Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/859

] &quot;The sierra presents four aspects: (1), smooth, with a defined outline ; (2), smooth, with no definite outline ; (3), fringed with filaments and (4), irregularly fringed with small flames. &quot; The prominences may be divided into three orders heaps, jets, and plumes. The heaped prominences need no special description. The jets are those to which alone Respighi s description is applicable. 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, fre quently but a few minutes, and they are only to be seen in the neighbourhood of the spots. Wherever the jet prominences are seen, there also are faculse. The plume prominences are distinguished from the jets in not being characterised 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 indiiferently over the sun s surface. It would seem that in jets a part of the photosphere is lifted up, whereas in the case of plumes only the sierra is disturbed.&quot; The theoretical inquiries of Zb llner, confirmed by his own and Respi^hi s observations, show the probability that the jet prominences are true phenomena of eruption. And more recently, Professor Young has witnessed what must be regarded as in all probability a veritable solar eruption, during which matter was propelled to a height of upwards of 200,000 miles above the sun s surface. The matter whose motion was actually observed was glowing hydro gen ; but it should be noticed that it does not necessarily follow that the matter erupted was hydrogen. The out- rush may have carried along with it portions of hydrogen which had before been quiescent in the solar atmosphere.

The solar corona comes next to be considered. It had long been known that around the black disc of the moon in total solar eclipses a halo or glory of light is seen, rather bright close by the sun, and fading away at first somewhat quickly, afterwards gradually, into the darkness of the surrounding sky. The records of some total eclipses described also radiations in this coronal glory, extending in many cases to a great distance from the sun. Various theories were propounded in explanation of the solar corona. According to one view, it was a mere ter restrial phenomenon, due to the passage of the solar rays through our own atmosphere. Others ascribed it to the effects produced by a lunar atmosphere. The theory that it is due to diffraction was also advanced. But the gener ality of astronomers, especially in recent times, regarded the corona as a true solar appendage. Space will not permit us to enter at length on the dis cussion of the various theories just mentioned, or on the description of the various observations which appeared to give support to one theory or another. Nor can we pre sent in full the interesting history of the observations made since 1869, when first the spectroscope was applied to this interesting phenomenon, and real evidence as to its nature and structure began to be obtained. We must be content with the brief statement of the salient points of the recent observations. During the solar eclipse of August 1869 the American astronomers, Young and Harkness, discovered that the spectrum of the corona is discontinuous (or that at least a portion of its light gives a discontinuous spectrum). Cer tainly one, and probably three bright lines appeared in the spectrum as they saw it, though the faintness of two of the three lines raised some doubt on the question whether they belonged to the corona. During the eclipse of December 1870, Young renewed his observations successfully, and other observers succeeded in seeing the bright-Hue spec trum of the corona. Young thus sums up the results of his own and other observations : &quot;There is,&quot; he said, &quot;sur rounding the sun, beyond any further reasonable doubt, a mass of self-luminous gaseous matter, whose spectrum is characterised by the green line 1474 Kirchhoff. The pre cise extent of this it is hardly possible to consider as de termined, but it must be many times the thickness of the red hydrogen portion of the chromatosphere, perhaps on an average 8 or 10, with occasional horns of twice that height. It is not at all unlikely that it may even turn out to have no upper limit, but to extend from the sun indefinitely into space.&quot; It was during this eclipse that for the first time photo graphy gave convincing evidence respecting the corona. Mr Brothers at Sicily, and Mr Willard (an American photo grapher) in Spain, obtained views which, though differing in extent, accorded so well in those parts which were common to both, as to leave no doubt that the corona is a solar and not a terrestrial phenomenon. A singular V-shaped gap, common to both the photographs, and seen also by several telescopists, attracted particular attention, and was regarded by the late Sir John Herschel as in itself demonstrative of the fact that the corona is a solar appendage. It was not, however, until the solar eclipse of December 1871 that the evidence on this point became so convincing as to satisfy even those who had most strenuously main tained the theory that the corona is merely a phenomenon of our own atmosphere. The spectroscopic and the photo graphic evidence were alike important. Janssen, with the spectroscope, succeeded in recognising, besides the bright lines already seen, others less bright, but manifestly belong ing to the corona. He also perceived a faint continuous spec trum, crossed by dark lines, and therefore presumably duo to reflected solar light, which, since our upper air near the sun s place, in total eclipse, is demonstrably not illuminated by sunlight, can have come only from matter in the true corona, such as meteoric flights, vapour clouds, or the like, capable of reflecting the light of the sun. The photographers met with equally decisive success. Lord Lindsay s photo grapher, Mr Davis, obtained a series of five pictures of the corona at successive stages of the totality, two of which were excellent and the remainder good. All these agreed perfectly in all respects, save only in the extent of the visible corona (depending, of course, on the conditions of illumination). It was demonstrated, therefore, that the features of the corona do not change during the progress of an eclipse at any given station ; hence the corona cannot be a phenomenon depending on the passage of light-rays athwart inequalities of the moon s limb. For whether we regard such rays as illuminating our own atmosphere or matter between the earth and the moon, the illumination would necessarily vary markedly, as the motions of the moon shifted the inequalities of her limb progressively athwart the light-rays. Again, Col. Tennant obtained a series of six photographs (five of which were very good), agreeing perfectly (always excepting differences of extent) with those obtained by Lord Lindsay s photo grapher. Now Col. Tennant s station was at Dodabetta, near the highest peak of the Neilgherries, more than 10,000 feet above the sea level, while Mr Davis was stationed at Baicull, close to the sea-shore, and hundreds of miles from Tennant s station. The agreement of two series taken under such diverse circumstances, proves to demonstration that the photographers were not dealing with the illumina-

