618 
STAFF COMMANDER E. W. CREAK ON THE DEVIATIONS OF THE 
Semicircular deviation. 
Coefficient 9B = Vctan (approximate value in degrees =B) is the maximum 
of semicircular deviation from fore and aft forces; 
— tan 9 arises from soft iron ; 
X 
AH 
from hard iron. 
Coefficient (5 = tan 9-\- ^) (approximate value in degrees =C) is the maximum 
Q_ 
AH 
H 
of semicircular deviation from transverse forces; 
tan 6 arises from soft iron, and is zero if the iron is symmetrically 
arranged ; 
from hard iron. 
For determining P and c separately, when 33 has been determined in two different 
magnetic latitudes, the foregoing equations are put under the form, 
tan #=33H 
A. X 
f+^H' tan0'=©'H' 
X X 
and similarly for Q and f when C has been determined in two different latitudes. 
From the values of 33 and C in the table, the constants P and c, Q and f have 
been calculated by the above formulae, and the results are given in Table I., which for 
convenience has been placed at the end of this paper. 
The explanation of this table is as follows :— 
The quantities found in the columns headed “Original P” and “Original Q ” are 
the constants P and Q, arising from the hard iron of the ship before correction by 
magnets, and which are mainly dependent for their values and sign upon the direction 
in which the ship’s head lay during building. This direction is given under each ship’s 
name. 
In the columns headed “ Corrected P ” and “ Corrected Q ” are shown the constants 
P and Q as altered by the correcting magnets. 
Q -P 
The quantities - tan 6 and - tan 6 (with their equivalents expressed in degrees) are 
X X 
the changing parts of the coefficients © and (5 respectively, for the South of England, 
which can only be corrected for all latitudes by vertical soft iron bars. 
From the constants c and f in the remaining columns, the changing part of the 
