355 
iron arising from its rotation. 
upon the needle, independent of the partial magnetism of 
particular points, I considered that if the plate were made to 
revolve the contrary way, the deviation ought to be on the 
opposite side, and this I found to be the case. I will illustrate 
this by the observations made when I first noticed the effect. 
The plate was divided at every 30° of its circumference 
(Fig. 4.) by lines drawn through the centre, and being placed 
on the arm, so that 0° coincided with the upper part of the 
limb, the north end of the needle pointed 10' east ; but when 
this point again coincided with the limb, by the upper edge 
of the plate revolving from west to east, the needle pointed 
30' east : making the plate revolve the contrary way, that is, 
its upper edge from east to west, when 0° coincided with the 
limb, the north end of the needle pointed 28' west: so that 
there was a difference of 58', when every point of the plate 
had the same position with respect to the needle, according 
as the plate was brought into that position by revolving from 
west to east, or from east to west. As this appeared extra- 
ordinary, I made repeated observations at the time, to ascer- 
tain that the effect was independent of any accidental 
circumstances, and found that the results always accorded 
with the first, the difference caused by the rotation of the 
plate being however greater or less according to the position 
of the plate. 
Having fully satisfied myself that, in whatever manner 
the rotation of the plate might cause this difference, such was 
really the effect, I next endeavoured to ascertain the nature 
and degree of the difference, according to the different situa- 
tions of the centre of the plate. For this purpose I made a 
great variety of experiments, of which I shall not however 
MDCCCXXV. 3 A 
