CORRECTOR EFFECTS 65 



tions. A sort of shake-down on other headings prior to taking 

 data would reduce such errors. 



(3) Deperming, structural changes, heavy gunfire, magnetic 

 cargoes, etc., subsequent to the first set of data will make the com- 

 parative results meaningless. 



(4) Inasmuch as the data will not be reliable if the ship's per- 

 manent magnetism changes between the two latitudes, it will like- 

 wise be unreliable if any of the binnacle correctors are changed, 

 including the heeling magnet. 



In the event that an intelligent approximation as to Flinders bar 

 length cannot be made, then the deviations at the two latitudes should 

 be taken with no Flinders bar in the holder. This procedure would 

 also simplify the resulting calculations. 

 97. Methods of determining Flinders bar length. 



(1) Having obtained reliable deviation data at two different 

 magnetic latitudes, the changes in the deviations, if any, may 

 justifiably be attributed to an incorrect Flinder's bar adjust- 

 ment. E,/W. and N./S. deviations are the ones which are sub- 

 ject to major changes from such an incorrect adjustment. If there 

 is no change in any of these deviations, the Flinders bar adjust- 

 ment is probably correct. A change in the E./W. deviations 

 indicates an unsymmetrical arrangement of vertical iron for- 

 ward or aft of the compass, which requires correction by Flinders 

 bar, forward or aft of the compass. A change in the N./S. de- 

 viations indicates an unsymmetrical arrangement of vertical iron 

 to port or starboard of the compass, which requires correction by 

 Flinders bar to port or starboard of the compass. This latter 

 case is very rare, but can be corrected, as indicated in chapter 

 XVIII. 



Determine the B deviations on magnetic east/west headings at 

 both latitudes. The constant c may then be calculated from the 

 following formula : 



, r^^i tan Bx — Ho tan B2 1 



^=1 zr^z^ J 



where 



A = shielding factor (0.7 to 1.0 average). 



5"! = earth's field, fi", at 1st latitude. 



j5i = degrees B deviation at 1st latitude (magnetic headings). 



Zi = earth's field, Z, at 1st latitude. 



^2 ^earth's field, H^ at 2d latitude. 



^2= degrees B deviation at 2d latitude (magnetic headings). 



^2 = earth's field, Z, at 2d latitude. 



This constant c represents a resultant mass of vertical iron 

 in the ship which requires Flinders bar correction. If Flinders 



