48 
CAPTAIN SABINE ON THE DIP OF THE 
of 1821 : the limb is a circle of twelve inches diameter, each quadrant being- 
graduated from 0° at the horizon to 90° at the vertical, and divided into spaces 
of 20'. The vertical edges of the agate supports, on which the axis of the 
needle rests, are rendered horizontal by a circular brass plate carrying a spirit- 
level ; the lower surface of the plate is carefully ground, and being placed on 
the supports and turned successively in the four principal directions, the ad- 
justment is made by the foot-screws of the instrument until the bubble of the 
level is stationary in every direction in which the plate is turned. The divi- 
sions 90°, and 90° of the circle marking its vertical points, ought then to coin- 
cide (and should they not do so, there is an adjustment to render them coin- 
dent,) with the points of conical radii, proceeding from the surfaces of the 
plate at right angles to it. By means of this plate, the level, by which the 
horizontal adjustment is effected, is applied directly to the supports, and the 
graduation of the circle made also to correspond with them. 
The needle which was first used, was a flat needle of the ordinary construc- 
tion 1 1.5 inches long, .4 broad, and .05 thick, rounded at the extremities. Three 
distinct observations were taken with it in each of the following positions ; 
and as the arc was read at both ends of the needle, six readings were obtained 
in each position. — 1st, with the face of the instrument to the east, and the 
marked side of the needle also to the east ; 2nd, with the face to the east, and 
the marked side of the needle to the west ; 3rd, with the face of the instrument 
to the west, and the marked side of the needle to the west ; 4th, with the face 
to the west, and the marked side of the needle to the east. The poles were 
then changed, and the same course of observation gone through as before. 
The poles were changed by bar magnets in the usual way ; the magnets were 
passed along the flat surfaces of the needle, ten times on each side ; the needle 
being laid in a groove, which confined the motion of the magnets to a direction 
parallel to the needle. The force of magnetism imparted to the needle, on 
each occasion when its poles were changed, was measured by the time of per- 
forming 10 vibrations always in similar arcs. The horizontality of the supports 
was examined afresh every time the instrument was turned in azimuth ; and 
no pains were spared to obtain results which might be consistent with each 
other, as I intended afterwards to apply the small screw and weight devised 
by Professor Meyer to the same needle, and wished to compare its perform- 
