DIMINUTION OF THE WATER OF RIVERS AND STREAMS. 201 



lu like manner the proportional luunidity of the woods difl'eis from 

 that of the open country, and at all seasons of the year is greater in the 

 forests. * 



In consideration of these facts, it is not surprising that the evapora- 

 tion in the woods is found to be far less than in the open fields, t Fur- 

 thermore the evaporation does not depend only upon the temperature- 

 bat also upon the motion of the air, which in the woods is proportion, 

 ably much less agitated. 



It was now important to determine accurately the portion of the de- 

 posit lost by evaporation, a very difficult matter, since the pro[)ortion 

 obtained by the atometer is entirely different from that of the evapora- 

 tion from the surface of plants, and from the soil. Besides, the propor- 

 tionate results of the atometric measurements by which the evaporation 

 of a water-surface is obtained are influenced by the different ways of 

 placing the instrument, sometimes exposed to the sun and rain, some- 

 times under shelter; by the difference in dimension of the evaporating 

 dishes, and by the difference in the material of which the latter are 

 compos«'d. It ought not to be surprising, under such circumstances, 

 if an atometer of small dimensions, made of metal and exposed to the 

 sun, givesyearly amounts of evaporation, which far exceed (2 or 3 times) 

 the amounts of precipitation. 



On account of the difficulty of determining the amount of evaporation 

 from the ground, and therewith the portion of the precipitation actually 

 received by the si)rings and running water, this should be sought by 

 coujpariug the quantities of water a stream carries away iu the course 

 of a year, and determining the corresponding amounts of rain fallen 

 within the drainage area of the stream; also on the other hand by 

 endeavoring to measure the amount of water absorbed by a certain por- 

 tion of earth, and the amount lost from a given stratum. | 



It' in the woods a considerable portion of the precipitation is received 

 upon the twigs and leaves, the rest (according to Ebermayei', 72 per 

 cent.. Journal of Meteorology, viii, 274) on this account remains longer 

 in the woods, and has time to sink into the soil and supply the springs. 

 This portion, at least that part of it not dissipated by evaporation, either 

 sinks into the ground or Hows off" of its surface. In the first instance it 

 serves especially for the supply of the springs; iu the second it is carried 

 directly to the water-courses, and produces a brief, more or less con- 

 siderable rising in the latter. 



* According to Eberiuayer, tlio dift'erence auiounts in tho four seasons, calculated 

 from the spring, to 5.7, OM, 5.2, and 5.2 per cent. 



t According to Eberniaycr the evaporation from a surface of water in tlie woods was 

 64 i)er cent, less tiian outside tlie .same. (Journal of Meteorology, vol. viii, ]>. 125:5. j 



I Molleudorf (The Kain-fall of Germany, p. 1:50-107) gives as the percentage of tlie do- 

 posits not evaporated and entered into tho supply of the springs and rivers, for Eng- 

 land, (4 determinations,) 31.7; for France, (2 determinations in mountainous regions,) 

 65.1; for Germany, (5 determinations on rivers, 7 determinations on earth-boxes and 

 artificial drainage,) 47.3 per cent. 



