462 Cambridge Philosophical Society. 



Advertiser at Guisborough, those of Professor Sedgwick at Dent 

 near Kendal, and those of Dr. Robinson at Armagh. The appa- 

 rent position of the arch was defined by all the observers with con- 

 siderable precision (especially in the aurora of October 12), by the 

 manner in which it passed between different stars ; and several im- 

 portant identifications were established by the accuracy with which 

 the time had been noted, especially at York and at Cambridge, 

 where the watches with which the observations were made had 

 been immediately corrected by comparison with transit-clocks. 

 These established beyond doubt the identity, and consequently the 

 greatness of distance of the arch which had been observed at these 

 distant stations ; at the same time the Professor expressed his con- 

 viction that the streamers in the aurora of September 17 were con- 

 nected with the clouds visible at Cambridge. The method by which 

 the elevation of the luminous arches was obtained was entirely gra- 

 phical. A map of the places of observation was made; through 

 one of these a line was drawn in the direction of the magnetic 

 meridian j ordinates from the different places were drawn perpen- 

 dicular to this line, and the observations were then treated as if 

 they had been made at the foot of the ordinate instead of the ac- 

 tual place of observation. For the determination of the distance, 

 &c, of the arch, a projection of the places of observation and the 

 direction of the visual rays on the vertical plane passing through 

 the magnetic meridian, was constructed. The direction of the 

 visual rays had been determined from the observations, by the use 

 of a celestial globe : in the projection of the magnetic meridian, 

 the earth's sphericity was taken into account. The general con- 

 clusion was that the two arches were nearly of the same height, 

 (that of September 17 being probably rather the higher,) and that 

 the height was less than sixty miles, but probably more than fifty ; 

 and that its situation was but a few miles to the north of Armagh. 

 Different combinations gave results slightly discordant ; the obser- 

 vations at Guisborough generally gave the altitude of the arch 

 greater than was consistent with the others ; the observations at 

 Armagh could not easily be reconciled with those made at other 

 places, either for the elevation or for the general appearance of the 

 phenomena ; but this did not appear surprising when it was consi- 

 dered that the arch there passed near the zenith, and that the part 

 of the arch seen there had at Cambridge a very confused appear- 

 ance. It seemed that the arch had approached rapidly from an 

 unknown distance, and had afterwards remained nearly stationary. 

 Its motion (from its apparently stationary character at Cambridge,) 

 seemed to be not parallel to the earth's surface, but inclined down- 

 wards, perhaps 15°. The Professor remarked that the elevation 

 must depend principally (among the observations here collected,) 

 upon those made at Cambridge. Fort he lower edge of the arch, 

 which was at one time extremely well defined at Cambridge, he 

 could find no satisfactory corresponding observations. He then 

 alluded to a beam or permanent streamer which was seen at the same 

 time at Cambridge and at York, and remained visible at both places 



