26o Mr, Christie on the magnetism of 
needle ; and the plate revolved, as in all other cases, in 
its own plane: they are a repetition of those by which I 
first discovered several of the facts I have mentioned, but 
made for the purpose of determining more precisely the 
deviation caused by the rotation. In making these, the in- 
strument was adjusted so that the index atg, fig. i, pointed 
to o°, that at K to 90°, and those at o, o' to Zero ; so that 
S N was horizontal and pointed east and west, as represented 
in fig. 3. 
In the following table, the numbers in the first column 
indicate the points of the plate which coincided with the plane 
of the meridian nearest the south, or upper pole of the 
sphere, when the several directions of the north end of the 
needle in the same lines with them were observed ; the 
latitudes and longitudes are those of the centre of the plate 
as referred to the centre of the needle, the longitudes being 
measured from east through north; the letters at the tops of 
the columns indicate the direction in which the edge of the 
plate, nearest the south pole of the sphere, moved ; the 
mean deviation of the needle, when the plate revolved in this 
direction, is placed in the line below the other deviations ; 
the direction in which the deviation due to rotation took place, 
in the following line ; and the whole deviation, arising from 
making the plate revolve in opposite directions, below this : 
the deviations observed always refer to the north end of the 
needle. The distance of the centre of the plate from that of 
the needle was 9.75 inches; the diameter of the plate, 17.88 
inches; thickness, .099 inch; weight, 112 oz.: so that its 
specific gravity appeared to be 7826. This plate I call 
No. I. 
