Magnetic Disturbances at Greenwich. 311 



Table II. shows, what Mr. Maunder already admits, that 

 the A (" active ") and still more the M ("moderate") storms 

 show a marked concentration of commencements between 

 noon and 7 p.m. ; but it further shows a similar though less 

 conspicuous concentration o£ the G (" great ") storms. In 

 fact, in the period 1848-81 the percentage of G storms 

 commencing between noon and 7 p.m. is greater than that 

 of the A storms *. 



§ 5. There is also reason to think that part of the apparent 

 difference between the different classes of storms arises from 

 a cause wholly unrecognized by Mr. Maunder. 



In Tables I. and II. it has been tacitly assumed that there 

 is no essential difference between one year and another, but 

 this is far from the case. In years of many sun-spots, 

 whilst the amplitude of the regular diurnal changes of the 

 magnetic elements in England seems enhanced at all hours, 

 the increase seems relatively greater by night than by day: 

 the distinction between day and night is thus less marked 

 than usual, As we shall presently see, a similar phenomenon 

 presents itself as regards the times of commencement of 

 magnetic storms, the diurnal incidence being notably more 

 regular in years of many than in years of few sun-spots. 

 We shall also show that the distribution of G, A, and M 

 storms is widely different in years of many and of few 

 sun-spots. In years of the latter type G storms are 

 exceedingly rare, and so the majority of storms of this 

 class are taken from the years of sun-spot maximum ; i. e. 

 from years which, as has already been stated, show an 

 exceptionally regular incidence of commencements. I now 

 proceed to prove these statements, dealing with the latter 

 first. 



The 56 years of Mr. Maunder's investigation included the 

 following 14 of exceptionally large sun-spot frequency : — 



1848, 49, 59, 60, 61, 70, 71, 72, 82, 83, 84, 92, 93, 94. 

 This group I shall refer to as S max. years. 



Amongst the 56 years the following 15 were of excep- 

 tionally small sun-spot frequency : — 



1854, 55, 56, 65, 66, 67, 77, 78, 79, 88, 89, 90, 99, 1900, 1901. 



This group I shall refer to as S min. years. 



* Mr. Maunder particularizes storms of sudden commence ment only 

 for 1882 to 1908. According to my figures in ' Terrestrial Magnetism/ 

 the percentage distribution of these was : — to 11 a.m., 47 ; noon to 

 7 p.m., 40 ; and 8 to 11 p.m., 13 (Mr. Maunder's figures would give 44, 

 40, and 16 respectively). This suggests a concentration between noon 

 and 7 p.m. even for sudden storms. 



