Let us premise that the depth of the crimson shell out of which they rise is usually less than five thousand miles, and that though the prominences vary greatly, the majority reach a height of nearly twenty thousand miles, while in exceptional cases this is immensely exceeded. Professor Young has seen one which grew to a height of three hundred and fifty thousand miles in an hour and a half, and in half an hour more had faded away.
These forms fall into two main classes,—that of the quiet and cloud-like, and that of the eruptive,—the first being almost exactly in form like the clouds of our own sky, sometimes appearing to lie on the limb of the sun like a bank of clouds on the horizon, sometimes floating entirely free; while sometimes “the whole under surface is fringed with down-hanging filaments, which remind one of a summer shower hanging from a heavy thunder-cloud.”
Here are some of the typical forms of the quieter ones:—
[Fig. 43], by Tacchini, the Director of the Roman Observatory, represents an ordinary prominence, or cloud-group in the chromosphere, whose height is about twenty-five thousand miles. The little spires of flame which rise, thick as grass-blades, everywhere from the surface, are seen on its right and left.
FIG. 46.—TACCHINI’S CHROMOSPHERIC FORMS. (“MEMORIE DEGLI SPETTROSCOPISTI ITALIANI.”)
[Fig. 44] (Tacchini) is one where the agitation is greater and the “filamentary” type is more marked. Besides the curiously thread-like forms (so suggestive of what we have already seen in the photosphere), we have here what looks like an extended cloudy mass, drawn out by a horizontally moving wind.
[Fig. 45] (by Vogel, at Bothkamp) represents another of these numerous types.
The extraordinary [Fig. 46] is from another drawing, by Tacchini, of a protuberance seen in 1871 (a time of great solar disturbance), and it belongs to the more energetic of its class.
