XV 



discovered in the Admiralty standard compass, " suggested the idea, that 

 " the arrangement of the needles in that compass might produce, in the 

 " case of deviations caused by a magnet or mass of soft iron in close 

 " proximity to it, a compensation of the sextantal and octantal deviations; 

 " and this, on the subject being mathematically investigated [on the 

 " approximate hypothesis that the intensity of magnetization is uniform 

 " through the length of each needle, and equal in the different needles], 

 " proved to be the case, this particular arrangement of needles reducing 

 " to zero the coefficients of the terms involving the square of the ratio 

 " of the length of the needle to the distance of the disturbing iron ; so 

 " that this remarkable result was obtained, that the arrangement of 

 " needles which produces the equality in the moments of inertia is, by a 

 " happy coincidence, the same as that which prevents the sextantal devia- 

 " tion in the case of correcting magnets, and the octantal deviation in 

 " the case of soft iron correctors. The consequence is, that with the 

 " Admiralty compass cards, or with cards with two needles each 30 

 " from the central line, correcting magnets and soft iron correctors 

 " may be placed much nearer the compass than can safely be done with a 

 " single-needle compass card, and that the large deviations found in iron 

 " ships may be thus far more accurately corrected." 



It will be understood that the preceding statement, even as an index 

 of subjects, gives but a very incomplete idea of Smith's thirty years' work 

 on magnetism. Further information is to be found in his papers in the 

 Transactions and Proceedings of the Royal Society, some of them contri- 

 buted in conjunction with Sabine or with Evans, others in his own name 

 alone. In 1850 he published separately* an account of his theoretical and 

 practical investigations on the correction of the deviations of a ship's 

 compass, which was afterwards given as a supplement to the Admiralty 

 " Practical Rules " in 1855. The large deviations found in iron-plated 

 ships of war " having rendered necessary the use of the exact instead of 

 " the approximate formulae," this article was rewritten by Smith for the 

 Compass Department of the Admiralty. It now forms Part III. of the 

 * Admiralty Manual for the deviations of the Compass,' edited by Evans 

 and Smith, to which are added appendices containing a complete mathe- 

 matical statement of the general theory, proofs of the practical formulae, 

 and constructions and practical methods of a more mathematical cha- 

 racter than those given in the body of the work for ordinary use* A 

 separate publication, of " Instructions for correcting the Deviation of the 

 Compass," by Smith, was made by the Board of Trade in 1857. 



It is satisfactory to find that the British Admiralty * Compass 

 Manual,' embodying as it does the result of so vast an amount of labour, 

 guided by the highest mathematical ability and the most consummate 



* Instructions for Computation of Tables of Deviations, by Archibald Smith. Pub- 

 lished for the Hydrographic Office of the Admiralty. 



