S6S 
iron arising from its rotation. 
in the four corresponding situations of lat. 19° so', the mean 
deviation due to rotation is 1° 32', and none of the deviations 
differ from this by more than 5', although in two cases the 
centre of the plate was in the axis of the horizontal needle, 
and its plane perpendicular to it, and in the two others the 
centre of the plate was at the distance of 39° from this axis, 
and its plane made an angle of 51° with it. The mean of the 
deviations due to rotation in the three* corresponding situa- 
tions of lat. 45° is 49', from which none of the deviations 
differ by 3', notwithstanding the difference in the situations 
of the centre and plane of the plate, in these cases, with re- 
spect to the axis of the horizontal needle. In long. 90° lat. 
45° S, the centre of the plate was 64° 30' above the horizontal 
axis, and its plane made an angle of 25° 30' with it ; in long. 
90° lat. 45° N, it made an angle of 64° 30' at 25° 30' below it ; 
and in long. 270° lat. 45° S, it was in a position above it 
similar to the last. Any doubt, however, on the subject 
will be removed, if we compare the deviation' in long. 90° 
lat. 39° N with that in long. 270° lat. o ; the one deviation 
being nearly double of the other, although the centre of the 
plate was at the distance of 19° 30' from the axis of the 
horizontal needle, and its plane made an angle of 70° 30' 
with it in both cases. The difference is even more striking, 
if we compare the deviation in lat. 70° 30' S, long. 270°, with 
that in lat. 31° so'S, long. 90°, the centre of the plate being 
in each case at the distance of 51° from the axis of the hori- 
zontal needle, and its plane making an angle of 39° with it. 
* The nature of the instrument would not admit of observations being made so 
near to the north pole in long. 270° as lat. 45°, or so near as lat. 70® 30' on the other 
side of the support G I. 
S B 
MDCCCXXV. 
