1 10 Mr, Christie on the Magnetism of Iron [Aug. 



the horizontal needle deviated by the rotation of the plate in 

 this position. 



** When the centre of the plate was in latitude 90° south, 

 contrary to what took place when it was in the equator, the 

 deviation of the south end of the needle is in the direction in 

 which the lower or north edge of the plate revolved ; and we 

 may therefore infer that the same would be the case if a needle 

 were suspended freely by its centre of gravity, and the plane of 

 the plate were in the plane of the secondary to the meridian 

 and equator, its centre being in latitude 90° S : which also 

 agrees exactly with the directions of the deviation of the hori- 

 zontal needle, due to rotation, in this position of the plate. 



" It is evident from these different experimentswith the dipping 

 needle, that whatever may be the peculiar effects produced on 

 the iron by its rotation, the deviations of the horizontal needle, 

 due to the rotation, are of the same nature as those that would 

 arise by referring the deviations of the dipping needle to the 

 horizontal plane. 



Theoretical Investigations, 



" It has in general been considered that the different deviations 

 of the horizontal needle, arising from the action of soft iron on 

 it in different positions, can only be accounted for on the sup- 

 position, that the iron is polarized by position, the upper part 

 being a north pole, and the lower a south one, each pole of the 

 iron attracting the pole of the needle of the same name, and 

 repelling that of a contrary name : but if we suppose that each 

 particle of the iron simply attracts indifferently either pole of a 

 magneiic particle, and refer the attraction of the iron to its centre, 

 then if the angular deviations of a magnetic particle in the centre 

 of the needle and in the line of the dip, arising from such at- 

 traction, be reduced to the horizontal plane, these reduced 

 deviations will agree with the actual deviations of the horizontal 

 needle. In investigating theoretically the effects that are pro- 

 duced by the rotation of a plate of iron, 1 will first suppose, 

 that, independently of rotation, the iron acts in this manner, 

 and that by the rotation it becomes polarized in a direction, 

 making a certain angle with the magnetic axis, since from such 

 a polarizing of the iron,, the law which I have shown to include 

 all the phaenomena, would evidently result. On this suppo- 

 sition, each pole of a magnetic particle in the centre of the 

 needle would be urged by an attractive force towards the centre 

 of the iron plate, by an attractive force towards the pole of a 

 contrary name, and by a repulsive force from the pole of the 

 same name in the iron." 



Mr. Christie here proceeds to investigate this theory by calcula- 

 tion, the results of which, he finds, "indicate that the effects are 

 not produced in precisely the manner we have supposed. In one 



