214 THE PRINCIPLES OF SOIL MANAGEMENT 



wind-breaks, results by King show that when the rate 

 of evaporation at 20, 40 and 60 feet to the leeward 

 of a black oak grove 15 to 20 feet high was 11.5 cc, 11.6 

 cc, and 11.9 cc, respectively from a wet surface of 27 

 square inches the evaporation was 14.5, 14.2 and 14.7 

 cc, at 280, 300 and 320 feet distant, or 24 per cent 

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

 scanty hedge-row reduced evaporation 30 per cent at 

 20 feet, and 7 per cent at 150 feet, below the evapora- 

 tion at 300 feet from the hedge. 



On sandy soil, wind-breaks prevent the blowing of 

 the dry surface soil, which would expose a fresh surface 

 of wet soil from which evaporation would be increased. 



The glass house reduces evaporation by preventing 

 winds. Some crops are grown only in the shade of other 

 crops, where they are not only protected from the sun 

 but from evaporation by the stagnating effect of the 

 surrounding vegetation on the atmosphere. Grass pro- 

 tects the surface of the soil from evaporation, acting 

 like a mulch. The largest application of this principle 

 is in the tents used in growing wrapper tobacco in Florida 

 and the Connecticut valley, and, to a less extent, for 

 other special crops in various parts of the country. The 

 most common form of the tent is a frame eight or nine 

 feet high, over which is spread a loosely woven cloth 

 cheese-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 per hour, 

 and 85 per cent when the outside velocity was twenty 

 miles per hour, there being a small regular decrease in 



