678 CONSTANTS OF MAGNETISATION. 



The correction of the quadrantal deviation by masses of iron 

 is easy, and when once made it is correct for all magnetic latitudes. 



This is not the case for the semicircular deflection corrected 

 with magnets. Experiment made in one and the same place does 

 not allow us to separate the subpermanent magnetism in the 

 coefficients B and C, from the magnetism produced by the vertical 

 component of the earth. As this latter varies with the latitude, the 

 correction for a given place is inexact away from this place. 



Sir W. Thomson remedies this defect by correcting the semi- 

 circular deflections both with magnets and with a vertical soft iron 

 rod called Flinder's bar; at starting an approximate correction is 

 made, and it is modified during the voyage by varying the ratio 

 of the bar and the magnets. When the value of this ratio has 

 been found by trial, the correction holds good for all ultimate 

 displacements. 



An analogous remark would apply to the correction of the 

 heeling error, but no account is taken of it in practice. 



When the correction is complete, the compass is only under the 

 action of the terrestrial field. By means of a deflecting magnet, put 

 on the lid of the compass in a given position, it is then ascertained 

 that the deflection produced when the deflector is at right angles 

 with the needle is independent of the direction of the vessel. 



The heeling error is itself corrected if Z' = Z ; that is to say, 

 if the apparent inclination determined on board is equal to the 

 magnetic inclination of the place. 



These two methods of verification give a means of re establishing 

 the correction even at sea, and assuming that the state of the sky 

 prevents a determination of the geographical meridian by astro- 

 nomical observations. It is sufficient for this purpose that the 

 deviation produced by the deflector is independent of the head, 

 and that the inclination measured on board is equal to the 

 real inclination which is known by the approximate position of 

 the vessel. 



