126 THIRD REPORT 1833. 



cately suspended, either by very fine wire, or by untwisted 

 fibres of silk. In order to render the changes in the direction 

 of the needle in the meridian more sensible, its directive force 

 should be diminished by means of two magnets north and south 

 of it, and having their axes in the meridian. These magnets 

 should be made to approach the needle until it points about 

 30° on either side of the meridian, and they should be so ad- 

 justed that the forces acting upon the needle will retain it i?i 

 equiUbrio with its marked end at about 30° to the east and 30° 

 to the west of north, and also at south. The needle is to be 

 left with its marked end pointing south, for the purpose of ob- 

 serving the changes occurring in its direction. If magnets are 

 employed to retain the second needle nearly at right angles to 

 the meridian, they should be made to approach its centre until 

 the points of equilibrium are at about 80° east, 80° west and 

 south, the observations being made with the needle at 80° east 

 or 80° west. An objection to this method of adjusting this 

 needle by means of magnets, and to which I have already re- 

 ferred in a note, is that any change in their temperature will 

 have a very sensible effect on the direction of the needle in this 

 position ; and should such change take place dui'ing the ob- 

 servations, corrections must be applied to the results before 

 any accurate conclusions can be drawn from them. As, how- 

 ever, an aurora is not generally of long continuance, any change 

 in the temperature of the magnets during the observations is 

 much more easily guarded against than where the observations 

 have to be continued during successive days and at different 

 seasons of the year. I have before remarked that this incon- 

 venience will be, in a great measure, obviated by employing the 

 torsion of a fine wire, or a very fine filament of glass, to retain 

 the needle at about 80° from the meridian. In this case, the 

 ratio of the force of torsion to the terrestrial force acting upon 

 the needle having been determined, a measure will be obtained 

 of the changes which take place in the intensity of the terres- 

 trial force during the occurrence of an aurora. It is very de- 

 sirable that it should be ascertained whether the effects on the 

 needle are simultaneous with any particular class of phaenomena 

 connected with the aurora ; whether these effects are dependent 

 on the production of beams and corruscations, or on the forma- 

 tion of luminous arches ; or whether any difference exists in the 

 effects produced by these. In order to determine this, it is ne- 

 cessary that the times of the occurrence of the different phae- 

 nomena, and also of the changes in the directions of the needles, 

 should be accurately noted ; and for such observations, three 

 observers appear to be indispensable. 



