Six Months in Ascension. An unscientific account of a scientific expedition, etc [With an introduction by Sir D. Gill.] With a map

Introduction

. A scientific expedition may be said to have two histories. The one treats of the special object of the expedition, the other of the personal adventures of those concerned in it. It is only the former which finds permanent record in the Transactions of scientific societies : the other too often remains unwritten. For many reasons I think this is a matter of regret. Mere details of observations are never looked at, ex cept by a very limited number of specialists ; to the general public such details are meaningless as well as inaccessible ; whilst the ordinary student usually accepts the result merely as he finds it quoted in some standard work or text-book. It is not because popular accounts of such expe ditions do not interest a sufficient circle of readers that they have not been more frequently written, but rather, I think, because the faculties of original research and popular exposition are seldom united in the same Introduction.

VIII individual. Besides this, I have found in my own experience, that on such expeditions there is so much actual work to be done, and the hours are so com pletely filled with it, that there is neither time nor inclination to write a diary. Thus, before the story can be committed to writing, it has lost its crispness — the interest has faded, and, from treacherous memory, inci dent is wanting to complete the narrative. On my expedition to Ascension last year, however, I had the good fortune to be accompanied by my wife, who found much pleasure and interest in making a daily record of our life and work there. This little book, compiled from her journal, she now lays before the public with much diffidence. It is an honest endeavour to tell a true story, and add somewhat to a neglected class of literature ; as such, she hopes that the faults incidental to a first work will meet with lenient judgment.

The story can boast of no stirring interest, no thrilling adventures by land or sea. It must derive its interest chiefly from its truthfulness as a record of an attempt to solve a great problem, viz., the distance of the Earth from the Sun. The nature of this problem my wife explains in Chapter I. sufficiently, I think, to make it of interest to the many that would gladly learn something of the history of scientific progress, but who are often deterred from so doing by the minute Introduction. IX details and heavy technicalities with which its every step is necessarily encumbered. To the best of her ability she has given one side of the history of one step. It now remains for me to preface her chronicle by a brief outline of the labours of those who have worked before in the same field of research.

The first attempt to measure the Sun*s distance was made as follows. In the diagram suppose s the Sun, m the Moon, and e an observer on the Earth. When the angle s M e is a right angle, the Moon will be exactly balf full. If it is less than a right angle tbe Moon will appear less than half illuminated to an observer at E, and vice versa. Hence, if the angle s E m is accurately measured at the instant when the Moon is half full, the proportions of the triangle sem will be known ; because the angle s m e being a right angle, and the angle s e M having been determined by measurement, one side and two angles are known ; tbe triangle sem can therefore be drawn on paper, or its proportions may be computed mathematically. Introduction. X Since e m represents the distance of the Earth from the Moon at the instant of observation, the pro portion of this distance to that of the Sun (represented by the line e s) is determined.

In this way Aristarchus of Samos concluded that the Sun was nineteen times more distant than the Moon. This distance we now know to be more than twenty times too small — and the reason of his failure was twofold. 1st. Because, from the irregularity of the Moon's surface, it is almost impossible to estimate when she is exactly half full ; 2nd. Because his means of measuring the angle of sem were rude and imperfect. The result of Aristarchus was however adopted by astronomers till the time of Kepler. About the year 1620, from the observations of Tycho Brahe" on the planet Mars, Kepler concluded that the distance of the Sun must amount at least to 1800 diameters of the Earth, upon which he was upbraided by his friend Criiger " for removing the Sun to such a huge distance." The first approximation to a true determination was the result of an expedition organized by the French astronomer, Cassini.

Richer had been sent to South America by the French Academy. On the 1st of October, 1672, the planet Mars approached very close to a bright star (*//■ Aquarii), aud, from Introduction. XI observations made by Richer in Cayenne, and by Picard and Romer in France, Cassini concluded that the Sun's distance must be at least 86 millions of miles. The results were, however, liable to very con siderable uncertainty. This was due to the imperfect instruments of the time, by means of which it was hardly possible to measure angles with the required accuracy. The first satisfactory step in advance is due to the Abbe de la Caille, whose celebrated expedition to the Cape of Good Hope took place in the year 1740. There he made a large number of observations of Mars, and from these, compared with corresponding observa tions in the Northern Hemisphere, deduced Sl-^ millions of miles for the Sun's distance.

He after wards combined this result with similar observations on the planet Venus, and arrived at 81TV millions of miles as a final result. In the meantime, however, the English astronomer Halley had gone to St. Helena, where he observed the Transit of Mercury on the 28th of October, 1677 (see Chap. III. p. 33), and this suggested to him the method of determining the Sun's distance by the Transits of Venus which would take place in the years 1761 and 1769. In two remarkable memoirs presented to the Royal Introduction. XII Society of London in 1691 and 1716, he pointed out the great advantages of this method, and urged upon astronomers the necessity of providing for the complete and accurate observation of these phenomena. Accordingly, in 1761, England sent Maskelyne to St. Helena, and Mason and Dixon were dispatched to Sumatra.

The two latter astronomers were so delayed by the way, that, fearing they would not reach the appointed station in time, they decided to remain at the Cape of Good Hope ; and the decision proved a fortunate one. The St. Petersburg Academy of Sciences sent Chappe to Tobolsk, and Rumowski to Selinghinsk near Lake Baikil in Siberia. The French sent Pingre to the Island of Rodriguez, and Le Gentil should have observed at Pondicherry. Poor Le Gentil ! He .duly reached Mauritius on the 10th of July, 1760 — nearly a year before the Transit. War having meanwhile broken out between France and England, he was unable to reach Pon dicherry ; so he resolved to go to the Island of Rodriguez instead, to join Pingre, who was already there. When on the point of starting for Rodriguez, he learned that a French frigate was about to sail from Mauritius for the coast of Coromandel.

Le Gentil decided to avail himself of the opportunity thus Introduction. XIII offered to reach the point chosen by the Academy ; but one delay after another occurred, and it was not until the middle of March that he sailed again from Mauritius. There was not much time to be lost, for the Transit would occur on the 6th of June. De tained by frequent calms he did not reach the coast of Malabar till the 24 th of May. Still there might have been time enough to prepare for the observation, had not the commander of the frigate learned that the English were masters of Mahe and Pondicherry. His only chance to escape capture was to make off as quickly as possible. This he did ; steering a course for Mauritius again, to Le Gentil's utmost despair. The 6 th of June arrived. The sky was gloriously clear.

From the deck of the vessel Le Gentil made the best observations he could ; but, from so unsteady a platform, they could be of little value to science. Other observers had better fortune ; but the results when computed proved far from satisfactory. Different astronomers obtained results varying from 81J to 9G| millions of miles ; not because there was any error in the method, but because the observations were dis cordant. The difficulties of actual observation proved to be far greater than had been anticipated. Instead ot precise phenomena at contact, only a gradual merging, Introduction. XIV or a gradual separation of the limbs of Venus aud the Sun could be observed. Thus different inter pretations could be put upon the language of the observers ; and, according to the interpretations, so was the result.

But the failure of the Transit of 1761 only urged to new effort for that of 1769. Astronomers through out the world felt that if the opportunity, which would then occur, was lost, another so favourable for determining the Sun's Distance would not occur again for 105 years. Accordingly, the most strenuous exer tions were made to provide for its proper observation. Encke has well said, " Whatever may be the future judgment as to the actual issue, posterity will never be able to reproach either the astronomers or the governments of that period with having neglected to call sufficiently careful attention to the more im portant points, or with having failed to further and support scientific efforts with sufficient readiness." It would occupy too much space to follow the adveu tures of all the observers, but some of them cannot be passed over without mention.

The French astronomer, Le Gentil — whose endeavours to observe the Transit of Venus in 1761 were defeated in the way already described — had no sooner returned to Mauritius than he set out again for Pondicherry, Introduction. XV determined to wait there, for eight years, till the next Transit of Venus. The eventful 3rd of June, 1769, at last arrived. The morning was fine, and everything promised a happy issue. But, just as the critical moment ap proached, an unfortunate cloud eclipsed the Sun, a torrent of rain descended, and the fruit of eight years' waiting was lost. Le Gentil had profitably employed his time in studying the astronomy of the Brahmins, so his eight years in Pondicherry had been well occu pied ; but the agony of disappointment he must have felt at the defeat of his noble endeavours cannot but enlist the sympathy of all who know his story.

It was intended that the French astronomer Chappe, who in 1761 had observed the Transit of Venus at Tobolsk, should now observe it at the Solomon Islands. The Spanish Government, however, refused the necessary permission, but offered to convey him, along with two Spanish observers, to Mexico or California by a Spanish fleet then about to sail for South America. This offer was accepted. Chappe, with his Spanish colleagues, selected Cape Lucas, in California, and there observed the Transit successfully. But he did not live to tell the tale at home. The plague visited the district, and Chappe was one of its first victims. Three days after the Transit, he was attacked by the Introduction. XVI malady, but had partly recovered, when his love of science led him to commit an imprudence which brought him to the grave.

Despite his feeble condition, he passed the night of the 18th of June in observ ing an eclipse of the moon. A relapse ensued, and on the 1st of August he died. A short time before his death he said to his friends : "I know I have but a short time to live, but I have fulfilled my mission, and I die content." The English Government took a holder course, and did not wait for the permission of the Spaniards to visit the South Seas. The celebrated Captain Cook in com mand of the Endeavour, with Green (a pupil of Brad ley) for astronomer, and Solander (a pupil of Linne) for naturalist, sailed on the 22nd of September, 1768, for an unknown destination. The result was one of the most brilliant and successful scientific expeditions ever undertaken. The Transit of Venus was successfully observed by Green and Cook at Tahiti, one of the Sandwich Islands ; and much valuable work in con nection with Natural History, Terrestrial Magnetism, and Hydrography, was accomplished.

Another English expedition was sent to Hudson's Bay. There Dymock and Wales, after encountering a good many hardships, successfully observed the Transit ; and their observations acquired a special value, because Introduction. XVII it unfortunately happened that those made at the other important northern station were liable to con siderable suspicion. Father Hell, with his assistant, Father Sainovicz, was invited by the King of Denmark to observe the Transit of Venus in his dominions. In the month of June, 1768, they left Copenhagen, accompanied by Borgrewing, a Danish observer. They reached Ward oehuus, in the north of Lapland, on the 1 1th of October, 1768. Here they passed the winter, and duly observed the Transit of Venus in 1769. But numerous circum stances tended to throw suspicion on Hell's observations.

In the first place, without any sufficient reason, he suppressed his observations for nine entire months, and many eminent astronomers did not hesitate to accuse him of having fabricated or changed them. In 1834, his original papers were presented to the Vienna Observatory by Baron Munch-Bellinghausen, into whose hands they had come through the death of his uncle, Baron von Penkler, an intimate friend and patron of Father Hell. Professor Littrow's investigation of these papers led to the discovery of Father Hell's original note-book for the days June 2 — 4, 1769.* "These notes fully * I quote Dr. Gould's account in his admirable introduction to tlie 0". S. Naval Astronomical Expedition to Chili. I Introduction. XVIII corroborate and justify previous suspicions. The chief figures, especially the times of entrance upon the solar disc, had been for the most part erased, and with a darker coloured ink.

Two passages, the one relating to the observations of Sainovicz, the other to those of Borgrewing, had been so thoroughly obliterated, that Professor Littrow was only able to conjecture the three first letters of the one and the first and last letter of the other. From an investigation of such figures as remained legible and unaltered, he succeeded in finding one observation of the Ingress by Borgrewing, and one of Egress by Hell, upon which reliance appears war rantable." " Although in reply to Lalande, Father Hell had publicly offered to exhibit the original note-book, free from erasures, and giving observations just as finally published by him, Littrow found both clear and undefaced documents containing the quantities as prepared for publication, and this note-book, which was as manifestly not designed for press.

It contains remarks, notes, and comments in chronological order ; the handwriting is unequal and frequently changing, observations never made public are here noted down, together with many jottings and memoranda which could not have been intended for the public. The important observations were chiefly obliterated with Introduction. XIX great care and thoroughness, as were also sundry remarks concerning them. There can be no doubt that the evidence is sufficient to establish this note book as being the identical one used at Wardoehuus, and that this establishment of identity discredits the published observations and the truthfulness of Father Hell, but provides few new figures upon which reliance may be placed." Encke also found another proof of Father Hell's dishonesty. An eclipse of the Sun occurred soon after the Transit of Venus of 1769, and afforded an excel lent means of checking the longitudes of the stations.

Father Hell observed this eclipse, and fortunately did not change the original record in his note-book. The time he published, however, differed from the time he recorded by 41 seconds; for, in his desire to publish better observations than he knew how to make, he altered his observation to agree with his computa tion, which proved to have been founded on erroneous elements. His original record was afterwards found to be a good observation. Such revelations must throw the gravest doubt upon all the observations of Father Hell, and give to the observations of Dymock and Wales an exceptional importance. Although the results obtained for the distance of the Sun from the Transit of 1769 did not differ so widely l Introduction. XX as those of 1761, still the agreement was by no means satisfactory.

Thus from the numerous results may be quoted the following Sun'a Distance Lalande obtained 96,3, millions of miles. Father Hell „ 93T»5 „ Hornsby „ 93^ Euler „ 92T'D Pingr<_ „ 92^ Laplace „ 92^ In 1835 Encke published his famous discussion of the Transits of Venus of 1761 and 1769. He found From the Transit of 17G1 95-^ millions of miles. 1769 95^s From both Transits combined 95^ Encke's discussion met with the general approval of astronomers at the time, and for many years was accepted as the standard determination of the Sun's distance. In 1832, Henderson observed, at the Cape of Good Hope, the favourable Opposition of Mars of that year, and his observations, combined with similar ones at Greenwich, Cambridge and Altona in the Northern Hemisphere, gave 90i millions of miles for the Sun's distance.

The results were, however, Introduction. XXI not very accordant, and were not generally accepted as satisfactory. In 1847 Professor Gerling proposed the observation of the planet Venus, at observatories in the Northern and Southern hemispheres, as a good means of de termining the Sun's distance. He argued that as Venus approaches nearer to the Earth than Mars, she pre sents a more favourable opportunity for determining parallax. He also contended that the delicate and faint crescent form of Venus (like a very young moon when the planet is near conjunction) formed a tele scopic object capable of the most accurate measurement. Professor Gerling's .idea, however, did not assume practical shape till it was taken up by a zealous observer, Lieut. Gilles of the United States Navy. He applied to his chief to ask from Congress a grant of 1000.. for the expenses of an expedition to Chili.

He proposed to observe there, not only the planet Venus, when near inferior conjunction (as suggested by Professor Gerling), but also the Oppositions of Mars which would occur in the years 1850 and 1852. His proposal was accepted. Gilles went to Chili, and there made more than 200 series of observations of Mars and Venus together. To combine with this splendid mass of work, only Introduction. XXII corresponding observations were made in the Northern Hemisphere, and even these do not appear to have been made with exceptional care, nor to possess the accuracy required in so delicate a research. The want of sympathy and support from astronomers, which Lieut. Gilles met with, is a blot upon the history of astro nomy, and Dr. Gould, in the work I have already quoted, has well said, " It is impossible to refrain from the expression of deep regret that, from all the observations of the well-equipped and richly-endowed observatories of the Northern Hemisphere, so few materials could be found toward rendering available, according to its original purpose, an expedition to which so much labour and enthusiasm had been consecrated, and to which an accomplished observer, already known for the precision of his measurements, had devoted his entire energies during so long a sojourn (three years) ; moreover, after the preparation and wide dissemination of ephemerides and charts of the comparison stars for both the planets during the whole period."

Dr. Gould has reduced the whole mass of observations with a loving care, and obtains the result 96jL millions of miles a result confessedly unsatisfactory from the non-agreement of the various observations. Introduction. XXIII Meanwhile theorists had been at work upon the motions of the Moon and Planets, — weighing one against the other, in fact, by finding how much their mutual attractions disturb each other. Some of these disturbances or inequalities depend upon the distance of the Earth from the Sun, that is to say, if the Earth were nearer the Sun, these inequalities would be greater and vice versa. It would be out of place to detail here the various results which have been derived from the application of these methods. I need only state that the results obtained about this time, by these methods, give from 91-,% to 91TV millions of miles for the Sun's distance ; instead of 95 ,*„ obtained by Encke from the Transits of Venus.

In 1862 there occurred a very favourable Opposition of Mars. Dr. Winnecke, an astronomer of Pulkowa (the Imperial Observatory of Russia), drew up a pro gramme of observations which was more or less per fectly carried out at six observatories in the Northern, and at three observatories in the Southern Hemisphere. Two partial discussions of some of these observations appeared, giving 91TV and 91-r*V millions of miles respectively for the Sun's distance. Then Mr. Stone rediscussed the Transit of Venus observations of 1769, employing only the observations in which both ingress and egress were observed. He Introduction. XXIV included the suspected observations of Father Hell, and by interpreting differently the language of various observers, and applying certain corrections for different phases observed, he obtained 91j'G millions of miles for the Sun's distance.

This quantity was in satisfac tory agreement with the results of the theoretical methods, and also with the recent results of the observations of Mars. Not only was this the case, but also all the observations were brought into the most beautiful accord. The largest error of observing the dura tion did not amount in any case to six seconds of time, and the probable error of one observation of duration taken by chance, was only two seconds of time. But the duration is made up of two contacts, so that two seconds combined the probable error of two contacts. According to the theory of probabilities, the probable error of one contact would then be two seconds divided by the square root oj two — in other words, the probable error of one observation of contact was only one second and four-tenths of a second of time.

This result was accepted at the time with enthu siasm, and Mr. Stone received the gold medal of the Royal Astronomical Society for his labours. About this time, however, Newcomb, the well-known Introduction. XXV American astronomer, published his masterly discus sion of the observations of Mars made at nine obser vatories in the year 1862. The result arrived at was 92j*j millions of miles for the Sun's distance. By his own rediscussion of some of the theoretical methods, Newcomb also showed that these could be reconciled with the result he found from the observations of Mars alone. He also pointed out that, according to a discussion by Powalky, the Transit of Venus of 1769 afforded a similar result, and by com bining all the various methods, he arrived at the con clusion that the true mean distance of the Sun is 92j*5 millions of miles.

An additional confirmation of Newcomb's result had been previously derived by Foucault from his determination of the velocity of light. The angular velocity of the Earth's motion round the Sun is accurately known ; hence, if the Earth's linear velocity can be determined, the radius of motion (i.e., the Sun's distance) will also be determined. Now, the proportion which the velocity of light bears to the velocity of the Earth's motion is pretty well determined by astronomical observation ; for the fact that light takes an appreciable time to travel, has the effect of Introduction. XXVI shifting the apparent places of the stars. By determining the maximum amount of this shifting (called the constant of aberration), the proportion of the velocity of light to the mean velocity of the Earth's motion becomes known.

Thus, if the velocity of light is determined, the velocity of the Earth's motion will become known, and hence the Sun's distance. The English astronomer, Bradley, was the first to trace out the cause of this shifting of the apparent places of the stars. The story is that the true explanation occurred to him as he was sailing in a boat on the Thames. The wind blew directly down the river, and, when the boat was at rest, the flag also was blown in a direction straight down the river ; but when he tacked to the right, the free end of the flag was carried to the left ; and when he tacked to the left, the free end of his flag was carried to the right. He had been much puzzled by changes in the apparent places of certain stars, which he could not account for. Now the truth flashed upon him suddenly.

The case is similar with my boat and its flag. The true direction of the wind represents the mean direction of a star ; the boat tacking from side to side represents the Earth in its yearly revolution going from one side of the Sun to the other. The deviation of the Introduction. XXVII flag from its true direction depends upon the velocity of the wind and the direction and velocity of the boat's motion. If, then, the velocity of light is not infinitely great, as compared with the velocity of the Earth's motion, the apparent place of a star must be changed according as the Earth is moving in one direction or the other ; in the same way that the apparent direction of the flag is changed according to the direction of the boat's motion. So Bradley set to work to reconcile his observations on this supposition, and succeeded perfectly, making, in so doing, the first determination of the const ant of aberration.

This constant he found to be 20".2, whence he con cluded that the velocity of light was 10,210 times as great as the velocity of the Earth's motion in its orbit. Succeeding astronomers have made careful determinations of the constant of aberration, with various results from 20".2 to 20".6. The value of this constant which has most generally received the confidence of astronomers is the result obtained by Struve, 20".44ol. The Sun's distance found by Encke from the Transit of Venus, combined with Struve's value of the constant of aberration, gave 191 thousand miles per second as the velocity of light. Later, Foucault, by most ingenious optical and me- Introduction. XXVIII chanical arrangements actually determined the velocity of light, and found it to be between 185 and 186 thousand miles per second, a velocity which, com bined with Struve's value of the constant of aberration, gave 92^ millions of miles for the Sun's distance; a quantity in satisfactory accord with Newcomb's result.

It was not until 1872, however, that Newcomb's conclusion received its most remarkable confirmation. In that year Leverrier communicated a paper to the Paris Academy of Sciences, in which he gave three values of the Sun's distance, which resulted from three independent re searches in the theory of the Planets. 1 . From the motions of Mars 92J", millions of miles. 2. „ „ Venus 92a 3. Other motions of Venus 92A In such estimation did Leverrier hold the accuracy of these results, that he conceived it almost impossible that any direct means of observation could furnish a better determination. He proposed, as preferable to the Transit of Venus, the mode of determining the Sun's distance by observations for the Velocity of Light and the Constant of Aberration, and recommended the Academy to take steps in these directions.

Introduction. XXIX Of the Transit of Venus, he says, " the determina tion of the Solar parallax by means of the Transit of Venus still retains all its interest, but conditionally on its being made with exceptional precision, so that the astronomer may be able to answer for it with an accuracy exceeding Ti „ of a second of arc." (This accuracy corresponds with T'o °f a million of miles in the Sun's distance). Leverrier did not believe that the Transit of Venus could yield such an accuracy ; and in this respect, as well as in regard to the accuracy of his theoretical con clusions (confirmed as they were by Newcomb's results), Leverrier's opinion was shared by many astronomers. By others, however, this latter opinion of Leverrier's was not held, but it seemed desirable to all that the Transit of Venus of 1874 should be observed in the best possible manner.

The aid of photography was called in ; the use of the most exquisite of all angle measuring instruments — the Heliometer — was dis cussed and adopted ; volumes were written, and papers were read and discussed as to the best stations for observation ; and all the astronomical talent of all nations busied itself in preparation. No scientific object ever before excited such widespread activity and interest ; or received from Governments and individuals such hearty and substantial assistance. Introduction. XXX The extent of labour and of toil expended upon the Transit' of 1874 may probably be gathered from a glance at the following table, prepared from the Eeport of the Council of the Royal Astronomical Society for the year 1876, showing the stations where the Transit was more or less successfully observed :— - Station.