Eclectic Magazine of Foreign Literature, Science, and Art

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 Langley’s 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 Langley’s 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 earth’s surface. This estimate was raised by Crova to 2·3, and by Violle in 1877 to 2·5;19 Professor Langley’s 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.

Still more unexpected is the answer supplied to the question: Were the earth wholly denuded of its aëriform covering, what would be the temperature of its surface? We are informed in reply that it would be at the outside 50 degrees of Fahrenheit below zero, or 82 of frost. So that mercury would remain solid even when exposed to the rays – undiminished by atmospheric absorption – of a tropical sun at noon.20 The paradoxical aspect of this conclusion – a perfectly legitimate and reliable one – disappears when it is remembered that under the imagined circumstances there would be absolutely nothing to hinder radiation into the frigid depths of space, and that the solar rays would, consequently, find abundant employment in maintaining a difference of 189 degrees21 between the temperature of the mercury and that of its environment. What we may with perfect accuracy call the clothing function of our atmosphere is thus vividly brought home to us; for it protects the teeming surface of our planet against the cold of space exactly in the same way as, and much more effectually than, a lady’s sealskin mantle keeps her warm in frosty weather. That is to say, it impedes radiation. Or, again, to borrow another comparison, the gaseous envelope we breathe in (and chiefly the watery part of it) may be literally described as a “trap for sunbeams.” It permits their entrance (exacting, it is true, a heavy toll), but almost totally bars their exit. It is now easy to understand why it is that on the airless moon no vapors rise to soften the hard shadow-outlines of craters or ridges throughout the fierce blaze of the long lunar day. In immediate contact with space (if we may be allowed the expression) water, should such a substance exist on our enigmatical satellite, must remain frozen, though exposed for endless æons of time to direct sunshine.

Amongst the most noteworthy results of Professor Langley’s observations in the Sierra Nevada was the enormous extension given by them to the solar spectrum in the invisible region below the red. The first to make any detailed acquaintance with their obscure beams was Captain Abney, whose success in obtaining a substance – the so-called “blue bromide” of silver – sensitive to their chemical action, enabled him to derive photographic impressions from rays possessing the relatively great wave-length of 1,200 millionths of a millimetre. This, be it noted, approaches very closely to the theoretical limit set by Cauchy to that end of the spectrum. The information was accordingly received with no small surprise that the bolometer showed entirely unmistakable heating effects from vibrations of the wave-length 2,800. The “dark continent” of the solar spectrum was thus demonstrated to cover an expanse nearly eight times that of the bright or visible part.22 And in this newly discovered region lie three-fifths of the entire energy received from the sun – three-fifths of the vital force imparted to our planet for keeping its atmosphere and ocean in circulation, its streams rippling and running, its forests growing, its grain ripening. Throughout this wide range of vibrations the modifying power of our atmosphere is little felt. It is, indeed, interrupted by great gaps produced by absorption somewhere; but since they show no signs of diminution at high altitudes, they are obviously due to an extra-terrestrial cause. Here a tempting field of inquiry lies open to scientific explorers.

On one other point, earlier ideas have had to give way to better grounded ones derived from this fruitful series of investigation. Professor Langley has effected a redistribution of energy in the solar spectrum. The maximum of heat was placed by former inquirers in the obscure tract of the infra-red; he has promoted it to a position in the orange approximately coincident with the point of greatest luminous intensity. The triple curve, denoting by its three distinct summits the supposed places in the spectrum of the several maxima of heat, light, and “actinism,” must now finally disappear from our text-books, and with it the last vestige of belief in a corresponding threefold distinction of qualities in the solar radiations. From one end to the other of the whole gamut of them, there is but one kind of difference – that of wave-length, or frequency in vibration; and there is but one curve by which the rays of the spectrum can properly be represented – that of energy, or the power of doing work on material particles. What the effect of that work may be, depends upon the special properties of such material particles, not upon any recondite faculty in the radiations.

These brilliant results of a month’s bivouac encourage the most sanguine anticipation as to the harvest of new truths to be gathered by a steady and well-organized pursuance of the same plan of operations. It must, however, be remembered that the scheme completed on Mount Whitney had been carefully designed, and in its preliminary parts executed at Alleghany. The interrogatory was already prepared; it only remained to register replies, and deduce conclusions. Nature seldom volunteers information: usually it has to be extracted from her by skilful cross-examination. The main secret of finding her a good witness consists in having a clear idea beforehand what it is one wants to find out. No opportunities of seeing will avail those who know not what to look for. Thus, not the crowd of casual observers, but the few who consistently and systematically think, will profit by the effort now being made to rid the astronomer of a small fraction of his terrestrial impediments. It is, nevertheless, admitted on all hands that no step can at present be taken at all comparable in its abundant promise of increased astronomical knowledge to that of providing suitably elevated sites for the exquisite instruments constructed by modern opticians.

Europe has not remained behind America in this significant movement. An observatory on Mount Etna, at once astronomical, meteorological, and seismological, was nominally completed in the summer of 1882, and will doubtless before long begin to give proof of efficiency in its threefold capacity. The situation is magnificent. Etna has long been famous for the amplitude of the horizon commanded from it and the serenity of its encompassing skies favors celestial no less than terrestrial vision. Professor Langley, who made a stay of twenty days upon the mountain in 1879-80, with the object of reducing to strict measurement the advantages promised by it, came to the conclusion that the “seeing” there is better than that in England (judging from data given by Mr. Webb) in the proportion of three to two – that is to say, a telescope of two inches aperture on Etna would show as much as one of three in England. Yet the circumstances attending his visit were of the least favorable kind. He was unable to find a suitable shelter higher up than Casa del Bosco, an isolated hut within the forest belt (as its name imports), at considerably less than half the elevation of the new observatory; the imperfect mounting of his telescope rendered observation all but impossible within a range of 30 degrees from the zenith, thus excluding the most serene portion of the sky; moreover, his arrival was delayed until December 25th, when the weather was thoroughly broken, high winds were incessantly troublesome, and only five nights out of seventeen proved astronomically available. It is, accordingly, reassuring to learn that while, with the naked eye, at ordinary levels, he could see but six Pleiades, with glimpses of a seventh and eighth, on Etna he steadily distinguished nine even before the moon had set;23 and that the telescopic definition though not uniformly good, was on December 31st such as he had never before seen on the sun, “least of all with a blue sky;”24 the “rice-grain” structure came out beautifully under a power of 212; and for the spectroscopic examination of prominences, the fainter orange light of their helium constituent served almost equally well with the strong radiance of the crimson ray of hydrogen (C) – a test of transparency which those accustomed to such studies will appreciate.

The Etnean observatory is the most elevated building in Europe. It stands at a height above the sea of 9,655 ft., or 1,483 ft. above the monastery of the Great St. Bernard. Its walls enclose the well-known “Casa Inglese,” where travellers were accustomed to spend the night before undertaking the final ascent of the cone, and occupy a site believed secure from the incursions of lava. Astronomical work is designed to be carried on there from June to September. For the Merz equatorial, 35 centimetres (13·8 inches) in aperture, which is facile primus of its instrumental equipment, a duplicate mounting has been provided at Catania, whither it will be removed during the winter months. The primary aim of the establishment is the study of the sun. Its great desirability for this purpose formed the theme of the representations from Signor Tacchini (then director of the observatory of Palermo, now of that of the Collegio Romano), which determined the Italian government upon trying the experiment. But we hear with pleasure that stellar spectroscopy will also come in for a large share of attention. The privilege of observation from the summit of Etna will not be enjoyed exclusively by the local staff. The Municipality of Catania who have borne their share in the expense of the undertaking, generously propose to give it somewhat of an international character, by providing accommodation for any foreign astronomers who may desire to enjoy a respite from the hampering conditions of low-level star-gazing. We cannot doubt that such exceptional facilities will be turned to the best account.

Eight years have now passed since General de Nansonty, aided by the engineer Vaussenat, established himself for the winter on the top of the Pic du Midi. Zeal for the promotion of weather-knowledge was the impelling motive of this adventure, which included, amongst other rude incidents, a snow-siege of little less than six months. It resulted in crowning one of the highest crests of the Pyrenees with a permanent meteorological observatory opened for work in 1881. It is now designed to render the station available for astronomical purposes as well.

The important tasks in progress at the Paris observatory have of late been singularly impeded by bad weather. During the latter half of 1882 scarcely four or five good nights per month were secured, and in December these were reduced to two.25 Moreover, M. Thollon, who, according to his custom, arrived from Nice in June for the summer’s work, returned thither in September without having found the opportunity of making one single spectroscopic observation. Yet within easy and immediate reach was a post, already in scientific occupation, where as General de Nansonty reported, ordinary print was legible by the radiance of the milky way and zodiacal light alone, and fifteen or sixteen Pleiades could be counted with the naked eye. At length Admiral Mouchez, the energetic director of the Paris observatory, convinced of the urgent need of an adjunct establishment under less sulky skies, issued to MM. Thollon and Trépied a commission of inquiry into telescopic possibilities on the Pic du Midi. Their stay lasted from August 17th, to September 22d, 1883, and their experiences were summarised in a note (preliminary to a detailed report) published in the “Comptes Rendus” for October 16th, glowing with a certain technical enthusiasm difficult to be conveyed to those who have never strained their eyes to catch the vanishing gleam of a “chromospheric line” through a “milky” sky, and dim and tremulous air. The definition, they declared, was simply marvellous. Not even in Upper Egypt had they seen anything like it. The sun stood out, clean-cut and vivid, on a dark blue sky, and so slight were the traces of diffusion, that, for observations at his edge the conditions approached those of a total eclipse. These advantages are forcibly illustrated by the statement that, instead of eight lines ordinarily visible in the entire spectrum of the chromosphere, more than thirty revealed themselves in the orange and green parts of it alone (Dto. F)! A fact still more remarkable is that prominences were actually seen, and their forms distinguished, though foreshortened and faint, on the very disc of the sun itself – and this not merely by such glimmering views as had previously, at especially favorable moments, tantalised the sight of Young and Tacchini, but steadily and with certainty. We are further told that, on the mornings of September 19th and 20th, Venus was discerned, without aid from glasses, within two degrees of the sun.

These extraordinary facilities of vision disappeared, indeed, as, with the advance of day, the slopes of the mountain became heated and set the thin air quivering; but were reproduced at night in the tranquil splendor of moon and stars.

The expediency of using such opportunities was obvious; and it has accordingly been determined to erect a good equatorial in this tempting situation, elevated 9,375 feet above the troubles of the nether air. The expense incurred will be trifling; no special staff will be needed; the post will simply constitute a dependency of the Paris establishment, where astronomers thrown out of work by the malice of the elements may find a refuge from enforced idleness, as well as, possibly, unlooked-for openings to distinction.

We must now ask our readers to accompany us in one more brief flight across the Atlantic. After a successful observation of the late transit of Venus at Jamaica, Dr. Copeland, the chief astronomer of Lord Crawford’s observatory at Dun Echt, took advantage of the railway which now crosses the Western Andes at an elevation of 14,666 feet, to make a high-level tour of exploration in the interests of science. Some of the results communicated by him to the British Association at Southport last year, and published, with more detail, in the astronomical journal “Copernicus,” are extremely suggestive. At La Paz, in Bolivia, 12,050 feet above the sea, a naked-eye sketch of the immemorially familiar star-groups in Taurus, made in full moonlight, showed seventeen Hyades (two more than are given in Argelander’s “Uranometria Nova”) and ten Pleiades. Now ordinary eyes under ordinary circumstances see six, or at most seven, stars in the latter cluster. Hipparchus censured Aratus – who took his facts on trust from Eudoxus – for stating the lesser number, on the ground that, in serene weather, and in the absence of the moon, a seventh was discernible.26 On the other hand, several of the ancients reckoned nine Pleiades, and we are assured that Moestlin, the worthy preceptor of Kepler, was able to detect, under the little propitious skies of Wurtemberg, no less than fourteen.27 An instance of keensightedness but slightly inferior is afforded by a contemporary American observer: Mr. Henry Carvil Lewis, of Germantown, Pennsylvania, frequently perceives twelve of this interesting sidereal community.28 The number of Pleiades counted is, then, without some acquaintance with the observer’s ordinary range of sight, a quite indeterminate criterion of atmospheric clearness; although we readily admit that Dr. Copeland’s detection of ten in the very front of a full moon gives an exalted idea of visual possibilities at La Paz.

During the season of tempestades– from the middle of December to the end of March – the weather in the Andes is simply abominable. Mr. Whymper describes everything as “bottled up in mist” after one brief bright hour in the early morning, and complains, writing from Quito, March 18th, 1880,29 that his exertions had been left unrewarded by a single view from any one of the giant peaks scaled by him. Dr. Copeland adds a lamentable account – doubly lamentable to an astronomer in search of improved definition – of thunderstorms, torrential rains merging into snow or hail, overcast nocturnal skies, and “visible exhalations” from the drenched pampas. At Puno, however, towards the end of March, he succeeded in making some valuable observations, notwithstanding the detention – as contraband of war, apparently – of a large part of his apparatus. Puno is the terminal station on the Andes railway, and is situated at an altitude of 12,540 feet.

Here he not only discovered, with a 6-inch achromatic, mounted as need prescribed, several very close stellar pairs, of which Sir John Herschel’s 18 inch speculum had given him no intelligence; but in a few nights’ “sweeping” with a very small Vogel’s spectroscope, he just doubled the known number of a restricted, but particularly interesting, class of stars – if stars indeed they be. For while in the telescope they exhibit the ordinary stellar appearance of lucid points, they disclose, under the compulsion of prismatic analysis, the characteristic marks of a gaseous constitution; that is to say, the principal part of their light is concentrated in a few bright lines. The only valid distinction at present recognisable by us between stars and “nebulæ” is thus, if not wholly abolished, at least rendered of a purely conventional character. We may agree to limit the term “nebulæ” to bodies of a certain chemical constitution; but we cannot limit the doings of Nature, or insist on the maintenance of an arbitrary line of demarcation. From the keen rays of Vega to the undefined lustre of the curdling wisps of cosmical fog clinging round the sword-hilt of Orion, the distance is indeed enormous. But so it is from a horse to an oak tree; yet when we descend to volvoxes and diatoms, it is impossible to pronounce off-hand in which of the two great provinces of the kingdom of life we are treading. It would now seem that the celestial spaces have also their volvoxes and diatoms – “limiting instances,” as Bacon termed such – bodies that share the characters, and hang on the borders of two orders of creation.

In 1867, MM. Wolf and Rayet, of Paris, discovered that three yellow stars in the Swan, of about the eighth magnitude possessed the notable peculiarity of a bright-line spectrum. It was found by Raspighi and Le Sueur to be shared by one of the second order of brightness in Argo (γ Argûs), and Professor Pickering, of Harvard, reinforced the species, in 1880-81, with two further specimens. Dr. Copeland’s necessarily discursive operations on the shores of Lake Titicaca raised the number of its members at once from six to eleven or twelve. Now the smaller “planetary” nebulæ – so named by Sir William Herschel from the planet-like discs presented by the first-known and most conspicuous amongst them – are likewise only distinguished from minute stars by their spectra. Their light, when analysed with a prism, instead of running out into a parti-colored line, gathers itself into one or more bright dots. The position on the prismatic scale of those dots, alone serves to mark them off from the Wolf-Rayet family of stars. Hence the obvious inference that both nebulæ and stars (of this type) are bodies similar in character, but dissimilar in constitution – that they agree in the general plan of their structure, but differ in the particular quality of the substances glowing in the vast, incandescent atmospheres which display their characteristic bright lines in our almost infinitely remote spectroscopes. Indeed, the fundamental identity of the two species are virtually demonstrated, by the “migrations” (to use a Baconian phrase) of the “new star” of 1876, which, as its original conflagration died out, passed through the stages, successively, of a Wolf-Rayet or nebular star (if we may be permitted to coin the term), and of a planetary nebula. So that not all the stars in space are suns – at least, not in the sense given to the word by our domestic experience in the solar system.