-^ 



prior to the Introduction of the Potentials. 55 



have mention'd any thing relating to this property of the Magnet, 

 have agreed, not only that the Attraction and Repulsion of 

 Magnets are not equal to each other, but that also, they do not 

 observe the same rule of increase and decrease." 



" The Attraction and Eepulsion of Magnets decreases, as the 

 Squares of the distances from the respective poles increase." 

 This great discovery, which is the basis of the mathematical 

 theory of Magnetism, was deduced partly from his own observa- 

 tions, and partly from those of previous investigators (e.g. 

 Dr. Brook Taylor and P. Muschenbroek), who, as he observes, 

 had made accurate experiments, but had failed to take into 

 account all the considerations necessary for a sound theoretical 

 discussion of them. 



After Michell the law of the inverse square was maintained 

 by Tobias Mayer* of Gottingen (&. 1723, d. 1762), better known 

 as the author of Lunar Tables which were long in use ; and by 

 the celebrated mathematician, Johann Heinrich Lambertf (b. 

 1728, d. 1777). 



The promulgation of the one-fluid theory of electricity, in 

 the middle of the eighteenth century, naturally led to attempts 

 to construct a similar theory of magnetism ; this was effected in 

 1759 by AepinusJ, who supposed the "poles "to be places at 

 which a magnetic fluid was present in amount exceeding or 

 falling short of the normal quantity. The permanence of 

 magnets was accounted for by supposing the fluid to be entangled 

 in their pores, so as to be with difficulty displaced. The particles 

 of the fluid were assumed to repel each other, and to attract the 

 particles of iron and steel ; but, as Aepinus saw, in order to satis- 

 factorily explain magnetic phenomena it was necessary to assume 

 also a mutual repulsion among the material particles of the 

 magnet. 



Subsequently two imponderable magnetic fluids, to which 



* Noticed in Gottinger Gelehrter Anzeiger, 1760 : cf. Aepinus, Nov. Comm. 

 Acad. Petrop., 1768, and Mayer's Opera Inedita, herausg. von G. C. Lichtenberg. 

 \-Histoirede V Acad. de Berlin, 1766, pp. 22, 49. 

 % In the Tentamen, to which reference has already been made. 



