VII. ON A NEW MAGNETICAL INSTRUMENT, FOR THE 

 MEASUREMENT OF THE INCLINATION, AND ITS 

 CHANGES. 



From the Proceedings of the Royal Irish Academy, 1842. 



IF a soft iron bar, perfectly devoid of magnetic polarity, be held in 

 a vertical position, it immediately becomes a temporary magnet 

 under the inducing action of the earth's magnetic force, the lower 

 extremity becoming a north pole, and the upper a south pole. Ac- 

 cordingly, if a freely- suspended horizontal magnet, whose dimen- 

 sions are small in comparison with those[of the bar, be situated near, 

 in a plane passing through one of these 'poles, it will be deflected 

 from the magnetic meridian. The deflecting force is the induced 

 force of the bar, which may be regarded as proportional to the 

 energy of the inducing cause, i. e. to the vertical component of the 

 earth's force ; while the counteracting force is the horizontal com- 

 ponent of the same force, acting directly on the magnet itself, to 

 bring it back to the magnetic meridian. Thus the magnet will 

 take up a position of equilibrium, under the action of these oppos- 

 ing forces; and this position will serve to determine the ratio 

 which subsists between them. When the right line connecting 

 the centre of the horizontal magnet, and the acting pole of the 

 bar, is perpendicular to the magnetic meridian, the tangent of the 

 angle of deflection will measure the ratio of the two forces, and will 

 therefore be proportional to the tangent of the. magnetic inclination. 

 Accordingly, by observing the changes of position of the horizontal 

 magnet, so circumstanced, we can infer those of the inclination 

 itself. 



But the iron bar may have (and generally will have) a certain 

 portion of permanent magnetism, which will concur with the in- 



