FISHERIES, GAME AND FORESTS. 415 



CONDITION CHIEFLY AFFECTING EVAPORATION. 



The evaporation from a water surface depends upon several elements outside of 

 temperature, as, for instance, the hygrometric state exercises a certain action, but 

 the agitation and constant change caused by wind movement exercises the greatest 

 influence of all, because of quickly removing layers of air in contact with the 

 liquid surface, substituting others in place of them.* 



DOUBLE SIGNIFICANCE OF THE TERM EVAPORATION. 



Mr. Beardmore points out in his Manual of Hydrology that the term evaporation 

 is, in reality, of double significance, the engineer regarding it as much with reference 

 to the quantity lost when the soil is wetted by rain or dew, as the amount merely 

 evaporated from exposed surfaces of water, which is the evaporation of the 

 meteorologist. Writers on general meteorology have thus far only casually taken 

 into account land evaporation. 



NEGATIVE EVAPORATION. 



This term may be taken to mean that when the temperature of the dew point is 

 higher than that of the evaporating surface, water is deposited on that surface. 

 Studies of run-off data in comparison with rainfall and without regard to forest 

 meteorology indicate that on some watersheds this is, perhaps, a frequent condition. 

 We need forest meteorology, therefore, to correct the uncertainty of run-off obser- 

 vations in this particular. 



EVAPORATION FROM NAKED SOIL. 



As already pointed out, evaporation from the ground follows substantially the 

 same law as evaporation from water surfaces, varying in accordance with temperature, 

 hygrometric state, wind movement, etc., and further varying with the nature of the 

 soil. Experiments on this line were made by Maurice at Geneva, Switzerland, in 

 1796, and by De Gasparin, at Orange, France, in 1821, from which were determined 

 the ratios between evaporation from the soil and rainfall for those years, as 0.61 for 

 the first, and 0.88 for the second. More recent studies show that these old determina- 

 tions have historical value only. Very recent French experiments are those of Marie- 

 Davy, made at the Municipal Observatory of Montsouris, as recorded in the Annual 

 Report of the Montsouris Observatory. Probably the best experiments are those 

 made in England, where the percolation through drain gages has been observed at the 



* Also refer to Fitz Gerald's paper, Beardmore's Hydrology, etc., for more extended statements 

 as to conditions affecting evaporation. 



