40 -Polarity of Magnets. [Book I. 



called the poles of the magnet : and at thefe extremi- 

 ties the magnetic virtues feem to exift in their great- 

 eft force, as a magnet will fupport a much more con- 

 fiderable weight near the poles than at any other 

 part *. 



This property in the magnet has proved the bafis 

 of an invention the moft ufeful to navigation that hu- 

 man fagacity ever difcovered j as before this infallible 

 guide was enlifted in the fervice of the mariner, the 

 moft adventurous pilot did not prefume to truft him- 

 felf out of the fight of land ; confequently commerce 

 is much facilitated by the difcovery, arid fhipwreck is 

 a much lefs frequent calamity. It was not till the 

 thirteenth century that this circumftance was known. 

 Authors are generally agreed at prefent, that a Nea- 

 politan, of the name of John de Gioja, if not the in- 

 ventor of the mariner's compafs, was at leaft the firft 

 who made ufe of it in conducting vefiels in the Medi- 

 terranean -j-. Some ridiculous pretences have been 

 made by the Chinefe to the honour of this, as well as 

 of other European inventions j but the fables of that 

 barbarous people, as well as of their encomiafts, the 

 jefuits and infidels, are little to be regarded J. 



Both the properties of attraction and repulfion have 

 an intimate connexion with this of polarity. Thus, 

 it is uniformly found to be the cafe, that in two mag- 

 nets an attractive force obtains between the oppo- 

 fite poles, and a repulfive force between the poles 

 of the fame denomination. If, for inftance, the north 

 pole of the one is brought near the north pole of the 

 other, a mutual repulfion will take place ; but if the 

 fouth pole of the one is applied to the north pole of the 

 other, they will be mutually attracted. And if, upon, 



* Adams on Mag. p. 387. | Cav. on Mag. 



J See Mr. Adams's iifiay, p. 409. 



the 



