Magnetic Disturbances at Greenwich. 315 



S. min. years can hardly be assigned in any large degree to 

 the fact that M storms form a larger percentage of the whole 

 in the latter than in the former group. 



§ 9. Returning to the subject of the diurnal inequality, 

 the conclusion to which our investigations point is that 

 whilst Mr. Maunder' s first paper may be, as he himself now 

 maintains, somewhat affected by a systematic error in the 

 estimate of times, the general agreement between it and his 

 second paper affords overwhelming evidence that a marked 

 diurnal inequality exists in the times of commencement of 

 magnetic storms at Greenwich as estimated by an unprejudiced 

 mind. The consequences of this are hardly, I think, adequately 

 recognized by Mr. Maunder. The "local peculiarities," 

 he says, "tend to blur the evidence for the Interval Rela- 

 tion;" but he adds, "they by no means efface it, for the 

 whole of the evidence ... in my two former papers ... is still 

 outstanding." But the certainty of the existence of a pro- 

 nounced diurnal period in the figures, and the admission of 

 uncertainty in the individual data, make the proof of 

 the existence of a true 27 to 28 day period more difficult, and, 

 supposing it to exist, increase the probable error in its estimate. 

 If the determining factor, as Mr. Maunder believes — 

 and as others before him, e. g. Arrhenius and Birkeland, 

 have supposed — be some form of electrical discharge from the 

 sun, the time at which that leaves the sun or reaches the 

 earth's atmosphere cannot well have anything to do with 

 whether it is day or night at Greenwich. We must thus 

 suppose that the effects at any given station often remain 

 almost entirely latent, for a time determined by circum- 

 stances which if not local, in the ordinary sense of the word, 

 have at least a marked relationship to local time. There is 

 of course nothing very improbable in this, if we suppose 

 that the emanation, or whatever it is that proceeds from the 

 sun, is not the immediate cause of the disturbance, but only 

 introduces conditions which facilitate the development in the 

 atmosphere of electrical currents, similar to but irregularly 

 distributed as compared with those to which the ordinary 

 diurnal inequality is due*. But evidently if this be the case, 

 the determination of the circumstances to which a particular 

 disturbance is due is a less simple matter than would appear 

 at first sight, and we cannot but view with increased reserve 

 estimates such as that of Ricco — quoted by Arrhenius f in 

 support of theoretical conclusions — of the time required for 

 the supposed electrical discharge to travel from the sun to 

 the earth. 



* Phil. Trans. A, vol. ccii. p. 436. 



t Terrestrial Magnetism, March 1905, p. 4. 



