321 
diurnal variation of the magnetic needle^ 
about 64°, and that the poles were at the distance 4‘5 inches 
from the centre. 
The same positions of the poles will account for the devia¬ 
tions when the compass was to the east, or to the west of the 
plate’s centre. It however appears from these, that the plate 
did not act with perfect uniformity ; that is, that the poles were 
not quite symmetrically situated with respect to the axis of 
heat. If the axis of polarity be supposed to cut the axis of heat 
about i’9 inch from the centre towards the place of heat, and 
to be inclined to it so that, supposing the place of heat down¬ 
wards, the angle below the axis of polarity on the western 
side is slightly obtuse, and on the eastern side, acute, it 
would perfectly account for the nature of the deviations when 
the angle between the place of heat and the compass was 
o, or 180°. 
It would appear then, that such deviuLioiis us those observed 
with the plane of the plate at right angles to the plane of the 
meridian would arise from four poles in the plate, two near 
each surface, in a line cutting the axis of heat nearly at right 
angles, between the centre and the place of heat, the south 
pole being on each surface of the plate on the right hand, 
looking on that surface from the place of heat. 
The same polarity will also account for the deviations 
that I observed with the plane of the plate in the plane of the 
meridian, and likewise with its plane horizontal. The obser¬ 
vations with the plane of the plate in the plane of the meri¬ 
dian were made in precisely the same manner as the pre¬ 
ceding, with the addition, that the deviations were likewise 
observed when the heated point was 45° on each side of the 
needle. In these the marked face of the plate was west, and 
MDCCCXXVII. T t 
