270 STAFF COMMANDER EVANS AND MR. A. SMITH ON THE MAGNETIC 
The effect of the coefficients on the deviation is most easily seen by considering the 
effect of the derivative coefficients X, 91, 93, 6, 2), ($, and of the heeling coefficients, which, 
for convenience of reference, are here arranged in a tabular form. These are as follows : — 
a + e . 
is a factor generally less than 1, giving the northern component of the mean 
directive force on the needle, or “ mean force to North.” 
d—b 
9t=-2^ (approximate value in degrees =A) is the constant term of the deviation; its 
real value is 0 when the iron is symmetrically placed on each side of the 
compass, and it is not in general distinguishable from an index error of the 
compass, or an error in the assumed variation of the compass (declination). 
93 =^ctan 0+ (approximate value in degrees =B) is the maximum of semicircular 
deviation from fore-and-aft forces ; - tan 6 arises from soft iron ; from 
aH 
hard iron. 
Q 
(5=^/tan 0-\- g^ (approximate value in degrees =C) is the maximum of semicircular 
deviation from transverse forces ; tan 6 arises from soft iron, and is zero if 
the iron is symmetrically arranged ; ^g from hard iron. 
v/93 2 +(5 2 (approximate value in degrees =>/ B 2 +C 2 ) is the maximum of semicircular 
deviation. 
is the tangent starboard angle, or of angle measured to right of fore and aft of line of 
ship, in which the force causing the semicircular deviation acts. 
2) = (approximate value in degrees =D) is the maximum of quadrantal deviation 
from soft iron symmetrically placed. 
— 1^ =~ is the part of 2> arising from fore-and-aft soft iron. 
® 1^ = — A is the part of 2) arising from transverse soft iron. 
($, = (approximate value in degrees =E) is the maximum of quadrantal deviation 
from soft iron un symmetrically placed. 
tan is the heeling coefficient, or the deviation to windward in degrees 
for one degree of heel when ship’s head North or South by disturbed 
compass. 
^2) + ^— 1^ tan 0 is the part of heeling coefficient from transverse soft iron. 
- — i j tan 6 is the part of heeling coefficient from vertical soft iron, and vertical force 
of hard iron. 
9 is the increase or decrease of vertical force above or below mean when ship’s head is 
tanS North or South. 
