iron arising from its rotation. s 61 
secondary both to the equator and meridian, the rotation of 
the plate, whatever might be its latitude, caused no deviation 
of the needle. In these experiments, the plate which I made 
use of was a circular one 17.88 inches in diameter, and .099 
inch in thickness, weighing 1 1 2 oz. The further I had pursued 
this inquiry, the more I was disposed to attribute the effects 
I have mentioned to a general magnetic action, arising in a 
peculiar manner from the rotation of the iron ; and my next 
experiments were with the view of ascertaining how far this 
idea was correct. As similar results might not be obtained 
with any other plate, I next made use of a plate 12.13 inches 
in diameter and .075 inch in thickness, weighing 38.75 oz., 
and with it obtained results precisely of the same nature, 
though considerably less in quantity. Another objection 
which occurred to me was this — that the iron being evidently 
slightly polarised in particular points, the effect might be 
supposed to arise from an impulse given to the needle by 
the motion of these points in a particular direction, and that 
the directive power of the needle not immediately over- 
coming the, slight friction on the pivot, a deviation might 
thus arise from the rotation of the plate. Had this, however, 
been the cause of the deviations, I should have expected that, 
when the centre of the plate was in the meridian, the greatest 
effect would be produced with the plate parallel to the 
horizon, and its centre vertical to that of the needle ; but I 
had seen that the greatest deviation took place when the 
centre of the plate was in the equator, its plane being per- 
pendicular to it ; and the deviation arising from the rotation, 
when the plate was parallel to the horizon, was not a fifth 
of the deviation when the plate was perpendicular to that 
