278 Mr. Christie on the Aurora Borealis. 



aurora might become sensible ; but as this is at best a very 

 imperfect instrument, and as these forces would probably in all 

 cases be small, compared with the other forces giving direction 

 to the needle, the effect produced would, very probably, be 

 quite insensible. To obviate this, the directive force of the 

 needle should be diminished, by placing magnets in the direc- 

 tion of the axis of the dipping needle, with their poles opposite 

 to the corresponding poles of the needle, the magnets being 

 placed at such distances, that the force acting upon the needle 

 in the direction of the dip should be extremely small. In 

 observing, however, with a needle so adjusted, it would be 

 necessary to be extremely cautious that the instrument and 

 the magnets should be so securely fixed, that their relative 

 positions could not alter during the observations, as a very 

 minute change would produce a very sensible deviation of the 

 needle. In order to observe the effects produced by forces not 

 acting in the meridian, it would be necessary also to adjust a 

 horizontal needle in a similar manner. 



As the mechanical difficulties, occurring in such an adjust- 

 ment of the dipping needle, are considerably greater than in 

 that of a horizontal needle, I consider that it would be better 

 to adjust two horizontal needles in this manner : 



1. A light needle being suspended by untwisted fibres of 

 silk, a fine hair, or a very fine wire, two bar magnets are to be 

 placed as I have described, and at such a distance that the 

 marked end or south pole of the needle may still point to zero. 

 Keeping the magnets in the meridian, and still at equal or 

 nearly equal distances from the needle, they are to be made to 

 approach it until the marked end deviates about 30 to the 

 east, or west of north. If the needle be now led towards 180 

 or south, by means of a small piece of iron held on the outside 

 of the compass box, it will remain at 180, provided the axes of 

 the magnets are in the meridian : if it does not point to 180, 

 the nearer ends of the magnets must be slightly moved east or 

 west, without changing their distances, until it does. The 

 magnets should now be firmly fixed in their positions by copper 

 nails, and it would be advisable to cover them up with some 

 bad conductor of heat, as a change in their temperature will 



