Mr. Airy on the Deviations of the Compass in Ships. 161 



June 21, 1855. — The Lord Wrottesley, President, in the Chair. 



The following communication was read : — 



Discussion of the observed Deviations of the Compass in several 

 Ships, Wood-built and Iron-built ; with General Tables for facili- 

 tating the examination of Compass-deviations." By G. B. Airy, 

 Esq., F.R.S., Astronomer Royal. 



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 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 

 isolated, produce "polar-magnet-deviation;" and remarking that 

 the combination of two polar-magnet-forces will produce a third 

 polar-magnet-force, the resulting deviations of which will be 

 "polar-magnet-deviations;" it is evident that the analysis of the 

 observed deviations of the compass in any given instance will con- 

 sist in resolving them into two systems, one of which follows the laws^ 

 of " polar-magnet-deviation," andthe other is a " quadrantal deviation." 



The practical solution of this problem, without the assistance 

 of tables, is troublesome. In order to diminish the difficulty, the 

 author has prepared a Table of polar-magnet-deviations, for the 

 whole circumference as regards azimuths, and for aU values of 

 " modulus " or proportion of the disturbing force to the earth's 

 horizontal force, up to 0'8. 



To discover the elements of " polar-magnet-deviation," that is, 

 the neutral point and the modulus, in any given case, it is neces- 

 sary so to combine the observations that the "quadrantal deviation" 

 shall be eliminated. Simple rules are given for this ; and the pro- 

 cess of investigating the elements, with the assistance of the Tables, 

 is illustrated by exhibiting the work from beginning to end, in an 

 actual instance. 



When the elements are found, the Tabular Polar-Magnet-Devia- 

 tiona arc to be formed from those elements ; and the excess of the 



