MOUNT WILSON OBSERVATORY. 



223 



The following table, prepared by Mr. Nicholson, indicates the polari- 

 ties of sun-spots observed in the northern and southern hemispheres 

 during the calendar year 1919 •.'^ 



SUN-SPOT HYPOTHESES. 



The various phenomena of the solar atmosphere, exhibiting some 

 analogies and many sharp contrasts when compared with their nearest 

 terrestrial equivalents, offer problems of extreme complexity. In 

 attempting to deal with these problems, it is much easier to subject 

 old hypotheses to destructive criticism than to substitute new ones 

 that satisfy all the requirements. In his investigations of the nature 

 of sun-spots, the Director has therefore made no effort to develop a 

 general theory, or even to establish a particular hypothesis accounting 

 for a part of the observed phenomena. The less ambitious plan has 

 been adopted of selecting for comparative study three working hypothe- 

 ses, two of which have been devised in the course of this investigation. 



Each of these hypotheses assumes, in harmony with the indications 

 of observation, that two vortices are involved in the system of a single 

 spot, the first representing the spot proper and giving rise to its mag- 

 netic field, the second lying above it, accounting for the pattern of the 

 hydrogen flocculi showTi by the spectroheliograph. As 60 per cent of 

 all spots are bipolar groups, while the great majority of other spots 

 show tendencies toward the double type, a complete hypothesis must 

 also explain this remarkable characteristic, and at the same time 

 account for the complex groups of mixed polarities which sometimes 

 appear on the sun. This demand, coupled with the ultimate necessity 

 of explaining the regular variation of spots in latitude dming the 

 eleven-year cycle, the law of magnetic polarities, and, to mention no 

 other general features, the sun-spot period itself, sets a sufficiently diffi- 

 cult task to the student of solar physics. When we reflect on the slow 

 progress of terrestrial meteorology, however, we may rest content for 

 the present with an unprejudiced examination of working hypotheses 

 that may account for even a small percentage of the intricate phenom- 

 ena presented by sun-spots. 



^In thf last annual report, on p. 229, the total number of spot-groups observed during the year 

 should have been given as 394 instead of 314. The number of regular polarities in the table on 

 p. 230 should have been given as 171, 187, and 358, instead of 131, 147, and 278 respectively. 



