THE aurora polaris, after long study, remains in many respects mysterious? of late years, however, scientific opinion has been nearly unanimous in regard to its height in the atmosphere, locating it at about 100 kilometres and above. St ¸rmer has found trustworthy evidence, in a few cases, of its occurrence so low as 80 km. Its upward extent is very variable, but at times it rises to a height of 800 km. These very high auror seem to occur exclusively in the part of the atmosphere which, though viewed from stations in the night hemisphere, is itself still illuminated by the sun's rays? no very likely explanation of this remarkable fact has yet been proposed.