CHARACTER OF THE ARMOUR-PLATED SHIPS OE THE ROYAL NAVY. 273 
paper requires the use of the “ exact coefficients ” 9(, S3, (5, 2), (S, which are not ex- 
pressed in degrees and minutes, but are nearly the sines of the corresponding angles 
A, B, C, D, E. 
For the purpose of this discussion we may confine our attention to A, B, C, D, E. 
A is the “ constant part of the deviation.” A real value of A can only be caused by 
elongated horizontal masses of soft iron unsymmetrically arranged with reference to the 
compass, and would be the same in all parts of the globe. An arrangement of hori- 
zontal soft iron rods such as that in fig. 1 would give a positive value to A and no 
other term in the deviation. This, however, is not an arrangement which would occur 
on shipboard. 
Fig. 1. Fig. 2. 
A soft iron rod such as that in fig. 2 would give -f A to the starboard compass, com- 
bined with +E; and — A, combined with — E, to the port compass. 
This arrangement is not unfrequent in the relative positions of the spindle of the 
steering-wheel and the binnacle compasses placed near it for the guidance of the 
helmsman. 
In compasses placed in the middle line of the ship such an arrangement is improbable, 
and in such case A has probably little or no real value. An apparent value may, how- 
ever, be given to A by index-error in the compass on board, index or other error in the 
shore compass with which it is compared, or error of observations generally. 
When the ship heels over, an elongated horizontal mass of iron, which was symme- 
trically placed from being below the compass, as the screw-shaft or the keel, is thrown 
to one side, and an A may then be introduced caused by and proportional to the angle 
of heel ; but this has not been found of sufficient amount to require attention in 
practice. 
The terms B sin £'+C cos £' make up together what is called the “semicircular devia- 
tion B depending on fore-and-aft forces, and having its zero when the ship’s head is 
North or South, its maximum when it is East or West ; C depending on transverse forces, 
and having its zero when the ship’s head is East or West, its maximum when it is North 
or South. 
B consists of two parts, one a coefficient arising from vertical induction in soft iron 
before or abaft the compass, and being multiplied by the tangent of the dip and a factor 
- hereafter explained ; the other a coefficient arising from permanent magnetism of the 
2 p 2 
