Popular Science Monthly/Volume 16/November 1879/The Recent Progress of Solar Physics
|THE RECENT PROGRESS OF SOLAR PHYSICS|
By Professor S. P. LANGLEY,
OF THE ALLEGHENY OBSERVATORY
LEAVING to those of wider knowledge the survey of the whole field of scientific labor, it has seemed to me that I could best present to you some account of that branch of it with which I am most familiar, which is that of "Solar Physics."
This study is essentially a modern one. Astronomy, which in the earliest times could only mark the annual path of the sun, or count the stars, with the invention of the telescope still concerned itself more with the motions of the heavenly bodies than with their physical nature. It sought out new methods of precision to fix the places of these stars and to mark out for the navigator the path of the moon on the celestial dial; it united itself intimately with the sister science of mathematics in predicting the places of the heavenly bodies from the law of gravitation, but it was still as a surveyor and marker of boundaries in the field of space that the observer chiefly labored, and we associate the most striking triumphs of the classic astronomy with this work of precision. It is this aspect that appeals even to the imagination, and which is seized as distinctive by the poet of Urania:
"That little Vernier on whose slender lines
The midnight taper trembles as it shines,
Tells through the mist where dazzled Mercury burns,
And marks the spot where Uranus returns."
These are noble aims, and noble results; but it is curious to see how observers of the last century, who had learned this excellent lesson of precision, had learned no other, and remained indifferent to a great question to which the old methods did not apply. We are called into existence by a great central fire, the sun, by which we continue to exist from one hour to another. What is it? what is this heat which it pours into space, and with whose cessation we shall cease? How long will it continue to feed our lives? A few years ago, with almost the sole exceptions of the Herschels and Pouillet, no one even asked these questions, much less intelligently sought their solution.
It is hard to say to whom the awakening of attention is due; yet if any one were to be named, it should perhaps be the Italian physicist Melloni, "the Newton of heat."
His book, "La Thermochrose," has to me an attraction of its own, for the author, with the ingenuous confidence of his nation, begins, not by describing his thermopile or galvanometer, but by taking the reader into his personal experience, and telling him how even as a child he felt an invincible curiosity about what we have just seen hardly any one else then cared for, and how, rising long before dawn, he loved to seek some quiet spot, to wait there in the silence of the sleeping world the first beams of the sun, and as he felt their warmth and heard the stir of life they awakened round him, how he too was stirred with wonder and interest as to the nature of that mysterious thing, radiant heat, and resolved to give his future to its study. If to distinguish a cause for wonder and inquiry in what to the common mind has called for neither be a characteristic of genius, then Melloni must be allowed its possession, and in his but too short years he showed the world how much interest and importance lay in this then neglected study, which so many with clearer knowledge and better methods follow to-day.
Fraunhofer's previous work had prepared the way for the spectroscope, and with the now awakened interest in these questions, its employment by Kirchhoff in 1860 may be said to inaugurate the present study of solar physics, as distinguished from the classic astronomy, which concerned itself with number and measure first, and in a wholly secondary degree with the physical characteristics of the heavenly bodies. This study occupies itself with the former, indeed, but chiefly in aid of other investigations, and by the study of solar physics then, we mean much more than a telescopic examination of the sun; we mean besides this the analysis of its radiations by the spectroscope, their summation by the photometer and thermopile, the determinations of its heat and the possible effects of changes in it on terrestrial meteorology, and generally the pursuit of all those problems which unite the methods of physics and astronomy.
In 1860 we already knew that the sun was surrounded by an envelope then visible only during total eclipses, and which was surmised to be gaseous; and of the sun itself we knew very little more than that it was a hot globe with spots upon it; for, though Schwabe had observed the periodicity of the spots, and Carrington was already at work, their results were not wholly public, and the facts of the variable velocity of the sun's rotation were rather the surmises of a few than part of the body of acquired knowledge. Since then this branch of astronomy has grown almost to the position of an independent science, and, though it has not yet been distinctly divided into specialties in its turn like its elder sisters, yet we already see a tendency to their formations. Thus, with the study of the motions of the solar surface we associate with the names just mentioned those of Sporer, De la Rue; and Wolf; with eye-studies of the photosphere or solar meteorology, those of Dawes, Secchi, and others; with the telescopic use of the spectroscope those of Huggins, Janssen, Lockyer, Secchi, Young, and Tacchini. The work of mapping the spectrum, begun by Kirchhoff, has been continued by Angstrom, Mascart, and Cornu, while photography, in the hands of Rutherfurd, Janssen, and Draper, has largely superseded telescopic studies of the photosphere, and the list might be enlarged indefinitely. Let us glance at part of the work done by these during the past twenty years, for their labors make the history of our study.
The work of Carrington, completed in 1861, taught us what had before been suspected—both the periodicity of the spots and that this great globe, so far as we can see it, has different periods of rotation, its equatorial zones completing a revolution in less time than its polar ones. We know very little more on this point now, the cause of both phenomena remaining wholly mysterious to-day.
In the next year (1862) an impulse was given to the study of the solar surface by the announcement of a supposed discovery of gigantic individual bodies in it, of from 500 to 1,000 miles in length, distinct from each other, and existing in countless numbers. This extraordinary statement was not easily disproved, as it is with great difficulty that the real structure is discernible by the best telescopes. Forms, we can scarcely call them "bodies," are undoubtedly there, of a size and in numbers which could only exist on so vast a surface, and which are no doubt the chief immediate cause of the sun's light and heat—but what are their causes in turn, and what is their real nature? The suggestion was made at the time by the then, perhaps, most eminent living astronomer, that they might be, in a sort, living things—beings, in fact, whose vital force gave us the solar heat; a suggestion which we may smile at now, but which was received at the time with a kind of awe, as adumbrating some possible truth. Of its author I would speak with all possible respect in citing it, which I do here, as nothing can better indicate the obscurity of our knowledge, even at so recent a period. We may look back on such a possible suggestion and its connection with that "vital force," now itself banished by physiology, as a kind of landmark on the road we have traveled. Our science, young as it is, is old enough to have had its age of fable.
Since that time, in France, in Italy, in England, and here, thousands of telescopic studies have been made with the purpose of defining these forms, and of learning more about the growth of those mysterious objects with which they are associated—the sun's spots, which drew the attention of Fabricius and Galileo, and which still attract our own more than ever to-day, with problems which seem nearly insoluble. Everything we see convinces us that the solar surface in which they are formed is neither a solid nor a liquid, but composed of volumes of whirling vapors; yet through this vapor, which seems to offer no resistance, come eruptions of explosive violence such as one would suppose must arise from the sudden bursting of some rigid shell. The turmoil within the areas of disturbance is so great, the area itself so vast and inclosing such diversities of action, that we are still doubtful how far this action is downward, how far upward.
Under the circumstances, we can hardly say that twenty years of observation in this department have brought us results commensurate with the labor expended, nor have we derived great aid from photography until some recent advances of which I have presently to speak.
A review of our past studies of the corona is a review of the solar eclipses during the past twenty years; for it is a fact, unparalleled in the sciences of observation, that the opportunities for this knowledge last only minutes, and are separated by intervals of years. Till 1860 it was uncertain whether the protuberances belonged to the sun or moon, but in that year the then newly applied photographic method made it nearly certain that they were parts of the former, and previous surmises that they were extensions of an envelope everywhere surrounding the sun were confirmed. In 1868 some traces of the corona were first photographed. The spectroscope was used upon the prominences, their gaseous nature was proved, and nine of the chromospheric lines were determined; and nearly together Messrs. Janssen and Lockyer made the discovery that these lines could be seen without an eclipse; 1869 brought that eclipse which traversed our own territory, and in this the distinctive coronal line was first observed by Young and by Harkness; while in this, and yet more in the eclipse of 1870 and 1871, we obtained better photographs of the corona, and greatly increased our knowledge of its apparent structure.
It is hardly possible to present even in the briefest way any review here of the separate history of spectroscopic research since 1860, during which time it has been connected with most of the important steps in every field of our study. It has, in the hands of Messrs. Huggins, Zöllner, and Young, made visible to us the forms of the chromosphere, and enabled us to measure the velocity of motions upon the sun otherwise beyond estimate, while at the same time it has given us independent data for the absolute velocity of other suns in space, and for that of the rotation of our own solar photosphere. It has, in the hands of Secchi and others, connected our knowledge of our sun's physical constitution, and perhaps of its past history with that of other suns, and even assumed to give us information whence we might infer something as to their mass, as well as physical constitution, while it has immensely increased the number of lines mapped twenty years since in the spectrum, and modified the ideas we then entertained as to the interpretation of these lines themselves.
The important question of the amount of heat received from the sun has been the subject of almost uninterrupted experiment and study during the period under review, but without essentially altering the data of Herschel and Pouillet which we already possessed. In this field the French physicists and our countryman, Mr. Ericsson, have been prominent workers, and we have attained results possessing all desirable certitude relatively to our knowledge in other branches.
Investigations on the solar temperature have been carried on by many observers, but with results which are thus far less satisfactory,
I am painfully sensible of the inadequacy of this review of the history of solar physics, but the brief time before me warns me to come from its past to its present. Within the last two years the difficulties I have alluded to, as so great in eye-studies of the solar surface, have been singularly modified by the remarkable advance of solar photography at the hands of M. Janssen. When I recently visited his observatory at Meudon, I found him producing original negatives on a scale of nearly thirty English inches to the solar diameter, and which bear enlargement to nearly ten feet with remarkable precision; and one of these negatives, which presents over a million discrete cloud-forms, can be taken in 3000 of a second. In another branch of photography, that of the reproduction of spectral lines, for which so much is due to Rutherfurd and Draper, I know nothing more surprising than the recent success of Captain Abney (of the Royal Engineers) at South Kensington, who has photographed the red end of the spectrum, and far beyond the red end, to a wave-length of about 12,000. As this statement may of itself convey no clear idea to some of my audience, let me explain in less technical language that it means we can now photograph objects in absolute darkness—objects which are not luminous—simply by the heat they give out. This is a discovery which obviously lends itself to important practical applications, while it is of further interest as bringing another proof of that identity of heat and light, with radiations differing only in wave-length, long since surmised by physicists, and asserted prominently by Dr. John Draper, whose photographs are also the earliest in the path which Captain Abney has carried on by independent methods. Theoretically, there would seem to be no limit to this power of photography so long as objects radiate any heat whatever.
Of recent coronal studies, I have only to speak of the opportunity afforded by the eclipse of last year in our own Western territory. Observed as it was. in the pure air of the Rocky Mountains, we found an immense and hitherto almost unsuspected extension of the corona in the direction of the solar equator, such as to make it increasingly probable that the outer corona and the zodiacal light are different appearances with a common origin. The physical constitution of the inner corona seemed to be modified by the weakness or absence of a former constituent, and perhaps we may say that some additional knowledge was gained as to its telescopic structure and its absolute light, while the polariscopic evidence was contradictory.
In the light of our latest knowledge, what, then, is the corona? We do not know. We have literally had but about twenty minutes in the last twenty years to look at it, and from that brief study it remains every way problematical. The extent of this vast solar appendage is unknown, its constitution is unknown, its function is unknown, and it is still uncertain whether we can devise any means for its study which will free us from this dependence upon momentary glimpses. Our only hope, since the most powerful telescope is useless in our lower atmosphere, seems to be to transport our observatory to some mountain-height, like that of Etna or the elevated table-lands of Colorado. There, even, we can not be sure of seeing it without an eclipse; but there, if anywhere, ingenuity will be hopefully employed in an endeavor to remove the difficulties which bar the way. After spending some weeks this year myself upon Mount Etna, on which the new solar observatory is to be built, I can testify to the excellence of such a station; and yet, when we have sites equally good, I can not but regret that it should be left to others to first enter such a promising field.
Of recent spectro-photographic observation, I may mention the valuable work of M. Cornu, who, working at the other extremity of the spectrum from Captain Abney, has extended it beyond the violet to a wave-length of 2,900, far beyond which the solar spectrum probably exists, but where M. Cornu finds our own atmosphere to interpose an almost impassable barrier. The solar spectrum, therefore, is now known by photography through three times the extent of the visible portion, and this great gain on our former knowledge may be said to have been completed for us in the past year.
In last November and subsequently, Mr. Lockyer has made the extremely important announcement that, reasoning from analogies furnished by known compounds, he has been able to show that many elements are really compound bodies, which, incompletely dissociated at the highest temperatures we can command, furnish under the form of feeble lines the spectra of their components.
I do not enter here into discussion of points still in debate; but that which has arisen round this and the recent communications of Dr. Henry Draper, at any rate elicits the evidence of the immense labor now requisite in establishing new facts in our science, and the refinement of some of the adverse explanations suggested in controversy shows us to how limited a company of specialists we must look as judges in matters so important.
The instrumental aids of our study have grown in the period under review with the demand for greater accuracy, until the detached prisms of Kirchhoff's apparatus are replaced by trains of automatically adjustable mechanism, giving us in Thollon's recent instrument the equivalent dispersion of thirty prisms of flint, or what has replaced the "gitter" of Fraunhofer, that wonderful product of skill, the Rutherfurd grating, which for a large variety of uses has already supplanted the prism. Observatories especially devoted to solar physics are being established by European governments, as at Potsdam by Prussia, and at Meudon by France. I have already alluded to that on Etna, and I hope it will not be long before we have a distinctly physical observatory within our own territory. There is no step in our power to take which promises so much for immediate advance as the installation of one in a suitably elevated station, for certain investigations can be made only under this condition, and no amount of instrumental appliance, patience, or skill, at a lower altitude, supplies their place.
In now reviewing the acquisitions which this twenty years' labor has brought us, we can not but agree that we have achieved a great deal, and yet must admit, with wonder at the field still before us, how little is our progress in comparison with what remains unknown.
We have found out how to detect daily the outbursts from the sun which were before invisible, but we watch these outpourings of enormous forms without yet knowing what drives them forth, without being sure how far our very view is not in part illusion.
We have learned how to study and fix many of the wonderful details of spot-actions without knowing what spots are. We see them presenting themselves in increasing importance through a term of years, and then diminishing, and we attempt to assign a period to these cycles of growth and decay. This period is often fixed at about eleven years, with a perhaps unjustifiable confidence, for we can not be said to know whether what we have seen in so brief a time is constant or variable, nor whether it be not the mere incident of some greater cycle, whose course began before man was here to see it, and whose term may not be complete till he has gone.
We are possibly now led to ask what our science has taught us on the connection of these remote changes with questions which affect our daily lives, and perhaps to put the utilitarian question, "What is all this worth?"
We find at the present time our study growing into a closer union, not merely with stellar astronomy on the one hand and terrestrial meteorology on the other, but with all the physical sciences, than would once have been supposed possible. Thus, to give a single instance, whatever be the result of the discussions aroused by Mr. Lockyer's statements, it seems likely that we are to look to the analysis of the solar radiations for the most favorable evidence of that resolvability of our so-called elements to simpler forms, which our chemists are now very generally ready to admit as possible.
It is in the solar spectrum that we are now searching for the laws of the molecular groupings which affect the ultimate constitution of matter, and in recent questions as to the real nature of certain terrestrial elements, which our laboratories can not yet deal with, the Mount Sherman observations of Professor Young on the appearance of their analogues in the sun have been accepted by both parties in debates before the Royal Society, as pertinent evidence, the only doubt as to which lies in its interpretation.
Of problems "practical" in the sense that their utility is apparent alike to the learned and the unlearned, there are two at least of the highest importance which now occupy us.
The solar heat, which grows for us the food by which we live, is no doubt in one sense the final cause of every meteorological change, bringing those years of want and years of plenty which are due to local variations of climate, that depend, through a chain of causes very remote and obscure, no doubt, yet finally, upon the sun. We have seen the magnetic needles vibrating all over the globe together at the time of a sudden commotion upon the solar surface; we watch the increase and decrease of auroras, and find we can almost predict their frequency, so apparently united are they by some mysterious bond with the changes of solar spots; and we look with natural hope for other signs of union which may enable us to anticipate more important effects on our meteorology. Extreme pains have been devoted—in some cases misdevoted—to researches aiming to establish such a connection, by collecting data as to the changes in rainfall, the movements of storms, the prices of grain, and of almost every feature of terrestrial meteorology, in order to see whether these run through periods coincident with those of known changes on the solar surface. It will be admitted by the most utilitarian that the end aimed at is a worthy one, for the practical result of success, such as some believe possible, would be to enable its attainer to predict the price of breadstuffs years in advance, to control the markets of the world; to bestow, if unselfish, an almost priceless knowledge to man, or, if self-seeking, to acquire wealth beyond wish.
I need hardly say that the attempt has thus far been unsuccessful. There is hardly any topic on which there is more popular interest, hardly any on which there is more popular error, than this of the supposed influence of the sun on the weather. By means of the study of what Professor Smythe terms the "rain-band" in the spectrum, we appear to have lately gained increased facility in predicting local weather-changes; but, excepting this comparatively unimportant contribution, studies connected with the sun have as yet done very little for us here, and it seems necessary to say that, as far as prophecy is concerned, none of us are yet prophets, or more able to tell from our knowledge of the sun what the weather will be next week than what the harvest will be next year.
There is another utilitarian aspect of our study about which there is less public interest, but more real promise—I mean that which concerns the direct application of solar heat to arts and manufactures. These are now all using it indirectly—by the water, for instance, which it lifts into the clouds to turn the mills of Lowell or Lawrence, as it flows back to the sea, or by the coal which it stored in former ages to drive our engines to-day. These indirect means use but the feeblest portion of the solar heat, which is in theory capable of furnishing nearly one horse-power for each square yard of the earth's surface under full sunshine.
What we have actually realized in experiment is still considerable.
The visitor to the last Paris Exposition may have seen upon its grounds a machine of strange appearance, in the open air, pointing sunward the axis of an immense reflector, shaped like a truncated cone, which gathered the rays to a linear focus upon the boiler of a working steam-engine, which it drove thus by direct solar heat. Many not dissimilar solar engines have been built in this country and in India, the particular one of which I speak, due to M. Mouchot, having actually realized about one horse-power to ten feet square of surface.
We are startled when we make the computation, to find the immensity of the force thus placed at our disposal, or to see what the utilization of the waste places of the earth would bring us. Upon the limited area of the Adirondack wilderness to the north of us, for instance, the daily wasted sun-power actually realizable, and after every allowance for loss, is many times that of all the estimated steam-power at present in use in the whole world. I am not myself so far utilitarian as to wish to see this use made of our pleasant summer haunts, but there are regions of the earth at present as entirely worthless as that great African desert which it is now proposed to partly reconvert to an inland sea, a sunburned area now apparently hopelessly useless to man, and yet on which an amount of power is every year poured in utter waste which could not be made good by the consumption of all the coal known to underlie the soil of Great Britain.
Such machines as those of M. Mouchot, owing to the expense of construction and attendance, cost more than an engine driven by coal, though the sun supplies its power gratis; but it is simply, it seems to me, a question of time when, with another form which I believe our researches already indicate, such engines may become an economical as well as a mechanical success, and in a larger sense it is still only a question of time when the rapidly consuming coal-beds of Great Britain yield their last, and her manufacturing empire is transferred to countries which have not exhausted their supply. But these will exhaust their own in turn; the stock, though great, is finite and not renewable; and we must look, for the only power we know which can replace coal, to those regions of the earth now desolated by solar heat, and to which future empire may probably tend.
We have considered the past and the present of our study; for its future, lies the solution of all the great problems I have already alluded to, but these questions are so interlocked that the complete answer to one will probably not be given till we are nearly ready to answer all,
I have spoken of the fallacy of the popular impression of the result of our study as enabling us to predict the weather, or to anticipate the character of coming harvests. Repeating my belief that we as yet know nothing here, or next to nothing, I yet do not mean to disparage the object of such researches, nor even to deny the possibility of their ultimate success. We can look forward, among other fair dreams for our science's future, to a time when it will enable us to predict the years of plenty or play the part of a beneficent Providence, by warning in season against those of famine, which have cost in our time so many million lives in China and in India. These are, I repeat, still dreams only, but we may call them hopes if we will—hopes of which increased knowledge may deprive us, but of which we can not say it may not bring fruition.
There remains among the greatest problems of the future of our science the all-important one to the whole human race of the future constancy of the sun's heat, of which we have, it seems to me, no assurance of the present rate of supply. We have, it is true, every assurance that in the contraction of the solar mass and in the supply of meteoric matter, we have heat to warm the human race for periods almost beyond limit; but we learn also that this heat is tempered to us by a solar envelope, which seems to be, as far as we know, in conditions which do not favor stability. It is constantly being added to by eruptions from within the sun, caused by we know not what, and constantly diminished by some counter-process which we understand as little. When we consider that the thickening of this solar atmosphere would bring back the age of ice, or its thinning carry our polar regions to tropical temperature, and when we remember that rhythmical action, not uniformity, seems to be the law of nature here, we can feel no certainty of the future constancy of the solar heat, nor of our protection against such changes as seem to have befallen other suns in space, and against which we are powerless to guard.
But such considerations of our ignorance and helplessness, while they may prevent us from any undue pride in what our science has already attained, may teach us renewed confidence from the very brevity of our life. These green fields around us were once covered with glacial ice, and the change has been absolute from that condition to the one of to-day. Yet in the lifetime of any one of the thousands of insect generations which have succeeded each other in these fields, there must have seemed no alteration; and, remembering what instants our own lives are, in a like comparison with the uncounted ages of the sun's history, we may well reckon that our generation shall see no change.
In the little span which is allowed us, however, we will try to learn something more of that source of light, life, and power of which we are materially the creatures; and, if we can leave a knowledge which will not die with ourselves, feel that we have left also the record of a something in us "which owes no homage to the sun."
- Address before the Physical Section of the American Scientific Association at Saratoga.