r82 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. t;i 



systems of wind into two classes. The systems of the first order 

 are those which extend only over quite a limited part of the surface 

 of the earth, and which at the same time exhibit variations of veloc- 

 ity so great that we can neglect friction and the deflecting force 

 of the rotation of the earth. For example we mention tornadoes, 

 waterspouts, whirlwinds of smoke, etc. The systems of wind of the 

 second order are those in which all the acting forces have some impor- 

 tance. As examples we mention cyclones, the trade winds, the sea 

 breezes and land breezes. 



Considering the motion of the air in the systems of wind we dis- 

 tinguish between the permanent systems and the variable systems. 

 In a permanent system the pressure and the velocity at any place are 

 independent of time and vary only from one place to another. In 





7>m- 



Po +~*« ?o 



m »- 



-w« 



P -« m. n> 



? 



mt- 



W;/77//7//////77>7////?/?7/777/777} 

 FIG. 1 8 



77777777777777777777777777777777777777777777?, 

 FIG. 19 



nature we never find a permanent system, but we may consider 

 the systems of wind which remain nearly invariable for quite a 

 long time as permanent. As examples we mention the trade winds, 

 an immovable anticyclone or cyclone, with constant pressure at 

 the center. The variable systems of wind are divided into movable 

 systems and immovable or fixed systems. In the variable fixed 

 systems the minimum or the maximum barometer does not change 

 position with the surface of the earth, but its value varies with the 

 time. 



In our following studies we shall consider four simple systems of 

 wind. 



(1) System of ascending parallel winds. 



This system (see fig. 18) has rectilinear isobars, a barometric 

 minimum at the surface of the earth and a barometric maximum in 

 the higher strata. The air flows inward along the surface of the 



