SUNSPOTS AND SOME OF THEIR PECULIARITIES. 67 



a spotless disc ; though, on the other hand, there were many 

 striking instances of apparent correspondence. At length I 

 decided that the only possible way of arriving at the solution was 

 to make a complete catalogue of all the magnetic disturbances 

 from 1882 onward (in which year Greenwich Observatory began 

 to publish reproductions of the traces of the chief storms) and 

 to ascertain the condition of the sun at the moment of the com- 

 mencement of each. To facilitate the comparison, I computed 

 the solar longitude of the centre of the sun's disc for the time of 

 commencement of each storm, and Table III. gives a small 

 extract from it. 



Table III. 

 List of Magnetic Disturbances. 

 1886, September— 1S87 , November. 



No. 



Character. 



Date of Commencement. 



Long, of 

 centre of 

 sun's disc. 



No. of 

 rotation 

 of sun. 







d. 



h. 



Deg. 





75 



Active 



1886, September 9 



13 



124-6 



440 



76 



Active 



October 6 



17 



126-0 



441 



77 



Moderate 



November 2 



15 



131-2 



442 



78 



Active 



30 



13 



123-3 



443 



79 



Moderate 



1887, February 12 



18 



225-6 



446 



80 



Moderate 



April 4 



14 



275-7 



448 



81 



Moderate 



August 1 



11 



143-4 



452 



82 



Moderate 



28 



20 



141-7 



453 



83 



Very active ... 



September 25 



16 



134-2 



454 



84 



Moderate 



October 22 



15 



138-5 



455 



The mystery was solved. Of the ten magnetic disturbances 

 that occurred in the 14 months from 1886, September, to 1887, 

 November, eight fell into two sets of four each, the members of the 

 same set occurring when the same meridian of the sun was near 

 the centre of the disc, that is to say, was fully presented to the 

 earth. When that meridian next came round to the same posi- 

 tion, the storm recurred. These two sets each showed a storm 

 occurring four times at intervals of a complete rotation of the 

 sun as seen from the earth. Further examination of the cata- 

 logue showed that similar recurrences were a feature of it. In 

 one particular instance, brought to light by a further examination 



F 2 



