MAGNETISM. 143 



arrangement, which they will lose as soon as the dynamic in- 

 ducing force is removed. 



Let us now suppose the vanes, instead of turning easily, 

 to be more stiffly fixed to the axles, so as to be turned with 

 difficulty : it will require a stronger wind to move them and 

 arrange them definitely ; but when so arranged, they will re- 

 tain their position ; and should a gentle breeze spring up in 

 another direction, it will not alter their position, but will it- 

 self be definitely deflected. Should the conditions of force 

 and stability be intermediate, both the breeze and the vanes 

 will be slightly deflected ; or, if there be no breeze, and the 

 spindles be all moved in any direction, preserving their linear 

 relation, they will themselves create a breeze. Thus it is 

 with the molecules of hard iron or steel in permanent mag- 

 nets ; they are polarised with greater difficulty, but, when so 

 polarised, they cannot be affected by a feeble current of elec- 

 tricity. Again, if the magnets be moved, they themselves 

 originate a current of electricity ; and, lastly, the magnetic 

 polarity and the electric current may be both mutually af- 

 fected, if the degrees of motion and stability be intermediate. 



The above instance will, of course, be taken only as an 

 approximation, and not as binding me to any closer analogy 

 than is generally expected of a mechanical illustration. It 

 is difficult to convey by words a definite idea of the dual or 

 antithetic character of force involved in the term polarity. 

 The illustration I have employed may, I hope, somewhat aid 

 in elucidating the manner in which magnetism acts on the 

 other dynamic forces ; i. e., definitely directing them, but not 

 initiating them, except while in motion. 



Magnets being moved in the direction of lines, joining 

 their poles, produce electrical currents in such neighbouring 

 bodies as are conductors of electricity, in directions trans- 

 verse to the line of motion ; and if the direction of motion 

 or the position of the magnetic poles be reversed, the current 

 of electricity flows in a reverse direction. So if the magnet 



