Popular Science Monthly/Volume 26/January 1885/Mountain Observatories
ON October 1, 1876, one of the millionaires of the New World died at San Francisco. Although owning a no more euphonious name than James Lick, he had contrived to secure a future for it. He had founded and endowed the first great astronomical establishment planted on the heights, between the stars and the sea. How he came by his love of science we have no means of knowing. Born obscurely at Fredericksburg, in Pennsylvania, August 25, 1796, he amassed some thirty thousand dollars by commerce in South America, and in 1847 transferred them and himself to a village which had just exchanged its name of Yerba Buena for that of San Francisco, situate on a long, sandy strip of land between the Pacific and a great bay. In the hillocks and gullies of that wind-blown barrier he invested his dollars, and never did virgin-soil yield a richer harvest. The gold-fever broke out in the spring of 1848. The unremembered cluster of wooden houses, with no trouble or tumult of population in their midst, nestling round a tranquil creek under a climate which, but for a touch of sea-fog, might rival that of the Garden of the Hesperides, became all at once a center of attraction to the outcast and adventurous from every part of the world. Wealth poured in; trade sprang up; a population of six hundred increased to a quarter of a million; hotels, villas, public edifices, places of business spread, mile after mile, along the bay; building-ground rose to a fabulous price, and James Lick found himself one of the richest men in the United States.
Thus he got his money; we have now to see how he spent it. Already the munificent benefactor of the learned institutions of California, he in 1874 formally set aside a sum of two million dollars for various public purposes, philanthropic, patriotic, and scientific. Of these two millions, seven hundred thousand were appropriated to the erection of a telescope "superior to and more powerful than any ever yet made." But this, he felt instinctively, was not enough. Even in astronomy, although most likely unable to distinguish the pole-star from the dog-star, this "pioneer citizen" could read the signs of the times. It was no longer instruments that were wanted; it was the opportunity of employing them. Telescopes of vast power and exquisite perfection had ceased to be a rarity; but their use seemed all but hopelessly impeded by the very conditions of existence on the surface of the earth.
The air we breathe is in truth the worst enemy of the astronomer's observations. It is their enemy in two ways. Part of the light which brings its wonderful, evanescent messages across inconceivable depths of space, it stops; and, what it does not stop, it shatters. And this even when it is most transparent and seemingly still; when mist-veils are withdrawn, and no clouds curtain the sky. Moreover, the evil grows with the power of the instrument. Atmospheric troubles are magnified neither more nor less than the objects viewed across them. Thus, Lord Rosse's giant reflector possesses—nominally—a magnifying power of 6,000; that is to say, it can reduce the apparent distances of the heavenly bodies to 1 their actual amount. The moon, for example, which is in reality separated from the earth's surface by an interval of about two hundred and thirty-four thousand miles, is shown as if removed only thirty-nine miles. Unfortunately, however, in theory only. Professor Newcomb compares the sight obtained under such circumstances to a glimpse through several yards of running water, and doubts whether our satellite has ever been seen to such advantage as it would be if brought—substantially, not merely optically—within five hundred miles of the unassisted eye.
Must, then, all the glowing triumphs of the optician's skill be counteracted by this plague of moving air? Can nothing be done to get rid of, or render it less obnoxious? Or is this an ultimate barrier, set up by Nature herself, to stop the way of astronomical progress? Much depends upon the answer—more than can, in a few words, be easily made to appear; but there is fortunately reason to believe that it will, on the whole, prove favorable to human ingenuity, and the rapid advance of human knowledge on the noblest subject with which it is or ever can be conversant.
The one obvious way of meeting atmospheric impediments is to leave part of the impeding atmosphere behind; and this the rugged shell of our planet offers ample means of doing. Whether the advantages derived from increased altitudes will outweigh the practical difficulties attending such a system of observation when conducted on a great scale, has yet to be decided. The experiment, however, is now about to be tried simultaneously in several parts of the globe.
By far the most considerable of these experiments is that of the "Lick Observatory." Its founder was from the first determined that the powers of his great telescope should, as little as possible, be fettered by the hostility of the elements. The choice of its local habitation was, accordingly, a matter of grave deliberation to him for some time previous to his death. Although close upon his eightieth year, he himself spent a night upon the summit of Mount St. Helena, with a view to testing its astronomical capabilities, and a site already secured in the Sierra Nevada was abandoned on the ground of climatic disqualifications. Finally, one of the culminating peaks of the Coast Range, elevated 4,440 feet above the sea, was fixed upon. Situated about fifty miles southeast of San Francisco, Mount Hamilton lies far enough inland to escape the sea-fog, which only on the rarest occasions drifts upward to its triple crest. All through the summer the sky above it is limpid and cloudless; and, though winter storms are frequent, their raging is not without highly available lucid intervals. As to the essential point—the quality of telescopic vision—the testimony of Mr. S. W. Burnham is in the highest degree encouraging. This well-known observer spent two months on the mountain in the autumn of 1879, and concluded, as the result of his experience during that time—with the full concurrence of Professor Newcomb—that "it is the finest observing location in the United States." Out of sixty nights he found forty-two as nearly perfect as nights can well be, seven of medium quality, and only eleven cloudy or foggy; his stay, nevertheless, embraced the first half of October, by no means considered to belong to the choice part of the season. Nor was his trip barren of discovery, A list of forty-two new double stars gave an earnest of what may be expected from systematic work in such an unrivaled situation. Most of these are objects which never rise high enough in the sky to be examined with any profit through the grosser atmosphere of the plains east of the Rocky Mountains; some are well-known stars, not before seen clearly enough for the discernment of their composite character; yet Mr. Burnham used the lesser of two telescopes—a 6-inch and an 18-inch achromatic—with which he had been accustomed to observe at Chicago.
The largest refracting telescope as yet actually completed has a light-gathering surface twenty-seven inches in diameter. This is the great Vienna equatorial, admirably turned out by Mr. Grubb, of Dublin, in 1880, but still awaiting the commencement of its exploring career. It will, however, soon be surpassed by the Pulkowa telescope, ordered more than four years ago on behalf of the Russian Government from Alvan Clark & Sons, of Cambridgeport, Massachusetts. Still further will it be surpassed by the coming "Lick Refractor." It is safe to predict that the optical championship of the world is, at least for the next few years, secured to this gigantic instrument, the completion of which may be looked for in the immediate future. It will have a clear aperture of three feet. A disk of flint-glass for the object-lens, 38·18 inches across, and one hundred and seventy kilogrammes in weight, was cast at the establishment of M. Feil, in Paris, early in 1882. Four days were spent and eight tons of coal consumed in the casting of this vast mass of flawless crystal; it took a calendar month to cool, and cost two thousand pounds sterling. It may be regarded as the highest triumph so far achieved in the art of optical glass-making.
A refracting telescope three feet in aperture collects rather more light than a speculum of four feet. In this quality, then, the Lick instrument will have—besides the Rosse leviathan, which, for many-reasons, may be considered to be out of the running—but one rival. And over this rival—the 48-inch reflector of the Melbourne Observatory—it will have all the advantages of agility and robustness (so to speak) which its system of construction affords; while the exquisite definition for which Alvan Clark is famous will, presumably, not be absent.
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, seventy-five 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 being rapidly 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 7, 1881, and Professor Todd obtained, December 6, 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 Jose 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 wagon can reach the observatory buildings without breaking his trot. 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 Joaquin, 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 astronomer 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. Toward the close of 1853 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." 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." 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. 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 downward from one magnitude to another means a decrease of luster in the proportion of two to five. A star of the fourteenth order of brightness sends us accordingly only one thirty-ninth 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. 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, traveled 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 one hundred and fifty, showed no longer the "amorphous figures" seen at Edinburgh, but such minute, sharply-defined disks 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 purposes of measurement.
The luster 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. 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, 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. The 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 focused 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 upward 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 greater 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. 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 marvelous 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 15, 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 unhappily 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 qua non is translucency. During the great "Indian eclipse" of August 18, 1868, the variously-colored lines were, by the aid of prismatic analysis, first descried, which reveal the chemical constitution of the flame-like "prominences," forming an ever-varying, but rarely absent, feature of the solar surroundings. Immediately afterward, M. Janssen, at Guntoor, and Mr. Norman Lockyer, in England, independently realized 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 disk—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 luster 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 of 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 now 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 water-spouts, 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") form invariable constituents of their substance. Of these unfailing lines Professor Young counts eleven. 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 tbey 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 scrutinized and compared, with a view to their eventual decipherment. The prodigious advantages afforded by great 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 two hundred and seventy-three. 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."
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," 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 twenty to twenty-five 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 eastward, to a height of 14,887 feet. Mount Whitney is thus entitled to rank as the Mont 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). Toward the end of July, 1881, the party reached the settlement of Lone Pine at the foot of the Sierras, where a camp of 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 11th. 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 those dusky-red veils were withdrawn, the true color of the photosphere would show as a very
distinct blue—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 a 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 unequaled precision, enable him to determine with some approach to finality. Pouillet valued the "solar constant" at 1·7 "calories"; in other words, he 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; 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° 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. 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° 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 seal-skin mantle keeps her warm in frosty weather. That is to say, it impedes radiation. Or, again, to borrow another comparison, the gaseous envelop 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 roons of time to direct sunshine.
Among the most noteworthy results of Professor Langley's observations in the Sierra Nevada was the enormous extension give by them to the solar spectrum in the invisible region below the red. The first to make any detailed acquaintance with these 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. 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 great 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 investigations. 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 textbooks, 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 anticipations 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 skillful 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 efforts 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.—Edinburgh Review.
- "Popular Astronomy," p. 145.
- "The Observatory," No. 43, p. 613.
- "Nature," vol. xxv, p. 537.
- Silvered glass is considerably more reflective than speculum-metal, and Mr. Common's 36-inch mirror can be but slightly inferior in luminous capacity to the Lick objective. It is, however, devoted almost exclusively to celestial photography, in which it has done splendid service. The Paris 4-foot mirror bent under its own weight when placed in the tube in 1875, and has not since been remounted.
- E. Holden, "The Lick Observatory," "Nature," vol. xxv, p. 298.
- "Monthly Notices," Royal Astronomical Society, vol. xiv, p. 133, 1854.
- "Philosophical Transactions," vol. cxlviii, p. 465.
- Captain Jacob unfortunately died August 10, 1862, when about to assume the direction of a hill observatory at Poonah.
- The height of the mercury at Guajara is 21·7 to 22 inches.
- "Philosophical Transactions," vol. cxlviii, p. 477.
- We are told that three American observers in the Rocky Mountains, belonging to the Eclipse Expedition of 1878, easily saw Jupiter's satellites night after night with the naked eye. That their discernment is possible even under comparatively disadvantageous circumstances is rendered certain by the well-authenticated instance (related by Humboldt, "Cosmos," vol. iii, p. 66, Otte's translation) of a tailor named Schön, who died at Breslau in 1837. This man habitually perceived the first and third, but never could see the second or fourth, Jovian moons.
- Sir W. Herschel's great undertakings, Bessel remarks ("Populäre Vorlesungen," p. 15), "were directed rather toward a physical description of the heavens than to astronomy proper."
- "American Journal of Science," vol. xiii, p. 89.
- The characteristic orange line (D3) of this unknown substance has recently been identified by Professor Palmieri in the spectrum of lava from Vesuvius—a highly interesting discovery, if verified.
- "The Sun," p. 193.
- R. D. Cutts, "Bulletin of the Philosophical Society of Washington," vol. i, p. 70.
- This instrument may be described as an electric balance of the utmost conceivable delicacy. The principle of its construction is that the conducting power of metals is diminished by raising their temperature. Thus, if heat be applied to one only of the wires forming a circuit in which a galvanometer is included, the movement of the needle instantly betrays the disturbance of the electrical equilibrium. The conducting wires or "balance-arms" of the bolometer are platinum-strips 1 of an inch wide, and 1 of an inch thick, constituting metallic antennæ sensitive to the chill even of the fine dark lines in the solar spectrum, or to changes of temperature estimated at 1 of a degree centigrade.
- Defined by the tint of the second hydrogen-line, the bright reversal of Fraunhofer's F. The sun would also seem—adopting a medium estimate—three or four times as brilliant as he now does.
- "Annales de Chimie et de Physique," tome x, p. 360.
- S. P. Langley, "Nature," vol. xxvi, p. 316.
- Sir J. Herschel's estimate of the "temperature of space" was 239° Fahr.; Pouillet's 224° Fahr. below zero. Both are almost certainly much too high. See Taylor, "Bulletin of the Philosophical Society of Washington," vol. ii, p. 73; and Croll, "Nature," vol. xxi, p. 521.
- This is true only of the "normal spectrum," formed by reflection from a "grating" on the principle of interference. In the spectrum produced by refraction, the red rays are huddled together by the distorting effect of the prism through which they are transmitted.