92 



THE AURORA BOREALIS. 



netic meridian, or very nearly so. It was not easy to determine this with pre- 

 cision, because of the motion of the bow, and the great magnitude of the circle, 

 of which it formed but a small segment : blackish streaks divided regularly the 

 luminous matter of the arc, and resolved it into a system of rays ; these rays 

 were alternately extended and contracted ; sometimes slowly, sometimes in- 

 stantaneously ; sometimes they would dart out, increasing and diminishing sud- 

 denly in splendor. The inferior parts, or the feet of the rays, presented always 

 the most vivid light, and formed an arc more or less regular. The length of 

 these rays was very various, but they all converged to that point of the heavens 

 indicated by the direction of the southern pole of the dipping needle, as indi- 

 cated in fi^r. 2. Sometimes they were prolonged to the point where their 



Fig. 2. 



directions intersected, and formed the summit of an enormous dome of light, as 

 represented in fig. 3. 



Fig. 3. 



The bow then would continue to ascend toward the zenith : it would suffer 

 an undulatory motion in its light that is to say, that from one extremity to the 

 other the brightness of the rays would increase successively in intensity. This 

 luminous current would appear several times in quick succession, and it would 

 pass much more frequently from west to east than in the opposite direction. 

 Sometimes, but rarely, a retrograde motion would take place immediately af- 

 terward ; and as soon as this wave of light would run successively over all the 

 rays of the aurora from west to east, it would return, in the contrary direction, 

 to the point of its departure, producing such an effect that it was impossible to 

 say whether the rays themselves were actually affected by a motion of transla- 

 tion in a direction nearly horizontal, or if this more vivid light was transferred 

 from ray to ray, the system of rays themselves suffering no change of position. 



The bow, thus presenting the appearance of an alternate motion in a direc- 

 tion nearly horizontal, had usually the appearance of the undulations or folds 

 of a riband or flag agitated by the wind, as represented in fig. 4. Sometimes 



