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shown in Plate VI. We have only to suppose each horizontal row of coloured prominences bent round a disc representing the Sun, and the prominences reduced to one-half their present dimensions, in order to have a complete picture of the Sun's condition on any of the days illustrated in the plate. It cannot be but that the application of Respighi's system throughout a long series of years will result in revealing laws in the number and magnitude of prominences at different times and in different solar latitudes. Such laws, once clearly, recognized, will probably serve to indicate relations, as yet wholly unthought of, between the prominences, the phenomena of the photosphere, the magnetic relations of the planetary scheme, and many other periodic phenomena.

Leaving the reader to study at his leisure the singularly interesting features shown in Plate V I., I will briefly indicate certain general conclusions to which, Professor Respighi's researches have led him.

He considers that the prominences, at least as regards their origin, bear no analogy whatever to terrestrial clouds, but are strictly phenomena of eruption.* He has noticed that where there are faculæ there are usually prominences, but he considers that the prominences are certainly not identical with the facula. Over Sun-spots there are low jets, but no high prominences. In the circumpolar regions great prominences are never

* This view is not altogether opposed to Dr. Zöllner's, who regards 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.

recognized, 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. 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

*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.

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.*

A few pages further on later evidence on this point will be considered. During the total solar eclipse of August, 1869, many interesting observations were made. But so much space has already been occupied by the subject of this chapter that I must deal very briefly with the observations on the prominences and sierra. In the next chapter I shall have occasion to quote at some length those narratives of the American observers which relate to the corona. The accompanying drawing (fig. 78) exhibits the prominences photographed by the American astronomers, the Moon's disc being reduced so as to permit all the prominences to be visible at once. The line F E indicates the course of the Moon's centre across the Sun, A B being a declination

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. [Prof. Young's account of the explosion he witnessed on September 7, 1871, p. 320, seems to accord well with the view here expressed.

circle, and C D a declination-parallel. The most successful photographing party was that headed by Dr. Mayer, at Burlington. But the two photographs by

FIG. 78.

[graphic]

The Eclipse of August 7, 1869. From Photographs by the
American astronomers.

Dr. Curtis possess a special value on account of their delicacy and the number of details they exhibit. He has shown that the sharply-defined outlines of prominences, and even of the coronal glare, seen in most photographs of eclipses, result from excessive development. He proves satisfactorily, also, that the encroachment of the prominence-bases on the black disc of the Moon is due to a similar cause.

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