THE TOTAL ECLIPSE IN AMERICA.
T is rather a singular coincidence that this year, as last year,
the session of the British Association should be in progress when news has arrived of the successful observation of a great 'Total Eclipse. Last year telegrams from Major Tennant and Lieu- tenant Herschel were placed in the hands of the President of the Mathematical Committee only a day or two after the occurrence of the great eclipse in India, and these telegrams announced the most important discovery which has been made in solar physics during the present century,—the revelation of the strange fact that the coloured prominences are vast tongues of flame. This year news of almost equal interest has been received, and again it -concerns the coloured prominences, teaching us to look on these enormous flames as far more complex in structure than they had been thought to be.
It is a particularly fortunate circumstance, we may notice in passing, that the great eclipse of August 7 has been witnessed ender favourable circumstances, for there will not be another total eclipse of the sun until the end of the year 1871, and then the duration of totality will be exceedingly short. After that, there will be no total eclipse till November, 1872, and this eclipse will be of no value at all, so far as observations to be made during totality are concerned, since the totality will not last more than a few seconds. Indeed, the eclipse will be of so singularly indefinite a character that astronomers cannot say for certain whether it will be total or annular. Probably it will begin as an annular eclipse, become total as the shadow sweeps rapidly across the earth's surface, and end as an annular eclipse again ; the point of the moon's true shadow just reaching the protuberant part of the earth's illuminated hemisphere. Ba this as it may, it is certain that there will be no possibility of observing the red prominences.
Thus, had it not been for the success with which, as we learn, the American astronomers have been able to observe the important eclipse of August 7, our solar physicists would have been forced to content themselves for several years with the results of the eclipse- expeditions of last year. This would have been the more unfor- tunate because that was the first eclipse during which astronomers had been able to avail themselves of the power of their new ally, the spectroscope. Many questions of extreme importance have arisen during the past year as to the significance of several observations made in India in 1868; and astronomers looked with interest to the eclipse which has just taken place to resolve their difficulties.
The eclipse of August 7 was in many respects inferior to the great eclipse of August 18, 1868. In 1868 the moon's disc over- lapped the sun (at the time of central eclipse) by an amount equal to nearly a thirtieth part of the sun's apparent diameter, and thus the totality lasted several minutes. This year the moon's overlap was about a fifth less, and the duration of totality was proportion- ately reduced. Still the eclipse was an important one, since it is comparatively seldom that even so near an approach is made to the exceptional magnitude of the great Indian eclipse. The recent eclipse, too, was characterized by certain very favourable features. The moon's shadow traversed a region of the earth's surface which was for the most part accessible to practical astronomers. The Americans had no occasion to under- take long and expensive journeys, since the eclipse visited them, so to speak, at their own doors. Some of the principal American observatories lay quite close to the line of central eclipse, others were not so far off but that large and powerful telescopes could readily be carried to some spot upon the central line. Then, again, the important processes of photography were not interfered with, as in India, by the tremendous heat of the climate. Major Tennant mentioned last year that he had had great difficulties to encounter owing to this circumstance. The American astronomers, among whom are some of the most successful professors of celestial photography, were subject to no such annoyances.
Thus, when we learn that good weather prevailed at every place to which observing parties were sent, that photographs were successfully taken, and that spectroscopic observations were made by several astronomers, we may assure ourselves that a rich fund of knowledge has been stored up for the eclipseless years that are approaching. Our solar physicists will not be without ample food for study and research. Nor, indeed, is it unlikely that as the eclipse of 1868 suggested new modes of inquiry, which have been successfully put in practice by our spectroscopists, so this eclipse may be similarly fruitful, and thus, besides the mere facts it has revealed, may set astronomers in the way of acquiring other facts.
We cannot hope that the photographic pictures of the eclipsed sun will be enlarged and laid before the scientific world before several months have passed. It will be remembered that last year Major Tennant judged, from the look of his photographs, that he had been unsuccessful, but he hoped "something might still be made of them." This was disheartening to those who had hoped for so much from the performance of the exquisite Newtonian reflector constructed for the expedition by Mr. Browning. But it turned out that the photographs were the best that had ever been taken during a solar eclipse, and under the skilful supervision of Mr. Warren de la Rue, the pictures of the eclipsed sun came out with singular clearness and beauty. As we write, we have before us a photograph enlarged from a portion of Major 'Pennant's, in which the largest of the prominences visible in 1868 is depicted without the intervention of any process of engraving. And it is impossible to look on the spiral convolutions of this great whorl of flame without feeling that there resides in these prominences a power of self-delineation by means of their chemical rays,* which must render their photographs infinitely more instructive than the best telescopic view we could obtain of them. We cannot but feel hopeful that the photographs which have recently been taken in
* Mr. de Is Rue was so impressed with the singular actinic power thus displayed by the prominences, as to be led to form the view that it might be possible to photo- graph the prominences' without the aid of a solar eclipse. The Astronomer Royal had tried to render the prominences visible by receiving the sun's image in a dark room upon a card sheet with a circular aperture cut out of it, so that the light from the sun's globe passed through "and was quenched in able& bag" The plan failed, because the light from our atmosphere still remained, and sufficed to blot out the prominences from view. But Mr. de la Rue thinks it not unlikely that the same plan might be applied successfully to obtain photographs of the prominences.
America will be even more valuable than Major Tennant's, since they were taken under circumstances so much more favourable.
One result of the American observations is very interesting. Last year there was some discrepancy between the various observers of the spectra of the prominences. as to the position of those bright lines which indicate the character of the prominence-flames. So important was this discrepancy, that many were disposed to consider that the observers had seen different lines, and in this way some eight or nine lines seemed to have appeared in the pro- minence-spectrum. One observer, indeed, M. Rayet announced that he had seen seven or eight lines. Now Jannsen (himself an observer of the eclipse) and Mr. J. Norman Lockyer, who indepen- dently discovered the fact that the bright lines of the prominences can be seen without an eclipse, have seen but three lines, two belonging to hydrogen, and the third near to but not coincident with the doable bright line of the metal sodium. But all doubt was not removed by this circumstance, since it was held to be not only possible, but highly probable that the fainter lines might escape a scrutiny made while the sun is blazing in full splendour : the peculiar sellective power of the spectroscope availing to render the brighter lines visible, without necessarily exhibiting the fainter ones. Now, all doubt on this point has been set at rest, since Profeeeor Winlock, who observed the eclipse at Stubbville, Ken- tucky, detected no less than eleven bright lines in the spectrum of a prominence. Doubtless, we shall soon learn what are the elements to which the new lines belong.
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