641 



We may regard as a step towards this solution the separation of 

 the disturbances of the declination into two distinct forces acting in 

 different directions and proceeding apparently from different foci; 

 the phenomena of distinct (though in so many respects closely allied) 

 variations exhibit the same peculiar features at all the stations to 

 which the analysis has hitherto extended, and have been exemplified 

 by the observations at Kew, as shown in the early part of this paper. 

 A similar separation into two independent affections, each having its 

 own distinct phenomenal laws, has followed from an analysis of the 

 same description applied to the disturbances of the magnetic dip and 

 force at the Colonial stations ; thus placing in evidence, and tracing 

 the approximate laws of the effects of six distinct forces (two in each 

 element) contemporaneously in action in all parts of the globe, and 

 pointing in no doubtful manner to the existence of two terrestrial 

 foci or sources in each hemisphere from which the action of the forces 

 emanating from the sun and communicated to the earth may be con- 

 ceived to proceed. Such an ascription naturally suggests to those 

 conversant with the facts of terrestrial magnetism the possibility that 

 Halley's two terrestrial magnetic foci in each hemisphere may be 

 either themselves the localities in question, or may be in some way 

 intimately connected with them. The important observations which 

 we owe to the zeal and devotion of Captain Maguire, R.N. and ths 

 Officers of H.M.S. ' Plover/ have made us acquainted with Point Bar- 

 row as a locality where the magnetic disturbances prevail with an 

 energy far beyond ordinary experience, indicating the proximity of 

 that station to the source or sources from which the action of the 

 forces may proceed. Now Point Barrow is situated in a nearly inter- 

 mediate position between what we believe to be the present localities of 



fall within the limits comprised by the observations. The period being decennial, 

 and the epoch of minimum occurring at the end of 1843 or beginning of 1844, 

 the epoch of maximum was necessarily waited for in order to ascertain the precise 

 duration of the cycle. The maximum took place in 1848-1849, the observations 

 in 1850 and 1851 showing that the aggregate value of the annual disturbances 

 was again diminishing as it had been in 1842 and 1843. The process of deter- 

 mining the proportion of disturbance in different years is a somewhat laborious 

 one, and requires time : but in March 1852, I was able to announce to the Royal 

 Society the existence of a decennial variation, based on the concurrent testimony 

 of the observations at Toronto and Hobarton ; deeming it proper that so remark- 

 able a fact should not be publicly stated until it had been thoroughly assured 

 by independent observations at two very distant parts of the globe. 



