"An aurora borealis is always preceded by the formation in the horizon of a sort of nebulous veil, which slowly ascends to a height of 4°, 6°, 8°, and even to 10°. It is towards the magnetic meridian of the place that the sky, at first pure, begins to get brownish. Through this obscure segment, the color of which passes from brown to violet, the stars are seen, as through a thick fog. A wider arc, but one of brilliant light, at first white, then yellow, bounds the dark segment. Sometimes the luminous arc appears agitated, for hours together, by a sort of effervescence, and by a continuous change of form, before the rising of the rays and columns of light, which ascend as far as the zenith. The more intense the emission of the polar light, the more vivid are its colors, which, from violet and bluish white, pass through all the intermediate shades of green and purple-red. Sometimes the columns of light appear to come out of the brilliant arc mingled with blackish rays, resembling a thick smoke; sometimes they rise simultaneously from different points of the horizon, and unite themselves into a sea of flames, the magnificence of which no painting could express; for, at each instant, rapid undulations cause their form and brilliancy to vary. Motion appears to increase the visibility of the phenomena. Around the point in the heaven which corresponds to the direction of the dipping needle produced, the rays appear to meet and form the boreal corona. It is seldom that the appearance is so complete, and is prolonged to the formation of the corona; but when the latter appears, it always announces the end of the phenomenon. The rays then become more rare, shorter, and less vividly colored. Soon nothing further is seen on the celestial vault than wide, motionless, nebulous spots, pale, or of an ashy color; they have already disappeared, when the traces of the dark segment whence the appearance originated still remain on the horizon."
The connection that seems to exist, says De la Rive, between the polar light and the appearance of a certain species of clouds is confirmed by all observers; all have affirmed that the polar light emitted its most brilliant rays when the high regions of the air contained heaps of cirri,--strata of sufficient tenuity and lightness to cause a corona to arise around the light. Sometimes these clouds are grouped and arranged almost like the rays of an aurora borealis; they then appear to disturb the magnetized needle. Father Secchi has remarked, that magnetic disturbances are manifested at Rome whilst the sky is veiled with clouds that are slightly phosphorescent, which, at night, present the appearance of feeble aurorae boreales.