326 THE POPULAR SCIENCE MONTHLY 



sailable evidence that our forests increase the frequency of precipita- 

 tion, although some excellent authorities incline to the view that they 

 do. No one can fairly be called unreasonable if he believes that, after 

 making all proper corrections, there remains no appreciable difference 

 in rainfall inside and outside of our temperate zone forests. Perhaps 

 even the slight remaining differences ought themselves to be " cor- 

 rected " away. On the other hand, no one can be called unduly 

 optimistic who, knowing the many uncertainties involved in any critical 

 study of rainfall records, gives the forest " the benefit of the doubt " 

 and holds that it really does rain a little more over forests than in the 

 open. But the " little " is, at best, very little, as the latest European 

 observations have shown. We can not, if we will, make it an excess 

 of more than a few hundredths of the total annual rainfall. The 

 margin of difference between the two points of view is thus seen to be 

 very slight indeed. One thing is clear. Granting that all of the 

 observed differences between the catch within forests and outside of 

 forests is due to an actual difference in rain fall, and not largely to the 

 difference in exposure, the excess over the forest still remains but a 

 small proportion of the annual rainfall. In other words, even the 

 uncorrected observations give a maximum value for forest effects which 

 is itself relatively slight. If, at best, forests can only produce such 

 slight differences over and among the trees themselves, we can not 

 suppose that they will have enough effect upon passing air currents to 

 influence the climate of more distant regions. Hellmann has shown 

 that an increase in the rainfall over a forest, resulting from the slack- 

 ening of the lower air currents and a readier descent of the raindrops, 

 is accompanied by a lessened fall to leeward. Thus there is equaliza- 

 tion; simply a slight difference in distribution. 



It is not altogether surprising that one writer has expressed the 

 opinion that " no definite and unassailable result can ever be obtained " 

 by means of such forest meteorological observations as those now made 

 in Europe, and that " there would be little to be gained by a further 

 study of the question." Yet this attitude will hardly commend itself 

 to those who are anxious to have the present uncertainty cleared up, 

 so far as possible. In view of what has already been said, it hardly 

 needs to be stated that, in spite of the deforestation, by lumbering and 

 fire, of large sections in the eastern United States, there is no reliable 

 evidence of any decrease in rainfall, nor of any other change of climate. 

 (It is, however, only fair to say that a good deal of this denuded area 

 has been covered by second-growth timber.) Nor, in spite of the 

 prevailing popular impression to the contrary, is there any reliable 

 evidence whatever that cultivation and tree-planting over extended 

 areas of the west and southwest have resulted in any increase in the 

 amount of precipitation. There is, of course, a better conservation of 

 moisture for plant use. We are surely within the bounds of reason when 



