HYDROLOGY OF NEW YORK 



135 



The temperatures indicated by the foregoing formula (31) are 

 above the freezing point. For temperatures below the freezing 

 point, the denominator of the fraction in the second member of 

 the formula should be 1240.2 — V . For Centigrade degrees, the 

 denominator of the fraction should be, when the temperature of 

 the dry bulb is above the freezing point, 610 — t', and when the 

 Temperature of the wet bulb is below the freezing point, the bulb 

 being covered by a film of ice, the denominator should be 689 — V. 1 



There is no difference between evaporation from a water surface 

 and evaporation from land, except that on a water surface it goes 

 on continuously, while on land evaporation may be interrupted 

 from lack of something to evaporate. The preceding formul? 

 shows that" the force of vapor is dependent upon the difference o/ 

 the dry and wet bulb thermometers, and not in any degree upoi. 

 the mean annual temperature. 



Evaporation relations. Prof. Cleveland Abbe 2 gives the follow- 

 ing relations of evaporation, as established by Prof. Thomas Tate : 



a) Other things being the same, the rate of evaporation is 

 nearly proportional to the difference of the temperatures indi- 

 cated by the wet-bulb and dry -bulb thermometers. 



b) Other things being the same, the augmentation of evapora- 

 tion due to air in motion is nearly proportional to the velocity of 

 the wind. 



c) Other things being the same, the evaporation is nearly in- 

 versely proportional to the pressure of the atmosphere. 



1 In the original discussion of this matter, in paper on Relation of Rain- 

 fall to Runoff, there is an error of statement in formula (31). The de- 

 nominator of the second member should be 1130 — f, instead of 689 — t'. 

 The former expression is for Fahrenheit degrees, while the latter is for 

 Centigrade degrees, and with the bulb covered by a film of ice. 



In formula (30), it will be noted that Mr. FitzGerald makes V the maxi- 

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

 of the water. Recent study of this matter indicates that there is considerable 

 doubt whether formula (31) strictly applies in the computation of V, but 

 since for present purposes an illustration of the matter is all that is needed, 

 it is not attempted to settle these difficult questions in physics here. 



-Preparatory Studies for Deductive Methods in Storm and Weather Pre- 

 dictions, by Prof. Cleveland Abbe : Ann. Rept. Chief Signal Officer for 1889, 

 Part I, Appendix 15. 



