166 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



the tangential force at the surface of the earth, expressed in milli- 

 meters per degree of the meridian. One should compare this force 

 with the gradients for horizontal movements in order to get a clear 

 idea of the force that acts in the vertical ascending movements. 



§16. Conditions of the existence of ascending and descending 



currents of air 



If the vertical currents preserve a steady motion, the pressure 

 within the currents and in the surrounding atmosphere must satisfy 

 certain conditions which we shall now consider. 



Ascending currents 



In ascending currents (fig. 12) the air enters along the surface of 

 the earth and consequently the pressure p\, of the atmosphere must 

 be greater than pressure p of the lowest part of the current; this 

 necessitates the existence of a barometric minimum at the surface of 



—K 



the earth. In the higher strata where the air flows out from the 

 vertical current, the pressure p in the current must remain greater 

 than the pressure p'oi the surrounding atmosphere and consequently 

 we shall find a barometric maximum at a certain elevation. 



We remark that great velocities may perhaps modify the phenom- 

 ena and that the air can flow outward even from a barometric min- 

 imum 7 but it is probable that in nature we shall always find baro- 

 metric maxima in such cases, because the velocities are slight at the 

 boundaries of the currents. 



7 By substituting 180 + a for a in the equations of §12, we shall have 



the formulae that belong to currents flowing from the center. This hypo- 



k r 

 thesis requires that v > ' 



