1826.] arising from its Rotation, SS 



measure to the magnetic centre of the needle not being in the 

 centre of suspension ; and it was to counteract their eflPects, that 

 I generally made observations on contrary sides of the centre. 



" With respect to the direction in which the deviation due to 

 rotation took place, it appears, that the rotation of the plate 

 always caused the north end of the needle to move in the same 

 direction as the edge of the plate nearest the south pole of the 

 magnetic sphere ; so that the deviation of the north end of the 

 needle was in the direction in which the south edge of the plate 

 moved, and that of the south end of the needle in the direction 

 in which the north edge moved, referring the edges to the poles 

 of the sphere. 



** Having ascertained, that when the centre of the plate was 

 in the pole, and its plane parallel to the equator, the deviation 

 due to rotation was nothing ; and some of the first experiments 

 which I had made having mdicated that this was also the case 

 when the centre of the plate was in the secondary to the equator 

 and meridian, and its plane, as before, a tangent to the sphere, 

 I wished to ascertain whether such were really the fact." 



The experiments made accordingly, the results of which are 

 given in another table, left no doubt in Mr. C.'s mind on the 

 subject, and from them, combined with the preceding, <'wc 

 may infer, that if the centre of the plate were made to describe 

 any parallel of latitude, the deviation due to rotation would be 

 nothmg when the longitude was 0° or 180°, and a maximum 

 when the longitude was 90° or 270", which is precisely the 

 reverse of the absolute deviations that would be produced by the 

 plate describing the parallel of latitude. 



" The next experiments which I made were with the view of 

 determining whether the rotation of the plate would produce any 

 deviation, when its plane coincided with the equator. For this 

 purpose an axis was fixed perpendicularly on the arm of the 

 instrument in such a manner, that when the plate revolved on 

 it, its plane was parallel to the limb. 



" In order to make these observations, it was necessary t« 

 adjust the whole instrument twice ; since the deviations for the 

 longitudes 90" and 270° could not be observed with the same 

 adjustment as those for the longitudes 0° and 180°. For the 

 longitudes 90° and 270°, the axis of the instrument was hori- 

 zontal, and pointed east and west, and the moveable limb 

 revolved on the axis until its plane, and therefore also 

 that of the iron plate, made an angle of 90° 30' with the 

 horizon, rising towards the north ; so that the compass being 

 elevated until the centre of the needle was in the plane of the 

 plate, the plate was then in the equator. For the other longi- 

 tudes, the axis of the instrument was inclined to the horizon at 

 an angle of 19° 30', and in the plane of the meridian ; and the 

 moveable limb adjusted at right angles to the fixed one : the 



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