tii Mr, Christie on the Magnetism of Iron [July, 



deviation being nothing when the centre of the plate was in the 

 pole of the dipping needle, and a maximum when in the equator, 

 as from its being very nearly equal at equal distances on each 

 side of the pole, and also of the equator ; that is, at very unequal 

 distances from the axis of the horizontal needle ; and from the 

 deviations at equal distances from the axis of the horizontal 

 needle being very unequal. For if we compare the deviation due 

 to rotation in lat. 70° 30' S, long. 90°, with that in lat. 70° 30' S, 

 long. 270°, the difference is only 1' ; in the first case, the centre 

 of the plate was at the distance of 90° from the axis of the hori- 

 zontal needle, and its plane parallel to it ; and in the other at the 

 distance of 51°, and its plane making an angle of 39° with this 

 axis. Again, in the four corresponding situations of lat. 19° 30', 

 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 situations 

 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 respect 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. ; 

 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 strik- 

 ing, if we compare the deviation in lat. 70° 30' S, long. 270°, 

 with that in lat. 31° 30' S, long. 90°, the centre of the plate 

 being in each case at the distance of 51° from the axis of the 

 horizontal needle, and its plane making an angle of 39° with it. 

 The differences which we have noticed in the deviations observed 

 at the same distance from the equator, is not more than I have 

 found to arise from a slight change in the adjustment of the 

 centre of the needle to the centre of the instrument, the plate 

 remaining in the same position. These errors of adjustment I 

 found it almost impossible to avoid, owing probably in a great 



" ♦ The nature of the instrument would not admit of observations being made so 

 near to the north pole in long. 270o as lat. 45°, or so near as lat. TO® 30' on the other 

 tide of the fupport O I.** 



