692 LECTUllE LV. 



Tlie dip of the nortli pole of the needle in the neighbourhood of London 

 is 72°. Hence the lower entl of a bar standing upright, as a poker, or a lamp 

 iron, becomes always a north pole, and the temporary south pole of apiece 

 of soft iron being uppermost, it is somewhat more strongly attracted by the 

 north pole of a magnet placed over it, than by its south pole ; the distribution 

 of tlie fluid in the magnet itself being also a little more favourable to the at- 

 traction, while its north pole is downwards. It is obvious that the magnetism 

 of the nortliern magnetic pole of the earth must resemble that of the south 

 pole of a magnet, since it attracts the north pole ; so that if we considered the 

 nature of the distribution of the fluid, rather than its situation in the earth, 

 we should call it a south pole. Although it is impossible to find any places 

 for two, or even for a greater number of magnetic poles, which will correctly 

 explain the direction of the needle in every part of the earth's surface, yet the 

 dip may be determined with tolerable accuracy, from the supposition of a 

 small magnet placed at the centre of the earth, and directed towards a point in 

 Baffin's Bay,about 75° north latitude, and 70° longitude west of London; and 

 the variation of the dip is so inconsiderable, that a very slow change of the po- 

 sition of this supposed magnet would probably be sufficient to produce it; 

 but the operation of such a magnet, according to the general laws of the forces 

 concerned, could not possibly account for the very irregular disposition of the 

 curves indicating the degree of variation or declination; a general idea of 

 these might perhaps be obtained from the supposition of two magnetic poles 

 situated in a line considerably distant from the centre of the earth ; but this 

 hypothesis is I)y no means sufficiently accurate to allow us to place any de- 

 pendence on it. (Plate XLL Fig. 577, 578) 



The art of making magnets consists in a proper application of the attractions 

 and repulsions of the magnetic fluid, by means of the ditferent conducting 

 powers of different kinds of iron and steel, to the production and preservation 

 of such a distribution of the fluid in a magnet, as is the best fitted to the ex- 

 hibition of its peculiar properties. 



We may begin with any bar of iron that has long stood in a vertical posi- 

 tion; but it is more common to employ an artificial magnet of greater 

 strength. When one pole of such a magnet touches the end of a bar of hard 



