476 



Royal Society :■ 



I have already, on a former occasion (Proceedings of the Royal So- 

 ciety, May 18, 1854), submitted to the consideration of the Society 

 the concurrent evidence from three stations, Toronto, Hobarton, and 

 St. Helena, of the existence of this inequality, and of the almost 

 uniform character of its phases at those stations, from which I ven- 

 tured to infer the probability that an inequality having a similar cha- 

 racter would be found to be a general phenomenon. I am now 

 able to add to the evidence which was then adduced, a representation 

 of the semiannual inequality at three additional stations, viz. at the 

 Cape of Good Hope, of which the particulars in detail will be found 

 in the 2nd volume of the * St. Helena Observations,' — at the Kew Ob- 

 servatory, taken from the hourly tabulations from the photographic 

 curves obtained by the self-recording declinometer at that station, — 

 and at Pekin, as shown in the following tabular view : — 



As the correspondence of such phenomena is often far better 

 judged of by the eye, when exhibited in the form of curves, than by 

 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 



