PRACTICAL ARBORICULTURE 145 
Siroccos, hurricanes, tornadoes, cyclones and wind currents of every 
character have laws 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. 
Natural 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 anv 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. 
