162 MESSES. SMITH A:N^D EVANS ON THE EFFECT OF THE LENGTH OF 
that they may be neglected, so that we have in all such cases 
sinS=B sin ^'+C cos ^'+D sin ; 
and if the deviation be of such an amount that we may take ^ for sin we have 
S=B sin ^'+C cos sin 
The first two terms of this expression represent a deviation which is zero when the 
ship’s head is on either of two opposite points, called the neutral points, and which is 
easterly when the ship’s head is in one of the semicircles terminated by these points, 
and westerly when it is in the other semicircle, and which deviation is therefore called 
the Semicircular Deviation. 
. . . 
The third term represents a deviation which is zero when — i- €. the azimuth of 
the ship’s head measm’ed from a line half-way between the correct magnetic north and 
the direction of the disturbed needle =0°, 90°, 180°, or 270°, and which has its maxi- 
mum (without regard to sign) when ^-^=45°, 135°, 225°, or 315°, or, in other words, 
Avhich has its zero-points when the ship’s head is near the four cardinal points, its maxi- 
mum values (independently of sign) when the ship’s head is near the centre of the N.E,, 
S.E., S.W., and N.W. quadrants, and is hence called the Quadrantal Deviation. 
The deviation may be corrected either mathematically or mechanically. 
In the Boyal Navy, as a rule, no mechanical correction is applied to the compasses. 
A standard compass, having a card 7‘6 inches in diameter, is fixed in a convenient posi- 
tion in the ship, at an elevation of about 5 feet above the deck, and as far as possible 
from any iron, and the deviations on each course are ascertained and allowed for by 
reference to a table or curve of deviations constructed for the particular ship from actual 
observation. In the Eoyal Navy, therefore, the distance of the nearest iron from the 
compass is in general so great that the assumption to which we have referred may be 
made without sensible error. 
In iron ships of the Mercantile Marine, the mechanical method of correction is 
extensively practised, which was originally proposed by Mr. Aiet in his well-known 
paper in the ‘Philosophical Transactions’ for 1839, and which is described in greater 
detail in his ‘ Results of Experiments on the Disturbance of the Compass in Iron-built 
Ships’ (Weale, 1840). In this method the semicircular deviation is corrected by two or 
more horizontal magnets, the part B being corrected by a magnet or magnets directed 
fore-and-aft, the part C by a magnet or magnets directed athwart-ships. Each magnet 
is fixed with its centre in either the fore-and-aft or the athwart-ships vertical plane pass- 
ing through the centre of the compass, or in the intersection of these planes. The 
quadrantal deviation D is corrected by masses of soft iron placed on each side of and at 
the same level with the compass-needle. When the distance and position of the ship’s 
iron and of the correctors are such that the square of the ratio of the projection of the 
needle on the direction of the disturbing iron to the distance of the disturbing iron may 
be neglected, the correction thus made may be considered as perfect for the place and 
