Popular Science Monthly/Volume 27/July 1885/Sketch of Professor S. P. Langley
By EDWARD S. HOLDEN.
I HAVE been asked to write a sketch of the life of Professor Langley, to accompany his portrait in this number of "The Popular Science Monthly."
Something of the life of every scholar and of every public man belongs to his audience; while most of that personality which endears him to his friends is their private possession, not to be set forth, except within narrow limits.
Professor Langley was born at Roxbury (now Boston), August 22, 1834. Like many another Boston boy, he was sent to the Boston Latin School, where Latin and Greek and little else was taught.
Latin and Greek was reputed to be the sum and end of learning, and Harvard College seemed to show dim perspectives of more Latin and Greek. It was no wonder that young Langley, whose genius lay in quite another direction, should look about him, after his graduation from the school, to see if there were not some practicable way in which he could pursue those mechanical and astronomical studies that already had fascinated him. He had little inclination to enter college, and the openings in astronomy proper were very rare in those years, even rarer than now. Since he was ten years old, he had been reading and studying astronomy, making small telescopes, using these and others, with various success, but always with ardor. The practical question of how to shape his life was one that had to be solved, and a variety of causes led to his determination not to go to college, but to become a civil engineer. Here at least was a profession whose basis was mathematical, and in which mechanical tastes and acquirements would have scope. So the practice of engineering was begun; special circumstances forced him into architecture, and for some years this was his pursuit. These were dull years, mostly spent in the West, where at that time there were few opportunities to display any real ability in this special calling.
There is little doubt but that the long and dreary hours spent over the drawing-table were an admirable though tedious preparation for the series of astronomical delineations which have been of so solid a use to science. But, finally, in the lack of real opportunities, architecture ceased to be a profession, and became a business, a means to live simply.
In 1864 Langley felt the need of some marked change in his life, and he spent the greater part of the years 1864 and 1865 in Europe.
In 1865 he returned to America, then thirty years old, and found himself entirely free, for the first time in his life, to follow his own inclinations. So, at thirty, instead of twenty, we find him as one of the regular assistants at the Harvard College Observatory. From this time forward he belongs to astronomy, although many an obstacle was yet to be overcome before he could freely exercise his special and high talents.
After a few months at Harvard, Langley was offered the position of Professor of Mathematics at the United States Naval Academy at Annapolis. Before the war, a small observatory had been founded at Annapolis by Professor Chauvenet. It contained a six-inch equatorial, and an exquisite meridian circle, by Repsold, with which Chauvenet had already made some observations. The removal of the Academy to Newport and the resignation of Professor Chauvenet left these instruments unused, and it was Langley's first business to remount them and to place the small observatory on a working basis. The next year was an apprenticeship in the practice of astronomy. In 1867 Professor Langley was invited to become the Professor of Astronomy in the Western University of Pennsylvania (at Pittsburg), and to take charge of its observatory on one of the high hills across the river (Allegheny City). The previous history of the observatory had been a checkered one, and its equipment was in the last degree inadequate and incomplete.
It had been built in a good situation; there was a dilapidated dwelling-house on the grounds; the observatory building itself was there; an equatorial of thirteen inches aperture was mounted; but this was all. Everything was bare; the equatorial was not provided with the necessary apparatus; the observatory was entirely empty, except for a table and three chairs; and the professor was expected to be active there, while at the same time he was to attend to the full duties of a chair at the college; no assistants were provided, and the observatory had no income! It is hardly possible to conceive a situation more tantalizing and less hopeful.
A way out soon suggested itself. For the prosperity of the observatory some definite income was essential, and it was absolutely requisite to earn this. What has an observatory to sell, that the business men of Pittsburg—the railways, the iron-masters, the glass-founders—will buy? Clearly, the only thing they want is the correct time. But will they pay for it? This was what Professor Langley set himself to provide, and by 1869 the full system was in successful operation and yielding a fair income to the observatory. For some years before, certain other observatories had established more or less complete time-services (at Albany, Washington and elsewhere), but the system at Allegheny was the most complete and elaborate of any, and the first which was looked to for an adequate support of an observatory.
Besides regulating the public time of Pittsburg and of numerous private offices, the observatory provided the standard time for the whole system of railways centering in Pittsburg, and daily sent (automatically by electricity) the beats of its standard clock over the telegraph lines from New York and Philadelphia west as far as Cincinnati and Chicago, north to Lake Erie, and south to Washington. This system is still in full operation, and has always maintained a high character for accuracy.
The United States Coast Survey organized several parties to observe the total eclipses of 1869 and 1870, and Professor Langley went to Oakland, Kentucky, in 1869, as a member of the party of his friend Professor Winlock, Director of Harvard College Observatory. In 1869 his station was upon the very edge of the shadow, and the object of his observation was to determine the limit of total eclipse. In 1870 the station assigned to Professor Langley was at Xeres, in Spain, where he determined the polarization of the solar corona to be radial.
During the year 1870 the affairs of the observatory began to assume such a shape that some time for original work in astronomy was available. The success of the time-service had created a small fund out of which the more pressing needs of instrumental equipment were provided; and Professor Langley now began a period of the most incessant work on the minute study of the features of the sun's disk. The situation of his observatory at Pittsburg, where dense clouds of smoke and dust and dirt obscure the heavens, and the meager state of his instrumental equipment, almost forced him to take up the study of the sun, which has light enough to penetrate even a Pittsburg fog. Fortunately, this study demanded very few auxiliary pieces of apparatus: the telescope has to be directed upon the sun, its motor-clock keeps it constantly pointed upon the same spot, and the observer has to follow, with infinite diligence and patience, the elusive details which the moments of best vision may allow him to glimpse. Two very important and rare qualifications are also necessary. The observer must be entirely unprejudiced and impartial; recording that which he sees, whether it is expected or not, and recording nothing which he does not see, no matter how firmly he may be convinced that it ought to be visible. This is the first qualification—one of unusual mental constitution; and the second is one of unusual manual skill. The observer must be able to delineate the most extraordinary and complex details justly and correctly. Both of these unusual qualifications Professor Langley possesses in a marked degree. His well-known and most beautiful drawing of a "Typical Sun-spot" illustrates this. This has since been copied in very many places, and it has received the very highest praises from all competent judges.Professor Langley's earliest published paper on the sun (February, 1874) may be taken as a type of his best work. It possesses that hardly-definable quality by which we become aware that it was written from a full mind. It is only fifteen pages long, yet we are not conscious of undue brevity. One has a sense, in reading, that every
A Typical Sun-spot.
statement of fact, or every expression of opinion, is based upon a hundred single instances like the one which is chosen, or upon a hundred concurring judgments. It is not that you are overborne by weight but convinced by character. This most important paper came at exactly the right time. It first summarizes the works of other recent observers, which, though important, had left the subject in an entirely unsatisfying condition, and then proceeds straight to the subject in hand.
The minute details, both of the general solar surface and of the extraordinarily complex spots, are one by one satisfactorily and lucidly described, with indications of the physical conditions to which they are due; and, finally, the general bearings of all this on the received solar theories are briefly set forth. We may fairly say that this paper is fundamental. It treats of a subject of which little had been accurately known, and it leaves this subject in a satisfactory and settled condition. Four years of labor on this subject had not failed to suggest many other researches.
A detailed study of the distribution of the heat of the solar surface was begun about this time, by means of the thermopile, and was quickly rewarded by the discovery of an unknown thermo-chroic action in the sun's atmosphere, such that it transmits the light less readily than the heat, owing to the difference in wave-length. An interesting consequence of this action is that, if, at any time, the sun's atmosphere should grow thicker, the color of the sun would tend toward red; if thinner, then toward blue. These changes, which are quite possible, suggest interesting explanations of some of the phenomena of the variable stars. The glacial epochs on the earth may be connected with changes in the solar atmosphere.
In 1877 we find another outcome of the series of measures of the heat from various parts of the sun's disk, and especially from the umbræ, etc., of sun-spots. The periodic changes in the spotted area of the solar disk, which had long been known, induced the inquiry whether changes in the amount of spotted surface bore any relation to changes of temperature on the earth's surface.
The result of the extremely delicate measures of Professor Langley led plainly to the conclusion that the direct effect of sun-spots on terrestrial temperature is sensible; that, when the spotted area is a maximum, the temperature is on that account lower, and the converse; but that the total direct effects of the periodic changes in the spotted area on the earth's mean temperature are extremely small, not more than a change of three tenths of 1° C. in eleven years, and not less than one twentieth of 1° C. The indirect effects are not here considered.
A thermopile used in connection with the most sensitive galvanometers is an extremely delicate instrument; and Allegheny Observatory now possessed a most complete outfit of this sort.
But the most important and pressing questions in solar physics demanded a means of measurement of heat still more delicate. When it was a question to measure the heat radiation from the different parts of the sun's disk, the thermopile was adequate. But if the heat from one of these parts is spread out into a heat-spectrum several feet or even yards long, it becomes necessary to devise new means of measuring the minute differences between the various parts. Such a device is the bolometer, which consists of two systems of extremely thin steel or platinum strips. Through these two systems an electric current passes. A sensitive galvanometer connected with both systems keeps its needle steady when the currents are equal.
If one system is now exposed to heat radiations while the other is protected from them, the temperature of the first is raised, its electric resistance is increased, and the battery-currents through the two systems and the galvanometer no longer balance. The needle then moves, and the amount of this motion measures the amount of heat disturbance. The sensitiveness of the instrument is from ten to thirty times greater than that of the most delicate thermopiles possible, and its constancy specially fits it for its work. The years 1879 and 1880 were given to perfecting this new and powerful instrument. Some of its first results were to show, by direct experiment, that the maximum of heat in the normal spectrum was in the orange, not the infra-red (then an interesting fact); and that the solar-constant, as determined by previous methods, was decidedly too small. The most suitable methods of determining this important constant were pointed out.
In 1881 Professor Langley organized an expedition to the top of Mount Whitney, in California, for the purpose of applying these new methods under the most favorable conditions. The expenses of the expedition were jointly borne by the United States Signal Service and by the private subscription of a wealthy gentleman in Pittsburg, who had now for some years taken the greatest interest in the researches of the observatory, and whose liberality had provided many of its instruments.
His name ought to be here mentioned. He has materially aided science in the most liberal and thoughtful way; but, against his expressed wish that he should be nameless in this connection (as he is in hundreds of other kind deeds), I have no right to contend.
The most important single result of the previous experiments with the bolometer had been the establishment of the fact of selective absorption of the solar rays by the earth's atmosphere. The results of this action are so important that I may be permitted to quote from Professor Langley an elementary exposition of them. He says: "Our observations at Allegheny had appeared to show that the atmosphere had acted with selective absorption to an unanticipated degree, keeping back an immense proportion of the blue and green, so that what was originally the strongest had, when it got down to us, become the weakest of all, and what was originally weak had become relatively strong, the action of the atmosphere having been just the converse of that of an ordinary sieve, or like that of a sieve which should keep back small particles analogous to the short wave-lengths (the blue and green), and allow freely to pass the large ones (the dark-heat rays). It seemed from these observations that the atmosphere had not merely kept back a part of the solar radiation, but had totally changed its composition in doing so—not by anything it had put in, but by the selective way in which it had taken out, as if by a capricious intelligence. The residue that had actually come down to us thus changed in proportion was what we know familiarly as 'white' light, so that white is not 'the sum of all the radiations,' as used to be taught, but resembles the pure original sunlight less than the electric beam which has come to us through reddish-colored glasses resembles the original brightness. With this visible heat was included the large amount of invisible heat, and, if there was any law observable in this 'capricious' action of the atmosphere, it was found to be this, that, throughout the whole range of the known heat-spectrum the large wave-lengths passed with greater facility than the shorter ones."
The effect of this selective absorption on the visible rays is to cut out the shorter wave-lengths proportionally more; so that to an eye outside of the earth's atmosphere the sun would be far bluer than to us. On the heat-rays taken together, the total amount of the absorption is very great, far greater than had been previously supposed. Professor Langley's experiments give a very great increase in the amount of solar heat reaching the earth over previous determinations, so that for example, according to him, the solar radiation is sufficient to annually melt an ice-shell one hundred and seventy-nine feet thick all round the earth. According to previous determinations, one hundred and ten feet in thickness could be melted. But while Professor Langley finds a vastly greater amount of heat supplied by the sun, his law of the selective absorption comes in to profoundly modify its terrestrial manifestations. Were there no such selective absorption, the temperature of the soil in the tropics, under a vertical sun, would probably not rise to that of the freezing-point of mercury. "The temperature of this planet, and with it the existence, not only of the human race but of all organic life on the globe, appears, from the results of the Mount Whitney expedition, to depend far less on the direct solar heat" than on the hitherto neglected quality of selective absorption.
The bearing of the observations at Mount Whitney on a great number of important questions, the temperature of the sun, the radiation from the sky, etc., etc., can not be here considered for want of space. The solar spectrum previously known was but half of that mapped out by the expedition, and there is good reason to believe that Professor Langley's observations have now revealed the whole of it to us.
The partial results of these investigations, published from time to time in foreign periodicals, have done much to make Professor Langley honored in other countries than his own. In 1882 he was invited to address the British Association for the Advancement of Science at Southampton, and did so. His paper on that occasion reminds one of that of February, 1874, in the astonishing fullness of experiment, thought, and judgment which seems to lie just back of the sentences. It comes from a full mind. In the spring of 1885 Professor Langley goes to England at the invitation of the Royal Institution to lecture before it.
There are many other most interesting researches of Professor Langley's which should be referred to here, were it not for limited space. His observations on the moon's heat; on the solar eclipse of 1878 (at the summit of Pike's Peak); his direct comparison of the sun with the molten metal of a Bessemer converter; his investigations at Mount Etna, Pike's Peak, and Mount Whitney, on the conditions of vision at great altitudes, all deserve more than this brief notice.
His published scientific papers are very numerous. A list of the more important of these follows this article. There are forty-six separate papers in the years from 1869 to 1885. Besides these, the magazines have contained many more popular articles; and his courses of lectures at the Lowell Institute, the Peabody Institute, and elsewhere, have been most successful.
Professor Langley is a member of the National Academy of Sciences and of numerous American and foreign bodies, and has received the recognition of honorary degrees from various universities.
Note.—The above list omits numerous minor publications. It includes original contributions to science (the more important in italics), and articles of a popular character (marked with *).