HALLEY AND HIS SUCCESSORS
There is no need to give the lives of the succeeding Astronomers Royal so fully as that of Flamsteed. Not that they were inferior men to him; on the contrary, there can be little doubt that we ought to reckon some of them as his superiors, but, in the case of several, their best work was done apart from Greenwich Observatory, and before they came to it.
This was particularly the case with Edmund Halley. Born on October 29, 1656, he was ten years the junior of Flamsteed. Like Flamsteed, he came of a Derbyshire family, though he was born at Haggerston, in the parish of St. Leonard's, Shoreditch. He was educated at St. Paul's School, where he made very rapid progress, and already showed the bent of his mind. He learnt to make dials; he made himself so thoroughly acquainted with the heavens that it is said, 'If a star were displaced in the globe he would presently find it out,' and he observed the changes in the direction of the mariner's compass. In 1673 he went to Queen's College, Oxford, where he observed a sunspot in July and August, 1676, and an occultation of Mars. This was not his first astronomical observation, as, in June, 1675, he had observed an eclipse of the moon from his father's house in Winchester Street.
EDMUND HALLEY.
(From an old print.)
A much wider scheme of work than such merely casual observations now entered his mind, possibly suggested to him by Flamsteed's appointment to the direction of the new Royal Observatory. This was to make a catalogue of the southern stars. Tycho's places for the northern stars were defective enough, but there was no catalogue at all of stars below the horizon of Tycho's observatory. Here, then, was a field entirely unworked, and young Halley was so eager to enter upon it that he would not wait at Oxford to obtain his degree, but was anxious to start at once for the southern hemisphere.
His father, who was wealthy and proud of his gifted son, strongly supported him in his project. The station he selected was St. Helena, an unfortunate choice, as the skies there were almost always more or less clouded, and rain was frequent during his stay. However, he remained there a year and a half, and succeeded in making a catalogue of 341 stars. This catalogue was finally reduced by Sharp, and included in the third volume of Flamsteed's Historia Cœlestis.
In 1678 he was elected Fellow of the Royal Society, and the following year he was chosen to represent that society in a discussion with Hevelius. The question at issue was as to whether more accurate observations of the place of a star could be obtained by the use of sights without optical assistance, or by the use of a telescope. The next year he visited the Paris Observatory, and, later in the same tour, the principal cities of the Continent.
Not long after his return from this tour, Halley was led to that undertaking for which we owe him the greatest debt of gratitude, and which must be regarded as his greatest achievement.
Some fifty years before, the great Kepler had brought out the third of his well-known laws of planetary motion. These laws stated that the planets move round the sun in ellipses, of which the sun occupies one of the foci; that the straight line joining any planet with the sun moves over equal areas of space in equal periods of time; and, lastly, that the squares of the times in which the several planets complete a revolution round the sun are proportional to the cubes of their mean distances from it. These three laws were deduced from actual examination of the movements of the planets. Kepler did not work out any underlying cause of which these three laws were the consequence.
But the desire to find such an underlying cause was keen amongst astronomers, and had given rise to many researches. Amongst those at work on the subject was Halley himself. He had seen, and been able to prove, that if the planets moved in circles round the sun, with the sun in the centre, then the law of the relation between the times of revolution and the distances of the planets would show that the attractive force of the sun varied inversely as the square of the distance. The actual case, however, of motion in an ellipse was too hard for him, and he could not deal with it. Halley therefore went up to Cambridge to consult Newton, and, to his wonder and delight, found that the latter had already completely solved the problem, and had proved that Kepler's three laws of planetary motion were summed up in one, namely, that the sun attracted the planets to it with a force inversely proportional to the square of the distance.
Halley was most enthusiastic over this great discovery, and he at once strongly urged Newton to publish it. Newton's unwillingness to do so was great, but at length Halley overcame his reluctance; and the Royal Society not being able at the time to afford the expense, Halley took the charges upon himself, although his own resources had been recently seriously damaged by the death of his father.
The publication of Newton's Principia, which, but for him, might never have seen the light, and most certainly would have been long delayed, is Halley's highest claim to our gratitude. But, apart from this, his record of scientific achievement is indeed a noble one. Always, from boyhood, he had taken a great interest in the behaviour of the magnetic compass, and he now followed up the study of its variations with the greatest energy. For this purpose it was necessary that he should travel, in view of the great importance of the subject to navigation. King William III. gave him a captain's commission in the Royal Navy—a curious and interesting illustration of the close connection between astronomy and the welfare of our navy—and placed him in command of a 'pink,' that is to say, a small vessel with pointed stern, named the Paramour, in which he proceeded to the southern ocean. His first voyage was unfortunate, but the Paramour was recommissioned in 1699, and he sailed in it as far as south latitude 52°.
In 1701 and the succeeding year he made further voyages in the Paramour, surveying the tides and coasts of the British Channel and of the Adriatic, and helping in the fortification of Trieste. He became Savilian Professor of Geometry at Oxford in 1703, having failed twelve years previously to secure the Savilian Professorship of Astronomy, mainly through the opposition of Flamsteed, who had already formed a strong prejudice against him, which some writers have traced to Halley's detection of several errors in one of Flamsteed's tide-tables, others to Halley's supposed materialistic views. Probably the difference was innate in the two men. There was likely to be but little sympathy between the strong, masterful man of action and society and the secluded, self-conscious, suffering invalid. At any rate, in the contest between Newton and Flamsteed, which has been already described, Halley took warmly the side of the former, and was appointed to edit the publication of Flamsteed's results, and, on the death of the latter, to succeed him at the Royal Observatory.
The condition of things at Greenwich when Halley succeeded to the post of Astronomer Royal in 1720 was most discouraging. The instruments there had all belonged to Flamsteed, and therefore, most naturally, had been removed by his widow. The Observatory had practically to be begun de novo, and Halley had now almost attained the age at which in the present day an Astronomer Royal would have to retire. More fortunate, however, than his predecessor, he was able to get a grant for instruments, and he equipped the Observatory as well as the resources of the time permitted, and his transit instrument and great eight-foot quadrant still hang upon the Observatory walls.
As Astronomer Royal his great work was the systematic observation of the positions of the moon through an entire saros. As is well known, a period of eighteen years and ten or eleven days brings the sun and moon very nearly into the same positions relatively to the earth which they occupied at the commencement of the period. This period was well known to the ancient Chaldeans, who gave it its name, since they had noticed that eclipses of the sun or eclipses of the moon recurred at intervals of the above length. It was Halley's desire to obtain such a set of observations of the moon through an entire saros period as to be able to deduce therefrom an improved set of tables of the moon's motion. It was an ambitious scheme for a man so much over sixty to undertake, nevertheless he carried it through successfully.
His desire to complete this scheme, and to found upon it improved lunar tables, hindered him from publishing his observations, for he feared that others might make use of them before he was in a position to complete his work himself. This omission to publish troubled Newton, who, as President of the Royal Society—the Greenwich Board of Visitors having lapsed at Queen Anne's death—drew attention at a meeting of the Royal Society, March 2, 1727, to Halley's disobedience of the order issued under Queen Anne, for the prompt communication of the Observatory results. That Newton should thus have put public pressure upon Halley, the man to whom he was so much indebted, and with whom there was so close an affection, is sufficient proof that his similar attitude towards Flamsteed was one of principle and not of arbitrariness. Halley, on his side, stood firm, as Flamsteed had done, urging the danger that, by publishing before he had completed his task, he might give an opportunity to others to forestall his results. It is said—probably without sufficient ground—that this refusal broke Newton's heart and caused his death. Certainly Halley's writings in that very year show his reverence and affection for Newton to have been as keen and lively as ever.
Halley's work at the Observatory went on smoothly, on the lines he had laid down for himself, for ten years after Newton's death; but in 1737 he had a stroke of paralysis, and his health, which had been remarkably robust up to that time, began to give way. He died January 14, 1742, and was buried in the cemetery of Lee Church.
As an astronomer, his services to the science rank higher than those of his predecessor; but as Astronomer Royal, as director, that is to say, of Greenwich Observatory, he by no means accomplished as much as Flamsteed had done. Professor Grant, in his History of Physical Astronomy, says that he seems to have undervalued those habits of minute attention which are indispensable to the attainment of a high degree of excellence in the practice of astronomical observation. He was far from being sufficiently careful as to the adjustment of his instruments, the going of his clocks, or the recording of his own observations. The important feature of his administration was that under him the Observatory was first supplied with instruments which belonged to it.
HALLEY'S QUADRANT.
(From an old print.)
His astronomical work apart from the Observatory was of the first importance. He practically inaugurated the study of terrestrial magnetism, and his map giving the results of his observations during his voyage in the Paramour introduced a new and most useful style of recording observations. He joined together by smooth curves places of equal variation, the result being that the chart shows at a glance, not merely the general course of the variation over the earth's surface, but its value at any spot within the limits of the chart.
Another work which has justly made his name immortal was the prediction of the return of the comet which is called by his name, to which reference will be made later. Another great scheme, and one destined to bear much fruit, was the working out of a plan to determine the distance of the sun by observations of the transit of Venus.
Of attractive appearance, pleasing manners, and ready wit, loyal, generous, and free from self-seeking, he probably was one of the most personally engaging men who ever held the office.
The salary of the Astronomer Royal remained under Halley at the same inadequate rate which it had done under Flamsteed—£100, without provision for an assistant. But in 1729 Queen Caroline, learning that Halley had actually had a captain's commission in the Royal Navy, secured for him a post-captain's pay.
JAMES BRADLEY.
(From the painting by Hudson.)
Halley's work is represented at the Observatory by two of his instruments which are still preserved there, and which hang on the west wall of the present transit room: the Iron Quadrant afterwards made famous by the observations of Bradley, and 'Halley's Transit,' the first of the great series of instruments upon which the fame of Greenwich chiefly rests. This transit instrument seems to have been set up in a small room at the west end of what is now known as the North Terrace. His quadrant was mounted on the pier which is now the base of the pier of the astrographic telescope. This pier was the first extension which the Observatory received from the original building.
On the breakdown of his health Halley nominated as his successor, James Bradley; indeed, it is stated that he offered to resign in his favour. He had known him then for over twenty years, and that keen and generous appreciation of merit in others which was characteristic of Halley had led him very early to recognize Bradley's singular ability.
James Bradley was born in 1692 or 1693, of an old North of England family. His birthplace was Sherbourne, in Gloucestershire, and he was educated at North Leach Grammar School and at Baliol College, Oxford. During the years of his undergraduateship he resided much with his uncle, the Rev. James Pound, Rector of Wanstead, Essex, an ardent amateur astronomer, a frequent visitor at the Observatory in Flamsteed's time, and one of the most accurate observers in the country. From him, no doubt, he derived his love of the science, and possibly some of his skill in observation.
Bradley's earliest observations seem to have been devoted to the phenomena of Jupiter's satellites and to the measures of double stars. The accuracy with which he followed up the first drew the attention of Halley, and so began a friendship which lasted through life. His observations of double stars, particularly of Castor, only just failed to show him the orbital movement of the pair, because his attention was drawn to other subjects before it had become sufficiently obvious.
In 1719 Bradley and his uncle made an attempt to determine the distance of the sun through observations of Mars when in opposition, observations which were so accurate that they sufficed to show that the distance of the sun could not be greater than 125 millions of miles, nor less than about 94 millions. The lower limit which they thus found has proved to be almost exactly correct, our best modern determinations giving it as 93 millions. The instrument with which the observations were made was a novel one, being 'moved by a machine that made it to keep pace with the stars;' in other words, it was the first, or nearly the first, example of what we should now call a clock-driven equatorial.
That same year he was offered the Vicarage of Bridstow, near Ross, in Monmouthshire, where, having by that time taken priest's orders, he was duly installed, July, 1720. To this was added the sinecure Rectory of Llandewi-Velgry; but he held both livings only a very short time. In 1721 the death of Dr. John Keill rendered vacant the Savilian Professorship of Astronomy at Oxford, for which Bradley became a candidate, and was duly elected, and resigned his livings in consequence.
It was whilst he was Savilian Professor that Bradley made that great discovery which will always be associated with his name. Though professor at Oxford, he had continued to assist his uncle, Mr. Pound, at his observations at Wanstead, and after the death of the latter he still lived there as much as possible, and continued his astronomical work. But in 1725 he was invited by Mr. Samuel Molyneux, who had set up a twenty-four-foot telescope made by Graham as a zenith tube at his house on Kew Green, to verify some observations which he was making. These were of the star Gamma Draconis, a star which passes through the zenith of London, and which, therefore, had been much observed both by Flamsteed and Hooke, inasmuch as by fixing a telescope in an absolutely vertical position—a position which could be easily verified—it was easy to ascertain if there was any minute change in the apparent position of the star. Dr. Hooke had declared that there was such a change, a change due to the motion of the earth in its orbit, which would prove that the star was not an infinite distance from the earth, the seeming change of its place in the sky corresponding to the change in the place of the earth from which the observer was viewing it.
Bradley found at once that there was such a change—a marked one. It amounted to as much as 1´´ of arc in three days; but it was not in the direction in which the parallax of the star would have moved it, but in the opposite. Whether, therefore, the star was near enough to show any parallax or not, some other cause was giving rise to an apparent displacement of the star, which entirely masked and overcame the effect of parallax.
So far, Bradley had but come to the same point which Flamsteed had reached. Flamsteed had detected precisely the same apparent displacement of stars, and, like Hooke, had ascribed it to parallax. Cassini had shown that this could not be the case, as the displacement was in the wrong direction; and there the matter had rested. Bradley now set to follow the question up. Other stars beside Gamma Draconis were found to show a displacement of the same general character, but the amount varied with their distance from the plane of the ecliptic, the earth's orbit. The first explanation suggested was that the axis of the earth, which moves very nearly parallel to itself as the earth moves round the sun, underwent a slight regular 'wobble' in the course of a year. To check this, a star was observed on the opposite side of the pole from Gamma Draconis; then Bradley investigated as to whether refraction might explain the difficulty, but again without success. He now was most keenly interested in the problem, and he purchased a zenith telescope of his own, made, like that of Molyneux, by Graham, and mounted it in his aunt's house at Wanstead, and observed continuously with it. The solution of the problem came at last to him as he was boating on the Thames. Watching a vane at the top of the mast, he saw with surprise that it shifted its direction every time that the boat was put about. Remarking to the boatmen that it was very odd that the wind should change just at the same moment that there was a shift in the boat's course, they replied that there was no change in the wind at all, and that the apparent change of the vane was simply due to the change of direction of the motion of the boat.
GRAHAM'S ZENITH SECTOR.
(From an old print.)
This supplied Bradley with a key to the solution of the mystery that had troubled him so long. It had been discovered long before this that light does not travel instantaneously from place to place, but takes an appreciable time to pass from one member of the solar system to another. This had been discovered by Römer from observations of the satellites of Jupiter. He had noted that the eclipses of the satellites always fell late of the computed time, when Jupiter was at his greatest distance from the earth; and Bradley's own work in the observation of those satellites had brought the fact most intimately under his own acquaintance. The result of the boating incident taught him, then, that he might look upon light as analogous to the wind blowing on the boat. As the wind, so long as it was steady, would seem to blow from one fixed quarter so long as the boat was also in rest, but as it seemed to shift its direction when the boat was moving and changed its direction, so he saw that the light coming from a particular star must seem to slightly change the direction in which it came, or, in other words, the apparent position of the star, to correspond with the movement of the earth in its orbit round the sun.
This was the celebrated discovery of the Aberration of Light, a triumph of exact observation and of clear insight. As to the exactness of Bradley's observations, it is sufficient to say that his determination of the value of the 'Constant of Aberration' gave it as 20·39´´; the value adopted to-day is 20·47´´.
On the death of Halley, in 1742, Bradley was appointed to succeed him. He found the Observatory in as utterly disheartening a condition as his predecessors had done. As already mentioned, Halley had not the same qualifications as an observer that Flamsteed had. He was, further, an old man when appointed to the post, he had no assistant provided for him, and the last five years of his life his health and strength had entirely given way. Under these circumstances, it was no wonder that Bradley found the instruments of the Observatory in a deplorable state. Nevertheless, he set to work most energetically, and in the year of his appointment he made 1500 observations in the last five months of the year. He was particularly earnest in examining the condition and the errors of his instruments; and as their defects became known to him, he was more and more anxious for a better equipment. He moved the Royal Society, therefore, to apply on his behalf for the instruments he required; and a petition from that body, in 1748, obtained what in those days must be considered the generous grant of £1000, the proceeds of the sale of old Admiralty stores. The principal instruments purchased therewith were a mural quadrant and a transit instrument, both eight feet in focal length, still preserved on the walls of the transit-room. It is interesting also to note that, following in the steps of Halley, and forecasting, as it were, the magnetic observatory which Airy would found, he devoted £20 of the grant to purchasing magnetic instruments.
Meantime he had continued his observations on aberration, and had discovered that the aberration theory was not sufficient entirely to account for the apparent changes in places of stars which he had discovered. A second cause was at work, a movement of the earth's axis, a 'wobble' in its inclination, technically known as Nutation, which is due to the action of the moon, and goes through its course in a period of nineteen years.
Beside these two great discoveries of aberration and nutation, Bradley's reputation rests upon his magnificent observations of the places of more than three thousand stars. This part of his work was done with such thoroughness, that the star-places deduced from them form the basis of most of our knowledge as to the actual movements of individual stars. In particular, he was careful to investigate and to correct for the errors of his instrument, and to determine the laws of refraction, introducing corrections for changes in the readings of thermometer and barometer. His tables of refraction were used, indeed, for seventy years after his death. Of his other labours it may be sufficient to refer to his determination of the longitudes of Lisbon and of New York, and to his effort to ascertain the parallax of the sun and moon, in combination with La Caille, who was observing at the Cape of Good Hope.
As Astronomer Royal, Bradley's great achievement was the high standard to which he raised the practical work of observation. From his day onwards, also, there was always at least one assistant. His first assistant was his own nephew, John Bradley, who received the munificent salary of ten shillings a week. Still, this was not out of proportion to the then salary of the Astronomer Royal, which practically amounted only to £90. However, in 1752, Bradley was awarded a Crown pension of £250 a year. He refused the living of Greenwich, which was offered him in order to increase his emoluments, on the ground that he could not suitably fulfil the double office. Bradley's later assistants were Charles Mason and Charles Green.
Bradley's last work was the preparation for the observations of the transit of Venus of 1761, according to the lines laid down by his predecessor, Halley. His health gave way, and he became subject to melancholia, so that the actual observations were taken by the Rev. Nathaniel Bliss, who succeeded him in his office after his death, in 1762. He was buried at Minchinhampton.
So far as we know Bradley's character, he seems to have been a gentle, modest, unassuming man, entirely free from self-seeking, and indifferent to personal gain. He was in many ways an ideal astronomer, exact, methodical, and conscientious to the last degree. His skill as an observer was his chief characteristic; and though his abilities were not equal as a mathematician or a mechanician, yet, on the one hand, he had a very clear insight into the meaning of his observations, and, on the other, he was skilful enough to himself adjust, repair, and improve his instruments.
Of Bradley's instruments, there are still preserved his famous twelve-and-a-half-foot zenith sector, with which he made his two great discoveries; his brass quadrant, which in 1750 he substituted for Halley's iron quadrant; his transit instrument, and equatorial sector. Bradley added to the buildings of the Observatory that portion which is now represented by the upper and lower computing rooms, and the chronometer room, which adjoins the latter. This room—the chronometer room—was his transit room, and the position of the shutters is still marked by the window in the roof.
The Rev. Nathaniel Bliss, who succeeded Bradley, only held the office for a couple of years, and during that time was much at Oxford. He, therefore, has left no special mark behind him as Astronomer Royal.
He was born November 28, 1700. His father, like himself, Nathaniel Bliss, was a gentleman, of Bisley, Gloucestershire.
NATHANIEL BLISS.
(From an engraving on an old pewter flagon.)
Bliss graduated at Pembroke College, Oxford, as B.A. in 1720, and M.A. in 1723. He became the Rector of St. Ebb's, Oxford, in 1736, and on Halley's death succeeded him as Savilian Professor of Geometry. He supplied Bradley with his observations of Jupiter's satellites, and from time to time, at his request, rendered him some assistance at the Royal Observatory. This was particularly the case, as has been already mentioned, with respect to the transit of Venus of 1761, the observations of which were carried out by Bliss, owing to Bradley's ill-health. It was natural, therefore, that on Bradley's death he should succeed to the vacant post; but he held it too short a time to do any distinctive work. Such observations as he made seem to have been entirely in continuation of Bradley's. He took a great interest, however, in the improvement of clocks, a department in which so much was being done at this time by Graham, Ellicott, and others.
Nevil Maskelyne, the fifth Astronomer Royal, was, like Bliss, a close friend of Bradley's. He was the third son of a wealthy country gentleman, Edmund Maskelyne, of Purton, in Wiltshire. Maskelyne was born in London, October 6, 1732, and was educated at Westminster School. Thence he proceeded to Cambridge, where he graduated seventh Wrangler in 1754. He was ordained to the curacy of Barnet in 1755, and, twenty years later, was presented by his nephew, Lord Clive, to the living of Shrawardine, in Shropshire. In 1782 he was presented by his college to the Rectory of North Runcton, Norfolk.
The event which turned his thoughts in the direction of astronomy was the solar eclipse of July 25, 1748; and about the time that he was appointed to the curacy of Barnet he became acquainted with Bradley, then the Astronomer Royal, to whom he gave great assistance in the preparation of his table of refractions.
Like Halley before him, he made an astronomical expedition to the island of St. Helena. This was for the special purpose of observing the transit of Venus of June 6, 1761, Bradley having induced the Royal Society to send him out for that purpose. Here he stayed ten months, and made many observations. But though the transit of Venus was his special object, it was not the chief result of the expedition: not because clouds hindered his observations, but because the voyage gave him the especial bent of his life.
Halley had actually held a captain's commission in the Royal Navy, and commanded a ship; Maskelyne, more than any of the Astronomers Royal before or since, made the improvement of the practical business of navigation his chief aim. None of all the incumbents of the office kept its original charter—'To find the so much desired Longitude at Sea, for the perfecting the Art of Navigation,' so closely before him.
The solution of the problem was at hand at this time—its solution in two different ways. On the one hand, the offer by the Government of a reward of £20,000 for a clock or watch which should go so perfectly at sea, notwithstanding the tossing of the ship and the wide changes of temperature to which it might be exposed, that the navigator might at any moment learn the true Greenwich time from it, had brought out the invention of Harrison's time-keeper; on the other hand, the great improvement that had now taken place in the computation of tables of the moon's motion, and the more accurate star-catalogues now procurable, had made the method of 'lunars,' suggested a hundred and thirty years before by the Frenchman, Morin, and others, a practicable one.
NEVIL MASKELYNE.
In principle, the method of finding the longitude from 'lunars,' that is to say, from measurements of the distances between the moon and certain stars, is an exceedingly simple one. In actual practice, it involves a very toilsome calculation, beside exact and careful observation. The principle, as already mentioned, is simply this: The moon travels round the sky, making a complete circuit of the heavens in between twenty-seven and twenty-eight days. It thus moves amongst the stars, roughly speaking, its own diameter, in about an hour. When once its movements were sufficiently well known to be exactly predicted, almanacs could be drawn up in which the Greenwich time of its reaching any definite point of the sky could be predicted long beforehand; or, what comes to the same thing, its distances from a number of suitable stars could be given for definite intervals of Greenwich time. It is only necessary, then, to measure the distances between the moon and some of these stars, and by comparing them with the distances given in the almanac, the exact time at Greenwich can be inferred. As has been already pointed out, the determination of the latitude of the ship and of the local time at any place where the ship is, is not by any means so difficult a matter; but the local time being known and the Greenwich time, the difference between these gives the longitude; and the latitude having been also ascertained, the exact position of the ship is known.
There are, of course, difficulties in the way of working out this method. One is, that whilst it takes the sun but twenty-four hours to move round the sky from one noon to the next, and consequently its movements, from which the local time is inferred, are fairly rapid, the moon takes nearly twenty-eight days to move amongst the stars from the neighbourhood of one particular star round to that particular star again. Consequently, it is much easier to determine the local time with a given degree of exactness than the Greenwich time; it is something like the difference of reading a clock from both hands and from the hour hand alone.
There are other difficulties in the case which make the computation a long and laborious one, and difficult in that sense; but they do not otherwise affect its practicability.
During this voyage to St. Helena, both when outward bound and when returning, Maskelyne gave the method of 'lunars' a very thorough testing, and convinced himself that it was capable of giving the information required. For by this time the improvement of the sextant, or quadrant as it then was, by the introduction of a second mirror, by Hadley, had rendered the actual observation at sea of lunar distances, and of altitudes generally, a much more exact operation.
This conclusion he put at once to practical effect, and, in 1763, he published the British Mariner's Guide, a handbook for the determination of the longitude at sea by the method of lunars.
At the same time, the other method, that by the time-keeper or chronometer, was practically tested by him. The time-keeper constructed by John Harrison had been tested by a voyage to Jamaica in 1761, and now, in 1763, another time-keeper was tested in a voyage to Barbadoes. Charles Green, the assistant at Greenwich Observatory, was sent in charge of the chronometer, and Maskelyne went with him to test its performance, in the capacity of chaplain to his Majesty's ship Louisa.
HADLEY'S QUADRANT.
(From an old print.)
The position which Maskelyne had already won for himself as a practical astronomer, and the intimate relations into which he had entered with Bradley and Bliss, made his appointment to the Astronomer Royalship, on the death of the latter, most suitable. At once he bent his mind to the completion of the revolution in nautical astronomy which his British Mariner's Guide had inaugurated, and in the year after his appointment he published the first number of the Nautical Almanac, together with a volume entitled, Tables Requisite to be Used with the Nautical Ephemeris, the value of which was so instantly appreciated, that 10,000 copies were sold at once.
The Nautical Almanac was Maskelyne's greatest work, and it must be remembered that he carried it on from this time up to the day of his death—truly a formidable addition to the routine labours of an Astronomer Royal who had but a single assistant on his staff. The Nautical Almanac was, however, in the main not computed at the Observatory; the calculations were effected by computers living in different parts of the country, the work being done in duplicate, on the principle which Flamsteed had inaugurated in the preparation of his Historia Cœlestis.
Maskelyne's next service to science was almost as important. He arranged that the regular and systematic publication of the observations made at Greenwich should be a distinct part of the duties of an Astronomer Royal, and he procured an arrangement by which a special fund was set apart by the Royal Society for printing them. His observations covering the years 1776 to 1811 fill four large folio volumes, and though, as already stated, he had but one assistant, they are 90,000 in number. Thus it was Maskelyne who first rendered effective the design which Charles II. had in the establishment of the Observatory. Flamsteed and Halley had been too jealous of their own observations to publish; Bradley's observations—though he himself was entirely free from this jealousy—were made, after his death, the subject of litigation by his heirs and representatives, who claimed an absolute property in them, a claim which the Government finally allowed. None of the three, however much their work ultimately tended to the improvement of the art of navigation, made that their first object. Whereas Maskelyne set this most eminently practical object in the forefront, and so gave to the Royal Observatory, which under his predecessors somewhat resembled a private observatory, its distinctive characteristics of a public institution.
It fell to Maskelyne to have to advise the Government as to the assignment of their great reward of £20,000 for the discovery of the longitude at sea. Maskelyne, while reporting favourably of the behaviour of Harrison's time-keeper, considered that the method of 'lunars' was far too important to be ignored, and he therefore recommended that half the sum should be given to Harrison for his watch, whilst the other half was awarded for the lunar tables which Mayer, before his death, had sent to the Board of Longitude. This decision, though there can be no doubt it was the right one, led to much dissatisfaction on the part of Harrison, who urged his claim for the whole grant very vigorously; and eventually the whole £20,000 was paid him. The whole question of rewards to chronometer-makers must have been one which caused Maskelyne much vexation. He was made the subject of a bitter and most voluminous attack by Thomas Mudge, for having preferred the work of Arnold and Earnshaw to his own.
Otherwise his reign at the Observatory seems to have been a singularly peaceful one, and there is little to record about it beyond the patient prosecution, year by year, of an immense amount of sober, practical work. To Maskelyne, however, we owe the practice of taking a transit of a star over five wires instead of over one, and he provided the transit instrument with a sliding eye-piece, to get over the difficulty of the displacement which might ensue if the star were observed askew when out of the centre of the field. To Maskelyne, too, we owe in a pre-eminent degree the orderly form of recording, reducing, and printing the observations. Much of the work in this direction which is generally ascribed to Airy was really due to Maskelyne. Indeed, without a wonderful gift of organization, it would have been impossible to plan and to carry the Nautical Almanac.
Beside the editing of various works intended for use in nautical astronomy or in general computation, the chief events of his long reign at Greenwich were the transit of Venus in 1769, which he himself observed, and for which he issued instructions in the Nautical Almanac; and his expedition in 1774 to Scotland, where he measured the deviation of the plumb-line from the vertical caused by the attraction of the mountain Schiehallion, deducing therefrom the mean density of the earth to be four and a half times that of water.
JOHN POND.
(From an old engraving.)
He died at the Observatory, February 9, 1811, aged 79, leaving but one child, a daughter, who married Mr. Anthony Mervin Story, to whom she brought the family estates in Wiltshire, inherited by Maskelyne on the deaths of his elder brothers, and, in consequence, Mr. Story added the name of Maskelyne to his own.
Maskelyne's character and policy as Astronomer Royal have been sufficiently dwelt upon. His private character was mild, amiable, and generous. 'Every astronomer, every man of learning, found in him a brother;' and, in particular, when the French Revolution drove some French astronomers to this country to find a refuge, they received from the Astronomer Royal the kindest reception and most delicate assistance.
Maskelyne added no instrument to the Observatory during his reign, though he improved Bradley's transit materially. He designed the mural circle, but it was not completed until after his death. His additions to the Observatory buildings consisted of three new rooms in the Astronomer Royal's house, and the present transit circle room.
John Pond was recommended by Maskelyne as his successor at Greenwich. At the time of his succession he was forty-four years of age, having been born in 1767. He was educated at Trinity College, Cambridge, and then spent some considerable time travelling in the south of Europe and Egypt. On his return home he settled at Westbury, where he erected an altazimuth by Troughton, with a two-and-a-half-foot circle. A born observer, his observations of the declinations of some of the principal fixed stars showed that the instrument which Maskelyne was using at Greenwich—the quadrant by Bird—could no longer be trusted. Maskelyne, in consequence, ordered a six-foot mural circle from Troughton, but did not live to see it installed, and in 1816 this was supplemented by Troughton's transit instrument of five inches aperture and ten feet focal length.
The introduction of these two important instruments, and of other new instruments, together with new methods of observation, form one of the chief characteristics of Pond's administration. Under this head must be specially mentioned the introduction of the mercury trough, both for determining the position of the vertical, and for obtaining a check upon the flexure of the mural circle in different positions; and the use in combination of a pair of mural circles for determining the declinations of stars.
Another characteristic of his reign was that under him there was the first attempt to give the Astronomer Royal a salary somewhat higher than that of a mechanic, and to support him with an adequate staff of assistants. His salary was fixed at £600 a year, and the single assistant of Maskelyne was increased to six.
This multiplication of assistants was for the purpose of multiplying observations, for Pond was the first astronomer to recognize the importance of greatly increasing the number of all observations upon which the fundamental data of astronomy were to be based.
In 1833 he finished his standard catalogue of 1113 stars, at that time the fullest of any catalogue prepared on the same scale of accuracy. 'It is not too much to say,' was the verdict of the Royal Astronomical Society, 'that meridian sidereal observation owes more to him than to all his countrymen put together since the time of Bradley.'
A yet higher testimony to the exactness of his work is given by his successor, Airy.
'The points upon which, in my opinion, Mr. Pond's claims to the gratitude of astronomers are founded, are principally the following. First and chief, the accuracy which he introduced into all the principal observations. This is a thing which, from its nature, it is extremely difficult to estimate now, so long after the change has been made; and I can only say that, so far as I can ascertain from books, the change is one of very great extent; for certainty and accuracy, astronomy is quite a different thing from what it was, and this is mainly due to Mr. Pond.'
The same authority eulogizes him further for his laborious working out of every conceivable cause or indication of error in his declination instruments, for the system which he introduced in the observation of transits, for the thoroughness with which he determined all his fundamental data, and for the regularity which he infused into the Greenwich observations.
One result of this great increase of accuracy was that Pond was able at once authoritatively to discard the erroneous stellar parallaxes that had been announced by Brinkley, Royal Astronomer for Ireland.
But Pond's administration was open, in several particulars, to serious censure, and the Board of Visitors, which had been for many years but a committee of the Royal Society, but which had recently been reconstituted, proved its value and efficiency by the remonstrances which it addressed to him, and which eventually brought about his resignation. His personal skill and insight as an observer were of the highest order; but either from lack of interest or failing health, he absented himself almost entirely from the Observatory in later years, visiting it only every ninth or tenth day. He had caused the staff of assistants to be increased from one to six, but had stipulated that the men supplied to him should be 'drudges.' His minute on the subject ran—
'I want indefatigable, hard-working, and, above all, obedient drudges (for so I must call them, although they are drudges of a superior order), men who will be contented to pass half their day in using their hands and eyes in the mechanical act of observing, and the remainder of it in the dull process of calculation.'
This was a fatal mistake, and one which it is very hard to understand how any one with a real interest in the science could have made. Men who had the spirit of 'drudges,' to whom observation was a mere 'mechanical act,' and calculation a 'dull process,' were not likely to maintain the honour of the Observatory, particularly under an absentee Astronomer Royal. Pond tried to overcome the difficulty by devising rules for their guidance of iron rigidity. The result was that after his resignation, in 1835, the First Lord and the Secretary of the Admiralty expressed their feeling to Airy, Pond's successor, 'that the Observatory had fallen into such a state of disrepute that the whole establishment should be cleared out.' A further evil was the excessive development of chronometer business, so as practically to swamp the real work of the Observatory, whilst the prices paid for the chronometers at this time were often much larger than would have been the case under a more business-like administration.
With all his merits, therefore, as an observer, the administration of Pond was, in some respects, the least satisfactory of all that the Observatory has known, and he alone of all the Astronomers Royal retired under pressure. He did not long survive his resignation, dying in September, 1836. He was buried by the side of Halley, in the churchyard at Lee.
Of Pond's instruments, the Observatory retains the fine transit instrument which was constructed by Troughton at his direction, and the mural circle, designed by Maskelyne, but which Pond was the first to use. Both of these have, of course, long been obsolete, and now hang on the walls of the transit room. The small equatorial, called, after its donor, the Shuckburgh equatorial, was also added in Pond's day, and though practically never used, still remains mounted in its special dome.