228 



AMERICAN FORESTRY 



upon precipitation." He further makes 

 "a plea for tolerance of opinion" and, 

 presumably in connection with this plea, 

 quotes the very intolerant opinion of 

 Prof. Cleveland Abbe : "In this day 

 and generation the idea that forests 

 either increase or diminish the quantity 

 of rain that falls from the clouds, is not 

 worthy to be entertained by rational, 

 intelligent men." If he were present- 

 ing a fair and a scientific discussion, 

 recognizing that the inductive or statis- 

 tical method will not give definite re- 

 sults, he would see that we must resort 

 to the deductive method, provided wc 

 can find some fundamental principles, 

 reasonably established, upon which to 

 base our deductions. He recognizes, 

 indeed, that the statistical method will 

 not work for he says: (page i6) "All 

 of these problems could be definitely 

 settled beyond the possibility of argu- 

 ment if we had accurate river gaugings 

 from day to day and from year to year, 

 together with a full knowledge of the 

 rainfall and of the proportion of wooded 

 to cleared areas, data which, unfortu- 

 nately, we do not have." But, he goes 

 on to say, incorrectly, that we must, 

 therefore, "reason empirically from the 

 best information at hand." Empirical 

 reasoning will scarcely lead to reliable 

 results. 



Now as a matter of fact, there are 

 several well established principles which 

 bear upon this matter, and which give 

 very good a priori reasons for believing 

 that forests do slightlv increase the rain- 

 fall. The first of 'these Mr. IMoore 

 mentions (page 22) quoting from Prof. 

 Bailey Willis : 



"The mountains are wet because thev 

 are high, and they are heavily forested 

 because they are wet. But there is also 

 a reciprocal action of the forests on the 

 wetness, for the radiation from the 

 dark-green expanse is comparatively 

 uniform and promotes frequent and 

 steady rains. Were the mountains bare 

 they would, like the bared sierras of 

 Spain, receive occasional but violent 

 downpours and send down excessive 

 and disastrous floods, even more disas- 

 trous than now. * * * por in-so- 



far as we clothe the surface with green 

 crops we lower the temperature of the 

 rising air and favor precipitation on the 

 verdure-covered plain." 



Regarding this statement by Profes- 

 sor W'illis he goes on to say "It would 

 be difficult to either confirm or disprove 

 this statement of Mr. Willis." If this 

 is the case, wdiy does he draw the con- 

 clusion that forests have no efifect? It 

 would be fairer, as well as more logical, 

 to simply say, what is the truth, that 

 many competent meteorologists believe 

 that forests increase the rainfall, that 

 there are good theoretical grounds in 

 favor of it, but that the effect is slight, 

 and difficult if not impossible to prove 

 by observations, considering the varia- 

 bility of the phenomenon. 



It is a reasonably established fact that 

 forests decrease the mean annual tem- 

 perature. This has been proved by 

 many observations. They therefore 

 bring the air nearer to the point of sat- 

 uration, and therefore tend to increase 

 the rainfall. Professor Moore argues 

 that this effect amounts to nothing be- 

 cause "the rain is precipitated largely 

 from air masses that exist at a consid- 

 erable distance from the surface of the 

 earth." The height above the ground 

 at which vapor is condensed in the air 

 is variable ; it may be at a great height 

 or it may be close to the ground. On 

 mountain sides, particularly, the con- 

 densation may occur at, or even below, 

 the level of the forest. We all have 

 seen rain falling from clouds lying at a 

 height considerably below the tops of 

 mountains or even high hills. Any effect 

 of the lowering of the temperature pro- 

 duced by forests, therefore, will be par- 

 ticularly noticeable in mountainous re- 

 gions. Just how much the effect w^ill 

 be will depend upon various circum- 

 stances ; among others, the distribution 

 of the rainfall through the year, for 

 the effect of forests on temperature 

 differs in summer and winter ; but with 

 a rainfall distributed with tolerable uni- 

 formity through the seasons, the resul- 

 tant effect should be to increase — even 

 if but slightly — the amount of rainfall. 



A second principle from which we 



