I 



1826.] arising from its Rotatiot}^^^ '''^^^' ZT 



quently parallel to the limb, this axis was inclined in such a 

 manner that the plane of the plate passed through the axis of 

 the instrument ; so that the axis of the instrument being hori- 

 zontal, and passing through the centre of the needle perpendicu- 

 larly to the meridian, when the arm of the instrument was 

 adjusted to zero on the limb, the revolution of the hmb caused 

 the centre of the plate to describe the magnetic meridian, and at 

 the same time the plane of the plate always passed through the 

 centre of the needle. The distance between the centre of the 

 plate and that of the needle was, as in the last, 13 '2 inches." 



" From these observations I find, directly contrary to what 

 took place when the plane of the plate was a tangent to the 

 sphere, that the deviation due to rotation increases from the 

 equator to the pole, where it is a maximum. In this case, how- 

 ever, as in the other, the deviations are very nearly equal at 

 equal distances on each side of the equator ; so that, as before^ 

 it appears that the horizontal needle was affected by the rotation 

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

 plate with respect to the poles and equator of the horizontal 

 needle, but with respect to tlie poles and equator of an imaginary 

 dipping needle passing through the centre of the horizontal 

 needle. 



" With regard to the direction of the deviation due to rotation^ 

 it appears, that when the centre of the plate had noith latitude, 

 the north end of the needle deviated in the direction of the motion 

 of the plate's inner edge ; and when it had south latitude^ the 

 north end deviated in a contrarij direction to that of the inner ed^e 

 of the plate, and therefore the south end deviated in the direction 

 of the inner edge : so that the end of the needle of the same name 

 as the latitude, always deviated in the direction of the motion of 

 the plate" s inner edge, 



" Let us compare this with the inference we have drawn from 

 the observations, viz. that when the centre of the plate is in the 

 meridian, and its plane a tangent to the s])here, the north end of 

 the needle, by the rotation of the plate, deviates in the direction 

 of the motion of the south edge, and the south end in the direc- 

 tion of the north edge, of the plate ; that is, either end of the' 

 needle deviates in a direction contrary to that of the motion of 

 the edge of the plate nearest to the pole of the sphere of the; 

 same name as that end. Now, if from the position which the' 

 plate had in the last experiments, namely, its plane passing 

 through the centre of the needle, it be conceived to revolve 

 about its diameter, which is perpendicular to the plane of the 

 meridian, until its plane be a tangent to the sphere, the direc- 

 tion of the revolution about this diameter being of the inner' 

 edge towards the pole of the same name as the latitude of the' 

 plate's centre, the inner edge will become the edge of the same ' 

 name as the end of the needle, which, in its first position,' 



