2l6 



ARBORICULTURE 



In an extensive planting of belts and 

 groups of trees of suitable character. 



The laws governint^ the flow of water 

 in streams are well understood by engi- 

 neers and countervailing forces are often 

 employed to modify their influence ; but 

 it remains for us to devise and apply 

 methods which will have a similar cfTect 

 upon the wind, the laws governing which 

 being in many respects identical with 

 those which controll the movement of 

 water. 



There are a few powerful forces which 

 set the atmosphere in motion and give 

 direction and velocity to wind currents. 



I. 



Heat, expanding the atmosphere in 



track in direction of prevailing snow 

 storms. Without these countervailing 

 obstacles the snow would fill the cuts 

 and cause endless delays of traffic. 



On the lee of these fences the snow is 

 accumulated instead of filling the cuts. 

 Often two or more lines of fences are 

 maintained, the more thoroughly to pro- 

 tect the track within the cut. 



Combinations of these various forces 

 guide the currents of wind with varying 

 direction and force according to the [nc- 

 (lominating influences. At times a gentle 

 zephyr, again the terrible tornado. 

 Where uncontrolled as upon the ocean, 

 a shij) may lie becalmed for weeks, mak- 

 ing no ])rogress, and afterwards be car- 

 ried to destruction by monster waves 



A 



VARIOUS FOIJCKS ol'KUATINC TO CONTROL WIND. 



some localities, causes it to rise ; desig- 

 nated by the arrow, "a." 



ir. 



Cooler air flowing in 



to prevent a 



vacuum, 



HI. 



Natural obstructions, such as mountain 

 ranges or forests, which tletlect the cur- 

 rents from a direct course, "c." 



IV. 



An eddy or reverse current, moving in 

 a circle in opposite direction to the main 

 current, on the lee of any obstacle, "d." 

 • V. 



Gravity, pressing the strata of air io 

 the surface, "e." 



The principle of the eddy is shown in 

 the railway cut and is taken advantage 

 of bv engineers in Nr>rthern localities who 

 erect fences a short distance from the 



lashed into mountains by the typhoon. 



We may illustrate this movement i)y 

 an ideal sketch. No. i. 



The same principle explains the snow 

 drifts upon a farm or on llie roadway in 

 prairie countries. A i)lain board fence 

 and fre(iuently a four wire fence sim- 

 ply causes the wind to form an eddy 

 which deposits a drift of snow to such 

 depth as to blockade the highway. It is 

 often necessary to open the fields for the 

 I)ublic travel until the roads are cleared 

 of snow. 



Builders of windmills understand that 

 a high building or clump of trees, a hill 

 or any obstruction near the wheel will 

 prevent its successful operation when the 

 direction of the wind is in line with 

 such obstacle, although operating per- 

 fectly while the direction is transverse 



