1910] on Magnetic Storms. 7<S8 



indicative of mao'netic disturbance. The followinsr davs were con- 

 siderably disturbed, August 29, 80, September 21, 25, 80, and October 

 18, 19, 28, 24 ; while a variety of other days, e.g. August 31 and 

 October 2, 8. and 9, were decidedly more disturbed than the average. 

 If we associate August 30 and September 25 we get a 26-day period, 

 if we associate August 29 and September 25, or September 21 and 

 October 18, we get a 27-day period, if we associate August 81 and 

 September 30 we get a 80-day period, and we have any number of 

 other possible combinations left. It should also be mentioned that 

 disturbed conditions are seldom limited to a few hours of a particular 

 day, and often extend over two or more days. Thus there is usually 

 a good deal that is arbitrary in the value deduced by observation for 

 the interval between two specified storms. 



One fact to which attention may be drawn is that the disturbances 

 of September 21, 25 and 80 led to a fall in the horizontal force, from 

 which it is doubtful whether the element had entirely recovered even 

 by the middle of November. 



§ 17. Mr. Maunder and Dr. Schmidt both associate their periods 

 with that of the revolution of the sun relative to a point on the earth. 

 This period exceeds the true period of the sun's rotation - which 

 varies considerably with solar latitude — because the earth is travelling 

 round the sun in the dii-ection in which the sun rotates. 



The view most in favour at the present time as to the source of 

 magnetic storms is that they are due to some solar dischaige, probably 

 from sun-spot areas and of an electrical nature. We may suppose 

 a solar discharge to traverse space like a jet of water ; when it 

 overtakes the earth a magnetic storm begins, which continues until 

 the full width of the jet has passed over. If the solar discharge 

 continues long enough, it may sweep over the earth during several 

 successive revolutions of the sun, and so give rise to a series of 

 magnetic storms at nearly equal intervals. 



Theories accepting a solar origin for magnetic storms differ as to 

 the nature of the solar discharge. 



Nordmann has suggested Rontgen rays, Birkeland cathode rays, 

 and Arrhenius negatively charged particles. On Nordmann's hypo- 

 thesis the terrestrial phenomena should follow the solar in a few 

 minutes, on Birkeland's hypothesis in a few hours, while according to 

 Arrhenius, the interval might be two days or more. 



From time to time observations have been reported favourable to 

 the several hypotheses. Thus Dr. J. S. Lockyer* has recently asso- 

 ciated the great storm of September 25 with " tremendous activity " 

 observed on the previous day at South Kensington, in a flocculus 

 associated with a sun-spot then visible. The greatest solar activity, 

 which according to Lockyer was " quite unique in the records made 

 with the spectroheliograph " at South Kensington, was about 10 a.m. 



* E. A. S. Notices, November 1909, p, 12. 



A'OL. XIX. (No. 104) 8 F 



