136 Mr Christie on the Effect of Rotation on Magnetic Forces. 



tion or the contrary, previously to being stationary, every 

 point in the plate being in precisely the same position, when 

 at rest, in the two cases. In order to discover the laws ac- 

 cording to which the rotation of the iron affected the needle, 

 he conceived a sphere to be described about its centre, and 

 the centre of the iron to be in the surface of this sphere; call- 

 ing the diameter of the sphere in the direction of the dip, the 

 axis, and the great circle having its plane at right angles to 

 this axis, the equator, the situation of the centre of the iron 

 would be determined by its latitude and longitude. When 

 the centre of the iron plate was in the meridian, and its 

 plane a tangent to the sphere, if the plate revolved so 

 that its upper edge moved from west to east, then, when it 

 was stationary, it was found that the rotation had caused a 

 deviation of the north end of the needle towards the east ; 

 and if the upper edge of the plate had been made to move 

 from east to west previously to the plate being brought to 

 rest, in precisely the same position as before, a deviation to- 

 wards the west was caused by the rotation. When the plane 

 of the plate was in the plane of the equator, or parallel to it, 

 the rotation caused no deviation of the needle. If the plane 

 of the plate was in the plane of the secondary to the equator 

 and meridian, then when its centre was in the equator, either 

 to the east or to the west of the needle, the deviation due to 

 rotation would be in a direction different from that in which 

 it took place when the centre of the plate was in the pole, the 

 direction of rotation being the same in all cases. Thus, sup- 

 posing the upper edge of the plate to have revolved from west 

 to east, then, when the centre of the plate was in the equator, 

 the deviation of the north end of the needle due to rotation 

 would be towards the west, whether the plate were to the 

 east or to the west of the needle; but if the centre of the 

 plate were in either pole, then the deviation under the same 

 circumstances of rotation would be towards the east. 



After making a great variety of experiments, Mr Christie 

 was enabled to connect all the phenomena arising from the 

 rotation under one general law, and he has found, that the 

 supposition of the mass of the iron acting from its centre, and 

 the rotation polarising it in a direction at right angles to the 



