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.