528 Mr. Christie on the Luminous Arch of the 



been supposed that the Aurora Borealis has generally a refer- 

 ence to the magnetic meridian, and that the luminous arches 

 are perpendicular to it : indeed, those which I subsequently 

 observed on the 7th of January, generally appeared to have 

 their summits in that direction. According to this supposi- 

 tion, these arches would consist of luminous bands, making 

 an angle, at this place, of about 65 30' with the meridian. If 

 this was the case with the arch which I observed to pass over 

 Mars, the value of y will be reduced to 1' 40", and the value 

 of h to 4*9 miles. ^1 do not, however, consider that I made 

 any very sensible error in the situation which I assigned to the 

 summit of this arch, and consequently the observations will not 

 warrant the adoption of such a value of h. I ought, notwith- 

 standing, to notice, that the altitudes of an arch seen later in 

 the evening, would assign nearly this value to h. Mr. Harris 

 states that ' at 10 p. M. the light gradually withdrew from the 

 zenith and settled very like a bank of cloud, extending from 

 west to east, about 10 in depth, the sky beneath for 20 being 

 perfectly black.' The altitude of the lower edge was therefore 

 20. On referring to my observations on the Aurora, it will be 

 seen that * at 9 h 50 m p. M. three concentric arches were dis- 

 tinctly visible, their highest points being still in the magnetic 

 meridian.' As Mr. Harris mentions but one arch at very 

 nearly the same time, we may infer, that it was the highest of 

 these arches whose lower edge he saw at an elevation of 20. 

 1 did not, at the time, make any particular observation on the 

 altitudes of these arches, but, from recollection of their rela- 

 tive positions, I consider that the altitude of the^ lower edge of 

 the highest was at least 50, which, I must remark, is the alti- 

 tude I assigned to it previous to making any calculations on 

 its absolute height. If this altitude be correct, and this was 

 the same luminous band which Mr. Harris observed, its height 

 above the surface of the earth must have been about 4*7 

 miles. I do not, however, lay much stress upon the coinci- 

 dence of this result with the former, as there must be great 

 uncertainty respecting the observations from which it is deduced. 

 As undoubtedly the summit of the first luminous arch was 

 not to the west of the meridian, we must conclude, from the 

 observations, that it was certainly not more than twenty-five 



