1826.] arising from its Rotation, 33 



30*^ coincided with the same line, and, after slightly agitating 

 the needle, noted the deviation ; and in the same manner were 

 the points 60°, 90°, 120°, 150°, 180°, 210°, 240°, 270°, 300°, 

 330 , 360°, or 0°, brought successively to coincide, and the 

 deviations noted. I now made the plate revolve once from west 

 to easty without noting the deviations, bringing 0° or 360° to 

 coincide with the same line, and then brought in succession 

 330°, 300°, 270°, 240°, 210°, 180°, 150°, 120°y 90°, 60°, 30°, 0° 

 to coincide, noting the deviations as before. The sum of the 

 first set divided by 12, 1 considered as the mean deviation, when 

 the? plate revolved from east to west ; and the sum of the others 

 divided by 12, as the mean deviation, when the plate revolved 

 from west to easl : their difference was the mean effect of the 

 rotation in contrary directions. This I call the Deviation due to 

 Rotation; and to distinguish it from the deviation caused simply 

 by the position of the iron, I call this last the Absolute Devia- 

 iion. When the change in the deviation from one point of the 

 plate to another was not so considerable, I made the observa- 

 tions only for the points 0°, 90°, 180°, 270° on the plate. 



"I now proceed to the detail of the expeiimeuts, and the 

 conclusions I draw from them. In those which I shall first 

 describe, the centre of the plate was always in the n^agnetic 

 meridian ; its plane was perpendicular to the meridian, and a 

 tangent to the sphere, whose centre was the centre of the needle; 

 and the plate revolved, as in all other cases, in its own plane : 

 they are a repetition of those by which I first discovered several 

 of the facts I have mentioned, but made for the purpose of deter- 

 mining more precisely the deviation caused by the rotation. In 

 making these, the instrument was adjusted so that the index at^, 

 fig. 1, pointed to 0°, that at K to 90°, and those at 0, 0' to zero." 



Mr. Christie now gives a table of observations, the results of 

 which he states as follows : 



^* From these observations it appears, that when the centre of 

 the plate was in the pole of the magnetic sphere, its plane being 

 parallel to the equator, the position of the needle, for any situa- 

 tion of the several points of the plate, was the same whether they 

 were brought into that situation by the plate revolving from 

 east through south to west, or from west through south to east ; 

 that is, that the deviation due to rotation was nothing : 



" That the deviation due to rotation increased from this point 

 towards the equator, where it was the greatest : 



'' And that the horizontal needle was affected by the rotation 

 of the plate, not according to the situation of the centre of the 

 plate as regarded the poles and equator of the horizontal needle, 

 but as regarded the poles and equator of an imaginary dipping 

 needle passing through the centre of the horizontal needle. 



" This last is not so evident, from the circumstance of the 



New Scries f vol. xii. o 



