Page:The American Cyclopædia (1879) Volume XV.djvu/493

 SUN 473 very edge of the sun, and projecting irregu- larly beyond the circular contour, reminding him of a ridge of low hills often seen at the enlightened limb of the moon. M. Chacornac, a most diligent French investigator, observed on one occasion a sudden transformation of the luminous part of the photosphere into dark parts ; luminous bridges were seen cross- ing the spots, and then gradually becoming dark. As these luminous bridges darkened, they at the same time plunged into the deeper parts, and became covered with other lumi- nous bridges which formed above them. The phenomena witnessed during total solar eclipses are next to be considered. The red prominen- ces were first seen during the solar eclipse of July 8, 1842. In the eclipse of July 28, 1851, it was shown that they belong to the sun, since the advancing moon visibly concealed those on one side and disclosed those on the other side. During the eclipse of June 18, 1860, Secchi and De la Rue photographed the prom- inences at two stations in Spain, and thence- forth the solar nature of these appendages was admitted by all. As some of them were seen to extend fully 3' from the edge of the sun on that occasion, it became manifest that they are objects of enormous dimensions, since 3' at the sun's distance corresponds to an extension of about 80,000 m. In the Indian eclipse of August, 1868, the prominences were examined with the spectroscope by Col. Tennant, Capt. Herschel, and Messrs. Janssen and Rayet. The spectrum was found to consist of bright lines, showing that the colored prominences are masses of glowing gas, the bright lines of hy- drogen were recognized, and an orange-yellow line was ascribed (mistakenly, however) to so- dium. But on the following day Janssen ap- plied a new method of research, the principle of which had been indicated earlier by Huggins (" Report of Council of Astronomical Society," "Monthly Notices," February, 1868). Since prismatic dispersion reduces the brightness of the solar spectrum, but only throws the lines of a gaseous spectrum further apart, it follows that by directing a tele-spectroscope toward the place of a prominence, the light from the air which usually obliterates the prominence light can be so reduced by sufficient dispersion that the prominence lines may be rendered visi- ble^ Janssen found this to be the case, and by noting the indications thus afforded he was able to determine the presence and even the shape of prominences at various parts of the sun's edge. Two months later, but before the news of Janssen' s success had reached England, Mr. Lockyer obtained a similar result. Before long Huggins, who had been the first to enunciate the principle of the method, showed how by opening the slit of the spectroscope the whole of a prominence could be seen at once. Since then the prominences have been successfully studied by Zollner, Respighi, Secchi, and oth- ers. Prof. Young of Dartmouth college has been particularly successful in applying this method of research. Even before the promi- nences were discovered, it was known that a border of red light surrounds the solar disk ; it had been seen on the eastern side at the be- ginning of total eclipse, and on the western side at the end. In 1860 this envelope was very clearly seen, and even photographed. It has been designated as the sierra, because of its serrated appearance; but recently the name chromosphere (for chromatosphere) has been given to it. The observations of prominences and sierra ^ as summarized by Secchi indicate the following results : " The sierra presents four aspects : 1, smooth, with defined outline ; 2, smooth, but no defined outline; 3, fringed with filaments ; and 4, irregularly fringed with small flames. The prominences may be divi- ded into three orders, heaps, jets, and plumes. The heaped prominences need no special de- scription. The jets are those to which alone the following description by Respighi (erro- neously given as generally applicable to all prominences) can be applied: ' They 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 80,000 m., 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 gi- gantic trees more or less rich in branches.' Their luminosity," proceeds Secchi, "is in- tense, 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 ex- ceedingly bright masses, which eventually sep- arate into fleecy clouds. The jet prominences last but a short time, rarely an hour, frequent- ly 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 faculas. The plume prominences are distinguished from the jets in not being char- acterized 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 jets a part of the photo- sphere is lifted up, whereas in the case of plumes only the sierra is disturbed." (It is here of importance to remark that these erup- tive prominences, particularly associated with spots, are of late becoming recognized as chief- ly due to metallic vapors, in distinction from the "plume" forms, which are largely com- posed of hydrogen.) This account would be in- complete without a description of the remark- able solar explosion actually witnessed by Prof. Young on Sept. 7, 1871. Fig. 1 represents a cloud prominence he had been observing on the eastern limb of the sun. It was about 100,- 000 m. long by 54,000 m. high. He was called