1826.] arising from its Rotation, 31 



" Having fully satisfied myself that, in whatever manner the 

 rotation of the plate might cause this difference, such was really 

 the effect, I next endeavoured to ascertain the nature and degree 

 of the difference, according to the different situations of the 

 centre of the plate. For this purpose I made a great variety of 

 experiments, of which I shall not however here give the details, 

 as I afterwards repeated them in a more convenient manner, and 

 with greater precision ; but shall merely point out the nature of 

 them in general, and the conclusions which I at the time drew 

 from them. The instrument being adjusted, and the arm fixed 

 so that the centre of the plate was in the position which I 

 required, I made the plate revolve so that its upper edge moved 

 from west to east, and noted the greatest and least deviation: of 

 the north end of the needle ; I then made the corresponding 

 observations when the plate revolved in the contrary direction ; 

 a mean of the differences between the two greatest and between 

 the two least I considered as the effect produced on the needle 

 by the rotation of the plate in opposite directions. Repeating 

 these in a variety of positions, I found that when the centre of 

 the plate was in the magnetic meridian, its plane being always 

 a tangent to the sphere circumscribed about the centre of the 

 needle, the deviation of the needle caused by the rotation of the 

 plate in its plane was the greatest when the centre of the plate 

 was in the equator, and that it decreased from there towards the 

 poles, where it was nothing; * that when its centre was on the 

 equator, this deviation was the greatest when the centre of the 

 plate was on the meridian, or in longitude 90°, and decreased to 

 nothing in the east and west points, or when the longitude of 

 the plate was 0° or 180°; and that when the centre of the plate 

 was in the secondary both to the equator and meridiun, thef. 

 rotation of the plate, whatever might be its latitude, caused nd 

 deviation of the needle. In these experiments, the plate which 

 I made use of was a circular one 17*88 inches in diameter, and- 

 •099 inch in thickness, weighing 112 oz. The further I had 

 pursued this inquiry, the more I was disposed to attribute the 

 effects I have mentioned to a general magnetic action, arisihg 

 in a peculiar manner from the rotation of the iron ; and my next 

 experiments were with the view of ascertaining how far this idea 

 was correct. As similar results might not be obtained with any 

 other plate, I next made use of a plate 12-13 inches in diameter- 

 and '075 inch in thickness, weighing 38*75 oz. and with it 

 obtained results precisely of the same nature, though consider- 

 ably less in quantity. Another objection which occurred to me' 



* I should here mention, that, from the nature of my instrument, I could not make' 

 observations at the north pole ; but as the results, as far as I could observe, were of the 

 same nature on this side of the equator as on the south side, I think 1 am warranted ia 

 concluding, that at the north pole the results would likewise be of the same nature as at 

 the south pole." * 



