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ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1931 



With the reappearance of the last sun-spot cycle it was firmly 

 established that the polarity of these spots completely reverses from 

 one cycle to the next.^ It has been recently suggested by a Nor- 

 wegian scientist, Bjerknes * that the sun spots are the visible ends of 

 a tubular vortex which may extend east and west for great distances 

 below the sun's surface. A reversal in the direction of whirl in 



FicuKE 4. — Tropical storms on the earth frequent latitudes correspond- 

 ing to the latitudes of many major spots on the sun 



this supposed vortex would account for a reversal of the magnetic 

 polarity of the sun spots with the change of cycle. 



No completely satisfactory explanation of the ultimate origin of 

 these whirls has yet been made. There is one peculiarity, however, in 

 the sun's behavior which doubtless has an important bearing on this 

 point. While the sun rotates on its axis from west to east in common 

 with the axial rotation of other bodies in the solar system, its period 

 of rotation is not the same for different parts of the solar surface. 



» Idem, vol. 49, p. 153, January to June, 1919. 

 *Idem, vol. 64, p. 93, July to December, 1926. 



