ON COMPASS CORRECTION. 31 



kinds of ships. One fact it brings out is, that the quadrantal error is 

 of such a kind that masses of soft iron placed not before and behind 

 the compass, but on the two sides of the binnacle (the starboard and 

 port side), will correct it. Where you have such an arrangement of 

 iron in such a ship, you could find a place (not the usual place for a 

 compass) in which the quadrantal error would be zero, or in which it 

 would be of the opposite kind, so that a mass of iron placed before it 

 and another behind it would be suitable for correcting it. But one 

 point brought out in the natural history of ships is, that the quad- 

 rantal correctors must be applied on each side for every ship in any 

 position of the compass. 



There is one other point of considerable theoretical interest with 

 respect to quadrantal corrections which has hitherto escaped investi- 

 gation, and which has not yet been published. It is this, that even 

 with the Admiralty compass and its perfectness with respect to 

 escaping the danger of octantal errors there is another action which 

 vitiates the perfect completeness of that correctness, viz., that the 

 induction produced by the compass needles themselves upon the soft 

 iron correctors produces a very sensible disturbance. I will tell you 

 what I have in my mind about that. I found to my surprise rather 

 some years ago, that out of i2- of quadrantal error, corrected with 

 the Admiralty standard compass by cylinders of soft iron of the 

 dimensions recommended by the Compass Committee, only 6| 

 were genuine, and 7 degrees depended on the influence of the 

 magnets of the compass card upon the correctors. That would not 

 in the slightest degree vitiate the correction as long as the same com- 

 pass is used and the ship remains in the same place. But if a 

 compass card with weaker magnets were used, then the correctors 

 would not be so efficacious, and if a compass card with more powerful 

 needles were introduced the correction would be overdone. 



Now I have to refer to one most important and valuable 

 quality of the method of correcting quadrantal error, introduced 

 by the Astronomer Royal, and that is, that when once made for 

 a ship in any latitude, it remains perfect for that ship as long 

 as the iron of the ship remains unchanged, to whatever part of 

 the world she may go ; but that is on the assumption that the 

 needles are infinitely small, and of infinitely small magnetic moment, 



