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even have gone beyond the requirements of the instrument, had not 
Dr. Lloyd removed a source of magnetic derangement, by which all the 
earlier observations were disturbed. 
This disturbance arose from the graduated circle of the instrument, 
which used formerly to be brought as close to the needle as possible. Dr. 
Lloyd was led, from an analysis of the observations he made in the mag- 
netic survey of 1828, closely to test this circle; and he found that, though 
made of materials which were supposed to be non-magnetic, it exhibited 
a trace of magnetism which injuriously affected the needle. He accord- 
ingly removed the circle to a sufficient distance from the needle, and as- 
certained the position of the needle by two microscopes carried by an 
arm which traverses the circle, and thus at once got rid of the magnetic 
disturbance, introduced greater accuracy into the reading of the position 
of the needle, and secured other important advantages, into which we need 
not now enter. 
This has again made the mechanical arrangements of the instrument 
the more defective; and I propose now to give some account of the de- 
gree of accordance which may be expected with the dip circle with which 
a magnetic survey of the southern half of Ireland was made in 1858, and 
of a slight modification of the apparatus for placing the axis ofthe needle 
on the agate planes, which would, I believe, render it possible to secure 
a higher degree of accordance. 
In determining the magnetic dips at any station, the axis of the 
needle is placed in eight distinct positions upon the upper horizontal 
edges of parallel agate plates, on which it can roll freely, and the posi- 
tion of each end of the needle observed about three times in each of the 
eight positions. 
Between each pair of readings the needle is lifted and replaced by a 
little frame, designed to lay the needle down with suitable precision on 
the edges of the agate plates. Each time that the magnet is replaced 
by this frame, it swings about for a short time, and the position it as- 
sumes when these oscillations are over is recorded. 
The axis of the magnet is thick in the centre, where it passes through 
the magnet. The ends, where it is to rest on the agate plates, are most 
carefully turned down to a needle-like thinness; and on either side, be- 
tween the thick and the thin parts, there are short pieces of intermediate 
thickness, which are to be placed in Ys, with which the lifting frame 
is fitted. The endlong adjustment of the axis is effected by the same 
Ys, since the thick part of the axis falls between them, resting 
against the sloped inner sides of the Ys by its cone-shaped ends. This 
little frame thus discharges distinct offices; it brings the proper parts of 
the axis of the needle down upon the agate plates, it places them upon 
the right parts of the edges of the agate plates, and it sets the magnet 
swinging between each pair of readings, since the frame turns on pivots 
at one end, and thus imparts a motion of rotation to the magnet in de- 
positing it. When the glass case of the instrument is shut, the ob- 
server can raise and lower the magnet from without by this lifting frame. 
This piece of apparatus needs to be manipulated with care. If the 
needle be lowered too rapidly, it comes down with a sensible impact 
