12 



A 25-mile wind is not uncommon on the Plains, and since it causes 

 six times the evaporation that would occur in a calm at the same 

 humidity and temperature, one can easily understand the rapidity 

 with which the moisture from a summer shower disappears. Taken 

 the year through, the wind averages more than 10 miles an hour, 

 which is sufficient to cause four times the evaporation there would be 

 in a calm. 



An experiment made by King furnishes some interesting infor- 

 mation in regard to the checking of evaporation by a windbreak. He 

 used a modified form of the Piche instrument, placed so as to give the 

 evaporation from a water surface 1 foot above the ground at varying 

 distances from an oak grove. Taking the evaporation at 20 feet from 

 the grove as unity, the following results were obtained: 



Neither the height of the grove nor the rate of wind is given, 

 though the statement is made that a light wind was blowing. The 

 table shows that the evaporation at 200 feet from the windbreak was 

 41 per cent more than at its base, and at 300 feet 66 per cent more. 

 The evaporation from 300 feet onward was practically constant, 

 showing that to be the limit of the influence of the windbreak in 

 this experiment, 



While a few. accurate experiments have been made to determine 

 how far a windbreak is effective, it is a safe practical assumption 

 that it protects the ground for a distance equal to ten or fifteen times 

 its height some observers say a rod for every foot. If a large field 

 were crossed by a series of windbreaks 30 feet high and 20 rods apart, 

 there is no doubt that they would be very effective, since the wind 

 would reach each succeeding break with diminished force. 



WOODLOTS. 



In many situations it will be a paying investment for the farmer 

 to put out a small plantation, simply to produce his own wood for 

 fuel and other purposes. It is true that some time must elapse be- 

 fore the plantation begins to be productive, but by choosing rapid- 

 growing species and planting closely the thinnings which will be 

 necessary in a few years, even though the trees be small, will do for 

 the wood pile and help make the owner independent of coal famines. 



F. H. King, Bulletin No, 42, Agric. Exp. Sta., University of Wisconsin, 

 October, 1894. 

 [Cir. 161.] 



