FOREST INFLUEN^CES. 25 



earth and at different times of the year; the distribution 

 of land and water areas, which produces a difference of in- 

 solation because the water has more heat capacity than the 

 land, and which also influences the direction of air- and sea- 

 currents ; the configuration of the earth, by which the den- 

 sity of the atmosphere is made unequal, and in consequence 

 of which differences of insolation and of air temperature are 

 induced. Thus we have not only climatic zones, but also 

 continental climates and mountain climates in opposition 

 to coast climates and plain or valley climates. 



*^ While this classification of cosmic climates satisfies the 

 climatologist, there are many local climates to be found 

 within the range of the cosmic, and the local climatic condi- 

 tions are those which affect human life and human occupa- 

 tions most sensibly. 



''The same causes, different only in degree, which modify 

 the cosmic climates, making a classification of the same 

 possible, effect further modifications and give rise to local 

 climates ; these causes are different in the degree of insola- 

 tion, obstruction to air-currents, presence of water surfaces, 

 or moisture-laden air strata. 



"Among the factors which thus modify the cosmic climate 

 and help to produce a local climate differing from other 

 local climates, the soil cover, and especially the presence of 

 forest areas, is claimed as one that, under certain conditions, 

 is potent: and this factor, being under the control of human 

 agency more than any other possible modifier of climate, 

 must therefore be of greatest interest to us. It is clear, 

 from what has been stated so far, that the influences of the 

 forest, if any, will be due mainly to its action as a cover 

 protecting the soil and air against insolation and against 

 winds. That the nature of a cover, its density, thickness, 

 and its proper position has everything to do with the 

 amount of protection it affords, everybody will admit. A 

 mosquito-net is a cover, so is a linen sheet or a woollen blan- 



