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



compass was then elevated to coincide with the plane of the 

 plate. 



" 111 these experiments the distance of the centre of the iron 

 from the centre of the needle was 13*2 inches ; but as its edge 

 was only 4*26 inches distant, the differences between the devia- 

 tions corresponding to the several points on the plate were 

 greatly increased ; and therefore to obviate any inaccuracies 

 that might arise, from the points not being brought into precisely 

 the same situation when the plate revolved in the opposite 

 directions, I increased the number of observations, making 

 twenty-four for each position, namely, twelve points on the plate, 

 as I have before described, the deviation for any point being 

 observed when that point coincided with the line joining the 

 centre of the plate and needle." 



The observations on this subject, which are given in a third 

 table, " show very clearly, that when the centre of the plate is 

 in the equator, and its plane also coincides with the plane of the 

 equator, the deviation due to rotation is always nothing, since the 

 small differences to be observed here in the revolutions in oppo- 

 site directions are only such as may justly be attributed to slight 

 errors in the adjustments of the centre of the needle or of the 

 plane of the plate, which are almost unavoidable. With regard 

 to the several deviations in the different columns, I should notice 

 that they are not those actually observed, but derived from them 

 by subtracting the same number from all the deviations observed 

 in two corresponding columns, so that they indicate the same 

 difference of deviations in the two revolutions as those actually 

 observed, and therefore give the same deviation due to rotation. 

 The necessity of this reduction arose from the circumstance of 

 my having to adjust the compass to the proper height, so that 

 its centre might be in the plane of the plate, while it was under 

 Jthe influence of the partial magnetismof particular points in the 

 plate ; and having done this, when zero of the compass was 

 brought to coincide with the point of the needle it was not 

 jiecessarily in the magnetic meridian, since the needle was 

 under the influence of this partial magnetism ; and as I wished 

 the deviations to be those from the meridian, I reduced the 

 observed deviations as I have mentioned. 



*' Being convinced that the rotation of the plate in the plane 

 of the equator caused no deviation of the needle, I proceeded to 

 determine the effects produced by its rotation in other planes* 

 In the first set of observations which I made, the centre of the 

 plate was in the meridian, and its plane perpendicular to the 

 plane of- the meridian and passing through the centre of the 

 needle. Before however making these, to avoid the necessity 

 of moving the compass as in the last, 1 made a shght alteration 

 in the instrument. Instead of having the axis on which the 

 plate revolved perpendicular to the arm, and the plate conse- 



