TEE CONTROL OF SOIL MOISTURE 285 



monly grown or retained for this purpose. Wooden fences 

 and walls of one sort or another have a similar effect. 

 Windbreaks composed of growing plants have the dis- 

 advantage that for a considerable distance beyond the 

 spread of their branches their roots penetrate the soil 

 and use the moisture, which is one reason for the smaller 

 growth of crops near trees. Bearing on the efficiency of 

 windbreaks, results by King^ show^ that when the rate 

 of evaporation at twenty, forty, and sixty feet to the 

 leeward of a black oak grove fifteen to twenty feet high 

 was 11.5, 11.6, and 11.9 cubic centimeters, respectively, 

 from a wet surface of twenty-seven square inches, the 

 evaporation was 14.5, 14.2, and 14.7 cubic centimeters, 

 at two hundred and eighty, three hundred, and three 

 hundred and twenty feet distant — or 24 per cent greater 

 at the outer stations than at the inner ones. A scanty 

 hedgerow reduced evaporation 30 per cent at twenty 

 feet and 7 per cent at one hundred and fifty feet, below 

 the evaporation at three hundred feet from the hedge. 



Very often tent shelters are used in the growing of 

 tobacco. The commonest form of the tent is a frame 

 eight or nine feet high, over which is spread a loosely 

 woven cloth. Investigations by Stewart ^ in Connecticut 

 showed : (1) That the tent greatly reduced the velocity 

 of the wind. This reduction amounted to 93 per cent 

 when the outside velocity was seven miles an hour, and 

 85 per cent when the outside velocity was twenty miles 

 an hour, there being a small regular decrease in relative 

 efficiency with increased velocity of the wind. (2) The 

 relative humidity under the tent was higher than outside, 



1 King, F. IL The Soil, p. 205. New York. 1906. 



2 Stewart, J. B. Effects of Shading on Soil Conditions, 

 U. S. D. A., Bur. Soils, BuL 39. 1907. 



