184 TERRESTRIAL PHENOMENA. 



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

 stated that Auroras decrease in frequency and brilliancy 

 with decreasing latitude, it must be understood of magnetic 

 latitude. Whilst an Aurora is a very rare occurrence in 

 Italy, it is extremely common in the same latitude in 

 Philadelphia (39 57'), owing to the vicinity of the Ame- 

 rican magnetic pole; and in Iceland, Greenland, New- 

 foundland, on the shores of the Slave Lake, and at Tort 

 Enterprise, the " merry dancers," ( 175 ) as the inhabitants 

 of the Shetland Islands call the quivering and variously- 

 coloured rays of the Aurora, are seen during certain seasons 

 of the year almost every night. But even in those parts 

 of the new continent and of Siberia which are distin- 

 guished by the frequency of the phenomenon, there may be 

 said to be different districts or zones of longitude in 

 which it shews itself with peculiar splendour. ( 176 ) Wrangel 

 saw its brilliancy diminish at Nishni Kolymsk, as he receded 

 from the coast of the Polar Sea ; in this and in similar 

 instances local influences are not to be denied. The expe- 

 rience of the various North Polar expeditions seems to shew 

 that in the immediate vicinity of the magnetic pole the 

 evolutions of light are, to say the least, not more intense or 

 frequent than at somewhat greater distances. 



What we know of the height of the Aurora is grounded 

 on measurements which, from their nature, and from 

 the incessant fluctuation of the phenomenon and con- 

 sequent uncertainty of the parallactic angle, cannot inspire 

 much confidence. Without including older statements, the 

 results of these measurements give heights varying from a few 

 thousand feet to several miles. ( 177 ) The most modem ob- 

 servers are inclined to place the seat of the phenomenon, not 



