220 REPORT ON MAGNETIC OBSERVATIONS. 



3°. The deviations of seven compasses have been observed, and compared with 

 the theory, at ten places so situated as to afford very great changes in the terrestrial 

 magnetic elements. For all these compasses the coefficients depending upon the 

 hard and soft iron have been so far separated from each other as to render it possible 

 to predict the deviations in any part of the world; and for the Admiralty Standard 

 and After Azimuth Compasses every one of the coefficients in Poisson's general 

 equations has been determined separately with a considerable degree of accuracy. 



The conclusions drawn from the discussion of the observations are that, in the 

 case of the Monadnock, 



■ a. The agreement between the theoretical and observed deviations is sufficiently 

 exact for the purposes of navigation, but is not entirely satisfactory in a scientific 

 point of view. 



h. It is questionable whether the theory really represents the semicircular as 

 well as it does the quadrantal deviation; and to settle this point there is great 

 need of more observations. 



c. The so-called permanent and subpermanent magnetism of the ship were 

 undergoing a constant and rapid change such as would correspond to a transfer 

 of magnetism from aft forward; and to a rotation from right to left in the direction 

 of the force. 



d. The ratio of the hard to the soft iron force was slowly varying at each com- 

 pass; and, for the different compasses it ranged between 4.3 and 67.4. 



e. There was not a compass on board at which the direction of the hard and 

 soft iron forces coincided; from which it follows that in no case was the ratio 

 of the hard to the soft iron force the same in the coefficient 33 as it was in the 

 coefficient (£. 



/. So far as can be judged from the observations discussed in this report, 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 33 and @ ; 

 and therefore, it is also impossible to make any reliable estimate of the changes the 

 deviations of the compasses will undergo upon a change of magnetic latitude. 



