146 PRACTICAL ARBORICULTURE 



Combinations of these various forces guide the currents of wind with 

 varying direction and force according to predominating influences. At times 

 a gentle zephyr, again the terrible tornado. Where uncontrolled as upon 

 the ocean, a ship may lie becalmed for weeks, making no progress, and after- 

 wards be carried to destruction by monster waves lashed into mountains by 

 the typhoon. 



The same principle explains the snow drifts upon a farm or on the road- 

 way in prairie countries. A plain board fence and frequently a four wire 

 fence simply 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 public to 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 operat- 

 ing perfectly while the direction is transverse to such lines. Here the eddy 

 or whirling removes the pressure from the wheel and also causes the vane to 

 vacillate. 



In the Ohio River, opposite the boyhood home of the author, is an im- 

 mense sand bar, caused by a projecting ledge of rocks from the limestone hills 

 of the Kentucky shore. In former years this bar extended entirely across the 

 river, both above and below the town of Rising Sun, very shallow water cov- 

 ering that portion called the channel, and steamboats were grounded on this 

 bar with frequency. Engineers have constructed two dams projecting from 

 the Indiana shore and one from the Kentucky side, in such manner as to con- 

 fine the current so that the force of the stream deepens the channel, in order 

 that boats may navigate more safely. 



Exactly the same principle governs the wind currents. A canyon in the 

 mountains diverts the wind, directs its course, and at times increases its 

 velocity. A range of high mountains or sharp hills deflect it upwards, while 

 belts of trees perform the same service. 



Locomotive engineers inform me that in time of strong head or side 

 winds these have a decided influence upon the train, retarding its progress, 

 and when a belt of timber intervenes the strain upon the engine is relieved 

 and the train shoots ahead with increased velocity. 



The whirlpool phenomenon in the Mississippi River, at Grand Gulf, Miss., 

 which fifty years ago was noted as a most extraordinary instance of this 

 reverse current and which gave the name to the then important locality, "Grand 

 Gulf," will be recognized by old river men, who knew the place in ante- 

 bellum times. 



The river at that time occupied the great bend, which, since the "cut-off" 

 and change in the river bed, has become an inland slough and island. The 

 Big Black River empties into the Mississippi at this point, while a hard clay 

 point projected from the Louisiana shore. The combination of forces during 

 periods of high water caused this vast pool to whirl violently. Flat boats 

 were often caught in this eddy, and it was with great difficulty they were 

 gotten out into the regular channel. The author, in his boyhood, has been in 



