78 ECONOMICS OF FORESTRY. 



attributable mainly to the rebuilding of the old 

 Roman drainage canals, which had been allowed 

 to collapse, and the malaria-breeding mosquitoes 

 have been reduced thereby. In any case, where 

 drainage is to be secured, artificial canalization 

 could probably be made more effective than forest 

 planting. Nevertheless, a sanitary influence exists, 

 as every one can experience, but it is mainly of a 

 negative character : absence of smoke, dust, obnox- 

 ious gases, and of strong winds which characterize 

 the air of cities, and which to some extent (at least 

 dust and winds) occur in the open, renders a forest 

 region more healthful. 



Furthermore, it has been found that forest air is 

 more free from pathogenic microbes. Especially 

 those bacilli which develop in the soil, like the 

 cholera, typhus, and yellow fever bacilli, find in 

 the forest soil less favorable conditions for develop- 

 ment, and, owing to the absence of strong winds, 

 are less apt to be carried into the air, where they 

 would be breathed by man. In fact, in the dense 

 forest, where the variation of soil moisture is small 

 and decomposing humus keeps the soil acid, no 

 pathogenic microbes have as yet been found. 

 Here r too, to be sure, the degree of effectiveness 

 must depend on the condition of the forest and 

 especially of the forest floor. 



It is also not impossible that the opening of 

 large swampy forest districts may improve health 

 conditions by changing moisture conditions; this 



