August 9, 1894] 



NATURE 



547 



the probability of their physical reality is increased by the 

 work of the magnetic survey. 



The large number of observations at our disposal has enabled 

 lis to calculate the secular change in a new way, by taking the 

 means of observations made about five years apart at numerous 

 though not identical stations scattered over districts about 150 

 miles square. The result thus obtained should be free from mere 

 local variations, but as calculated for the south-east of England 

 for the five years 18S6-91 it differs by nearly 5 from the change 

 actually observed at Kew. 



We have also determined the secular change at twenty-five 

 stations by double sets of observations made as nearly as possible 

 on the same spot at intervals of several years. The results 

 must be interpreted with caution. In districts such as Scotland, 

 where strong IoctI disturbances are frequent, a change of a few 

 yatds in the position of the observer might introduce errors far 

 larger than the fluctuations of secular change. But when all 

 such changes are eliminated, when all allowance is made for the 

 possible inaccuracy of field observations, there are outstanding 

 variations which can hardly be due to anything but a real 

 difference in the rate of change of the magnetic elements. 



A single example will suffice. St. Leonards and Tunbridge 

 Wells are about thirty miles apart. Both are situated on the 

 Hastings Sand formation, and on good non-magnetic observing 

 ground. At them, as at the stations immediately anund them 

 • — Lewes, Eastbourne, Appledore, Etchingham, Heathfield, 

 and Maidstone— the local disturbing forces are very small. All 

 these places lie within a district about forty miles square, at no 

 point of which has the magnet been found to deviate by 5' 

 from the true magnetic meridian. No region could be more 

 favourably situated for the determination of the secular change, 

 vet according to our observations the alteration in the de- 

 clination at St. Leonards in six years was practically equal to 

 that at Tunbridge Wells in five. It is difficult to assign so 

 great a variation to an accumulation of errors, and this is only 

 one amongst several instances of the same kind which might be 

 quoted. 



We find, then, when we consider the earth as a whole, grave 

 reason to question the old idea of a secular change caused by a 

 magnetic pole or focus pursuing an orderly orbit around the 

 geographical axis of the earth, or oscillating in some regular 

 period in its neighbourhood. It would, of course, be absurd 

 to admit the possibility of change in the tropics and to deny 

 that possibility in the arctic circle, but the new facts lead us to 

 look upon the earth not as magnetically inert, but as itself — at 

 the equator as well as at the pole — producing or profoundly 

 modifying the influences which give rise to secular change. 

 And then, when we push our inquiry further, accumulating 

 experience tells the same tale. The earth seems as it were 

 alive with magnetic forces, be they due to electric currents or 

 to variations in the state of magnetised matter. We need not 

 now consider the sudden jerks which disturb the diurnal sweep 

 of the magnet, which are simultaneous at places far apart, and 

 probably originate in causes outside our globe. But the slower 

 secular change, of which the small part that has been observed 

 has taken centuries to accomplish, is apparently also in- 

 terfered with by some slower agency the action of which 

 is confined within narrow limits of space. Between Kew, 

 Greenwich, and Stonyhurst, between St. Leonards and 

 Tunbridge Wells, and I may add between Mablethorpe and 

 Lincoln, Enniskillen and Sligo, Cbarleville and Bantry, the 

 measured differences of secular variation are so large as to 

 suggest that we are dealing not with an unruflled tide of change, 

 which, unaltered by its passage over continent or ocean, sweeps 

 slowly round the earth, but with a current fed by local springs 

 or impeded by local obstacles, furrowed on the surface by 

 billows and eddies, from which the magnetician, if he will but 

 study them, may learn much as to the position and meaning of 

 the deeps and the shallows below. But if this is the view 

 which the facts I have quoted suggest, much remains to be done 

 before it can be finally accepted ; and in the first place — to 

 come back to the point from which I started — -we want, for 

 some years at all events, a systematic and repeated comparison 

 of the standard instruments in use at the difTerent observatories. 

 That they are not in accord is certain ; whether the relations 

 between them are constant or variable is doubtful. If constant, 

 the suggestions I have outlined are probably correct ; if variable, 

 then the whole or jiart of the apparent fluctuations of secular 

 change may be nothing more than the irregular shiftings of 

 inconstant standards. 



NO. 



1293, '^'f^I- 50] 



I cannot myself believe that this is the true explanation, but 

 in any case it is important that the doubt should be set at rest, 

 and that if the apparent fluctuations of secular change are not 

 merely instrumental, the inquiry as to their cause should be 

 undertaken in good earnest. 



The question is interesting from another point of view. It is 

 now fully establi-hed that even where the surface soil is non-mag- 

 netic, and e»en where geologists have every reason to believe that 

 it lies upon non-magnetic strata of great thickness, there are 

 clearly-defined lines and centres towards which the north-seek- 

 ing pole of a magnet is attracted, or from which it is repelled. 

 To the magnetic surveyor fluctuations in secular change would 

 appear as variations in the positions of these lines, or as changes 

 in the forces in play in their neighbourhood. 



Greenwich and Kew are both under the influence of a 

 widespread local disturbance which culminates near Reading. 

 At both places the needle is deviated to the west of the normal 

 magnetic meridian, and if the westerly declination diminishes 

 sometimes faster and sometimes more slowly at one observatory 

 than at the other, this must be, or, at all events, would in the 

 first instance appear to be, due to local changes in the regional 

 disturbing force*. The questions of the nature of the irregu- 

 larities of secular change and of the causes of local disturbances 

 are therefore intermingled ; and information gained on these 

 points may in turn be useful in solving the more difficult problem 

 of world-wide secular variations. 



Two causes of regional and local disturbances have been sug- 

 gested, viz. earth currents, and the presence of visible or con- 

 cealed magnetic rocks. The two theories are not mutually 

 exclusive. Both causes of the observed effects may, and probably 

 do, coexist. I have, however, elsewhere explained my reasons 

 for believing that the presence of magnetic matter, magnetised 

 by induction in the earth's field, is the principal cause of the 

 existence of the magnetic ridge-lines and foci of attraction 

 which for so many years we have been carefully tracing. I will 

 only now mention what appears to me to be the final and con- 

 clusive argument, which, since it was first enunciated, has been 

 strengthened by the results of our more recent work. We find 

 that every great mass of basic rock, by which the needle 

 is affected at considerable distances, attracts the north-seeking 

 pole. Captain Creak some years ago showed that the same 

 statement is true of those islands in the northern hemisphere 

 which disturb the lines of equal declination, while islands in the 

 southern hemisphere repel the north pole and attract the south. 

 In other words, these disturbances are immediately explained if 

 we suppose that they are due to magnetic matter magnetised by 

 induction. The theory of earth currents would, on the other 

 hand, require that round the masses of visible basalt, and round 

 the island investigated by Captain Creak, currents, or eddies in 

 currents, should circulate in directions which are always the 

 same in the saine hemisphere, and always opposed on opposite 

 sides of the equator. For this supposition no satisfactory ex- 

 planation is forthcoming, and, therefore, with all reserve and a 

 full consciousness that in such matters hypothesis difl'ers but 

 little from speculation, it appears to me that the theory that in- 

 duced magnetism is the main cause of the disturbance has the 

 greater weight of evidence in its favour. 



If this be granted, it is evident that the positions of the main 

 lines and centres of attraction would be approximately constant, 

 and, so far as it is possible to form an opinion, these conditions 

 seem to be satisfied. There has certainly been no noticeable 

 change in the chief loci of attraction in the five years which 

 have elapsed between the epochs of our two surveys. Mr. 

 Welsh's observations made in Scotland in 1S57-S fit in well 

 with our own. Such evidence is not, however, inconsistent 

 with minor changes, and it is certain that as the directions and 

 magnitude of the inducing forces alter, the disturbing induced 

 forces must alter also. But this change would be slow, and as 

 the horizontal force is in these latitudes comparatively weak, the 

 change in the disturbing forces would also be small, unless the 

 vertical force altered greatly. It is at all events impossible to 

 attribute to this cause oscillations which occupy at most eight 

 or ten years. It is possible to suggest other changes in the 

 state of the concealed m.ignetic matter — alterations of 

 pressure, temperature, and the like— to which the oscilla- 

 tions of secular change might be due, but probably 

 there will be a general consensus of opinion that if the 

 slowly changing terms in the disturbance function are due to 

 I magnetic matter, the more rapid fluctuations of a few years' 

 I period are more likely to be connected with earth currents. It 



