ON MAGNETISM, GSQ 



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 out- 

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



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

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

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

 tallic and sfightly 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 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 the meridian and to the 

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

 ing: a needle, whichis poised with a liberty of horizontal motion, assuming \ 

 the direction of the magnetic meridian, which for a certain tiaie remains 

 almost invariable for the same place; and a similar property is also observa- 

 ble in the dipping needle, whichis moveable only in a vertical plane; for 

 when this plane is placed in the magnetic meridian, the needle acquires 

 an inclination to the horizon, which varies according to the situation of the 

 place with respect to the magnetic poles. (Plate XLI. Fig. 57'2, 57'o.) 



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 \yith much 

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



