^ Mr, Christie on the Magnetism of Irony ^c. [July, 



centre of the plate is equal lo half the angle which the dipping 

 needle makes with the horizon. Whether this coincidence is 

 purely accidental, or is a necessary consequence of the manner 

 m which the effect is produced, must remain doubtful, until it 

 can be shown how the action takes place ; it, however, led me 

 to ascertain precisely the point at which the deviation due to 

 rotation vanishes;' 



" General Law of the Deviation due to Rotation deduced from 

 the Experiments. 



" Having now ascertained the nature of the effects produced 

 on the horizontal needle by the rotation of the plate in different 

 planes, 1 endeavoured to discover some general law, according 

 to which the direction of the deviation depended on the direc- 

 tion of the rotation of the plate ; so that the situation of the 

 centre of the plate, the plane in which it revolved, and the 

 direction of rotation being given, we might point out imme- 

 diately the direction in which the deviation would take place. 



" On comparing together all the facts which I have detailed, 

 I found that this might be effected in the following manner. 1 

 refer the deviations of the horizontal needle to the deviations of 

 magnetic particles in the direction of the dip, or to those of a 

 dipping needle passing through its centre ; so that, in whatever 

 direction this imaginary dipping needle would deviate by the 

 action of the iron, the horizontal needle would deviate in such a 

 manner as to be in the same vertical plane with it : thus, when 

 the north end of the horizontal needle deviates towards the west, 

 and consequently the south end towards the east, I consider 

 that it has obeyed the deviation of the axis of the imaginary 

 dipping needle, whose northern extremity has deviated towards 

 the west and its southern towards the east ; so that the western 

 side of the equator of this dipping needle has deviated towards 

 the south pole of the sphere, and its eastern side towards the 

 north pole. It would follow from this, that if the north and 

 south sides of the equator of the dipping needle (referring to 

 these points in the horizon) deviated towards the poles, no cor- 

 responding deviation would be observed in the horizontal 

 needle ; the effect, in this case, taking place in the meridian, 

 would only be observable in the angle which the dipping needle 

 made with the horizon. As it is not my intention at present to 

 advance any hypothesis on the subject, I wish this to be consi- 

 dered only as a method of connecting all the phenomena under 

 one general view. Assuming it then for this purpose, it will be 

 found that the deviations of the horizontal needle due to rotation 

 are always such as would be produced bi/ the sides of the equator 

 of this imaginary dipping needle deviating in directions contrary 

 to the directions in which the edges oj the plate move, that edge of 

 the plate nearest to either edge of the equator producing the 



