The solar protuberances appear under various shapes, and are often so complicated in appearance that they defy description. Some resemble huge clumsy masses having a few perforations on their sides; while others form a succession of arches supported by pillars of different styles. Others form vertical or inclined columns, often surmounted by cloud-like masses, or by various appendages, which sometimes droop gracefully, resembling gigantic palm leaves. Some resemble flames driven by the wind; others, which are composed of a multitude of long and narrow filaments, appear as immense fiery bundles, from which sometimes issue long and delicate columns surmounted by torch-like objects of the most fantastic pattern. Some others resemble trees, or animal forms, in a very striking manner; while still others, apparently detached from the solar limb, float above it, forming graceful streamers or clouds of various shapes. Some of the protuberances are very massive, while others are so thin and transparent as to form a mere veil, through which more distant flames can easily be seen.
Notwithstanding this variety of form, two principal classes of solar protuberances may be recognized: the cloud-like or quiescent, and the eruptive or metallic protuberances.
The first class, which is the most common, comprises all the cloud-like protuberances resting upon the chromosphere or floating about it. The protuberances of this type often obtain enormous horizontal proportions, and it is not rare to see some among them occupying 20° and 30° of the solar limb. The height attained by protuberances of this class does not correspond in general to their longitudinal extent; although some of their branches attain considerable elevations. These prominences very seldom have the brilliancy displayed by the other type, and are sometimes so faint as to be seen with difficulty. Although it is generally stated by observers that some of the protuberances belonging to this class are detached from the solar surface, and kept in suspension above the surface, like the clouds in our atmosphere, yet it seems to me very doubtful whether protuberances are ever disconnected from the chromosphere, since, in an experience of ten years, I have never been able to satisfy myself that such a thing has occurred. Many of them have appeared to me at first sight to be detached from the surface, but with a little patience and attention I was always able to detect faint traces of filamentary elements connecting them with the chromosphere. Quite often I have seen bright protuberances gradually lose their light and become invisible, while soon after they had regained it, and were as clearly visible as before. Observations of this kind seem to show that while the prominences are for the most part luminous, there are also a few which are non-luminous and invisible to the eye. These dark and invisible forms are most generally found in the vicinity of Sun-spots in great activity. When observing such regions with the spectroscope, it is not rare to encounter them in the form of large dark spots projecting on the solar spectrum near the hydrogen lines. On July 28th, 1872, I observed with the spectroscope a dark spot of this kind issuing from the vicinity of a large Sun-spot, and extending over one-fifth of the diameter of the Sun. This object had been independently observed in France a little earlier by M. Chacornac with the telescope, in which it appeared as a bluish streak.
The second class of solar protuberances, comprising the eruptive type, is the most interesting, inasmuch as it conveys to us a conception of the magnitude and violence of the solar forces. The protuberances of this class, which are always intensely bright, appear for the most part in the immediate vicinity of Sun-spots or faculæ. These protuberances, which seem to be due to the outburst of the chromosphere, and to the violent ejection of incandescent gases and metallic vapors from the interior of the Sun, sometimes attain gigantic proportions and enormous heights.
While the spectrum of the protuberances of the cloudy type is simple, and usually composed of four hydrogen lines and the yellow line D3, that of the eruptive class is very complicated, and, besides the hydrogen lines and D3, it often exhibits the bright lines of sodium, magnesium, barium, titanium, and iron, and occasionally, also, a number of other bright lines.
The phenomena of a solar outburst are grand and imposing. Suddenly immense and acute tongues and jets of flames of a dazzling brilliancy rise up from the solar limb and extend in various directions. Some of these fiery jets appear perfectly rigid, and remain apparently motionless in the midst of the greatest disorder. Immense straps and columns form and rise in an instant, bending and waving in all sorts of ways and assuming innumerable shapes. Sometimes powerful jets resembling molten metal spring up from the Sun, describing graceful parabolas, while in their descent they form numerous fiery drops which acquire a dazzling brilliancy when they approach the surface.
The upward motion of the protuberances in process of formation is sometimes very rapid. Some protuberances have been observed to ascend in the solar atmosphere at the rate of from 120 to 497 miles a second. Great as this velocity may appear, it is nevertheless insignificant when compared with that sometimes attained by protuberances moving in the line of sight instead of directly upwards. Movements of this kind are indicated by the displacement of the bright or dark lines in the spectrum. A remarkable instance of this kind occurred on the 26th of June, 1874. On that day I observed a displacement of the hydrogen C line corresponding to a velocity of motion of 1,600 miles per second. The mass of hydrogen gas in motion producing such a displacement was, according to theory, moving towards the Earth at this incredible rate, when it instantly vanished from sight as if it had been annihilated, and was seen no more.
Until recently the protuberances had not been observed to rise more than 200,000 miles above the solar surface; but, on October 7th, 1880, a flame, which had an elevation of 80,000 miles when I observed it at 8h. 55m. A. M., had attained the enormous altitude of 350,000 miles when it was observed at noon by Professor C. A. Young. If we had such a protuberance on the Earth, its summit would be at a height sufficient not merely to reach, but to extend 100,000 miles beyond the Moon.
Although the solar protuberances represented in Plate II. have not the enormous proportions attained by some of these objects, yet they are as characteristic as any of the largest ones, and afford a good illustration of the purely eruptive type of protuberances. The height of the largest column in the group equals 4' 43", or a little over 126,000 miles. A large group of Sun-spots was in the vicinity of these protuberances when they were observed and delineated.