Terrestrial magketism. 185 



nonmagnetic strata of great thickness, there are clearly defined lines 

 and centers toward wliich the north-seeking pole of a magnet is 

 attracted, or from which it is repelled. To the magnetic surveyor tlnc- 

 tuations in secular change would ai)pear as variations in the positions 

 of these lines, or as changes in the forces in play in their neighborhood. 



Greenwich and Kew are both under the influence of a widespread 

 local disturbance which culminates near Eeading. At both pLices 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 apjjear 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 difficult 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 believing that the presence of 

 magnetic matter, magnetized by induction in the earth's held, 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 conclu- 

 sive argument, which, since it was first enunciated, lias been strength- 

 ened 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 dis- 

 tances, attracts the north-seeking i)ole. Captain Creak some years ago 

 showed that the same statement is true of those islands in the ]Srorthern 

 Hemisphere which disturb the lines of equal declination, while islands 

 in the Southern Hemisphere rei)el the north pole and attract the south. 

 In other words, these disturbances are inmiediately explained if we 

 suppose that they are dim to magnetic matter magnetized 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 ai"e always the same in the same hemisphere, and 

 always opposed on opposite sides of the equator. For this supposition 

 no satisfactory explanation is forthcoming, and, therefore, with all 

 reserve and a full consciousness that in such matters hypothesis differs 

 but little from speculation, it appears to me that the theory that 

 induced magnetism is the main cause of the dfsturbance has the greater 

 weight of evidence in its favor. 



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

 and centers of attraction would be approximately constant, and, so far 



