1873.] Messrs. De La Rue, Stewart, and Loewy on Sun-spots. 399 



XII. " On a tendency observed in Sun-spots to change alternately 

 from the one Solar Hemisphere to the other." By Warren 

 De La Rue,D.C.L.,F.R.S.,Balfour Stewart, LL.D.,F.R.S., 

 and Benjamin Loewy, F.R.A.S. Received June 12, 1873. 



1. Hitherto in our reductions we have summed up the spotted areas 

 of the various groups occurring on the sun's surface on any day, and 

 have regarded their sum as a representation of the spot-activity for that 

 day. It has occurred to us to see what result we should obtain by 

 taking instead for each day the excess of the spotted area in the one 

 solar hemisphere above that in the other. 



2. On adopting this method, it soon became evident that during periods 

 of great disturbance there is a tendency in spots to change alternately 

 from the north or positive to the south or negative hemisphere, and 

 vice versa, the period of such change being about 25 days. When, on 

 the other hand, the solar disturbance is inconsiderable, the spots do not 

 present any such systematic oscillation. 



3. We have graphically represented on the annexed diagram the 

 results derived from this method during three of the most considerable 

 periods of solar disturbance. 



In this diagram the observed values of hemispherical excess are marked 

 with an asterisk, and a curve is drawn so as to equalize their smaller 

 irregularities. The northern hemisphere is reckoned positive, and the 

 southern negative. The unit of area is, as before, the one millionth of 

 the sun's visible hemisphere. 



4. The first of these three periods extends from the beginning of 

 August to the end of December 1859. We derive from our diagram the 

 following Table, exhibiting the maximum amounts of hemispherical excess, 

 with their respective dates : — 



Hemispherical excess. 



Date, 



North. 



South. 



1859, July 31 



+4180 





Aug. 18 





(+ 40) 



Aug. 27 



+2580 





Sept. 11 





-2920 



Sept. 17 



+920 





Oct. 3 





-1420 



Oct. 16 



...... +1000 





Nov. 3 





-2480 



Nov. 15 



+ 120 





Nov. 20 





-1320 



Dec. 7 



+1050 





Dec. 22 . , 





-1400 



