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CHAl'TER 11. 



MOTION OF AIR. 



SECTION I. 



WINDS. 



Wind is air in motion. Its force depends on its 

 speed. When its motion is slow, it constitutes the 

 soft, gentle breeze. As the velocity increases, the force 

 becomes greater, and the strong gale sweeps round the 

 arms of the wind-mill with the strength of many horses, 

 and huge ships are driven swiftly through the waves 

 by its pressure. By a still greater velocity of the air, 

 its power becomes more irresistible, and solid buildings 

 totter, and forest trees are torn up by the roots in the 

 track of the tornado. 



The force of wind increases dh*ectly as the square 

 of the velocity. Thus a wind blowing ten miles an 

 hour exerts a pressure four times as great as at five 

 miles an hour, and twenty-five times as great as at two 

 miles an hour. The following table exhibits the force 

 of wind at different degrees of velocity : 



Description. 

 Hardly perceptible. 

 Just perceptible. 



Light breeze. 

 Gentle, pleasant wind. 



