Forest Planting in Eastern Nebraska. 185 



the protection they afford from strong, southerly winds by checking 

 evaporation. Not only common experience attests the effectiveness of 

 forest belts in this regard, but the point has been demonstrated by ex- 

 perimentation. 



In the first place, it has been shown by experiments that there is a 

 close relation between wind velocity and the rate of evaporation. The 

 United States Signal Service has made investigations on this part, and 

 their observations prove conclusively that the rate of evaporation in- 

 creases very rapidly with increase in the velocity of the wind. Taking 

 the rate of evaporation in a perfect calm as unity, it was found that 

 ■when the velocity of the wind was ten miles per hour, the rate of 

 evaporation increased to 3.8; to 5.7, when the wind velocity per hour 

 was twenty miles, and to 6.3 when the wind velocity was thirty miles 

 per hour. 



The importance of any agency therefore that will retard the ve- 

 locity of the wind and thus check evaporation becomes apparent at 

 once. That windbreaks will do this has been demonstrated. Card and 

 Emerson, at the Nebraska Station, made quite an exhaustive study of 

 this question in 1896, in which the ability of windbreaks to retard 

 evaporation was clearly demonstrated. In one of their experiments, 

 for example, conducted during the night of August 3, 1896, when there 

 was a strong south wind, with a high temperature and a low humidtiy, 

 the evaporation was 71 per cent less three rods north, and 33 per cent 

 less twelve rods north of a forest belt than twenty rods south. The 

 "belt consisted of twenty-two rows of trees, running east and west. 



Experiments by these investigators, as also by King of the Wis- 

 consin Station, show that the relative humidity of the atmosphere and 

 the per cent of water retained in the soil are greater to the leeward of 

 windbreaks than elsewhere. 



As to the width of the zone affected by forest belts, Newell, in speak- 

 ing of the influence of windbreaks on the plains, in "Irrigation," says: 

 "It is estimated that every foot of height of compact trees protects a 

 rod; hense a Lombardy poplar windbreak of an average height of sixty 

 feet, properly set out, has a beneficial influence extending practically 

 1,000 feet to the leeward." 



It is interesting in this connection to note that the United States 

 "Weather Bureau and the United States Eorest Service are planning a 

 series of experiments on a comprehensive scale to determine the rela- 

 tion of , windbreaks to crop production. 



COMMERCIAL PLANTING. 



From the data gathered in the field season of 1904, the United 

 . States Forest Service is in a position to give intelligent advice as to the 



