ON MAGNETISM. 535 



acquires an inclination to the horizon, which varies according to the situa- 

 tion of the place with respect to the magnetic poles. (Plate XLI. Fig. 572, 

 573.) 



The natural polarity of the needle may be in some measure illustrated 

 by inclosing an artificial magnet in a globe ; the direction of a small needle, 

 suspended over any part of its surface, being determined by the position of 

 the poles of the magnet, in the same manner as the direction of the compass 

 is determined by the magnetical poles of the earth, although with much 

 more regularity. In either case the whole needle is scarcely more or less 

 attracted towards the globe than if the influence of magnetism were 

 removed ; except when the small needle is placed very near to one of the 

 poles of the artificial magnet, or, on the other hand, when the dipping 

 needle is employed in the neighbourhood of some strata of ferruginous sub- 

 stances, which, in particular parts of the earth, interfere materially with 

 the more general effects, and alter the direction of the magnetic meridian. 



A bar of soft iron, placed in the situation of the dipping needle, acquires 

 from the earth, by induction, a temporary state of magnetism, which may 

 be reversed at pleasure by reversing its direction ; but bars of iron, which 

 have remained long in or near this direction, assume a permanent polarity ; 

 for iron, even when it has been at first quite soft, becomes in time a little 

 harder. A natural magnet is no more than a heavy iron ore, which, in the 

 course of ages, has acquired a strong polarity from the great primitive 

 magnet. It must have lain in some degree detached, and must possess but 

 little conducting power, in order to have received and to retain its mag- 

 netism. 



We cannot, from any assumed situation of two or more magnetic 

 poles, calculate the true position of the needle for all places ; and even in the 

 same place, its direction is observed to change in the course of years, accord- 

 ing to a law which has never yet been generally determined, although the 

 variation which has been observed, at any one place, since the discovery of 

 -the compass, may perhaps be comprehended in some very intricate expres- 

 sions ; but the less dependence can be placed on any calculations of this 

 kind, as there is reason to think that the change depends rather on chemical 

 than on physical causes. Dr. Halley* indeed conjectured that the earth 

 contained a nucleus, or separate sphere, revolving freely within it, or rather 

 floating in a fluid contained in the intermediate space, and causing the 

 variation of the magnetic meridian ; and others have attributed the effect 

 to the motions of the celestial bodies : but in either case the changes pro- 

 duced would have been much more regular and universal than those which 

 have been actually observed. Temporary changes of the terrestrial mag- 

 netism have certainly been sometimes occasioned by other causes ; such 

 causes are, therefore, most likely to be concerned in the more permanent 

 effects. Thus, the eruption of Mount Hecla was found to derange the 

 position of the needle considerably ; the aurora borealist has been observed 

 to cause its north pole to move 6 or 7 degrees to the westward of its usual 



*.?h.'Tr. 1693, p. 563. 



f See Arago, Ann. de Ch. xxxix. 369. Fox, Ph. Tr. 1831, p. 199. Sabine, 

 Obs. on Days of unusual Mag. Disturbance, 4to, 1843. 



