"The sierra presents four aspects: 1, smooth with defined outline; 2, smooth but with no defined outline; 3, fringed with filaments; and, 4, irregularly fringed with small flames. The prominences may be divided into three orders,--heaps, jets, and plumes. The heaped prominences need no special description. The jets ... originate generally in rectilinear jets either vertical or oblique, very bright and very well defined. They rise to a great height, often to a height of at least eighty thousand miles, and occasionally to more than twice that; then bending back, fall again upon the sun like the jets of our fountains. Then they spread into figures resembling gigantic trees more or less rich in branches. Their luminosity is intense, insomuch that they can be seen through the light clouds into which the sierra breaks up. Their spectrum indicates the presence of many elements besides hydrogen. When they have reached a certain height they cease to grow, and become transformed into exceedingly bright masses, which eventually separate into fleecy clouds. The jet prominences last but a short time--rarely an hour, frequently but a few minutes,--and they are only to be seen in the neighborhood of the spots. Wherever there are jet prominences there also are faculæ. The plume prominences are distinguished from the jets in not being characterized by any signs of an eruptive origin. They often extend to an enormous height; they last longer than the jets, though subject to rapid changes of figure; and, lastly, they are distributed indifferently over the sun's surface. It would seem that in the jets a part of the photosphere is lifted up, whereas in the case of plumes only the sierra is disturbed." Of these eruptions Professor Ball says, "Vast masses of vapors are frequently expelled from the interior of the sun by convulsive throes with a speed of three hundred, four hundred, and sometimes nearly a thousand miles a second.... The spectroscope enables the observer actually to witness the ascent of these solar prominences."