84 On the Height of the Aurora Borealis. 



"According to an estimate made conjointly by myself and a 

 friend at 10'' lO""* Cambridge mean time, it preceded the 

 bright starMirach, or /3 Andromedce, 10™ in right ascension, 

 and had greater north joolar distance by two degrees : conse- 

 quently, by calculation, its azimuth was 80° 41' from south 

 towai'ds east, and its altitude 69° 51'. The azimuth appeared 

 not to vary with the diurnal motion of the heavens." 



Here, then, are two observations on an auroral appearance, 

 the most distinct and unmistakeable. First at Durham (lat. 

 54° 46' 30") a corona is seen, the centre of which is in a line 

 with the star Mirach. At Cambridge (lat. 52° 13') about the 

 same time a similar corona is seen, and its position in the 

 heavens distinctly marked out in reference to the same star. 

 Nothing could be more satisfactory for the determination of 

 the height of the aurora, as the places of observation were 

 distant full 2^-° the one from the other; the object of obsei'- 

 vation had been noted carefully, and was not a mere fleeting 

 point ; as at Durham a corona had been seen " in the same 

 part of the sky" about an hour and a half previous, and at 

 Cambridge the corona was seen for some time, and its " azi- 

 muth appeared not to vary with the diurnal motion of the 

 heavens." 



Under these circumstances there can be no doubt, that had 

 the corona been observed at Cambridge to the north of Mi- 

 rach, these observations would have been taken as sufficient 

 to determine the height of the aurora. But instead of the 

 corona appearing some 60° north of Mirach, as it should have 

 been, had the height of the aurora been IGO miles, and 

 more or less, according to its height, the corona appeared 

 2° south of that star ; thus having very nearly the same alti- 

 tude as at Durham, and therefore giving the most convincing 

 proof that no dependence can be placed on any observation 

 on the apparent altitude of an aux'ora, for the purpose of de- 

 termining its height from the earth. 



I intended, looking over the various reports on aurorre, 

 given in the Philosophical Transactions, to see how far they 

 coincided with the opinions I now advance ;" but have only 

 had time to go through Halley's celebrated report on the 

 aurora of March 6, 1716 ; and although this report is often 

 referred to, to piove the great height of aurora?, yet I believe 



