370 Mr, Christie on the magnetism of 
centre of the needle. Before however making these, to 
avoid the necessity of moving the compass as in the last, I 
made a slight alteration in the instrument. Instead of having 
the axis on which the plate revolved perpendicular to the 
arm, and the plate consequently parallel to the limb, this 
axis was inclined in such a manner that the plane of the plate 
passed through the axis of the instrument, as represented 
fig. 6 ; so that the axis of the instrument being horizontal, 
and passing through the centre of the needle perpendicularly 
to the meridian, as in fig. 3, when the arm of the instrument 
was adjusted to zero on the limb, the revolution of the limb 
caused the centre of the plate to describe the magnetic 
meridian, and at the same time the plane of the plate always 
passed through the centre of the needle. The distance be- 
tween the centre of the plate and that of the needle was as in 
the last 13.2 inches. The observations are given in the fol- 
lowing table, where the letters above the columns indicate 
the direction of rotation of the plate's inner edge. 
