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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 51 



panied by a movement of the ascending or descending vertical cur- 

 rent, and the cause of this is due to the heterogeneity of the air that 

 enters the barometric minimum either at the surface of the earth or 

 in the upper strata. The alimentary air on entering, produces a 

 new vertical current at the same time that the supplementary air 

 suppresses the existing current, and consequently the vertical cur- 

 rent moves in advance of the barometric minimum and causes its 

 change of position. When the barometric minimum is situated 

 in the upper strata its movement is accompanied by the movement 

 of the barometric maximum at the surface of the earth, and in- 

 versely. 



In any movable system of wind the pressure at any point what- 

 ever varies with the time and this variation of pressure is closely 

 connected with the velocity of propagation of the barometric min- 

 imum or of the central calm region. 



Let x and y be the coordinates of any point, whatever; $ and rj 

 the coordinates of the movable origin which represents the baro- 

 metric minimum; we can generally express the pressure as a function 

 of the location and the time, or 



p = f(x - $,t) - y,t) 



Differentiating we shall have 



dp dp <i£ dp drj 

 dt ds dt drj dp 



dp 

 dt 



(1) 



FIG. 31 



Denoting the velocity of propagation the [movement of the mini- 

 mum] by W and its angle with the axis X by /? (see fig. 31) the 

 gradient by G and the angle of the direction of the gradient with the 



