202 



REPORT ON 



The following table shows the change, in amount and direction, of the hard iron 

 force betAveen November 1, 1865, and June 23, 1866; the ratio of the hard to the 

 soft iron force on each of these dates; and also the mean ratio of the same forces. 





Change of Hard Iron Force. 



Ratio of Hard to Soft Iron Force. 



Compass. 











Amount. 



Direction. 



Nov. 1, 1865. 



June 23, 1866. 



Mean. 



Admiralty Standard . 



— 0.234 



— 12°. 8 



19.2 



9-4 



14 3 



After Binnacle 







— 0.025 



— 7.2 



68.8 



66.1 



67.4 



After Ritchie . 







■ — 0.299 



+ 5-o 



42.1 



26.0 



34-o 



After Azimuth . 







+ 0.074 



— 12.9 



52.6 



62.8 



57-7 



Forward Alidade 







+ 0.071 



— 10.3 



6.6 



11. 



8.8 



Forward Binnacle 







+ 0.095 



_ 5,1.8 



. 3-3 • 



5-3 



4-3 



Forward Ritchie 







-j- O.OII 



-58.1 



17.1 



17.6 



17-3 



An examination of the last two tables shows that during the whole cruise the 

 hard iron force was changing in a very remarkable manner, both in amount and 

 direction. In the case of the three compasses mounted above the forward turret, 

 the force was increasing: while in the case of those mounted above the after turret, 

 it was decreasing. In other words, there seems to have been a transfer of hard 

 iron force from aft forward. Now, looking at the change in direction of the force, 

 we see that in every case, excepting only that of the After Eitchie, it took place 

 in such a manner as to correspond to a rotation from right to left. Further, the 

 ratio of the hard to the soft iron force was slowly varying at each compass; and 

 for the different compasses it ranged between 4.3 and 67.4. Finally, there was not 

 a single compass on board at which the direction of the hard and soft iron force 

 coincided; from which it follows that in no case was the ratio of the hard and soft 

 iron forces the same in the coefficient 35 as it was in the coefficient (£. Under these 

 circumstances we are forced to conclude that, so far as can be judged from the 

 observations here given, in the case of a vessel swung for the first time it is 

 impossible to make any reliable estimate of the ratio of the hard to the soft iron 

 force in the coefficients 23 and (J; and, therefore, it is also impossible to make any 

 reliable estimate as to what changes her deviations will undergo upon a change of 

 magnetic latitude. As a further proof of this, we see that the After Azimuth 

 Compass, with a maximum deviation of 10° 5', changed its deviation during the 

 cruise by only 1° 43', that is, by about one-sixth of its whole amount; while the 

 Forward Binnacle Compass, with a maximum deviation of only 7° 43' changed its 

 deviation during the cruise by 9° 42', that is, by about one and a quarter times its 

 whole amount. 



In the beginning of this section it was stated that, at the positions occupied by 

 the Admiralty Standard and After Azimuth Compasses, observations of deflection 

 and dip were made in order to determine the absolute magnetic force; and the 

 details of the method followed in taking these observations were explained. We 

 will now proceed to reduce and discuss the observations themselves, and for that 

 purpose the first thing necessary to be known is the magnetic moment of the 

 deflecting magnets. For its determination we have the observations recorded in 

 the following table, which were all made on shore. The first and second columns 



