370 



the comparison of tables, I have exhibited in a diagram the phases of 

 the semiannual inequality at the six stations, by which it will be seen 

 that they add their confirmation to the inference which I had previ- 

 ously drawn. With this additional evidence of its uniform character 

 in different parts of the globe, it may be hoped that the claim of the 

 semiannual inequality to be received as a successful generalization 

 from a careful and comprehensive induction may be admitted, and 

 that as an accession to our positive knowledge it may have a recognized 

 place amongst the facts of the diurnal variation, which have to be ac- 

 counted for in the theories which may be hereafter adduced for their 

 physical explanation. 



We now, therefore, recognize three classes of phenomena derived 

 from three different sources, which are superposed in the diurnal va- 

 riation obtained from the unreduced observations, and which for a 

 proper understanding of the whole, require to be separated from each 

 other by a proper analysis, so that the part due to each may be di- 

 stinctly ascertained : these are 1st, the mean effects of the magnetic 

 storms ; 2nd, the semiannual inequality of the regular solar-diurnal 

 variation; and 3rd, the mean solar-diurnal variation of the year 

 into which the semiannual differences merge. The distinctive 

 characteristics of the first, viz. the disturbance diurnal variation, 

 have already been stated in the early part of this paper, together 

 with the evidence they supply of being due to some modification 

 of the solar action, justifying their being treated as distinct and 

 separate from the affections which constitute the more regular 

 variation. There are also distinctive characters in the pheno- 

 mena of the semiannual inequality, and in those of the mean va- 

 riation, which appear to point out a difference in the mode in which 

 the primary cause operates in producing the two classes of pheno- 

 mena. For the purpose of explaining this difference, we may em- 

 ploy, as more likely to be generally understood, the usual custom of 

 referring all deflections, whether in the northern or the southern 

 hemisphere, to the north end of the magnet ; we say then that, in 

 the mean variation, the directions of the deflection are uniform 

 throughout the year in the middle latitudes of the one hemisphere, 

 and (although opposite) are also uniform throughout the year in the 

 middle latitudes of the other hemisphere ; whilst in the semiannual 

 inequality, the directions of the deflection are uniform in the two 



