1891.] Professor A. W. Hiicker on Magnetic Bocks. 417 



WEEKLY EVENING MEETING, 



Friday, April 17, 1891. 



Edward Feankland, Esq. D.C.L. LL.D. F.R.S. Vice-President, 



in the Chair. 



Professor A. W. HDgker, M.A. F.R.S. M.B.I. 



Magnetic Bocks. 



The cause of terrestrial magnetism is still unknown, and the 

 problem of attempting to discover it is not rendered more easy by 

 the fact that a solution may be looked for in either of two different 

 directions. 



On the one hand the earth is partly composed of magnetic 

 material, and if vast masses of this were permanently magnetised, the 

 principal phenomena observed upon the surface might be produced. 

 On the other hand, we know that different points on the earth's crust 

 are at different electrical potentials, and it is conceivable that the 

 directive forces exerted on the magnet might be due to a world-wide 

 system of earth currents. Both theories are beset with difficulties, 

 and at present we are accumulating facts, in the hope that a clue to 

 an explanation may hereafter be found. 



A mere dry record of observations is, however, hardly a subject 

 for a lecture, and I should not have mooted the question if there had 

 not been another problem, related to, though differing from that of 

 terrestrial magnetism, with regard to which it is perhaps possible to 

 form an opinion as to the direction in which the balance of evidence 

 inclines. 



If the magnetic declination be determined at a number of stations 

 scattered all over the surface of the globe, lines can be drawn 

 through those places at which the deviation from true north is the 

 same. If the scale of the map on which they are depicted is small, 

 and if the distances between the stations are measured in scores or 

 in hundreds of miles, these isogonal lines are smooth curves ; but if 

 the number of stations be multiplied, and the scale on which the 

 results are represented increased, the curves are found to be irregular, 

 and to be complicated by unexpected bends and twists. 



These irregularities must be due to disturbing magnetic forces, 

 produced by local causes, which deflect the needle from its normal 

 direction ; and if the number of stations be sufficiently great there 

 is no difficulty in sifting out the disturbing from the normal forces 

 and determining with approximate accuracy the directions in which 

 they act. 



If this is done, the question may be asked whether these local 

 peculiarities are due to rock magnetism or to earth currents. There 

 is no reason why both should not in some cases coexist, but as there 



