TRANSACTIONS OF SECTION A. 519 



trace the general course of the secular changes all over the world for forty years. 

 The negative results may be shortly stated. There is no evidence of any motion 

 either of magnetic pole or focus. The positive conclusions are still more curious. 

 There are certain lines on the surface of the earth towards which in the interval 

 under consideration the north pole of the needle was attracted. From each sidft 

 the compass veered or backed towards them. Above them the north pole of dip 

 needle moved steadily down. 



There are other lines from which, as tested by compa.'ss and dip circle, a north 

 pole was in like manner repelled. The two principal points of increasing attrac- 

 tion are in China and near Cape Horn ; the chief points of growing repulsion are 

 in the North of Canada and the Gulf of Guinea. 



I am sure that my friend Captain Creak would be the first to urge that we 

 should not generalise too hastily from this mode of presenting the facts, but there 

 can be no doubt that they cannot be explained by any simple theory of a rotating or 

 oscillating pair of poles. Prima facie they suggest that the secular change is due 

 not so much to changes at the principal magnetic points, as to the waxing and 

 waning of the forces apparently exerted by secondary lines or points of attraction 

 or repulsion. 



All down the west coast ol America, close — be it noted — to one of the great 

 lines of volcanic activity, north hemisphere magnetism has since 1840 been grow- 

 ing in relative importance. Near Cape Horn a weak embryonic pole is developing 

 of the same kind as the well-known pole at the other end of the continent near 

 Hudson's Bay. Along a line which joins Newfoundland to the Cape of Good 

 Hope, precisely the reverse effects have been experienced ; while in the Gulf of 

 Guinea a south hemisphere pole is growing within the tropics. Of course I do not 

 suggest that these secondary systems can ever determine the principal phenomena 

 of terrestrial magnetism, or reverse the magnetic states of the hemispheres in 

 which they occur. These are no doubt fixed by the rotation of the eartti. I do, 

 however, wish to emphasise the fact that they show that either secular change 

 is due to the conjoint action of local causes, or that if some single agent such as 

 a current system within the earth, or a change of magnetic conditions outside it, 

 be the primary cause, the eflects of this cause are modified and complicated by 

 local peculiarities. 



Mr. Henry Wilde has succeeded iu representing with approximate accuracy 

 the secular change at many points on the surface of the earth by placing two 

 systems of currents within a globe, and imparting to the axis of one of them a 

 motion of rotation about the polar axis of the earth. Eut he has had to supplement 

 uhis comparatively simple arrangement by local features. He has coated the seas 

 witli thin sheet iron. The ratio between the two currents which serves to depict 

 the secular change near the meridian of Greenwich fails in the West Indies. Thus 

 this ingenious attempt to imitate the secular change by a simple rotation of the 

 magnetic pole supports the view that local peculiarities play a powerful part in 

 modifying the action of a simple first cause, if such exist. I need hardly say that 

 I think the proper attitude of mind on this difficult subject is that of suspended 

 judgment ; but there is no doubt that recent investigation has, at all events, definitely 

 raised the question how far secular change is either due to or modified by special 

 magnetic features of diflerent parts of the earth. 



It is possible that light may be thrown upon this point by observations on a 

 smaller scale. Assuming for the moment that the difference in the secular changes 

 on opposite sides of the Atlantic is due to a dift'erence of local causes, it is conceiv- 

 able that similar causes, though less powerful and acting through smaller ranges, 

 might produce similar though less obvious difierences between places only a few 

 miles apart. For testing this Greenwich and Kew are in many respects most 

 favourably situated. Nowhere else are two first-class observatories so near to- 

 gether. Differences in the methods of publishing the results have made it somewhat 

 ditlicultto compare them, but the late Mr. Whipple furnished me with figures for 

 several years which made comparison easy. Without entering into details it may 

 be sufScient to say that the declination needles at the two places do not from year 

 to year run parallel courses. Between 1880-82 Kew outstripped its rival, between 



