The Peak of Teneriffe, as we have seen, offers a marked exception to this rule, the impalpable dust diffused through the air giving, even at its summit, precisely the same kind of detailed reflection as aqueous vapors at lower levels. It is accordingly destitute of one of the chief qualifications for serving as a point of vantage to observers of the new type.
The changes in the spectra of chromosphere and prominences (for they are parts of a single appendage) present a subject of unsurpassed interest to the student of solar physics. There, if anywhere, will be found the key to the secret to the suns internal economy; in them, if at all, the real condition of matter in the unimaginable abysses of heat covered up by the relatively cool photosphere, whose radiations could, nevertheless, vivify 2,300,000,000 globes like ours, will reveal itself; revealing, at the same time, something more than we know of the nature of the so-called elementary substances, hitherto tortured, with little result, in terrestrial laboratories.
The chromosphere and prominences might be figuratively described as an ocean and clouds of tranquil incandescence, agitated and intermingled with waterspouts, tornadoes, and geysers of raging fire. Certain kinds of light are at all times emitted by them, showing that certain kinds of matter (as, for instance, hydrogen and helium14) form invariable constituents of their substance. Of these unfailing lines Professor Young counts eleven.15 But a vastly greater number appear only occasionally, and, it would seem, capriciously, under the stress of eruptive action from the interior. And precisely this it is which lends them such significance; for of what is going on there, they have doubtless much to tell, were their message only legible by us. It has not as yet proved so; but the characters in which it is written are being earnestly scrutinised and compared, with a view to their eventual decipherment. The prodigious advantages afforded by high altitudes for this kind of work were illustrated by the brilliant results of Professor Youngs observations in the Rocky Mountains during the summer of 1872. By the diligent labor of several years he had, at that time, constructed a list of one hundred and three distinct lines occasionally visible in the spectrum of the chromosphere. In seventy-two days, at Sherman (8,335 feet above the sea), it was extended to 273. Yet the weather was exceptionally cloudy, and the spot (a station on the Union Pacific Railway, in the Territory of Wyoming) not perhaps the best that might have been chosen for an astronomical reconnaissance.16
A totally different kind of solar research is that in aid of which the Mount Whitney expedition was organized in 1881. Professor S. P. Langley, director of the Alleghany observatory in Pennsylvania, has long been engaged in the detailed study of the radiations emitted by the sun; inventing, for the purpose of its prosecution, the bolometer,17 an instrument twenty times as sensitive to changes of temperature as the thermopile. But the solar spectrum as it is exhibited at the surface of the earth, is a very different thing from the solar spectrum as it would appear could it be formed of sunbeams, so to speak, fresh from space, unmodified by atmospheric action. For not only does our air deprive each ray of a considerable share of its energy (the total loss may be taken at 20 to 25 per cent. when the sky is clear and the sun in the zenith), but it deals unequally with them, robbing some more than others, and thus materially altering their relative importance. Now it was Professor Langleys object to reconstruct the original state of things, and he saw that this could be done most effectually by means of simultaneous observations at the summit and base of a high mountain. For the effect upon each separate ray of transmission through a known proportion of the atmosphere being (with the aid of the bolometer) once ascertained, a very simple calculation would suffice to eliminate the remaining effects, and thus virtually secure an extra-atmospheric post of observation.
The honor of rendering this important service to science was adjudged to the highest summit in the United States. The Sierra Nevada culminates in a granite pile, rising, somewhat in the form of a gigantic helmet, fronting eastwards, to a height of 14,887 feet. Mount Whitney is thus entitled to rank as the Mount Blanc of its own continent. In order to reach it, a railway journey of 3,400 miles, from Pittsburg to San Francisco, and from San Francisco to Caliente, was a brief and easy preliminary. The real difficulty began with a march of 120 miles across the arid and glaring Inyo desert, the thermometer standing at 110° in the shade (if shade there were to be found.) Towards the end of July 1881, the party reached the settlement of Lone Pine at the foot of the Sierras, where a camp for low-level observations was pitched (at a height, it is true, of close upon 4,000 feet), and the needful instruments were unpacked and adjusted. Close overhead, as it appeared, but in reality sixteen miles distant, towered the gaunt, and rifted, and seemingly inaccessible pinnacle which was the ultimate goal of their long journey. The illusion of nearness produced by the extraordinary transparency of the air was dispelled when, on examination with a telescope, what had worn the aspect of patches of moss, proved to be extensive forests.
The ascent of such a mountain with a train of mules bearing a delicate and precious freight of scientific apparatus, was a perhaps unexampled enterprise. It was, however, accomplished without the occurrence, though at the frequent and imminent risk, of disaster, after a toilsome climb of seven or eight days through an unexplored and, to less resolute adventurers, impassable waste of rocks, gullies, and precipices. Finally a site was chosen for the upper station on a swampy ledge, 13,000 feet above the sea; and there, notwithstanding extreme discomforts from bitter cold, fierce sunshine, high winds, and, worst of all, mountain sickness, with its intolerable attendant debility, observations were determinedly carried on, in combination with those at Lone Pine, and others daily made on the highest crest of the mountain, until September 11. They were well worth the cost. By their means a real extension was given to knowledge, and a satisfactory definiteness introduced into subjects previously involved in very wide uncertainty.
Contrary to the received opinion, it now appeared that the weight of atmospheric absorption falls upon the upper or blue end of the spectrum, and that the obstacles to the transmission of light waves through the air diminish as their length increases, and their refrangibility consequently diminishes. A yellow tinge is thus imparted to the solar rays by the imperfectly transparent medium through which we see them. And, since the sun possesses an atmosphere of its own, exercising an unequal or selective absorption of the same character, it follows that, if both these dusky-red veils were withdrawn, the true color of the photosphere would show as a very distinct blue18 not merely bluish, but a real azure just tinted with green, like the hue of a mountain lake fed with a glacier stream. Moreover, the further consequence ensues, that the sun is hotter than had been supposed. For the higher the temperature of a glowing body, the more copiously it emits rays from the violet end of the spectrum. The blueness of its light is, in fact, a measure of the intensity of its incandescence. Professor Langley has not yet ventured (that we are aware of) on an estimate of what is called the effective temperature of the sun the temperature, that is, which it would be necessary to attribute to the surface of the radiating power of lamp-black to enable it to send us just the quantity of heat that the sun does actually send us. Indeed, the present state of knowledge still leaves an important hiatus only to be filled by more or less probable guessing in the reasoning by which inferences on this subject must be formed; while the startling discrepancies between the figures adopted by different, and equally respectable, authorities sufficiently show that none are entitled to any confidence. The amount of heat received in a given interval of time by the earth from the sun is, however, another matter, and one falling well within the scope of observation. This Professor Langleys experiments (when completely worked out) will, by their unequalled precision, enable him to determine with some approach to finality. Pouillet valued the solar constant at 1·7 calories; in other works, had calculated that, our atmosphere being supposed removed, vertical sunbeams would have power to heat in each minute of time, by one degree centigrade, 1·7 gramme of water for each square centimetre of the earths surface. This estimate was raised by Crova to 2·3, and by Violle in 1877 to 2·5;19 Professor Langleys new data bring it up (approximately as yet) to three calories per square centimetre per minute. This result alone would, by its supreme importance to meteorology, amply repay the labors of the Mount Whitney expedition.