WATER CIRCULATION AND ITS CONTROL 265 



that which runs off may be useful, but ing condensed is seen as a cloud. Blown 



is frequently so violent as to be harm- farther, the water particles are redis- 



ful ; that which sinks in flows slowly solved in warm air currents and the 



to the springs and streams and becomes cloud ends ; but as the conditions and 



available when most needed. changes are continuous the banner is 



Now man can send more water back not blown away. 



at once to the air and down to the A similar phenomenon may be ob- 



streams, or he can lead a larger pro- served over such regions as our great 



portion into the soil to enrich his crops, plains, where the column of rising air 



to feed springs, and to bear his boats becoming dilated and cooled forms a 



during low river stages, for he can strip rain cloud at great height, yet the fall- 



the ribbed rocks of soil, as witness ing rain is dissolved in the hot air below 



Spain, France, Italy, Greece and China, before it can reach the ground. No one 



or he can clothe the surface with forest who has seen the shower vanish into 



or verdure or thirsty fallow. thin air will question the effectiveness 



It is the object of this paper to con- of radiation from treeless plains under 

 sider the methods and efficiency of a summer sun to prevent precipitation, 

 man's control, through engineering. \Yho does not know a drying wind- 

 agriculture and forests. But before en- such an one as blows cold from the 

 tering upon that subject we may take Canadian snows and warming up 

 a general view of moisture in circula- drinks up moisture from every pore 

 tion along the several parts of the route of the surface? Or if you have crossed 

 that are commonly known as precipita- the Cascade Mountains of Oregon or 

 tion, ground water, evaporation, and Washington you know ho\v the wet air 

 surface flow. of the western ranges contrasts with 



the furnace dryness of the eastern 



PRECIPITATION plains. The mountains are wet because 



they are high, and they are heavily for- 



Air and moisture may be mixed, as ested because tbey are we t. But" there 



in clouds and mists, and some amount is a]so a rec i p rocal action of the forest 



of moisture is dissolved in the air, no on the wetness , f or the radiation from 



matter how dry it may seem. These the dark ?reen ex panse is comparatively 



two states of moisture in the air, uniform and promotes frequent and 



whether mixed or dissolved, depend steady rains Were the mountains bare 



chiefly on the temperature. Changes of they woukl Hke the bared sierras of 



the barometer change the temperature, Spain< rece i v e occasional but violent 



and thus pressure has an effect on the downpours and sen d down excessive 



condition. And for any particular tern- and disastrous flo . odSi ev en more disas- 



perature and pressure the air will hold trous than now 



a certain amount of dissolved moisture Qn the other hand< the pla}n? are drv 



and no more. 1 because they are low and situated 



Any effect which changes the tern- beyond the "heights in the path of 



perature of the air makes it either more moisture-bearing currents, and they are 



or less thirsty. A familiar instance is without forests because they are dry. 



the cloud banner which seems nailed to But here also there might be, yes. 



a mountain top, especially to a snow may be, a reciprocal action of vegeta- 



peak, in spite of a strong wind. In that tion on the dryness. For in so far as 



case a warm air current containing dis- we clothe the surface with green crops 



solved moisture is cooled by rising we lower the temperature of the rising 



along the slope or by contact with the air and favor precipitation on the ver- 



snow, and a part of the moisture becom- dure-covered plain. 



'Dissolved here means invisibly dispersed in the air. The physical or chemical nature 

 of solution is not in point. 



3 



