REPORT ON MAGNETIC OBSERVATIONS. 



SECTION I. 



INTRODUCTION. 



On the fifth of October, 1865, I was ordered to the U. S. Iron-clad Monad- 

 nock 1 for the purpose of making observations on the action of her compasses 

 during the cruise which she was about to undertake from Philadelphia to San 

 Francisco, by way of the Straits of Magellan. She was then fitting out at the 

 Philadelphia Navy Yard, and the work on her was so far advanced that it was 

 expected she would sail in about two weeks. As the department had not previ- 

 ously intimated its intention of assigning me to this duty, and as everything 

 relating to the number and kind of observations to be made, and the instru- 

 ments required, was left entirely to my own discretion, it will be seen that the 

 time available for making plans and collecting the necessary apparatus was very 

 limited. 



The plan of observation ultimately adopted was that at every port in which 

 we remained for more than twenty-four hours the following operations should be 

 gone through with. 1st. The ship should be swung, and as her head pointed 

 successively to each of the thirty-two true magnetic points, the reading of every 

 compass on board should be recorded for each point, 2d. That at such of the 

 compasses as were so situated as to render it possible, the horizontal force and 

 inclination should be determined. 3d. The position of the dividing line be- 

 tween the north and south polarity should be traced on each turret. 4th. The 

 magnetic declination, inclination, and horizontal force should be determined on 

 shore. While at sea it was intended to observe the declination— and consequently 

 the deviation— and horizontal force daily, by means of the standard compass; but 

 this turned out to be impracticable, because the only place in the ship where it was 

 possible to mount that instrument was on top of the after pilot-house ; a situation 



1 The Monadnock is a doublc-tnrreted vessel of the monitor type. During the cruise in question, 

 Lieutenant Commander Francis M. Bunce, U. S. N., was her captain, and she was attached to the 

 squadron commanded by Commodore (now Rear- Admiral) John Rogers, U.S.N., at whose special 

 request I was detailed by the Navy Department to make the observations which are the subject of 

 this paper. 



1 December, 1871. ( 1 ) 



