80 TREE PLANTER'S MANUAL. 



FORCE OF WINDS. 



This necessity becomes the more apparent when we consider the disastrous 

 effect of our prairie winds. Interesting experiments for the purpose of 

 ascertaining the changes in the rate of evaporation affected by the velocity 

 of the wind were made by Prof. T. Russell, Jr., of the signal service, in 

 1887. "The results of these experiments show that with the temperature 

 of the air at eighty-four degrees and a relative humidity of fifty per cent, 

 evaporation with the wind blowing at the rate of five miles an hour, was 

 2.2 times greater than at calm; at ten miles, 3.8; at fifteen miles, 4.9; at 

 twenty miles, 5.7; twenty-five miles, 6.1; and at thirty miles per hour the 

 wind would evaporate 6.3 times as much water as a calm atmosphere of the 

 same temperature and humidity. Now, if it is considered that the winds 

 which sweep the western sub-arid and arid plains is from ten to fifteen 

 miles, not rarely attaining a maximum of fifty or more miles, the cause of 

 the aridity is not far to seek and the function of the timber belt or even 

 simple wind-break can be readily appreciated." 



Not only does the wind dissipate the water, but it falls upon it with 

 weight, often uncovering our grains in the early spring, beating our plants 

 down and hardening the ground to the great injury of all our vegetations. 

 Smeaton, in his ''Philosophical Transactions," gives us some figures as to 

 the force of wind, at special rates, on one square foot of avoirdupois 

 pounds. Thus, the force or weight of a wind blowing at thirty miles an 

 hour is 4.429 pounds; at forty miles an hour, it is 7.873 pounds; at one 

 hundred miles an hour, a hurricane, it is 49.200 pounds per square foot, 

 carrying trees and buildings before it. 



VALUE OF ACCUMULATION. 



While the surface exposed determines the amount of evaporation from 

 water courses and reservoirs, it is found that the smaller and slower run 

 loses proportionally more than the larger, illustrating the value and pro- 

 tection of accumulation. A like ratio applies to a forest. If the trees are 

 scattered and far apart, or great gaps obtain by clean-sweep cuttings and 

 fi res _ as is the resultant condition of our native forests to a large extent 

 the evaporation is, of course, far greater than where the trees stand close 

 and high. 



FOREST EVAPORATION. 



Owing to cover conserving moisture and the wind-breaking power of the 

 trees, the evaporation from the forest is considerably less than from the 

 plants in our gardens and fields. The annual evaporation within a forest 

 is estimated on data of experimentation at about half that of the open 

 field. The forest, therefore, is nature's best economist, steady in action, 

 giving when most needed. Evaporation is one of its distributive forces. 

 "A forest through its leaves," says L. H. Wilcox, "gives far more moisture 

 to the air than the same area covered with water" (about three times 

 more), and in Minnesota it measures among the millions of tons of water. 



But plants do not live by water alone, water serves them as a solvent, 

 dissolving and preparing substances in the air and ground for vegetation. 



