FORESTS AND WATER SUPPLY. 31 



"We must therefore enter into our discussions acknowledg- 

 ing ignorance of one of the most important factors, at least as 

 to its numerical or quantitive value. 



"The distribution of the circulating water capital is influenced 

 by various agencies. The main factor which sets the capital 

 afloat is the sun, which, by its heat and the air currents caused 

 by it, and by the rotation of the earth, produces the evaporation 

 which fills the atmosohere with vapor. Anything, therefore, 

 that influences the intensity of insolation, the action of the sun, 

 or obstructs the passage of winds, must influence the local dis- 

 tribution of the water capital. The great cosmic influences 

 which produce the variability of all climatic conditions, and 

 therefore also of the circulating water capital, are the position of 

 the earth's axis to the sun, by which the angle and therefore the 

 heat value of the sun's rays vary in different parts of the earth 

 and at different times of the year; the distribution of land and 

 water areas, which produces a difference of insolation because 

 the water has less heat capacity than the land, and which also 

 influences the direction of air and sea currents; the configura- 

 tion of the earth, by which the density of the atmosphere is 

 made unequal, and in consequence of which differences of inso- 

 lation 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 cli- 

 mates. 



"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 conditions are 

 those which affect human life and human occupations most sen- 

 sibly. 



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

 the cosmic climates, making a classification of the same possi- 

 ble, effect further modifications and give rise to local climates; 

 these causes are different in the degree of insolation, 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: 



