492 



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 he 

 " 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," and the 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 all 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- 

 tions are to be formed from those elements ; and the excess of the 

 Observed Deviations over these Tabular Deviations ought to consist 

 simply of Quadrantal Deviation and Errors of Observations 



From the neutral point and the modulus, with the absolute mea- 

 sure of the terrestrial horizontal force, it is easy to form the abso- 

 lute measures of the apparent permanent forces of the ship, in the 

 directions of " headward " and " starboard " respectively. The star- 

 board force (on the assumption of general symmetry) can arise from 

 nothing but subpermanent magnetism ; the headward force will con- 

 sist of subpermanent magnetism added algebraically to a multiple of 

 the terrestrial vertical force, the multiplier being an unknown con- 

 stant different for each different ship. 



The process is then applied by the author to four wood-built sail- 

 ing ships, two wood-built steamers, and five iron-built steamers, 

 whose compass-deviations have been observed at twenty-nine sta- 

 tions in all. The results are as follows : 



