These singular forms, which could, until recently, be observed only during the short duration of the total eclipses of the Sun, can now be seen on every clear day with the spectroscope, thanks to Messrs. Janssen and Lockyer, to whose researches solar physics is so much indebted.
PLATE II.—SOLAR PROTUBERANCES.
Observed on May 5, 1873 at 9h, 40m. A.M.
The solar protuberances, the Sun-spots, and the faculæ to which they are closely related, are confined within the same general regions of the Sun, although the protuberances attain higher heliocentric latitudes.
There is certainly a very close relation between the faculæ and the solar protuberances, since when a group of the faculæ traverses the Sun's limb, protuberances are always seen at the same place. It seems very probable that the faculæ and the protuberances are in the main identical. The faculæ may be the brighter portion of the protuberances, consisting of gases which are still undergoing a high temperature and pressure; while the gases which have been relieved from this pressure and have lost a considerable amount of their heat, may form that part of the protuberances which is only visible on the Sun's limb.
A daily study of the solar protuberances, continued for ten years, has shown me that these objects are distributed on two zones which are equidistant from the solar equator, and parallel with it. The zone arrangement of the protuberances is more easily recognized during the years of minimum solar activity, as in these years the zones are very narrow and widely separated. During these years the belt of protuberances is situated between 40° and 45° of latitude, north and south. In years of great solar activity the zones spread considerably on either side of these limits, especially towards the equator, which they nearly reach, only a narrow belt, usually free from protuberances, remaining between them. Towards the poles the zones do not spread so much, and there the space free from protuberances is considerably greater than it is at the equator.
During years of maximum solar activity, the protuberances, like the Sun-spots and the faculæ, are very numerous, very large, and very complicated—sometimes occupying a great part of the whole solar limb. As many as twenty distinct flames are sometimes observed at one time. In years of minimum solar activity, on the contrary, the prominences are very few in number, and they are of small size; but, as far as my observations go, they are never totally absent.
In general, the solar flames undergo rapid changes, especially those which are situated in the vicinity of Sun-spots, although they occasionally remain unchanged in appearance and form for several hours at a time. The protuberances situated in higher latitudes are less liable to great and sudden changes, often retaining the same form for several days. The changes observed in the protuberances of the equatorial regions are due in part to the comparatively great changes in their position with respect to the spectator, which are occasioned by the rotation of the Sun. This rotation, of course, has a greater angular velocity on the equator than in higher latitudes. In most cases, however, the changes of the equatorial protuberances are too great and too sudden to be thus explained. They are, in fact, due to the greater solar activity developed in the equatorial zones, and wherever spots are most numerous.