228 THE POPULAR SCIENCE MONTHLY. 



soil, and substituting debris and unfertile ground from the mountains, 

 and pauperizing the once productive lands, inflicting damages, which 

 now, by the expenditure of millions of dollars yearly, and with the 

 exercise of the greatest ingenuity, can hardly be repaired. Within 

 the last twenty years, France has i*eforested about two hundred and 

 fifty thousand acres of mountain-lands, at a cost of $30,000,000 of 

 which the Government j)aid one half, the local communities the other 

 half. In addition, two hundred thousand acres of sand-dunes, which 

 were the result of injudicious clearing, have been reforested since 

 1862, and thus been made productive. 



An extensive literature on the subject of forest benefits has now 

 accumulated. But I do not intend to rehearse these often-cited argu- 

 ments, which are so well elaborated in George P. Marsh's classical 

 book, "The Earth as modified by Man." We must admit, however, 

 that historical evidence alone can not be held sufficient proof of any 

 natural law, and a problem of natural science needs for its solution 

 to be subjected to scientific methods of investigation and reasoning. 

 This has only lately been done with regard to forest influences ; and, 

 though systematic and continuous observations have not yet extended 

 over a long period, we are already prepared at least to understand the 

 character, though not always the extent, of the part which the forest 

 plays in the economy of Nature. We have learned to discriminate be- 

 tween the different functions of forest influences. We have learned 

 that the mechanical influence of forest-cover upon hydrologic and soil 

 conditions is undeniable ; we have learned that climatic changes due 

 to deforestation may be favorable as well as unfavorable ; that the 

 great characteristics of a climate, due to cosmic conditions, such as the 

 twofold movement of the earth, the presence of water-surfaces, eleva- 

 tion, the prevailing winds, etc., are probably beyond the reach of for- 

 est influence ; that such influence must, in the main, be local, and its 

 nature and extent be dependent largely upon the geography of the 

 locality. 



The rationale of forest influences is easily enough understood, if 

 we consider them step by step. The climate of a locality — i. e., the in- 

 terdependent oscillations of temperature and humidity of the air (not 

 as it is popularly expressed the mean condition of these factors) — is, in 

 the first place, dependent upon the heating effect of the sun's rays ; in 

 one word, upon "insolation." The temperature of the air derives its 

 heat, for the most part, only from contact with the heated earth or ob- 

 jects on the earth, and by radiation from these. Any mechanical bar- 

 rier, then, against insolation of the soil, like a shady, dense forest, 

 must have the effect of lowering the temperature of the soil and con- 

 sequently of the air above it. The immediate consequence of this is 

 diminished evaporation from the surface of the soil ; while, on the 

 other hand, the transpiration through the leaves makes the ground- 

 water of greater depths available to the atmosjihere. Thus cooler and 



