12 THE VOYAGE OF H.M.S. CHALLENGER. 



On the Observations required to determine the effect of the Ship's Iron on the Magnetic Instruments 



used on board. 



Positions have been selected in the ship in the most favourable locality with respect to neigh- 

 bouring iron for the Standard compass and for Fox's circle. Although the instruments are within a 

 few feet of each other, the effect of the ship's iron is sufficiently dissimilar at each position as to 

 require the determination of the requisite coefficients for both instruments. 



The determination of the deviations of the Standard compass by swinging the ship is an operation 

 now familiar to navigators, the several processes are given in detail in the " Admiralty Compass 

 Manual," and need not here be further referred to. 



A few words are however necessary as to the determination of the deviations of a compass placed 

 at the exact position of the Fox circle. At the time the deviations of the Standard compass are being- 

 observed, the compass specially furnished for the purpose should be placed on the Fox gimbal table, 

 and an exact comparison made of the azimuth of the ship's head by it and the Standard compass : by 

 the differences resulting from this comparison the deviations of the compass at the position of the Fox 

 circle are accurately obtained. 



When the ship is in all respects prepared for sea, that is, with the boats, guns, shot, and all other 

 ironwork in the positions in which they are to remain at sea, the deviations of the Standard compass 

 and Fox position compass would be observed, and when completed, the ship's head should be again 

 successively placed on the sixteen (or eight) principal points of the compass, as woidd be indicated by 

 the Fox position compass (this can of course be effected by aid of the Standard compass, a table of 

 azimuths being prepared beforehand). 



On each of these points the magnetic Inclination by the direct observation of Fox's circle on its 

 gimbal table is then observed, as also the angle of deflection by one or both of the deflectors (as more 

 convenient than weights), in the manner described in Appendix No. 3, paragraph 4, subparagraphs 1 

 and 2. Unless time and other favourable circumstances allow of these observations being made with 

 face of instrument east and west, both on board and on shore, good results can be obtained with the 

 face of the instrument in one direction, but it is then necessary to observe the Inclination on shore 

 with the same needle and with the face of the circle in the same direction (to the east or west) 

 precisely as when used on board, as well as the angle of deflection produced by the deflector also 

 employed on board. 



The observations which have been thus detailed should be repeated whenever the ship has so 

 materially changed her geographical position as to have altered the magnetic Inclination 30° or 40°, or 

 whenever she has undergone any change which may have made considerable alteration either in the 

 amount or the distribution of her iron. It is also particularly desirable to repeat them in regions 

 where the Inclination is very small, or where it is very large, and finally, they should be repeated 

 without fail whenever the magnetic observations made on board are brought to a termination. 



When the observations thus described have been carefully made and recorded, they furnish the 

 means of calculating approximately all the corrections required to clear the magnetic observations 

 made on board the ship in her successive passages from port to port, from the effect of the iron upon 

 the needles of the Standard compass and of Fox's circle. 



