540 MAGNl'.TICAL OBSERVATIONS. [aPP. N* IX. 



and tlie fol lowing effects will be invariably produc- 

 ed. If the upper end of the nail, when in an erect 

 position, be presented to the north end of the 

 needle, it will attract the needle, and draw it aside; 

 but if presented to the south pole, the nail will repel 

 the needle. If the nail be then reversed, and the con- 

 trary end now upwards, (which before the change of 

 position would have attracted the south pole of the 

 needle, and repelled the north), be presented to the 

 compass, it will be found to attract the north end of 

 the needle. Hence, it is clear, that the moment the 

 soft iron is placed nearly in the position of the dip- 

 ping needle, it becomes k magnet, the upper end be- 

 ing a south pole, and the lower end a north pole. 

 But as it has no power to retain the magnetism that 

 it has acquired, its poles are immediately changed, if 

 its position be reversed. And if the nail be laid in a 

 position almost horizontal, or rather, if it be placed in 

 a position at right angles to that of the dipping needle, 

 it ceases to be a magnet, and, excepting when very 

 near the magnetic needle, evidences no attraction for 

 it whatever. For illustrating these properties of bar- 

 iron, I have constructed a small apparatus, in which 

 a piece of thick iron-wire, 65 inches long, is attached 

 to a plane of mahogany that is moveable through 260 

 decrees of a vertical circle, so that the end of the bar 

 is always at the same distance from a magnetic needle, 

 and can bp presented to it at any given angle, by 

 means of a vertical graduated circle of brass. On 

 performing some experiments with this apparatus, the 

 foUowino; results were obtained : — 1. When the bar 

 was in the plane of the magnetic meridian, and the 

 north pole of the magnetic needle a\ as placed about 

 an inch to one side of the end of the bar, no deflection 

 whatever was produced, so long as the bar was in the 

 direction of tlie magnetic equator ; but an attraction 

 of 22" \va^: produced when the upper end of the bar 



