4 ON THE RECENT SECULAR PERIOD 



one view in separate groups, such as are similar, forming a full and accurate classi- 

 fication ; thirdly, to inquire what general truths these facts reveal, since these deduc- 

 tions form the proximate laws of the phenomenon ; and, finally, to make the 

 laws the groundwork of a general theory, which shall assign the true cause of all. 



CLASSIFICATION OF AURORAS. 



The Aurora Borealis presents itself to us under six different forms. 



1. Auroral TuoUigM. — A light in the north resembling the dawn of day, and of 

 various degrees of intensity. 



2. Arches. — Arcs of circles or zones, formed at various altitudes between N. E. 

 and N. W., being sometimes the mere boundary of a segment, at other times a 

 dense pillar of light, forming a grand columnar arch which spans the heavens from 

 east to west. It frequently moves from north to south, usually advancing but 

 little further than the zenith, but in a few instances, in our latitude, it has been 

 seen to reach within twenty degrees of the southern horizon. 



3. Streamers. — Acute cones or spindles, usually shooting up from an arch, or 

 from a dark smoky cloud which lies along the northern horizon, or rises a few 

 degrees above it. 



4. Corona. — A circular zone around the pole of the dipping needle, formed of 

 wreaths of auroral vapor, either of pure white, or of various prismatic colors, with 

 streamers radiating from the circumference. 



5. Waves. — Undulations which commonly flow upwards towards the centre of 

 the corona, along the line of the streamers, but sometimes course along the line 

 of an arch from east to west. 



G. Auroral Clouds. — A milky vapory bank in the north, the quantity and appa- 

 rent depth of which afford a prognostic of the intensity of the approaching aurora. 

 These clouds are sometimes of a smoky hue, especially in front, while the margins 

 are luminous. The term Merry Dancers is loosely applied to several different 

 appearances, constituting the more active portions of the phenomenon. Thus 

 rapid coruscations, Bickerings, and swift horizontal movements across a forest of 

 streamers, have been, by different writers, severally denominated merry dancers. 



In different exhibitions of the aurora borealis, the various forms above enumer- 

 ated, are sometimes seen single, but commonly more or less combined. In the 

 most magnificent examples they all are seen in company. At first, usually at an 

 early hour of the evening, appears the northern twilight, as though the sun after 

 he had set was rising prematurely in the north. If a large bank of luminous vapor 

 (which is so peculiar in its external properties, and so distinct from watery vapor, 

 as to warrant the denomination of auroral vapor), rests on the northern horizon, 

 we may expect to see the aurora put on successively more of its higher forms — 

 streamers will begin to shoot upwards; a dark smoky front will cover the auroral 

 vapor, exhibiting here and there changeable and transient white spots, which 

 suddenly swell out, and often as suddenly disappear — then large columns of a clear 

 silvery lustre will form in the northwest and northeast simultaneously, which will 



