348 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



from near the Thames a few miles east of London in a northwest direc- 

 tion, would probably have the effect of stopping a considerable amount 

 of fog, which often moves from the Essex marshes toward the metrop- 

 olis. It would somewhat increase the annual rainfall on its westerly 

 side. A wall stretching from northwest to southeast across some of 

 the heaths in the neighborhood of Woking would reduce the rainfall of 

 northeast Surrey and of London. 



The effect of a wall, like that of a perpendicular cliff, would be to 

 drive the impinging air vertically upward, so that the increased rain- 

 fall would take place near the wall and a little to leeward. 



Experimental barriers might be first erected across the mouths of 

 valleys open toward the west or southwest, for in many such situa- 

 tions a wall 1 or 2 miles long and 500 or 600 feet high would cause 

 increased precipitation near the ocean, and a considerably drier climate 

 in nearly the whole of the remainder of the valley. For example, a 

 wall across the valley, a little to the north of the town of Neath, would 

 reduce the rainfall of the Vale of Neath for a long distance, and many 

 of the Welsh valleys opening westward to Cardigan Bay might be 

 equally protected from excessive winter rains. 



With regard to other countries, there are localities where a structure 

 a few miles long based on rocks or ridges already some hundred feet 

 above the sea would prove very beneficial in reducing rainfall farther 

 inland. In other exceptional cases, where precipitation is deficient, it 

 might be promoted on the windward side by similar means. 



In parts of Australia, local rainfall might be appreciably increased 

 by raising the height of ridges. Wherever water is scarce and valu- 

 able and the climatic conditions favorable, experimental barriers would 

 give interesting results. 



Some American cities are very liable to be attacked and partially 

 destroyed by violent tornadoes or whirlwinds. These storms usually 

 proceed from about the same direction, and it might possibly be an 

 experiment worth making to set up a wall, say 300 feet high and 2 miles 

 long, on the dangerous quarter, with the object of breaking their force. 

 The clearing of forests seems to favor the development and progress 

 of American tornadoes by allowing the surface of the earth to become 

 more highly heated and by reducing friction, for they are caused 

 chiefly by the breaking of unstable equilibrium when the lowest strata 

 are highly heated and a cold current prevails within a few miles of the 

 earth's surface. 



