In 1840 the distinguished astronomer Bessel declared that attempts to explain the discrepancies "must be based on the endeavor to discover an orbit and a ma.s.s for some unknown planet, of such a nature, that the resulting perturbations of Ura.n.u.s may reconcile the present want of harmony in the observations." Two years later he undertook researches in reference to the new planet of whose existence he felt certain. His labors, however, were interrupted by the death of his a.s.sistant Flemming, and by his own illness, which proved fatal in 1846, a few months before the actual discovery of Neptune. It is evident that the quest of the new planet had become general. The error of Ura.n.u.s still amounted to less than two minutes. This deviation from the computed place is not appreciable by the naked eye, yet it was felt, by the scientific world, to challenge the validity of the Newtonian theory, or to foreshadow the addition of still another planet to our solar system.
In July, 1841, John Couch Adams, a young undergraduate of St. John"s College, Cambridge, whose interest had been aroused by reading Airy"s paper on the _Progress of Astronomy_, made note of his resolution to attempt, after completing his college course, the solution of the problem then forming in so many minds. After achieving the B.A. as senior wrangler at the beginning of 1843, Adams undertook to "find the most probable orbit and ma.s.s of the disturbing body which has acted on Ura.n.u.s." The ordinary problem in planetary perturbations calls for the determination of the effect on a known orbit exerted by a body of known ma.s.s and motion. This was an inverse problem; the perturbation being given, it was required to find the position, ma.s.s, and orbit of the disturbing planet. The data were further equivocal in that the elements of the given planet Ura.n.u.s were themselves in doubt; the unreliability of its planetary tables, in fact, being the occasion of the investigation now undertaken. That thirteen unknown quant.i.ties were involved indicates sufficiently the difficulty of the problem.
Adams started with the a.s.sumptions, not improbable, that the orbit of the unknown planet was a circle, and that its distance from the sun was twice that of Ura.n.u.s. This latter a.s.sumption was in accord with the so-called "Bode"s Law," which taught that a simple numerical relationship exists between the planetary distances (4, 7, 10, 16, 28, 52, 100, 196), and that the planets as they lie more remote from the sun tend to be more nearly double the distance of the next preceding. Adams was encouraged, by his first attempt, to undertake a more precise determination.
On his behalf Professor Challis of Cambridge applied to Astronomer Royal Airy, who furnished the _Reductions of the Planetary Observations_ made at Greenwich from 1750 till 1830. In his second endeavor Adams a.s.sumed that the unknown planet had an elliptical orbit. He approached the solution gradually, ever taking into account more terms of the perturbations. In September, 1845, he gave the results to Challis, who wrote to Airy on the 22d of that month that Adams sought an opportunity to submit the solution personally to the Astronomer Royal. On the 21st of October, 1845, the young mathematician, twice disappointed in his attempt to meet Airy, left at the Royal Observatory a paper containing the elements of the new planet. The position a.s.signed to it was within about one degree of its actual place.
On November 5 Airy wrote to Adams and, among other things, inquired whether the solution obtained would account for the errors of the radius vector as well as for those of heliocentric longitude. For Airy this was a crucial question; but to Adams it seemed unessential, and he failed to reply.
By this time a formidable rival had entered the field. Leverrier at the request of Arago had undertaken to investigate the irregularities in the tables of Ura.n.u.s. In September of the same year Eugene Bouvard had presented new tables of that planet. Leverrier acted very promptly and systematically. His first paper on the problem undertaken appeared in the _Comptes Rendus_ of the Academie des Sciences November 10, 1845. He had submitted to rigorous examination the data in reference to the disturbing influence of Jupiter and of Saturn on the orbit of Ura.n.u.s. In his second paper, June 1, 1846, Leverrier reviewed the records of the ancient and modern observations of Ura.n.u.s (279 in all), subjected Bouvard"s tables to severe criticism, and decided that there existed in the orbit of Ura.n.u.s anomalies that could not be accounted due to errors of observation. There must exist some extraneous influence, hitherto unknown to astronomers. Some scientists had thought that the law of gravitation did not hold at the confines of the solar system (others that the attractive force of other systems might prove a factor), but Leverrier rejected this conception. Other theories being likewise discarded he asked: "Is it possible that the irregularities of Ura.n.u.s are due to the action of a disturbing planet, situated in the ecliptic at a mean distance double that of Ura.n.u.s? And if so, at what point is this planet situated? What is its ma.s.s? What are the elements of the orbit which it describes?" The conclusion reached by the calculations recorded in this second paper was that all the so-called anomalies in the observations of Ura.n.u.s could be explained as the perturbation caused by a planet with a heliocentric longitude of 252 on January 1, 1800.
This would correspond to 325 January 1, 1847.
Airy received Leverrier"s second paper on June 23, and was struck by the fact that the French mathematician a.s.signed the same place to the new planet as had Adams in the preceding October. He wrote to Leverrier in reference to the errors of the radius vector and received a satisfactory and sufficiently compliant reply. At one time the Astronomer Royal had felt very skeptical about the possibility of the discovery which his own labors had contributed to advance. He had always, to quote his own rather nebulous statement, considered the correctness of a distant mathematical result to be the subject of moral rather than of mathematical evidence. Now that corroboration of Adams"s results had arrived, he felt it urgent to make a telescopic examination of that part of the heavens indicated by the theoretical findings of Adams and Leverrier. He accordingly wrote to Professor Challis, July 9, requesting him to employ for the purpose the great Northumberland equatorial of the Cambridge Observatory.
Professor Challis had felt, to use his own language, that it was so novel a thing to undertake observations in reliance upon merely theoretical deductions, that, while much labor was certain, success appeared very doubtful. Nevertheless, having received fresh instructions from Adams relative to the theoretical place of the new planet, he began observations July 29. On August 4 in fixing certain reference points he noted, but mistook for a star, the new planet. On August 12, having directed the telescope in accordance with Adams"s instructions he again noted the same heavenly body, as a star. Before Challis had compared the results of the observation of August 12 with the results of an observation of the same region made on July 30, and arrived at the inference that the body in question, being absent in the latter observation, was not a star but a planet, the prize of discovery had fallen into the hands of another observer.
On August 31 had appeared Leverrier"s third paper, in which were stated the new planet"s...o...b..t, ma.s.s, distance from the sun, eccentricity, and longitude. The true heliocentric longitude was given as 326 32" for January 1, 1847. This determination placed the planet about 5 to the east of star d of Capricorn. Leverrier said it might be recognized by its disk, which, moreover, would subtend a certain angle.
The systematic and conclusive character of Leverrier"s research, submitted to one of the greatest academies of science, carried conviction to the minds of astronomers. The learned world felt itself on the eve of a great discovery. Sir John Herschel, in an address before the British a.s.sociation on September 10, said that the year past had given prospect of a new planet. "We see it as Columbus saw America from the sh.o.r.es of Spain. Its movements have been felt trembling along the far-reaching line of our a.n.a.lysis with a certainty hardly inferior to ocular demonstration."
On September 18 Leverrier sent a letter to Dr. Galle, of the Berlin Observatory, which was provided with a set of star maps, prepared at the instance of Bessel. Galle replied one week later. "The planet, of the position of which you gave the indication, really exists. The same day that I received your letter [September 23] I found a star of the eighth magnitude, which was not inscribed in the excellent map (prepared by Dr.
Bremiker) belonging to the collection of star maps of the Royal Academy of Berlin. The observation of the following day showed decisively that it was the planet sought." It was only 57" from the point predicted.
Arago said that the discovery made by Leverrier was one of the most brilliant manifestations of the precision of modern astronomic science.
It would encourage the best geometers to seek with renewed ardor the eternal truths which, in Pliny"s phrase, are latent in the majesty of theory.
Professor Challis received Leverrier"s third paper on September 29, and in the evening turned his magnificent refractor to the part of the heavens that Leverrier had so definitely and so confidently indicated.
Among the three hundred stars observed Challis was struck by the appearance of one which presented a disk and shone with the brightness of a star of the eighth magnitude. This proved to be the planet. On October 1 Challis heard that the German observer had antic.i.p.ated him.
Arago, while recognizing the excellent work done by Adams in his calculations, thought that the fact that the young mathematician had failed to publish his results should deprive him of any share whatever in the glory of the discovery of the new planet, and that history would confirm this definite judgment. Arago named the new planet after the French discoverer, but soon acquiesced in the name Neptune, which has since prevailed.
Airy, in whose possession Adams"s results had remained for months unpublished and unheeded, wrote Leverrier: "You are to be recognized beyond doubt as the predictor of the planet"s place." A vigorous official himself, Airy was deeply impressed by the calm decisiveness and definite directions of the French mathematician. "It is here, if I mistake not, that we see a character far superior to that of the able, or enterprising, or industrious mathematician; it is here that we see the philosopher." This explains, if anything could, his view that a distant mathematical result is the subject of ethical rather than of mathematical evidence.
Adams"s friends felt that he had not received from either of the astronomers, to whom he confided his results, the kind of help or advice he should have received. Challis was kindly, but wanting in initiative.
Although he had command of the great Northumberland telescope, he had no thought of commencing the search in 1845, for, without mistrusting the evidence which the theory gave of the _existence_ of the planet, it might be reasonable to suppose that its position was determined but roughly, and that a search for it must necessarily be long and laborious. In the view of Simon Newcomb,[3] Adams"s results, which were delivered at the Greenwich Observatory October 21, 1845, were so near to the mark that a few hours" close search could not have failed to make the planet known.
Both Adams and Leverrier had a.s.sumed as a rough approximation at starting that the orbit of the new planet was circular and that, in accordance with Bode"s Law, its distance was twice that of Ura.n.u.s. S. C.
Walker, of the Smithsonian Inst.i.tution, Washington, was able to determine the elements of the orbit of Neptune accurately in 1847. In February of that year he had found (as had Petersen of Altona about the same time) that Lalande had in May, 1795, observed Neptune and mistaken it for a fixed star. When Lalande"s records in Paris were studied, it was found that he had made two observations of Neptune on May 8 and 10.
Their failure to agree caused the observer to reject one and mark the other as doubtful. Had he repeated the observation, he might have noted that the _star_ moved, and was in reality a planet.
Neptune"s...o...b..t is more nearly circular than that of any of the major planets except Venus. Its distance is thirty times that of the earth from the sun instead of thirty-nine times, as Bode"s Law would require.
That generalization was a presupposition of the calculations leading to the discovery. It was then rejected like a discredited ladder. Man"s conception of the universe is widened at the thought that the outmost known planet of our solar system is about 2,796,000,000 miles from the sun and requires about 165 years for one revolution.
Professor Peirce, of Harvard University, pointing to the difference between the calculations of Leverrier and the facts, put forward the view that the discovery made by Galle must be regarded as a happy accident. This view, however, has not been sustained.
REFERENCES
Sir Robert Ball, Neptune"s Jubilee Year, _Scientific American_, Supplement, Oct. 10, 1896.
Sir Robert Ball, _The Story of the Heavens_, chap. XV.
B. A. Gould, _Report on the History of the Discovery of Neptune, Smithsonian Contributions to Knowledge_, 1850.
Robert Grant, _History of Physical Astronomy_.
Simon Newcomb, _Popular Astronomy_.
Benjamin Peirce, _Proceedings of the American Academy of Arts and Sciences_, vol. I, pp. 57-68, 144, 285, 338-41, etc.
FOOTNOTES:
[3] See article "Neptune," _Encyc. Brit._
CHAPTER XV
SCIENCE AND TRAVEL--THE VOYAGE OF THE BEAGLE
Sir Charles Lyell, in his _Principles of Geology_, the first edition of which appeared in 1830-1833, says: "If it be true that delivery be the first, second, and third requisite in a popular orator, it is no less certain that travel is of first, second, and third importance to those who desire to originate just and comprehensive views concerning the structure of our globe." The value of travel to science in general might very well be ill.u.s.trated by Lyell"s own career, his study of the mountainous regions of France, his calculation of the recession of Niagara Falls and of the sedimentary deposits of the Mississippi, his observations of the coal formations of Nova Scotia, and of the composition of the Great Dismal Swamp of Virginia--suggestive of the organic origin of the carboniferous rocks.
Although it is not with Lyell that we have here princ.i.p.ally to deal, it is not irrelevant to say that the main purpose of his work was to show that all past changes in the earth"s crust are referable to causes now in operation. Differing from Hutton as to the part played in those changes by subterranean heat, Lyell agreed with his forerunner in ascribing geological transformations to "the slow agency of existing causes." He was, in fact, the leader of the uniformitarians and opposed those geologists who held that the contemporary state of the earth"s crust was owing to a series of catastrophes, stupendous exhibitions of natural force to which recent history offered no parallel. Also enlightened as to the significance of organic remains in stratified rock, Lyell in 1830 felt the need of further knowledge in reference to the relation of the plants and animals represented in the fossils to the fauna and flora now existing.
It is to Lyell"s disciple, Charles Darwin, however, that we turn for our main ill.u.s.tration of the value of travel for comprehensive scientific generalization. Born, like another great liberator, on February 12, 1809, Darwin was only twenty-two years old when he received appointment as naturalist on H.M.S. Beagle, about to sail from Devonport on a voyage around the world. The main purpose of the expedition, under command of the youthful Captain Fitzroy, three or four years older than Darwin, was to make a survey of certain coasts in South America and the Pacific Islands, and to carry a line of chronometrical measurements about the globe. Looking back in 1876 on this memorable expedition, the naturalist wrote, "The voyage of the Beagle has been by far the most important event in my life, and has determined my whole career." In spite of the years he had spent at school and college he regarded this experience as the first real training or education of his mind.
Darwin had studied medicine at Edinburgh, but found surgery distasteful.
He moved to Cambridge, with the idea of becoming a clergyman of the Established Church. As a boy he had attended with his mother, daughter of Josiah Wedgwood, the Unitarian services. At Cambridge he graduated without distinction at the beginning of 1831. It should be said, however, that the traditional studies were particularly ill suited to his cast of mind, that he had not been idle, and had developed particular diligence in different branches of science, and above all as a collector.
He was six feet tall, fond of shooting and hunting, and able to ride seventy-five or eighty miles without tiring. He had shown himself at college fond of company, and a little extravagant. He was, though a sportsman, extremely humane; had a horror of inflicting pain, and such repugnance at the thought of slavery that he quarreled violently with Captain Fitzroy when the latter condoned the abomination. Darwin was not, however, of a turbulent disposition. Sir James Sulivan, who had accompanied the expedition as second lieutenant, said many years after: "I can confidently express my belief that during the five years in the Beagle, he was never known to be out of temper, or to say one unkind or hasty word _of_ or _to_ any one."
Darwin"s father was remarkable for his powers of observation, while the grandfather, Erasmus Darwin, is well known for his tendency to speculation. Charles Darwin possessed both these mental characteristics in an eminent degree. One who has conversed with him reports that what impressed him most in meeting the great naturalist was his clear blue eyes, which seemed to possess almost telescopic vision, and that the really remarkable thing about Darwin was that he saw more than other people. At the same time it will scarcely be denied that his vision was as much marked by insight as by careful observation, that his reasoning was logical and singularly tenacious, and his imagination vivid. It was before this supreme seer that the panorama of terrestrial creation was displayed during a five years" voyage.
No one can read Darwin"s _Journal_ descriptive of the voyage of the Beagle and continue to entertain any doubts in reference to his aesthetic sense and poetic appreciation of the various moods of nature. Throughout the voyage the scenery was for him the most constant and highest source of enjoyment. His emotions responded to the glories of tropical vegetation in the Brazilian forests, and to the sublimity of Patagonian wastes and the forest-clad hills of Tierra del Fuego. "It is easy,"
writes the gifted adolescent, "to specify the individual objects of admiration in these grand scenes; but it is not possible to give an adequate idea of the higher feelings of wonder, astonishment, and devotion, which fill and elevate the mind." Similarly, on the heights of the Andes, listening to the stones borne seaward day and night by the mountain torrents, Darwin remarked: "The sound spoke eloquently to the geologist; the thousands and thousands of stones, which striking against each other, made the one dull uniform sound, were all hurrying in one direction. It was like thinking on time, where the minute that now glides past is irrecoverable. So was it with these stones, the ocean is their eternity, and each note of that wild music told of one more step towards their destiny."
When the Beagle left Devonport, December 27, 1831, the young naturalist was without any theory, and when the ship entered Falmouth harbor, October 2, 1836, though he felt the need of a theory in reference to the relations of the various species of plants and animals, he had not formulated one. It was not till 1859 that his famous work on the _Origin of Species_ appeared. He went merely as a collector, and frequently in the course of the voyage felt a young man"s misgivings as to whether his collections would be of value to his Cambridge professors and other mature scientists.
Professor Henslow, the botanist, through whom Darwin had been offered the opportunity to accompany the expedition, had presented his pupil with the first volume of Lyell"s _Principles of Geology_. (Perhaps, after Lyell, the most potent influence on Darwin"s mind at this time was that of Humboldt and other renowned travelers, whose works he read with avidity.) At the Cape Verde Islands he made some interesting observations of a white calcareous stratum which ran for miles along the coast at a height of about forty-five feet above the water. It rested on volcanic rocks and was itself covered with basalt, that is, lava which had crystallized under the sea. It was evident that subsequently to the formation of the basalt that portion of the coast containing the white stratum had been elevated. The sh.e.l.ls in the stratum were recent, that is, corresponded to those still to be found on the neighboring coast. It occurred to Darwin that the voyage might afford material for a book on geology. Later in the voyage, having read portions of his _Journal_ to Captain Fitzroy, Darwin was encouraged to believe that this also might prove worthy of publication.
Darwin"s account of his adventures and manifold observations is so informal, so rich in detail, as not to admit of summary. His eye took in the most diverse phenomena, the color of the sea or of rivers, clouds of b.u.t.terflies and of locusts, the cacique with his little boy clinging to the side of a horse in headlong flight, the great earthquake on the coast of Chile, the endless variety of plant and animal life, the superst.i.tion of savage and _padre_, the charms of Tahiti, the unconscious humor of his mountain guides for whom at an alt.i.tude of eleven thousand feet "the cursed pot (which was a new one) did not choose to boil potatoes"--all found response in Darwin"s open mind; everything was grist to his mill. Any selection from the richness of the original is almost sure to show a tendency not obvious in the _Journal_.
On the other hand, it is just such multiplicity of phenomena as the _Journal_ mirrors that impels every orderly mind to seek for causes, for explanation. The human intellect cannot rest till law gives form to the wild chaos of fact.
No disciple of Lyell could fail to be convinced of the immeasurable lapse of time required for the formation of the earth"s crust. For this principle Darwin found abundant evidence during the years spent in South America. On the heights of the Andes he found marine sh.e.l.l fossils at a height of fourteen thousand feet above sea-level. That such an elevation of submarine strata should be achieved by forces still at Nature"s command might well test the faith of the most ardent disciple. Of how great those forces are Darwin received demonstration on the coast of Chile in 1835. Under date of February 12, he writes: "This day has been memorable in the annals of Valdivia for the most severe earthquake experienced by the oldest inhabitant.... A bad earthquake destroys our oldest a.s.sociations; the earth, the very emblem of solidity, has moved beneath our feet like a thin crust over a fluid." He observed that the most remarkable effect of this earthquake was the permanent elevation of the land. Around the Bay of Concepcion it was raised two or three feet, while at the island of Santa Maria the elevation was much greater; "on one part Captain Fitzroy found beds of putrid mussel sh.e.l.ls _still adhering to the rocks_, ten feet above high-water mark." On the same day the volcanoes of South America were active. The area from under which volcanic matter was actually erupted was 720 miles in one line and 400 in another at right angles to it. Great as is the force at work, ages are required to produce a range of mountains like the Cordilleras; moreover, progress is not uniform and subsidence may alternate with elevation. It was on the principle of the gradual subsidence (and elevation) of the bed of the Pacific Ocean that Darwin accounted for the formation of coral reefs. Nothing "is so unstable as the level of the crust of this earth."
Closely a.s.sociated with the evidence of the immensity of the force of volcanic action and the infinitude of time elapsed, Darwin had testimony of the mult.i.tude of plant and animal species, some gigantic, others almost infinitely small, some living, others extinct. We know that his thought was greatly affected by his discovery in Uruguay and Patagonia of the fossil remains of extinct mammals, all the more so because they seemed to bear relationship to particular living species and at the same time to show likeness to other species. The Toxodon (bow-tooth), for example, was a gigantic rodent whose fossil remains were discovered in the same region where Darwin found living the capybara, a rodent as large as a pig; at the same time the extinct species showed in its structure certain affinities to the Edentata (sloths, ant-eaters, armadillos). Other fossils represented gigantic forms distinctly of the edentate order and comparable to the Cape ant-eater and the Great Armadillo (_Dasypus gigas_). Again, remains were found of a thick-skinned non-ruminant with certain structural likeness to the Camelidae, to which the living species of South American ruminants, the _guanacos_, belong.