346 



NA TV RE 



[August 9, 1S94 



the Society only, but the Royal Society has now consented to 

 publish them in the Proceedings. Before long, therefore, the 

 Kew and Falmouth records, which are already worked up in 

 the same way, will be given to the world side by side. Is it 

 too much to hope that thi< may be the first step towards the 

 production of a British Magnetic Year Book, in which obser- 

 vations whose chief interest lies in their comparison, may be so 

 published as to be easily compared ? 



We owe to private enterprise another advance of the same 

 kind. The managers of the new journal Science Progress have 

 made arrangements with the Kew Committee for the yearly 

 publication of a table showing the mean annual values of the 

 magnetic elements as determined at the various magnetic obser- 

 vatories of the world. It will therefore in future be possible to 

 get a general idea of the rate of secular change in different 

 localities without searching through a number of reports in 

 different languages, which can only be consulted in the rooms 

 of the few societies or institutions to which they are annually i 

 sent. The present state of our knowledge of the secular change 

 in the magnetic elements affords indeed very strong support to 

 the arguments I have already adduced in favour of a com- 

 parison between the instruments of our magnetic observatories. 



TTie whole question of the cause of this phenomenon has 

 entered on a new stage. It has long been recognised that the 

 earth is not a simple magnet, but that there are in e.ich hemi- 

 sphere one pole or point at which the dip needle is vertical, 

 and two foci of ma.ximum intensity. A comparison of earlier 

 with later magnetic observations led to the conclusion that 

 one or both of the foci in each hemisphere is in motion, and that 

 to this motion — however caused — the secular change in the 

 values of the magnetic element is due. Thus the late Prof. 

 Balfour Stewart, writing in 18S3, says, " While there is no 

 well-eitablished evidence to show that either the poleofverticily 

 or the centre of force to the North of America has perceptibly 

 changed its place, there is on the other hand very strong 

 evidence to show that we have a change of place on the part of 

 the Siberian focus.'' The facts in favour of this conclusion are 

 there discussed. The arguments are based, not on the results 

 of any actual observations near to the focus in question, but on 

 the behaviour of the magnet at points far distant from it in 

 Europe and Asia. The westerly march of the declination 

 needle, which lasted in England up to iSiS, and the easterly 

 movement which has since replaced it, are connected with a 

 supposed easterly motion of the Siberian focus, which, it is 

 added, " there is some reason to believe .... has recently 

 been reversed." In opposition, therefore, to the idea of the 

 rotation of a magnetic focus round the geographical poles which 

 the earlier magneticians adopted, Stewart seems to have regarded 

 the motion of the Siberian focus as oscillatory. 



A very different aspect is put upon the matter by a compari- 

 son of the magnetic maps of the world piepared by Sabine and 

 Creak for the epochs 1840 and 18S0 respectively. Captain 

 Creak, having undertaken to report on the magnetic observations 

 made during the voyage of the Challenger, supplemented them 

 with the unrivalled wealth of recorded facts at the disposal of 

 the Ilydrographic Department of the Admiralty He was thus 

 able, by a comparison with .Sabine s map, to trace the general 

 course of the secular changes all over the world for forty 

 years. The negative results may be shortly staled. 

 There is no evidence of any motion either of magnetic pole 

 or focus. The positive conclusions are still mote curious. 

 There arc certain lines on the surface of the earth 

 toward! which in the interval under consideration the 

 north pole of the needle was attracted. From each side 

 the compass veered or backed towards them. .\l)ove them the 

 north pole of dip needle moved steadily down. 



There are other lines from which, as tested by compass and 

 dip circle, a north pole was in like manner repelled. The two 

 principal points of increasing attraction arc 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 ptcscnting the facts, but there can be no doubt that ihcy 

 cannot be explained by any simple theory of a rotating or 

 oscillatiog pair of poles. Prima facte they suggest that the 

 •ccular change ii due not so much to changes at the principal 



1 l.n. \.tc^dia Brit,, 9th cditioo An. " M:tcorology— Terrcitrial 



magnetic points, as to the waxing and waning of the forces 

 apparently exerted by secondar)- lines or points of attraction or 

 repulsion. 



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

 one of the great lines of volcanic activity, north hemisphere 

 magnetism h.TS since 1S40 been growing in relative im- 

 portance. Near Cape Horn a weak embyronic pole is 

 developing of the same kind .is the well-known pole at 

 the other end of the continent near Hudson's Bay. .■\long 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 couise 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 earth. I do, however, wish to emphasise the f.ict 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 eflecis of this cause are 

 modified and complicated by local peculiarities. 



Mr. Henry Wilde has succeeded in representing with .ap- 

 proximate 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. But he has had to 

 supplement this compar.atively simple arrangement by loc.il 

 features. He has coated the seas with 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 simph; rotation of the magnetic pole sup- 

 ports the \iew 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 difticult subject is that of suspended judgment, but there 

 is no doubt that recent investigation h.is, at all events, definitely 

 raised the question how far secular change is cither due to, or 

 modified by, special magnetic features of different parts of the 

 earth. 



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

 observations on a smaller scale. .Vssuming for the moment that 

 the diflerence in the secular changes on opposite sides of the 

 Atlantic is due to a difference of local causes, it is conceivable 

 that similar causes, though less powetful and acting through 

 smaller ranges, might produce similar though less obvious 

 differences between places only a few miles .ipart. For testing 

 this Greenwich and Kew are in many respects most favourably 

 situated. Nowhere else ate two firsi-cl.iss observatories so near 

 together. Differences in ilie methods of publishing the results 

 have made it somewhat diilicult to compare theui, but the late 

 Mr. Whipple fuinished inc with figures for several years which 

 made comparison e.osy. Without entering into details it may 

 be sufficient to say that the declination needles at the two 

 places do not fiom year to year run parallel courses. Between 

 18S0-S2 Kew outstripped iis rival, between 18S5 and iSSg it 

 lost, so that the gain was rather more than compensated. The 

 difference of the declination of the two pl,icesa|ipears to increase 

 and diminish through a range of five minutes of arc. 



This evidence can be supplemented by other ciually signifi- 

 cant examples. No fact connected with terrestrial magnetism 

 is more certain than that at present the rale of secular change 

 of declination in this part of Europe incic.iscs as we go north. 

 This is shown by a comparison of our survey with those of 

 our predecessors fifty and thirty years ago, by M. .Moureaux's 

 results in France, and by Captain Creak's collation of previous 

 observations. \ el, in spite of this, Stonyhurst, which is some 

 200 miles north of Greenwich and Kew, and should therefore 

 outrun them, sometimes lags behind and then makes up for lost 

 lime by prodigious bounds. lictween 1SS2 and 1886 the total 

 secular change of declination at Stonyhurst w.as about 3' 5 less 

 than that at Greenwich and Kew, whereas in the two years 

 1890-1892 it reached at Stonyhurst the enormous amount of 

 28', just doubling the corresponding alter.ilion registered in the 

 same time at Kew. If these fluctuations are caused by the 

 instruments or methods of reduction, my argument in favour 

 of frequent comparisons and uniform treatment would be much 

 strengthened, but, apart from the inherent improbability of 

 such large differences being due to the methods of observation, 



NO. 1293, VOL. 50] 



