124 FOREST INFLUENCES. 



A singular illiistration of these errors is presented in a hydrograpliic 

 investigation of tlie river Rhone and its watershed. While the amount 

 of annual discharge of the river corresponds to a rainfall of 44 inches 

 over the watershed, the rainfall records themselves for a certain period 

 gave a precipitation of only 27.6 inches. Even the close estimate of 

 the waters of the upper Elbe, according to which they drain one-fourth 

 of the total rainfall, calculated by the ingenious methods of Prof. Stud- 

 nicka, does not inspire confidence. The attempts of the U. S. Weather 

 Bureau to relate river stages to rainfall measurements have also so far 

 failed to yield satisfactory results. Again, much of the moisture which 

 is condensed and precipitated in dews escapes our observation, or at 

 least our measurements, entirely; * this is, however, so small a quantity 

 that it may be neglected in its relation to the total precipitation. 



Late investigations have brought convincing proof not only that the 

 amount of dew is much smaller than was supposed, but also that the 

 larger part of the deposit is derived from the moisture of the soil and 

 not from that of the atmosphere; that, therefore, dew formation in 

 many cases can not be considered an additional water supply, but 

 rather an element of dissipation. While G. Dines calculated the 

 annual amount of dew at best equivalent to 38 mm. rainfall as against 

 127 mm. as formerly estimated for England, Wollny, upon careful 

 measurements, calculated it for Munich at 28 to 32 mm. or less than 3J 

 per cent of the total rainfall. The theories regarding dew formation, 

 according to which the moisture is deposited from the atmosphere in a 

 manner corresponding to the familiar phenomenon on the ice pitcher, 

 which was first antagonized in 1833 by Gerster and later by others^ 

 among whom our countryman, Stockbridge, seem in part at least incom- 

 plete and needing revision. 



The distribution of the circulating water capital is influenced by 

 various agencies. The main fiictor which sets the capital afloat is the 

 sun, which, by its heat and the air currents caused by it, produces the 

 evaporation which fills the atmosphere with vapor. Anything, there- 

 fore, that influences the intensity of insolation or obstructs the passage 

 of winds must influence the local distribution of the water capital, and 

 hence a forest cover, which withdraws a portion of the soil from the 



*A few experiments on condensation of aqueous vapor made by L. Hampel witli 



forest tree leaves are of interest : ^ ^. 



Centigrams. 



Austrian pine (4 needles) condensed per day in the average 4. 84 



Linden (one leaf) condensed per day in the average 24. 40 



Oak (one leaf) condensed per day in the average 25. o6 



Spruce (a branchlet) condensed per day in the average 9. 80 



The linden, of which one leaf condensed 24.40 centigrams of dew, had 1,763 leaves. 

 It would, therefore, if all leaves had done the same, which is to be sure not the 

 case, have condensed 430 grams (nearly 1 pound). 



On grass the amount of dew per year was found by G. Dines to be 27 millimeters ; 

 t. c, if collected an amount corresponding to 27 millimeters (over 1 inch) height of 

 water would have resulted. 



