Chap. 6.] Theories of^Magmtlfm. $3 



rather for that immenfe mafs of ferrugineous matter 

 which the earth contains. The attraction of the 

 magnet is commonly fuppofed to depend upon the 

 agency of a fubtile fluid which circulates around it, en- 

 ters the pores of the magnet itfelf, and of all the 

 bodies which it attracts. I cornels that the theories 

 which are founded upon this hypothefis appear to me 

 fo deficient in the only proof that ought to be admitted 

 in natural philofophy, I mean actual obfervation, that 

 I am frill inclined to account the caufe of magnetifm 

 as one of the undifcovered principles of philofophy. 

 I am not fond of indulging the imagination in its fa- 

 vourite propenfity to create invifible agents in order 

 for the fabrication of plaufibie theories, which fome 

 flight and cafual experiment may mortly overturn. 

 We appear to^be equally ignorant of the nature of 

 gravitation, and of the common attraction of cohefion 

 and combination. It is a trite remark, that there are 

 certain points at which the human faculties muft flop 

 in all our fpeculations. This would be a dangerous 

 tenet, if it promoted indolence, or difcouraged our 

 ardour in the puriuit of natural knowledge by the 

 only fecure path, I mean that of experiment; but it is 

 a faiutary maxim when applied to the imagination, and 

 when it only ferves to reftrain our ardour for fabricat- 

 ing fyftems, which have no other end but to remove 

 fora moment the uneafy but ufeful fenfation of doubt 

 and curiofity. 



I lhall not therefore incumber my work with the 

 detail of fyftems to which I do not feel inclined to* 

 afient j but for a clear, and, I think, correct, ftatement 

 of the moft plaufibie theories concerning" the caufc? of 

 magnetifm, fhall content inyfelf with referring my rea- 

 der to an author to whom I have many obligations, 

 E 3 and 



