286 Proceedings of the Royal Society 
ponding magnetic observations which had been then recently made 
on board the “ Erebus” and “ Terror” in Captain Ross’s Antarctic 
Expedition of 1840-41. The systematic character of the devia- 
tions, unprecedented in amount, experienced by the “Isabella” 
and “ Alexander ” in the course of their Arctic voyage, had attracted 
the attention of Poisson, who published in 1824, in the “ Memoirs 
of the French Institute,” three papers containing a mathematical 
theory of magnetic induction with application to ships’ magnetism. 
The subsequent magnetic survey of the Antarctic regions, of which 
by far the greater part had to be executed by daily observations of 
terrestrial magnetism on ship-board, brought into permanent view 
the importance of Poisson’s general theory ; but at the same time 
demonstrated the necessity for replacing his practical formulae by 
others, not limited by certain restrictions as to symmetry of the 
ship, which he had assumed for the sake of simplicity. This was 
the chief problem first put before Smith by Sabine, and his solution 
of it was the first great service which he rendered to the practical 
correction of the disturbance of the compass caused by the 
magnetism of ships. 
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 devia- 
tions 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 mathematical 
statement of the general theory, proofs of the practical formulae, 
and constructions and practical methods of a more mathematical 
character 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 
