326 Mr. Barlow on the temporary magnetic effect 
same at all points between the horizon and an altitude of 
6o°; beyond this, the north end will point to the east till 
we have passed the zenith 30° on the west side ; and then 
again from this point to the west horizon the north end will 
, again point to the west; and similar changes will take place 
below the ball. This, which is a necessary* consequence of 
our hypothesis, is completely verified by experiment. 
It will of course be understood, that the supposition of the 
axis being in the meridian is merely to fix the idea; for a 
similar motion takes place whatever direction the axis of the 
shell may have. 
It is presumed, that what has now been stated is sufficient, 
without referring to any further experiments, to establish the 
principal fact adverted to in this letter, viz. that when any 
iron body is put in rapid rotation on any line not coinciding 
with its magnetic axis, a temporary derangement takes place 
in its magnetic powers, which in its effects is equivalent to 
a new axis of polarization perpendicular to the plane passing 
through its axis of polarization and rotation. 
I have stated in the beginning of this letter the motives 
which led me to undertake these experiments ; but notwith- 
standing I have certainly found a stronger effect produced 
by the rotation than I anticipated, yet it does not appear to 
be of a kind to throw any new light upon the difficult sub- 
ject of terrestrial magnetism. I think there are strong rea- 
sons for assuming, that the magnetism of the earth is of 
that kind which we call induced magnetism ; but at present 
we have no knowledge of the inductive principle, and are 
therefore unable to judge, how far the earth's rotation may 
be influential in producing those discrepancies from the gene- 
