86 



Ocean Magnetic Observations, 1905-16 



Adjustment of Deviation-Coefficients and Derivation of Parameters. 



The adjustments of all observed coefficients obtained from the swings for each magnetic 

 element were made after dividing the coefficients into groups, each group being adjusted 

 separately. Group 1 of Cruise I was experimental. Parameters were not used to compute 

 deviations for this group. Group 2 includes the rest of this short cruise. The coefficients 

 for Cruises II and III were grouped from consecutive portions of a cruise during which the 

 vessel's course lay in the same general direction. This method of grouping was adopted on 

 the assumption that the ship's parameters are liable to alterations as the ship changes 

 from one course, whereon she has experienced continued buffeting, to a widely different 

 one, where she again experiences continued buffeting by the sea. 



The arrangement of the groupings and the periods of time to which the coefficients and 

 parameters of each group apply are shown in Table 29. 



The overlappings of dates in the above groups indicate that the same coefficients have 

 occasionally been included in each adjustment of two adjacent groups. 



For convenience in making the least-square adjustment of the coefficients and in com- 

 puting coefficients from the parameters determined by this least-square adjustment for 

 any place at which the deviation corrections are required, the expressions for the coefficients 

 have been modified as follows : 



Decimation. — Equations (8) to (13), page 79, are simplified by expressing the quan- 

 tities in degrees, making 



c P , f ., Q 



X = 



y = 



X = 



X sin 1° " X sin 1° ~ X sin 1° 



and dividing the expressions for At, Dt, and Ea by sin 1°. Then 



1 d-b 



y = 



X sin 1° 



A,= 



X 2 sin 1° 



Ba == X i&n I + y jj 

 Ca== x' t&n I + y'^ 



^'' X 2 sin 1° 



£<,= 



1 d + b 



X 2 sin r 



(27) 

 (28) 

 (29) 

 (30) 

 (31) 



The numerical values of the coefficients B^ and C^, which are functions of the horizontal 

 intensity and inclination, may readily be computed from the values of x, y, x', and y' given 

 for the period of time covered by each group-adjustment in Table 30 containing the dec- 

 lination deviation-constants and parameters for standard-compass position. The numerical 



