Magnetic Disturhances at Greenwich. 313 



Whether we consider all the storms, or only the M storms 

 in Table IV., we see a notable difference of the kind already 

 indicated between S max. and S min. years. If the difference 

 had existed between the " All " storms, but not between 

 the M storms, then of course it would naturally have been 

 suggested that the difference was solely a matter of the 

 absolute size of the average storm in the two groups of years ; 

 but the fact that the difference is conspicuous when we 

 confine ourselves to M storms renders this position untenable. 

 The Gand A storms are so few in the S min. years that com- 

 parative data based on them are too uncertain, but the 

 M storms number §§ in this group and 112 in the S max. 

 group ; so that the difference shown in Table IV. can hardly 

 be assigned in any large measure to chance. 



We have now shown (Table IV.) that the diurnal incidence 

 of commencements is notably more uniform in years of 

 S max. than in years of* S min., and (Table III.) that an 

 unduly large proportion of the G storms come from the S max. 

 years. It is thus clear that a difference of the kind shown 

 in Table II. between storms of the three classes would 

 naturally arise, even if the size of the storm were absolutely 

 without effect on the hour of commencement or its estimate. 



At the same time, it should be clearly understood that our 

 investigation has not shown that the difference of behaviour 

 in S max. and S min. years is the sole cause of the phenomena 

 appearing in Table II. In fact it suggests that there is a real 

 difference depending on the size of the disturbance ; for 

 Table IV. shows in both groups of years a greater concentra- 

 tion of M than of " All" storms between noon and 7 p.m., and 

 in the case at least of the S max. group the number of G 

 and A storms is sufficient to render chance an improbable 

 explanation. 



§ 7. The difference between S max. and S min. years 

 extends, as will be presently shown, to the annual distribution,, 

 a subject to which Mr. Maunder also refers in his third 

 paper. Before dealing with this, I might explain that the 

 tendency to a maximum near the equinoxes in the number of 

 disturbances, to which Mr. Maunder refers on his pp. 680-1, 

 has long been known to occur at certain stations, including 

 Toronto and Greenwich — whose long series of results have 

 been so fully dealt with by Mr. Ellis ; but other stations of 

 whose data Mr. Maunder seems unaware show different 

 phenomena. Some of the older results are discussed in 

 Balfour Stewart's article in the Encyclopaedia Brilannica, and 

 some more recent results are given by Mascart in his 

 Magnetisme Terrestre (pp. 296 et seq.). 



