202 Mr. Christie’s remarks on the repetition 
The situation of the point where the deviation due to rota- 
tion vanishes, is somewhat different in the two cases ; Mr. 
Foster’s observations giving its latitude 52°j and mine 54°!. 
The method by which Mr. Foster was under the necessity 
of determining the situation of the plate’s centre, as referred 
to that of the needle, did not, as he states, admit of consider- 
able accuracy, but the errors to which it was liable would 
scarcely account for the difference in the two cases. I can- 
not attribute this difference to errors in estimating the situa- 
tion of the plate’s centre in my own observations, since this 
was determined on the graduated limb of the instrument by 
the index on the arm on which the plate was carried, and 
the effect of any error of centering in the compass would be 
counteracted by the opposite readings. As, however, the 
situation of this point is by no means an indifferent question 
in the theoretical investigation of the phenomena dependant 
upon rotation, I shall, when I have sufficient leisure, repeat 
my observations. 
When the plane of the plate was a tangent to the sphere, 
and its centre in the meridian, I had found that the deviation 
due to rotation vanished when the plate’s centre was at the 
pole, and was a maximum when in the equator : according 
to Mr. Foster’s observations it likewise vanishes at the 
pole, but the maximum takes place at a point intermediate 
to the equator and south pole in longitude 90°, and to the 
equator and north pole in longitude 270°. The situation of 
the point of maximum deviation at Port Bowen, I have no 
doubt arose, as I pointed out to Mr. Foster, from this cir- 
cumstance, that when the centre of the plate is in south 
latitude in longitude 90°, or in north latitude in longitude 
