222 HISTORY OF MAGNETISM. 



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 ^Epir.ian theory, in the case of magnetism, 

 as in that of electricity, substituted two fluids (an austral and a boreal 

 fluid,) for the single fluid ; and in this way removed the necessity under 

 which ^Epinus found himself, of supposing all the particles of iron and 

 steel and other magnetic bodies to have a peculiar repulsion for each 

 other, exactly equal to their attraction for the magnetic fluid. But in 

 the case of magnetism, another modification was necessary. It was 

 impossible to suppose here, as in the electrical phenomena, that one of 

 the fluids was accumulated on one extremity of a body, and the other 

 fluid on the other extremity ; for though this might appear, at first 

 sight, to be the case in a magnetic needle, it was found that when the 

 needle was cut into two halves, the half in which the austral fluid had 

 seemed to predominate, acquired immediately a boreal pole opposite to 

 its austral pole, and a similar effect followed in the other half. The 

 same is true, into however many parts the magnetic body be cut. The 

 way in which Coulomb modified the theory so as to reconcile it Avith 

 such facts, is simple and satisfactory. He supposes 6 the magnetic body 

 to be made up of " molecules or integral parts," or, as they were after- 

 wards called by M. Poisson, " magnetic elements." In each of these 

 elements, (which are extremely minute,) the fluids can be separated, so 

 that each element has an austral and a boreal pole ; but the austral 

 pole of an element which is adjacent to the boreal pole of the next 

 neutralizes, or nearly neutralizes, its effect ; so that the sensible mag 

 netism appears only towards the extremities of the body, as it would do 

 if the fluids could permeate the body freely. We shall have exactly 

 the same result, as to sensible magnetic force, on the one supposition 

 and on the other, as Coulomb showed. 7 



The theory, thus freed from manifest incongruities, was to be reduced 

 to calculation, and compared with experiment ; this was done in Cou- 

 lomb's Seventh Memoir. 8 The difficulties of calculation in this, as in the 

 electric problem, could not be entirely surmounted by the analysis of 

 Coulomb ; but by various artifices, he obtained theoretically the rela- 



p. G03. 6 Mem. A. P. 1780, p. 488. T Jfcm. A. P. p. 492. D A. P. 1789. 



