166 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1923 



Sunspots, if looked at casually, appear to differ widely in their 

 form and behavior, but a little systematic observation shows that, for 

 the most part, they conform in their history to a single type. As an 

 illustration we may take the group which attained its maximum de- 

 velopment in March, 1920, and of which the Rev. A. L. Cortie has 

 summarized the life history in the Monthly Notices, 80, 574^-578, 

 where it is accompanied by seven drawings of its appearance. This 

 type is that of a more or less regular stream, of which the first and 

 last spots are usually the largest, best denned, and the most stable. 

 The straight line joining the two chief spots — sometimes distin- 

 guished as the " leader " and " trailer " — was in this case parallel to 

 the sun's equator. 



Professor Hale has shown that all spots contain magnetic fields, 

 and that the strength of this field (up to a certain maximum) in- 

 creases with the diameter of the spot ; and that the polarities in any 

 one cycle follow a definite law with respect to the position of the 

 spot on the sun according as it is north or south of the equator. If 

 the sign of the dominant charge remains always the same, then 

 opposite polarities may be regarded as representing opposite direc- 

 tions of whirl. Now in streams in which the " leader " and the 

 " trailer " are dominant, Professor Hale has pointed out that these 

 two are of opposite polarities; that is, of opposite directions of 

 whirl. Where, then, we have two " bipolar " streams, one north and 

 one south of the solar equator, we find that the two leaders have 

 whirls in directions opposite to each other, and, similarly, the two 

 " trailers " have whirls opposite. Further, during the cycle ending 

 in 1913, the leader spots in the northern and the trailer spots in the 

 southern hemisphere were of negative polarity; the sign of the 

 polarity being positive for the southern leaders and northern trailers. 

 But during the cycle after 1914, these relative whirls were inter- 

 changed, the northern leaders and southern trailers becoming positive 

 and the southern leaders and northern trailers negative, until June 

 24, 1922, when a small single spot (probably a leading spot whose 

 trailer had become invisible) in north latitude 31°, was found to have 

 south or negative polarity. This was the first indication that the 

 new cycle just beginning was again experiencing a general reversal 

 of polarities. 



Thus during the minimum years of 1913-14 and of 1922-24, we 

 have not only the high latitude spots of the new cycle overlapping the 

 low latitude spots of the old, but also four spot zones, characterized 

 by distinct magnetic polarities coexisting together. But the general 

 magnetic field of the sun shows no reversal of polarity. 



Professor Hale also points out (Contributions from Mount Wilson 

 Solar Observatory, No. 165) that the inclination of the axis of the 

 spot stream, which is in general represented by the line joining the 



