182 TERRESTRIAL MAGNETISM. 



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 emphasize the fact 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 effects of this cause are modified and complicated by local 

 peculiarities. 



Mr. Henry Wilde has succeeded in rei)resenting with approximate 

 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 compariti vely simple arrange- 

 ment by local 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. Ihus 

 this ingenious attempt to imitate the secular change by a simple rota- 

 tion of the magnetic pole sup})orts the view that local peculiarities 

 play a powerful irdvt 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 difficult subject is that of suspended judgment, but there is no 

 doubt that recent investigation has, at all events, definitely raised the 

 question how far secular change is either due to, or modified by, special 

 magneto features of different parts of the earth. 



It is possible that light may be thrown ui)on this point by observa- 

 tions on a smaller scale. Assuming for the moment that the difference 

 in the secular changes on opposite sides of the Atlantic is due to a dif- 

 ference of local causes, it is conceivable that similar causes, though 

 less powerful and acting through smaller ranges, might produce similar 

 though less obvious differences between places only a few miles apart. 

 For testing this Greenwich and Kew are in many respects most favora- 

 bly situated. JSTo where else are two first-class observatories so near 

 together. Differences in the methods of publishing the results have 

 made it somewhat difficult to compare them, but the late Mr. Whipple 

 furnished me with figures for several years, which made comparison 

 easy. Without entering into details it may be sufficient to say that 

 the declination needles at the two i)laces do not from year to year run 

 parallel courses. Between 1880 and 1882 Kew outstripped its rival; 

 between 1885 and 1889 it lost, so that the gain was rather more than 

 compensated. The difference of the declination of the two places 

 appears to increase and diminish through a range of 5 minutes of arc. 



This evidence can be supplemented by other equally significant 

 examples. Ko fact connected with terrestrial magnetism is more cer- 

 tain than that at present the rate of secular change of declination in 

 this part of Europe increases as we go north. This is shown by a com- 



