has noticed that where there are facule there are usually prominences, but he considers that the prominences are certainly not identical with the faculæ. Over Sun-spots there are low jets, but no high prominences. In the circumpolar regions great prominences are never recognised, and the prominences actually seen, besides being small, are few in number, and last but a short time. At the solar equator, also, the prominences are less frequent, less active, and less developed than in higher solar latitudes. He noticed some prominences exceeding three minutes, or ten terrestrial diameters, in altitude; and one prominence observed by him had an elevation of no less than twenty terrestrial diameters, or about 160,000 miles. He found that the formation of a prominence is usually preceded by the appearance of a rectilinear jet, either vertical or oblique, and very bright and well defined. This jet rising to a great height, is seen to bend back again, falling upon the Sun like the jets of our fountains, and presently the sinking matter is seen to assume the shape of gigantic trees more or less rich in branches and foliage. Gradually the whole sinks down upon the Sun, sometimes forming isolated clouds before reaching the solar surface. It is in the upper portions of such prominences that the most remarkable and rapid transformations are witnessed; but a great difference is observed in the rate with which prominences change in figure. the two types into which he divides prominences as different forms of the same objects, and, therefore (necessarily), the cloud form as the sequel of the eruptive. Their duration, too, is very variable. Some develope and disappear in a few minutes, while others remain visible for many successive days. He considers that the sharply-defined bases of the eruptive jets prove that the eruption takes place through some compact substance forming a species of solar crust.* He agrees with Zöllner in considering that the enormous velocity with which these gaseous masses rush through the solar atmosphere implies that the latter is of exceeding tenuity. His conclusion that repulsive forces must balance the solar gravity, is not shared, however, by Zöllner, who prefers to regard the forces at work in producing the prominences as strictly analogous to those which produce eruptive action on our Earth.† I had proposed to give a somewhat full account of the observations made by the American astronomers upon the solar prominences visible during the total solar eclipse of August 1869. But so much space has already been occupied by the subject of this chapter that I prefer to introduce only those narratives of the American observers which relate to the corona. These will be found in the * This corresponds to the Trennungschicht of Zöllner's theory, who, however, does not agree with Respighi in regarding some form of electric action as the probable cause of these eruptions. It seems conceivable that the phenomena of geysers may have their counterpart in these solar prominences. It is true that a gaseous, not a liquid, mass is erupted; so that there is here nothing comparable to the rapid change of a column of liquid into vapour which causes the outbursts of geysers. But the temperature and pressure at which dissociation occurs may bear the same relation to the outbursts of these gaseous masses that the temperature and pressure at which water boils, bear, in Bunsen's theory of geysers, to the occurrence of geyser eruptions. |