Mr. R. Potter on the Aurora Boreal is. 423 



altitude, therefore, at 45 m past 8 o'clock, was from 52° to 56° 

 (its upper and lower edges). It faded with great rapidity 

 after I first observed it ; but I readily detected its southward 

 motion, as I have done on similar occasions." " At 8 h 55 m it 

 passed through a Leonis." "After this it was very imperfectly 

 visible. There was a bright auroral light in the north, with 

 occasional corruscations. I shall be very anxious to hear if 

 you have observed this beautiful phenomenon." 



Of the Earl of Darnley's observation Professor Forbes gives 

 me the following particulars. " Seen at Athboy, Ireland, lati- 

 tude 53° 47' N., longitude 6° 54 / W." Observed about 9 p.m.: 

 " reached from the eastern to the western horizon which it 

 entered to the north of the constellation of Orion, passing 

 about midway between the Great Bear and Arcturus, and 

 directly over the two principal stars of Gemini'" most brilliant 

 at the east, where 1° wide; but increased to 5° to 6° at west. 

 " During the twenty minutes that Lord Darnley observed the 

 phenomenon it seemed to proceed through its whole extent 

 from N. to &, its edges, when first observed, extending equally 

 on either side of Castor and Pollux, having in that time en- 

 tirely left the most northern of those stars." At 10 it had 

 disappeared. An account of a similar phenomenon, it is 

 stated, was given in a Carlisle paper. I hope you will be able 

 to find this out." His Lordship, it appears, also notices the 

 aurora having been seen at Castlereah, about 60 miles distant 

 from Athboy. 



The following are extracts from Dr. Robinson's letter to 

 me, dated Armagh Observatory, July 26, 1833. " The ob- 

 servation I made of the aurora on March 21st last, was this: 

 At 8 h 44 m 10 s , Armagh time (or 9 h 10 m 45 s Greenwich), the 

 luminous arch was bisected by Arcturus and by y Leonis; at 

 the same time its upper edge was on a Orionis, and its lower 

 on 5 Orionis. As to se?ise, its highest point seemed on the 

 magnetic meridian (which I found, in January 1829, to deviate 

 29° 7' west)." At exactly 5 minutes later he made another 

 observation of the arch's place, from which we learn that its 

 upper edge was then on 8 Orionis, and its lower on £ Orionis, 

 which indicates a considerable southward motion, the upper 

 edge at the same time being on y Leonis, and the lower on 

 y Leonis. Dr. Robinson did not observe the aurora longer, 

 being at the time very much indisposed. 



The above observations must be allowed to demonstrate, if 

 there were need of any further demonstration, a most import- 

 ant point in the theory of the aurora borealis ; namely, that the- 

 symmetrical arches are rings, or portions of rings, in planes 

 perpendicular to the magnetic axis. If we take in round num- 



