MOUNT WILSON OBSERVATORY. 225 



Mr. St. John's spectroscopic investigations of the solar rotation, 

 which have now been continued for some years, give a mean linear 

 velocity at the equator of 1.93 km. per second, with slight irregular 

 fluctuations. He sees no reason to believe in a progressive change 

 from year to year, and is inclined to attribute differences between 

 observers to systematic errors (p. 241). 



Daily observations of the magnetic polarities and field-strengths of 

 all sun-spots have been made, as in previous years, with the 150-foot 

 tower telescope by Messrs. Ellerman, Nicholson, Pettit, Hoge, and 

 Benioff. Special mention should be made of the great spot of last 

 ]May, which was notable for its size, its position exactly on the equator, 

 its eruptive activity, its mixed polarity, and its connection with the 

 brilliant auroras and violent magnetic storms which occurred while 

 the spot was crossing the disk. All of the evidence favors the view that 

 the terrestrial phenomena were due to the eniptions which took place 

 repeatedly in the area surrounding the spot. Although appearmg to 

 the eye as an ordinarj^ bipolar group, the spot was shown by our 

 observations of the Zeeman effect to comprise two large areas of oppo- 

 site polarity in each of its chief membere. As any direct magnetic 

 effect at a distance would thus be largely annulled, the evidence against 

 appreciable influence on the earth, already sufficiently^ conclusive, is 

 still further strengthehed by this interesting case (p. 235). 



The writer's study of the nature of sun-spots has included the 

 further examination of the three spot hjiDotheses outlined in the last 

 report; the search for an electric field, now almost conclusively settled 

 in the negative (p. 288) ; and the detailed investigation of the Zeeman 

 effect in the spot spectrum. jVIuch evidence may be assembled (p. 237) 

 in support of the deep penetration of the spot-vortex, which would rule 

 out the shallow vortex called for by the second hj^Dothesis. The 

 requirement that the length of the vortex shall be of the same order 

 as the diameter of the spot, which was deduced by Stormer from his 

 theoretical investigation of the direction of the lines of force, is strongly 

 opposed to this hypothesis. Dr. Russell has also shown that to account 

 for the low temperature of spots by expansion, the ascending gases 

 must have come from depths where the temperature ranges from 

 10,000° to 20,000° C. (p. 240). Detailed investigation of the Zeeman 

 effect in the spot spectrum has involved the measurement, by Dr. H. C. 

 Wilson and Miss Mayberry, of displacements on photographs taken 

 with Nicol and quarter-wave plate of about 6,500 lines in the regions 

 X3900 to X4700 and X5200 to X5300. The vrork is being continued 

 by Miss Mayberry and the writer toward the red, about 700 lines 

 beyond X6100 having been measured thus far. The peculiar displace- 

 ments of the p-components of spot triplets mentioned in previous 

 reports may be due in part, if not exclusively' , to the mutual influence of 

 closely adjoining lines rather than to some abnormality of the Zeeman 

 effect; but this question is not yet settled (p. 237). 



