MAONETISM. 07 



iii the internal parts of the earth, is probably of a far more intricate 

 structure, and we can only judge of its nature from the various phae. 

 noineoa derived from its influence. 



The accumulation and the deficiency of the magnetic fluid, which 

 determine the place of the poles of this magnet, are probably in 

 fact considerably diffused, but they may generally be imagined, 

 without much error in the result, to centre in two points, one of 

 them nearer to the north pole of the earth, the other to the south 

 pole. In consequence of their attractions and repulsions, a needle, 

 whether previously magnetic or not, assumes always, if freely poised, 

 the direction necessary for its equilibrium ; which, in various parts 

 of the globe, is variously inclined to tlie meridian and to the horizon. 

 Hence arises the use of the compass in navigation, and in surveying: 

 a needle, which is poised with a liberty of horizontal motion, assum- 

 ing the direction of the magnetic meridian, which for a certain time 

 remains almost invariable for the same place ; and a similar pro- 

 perty is also observable in the dipping needle, which is moveable 

 only in a vertical plane ; for when this plane is placed in the mag. 

 netic meridian, the needle acquires an inclination to the horizon, 

 which varies according to the situation of the place with respect 

 to the magnetic poles. 



The natural polarity of the needle may be in some measure illus- 

 trated by inclosing an artificial magnet in a globe ; the direction of 

 a small needle, suspended over any part of its surface, being deter- 

 mined by the position of the poles of the magnet, in the same man. 

 ner 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 magne- 

 tism, which may be reversed at pleasure by reversin 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 



