103 



ration, and from which the water flows with greater rapidity after the 

 ground is saturated, are being increased. This period of eighty years, 

 from 1816 to 1895, during which the autumn flow of Ithe Schuylkil 

 seems to have been gradually diminishing, is too long a reach to be ex- 

 plained by any cyclic change of which we have any exact instrumental 

 record and certainly the cyclic explanation must be open to ques- 

 tion. The decrease in water flow becomes all the more striking when 

 it is noted that the deficiency of water occurs just at the time when we 

 should expect the effects of the summer evaporation over cleared areas 

 to be most marked. 



3d. Any excess of water which flows out of the country in a freshet 

 leaves so much less in the country to maintain the even, average flow 

 of springs and streams. 



4th. Competent engineers inform us that as between two similarly 

 situated and conditioned regions, the one, however, being treeless, and 

 the other being timber clad, the latter will absorb of the water which 

 falls, and of the snow which melts, about three-fifths more than the 

 former. 



5th. Many of our smaller streams, which once flowed the year 

 through, are now, for a portion of each summer season, either ab- 

 solutely dry, or nearly so. This is ordinarily supposed to mean merely 

 so much water cut off from the larger streams by previous stages of 

 high water. But as a matter of fact it means also a dry atmosphere 

 over a large portion of our cleared areas, which dry air evaporates, by 

 so much, the water from the larger streams. We cannot here give 

 the exact ratio of evaporation, but it probably aggregates an immense 

 volume of water.* 



6th. It is now frequently seen that the "green woods" burn in our 

 forest fires, and that formerly they very seldom did so. This means 

 that the normal condition of a saturated atmosphere which once ex- 

 isted has changed, and that, instead, we have an air from which much 

 of the moisture has been evaporated, because the forest areas of slow 

 evaporation have been changed into cleared areas of rapid evapora- 

 tion, and that the latter areas tend to a constant equalization of the 

 quantity of moisture in the air by drawing upon and drying out even 

 the green woods. 



7th. While our river guages have thus far not clearly indicated 

 periods of higher water, the fact remains that our county commis- 

 sioners are continually raising the height of our bridges above the 

 stream to keep them out of the way of the flood. 



It would thus appear that in the loss of water retaining power in 

 the cleared ground and in the increase in the rate of evaporation over 

 the remaining water surfaces there is good reason to associate, at 



* Mr. llafter (op. cit. p. 157) writes, "I reach the conclusion that the deforestation of a drainage area 

 wilt, in the state of New York, probably decrease the annual water yield of that area from four to six 

 inches." 



