Land Covers to Conserve Moisture. 49 



potato culture; but while these lands are giving fair 

 yields of potatoes of good quality, they are in many 

 places suffering great injury from the destructive 

 effects of winds. On these lands, wherever broad, 

 open fields lie unprotected by wind-breaks of any 

 sort, the clearing west and northwest winds after 

 storms often sweep entirely away crops of grain after 

 they are 4 inches high, uncovering the roots by the 

 removal of from 1 to 3 inches of the surface soil. 

 It has been observed, however, that such slight bar- 

 riers as fences and even fields of grass afford a 

 marked protection against drifting for several hun- 

 dred feet to the leeward of them. 



"In the case of groves, hedge -rows and fields of 

 grass, their protection results partly through their 

 tendency to render the air which passes across them 

 cooler and more moist, and partly by diminishing 

 the surface velocity of the wind. The writer has 

 observed that when the rate of evaporation at 20, 

 40 and 60 feet to the leeward of a grove of black 

 oak 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, it was 14.5, 14.2 and 14.7 at 280, 

 300 and 320 feet distance, or 24 per cent greater 

 at the three outer stations than at the nearer ones. 

 So, too, a scanty hedge -row produced observed dif- 

 ferences in the rate of evaporation, as follows, dur- 

 ing an interval of one hour: 



"At 20 feet from the hedgerow the evaporation was 10.3 cc. 

 " 150 " " " " " " " 12.5 " 



" 300 " " " " " " " 13.4 " 



