PROGRESS OF MAGNETIC THEORY; 221 



~nost persuasive of his physical attempts. If a magnet be placed 

 imong iron filings, these arrange themselves in curved lines, which 

 proceed from one pole of the magnet to the other. It was not difficult 

 io conceive these to be the traces of currents of ethereal matter which 

 circulate through the magnet, and which are thus rendered sensible 

 even to the eye. When phenomena could not be explained by means 

 of one vortex, several were introduced. Three Memoirs on Magnetism, 

 written on such principles, had the prize adjudged 3 by the French 

 Academy of Sciences in 1*746. 



But the Cartesian philosophy gradually declined ; and it was not 

 difficult to show that the magnetic curves, as well as other phenomena 

 would, in fact, result from the attraction and repulsion of two poles. 

 The analogy of magnetism with electricity was so strong and clear, 

 that similar theories were naturally proposed for the two sets of facts ; 

 the distinction of bodies into conductors and electrics in the one case, 

 corresponding to the distinction of soft and hard steel, in their rela- 

 tions to magnetism. ^Epinus published a theory of magnetism and 

 electricity at the same time (1*759); and the former theory, like the 

 latter, explained the phenomena of the opposite poles as results of the 

 excess and defect of a magnetic " fluid," which was dislodged and 

 accumulated in the ends of the body, by the repulsion of its own par- 

 ticles, and by the attraction of iron or steel, as in the case of induced 

 electricity. The ^Epinian theory of magnetism, as of electricity, was 

 recast by Coulomb, and presented in a new shape, Avith two flu-ids in- 

 stead of one. But before this theory was reduced to calculation, it 

 was obviously desirable, in the first place, to determine the law of 

 force. 



In magnetic, as in electric action, the determination of the law of 

 attraction of the particles was attended at first with some difficulty, 

 because the action which a finite magnet exerts is a compound result 

 of the attractions and repulsions of many points. Newton had ima- 

 gined the attractive force of magnetism to be inversely as the cube of 

 the distance; but Mayer in 1760, and Lambert a few years later, 

 asserted the law to be, in this as in other forces, the inverse square. 

 Coulomb has the merit of having first clearly confirmed this law, by 

 the use of his torsion-balance. 4 He established, at the same time, other 

 very important facts, for instance, " that the directive magnetic force, 

 which the earth exerts upon a needle, is a constant quantity, parallel 



8 Coulomb, 1789, p. 4S2. 4 Jfem. A. P. 17S4, 2d Mem. p. 5C>3. 



