176 MESSES. SMITH AJS’D EVANS ON THE EFFECT OF THE LENGTH OF 
Conclusions. 
The conclusions which we draw from these investigations are the following : — 
I. All mechanically corrected compasses should have compound needles arranged as 
in the Admiralty Standard compass, or two parallel needles the extremities of which are 
60° distant from each other. 
If the needles are rather nearer each other than the distance given by the above rule, 
the correction of the sextantal and octantal errors will be rather more perfect without 
injuring the quality of the compass in other respects. If the longer needles in the 
Admiralty Standard compass are a little nearer each other, the decantal and dodecantal 
errors will be likewise more perfectly corrected. 
II. If a single-needle compass be corrected, the following rules should be attended 
to : — 
1. The needle should not be more than 6 or 7 inches long. 
2. A magnet at the same level should not be nearer the centre of the needle than six 
lengths of the needle. 
3. Let the last-mentioned distance be called the “ nearest horizontal distance then 
if the magnet be below the level of the compass it may be brought nearer the compass, 
but not nearer than the foot of the perpendicular dropped from the “ nearest horizontal 
distance” on the line joining the centre of the needle and the magnet. So that if this 
direction makes an angle of 30° with the vertical, the magnet should not come nearer 
than three lengths of the needle. 
4. If possible no soft iron correctors should be brought within two lengths of the 
needle from the centre of the needle, and on no account within one and a half length. 
III. In correcting the quadrantal deviation by the reciprocal action of two compasses, 
the following rules are to be attended to : — 
The two compasses are to be placed, as in the common double binnacle, at a distance 
from each other, to be determined in a manner to be described. 
A place must be selected for the double binnacle, such that no iron will be very near 
the compasses, in order that the independent deviations of the two compasses may be 
the same. 
The ship must be swung so as to ascertain the amount of the quadrantal deviation in 
the positions of the double binnacle before the operation of correction is commenced, 
experience showing that, except in the immediate neighbourhood of masses of iron, the 
quadrantal deviation is nearly the same in all the positions in which a compass could be 
placed. 
The compass-maker should have previously matched his compasses in pairs, the com- 
passes of each pair being the same in all respects, and having the same power. He 
should also have ascertained, by previous trial in his workshop, with the compasses 
placed so as to bear N.E. and S.W. and N. W. and S.E. from each other (by the distm'bed 
needles), and should have marked on a scale to accompany the compasses, the distances 
from each other at which they produce deviations of 2°, 3°, 4°, 5°, 6°, 7°, 8°. 
