491 



June 21, 1855. (Continued.) 

 The LORD WROTTESLEY, President, in the Chair. 



The following communications were read : 



XVII. " Discussion of the observed Deviations of the Compass 

 in several Ships, Wood-built and Iron-built ; with General 

 Tables for facilitating the examination of Compass-devia- 

 tions." By G. B. AIRY, Esq., F.R.S., Astronomer Royal. 

 Received June 2, 1855. 



The author refers, in the first place, to his paper in the Philoso- 

 phical Transactions for 1839, on the Disturbance of the Compass 

 in Iron Ships, for a theory of the forces produced by the transient 

 induced magnetism of iron. Using the term " polar-magnet-devia- 

 tion " to express a deviation similar to that which would be pro- 

 duced by a magnetized steel bar partaking of the movements of the 

 ship ; and using the term " quadrantal deviation " for a deviation 

 following the law of the sine of double the azimuth, and thus 



having, if " positive," the signs -\ 1 in the four successive 



quadrants of azimuth, or, if " negative," the signs 1 \- in the 



four successive quadrants : then it appears that the deviation pro- 

 duced by the transient induced magnetism of a ship will consist of 

 two parts ; of which one will be a " polar-magnet-deviation," such 

 as would be produced by a magnetized steel bar whose axis is par- 

 allel to the keel of the ship, and whose absolute intensity is propor- 

 tional to the terrestrial vertical force at the place ; and the other 

 will be a " quadrantal deviation," which, in angular deviation, will 

 be absolutely the same in all magnetic latitudes and with all mag- 

 nitudes of terrestrial magnetic force, and will usually be " posi- 

 tive." Now combining these forces with the force of the " sub- 

 permanent magnetism" of a ship, which in its nature is essentially 

 similar to the magnetism of a steel bar, and would therefore, if 



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