Ship Deviation-Coefficients 79 



D', I', H', Z' are, respectively, the observed ship values of the declination, inclination, 

 horizontal intensity, and vertical intensity; D, I, H, Z are the true, or undisturbed, values^ 

 those which would be observed if the ship were wholly non-magnetic. 



The deviation-correction is the quantity to be apphed to the magnetic element ob- 

 served aboard ship to obtain the true or undisturbed value. It is of opposite sign to the 

 deviation; thus, e. g., D — D' — dD; etc. 



In the above formulae f , because of the smallness of the deviation-effect on the compass 

 aboard the Galilee, may be taken directly as the ship's indicated magnetic course, or as the 

 indicated magnetic azimuth of the ship's head, measured continuously from the magnetic 

 north through east. 



Let X = H'/H, fjL = Z'/Z, and let the so-called "exact deviation-coefficients" be indi- 

 cated by primes, e. g., A' a, B'a, etc.; then the relations existing between the parameters 

 and the deviation-coefficients are : 



For Declination 



X=l+\ia + e) (8) 



^>sinA, = -(^) (9) 



B; = sin5<, = J(ctan/ + ^) (10) 



C: = sinC,= J(/tan/ + |) (11) 



D', = ^mD,= l(^) (12) 



£j:=sin£:.= J(^) (13) 



For Inclination 



A', = sin A, = ^{\-li) sin 2/ = |(x-it-l- |)sin 27 (14) 



B: = sin B, = \(XB',-g cot I) sm2 1 = \(c-g)- \(c+g) cos2I+^^sm2I (15) 



C; = sm C. = |(;icot7-XQ sin2/=|(/i-/)+|(/i-f/)cos2/-|^sin2/ (16) 



D;=sinA = + |xZ):sin2/=|(^)sin2/ (17) 



E', = sinEi = - ^XE'i sin 27= - | (^)sin 27 (18) 



For Horizontal Intensity 

 ^ = f (a+e)=77(X-l) (19) 



B„ = cH tan I +P = XH ■ B'^=XH- sin Ba (20) 



C« = -/77tan7-Q = -X77-Ci= -Xff-sinC, (21) 



Z), = :| (a - e) = X/7- D'^ = X77- sin 7), (22) 



E, = -^{d-^h)= -\HE',= -\HsmE, (23) 



