Part 2

Already preparations are being made for its reception at Mount Hamilton. The scabrous summit of “Observatory Peak” has been smoothed down to a suitable equality of surface by the removal of 40,000 tons of hard trap rock. Preliminary operations for the erection of a dome, 75 feet in diameter, to serve as its shelter, are in progress. The water-supply has been provided for by the excavation of great cisterns. Buildings are rapidly being pushed forward from designs prepared by Professors Holden and Newcomb. Most of the subsidiary instruments have for some time been in their places, constituting in themselves an equipment of no mean order. With their aid Professor Holden and Mr. Burnham observed the transit of Mercury of November 7th, 1881, and Professor Todd obtained, December 6th, 1882, a series of 147 photographs (of which seventy-one were of the highest excellence) recording the progress of Venus across the face of the sun.

We are informed that a great hotel will eventually add the inducement of material well-being to those of astronomical interest and enchanting scenery. No more delightful summer resort can well be imagined. The road to the summit, of which the construction formed the subject of a species of treaty between Mr. Lick and the county of Santa Clara in 1875, traverses from San José a distance, as a bird flies, of less than thirteen miles, but doubled by the windings necessary in order to secure moderate gradients. So successfully has this been accomplished, that a horse drawing a light waggon can reach the observatory buildings without breaking his trot.[5] As the ascending track draws its coils closer and closer round the mountain, the view becomes at every turn more varied and more extensive. On one side the tumultuous coast ranges, stooping gradually to the shore, magnificently clad with forests of pine and red cedar; the island-studded bay of San Francisco, and, farther south, a shining glimpse of the Pacific; on the other, the thronging pinnacles of the Sierras—granite needles, lava-topped bastions—fire-rent, water-worn; right underneath, the rich valleys of Santa Clara and San Joaquim, and, 175 miles away to the north (when the sapphire of the sky is purest), the snowy cone of Mount Shasta.

Thus, there seems some reason to apprehend that Mount Hamilton, with its monster telescope, may become one of the show places of the New World. _Absit omen!_ Such a desecration would effectually mar one of the fairest prospects opened in our time before astronomy. The true votaries of Urania will then be driven to seek sanctuary in some less accessible and less inviting spot. Indeed, the present needs of science are by no means met by an elevation above the sea of four thousand and odd feet, even under the most translucent sky in the world. Already observing stations are recommended at four times that altitude, and the ambition of the new species of climbing astronomers seems unlikely to be satisfied until he can no longer find wherewith to fill his lungs (for even an astronomer must breathe), or whereon to plant his instruments.

This ambition is no casual caprice. It has grown out of the growing exigencies of celestial observation.

From the time that Lord Rosse’s great reflector was pointed to the sky in February, 1845, it began to be distinctly felt that instrumental power had outrun its opportunities. To the sounding of further depths of space it came to be understood that Atlantic mists and tremulous light formed an obstacle far more serious than any mere optical or mechanical difficulties. The late Mr. Lassell was the first to act on this new idea. Towards the close of 1852 he transported his beautiful 24-inch Newtonian to Malta, and, in 1859-60, constructed, for service there, one of four times its light capacity. Yet the chief results of several years’ continuous observation under rarely favorable conditions were, in his own words, “rather negative than positive.”[6] He dispelled the “ghosts” of four Uranian moons which had, by glimpses, haunted the usually unerring vision of the elder Herschel, and showed that our acquaintance with the satellite families of Saturn, Uranus, and Neptune must, for the present at any rate, be regarded as complete; but the discoveries by which his name is chiefly remembered were made in the murky air of Lancashire.

The celebrated expedition to the Peak of Teneriffe, carried out in the summer of 1856 by the present Astronomer Royal for Scotland, was an experiment made with the express object of ascertaining “how much astronomical observation can be benefited by eliminating the lower third or fourth part of the atmosphere.”[7] So striking were the advantages of which it seemed to hold out the promise, that we count with surprise the many years suffered to elapse before any adequate attempt was made to realize them.[8] Professor Piazzi Smyth made his principal station at Guajara, 8,903 feet above the sea, close to the rim of the ancient crater from which the actual peak rises to a further height of more than 3,000 feet. There he found that his equatorial (five feet in focal length) showed stars fainter by _four magnitudes_ than at Edinburgh. On the Calton Hill the companion of Alpha Lyræ (eleventh magnitude) could never, under any circumstances, be made out. At Guajara it was an easy object twenty-five degrees from the zenith; and stars of the fourteenth magnitude were discernible. Now, according to the usual estimate, a step downwards from one magnitude to another means a decrease of lustre in the proportion of two to five. A star of the fourteenth order of brightness sends us accordingly only 1/39th as much light as an average one of the tenth order. So that, in Professor Smyth’s judgment, the grasp of his instrument was virtually _multiplied thirty-nine times_ by getting rid of the lowest quarter of the atmosphere.[9] In other words (since light falls off in intensity as the square of the distance of its source increases), the range of vision was more than sextupled, further depths of space being penetrated to an extent probably to be measured by thousands of billions of miles!

This vast augmentation of telescopic compass was due as much to the increased tranquillity as to the increased transparency of the air. The stars hardly seemed to twinkle at all. Their rays, instead of being broken and scattered by continual changes of refractive power in the atmospheric layers through which their path lay, travelled with relatively little disturbance, and thus produced a far more vivid and concentrated impression upon the eye. Their images in the telescope, with a magnifying power of 150, showed no longer the “amorphous figures” seen at Edinburgh, but such minute, sharply-defined discs as gladden the eyes of an astronomer, and seem, in Professor Smyth’s phrase, to “provoke” (as the “cocked-hat” appearance surely baffles) “the application of a wire-micrometer” for the purposes of measurement.[10]

The lustre of the milky way and zodiacal light at this elevated station was indescribable, and Jupiter shone with extraordinary splendor. Nevertheless, not even the most fugitive glimpse of any of his satellites was to be had without optical aid.[11] This was possibly attributable to the prevalent “dust-haze”, which must have caused a diffusion of light in the neighborhood of the planet more than sufficient to blot from sight such faint objects. The same cause completely neutralized the darkening of the sky usually attendant upon ascents into the more ethereal regions, and surrounded the sun with an intense glare of reflected light. For reasons presently to be explained, this circumstance alone would render the Peak of Teneriffe wholly unfit to be the site of a modern observatory.

Within the last thirty years a remarkable change, long in preparation,[12] has conspicuously affected the methods and aims of astronomy; or, rather, beside the old astronomy—the astronomy of Laplace, of Bessel, of Airy, Adams, and Leverrier—has grown up a younger science, vigorous, inspiring, seductive, revolutionary, walking with hurried or halting footsteps along paths far removed from the staid courses of its predecessor. This new science concerns itself with the _nature_ of the heavenly bodies; the elder regarded exclusively their _movements_. The aim of the one is _description_, of the other _prediction_. This younger science inquires what sun, moon, stars, and nebulæ are made of, what stores of heat they possess, what changes are in progress within their substance, what vicissitudes they have undergone or are likely to undergo. The elder has attained its object when the theory of celestial motions shows no discrepancy with fact—when the calculus can be brought to agree perfectly with the telescope—when the coursers of the heavens come strictly up to time, and their observed places square to a hair’s-breadth with their predicted places.

It is evident that very different modes of investigation must be employed to further such different objects; in fact, the invention of novel modes of investigation has had a prime share in bringing about the change in question. Geometrical astronomy, or the astronomy of position, seeks above all to measure with exactness, and is thus more fundamentally interested in the accurate division and accurate centering of circles than in the development of optical appliances. Descriptive astronomy, on the other hand, seeks as the first condition of its existence to _see_ clearly and fully. It has no “method of least squares” for making the best of bad observations—no process for eliminating errors by their multiplication in opposite directions; it is wholly dependent for its data on the quantity and quality of the rays focussed by its telescopes, sifted by its spectroscopes, or printed in its photographic cameras. Therefore, the loss and disturbance suffered by those rays in traversing our atmosphere constitute an obstacle to progress far more serious now than when the exact determination of places was the primary and all-important task of an astronomical observer. This obstacle, which no ingenuity can avail to remove, may be reduced to less formidable dimensions. It may be diminished or partially evaded by anticipating the most detrimental part of the atmospheric transit—by carrying our instruments upwards into a finer air—by meeting the light upon the mountains.

The study of the sun’s composition, and of the nature of the stupendous processes by which his ample outflow of light and heat is kept up and diffused through surrounding space, has in our time separated, it might be said, into a science apart. Its pursuit is, at any rate, far too arduous to be conducted with less than a man’s whole energies; while the questions which it has addressed itself to answer are the fundamental problems of the new physical astronomy. There is, however, but one opinion as to the expediency of carrying on solar investigations at higher altitudes than have hitherto been more than temporarily available.

The spectroscope and the camera are now the chief engines of solar research. Mere telescopic observation, though always an indispensable adjunct, may be considered to have sunk into a secondary position. But the spectroscope and the camera, still more than the telescope, lie at the mercy of atmospheric vapors and undulations. The late Professor Henry Draper, of New York, an adept in the art of celestial photography, stated in 1877 that two years, during which he had photographed the moon at his observatory on the Hudson on every moonlit night, yielded _only three_ when the air was still enough to give good results, nor even then without some unsteadiness; and Bond, of Cambridge (U. S.) informed him that he had watched in vain, through no less than seventeen years for a faultless condition of our troublesome environing medium.[13] Tranquillity is the first requisite for a successful astronomical photograph. The hour generally chosen for employing the sun as his own limner is, for this reason, in the early morning, before the newly emerged beams have had time to set the air in commotion, and so blur the marvellous details of his surface-structure. By this means a better definition is secured but at the expense of transparency. Both are, at the sea-level, hardly ever combined. A certain amount of haziness is the price usually paid for exceptional stillness, so that it not unfrequently happens that astronomers see best in a fog, as on the night of November 15th, 1850, when the elder Bond discovered the “dusky ring” of Saturn, although at the time no star below the fourth magnitude could be made out with the naked eye. Now on well-chosen mountain stations, a union of these unhappy divorced conditions is at certain times to be met with, opportunities being thus afforded with tolerable certainty and no great rarity, which an astronomer on the plains might think himself fortunate in securing once or twice in a lifetime.

For spectroscopic observations at the edge of the sun, on the contrary, the _sine quâ non_ is translucency. During the great “Indian eclipse” of August 18th, 1868, the variously colored lines were, by the aid of prismatic analysis, first described, which reveal the chemical constitution of the flamelike “prominences,” forming an ever-varying, but rarely absent, feature of the solar surroundings. Immediately afterwards, M. Janssen, at Guntoor, and Mr. Norman Lockyer, in England, independently realised a method of bringing them into view without the co-operation of the eclipsing moon. This was done by _fanning out_ with a powerfully dispersive spectroscope the diffused radiance near the sun, until it became sufficiently attenuated to permit the delicate flame-lines to appear upon its rainbow-tinted background. This mischievous radiance—which it is the chief merit of a solar eclipse to abolish during some brief moments—is due to the action of the atmosphere, and chiefly of the watery vapors contained in it. Were our earth stripped of its “cloud of all-sustaining air,” and presented, like its satellite, bare to space, the sky would appear perfectly black up to the very rim of the sun’s disc—a state of things of all others (vital necessities apart) the most desirable to spectroscopists. The best approach to its attainment is made by mounting a few thousand feet above the earth’s surface. In the drier and purer air of the mountains, “glare” notably diminishes, and the tell-tale prominence-lines are thus more easily disengaged from the effacing lustre in which they hang, as it were suspended.

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 sun’s 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 “helium”[14]) 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 Young’s 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 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 _blue_[18]—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.