374 Mr. J. R, Napieu on Ships' Compasses. 



Showing a difference of only l°f- from the correct bearing. When the 

 bearings are taken on only four pivots these are best to be the diagonal 

 points. 



N.W N. 'o W. 



N.B N. 51 W. 



S.E N. 60 W. 



S.W N. 26 W. 



4 )137 

 N. 34-25 W. 

 Showing a difference of only 1°| from the truth. 



Care must be taken that the deviations thus found be correctly allowed 

 for, as Sir Snow Harris and the late Captain Johnstone have both erred 

 in their directions for steering given courses. 



The most simple and at the same time correct method of showing and 

 applying the deviations, is the graphic method, suggested by Mr. Archi- 

 bald Smith, and described in his " Supplement to the Practical Eules for 

 Ascertaining the Deviations of the Compass, &c,," and also in the late 

 Captain Johnstone's work on Compasses. (Vide diagram.) 



I am not prepared to enter upon the subject of correcting compasses 

 by magnets, as recommended by Professor Airy. Trustworthy observa- 

 tions have shown that in some instances compasses so adjusted have been 

 correct in all the latitudes into which the vessel has sailed, while other 

 observations have shown that the magnetic corrections have altogether 

 failed. The accompanying remarks, with which Mr. Smith has kindly 

 furnished me, puts the subject in a clearer and more satisfactory light : — 



" Lincoln's Inn, January 22, 1855. 



" My dear Napier, — The results I have got are these : — In wooden 

 ships the magnetism is almost entirely that of soft iron, which only 

 becomes magnetic by induction, but on a wooden ship changing her 

 latitude it requires some weeks for the iron to get into its new magnetic 

 condition. In such ships the principal part of the deviation varies as the 

 tangent of the dip, and therefore becomes nil at the magnetic equator, 

 and changes its sign on a change of hemisphere. 



"In iron ships the greater part of the deviation seems to arise from 

 permanent magnetism, and the deviation in the south to be in the same 

 direction generally as in the north ; but then the ships whose deviations 

 I have examined had not been long in the south, and it may be that in 

 time, and with blows or strains, the magnetism would have changed. 



" The following considerations seem to show, conclusively, that no con- 

 fidence can be placed in any prediction as to the changes an iron ship 

 will undergo on a chancre of latitude. 



