PNEUMATICS. 311 



ving an impulse from the great body of air de- 

 scending from the north, it goes round wholly to 

 the west, and becomes, as it were, an eddy of the 

 Trade Wind. 



These periodical winds are called Monsoons. 



The change, or the setting in of the monsoons, does 

 not happen all at once. In some places the shift- 

 ing of the wind is accompanied with calms ; in others 

 with variable winds, heavy rains, thunder, and vio- 

 lent storms. 



420. The tract from the parallel of 30 to the 

 Pole, in each hemisphere, is the region of variable 

 winds ; and their unsteadiness and violence seem 

 to increase, the nearer they approach the Polar 

 Circles. 



The irregularity of winds proceeds from inequalities in 

 the motion of the general currents above mentioned, 

 and from a variety of local causes ; also from the 

 chemical changes that are carried on in the air, 

 such as the solution and precipitation of moisture, 

 and the action of the electric fluid on the different 

 gases that compose the atmosphere. 



421. Sudden and strong gales of wind appear 

 almost always to arise from a diminution of the 

 weight of the air in the tract where the wind pre- 

 vails, and are accompanied, or preceded, by a fall of 

 the barometer. 



The 



