

ACTICAL ARBORICULTURE 



Siroccos, hurricanes, tornadoes, cyclones and wind currents of every 

 character have 1; s governing their movements, and such storms may be con- 

 trolled or modified to great extent by proper efforts upon the part of people 

 who reside in the locations where they prevail, by an extensive planting of 

 belts and groups of trees of suitable character. 



The laws governing the flow of water in streams are well understood by 

 engineers 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 effect upon the wind, the laws governing which being in many 

 respects identical with those which control the movement of water. 



There are a few powerful forces which set the atmosphere in motion and 

 give direction and velocity to wind currents. 



Heat, expanding the atmosphere in some localities, causes it to rise. 



Cooler air flowing in to prevent a vacuum. 



Xatural obstructions, such as mountain ranges or forests, which deflect 

 the currents from a direct course. 



An eddy or reverse current, moving in a circle in opposite direction to the 

 main current, on the lee of any obstacle. 



Gravity, pressing the strata of air to the surface. 



DOUBLE LINE OF SNOW FENCES ON RIO GRANDE WESTERN RAILWAY, UTAH. 



The principle of the eddy is shown in the railway cut and is taken advan- 

 tage of by engineers in northern localities who erect fences a short dis- 

 tance from the 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 protect the track within the cut. 



