ON MAGNETISM. 537 



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 position of this 

 supposed magnet would probably be sufficient to produce it ; but the ope- 

 ration of such a magnet, according to the general laws of the forces con- 

 cerned, 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 by no means sufficiently accurate to allow us to place any 

 dependence on it. (Plate XLI. Fig. 577, 578.) 



The art of making magnets consists in a proper application of the attrac- 

 tions and repulsions of the magnetic fluid, by means of the different con- 

 ducting 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 exhibition of its peculiar properties. 



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

 position ; 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 

 iron or steel ; that end assumes in some degree the opposite character, and 

 the opposite end the same character : but in drawing the pole along the 

 bar, the first end becomes neutral, and afterwards has the opposite polarity ; 

 while the second end has its force at first a little increased, then becomes 

 neutral, and afterwards is opposite to what it first was. When the opera- 

 tion is repeated, the effect is at first in some measure destroyed, and it is 

 difficult to understand why the repetition adds materially to the inequality 

 of the distribution of the fluid ; but the fact is certain, and the strength of 

 the new magnet is for some time increased at each stroke, until it has 

 acquired all that it is capable of receiving. Several magnets, made in this 

 manner, may be placed side by side, and each of them being nearly equal 

 in strength to the first, the whole collection will produce together a much 

 stronger effect ; and in this manner we may obtain from a weak magnet 

 others continually stronger, until we arrive at the greatest degree of polarity 

 of which the metal is capable. It is, however, more usual to employ the 

 process called the double touch ; placing two magnets, with their opposite 

 poles near to each other, or the opposite poles of a single magnet, bent into 

 the form of a horseshoe, in contact with the middle of the bar ; the opposite 

 actions of these two poles then conspire in their effort to displace the mag- 

 netic fluid, and the magnets having been drawn backwards and forwards 

 repeatedly, an equal number of times to and from each end of the bar, with 

 a considerable pressure, they are at last withdrawn in the middle, in order 

 to keep the poles at equal distances. 



