induced in iron bodies by rotation. 
323 
needle in its various positions prior to the motion, and ri d, 
n' s', &c. its direction as resulting from the motion ; the 
rotation of the shell being from c towards d. . of course with 
the rotation reversed, the effect will be reversed also. 
Now this effect you will, I think, find to be perfectly con- 
sistent with the view you have taken of the subject, in your 
letter of Jan. 13th, where you say in reference to your for- 
mer query, and to the views I then entertained, I should 
rather have expected a diminution of the magnetic polarity, 
commensurate to the rapidity of rotation and a change in the 
direction of the magnetic axis of the globe, from parallelism 
to that of the earth, to a position somewhere intermediate 
between that and the axis of rotation, but approaching nearer 
the latter as the velocity increased, &c.'' 
' The fact is, that the needle in my experiments being under 
no influence prior to the rotation from either the iron or the 
earth, the direction which it takes up in consequence of the 
motion, enables us to discover the precise direction of the 
new forces thus impressed upon the shell, and it will be 
seen immediately to indicate a polarization of the latter in the 
direction c d; that is, in a direction perpendicular to the axis 
of motion, and to the plane passing through that axis and the 
actual poles of the ball. 
You will of course understand that I do not mean that 
such a polarization actually takes place ; I mean merely that 
the cohesive power of the iron is such, as to resist in a cer- 
tain degree the inductive powers of the earth, whereby the 
magnetic forces are changed, as you have suggested, from 
their original direction, parallel to the magnetic axis of 
the ball into a position oblique to it, which oblique forces 
MDCCCXXV. U u 
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