ao4 



cosines respectively, of the apparent magnetic bearings of the distant 

 object, observed with the ship's head successively on the sixteen 

 principal points of the compass (or on eight principal points, as the 

 case may be). He considers that this method may prove useful in 

 magnetic surveys of the ocean. 



Additional remarks to the foregoing paper. Received April 14, 

 1853. 



In consequence of a suggestion of Professor William Thomson, 

 the author here investigates the modifications required in the formulae 

 of the previous part of his paper, when the compass-needle produces 

 by induction a sensible effect on the mutual magnetic action of the 

 earth and the ship. 



Let A, as in Mr. Archibald Smith's formulae, represent the mean 

 of the sines of the deviations of the compass-needle observed during a 

 complete revolution of the ship. As there is reason to believe that 

 this quantity does not vary for a given ship in different parts of the 

 earth so long as the quantity and distribution of her iron are un- 

 changed, it may be determined, once for all, while in port, in the 

 usual way. 



When the ship is out of reach of land, let s be the mean of the 

 sines, and c the mean of the cosines, of the apparent magnetic azi- 

 muths of a distant object observed during a complete revolution of 

 the ship. Then the sine of the true magnetic azimuth of the object 

 is given by the formula 



sa/(c2 + s2_A2)_cA 

 sm a= ——^ 



When A=0, this formula becomes 



s 



tana= — » 



c 



being identical with that of the previous part of this paper. 



May 12, 1853. 

 The EARL OF ROSSE, President, in the Chair. 



In compliance with the Statutes 

 that the following Candidates are 

 election into the Society ; — 



James Apjohn, M.D. 

 John George Appold, Esq. 

 John Allan Broun, Esq. 

 Antoine Jean Francois Claudet, 

 Esq. 



Edward J. Cooper, Esq. 

 E. Franldand, Esq. 

 John Hall Gladstone, Esq. 

 Joseph Beete Jukes, Esq. 



, it was announced from the Chair, 

 recommended by the Council for 



Robert MacAndrew, Esq. 

 Charles Manby, Esq. 

 Joseph Prestwich, Esq. 

 William John Macquorn Ran- 



kine, Esq. 

 William Wilson Saunders, Esq. 

 William Spottiswoode, Esq. 

 Count P. de Strzelecki. 



