PROGRESS OF MAGNETIC THEORY. 57 



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 deter- 

 mination 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 com- 

 pound result of the attractions and repulsions of 

 many points. Newton had imagined 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 in- 

 stance, "that the directive magnetic force, which 

 the earth exerts upon a needle, is a constant quan- 

 tity, parallel to the magnetic meridian, and passing 

 through the same point of the needle whatever be 

 its position." This was the more important, because 

 it was necessary, in the first place, to allow for the 

 effect of the terrestrial force, before the mutual 

 action of the magnets could be extricated from the 

 phenomena 5 . Coulomb then proceeded to correct 

 the theory of magnetism. 



Coulomb's reform of the ^Epinian theory > in the 

 case of magnetism, as in that of electricity, substi- 

 tuted two fluids (an austral and a boreal fluid,) for 



4 Mem. A. P. 1784, 2d Mem. p. 593. 5 p. 603. 



