$o Diminution of Magnetic Power. [Book I. 



of iron, confequemly it is rendered capable of a greater 

 degree of attraction for the original magnet*. To 

 make this perfectly clear, it is necefiary to be obferv- 

 ed, that a piece of iron, brought within a certain dif- 

 tance of a magnet, becomes itfelf porTeflfed of all the 

 magnetic properties, and that part of the iron which 

 is neareft the magnet acquires a contrary polarity. 

 Thus, if a magnetic chain is compoied of feveral 

 pieces of iron, each piece is in itfelf a complete mag- 

 net, and they mutually ftrengthen the magnetic virtue 

 of each other j-, as all magnets in contact are known, 

 to do. 



The magnetic virtue is DIMINISHED : 

 i ft. By dtfufe : particularly if the magnet is laid 

 amongft iron, or permitted to ruft. Magnets will 

 alfo be injured, unlefs they are kept together with the 

 oppofite poles correiponding, the ends being connect- 

 ed by pieces of iron j and they ought never to touch, 

 except when in this pofition. The fouth pole mould 

 always be employed in this hemifphere to lift iron ; 

 and a (trait magnet will be weakened, unlefs kept 

 with its fouth pole to, the north in the direction of the 

 magnetic needle, or downwards in that of the dip- 

 ping needle J. 



sdly. Heat weakens the power of a magnet ; and that 

 high degree which is called by chemifts a white heat, 

 , entirely deftroys it . On this principle Mr. Canton 

 endeavoured to account for the daily variation of the 

 compafs; as fuppofing it to depend on the heating 

 and cooling of the magnetic fubftances within the 

 earth. This theory he illuftrated by the following 

 experiment : About E. N. E. from a compafs he 



Cav. 200. f See Cav. p. 30 and 203. 



Adams on Mag. 397, 44.3. . Cav. 35. 



placed 



