534 LECTURE LV. 



oval figure passing through those poles, of which the properties have been 

 calculated by various mathematicians. This polarity may easily be 

 imitated by electricity ; a suspended wire being brought near to the ends 

 of a positive and negative conductor, which are placed parallel to each 

 other, as in Nairne's electrical machine, its position is perfectly similar to 

 that of a needle attracted by a magnet, of which those conductors repre- 

 sent the poles. (Plate XLI. Fig. 569.) 



The same effect is observable in iron filings placed near a magnet, and 

 they adhere to each other in curved lines, by virtue of their induced mag- 

 netism, the north pole of each particle being attached to the south pole of 

 the particle next it. This arrangement may be seen by placing the filings 

 either on clean mercury, or on any surface that can be agitated ; and it 

 may be imitated by strewing powder on a plate of glass supported by 

 two balls, which are contrarily electrified.* (Plate XLI. Fig, 570.) 



The polarity of a needle may often be observed when it exhibits no sen- 

 sible attraction or repulsion as a whole ; and this may easily be understood 

 by considering that when one end of a needle is repelled from a given point, 

 and the other is attracted towards it, the two forces, if equal, will tend to 

 turn it round its centre, but will wholly destroy each other's effects with 

 respect to any progressive motion of the whole needle. Thus, when the 

 end of a magnet is placed under a surface on which iron filings are spread, 

 and the surface is shaken, so as to leave the particles for a moment in the 

 air, they are not drawn sensibly towards the magnet, but their ends, which 

 are nearest to the point over the magnet, are turned a little downwards, so 

 that they strike the paper further and further from the magnet, and then 

 fall outwards, as if they were repelled by it. (Plate XLI. Fig. 571.) 



The magnets, which we have hitherto considered, are such as have a sim- 

 ple and well determined form ; but the great compound magnet, which 

 directs the mariner's compass, and which appears to consist principally of 

 the metallic and slightly oxidated iron, contained in 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 phenomena derived from its influence. 



The accumulation and the deficiency of the magnetic fluid, which deter- 

 mine the place of the poles of this magnet, are probably in fact consider- 

 ably 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 the meridian and to 

 the horizon. Hence arises the use of the compass in navigation and in 

 surveying : a needle, which is poised with the liberty of horizontal motion, 

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

 remains almost invariable for the same place ; and a similar property is 

 also observable in the dipping needle, which is moveable only in a vertical 

 plane ; for when this plane is placed in the magnetic meridian, the needle 



* Bazin, Descrip. des Courans Mag. en 15 Planches, 4to, Strasb. 1753. Roget, 

 Jour, of the Roy. Inst. 1831, p. 311. 



