WATER CIRCULATIOX AXD ITS COXTROL 



265 



that wliich runs olT may be useful, but 

 is frec|uently so violent as to be harm- 

 ful : that which sinks in flows slowly 

 to the s|)rinos and streams and becomes 

 available when most needed. 



X^ow man can send more water back 

 at once to the air and down to the 

 streams, or he can lead a larger pro- 

 portion into the soil to enrich his crops, 

 to feed springs, and to bear his boats 

 during' low river stag'cs, for he can strip 

 the ribbed rocks of soil, as witness 

 Spain, bTance, Italy, Greece and China, 

 or he can clothe the surface with forest 

 or verdure cr thirsty fallow. 



It is the object of this paper to con- 

 sider the methods and efficiency of 

 man's control, through engineering, 

 agriculture and forests. But before en- 

 tering upon that subject we may take 

 a general view of moisture in circula- 

 tion along the several parts of the route 

 that are commonly known as precipita- 

 tion, ground water, evaporation, and 

 surface flow. 



PKECIPITATION 



Air and moisture may be mixed, as 

 in clouds and mists, and some amount 

 of moisture is dissolved in the air, no 

 matter how dry it may seem. These 

 two states of moisture in the air, 

 whether mixed or dissolved, depend 

 chiefly on the temperature. Changes of 

 the barometer change the temperature, 

 and thus pressure has an efl:"ect on the 

 condition. And for any particular tem- 

 perature and pressure the air will hold 

 a certain amount of dissolved moisture 

 and no more.^ 



Anv effect which changes the tem- 

 perature of the air makes it either more 

 or less thirsty. A familiar instance is 

 the cloud banner which seems nailed to 

 a mountain top, especially to a snow 

 peak, in spite of a strong wind. In that 

 case a warm air current containing dis- 

 solved moisture is cooled by rising 

 along the slope or Ijy contact with the 

 snow, and a part of the moisture becom- 



ing condensed is seen as a cloud. Blown 

 farther, the water particles are redis- 

 solved in warm air currents and the 

 cloud ends ; but as the conditions and 

 changes are continuous the banner is 

 not blown away. 



A similar phenomenon may be ob- 

 served over such regions as our great 

 plains, where the column of rising air 

 becoming dilated and cooled forms a 

 rain cloud at great height, yet the fall- 

 ing rain is dissolved in the hot air below 

 l)efore it can reach the ground. X'^o one 

 who has seen the shower vanish into 

 thin air will (juestion the effectiveness 

 of radiation from treeless plains under 

 a summer sun to prevent precipitation. 



Wdio does not know a drying wind — 

 such an one as blows cold from the 

 Canadian snows and warming up 

 drinks u]) moisture from . every pore 

 of the surface? Or if you have crossed 

 the Cascade Mountains of Oregon or 

 Washington you know how the wet air 

 of the western ranges contrasts with 

 the furnace dryness of the eastern 

 plains. The mountains are wet because 

 they are high, and they are heavily for- 

 ested because they are wet. But there 

 is also a reciprocal action of the forest 

 on the wetness, for the radiation from 

 the dark green expanse is comparatively 

 unif(M-m and ]iromotes frequent and 

 stead v rains. \\'ere the mountains bare 

 thev would, like the bared sierras of 

 Spain, receive occasional but violent 

 downpours and send down excessive 

 and disastrous floods, even more disas- 

 trous than now. 



On the other hand, the plains are dry 

 because they are low and situated 

 beyond the heights in the path of 

 moisture-bearing currents, and they are 

 without forests because they are dry. 

 But here also there might be, yes. 

 may be, a reciprocal action of vegeta- 

 tion on the dryness. For in so far as 

 we clothe the surface with green crops 

 we lower the temperature of the rising 

 air and favor ])recii)itation on the ver- 

 dure-covered plain. 



'Dissolved here means imisibly dispersed in the air. 

 of solution is not in point. 



The physical or clieniical nature 



