l84 TERRESTRIAL MAGNETISM. 



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

 to question the ohl 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 regalar period in its neighborhood. 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 magnet- 

 ized matter. We need not now consider the sudden jerks which dis- 

 turb the diurnal sweep of the magnet, which are simultaneous at 

 places far apart, and i^robably 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 inter- 

 fered with by some slower agency the action of which is confined 

 within narrow limits of space. Between Kew, Greenwich, and Stony- 

 hurst, between St. Leonards and Tunbridge Wells, ami I may add, 

 between Mablethorpe and Lincoln, Enniskillen and Sligo, Charleville 

 and Bantry, the measured diftercnces of secular variation are so large 

 as to suggest that we are dealing not with an nnrulfled 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 j^oint 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 can not 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 instru- 

 mental, the iuc^uiry as to their cause should be undertaken in good 

 earnest. 



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

 fully established that even where the surface soil is nonmagnetic, and 

 even where geologists have every reason to believe that it lies upon 



