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imi arising from its rotation. 
as in fig. 1, the compass being however removed ; the indexes 
at o, o' were brought to ce, on the moveable limb, and 
that limb was placed at right angles to the fixed limb, so that 
the plane of the plate was parallel to the magnetic meridian. 
The dipping needle was then placed as nearly as possible in 
the required position, and the levels being carefully adjusted, 
the needle was made to vibrate freely and left to settle. After 
the plate had been made to revolve several times in the same 
direction, the point marked o was brought to coincide with 
the upper part of a line parallel to the magnetic axis, and 
passing through the centre of the plate. The needle was 
then slightly agitated, or made to vibrate through a small 
arc ; and when it settled, the dip was noted both at the upper 
and lower extremity, or the south and north end of the 
needle. This was repeated for the points marked 6o, 120, 
180, 240, 300. The plate was now made to revolve in the 
contrary direction, and similar observations made of the dip 
of the needle when the several points 300, 240, 180, 120, 60, 
o, coincided with the upper part of the line parallel to the 
magnetic axis. Continuing the revolution of the plate in this 
direction, a second set of observations of the dip were made 
for the several points from 300 to o. After this, the plate 
was again made to revolve in its first direction, and a second 
set of observations made of the dip for the points from o to 
300. I considered the mean of all the observations in the 
two sets, when the plate revolved from o to 300, as the mean 
dip when the plate revolved in this direction ; the mean of 
all the observations in the two sets, when the plate revolved 
from 300 to o, as the mean dip when the plate revolved in 
