^^^ AURORA BOEEALIS. 



have seen the stars without any dimhiution in their brilliancy. Its dark apuear- 

 ance is the eifect of contrast with the luminous arc.*" ^^ 



In the year 1S3S the French government sent out a scientific expedition to 

 make explomtions in the Arctic seas. Five members of this commission Zn^ 



ongiuule 1 oo - E. ot Greenwich,) for the purpose of making observations upon 



mV Totdn^E f ' '"^'^r", '-f ^--^^H^-f phenomena. The observers ITe 

 i\lM. Lottii, Bravais, Lilhehook, and Siljestrom, while M. Bevalet made 

 sketches or the most remarkable auroras. These observers, in their final report 

 say : i he dark segment was situated near the magnetic meridian toward, the 

 north, and was generally illumined by auroral hghr. Sometimes the illumina! 

 tion prevailed throughout the entire extent of the upper border of the seo-ment • 



t . oulinaiy appearance of an auroral arch; but generally the source of the 

 dlummation seemed to be 6e/M the segment, nearihe horfzon, or even below 

 It. ihe lower edge of the luminous band which crowned the segment was dif- 



Sv;"d^T' '' 'V' "^"^^ '' '^' ^"-^^^^"S ''^'''y '' the hazy ISlt^m 

 travel sea by the visual ray. "^ 



" Once the dark segment was observed at the souik point of the horizon, and 

 appeared bordered by the auroral light. At other times the haze exteVd^d 

 towards the east and west part of the horizon; it then appeared to overlan he 

 lower coctremity of thearches which passed neir the zenilh, an I conc'^el^h 

 point of intersection with the horizon. 



" The appearance of an aurora is not necessarily preceded by this dark seer, 

 ment, several brilliant auroras having been seen when the sky was clear and ff 

 a deep blue quite down to the horizon. 



" The light of the stars is but little diminished by passing throu-h the sub- 

 stance of the aurora The smallest stars can be seeVthrough the ?ays of ho 

 curora, especially when its light is feeble and diffuse."t ^ ^ « 



meri^nn^\<'' ^"f "\ '^'" '^7^' ''S^ent is generally found in the magnetic 



meiidian. Exceptional cases, however, frequently occur, and in certain rc-ions 



here appear tooe constant causes which tend to deviate this point uniformly in 



he same direction. Ihus at Abo, (latitude 60O 27' N.,) M. Argelander iound 



that this sumnnt was lio west of the magnetic meridian t 



hroM'tZ'^f-'^^^^f" ''^^'^' ^' ^"""^^^'^ by a luminous arc, whose 

 bic adth vanes from a half degree to one or two degrees. The lower edge is well 

 defanec, but the upper edge is only so when the breadth is very small As the 



brl'V"'T'''/^f W "'^-^ ^''''''^'' ^''' ^^^'""te, and at length its light 

 becomes confounded with a general brightness of the sky. If the aurora be- 



z:t!;^;i:nti sir^ ''-' ^^^^"^ ^^-^ ^^^ ^-^^- ^-^''-' -t''^- p--^ 



arc^sonv';!!!. ' Vt^^ '^T ^''' '' '■'''T'^ "'^"1^' ^" ^^' ^'"^-"^tic meridian, and the 

 ai c sometimes extends symmetrically on each side towards the horizon. Durino- 

 thewmterof 18oS and 1839 numerous measurements of auroral arches were 

 njade in Scandinavia (latitude 70^) by MM. Lottin and Bravais, with the ad 

 foo tl™l h 'VT^' '^' -' ^b^'^-'^-^tioiis gave an average deviation of 



lOO tovvaids the west of the magnetic meridian. The deviation was only about 

 60 for arcs rising but little above the northern horizon; it was about 120 for 

 arcs passing near the zenith. This result is almost indentical with that obtained 

 by Aigeiandcr at Abo. 



The auroral arches observed by Captain Parry, in his Arctic voyages, did not 

 always have their centres in the magnetic meridian.§ A splendid aurond arch 



* Pogg-endorff Aim., XXII, p. 4.56. 

 t Voyages eu Scandiuavie, pp. 437-442. 

 ■tKaenitz's xMetcorologv, by Walker, p. 453. 

 5 Parry's Secoud Voyage, p. 135. 



