am. Soc. 0. E., Vol. XV, pp. 581-646. 



