203 
of his experiments by Lieut. Foster. 
2 70°, the directive intensity of the horizontal needle is dimi- 
nished by the attraction of the iron plate ; and although this 
diminution would produce effects scarcely observable here, 
where the intensity of the horizontal needle is great, and the 
deviation due to rotation very small, yet when the case is 
reversed, as in the Port Bowen observations, the effect will 
be so sensible, that the increase in deviation from this cause 
will much more than counterbalance the diminution which 
arises from the centre of the plate being nearer to the pole. 
The effects that would be produced under these circum- 
stances will be most evident, by considering how a dipping 
needle would be affected, and referring its deviations to the 
horizontal plane, remembering that in all cases an increase 
of dip causes an increase in horizontal deviation, and the 
contrary. When the centre of the plate is in south latitude 
longitude 90°, and in north latitude longitude 270°, the 
attraction of the plate tends to increase the dip, and to dimi- 
nish it when in south latitude longitude 270°, and north 
latitude longitude 90° ; so that in the former cases the 
deviation will be increased from this cause, and in the latter 
diminished. This effect was so great that in one instance 
the zero, or reading of the north end of the needle previous 
to rotation, corresponding to the point 240 on the plate, was 
97° W, 36° E, after rotation in one direction, and 144 0 E, 
after rotation in the other, giving no less than 108° for the 
deviation due to rotation in opposite directions : correspond- 
ing to the point 180 on the plate, these were 86° 40' E, 
42 0 io' W, and 20° io' W, giving only 22 0 for the deviation 
due to rotation. By referring to Table I. in my Paper, it 
will be seen that there are indications of the same effect, 
