Mr. Christie on the Aurora Borealis. 279 



cause a change in their intensity, and should this be much 

 diminished, the needle will quit its position at 180, and resume 

 that at zero. This needle being left in this position, its devia- 

 tions will indicate corresponding changes in the direction of 

 the magnetic meridian. 



2. Another needle should be adjusted in a similar manner, 

 but the magnets should be brought so near to it that the posi- 

 tions in which the needle will rest become 180, N. 70 E. and 

 N. 70 W. 5 and instead of being left with the marked end 

 pointing to 180, like the former, it should be led to the posi- 

 tion between N. and E., or N. and W. (whichever happens to be 

 most conveniently circumstanced for observation), at which it 

 will remain. The changes in the direction of this needle will 

 principally indicate changes in the terrestrial horizontal inten- 

 sity, corresponding to changes in the dip. 



To those who are desirous of observing the magnetical effects 

 produced during an aurora, and who have leisure to watch for 

 the occurrence of this phenomenon, I would recommend such 

 adjustments of two horizontal needles. During the time of 

 an aurora, these needles should be carefully watched, the ob- 

 server being very careful to remove from his person every 

 article of steel or iron. Their directions should be noted at 

 very short intervals ; if their motion be vibratory, the limits 

 should be marked ; and the precise time when any change in 

 the direction of their motion takes place should be carefully 

 noted. Another observer should at the same time note any 

 remarkable circumstance in the aurora, as the appearance of 

 columns or arches of light, their magnetic bearings and atti- 

 tudes nearly, with the precise time of their occurrence. It is 

 desirable, also, that the directions of the needles should be ob- 

 served every day at intervals of an hour, throughout the 

 twenty-four hours, particularly that of the needle pointing 

 south, as this would be but little influenced by changes in the 

 temperature of the magnets. By means of the latter observa- 

 tions, not only the times of maximum east and west variation, 

 and the relative extent of the variation each day would be deter- 

 mined, but a comparison would be afforded between the ordi- 

 nary diurnal excursions of the needle, and those during an 

 aurora ; and they would besides enable us to determine with 



VOL. II. Nov. 1831. U 



