iron arising from its rotation. 
393 
Theoretical Investigations. 
It has in general been considered that the different devia- 
tions of the horizontal needle, arising from the action of soft 
iron on it in different positions, can only be accounted for on 
the supposition, that the iron is polarised 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 in- 
differently either pole of a magnetic particle, and refer the 
attraction of the iron to its centre, then if the angular devia- 
tions of a magnetic particle in the centre of the needle and 
in the line of the dip, arising from such attraction, be reduced 
to the horizontal plane, these reduced deviations will agree 
with the actual deviations of the horizontal needle. In in- 
vestigating theoretically the effects that are produced by the 
rotation of a plate of iron, I will first suppose, that, inde- 
pendently of rotation, the iron acts in this manner, and that 
by the rotation it becomes polarised in a direction, making a 
certain angle with the magnetic axis, since from such a 
polarising of the iron, the law which I have shown to include 
all the phcenomena, 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. 
Suppose now that the centre and plane of the plate are in 
the secondary to the meridian and equator, that its centre is 
