FISHERIES, GAME AND FORESTS. 413 



The lowest annual temperature at Philadelphia, 48. 2°, was in 1836. 



A study of temperature records, so far as the author has carried such a study, 

 seems to show that there is no direct relation between the run-off of streams and mean 

 annual temperature, although on this point he does not wish to be understood as yet 

 passing a final judgment. * 



HYGROMETRY. 



Hygrometry may be defined as that branch of meteorology which is concerned 

 with the determination of the amount of water present in the air in the form of vapor. 

 The degree of humidity is its hygrometric state. When expressed as a per cent., it 

 is relative humidity, but when the tension or elastic force of aqueous vapor which 

 represents the pressure of all the vapor in the air above the point of observation is 

 expressed in terms of inches of a mercury column, it represents the absolute 

 humidity of the air.f 



VARIATIONS IN THE HYGROMETRIC STATE. 



The hygrometric state varies greatly at different times of day and different seasons. 

 For diagrams and tables illustrating these variations, see Durand-Claye's Hydrau- 

 lique Agricole et Genie Rural, Chapter 7. 



EVAPORATION. 



GENERAL FORMULA FOR EVAPORATION. 



So far as the English literature of the subject is concerned, the most satisfactory 

 discussion of evaporation is that of Mr. Fitz Gerald, % whose formula for evaporation 

 is as follows: 



[v-^ (1 + " 



E = 



60 



In this formula, V means the maximum force of vapor in inches of mercury 

 corresponding to the temperature of the water ; v, the force of vapor present in the 



* For a large number of temperature records in the State of New York, tabulated with reference 

 to a water year from December to November, inclusive, and divided into storage, growing and 

 replenishing periods, the same as for rainfall records, see the author's Reports on the Genesee and 

 Hudson Rivers Storage Surveys, in the Annual Reports of the State Engineer and Surveyor, as pre- 

 viously cited. 



t Refer to Moore's Meteorology, Chapters XVI to XX. 



X See paper on Evaporation. By Desmond Fitz Gerald. Trans. Am. Soc. C. E., Vol. XV 

 (Sept. 1886), pp. 581-646. 



