

1873.] Mr. J. A. Broun on Sun-spots and Terrestrial Magnetism. 43 



The following communications were read : 



I. "On the Period of Hemispherical Excess of Sun-spots, and 

 the 26-day Period of Terrestrial Magnetism." By J. A. 

 BROUN, F.E.S. Received September 13, 1873. 



It appears from the interesting communication to the Eoyal Society, 

 June 19th, by Messrs. De La Eue, Stewart, and Loewy*, that the 

 difference of the area of spots on the visible northern and southern 

 quarter-spheres of the sun seems, during periods of considerable solar 

 disturbance, to obey a law such that the difference is a maximum in the 

 same quarter-sphere during several successive rotations of the sun, the 

 difference being a maximum alternately in the northern and southern 

 hemispheres the time from maximum to maximum, for the same 

 hemisphere, being variable between 18 and 32 days, but having a mean 

 value of about 25' 2 days. 



It occurs at once that if the variations of the mean terrestrial mag- 

 netic force are connected in any way with the solar spots, or the causes 

 which produce them, we might here find some explanation of the mag- 

 netic period of 26 days, the difference of spot-area in one hemisphere 

 from that in the other being related to a difference of the solar magnetic 

 action. 



In order to determine whether such a connexion existed, I projected 

 first the curves of excess of spot-area given in the paper of Messrs. 

 De La Eue, Stewart, and Loewy, and below them the daily mean hori- 

 zontal force of the earth's magnetism during the same periods. The 

 conclusion from these projections is, that there is no relation ivhatever 

 between the two classes of cwves. The maxima and minima of the one 

 agree in no way with those of the other : the greatest excesses of sun- 

 spot area in the one hemisphere over those in the other occur when the 

 earth's magnetic force is the most constant ; the greatest variations of 

 the earth's magnetic force from the mean occur in several instances 

 when the sun-spot area is equal in the two visible quarter-spheres. 



It should be remembered, in considering the curves of sun-spot excess, 

 that the minima and maxima are in some cases only relative, sometimes 

 the one, sometimes the other being really cases in which there is neither 

 maximum nor minimum that is to say, cases in which the sun-spot area 

 is equal, or nearly so, in the two visible quarter-spheres. 



It would be hasty to conclude from this comparison that the variations 

 of the mean magnetic force are really unconnected with the mode of 

 distribution of the sun-spots. Other methods of grouping the spots 

 may perhaps be employed with advantage relatively to this and other 

 questions ; for example, were the position of the centre of gravity of 

 the sun-spots determined for the visible quarter-spheres and hemisphere, 



* Proc. Eoy. Soc. vol. xxi. p. 399. 



