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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 51 



By the aid of these formulae and t he equations of § 16 we have 

 calculated the following tables. 



Table II. Descending currents 



In whirlwinds of small dimensions we can neglect the action of 

 the rotation of the earth. Assuming the altitude / of the horizontal 

 current to be very small it is necessary to attribute to the coefficient 

 of friction a large value and consequently the air enters into the 

 current almost radially. In this case denoting by r the radius of 

 the whirl, the equation of continuity gives 



2K r I v = 71 r 2 w 



and supposing v = w we shall have r = 2 I. 



In this case the radius of the whirlwind will be equally small, as 

 is proved by observations of whirls of smoke, the whirls of dust over 

 roads, and whirls of sand over deserts. In order to calculate the 

 horizontal velocity we neglect the work of friction, because the dis- 

 tance traversed is very short, then we shall find 



^o= >j 



,Po -P< 



If we suppose the altitude z to be less than iooo m , we can develop 

 f(z) as determined by equation (3) of §16 in a series and introduc- 



