Page:Popular Science Monthly Volume 82.djvu/330

326 evidence that our forests increase the frequency of precipitation, 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 "corrected" 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 slackening of the lower air currents and a readier descent of the raindrops, is accompanied by a lessened fall to leeward. Thus there is equalization; 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