33o THE POPULAR SCIENCE MONTHLY 



than to forest trees, and that the damage in the former case would at- 

 tract, in the latter would largely escape notice. Further, as regards 

 storms and high winds in general, forests do, as has been seen, tend to 

 check wind velocity, and thus to reduce the local violence of a gale. 

 On the other hand, however, recent investigations in Germany have 

 shown that in thunderstorms the obliquely descending component of the 

 wind can be but slightly, if at all, affected by forests, whose trees are 

 easily uprooted by these winds. 



The Hygienic Influence of Forests 



There are several ways in which forests have a hygienic significance, 

 and the location of many of our well-known health resorts in or near 

 extended forest areas is, therefore, well planned and logical. The re- 

 duced wind movement; the protection against the severest extremes of 

 summer heat and of winter cold; the marked decrease of dust and of 

 other atmospheric impurities; the grateful shade and lack of glare on 

 sunny days; the relatively small number of microorganisms — all these 

 are helpful, not only to those who are ill or convalescent, but to per- 

 sons in good health; all these are arguments in favor of wooded parks 

 in and in close proximity to our cities. In addition, but of non-climatic 

 importance, there are the scenic attractions of the forests; the relief 

 from the noise and the bustle of the city; the fragrance of the air among' 

 the evergreen trees, and the frequent intermingling of river and lake 

 and mountain, all of which features contribute to the popularity of 

 forest sanitaria and pleasure resorts. So far as the composition of for- 

 est air is concerned, there is no further notable difference between it 

 and the air outside. We can not, therefore, look for any marked cura- 

 tive effects on that account. The much-discussed beneficial effects of the 

 ozone in the forest air seem to lack the support of observation. 



The Influence of Forests upon Water-supply, Erosion and 



Floods 



The preceding discussion has dealt with the influence of forests upon 

 climate. Therefore no mention has been made of their relation to the 

 conservation of the water-supply, to erosion and to floods, all of which 

 are non-climatic, or at any rate only indirectly climatic effects. There 

 is still a great deal to be learned about the use of forests in connection 

 with water-supplies; their effects in holding back rainfall and in stor- 

 ing the winter snow; their relation to floods, and ground-water, and 

 springs and erosion. The " last word " in this discussion is to be found 

 in the "Final Eeport of the National Highways Commission" (Sen. 

 Doc. No. 469, 62d Cong., 2 sess., 1912). From this report we take the 

 following statements, which are of peculiar interest, because they repre- 

 sent the conclusions and recommendations which have been reached 



