192 
Lieut, Foster's account of the 
centre of the needle, and also in lat. 52°i S, long. 90°. These 
unusual quantities are doubtless attributable to a circum- 
stance I had previously noticed in the voyage of H. M. S. 
Griper to Spitsbergen, where it was found, that with the 
ship’s head to the southward, the iron in the vessel neutral- 
ized the needle, or nearly so, and thereby left it free to obey 
any new force impressed upon it ; and so in these cases. In 
both the positions specified, it will be seen that the needle was 
nearly neutralized by the plate, and therefore the effect of 
rotation was more strongly exhibited ; the character of these 
deflections were generally to the east of zero, or reading 
previous to rotation : but when the actioiTof the plate co-ope- 
rates with that of the earth, the contrary to the above effect 
of rotation of course takes place. In this case the horizontal 
intensity of the needle being increased, the effect produced by 
rotation is diminished, as will be seen when the plate was 
placed in lat. 52°^- N, long. 90°, and in lat. 52°^ S, long. 270° : 
in both these positions the upper edge being made to revolve 
from east to west, the needle was deflected to the west. The 
centre of the plate placed in lat. 52°-yN, long. o°, and in 
lat. 52°! S, long. 180°, the upper edge revolving from 
south to north, the deviations were to the west, and of 
greater amount than those to the east, caused by the rotation 
of the plate in the opposite direction. Effects, however, pre- 
cisely contrary to these last mentioned were produced by the 
revolutions of the plate, when fixed with its centre in 
lat. 52°-jS, long. o°, and in lat. 52°^ N, long. 180°. When 
the plate was adjusted with its plane in that of the equator, 
and its centre in the various magnetic positions specified in 
the experiments, very trifling deviations due to rotation were 
