208 Infix Terrestrial Magnetism on Iron, 



tral pole ; because, say they, it is attracted by the magnetic 

 pole of the earth situated at the north, and which is naturally 

 the boreal pole ; for the same reason they have given the 

 name of boreal pole to that end of the needle that is directed 

 towards the south ; because, say they, it is attracted by the 

 austral pole of the globe. With regard to ourselves, as we 

 prefer a denomination founded upon fact to that which rests 

 upon theory more or less contestable, we shall continue to 

 call the north pole of the needle that which is directed to- 

 wards the north, and the south pole that which is directed 

 towards the south. 



The magnetisation produced by terrestrial magnetism is 

 facilitated by all actions, whether mechanical or physical, 

 which derange the molecules of iron from their natural posi- 

 tion of equilibrium. Thus percussion, torsion, every kind of 

 vibration, impressed upon a bar of iron determines in it the 

 presence of the two magnetic poles ; simple^ oxidation in the 

 air produces the same effect. To prove that this magnetisation 

 is entirely due to the influence of terrestrial magnetism, and 

 not to these actions themselves, we have merely to examine 

 the position of the poles in the bars submitted to experiment, 

 and we find that this position is always that which would re- 

 sult from the immediate action of the globe ; thus, the north 

 pole is always the one found at the extremity of the bar that 

 is inclined below the horizon, or at that which is turned to- 

 wards the side of the north, if the bar is horizontal. We 

 may even, if the bar is of very soft iron, immediately change 

 the poles by suddenly turning it so that the extremity which 

 was directed towards the north shall be to the south, and 

 that which was directed towards the south shall be to the 

 north. Furthermore, it is easy to prove that, whatever be 

 the action by which the body is constrained, the magnetism 

 that it acquires is the more intense as its position approaches 

 more the direction of the dipping needle ; and that it becomes 

 altogether null if the bar is placed in a position perfectly 

 perpendicular to this direction. We have thus the evident 

 proof that the effect does not arise in an immediate manner 

 from the action to which the bar is subjected, but simply 



