3io POPULAR LECTURES AND ADDRESSES. 



the four quadrantal points, N.E., S.E., S.W. 

 and N.W. The great lengths and the great mag- 

 netic moments of the needles hitherto used in the 

 marine compass rendered it practically impossible 

 for the latter part of Sir George Airy's method to 

 be carried out correctly in practice, except in cases 

 in which the quadrantal error was much smaller 

 than it generally is in modern iron ships. The 

 primary object of my new form of compass de- 

 scribed above, is to permit complete correction of 

 the quadrantal error, not merely when its amount 

 is from 5 to 7 or 8, which it generally is in iron 

 sailing ships or steamers of ordinary modern types ; 

 but even when it amounts to as much as 15 or 

 20, as it is sometimes found to be in ironclads. 

 A complete realization of Airy's method is thus 

 now for the first time rendered practically possible 

 for all classes of ships. The whole method essen- 

 tially includes some plan for gradually changing 

 the positions of the correcting magnets at sea to 

 correct on the north, or south, or east, or west course 

 when error is found to have sprung up, whether 

 through change in the ship's sub-permanent mag- 



