ADAMS AND EVANS. — TERRESTRIAL MAGNETISM. 13 



Following the course pursued at Point Barrow, in H. M.S. Plover 

 in 1852-4, and at Port Kennedy in the Fox, Sir Leopold 

 McClintock 1858-9, it is anticipated that hourly observations 

 of the declination can be continuously made, and if so, the obser- 

 vations should be made at the commencement of each hour local 

 mean time. 



On all occasions of marked disturbance of the magnet and 

 especially during the occurrence of an aurora, extraordinary obser- 

 vations should be made. If the changes are rapid and irregular, 

 instead of recording the observations at stated intervals of time, 

 observe the extreme readings of the scale and the times at which 

 the magnet commences its return movement towards an opposite 

 extreme, so as to determine the extent and duration of the move- 

 ments in opposite directions. 



At stations where the stay of the ship is only of a few days 

 duration, the same observations for the absolute declination, in- 

 clination, and intensity, should be made, omitting the differential 

 observations of declination. 



Observations to be made by Sledge or Travelling Parties. 



The apparatus devised by Mr. R. W. Fox, F.R.S., for observing 

 inclination and force [Fox's circle] proved of such great value under 

 exceptional conditions in the Antarctic Magnetic Survey, performed 

 in H.M. ship Erebus and Terror (1840-3) under Sir James Ross, 

 that it has been deemed expedient to furnish it to the present 

 Arctic Expedition. The instrument from its construction, will, it 

 is considered, be found invaluable to travelling parties as it can 

 be carried safely (and manipulated) under circumstances which 

 would be fatal to more delicate instruments. 



Preparatory to travelling parties setting out from winter quarters 

 (or from a fixed station when absolute determinations have been 

 made) comparative observations for inclination and force, should 

 be made with the Fox circles which accompany them, and the 

 same repeated on return. 



The comparative observations for inclination are requisite to 

 determine the index errors of the Fox needles as they are 7iot 

 reversible. The observations made with the deflectors and the 

 weights should be sufficiently extended to embrace the circum- 

 stances of all possible observations whilst travelling, so as to 

 ensure that no travelling observation be lost for want of compara- 

 tive observations. Experience will soon determine when travelling 

 the extent to which deflectors and weights may be conveniently 

 used. 



The small prismatic compasses furnished for the use of travelling 

 parties are exclusively intended for observing the declination (or 

 variation of the compass). On account of the large changes in 

 declination, consequent on a small change of geographical position 

 in the regions of high latitudes which will be explored, these 

 observations will, as a matter of necessity, be frequently required.* 



* At the magnetic pole, the horizontal force vanishes and consequently 

 the direction of the magnetic meridian is indeterminate. At the geographical 



