TRANSACTIONS OF SECTION A. 551 



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 interfered with by some slower agency the action of which is 

 contined within narrow limits of space. Between Kew, Greenwich, and Stonyhurst, 

 between St. Leonards and Tunbridge Wells, and I may add between Mablethorpe and 

 Lincoln, Euniskillen and Sligo, Oharleville and Bantry, the measured diflerences 

 of secular variation are so large as to suggest that we are dealing not with an 

 unruffled 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 

 luagnetician, 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 different 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 part of the apparent fluctuations of secular change may 

 be nothing more than the irregular shiftings of inconstant standards. 



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 fluctua- 

 tions 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. Tt is now fully estab- 

 lished that even where the surface soil is non-magnetic, and even where geolofiists 

 have every reason to believe that it lies upon non-magnetic strata of great thick- 

 ness, there are clearly-defined lines and centres towards which the north-seeking 

 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 i'orces in play in their neighbourhood. 



Greenwich and Kew are both under the influence of a widespread local disturb- 

 ance 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 forces. The questions of the nature of the 

 irregularities 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 diflicult problem of world-wide secular variations. 



Two causes of regional and local disturbances have been suggested — viz., earth 

 currents, and the presence of visible or concealed 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 believ- 

 ing 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 conclusive 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 same hemisphere, 



