MAGNETIC DECLINATION AT KEW AND NERTSCHINSK. 
243 
we had learnt to recognize in the sun itself a source of magnetic energy, — before we had 
been informed by the sun-spots of the existence of periodical variations in the physical 
aspect, and consequently in the physical condition of that luminary, — and before we had 
succeeded in connecting these by their identity in period and epoch with the magnetic 
variations of our terrestrial sphere, — are no longer tenable. The solar origin of the 
variations in the magnetic phenomena of the earth’s surface is indeed legitimately infer- 
rible from their correspondence to solar hours ; but in the decennial cycle, discovered 
in the solar spots and in the terrestrial magnetic disturbances, we have the absolute 
evidence and the ocular demonstration of a periodical variation common to the sun and 
to the earth, which in the sun is cognizable by our visual organs, and which, in the case 
of the earth, we know to be a magnetic variation. 
We do not, as yet at least, possess a similar ocular demonstration of a connexion 
between the sun and the earth in the cycle of longer duration corresponding to the 
earth’s secular magnetic change. But careful observations of the variable phenomena 
of the solar disk can only be said to be in their commencement ; and it would be prema- 
ture to assume that no visible phenomena will ever be discovered in the sun which will 
render the evidence of connexion as complete in the one case as in the other. But such 
evidence is not a necessary condition of an existing connexion; the decennial period 
would have been equally true (though not so readily perceived by us) if the sun-spots 
had been less conspicuous. 
In the cosmical hypothesis here imagined, the north “ pole or point of greatest 
attraction” (adopting Halley’s phraseology) of the induced terrestrial system at this 
epoch is in the north of the Europgeo-Asiatic continent, whilst that of the magnetism 
proper of the globe is in the north of the American continent ; the direction of the 
magnet “ in those parts which lie adjacent to either being governed thereby, the nearest 
pole being always predominant over the more remote”*. 
In the references made in this paper to the existence of a Theory of Terrestrial Mag- 
netism, and to the advantage which I have myself endeavoured to derive from it in 
guiding experimental inquiry, I wish it to be understood that I employ the term 
“Theory,” and regard its office in the work of inductive research, in the same light in 
which both were viewed by the late Professor Playfaie. “In physical inquiries the 
work of theory and observation must go hand in hand, and ought to be carried on at 
the same time ; more especially if the matter is very complicated, for then the clue of 
* I have recalled these words of Halley in the text, because they show that he already recognized what has 
since been dwelt on by other magneticians, viz., that we must discriminate between the true poles or points of 
greatest force of the terrestrial and induced systems, and the apparent poles or centres of the isodynamic loops, 
which are the resultants of the double system. It is not improbable that the further observation and study of 
the magnetic disturbances, when those of the three elements are brought to bear together on the question, may 
guide us directly to a knowledge of the geographical positions of the true foci, as distinguished from the resultant 
foci. We have now learnt experimentally, i. e. by the observations of Captains Maguire and M'Clixtock 
(Phil. Trans. 1863, p. 657), that the resultant foci are not themselves the points of origin of the disturbances. 
MDCCCLXIV. 2 L 
