Page:Popular Science Monthly Volume 53.djvu/487

Rh "nordlicht" in German, "nordljus" in Swedish, and "nordlys" in Danish, all meaning northern lights.

The aurora in its most varied and interesting forms is either motionless or rapidly and incessantly scintillating. Of the first class there are three forms: (1) Faint lights without very defined form; (2) more distinct, in patches or clouds; (3) clearly defined arcs touching the horizon at either end. Of the second or moving class there are: (4) irregular arcs, formed of intermittent rays; (5) rays isolated from each other at a greater or less distance, converging to a fixed point in the sky, and sometimes forming around this center a crown or glory; (6) non-homogeneous bands, formed of rays pressed close together, which have not all the same degree of brilliancy; sometimes these bands fold over on themselves, becoming draped auroras, the most beautiful of their manifestations, aside from the boreal crown.

It sometimes happens that these arcs, instead of being circular, are distinctly elliptical; the two curves which form the upper and lower edge of the arc may or may not be parallel; sometimes, instead of a single circular arc, there are two, three, or four arcs, all perfectly concentric. Multiple striped arcs are not uncommon. Stars have been discerned through the rays, as also through the arcs, proving their extremely diaphanous character.

The chief characteristic of the auroral rays is their extreme variability. They are subject to two kinds of movements: the lateral or sidewise, and the longitudinal or upward; both movements are very rapid. A ray in twenty-seven seconds covered a distance of ninety degrees, or half the heaven, in one instance. At times a ray remaining nearly in the same place is seen at the upper extremity to dart toward the zenith or lengthen itself upward. At other times it rises and falls alternately, vibrating, and it is then said to dance. By sixteenth-century writers these rays were called "leaping goats" and "flying fires"; such rays are still known in Canada as "marionettes," and in the Shetlands as "merry dancers."

The direction of all the rays passes, as a rule, close to the magnetic zenith of the locality, or may radiate therefrom, as we have seen, forming a crown; the center of this crown may be either luminous or obscure. At certain moments the rays which compose this crown or glory enter into rapid movement, become very brilliant, and take on, instead of the usual yellowish-white color, vivid tints of red and green. This is one of the finest auroral effects, if not the finest. When one of these crowns forms in the midst of an already existing aurora, all the other lights of the aurora pale, to reappear when the crown is dissipated.

So much for the forms of the aurora. Now as to the colors. The