ADJUSTMENT PROCEDURE 



53 



are rare, and others have been corrected by adjustments at dockside. 

 Therefore, for most ships, there remain only three errors to be cor- 

 rected at sea, namely, the B^ C^ and D errors. These are corrected by 

 the use of fore-and-aft B magnets, athwartship G magnets, and quad- 

 rantal spheres respectively. 



83. Study of adjustment procedure. — Inspection of the general 

 5, C^ and D combination of errors, pictured in figure 26, will reveal 

 that there is a definite holation of the deviation effects on cardinal 

 compass headings. 



East 



Dog. 

 Dev. 



West 



180O 



270° 



360' 



Compass Hd." Degrees 



Figure 26. — B, G, and D deviation effects. 



For example, on 090° or 270° compass headings, the only deviation 

 which is effective is that due to B. This isolation, and the fact that 

 the B effect is greatest on these two headings, make these headings 

 convenient for B correction. A correction of the B deviation on a 090° 

 heading will correct the B deviation on the 270° heading by the same 

 amount but in the opposite direction ; and naturally, it will not change 

 the deviations on the 000° and 180° headings, except where B errors 

 are large. However, the total deviation on all the intercardinal head- 

 ings will be shifted in the same direction as the adjacent 090° or 270° 

 deviation correction, but only by seven-tenths (0.7) of that amount, 

 since the sine of 45° equals 0.707. The same convenient isolation of 

 effects and corrections holds true for the C error on 000° and 180° 

 headings ; and the correction of G error will also change the deviations 

 on all the intercardinal headings by the seven-tenths rvle^ as before. 

 It will now be observed that only after correcting the B and G errors 

 on the cardinal headings, and consequently their proportional values 

 of the total curve on the intercardinal headings, can the D error be 

 observed separately on any of the intercardinal headings. The D 

 error may then be corrected by use of the spheres on any intercardinal 

 heading. Correcting D error will, as a rule, change the deviations 

 on the intercardinal headings only and not on the cardinal headings. 

 Only when the D error is excessive, the spheres are magnetized, or 

 the permanent magnet correctors are so close as to create much indue- 



