induced in iron bodies in rotation, 525 
did before I was aware of the cause, a point of change 
at 30° distance on each side of the meridian passing through 
the axis ; which meridian, as respects the induced power, is 
actually the equator of the new magnetic sphere. 
To render this more obvious, let us refer to fig. 4, in 
which AB represents the axis of rotation of the shell, the 
black lines the needle in its natural direction, and the dotted 
lines the direction the needle has a tendency to assume ac- 
cording to the law' above named, in consequence of the mag- 
netism impressed by the rotation in the line n s. Beginning 
at the point A, if we say the motion is from left to right, 
that is from n to n' , it will be from right to left at 60°, 75°, 
90'', &c. till we arrive again at 30* ; at this point as at the 
former the new power is exerted in the actual direction of 
the needle, and if it were greater than its natural directive 
power, it would wholly invert it ; in this case it would pass to 
either hand ; but as the new power cannot invert it, it has no 
tendency to deflect it, and it therefore remains stationary. 
Thus one of the results which was at first the most perplex- 
ing, serves to confirm the law we have established. 
On similar principles, if we conceive a circle passing ver- 
tically from 90° to 90®, and if the needle be perfectly neu- 
tralized at different positions in this circle, and rendered 
parallel to the axis at each, then in every case the needle 
will have a tendency to take up a position directly at right 
angles to the axis of the shell, and it will point in opposite 
directions at certain parts of this circle: thus, if to fix the 
idea we conceive the axis to be in the meridian,. and the motion 
of the shell from west to east, then at the east point of the 
horizon the needle will point to the west, and it will do the 
