404 Mr. Christie on the magnetism of 
presenting the different ends of a mass of iron to the poles of 
a magnetic needle, in addition to the hypothesis, that the 
upper part becomes a north, and the lower a south pole, by 
position, it is necessary to suppose also, that in every change 
of position of the iron there is a corresponding and imme- 
diate change of its poles ; that is, the upper end becoming 
the lower, it also immediately becomes a south pole. Now 
it appears to me, that if we attempt to explain, on this hypo- 
thesis, the phcenomena arising from the rotation of the iron, 
we shall find that there are circumstances which are wholly 
incompatible with it. If on turning a mass of iron end for 
end, the poles are immediately transferred from one end to 
the other, how can we suppose that the revolution of the iron 
will cause these poles to move forwards, so that the line 
joining them shall describe an angle from the line of the dip ? 
or even granting that during the revolution of the iron they 
may be carried forward, they must, as soon as the iron 
ceases to revolve, resume their original position in the line 
of the dip, if they are so immediately transferred from one 
end of the iron to the other, as it is necessary to suppose 
in order to account for the phsenomena which take place of 
attraction and repulsion, as they have been called. Imme- 
diately, then, that the iron becomes stationary in any position, 
the deviation of the needle ought, on this hypothesis, to be- 
come the same, whether the iron has been brought into that 
position by revolving in one direction, or in the contrary. It 
is hardly necessary for me to say that this would not be the 
case, since I have stated, that, in all the preceding observa- 
tions, the iron was stationary previous to the observation 
being made. 
