413 Mr. Christie on the magnetism of 
earth and other planets may receive polarity from it ? If so, 
further experiments and observations on the magnetic effects 
produced by the rotation of bodies may indicate the cause of 
the situations of the earth's magnetic poles, and of their 
progressive movements or oscillations. 
Comparison of the magnetical effects produced by slow and by 
rapid rotation. 
With the view of ascertaining how far the effects pro- 
duced on a magnetic needle by a plate of iron during its 
rapid rotation, corresponded with those that I have described 
as nearly independent of the velocity of rotation, and as con- 
tinuing after the rotation had ceased, I placed the same plate 
of iron, which I had used in my former experiments, in the 
plane of the magnetic meridian, on an axis perpendicular to 
its plane, and about which it could be made to revolve with 
any velocity, not exceeding lo revolutions in a second. I 
then placed a small compass, with a light needle delicately 
suspended, on a platform wholly detached from the iron 
plate, in certain positions opposite to the edge of the plate, 
both to the east and to the west of it, as near to the surface as 
the compass box would admit. The compass being adjusted, 
the plate was made to revolve once, slowly, so that its upper 
edge moved from north to south, and the point o coinciding 
with the plane perpendicular to the plane of the plate, and 
passing through its centre and that of the needle, the direc- 
tion of the north end of the needle was observed ; and also 
when 180 coincided with the plane, the same observation was 
made. The plate was now made to revolve rapidly in the 
same direction, about 8 times in a second ; and when the 
