312 Royal Society. 



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 con- 

 siderable 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 maxi- 

 mum 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 

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

 the causes which produce them, we might here find some explana- 

 tion of the magnetic period of 26 days, the difference of spot-area 

 in one hemisphere from that in the other being related to a differ- 

 ence of the solar magnetic action. 



In order to determine whether such a connexion existed, I pro- 

 jected 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 horizontal force of the earth's magnetism during the 

 same periods. The conclusion from these projections is, that there 

 is no relation whatever between the two classes of curves. 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 va- 

 riations 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 re- 

 latively 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, giving each spot a weight in pro- 

 portion to its area, the variation of these positions in latitude 

 and longitude, and their weights, might give a more satisfactory 

 base for this comparison and for other deductions. 



It will be obvious also that this investigation refers only to the 

 visible hemisphere of the sun ; an approximation to the spot-dis- 

 tribution on the other hemisphere, however, will be frequently pos- 

 sible. 



* Proc. Koy. Soc. toI. xxi. p. 399. 



