380 
Mr. Christie on the magnetism of 
side of the equator, though not so multiplied as those on 
the south, were sufficient to show, that the deviations due to 
rotation observed the same laws on that side of the equator 
as I had noticed on the south side. 
The deviation due to the rotation of the plate, when its cen- 
tre is in the secondary to the equator and meridian, having 
a peculiar character, namely, two greater maxima when the 
centre is in the equator, two less maxima, in a contrary 
direction, when the centre is in either pole, and four points 
where it vanishes, I consider to be particularly well adapted 
for forming an estimate of the correctness of any theory 
which may be adopted for the explanation of the phsenomena 
in general ; since the theory must be perfectly compatible 
with these peculiarities, before it can be applied to the expla- 
nation of the less marked ph£enomena. 
As it appeared from these observations, that the point 
where the deviation due to rotation vanishes, is not far from 
lat. 55°, the complement of which, 35°, is nearly half the angle 
of the dip, I wished to ascertain whether the deviation were 
really o in latitude 54° 45', which I considered to be correctly 
the complement of half the dip 70° 30', although I could not 
see how the angle which the plane makes with the horizon 
could have an influence on an angle in the plane itself. The 
following observations show, that in this instance the devi- 
ation due to rotation vanishes, or nearly so, when the polar 
distance of the centre of the plate is equal to half the angle 
which the dipping needle makes with the horizon. Whether 
this coincidence is purely accidental, or is a necessary conse- 
quence of the manner in which the effect is produced, must 
remain doubtful, until it can be shown how the action takes 
