XUBORA BOKEALIP. 193 



nightly. Every observer no doubt sees a separate auiora 

 of his own, as he sees a separate rainbow. A great portion of 

 the earth simultaneously engenders these phenomena of emana- 

 tions of light. Many nights may be instanced in which the 

 phenomenon has been simultaneously observed in England 

 and in Pennsylvania, in Rome and in Pekin. When it is 

 stated that auroras diminish with the decrease of latitude, 

 the latitude must be understood to be magnetic, and as mea- 

 sured by its distance from the magnetic pole. In Iceland, in 

 Greenland, Newfoundland, on the shores of the Slave Lake, and 

 at Fort Enterprise in Northern Canada, these lights appear 

 almost every night at certain seasons of the year, celebrating 

 with their flashing beams, according to the mode of expression 

 common to the inhabitants of the Shetland Isles, " a merry 

 dance in heaven."* Whilst the aurora is a phenomenon of 

 rare occurrence in Italy, it is frequently seen in the latitude of 

 Philadelphia (39 57'), owing to the southern position of the 

 American magnetic pole. In the districts which are remark- 

 able, in the New Continent and the Siberian coasts, for the 

 frequent occurrence of this phenomenon, there are special 

 regions or zones of longitude, in which the polar light is 

 particularly bright and brilliant, f The existence of local 

 influences cannot, therefore, be denied in these cases. Wran- 

 gel saw the brilliancy diminish as he left the shores of the 

 Polar sea, about Nischne-Kolymsk. The observations made 

 in the North Polar expedition appear to prove that in the 

 immediate vicinity of the magnetic pole the development of 

 light is not in the least degree more intense or frequent than 

 at some distance from it. 



The knowledge which we at present possess of the altitude 

 of the polar light is based on measurements, which, from their 

 nature, the constant oscillation of the phenomenon of light, 

 and the consequent uncertainty of the angle of parallax, are 

 not deserving of much confidence. The results obtained, set- 

 ting aside the older data, fluctuate between several miles, and 



* The northern lights are called by the Shetland Islanders "tho 

 merry dancers." (Kendal, in the Quarterly Journal of Science, ne* 

 series, vol. iv. p. 395,) 



t See Muncke's excellent work in the new edition of Gehler's Physik, 

 W&rterbuch. bd. vii, i. s. 113-28, and especially s. 158. 



o 



