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APPENDIX. 
some additions to the Instrument, to facilitate its adjustment, and to conduce 
towards greater accuracy in the Observation, it has been thought proper to 
annex a short description of them. 
The perfect horizontality of the agate planes which support the axis of the 
Needle is essential to a correct observation ; it has been usual in Dipping 
Needles hitherto, to intrust this adjustment to spirit levels attached to some 
part of the frame of the Instrument, placed originally by the maker parallel to 
the planes; whilst the parallelism is maintained, the adjustment by the spirit 
levels is correct; but no means were afforded of detecting any change which 
might have taken place either in the planes, or in the levels, nor of re-adjusting 
them to each other on any subsequent occasion; to obviate this incon- 
venience, Mr. Dollond has contrived the following apparatus ; it is a double 
cone, the bases of which are united by a plate, having a diameter greater 
than the bases, for the purpose of placing the cones upon the agate planes ; 
the apexes of the cones are equal to the diameter of the divided circle 
of the instrument, and are perfectly at right angles to the plate which unites 
them ; one of the cones is perforated to admit a level, which has the proper 
adjustments to bring it parallel to the plate. 
In using the apparatus the cones are placed upon the agate planes, which are 
brought into the horizontal position by the foot-screws of the instrument, and 
by the screws which attach the pieces in which they are held to the cross 
bars ; the errors of the level are shewn by placing the double cone in various 
positions horizontally, and of the planes by turning the instrument upon 
its horizontal centre; when these errors are adjusted, the apexes of the 
cones should coincide with the divisions 90°, and 90° of the circle ; if they 
do not, the adjustments used in levelling must be again referred to, and 
half the error corrected by the foot-screws, and the remaining half by the 
screws in the cross bars. 
A second addition is for the purpose of retaining and releasing the needle 
at any arc at which it may be desired to commence the vibrations for deter- 
mining the intensity of the magnetic force ; and is instead of the usual incon- 
venient and faulty mode of drawing the needle from its natural position by 
a magnet ; it is composed of a moveable ring fitted into the great ring of the 
instrument, and close to the back of the divided circle ; to one part of this 
ring is attached a double lever, which is acted upon by a spring throwing 
one end of the lever within the divided circle, so as to support the needle at 
any division that may be required ; to the other end of the lever is attached 
