Page:All the Year Round - Series 2 - Volume 1.djvu/262

252[February 13, 1869] would become encrusted with a solid shell whose Ætnas and Vesuviuses, whose German and Pyreneean thermal waters betray the fires still smouldering within. We are, fortunately for us, behindhand with regard to the moon, but considerably in advance of the sun. Every heavenly body must undergo the same successive transformations in the eternal harmony of the universe.

Another mode of investigating the sun's constitution is the observation of total eclipses. At such times, the moon, by screening almost the whole of the solar disc, prevents the observer from being blinded by excess of light. The outline and profile of the sun, which could not be examined under ordinary circumstances, are then distinguished with perfect clearness. During the eclipse of 1860 the French astronomers who went to Spain distinctly saw the different peculiarities presented by the brilliant crown which surrounds the moon's black disc. In the middle of the luminous ring they perceived what looked like pink or red clouds.

These "protuberances," to retain the name first given to them, appeared under very varied guise. Before the eclipse of 1860, some observers were inclined to take them for the summits of exceedingly lofty mountains rising above the solar atmosphere. But after it, doubt was no longer possible. The semblances of mountain peaks, it is true, were seen; but pointed were less frequent than rounded and lengthy forms. Many protuberances, moreover, were absolutely detached from the sun, like the cumulus clouds which float in our atmosphere. Others were bent in a sidewise direction, recalling flames under the influence of a current of air. A few French savans considered the protuberances as optical illusions produced by the moon's interposition; but the majority regarded them as appendages composed of fiery vapours streaming into the upper regions of the solar atmosphere. Spectral analysis, still in its infancy, had not yet said its say.

The eclipse of 1868 was therefore impatiently awaited. M. Janssen, well known by his spectroscopic researches, directed his course to Masulipatam. Afterwards, advised by persons who knew the country, he determined to proceed to Guntoor. Scarcely a week after the event, he sent by telegraph the agreeable news, "Eclipse well observed; protuberances gaseous." M. Stéphan, who directed the Malacca expedition, announced, a month afterwards, that four protuberances had been seen on the corona, and their gaseous nature determined by their spectrum. Moreover, from the undue prolongation of brilliant lines in the spectrum, M. Rayet deduced the conclusion that a certain portion of the incandescent gaseous matter of which the protuberances consist, extends to a height in the solar atmosphere beyond the limits assigned to it by human eyes.

In a letter to the Minister of Public Instruction, dated 19th September, M. Janssen wrote: "Not having the time to send a detailed account to-day, I will have the honour of doing so by next post. Guntoor has doubtless been the most favoured station. The sky was clear, especially during the totality, and my powerful nine-feet-focus telescopes allowed me to pursue the analytical study of all the phenomena of the eclipse. Immediately after the totality, two magnificent protuberances appeared: one of them, more than three minutes in height, shone with a splendour difficult to conceive. The analysis of its light immediately informed me that it consisted of an immense incandescent gaseous column, principally composed of hydrogen.

"But the most important result of these observations is the discovery of a method which suggested itself during the actual occurrence of the eclipse, and which permits the study of the protuberances and the circumsolar regions at all times, without having to wait for the passage of an opaque body in front of the sun's disc.

"The very day after the eclipse, the new method was successfully put in practice, and I was able to witness the phenomena presented by a fresh eclipse, which lasted the whole day long. Yesterday's protuberances were profoundly modified. Of the grand protuberance there scarcely remained a trace, and the distribution of the gaseous matter was quite different to what it had been. From that day up to the 4th of September I have constantly studied the sun with this object in view. I have drawn up maps of the protuberances, which show with what rapidity (often in a few minutes) these immense masses of gas change their form and place. Lastly, during this period, which has been, as it were, an eclipse lasting seventeen days, I have collected a great number of facts, which presented themselves spontaneously, respecting the physical constitution of the sun."

M. Janssen's method, like Columbus's finding America, will appear very simple now it is known. On looking at the sun near its outer edge, but a little outside it, where the protuberances show themselves, it is clear that not only the radiations from the great luminary itself will meet the eye, but also, mingled with them, the light of the protuberances. Consequently, the spectrum of the mixed radiations ought to contain both the black stripes of the solar rays and the bright stripes of the protuberances. And, in fact, when M. Janssen had recognised the bright lines of the protuberances, he found them again below the black lines of the solar spectrum and in their prolongations. Consequently, there are two systems of lines: one lying above the other. By reading the lower scale, you have before you the characteristic features of the protuberances. But we do not yet know our spectral alphabet; we are only beginning to learn to read.

M. Janssen expressses his high appreciation of the reception given him by the English authorities in India. A steamer was placed at his disposal to take him from Madras to Masulipatam; another for the Godavery; and a young sub-collector was attached to his mission, to smooth any difficulties that might arise. He ought now to have reached Calcutta, and proposes to carry out in the Himalayas certain