REPORT ON THE MAGNETICAL RESULTS. 17 



In like manner, in China there is a minor blue pole of increasing power attracting 

 the freely suspended needle over a large area. 



As there does not appear to be any secular change of importance found in Siberia, 

 and the horizontal needle is moving somewhat rapidly to the eastward at, and in the 

 regions surrounding, the focus of change in the declination situated in the German 

 Ocean and similarly to the westward in Alaska, a decrease in the vertical force in 

 the high latitudes of North America, or the equivalent to a red pole of increasing 

 power repelling the freely suspended needle for a large area around it, may by analogy 

 be looked for. 



Data of sufficient precision are still wanting for the determination of how far the 

 vertical force of the earth at and about these poles or foci of attraction and repul- 

 sion varies at different epochs ; yet if the hypothesis of their translation be given up 

 or only accepted as existing over small areas, it is not unreasonable to suppose that 

 the vertical force at these poles has a distinct variation, and that the phenomena of 

 the angular movements of the freely suspended needle, as shown by the secular 

 changes in the declination and inclination, are chiefly dependent upon changes in 

 the relative power of these poles. It must further be remembered that the move- 

 ments of the horizontal needle are also modified by changes in the horizontal component 

 of the earth's force, increasing force retarding and decreasing force accelerating them. 



If the case be thus, the cpiestion arises : What are the causes of these remarkable 

 changes in the earth's magnetic force as measured on its surface ? 



No satisfactory explanation has yet been given, and in the present instance only 

 suggestions can be made based on the far from complete facts available. 



The voyage of the Challenger has shown that, in addition to the remarkable 

 local magnetic disturbances which have been found on the great continents, in 

 the sobtary islands of the sea surrounded by apparently normal conditions similar 

 local disturbance is found. It has also been suggested that the magnetic portions 

 of these islands causing the disturbance may possibly " have been raised to the 

 earth's surface from the magnetised portion of the earth forming the source of its 

 magnetism," and tending to prove Airy's conclusion, " that the source of magnetism 

 lies deep." 



Considering, therefore, the changes which are in progress and have taken place in 

 ages past in the distribution of land in the world, it may fairly be conceived, not only 

 that large changes have likewise occurred in the distribution of its magnetic portions 

 appearing here and there on the surface and producing local magnetic disturbance, but 

 that there are others of a more progressive character below the earth's surface which are 

 only made manifest by the secular change observed in the magnetic elements. 

 Although prominence is thus given to the conception that the secular change is 

 chiefly due to continuous redistribution of magnetised matter in the interior of the 



(PHYS. CHEM. CHALL. EXP. PAET VI. 1888.) 3 



