Aurora Borealis of the 1th of Jan., 1831. 527 



As nearly as I could judge, circumstanced as I was at the 

 time, the highest part of the arch passed over the planet Mars, 

 then a few degrees from the meridian, and Mr. Harris's obser- 

 vations likewise indicate that the arch was nearly at right 

 angles to the meridian. Assuming then, in the first instance, 

 that this band of light was perpendicular to the meridian, 

 the difference of latitude of Blackheath and Heron Court will 

 be the arc between the two places. The latitude of Heron 

 Court is nearly 50 47' N., and that of my place of observa- 

 tion 51 28' N., so that y = 41'. , 



Mr. Harris observing towards the north, and my observa- 

 tion being towards the south, the highest point of the arch in 

 the one case would be nearly the lowest point in the other. 

 The altitude of the highest point, according to Mr. Harris's 

 observation, was 55, and the depth of the arch 10 ; so that 

 the altitude of the middle point was 50 from the northern 

 horizon. The altitude of Mars was, at the time I observed, 

 46 nearly, and the middle point of the arch about 1 30' 

 below the planet $ so that the altitude of this point was about 

 44 30' from the southern horizon. We have then from these 

 observations = 44 30', /3 = 50, <y = 41'. Substituting 

 these values, and 3960 miles for r, we have 



6 = 45 8', and h =25-7 miles. 



This determination of the height of the luminous band 

 above the earth's surface is on the supposition that it was per- 

 pendicular to the meridian : if we suppose that the highest part 

 of the arch was accurately in the same vertical plane as Mars, 

 whose azimuth was then about 13 east, or that the band made 

 an angle of 77 with the meridian, the value of h will be consi- 

 derably diminished. In this case, we must, instead of the differ- 

 ence of latitude, take for y the arc which would be a perpendi- 

 cular from Blackheath on the arc passing through Heron 

 Court, and making an angle of 77 with the meridian of 

 Blackheath. The longitude of Heron Court being 1 50' W., 

 this arc will be about 23J'. Substituting this value of y, we 

 shall have h = 14-86 miles. 



There is another supposition, with regard to the direction of 

 the luminous band forming the arch, which would still further 

 reduce its height, as determined from these observations. It has 



