CHAPTER III BRADLEY'S DISCOVERIES OF THE ABERRATION OF LIGHT AND OF THE NUTATION OF THE EARTH'S AXIS
In examining different types of astronomical discovery, we shall find certain advantages in varying to some extent the method of presentation. In the two previous chapters our opportunities for learning anything of the life and character of those who made the discoveries have been slight; but I propose to adopt a more directly biographical method in dealing with Bradley’s discoveries, which are so bound up with the simple earnestness of his character that we could scarcely appreciate their essential features properly without some biographical study. But the record of his life apart from his astronomical work is not in any way sensational; indeed it is singularly devoid of incident. He had not even a scientific quarrel. There was scarcely a man of science of that period who had not at least one violent quarrel with some one, save only Bradley, whose gentle nature seems to have kept him clear of them all. Judged by ordinary standards his life was uneventful: and yet it may be doubted whether, to him who lived it, that life contained one dull moment. Incident came for him in his scientific work: in the preparation of apparatus, the making of observations, above all in the hard-thinking which he did to get at the clue which would explain them; and after reviewing his biography,[2] I think we shall be inclined to admit that if ever there was a happy life, albeit one of unremitting toil, it was that of James Bradley.
Bradley’s birth and early life.
He was born at Sherbourn, in Gloucestershire, in 1693. We know little of his boyhood except that he went to the Grammar School at Northleach, and that the memory of this fact was preserved at the school in 1832 when Rigaud was writing his memoir. [The school is at present shut up for want of funds to carry it on; and all inquiries I have made have failed to elicit any trace of this memory.] Similarly we know little of his undergraduate days at Oxford, except that he entered as a commoner at Balliol in 1710, took his B.A. in the regular course in 1714, and his M.A. in 1717. As a career he chose the Church, being ordained in 1719, and presented to the vicarage of Bridstow in Monmouthshire; but he only discharged the duties of vicar for a couple of years, for in 1721 he returned to Oxford as Professor of Astronomy, an appointment which involved the resignation of his livings; and so slight was this interruption to his career as an astronomer that we may almost disregard it, and consider him as an astronomer from the first.Brief clerical career. But to guard against a possible misconception, let me say that Bradley entered on a clerical career in a thoroughly earnest spirit; to do otherwise would have been quite foreign to his nature. When vicar of Bridstow he discharged his duties faithfully towards that tiny parish, and moreover was so active in his uncle’s parish of Wansted that he left the reputation of having been curate there, although he held no actual appointment. And thirty years later, when he was Astronomer Royal and resident at Greenwich, and when the valuable vicarage of Greenwich was offered to him by the Chancellor of the Exchequer, he honourably refused the preferment, “because the duty of a pastor was incompatible with his other studies and necessary engagements.”
Learnt astronomy not at Oxford,
But now let us turn to Bradley’s astronomical education. I must admit, with deep regret, that we cannot allow any of the credit of it to Oxford. There was a great astronomer in Oxford when Bradley was an undergraduate, for Edmund Halley had been appointed Savilian Professor of Geometry in 1703, and had immediately set to work to compute the orbits of comets, which led to his immortal discovery that some of these bodies return to us again and again, especially the one which bears his name—Halley’s Comet—and returns every seventy-five years, being next expected about 1910. But there is no record that Bradley came under Halley’s teaching or influence as an undergraduate. In later years the two men knew each other well, and it was Halley’s one desire towards the close of his life that Bradley should succeed him as Astronomer Royal at Greenwich; a desire which was fulfilled in rather melancholy fashion, for Halley died without any assurance that his wish would be gratified. But Bradley got no astronomical teaching at Oxford either from Halley or others.but from his uncle, James Pound. The art of astronomical observation he learnt from his maternal uncle, the Rev. James Pound, Rector of Wansted, in Essex. He is the man to whom we owe Bradley’s training and the great discoveries which came out of it. He was, I am glad to say, an Oxford man too; very much an Oxford man; for he seems to have spent some thirteen years there migrating from one Hall to another. His record indeed was such as good tutors of colleges frown upon; for it was seven years before he managed to take a degree at all; and he could not settle to anything. After ten years at Oxford he thought he would try medicine; after three years more he gave it up and went out in 1700 as chaplain to the East Indies. But he seems to have been a thoroughly lovable man, for news was brought of him four years later that he had a mind to come home, but was dissuaded by the Governor saying that “if Dr. Pound goes, I and the rest of the Company will not stay behind.” Soon afterwards the settlement was attacked in an insurrection, and Pound was one of the few who escaped with his life, losing however all the property he had gradually acquired. He returned to England in 1706, and was presented to the living of Wansted; married twice, and ended his days in peace and fair prosperity in 1724. Such are briefly the facts about Bradley’s uncle, James Pound;Pound a first-rate observer. but the most important of all remains to be told—that somehow or other he had learnt to make first-rate astronomical observations, how or when is not recorded; but in 1719 he was already so skilled that Sir Isaac Newton made him a present of fifty guineas for some observations; and repeated the gift in the following year; and even three years before this we find Halley writing to ask for certain observations from Mr. Pound.
With this excellent man Bradley used frequently to stay. To his nephew he seems to have been more like a father than an uncle. When his nephew had smallpox in 1717, he nursed him through it; and he supplemented from his own pocket the scanty allowance which was all that Bradley’s own father could afford. But what concerns us most is that he fostered, if he did not actually implant, a love of astronomical observation in his nephew.Bradley worked with him. The two worked together, entering their observations one after the other on the same paper; and it was to the pair of them together, rather than to the uncle alone, that Newton made his princely presents, and Halley wrote for help in his observations. There seems to be no doubt that the uncle and nephew were about this time the best astronomical observers in the world. There was no rivalry between them, and therefore there is no need to discuss whether the partnership was one of equal merit on both sides; but it is interesting to note that it probably was. The ability of Pound was undoubted; many were keenly desirous that he, and not his nephew, should be elected to the Oxford Chair in 1721, but he felt unequal to the duties at his advanced age. On the other hand, when Bradley lost his uncle’s help, there was no trace of faltering in his steps to betray previous dependence on a supporting or guiding hand. He walked erect and firm, and trod paths where even his uncle might not have been able to follow.
The work done by Pound and Bradley.
A few instances will suffice to show the kind of observations made by this notable firm of Pound and Bradley. They observed the positions of the fixed stars and nebulÆ: these being generally the results required by Halley and Newton. They also observed the places of the planets among the stars, and especially the planet Mars, and determined its distance from the Earth by the method of parallax, thus anticipating the modern standard method of finding the Sun’s distance; and though with their imperfect instruments they did not obtain a greater accuracy than 1 in 10, still this was a great advance on what had been done before, and excited the wonder and admiration of Halley. They also paid some attention to double stars, and did a great deal of work on Jupiter’s satellites. We might profitably linger over the records of these early years, which are full of interest, but we must press on to the time of the great discoveries, and we will dismiss them with brief illustrations of three points: Bradley’s assiduity, his skill in calculation, and his wonderful skill in the management of instruments. Of his assiduity an example is afforded by his calculations of the orbits of two comets which are still extant. One of them fills thirty-two pages of foolscap, and the other sixty; and it must be remembered that the calculations themselves were quite novel at that time. Of his skill in calculation, apart from his assiduity, we have a proof in a paper communicated to the Royal Society rather later (1726), where he determines the longitudes of Lisbon and New York from the eclipses of Jupiter’s satellites, using observations which were not simultaneous, and had therefore to be corrected by an ingenious process which Bradley devised expressly for this purpose.Use of very long telescopes. And finally, his skill in the management of instruments is shown by his measuring the diameter of the planet Venus with a telescope actually 212¼ feet in length. It is difficult for us to realise in these days what this means; even the longest telescope of modern times does not exceed 100 feet in length, and it is mounted so conveniently with all the resources of modern engineering, in the shape of rising floors, &c., that the management of it is no more difficult than that of a 10-foot telescope. But Bradley had no engineering appliances beyond a pole to hold up one end of the telescope and his own clever fingers to work the other; and he managed to point the unwieldy weapon accurately to the planet, and measure the diameter with an exactness which would do credit to modern times.Reason for great length. A few words of explanation may be given why such long telescopes were used at all. The reason lay in the difficulty of getting rid of coloured images, due to the composite character of white light. Whenever we use a single lens to form an image, coloured fringes appear. Nowadays we know that by making two lenses of different kinds of glass and putting them together, we can practically get rid of these coloured fringes; but this discovery had not been made in Bradley’s time. The only known ways of dealing with the evil then were to use a reflecting telescope like Newton and Gregory, or if a lens was used, to make one of very great focal length; and hence the primary necessity for these very long telescopes. They had another advantage in producing a large image, or they would probably have given way to the reflector. This advantage is gradually bringing them back into use, and perhaps in the eclipse of 1905 we may use a telescope as long as Bradley’s; but we shall not use it as he did in any case. It will be laid comfortably flat on the ground, and the rays of light reflected into it by a coelostat.
Bradley appointed at Oxford,
In 1721 Bradley was appointed to the Savilian Professorship of Astronomy at Oxford, vacant by the death of Dr. John Keill. Once it became clear that there was no chance of securing his uncle for this position, Bradley himself was supported enthusiastically by all those whose support was worth having, especially by the Earl of Macclesfield, who was then Lord Chancellor; by Martin Foulkes, who was afterwards the President of the Royal Society; and by Sir Isaac Newton himself. He was accordingly elected on October 31, 1721, and forthwith resigned his livings. His resignation of the livings was necessitated by a definite statute of the University relating to the Professorship, and not by the existence of any very onerous duties attaching to it; indeed such duties seem to have been conspicuously absent,but continues to work at Wansted. and after Bradley’s election he passed more time than ever with his uncle in Wansted, making the astronomical observations which both loved; for there was not the vestige of an observatory in Oxford. His uncle’s death in 1724 interrupted the continuity of these joint observations, and by an odd accident prepared the way for Bradley’s great discovery. He was fain to seek elsewhere that companionship in his work which had become so essential to him, and his new friend gave a new bent to his observations.
Samuel Molyneux.
Samuel Molyneux was a gentleman of fortune much attached to science, and particularly to astronomy, who was living about this time at Kew. He was one of the few, moreover, who are not content merely to amuse themselves with a telescope, but had the ambition to do some real earnest work, and the courage to choose a problem which had baffled the human race for more than a century. The theory of Copernicus, that the earth moved round the sun, necessitated a corresponding apparent change in the places of the stars, one relatively to another; and it was a standing difficulty in the way of accepting this theory that no such change could be detected. In the old days before the telescope it was perhaps easy to understand that the change might be too small to be noticed, but the telescope had made it possible to measure changes of position at least a hundred times as small as before, and still no “parallax,” as the astronomical term goes, could be found for the stars. The observations of Galileo, and the measures of Tycho BrahÉ, as reduced to systematic laws by Kepler, and finally by the great Newton, made it clear that the Copernican theory was true: but no one had succeeded in proving its truth in this particular way.Attempts to find stellar parallax. Samuel Molyneux must have been a man of great courage to set himself to try to crack this hard nut; and we can understand the attraction which his enterprise must have had for Bradley, who had just lost the beloved colleague of many courageous astronomical undertakings. His co-operation seems to have been welcomed from the first; his help was invited and freely given in setting up the instrument, and he fortunately had the leisure to spend considerable time at Kew making the observations with Molyneux, just as he had been wont to observe with his uncle.
I must now briefly explain what these observations were. There is a bright star ? Draconis, which passes almost directly overhead in the latitude of London. Its position is slowly changing owing to the precession of the equinoxes, but for two centuries it has been, and is still, under constant observation by London astronomers owing to this circumstance, that it passes directly overhead, and so its position is practically undisturbed by the refraction of our atmosphere.
It was therefore thought at the time that, there being no disturbance from refraction, the disturbance from precession being accurately known, and there being nothing else to disturb the position but “parallax” (the apparent shift due to the earth’s motion which it was desirable to find), this star ought to be a specially favourable object for the determination of parallax. Indeed it had been announced many years before by Hooke that its parallax had been found; but his observations were not altogether satisfactory, and it was with a view of either confirming them or seeing what was wrong with them that Molyneux and Bradley started their search. They set up a much more delicate piece of apparatus than Hooke had employed.The instrument. It was a telescope 24 feet long pointed upwards to the star, and firmly attached to a large stack of brick chimneys within the house. The telescope was not absolutely fixed, for the lower end could be moved by a screw so as to make it point accurately to the star, and a plumb-line showed how far it was from the vertical when so pointing. Hence if the star changed its position, however slightly, the reading of this screw would show the change.Expected results. Now, before setting out on the observations, the observers knew what to expect if the star had a real parallax; that is to say, they knew that the star would seem to be farthest south in December, farthest north in June, and at intermediate positions in March and September; though they did not know how much farther south it would appear in December than in June—this was exactly the point to be decided.
Fig. 2.
The reason of this will be clear from Fig. 2. [Remark, however, that this figure and the corresponding figure 4 do not represent the case of Bradley’s star, ? Draconis: another star has been chosen which simplifies the diagram, though the principle is essentially the same.] Let A B C D represent the earth’s orbit, the earth being at A in June, at B in September, and so on, and let K represent the position of the star on the line D B. Then in March and September it will be seen from the earth in the same direction, namely, D B K; but the directions in which it is seen in June and December, viz. A K and C K, are inclined in opposite ways to this line. The farther away the star is, the less will this inclination or “parallax” be; and the star is actually so far away that the inclination can only be detected with the utmost difficulty: the lines C K and A K are sensibly parallel to D B K. But Bradley did not know this; it was just this point which he was to examine, and he expected the greatest inclination in one direction to be in December. Accordingly when a few observations had been made on December 3, 5, 11, and 12 it was thought that the star had been caught at its most southerly apparent position, and might be expected thereafter to move northwards, if at all.Unexpected results. But when Bradley repeated the observation on December 17, he found to his great surprise that the star was still moving southwards. Here was something quite new and unexpected, and such a keen observer as Bradley was at once on the alert. He soon found that the changes in the position of the star were of a totally unexpected character. Instead of the extreme positions being occupied in June and December, they were occupied in March and September, just midway between these. And the range in position was quite large, about 40—not a quantity which could have been detected in the days before telescopes, but one which was unmistakable with an instrument of the most moderate measuring capacity.
Tentative explanations.
What, then, was the cause of this quite unforeseen behaviour on the part of the star? The first thought of the observers was that something might be wrong with their instrument, and it was carefully examined, but without result. The next was that the apparent movement was in the plumb-line, the line of reference. If the whole earth, instead of carrying its axis round the sun in a constant direction, were to be executing an oscillation, then all our plumb-lines would oscillate, and when the direction of a star like ? Draconis was compared with that of the plumb-line it would seem to vary, owing actually to the variation in the plumb-line. The earth might have a motion of this kind in two ways, which it will be necessary for us to distinguish, and the adopted names for them are “nutation of the axis” and “variation of latitude” respectively. In the case of nutation the North Pole remains in the same geographical position, but points to a different part of the heavens. The “variation of latitude,” on the other hand, means that the North Pole wanders about on the earth itself. We shall refer to the second phenomenon more particularly in the sixth chapter.
Nutation?
But it was the first kind of change, the nutation, which Bradley suspected; and very early in the series of observations he had already begun to test this hypothesis. If it was not the star, but the earth and the plumb-line, which were in motion, then other stars ought to be affected. The telescope had been deliberately restricted in its position to suit ? Draconis; but since the stars circle round the Pole, if we draw a narrow belt in the heavens with the Pole as centre, and including ? Draconis, the other stars included would make the same circuit, preceding or following ? Draconis by a constant interval. Most of them would be too faint for observation with Bradley’s telescope; but there was one bright enough to be observed, which also came within its limited range, and it was promptly put under surveillance when a nutation of the earth’s axis was suspected. Careful watching showed that it was not affected in the same way as ? Draconis, and hence the movement could not be in the plumb-line. Was there, then, after all, some effect of the earth’s atmosphere which had been overlooked? We have already remarked that since the star passes directly overhead there should be practically no refraction; and this assumption was made by Molyneux and Bradley in choosing this particular star for observation. It follows at once, if we assume that the atmosphere surrounds the earth in spherical layers.Anomalous refraction. But perhaps this was not so? Perhaps, on the contrary, the atmosphere was deformed by the motion of the earth, streaming out behind her like the smoke of a moving engine? No possibility must be overlooked if the explanation of this puzzling fact was to be got at.
Fig. 3.
The way in which a deformation of the atmosphere might explain the phenomenon is best seen by a diagram. First, it must be remarked that rays of light are only bent by the earth’s atmosphere, or “refracted,” if they enter it obliquely.
If the atmosphere were of the same density throughout, like a piece of glass, then a vertical ray of light, A B (see Fig. 3), entering the atmosphere at B would suffer no bending or refraction, and a star shining from the direction A B would be seen truly in that direction from C. But an oblique ray, D E, would be bent on entering the atmosphere at E along the path EF, and a star shining along D E would appear from F to be shining along the dotted line G E F. The atmosphere is not of the same density throughout, but thins out as we go upwards from the earth; and in consequence there is no clear-cut surface, B E, and no sudden bending of the rays as at E: they are gradually bent at an infinite succession of imaginary surfaces. But it still remains true that there is no bending at all for vertical rays; and of oblique rays those most oblique are most bent.
Fig. 4.
Now, suppose the atmosphere of the earth took up, owing to its revolution round the sun, an elongated shape like that indicated in diagram 4, and suppose the star to be at a great distance away to the right of the diagram. When the earth is in the position labelled “June,” the light would fall vertically on the nose of the atmosphere at A, and there would be no refraction. Similarly in “December” the light would fall at C on the stern, also vertically, and there would be no refraction. [The rays from the distant star in December are to be taken as sensibly parallel to those received in June, notwithstanding that the earth is on the opposite side of the sun, as was remarked on p. 98.] But in March and September the rays would strike obliquely on the sides of the supposed figure, and thus be bent in opposite directions, as indicated by the dotted lines; and the extreme positions would thus occur in March and September, as had been observed. The explanation thus far seems satisfactory enough.
But we have assumed the star to lie in the plane of the earth’s orbit; and the stars under observation by Bradley did not lie in this plane, nor did they lie in directions equally inclined to it. Making the proper allowance for their directions, it was found impossible to fit in the facts with this hypothesis, which had ultimately to be abandoned.
Delay in finding real explanation.
It is remarkable to find that two or three years went by before the real explanation of this new phenomenon occurred to Bradley, and during this time he must have done some hard thinking. We have all had experience of the kind of thinking if only in the guessing of conundrums. We know the apparent hopelessness of the quest at the outset: the racking of our brains for a clue, the too frequent despair and “giving it up,” and the simplicity of the answer when once it is declared. But with scientific conundrums the expedient of “giving it up” is not available. We must find the answer for ourselves or remain in ignorance; and though we may feel sure that the answer when found will be as simple as that to the best conundrum, this expected simplicity does not seem to aid us in the search. Bradley was not content with sitting down to think: he set to work to accumulate more facts. Molyneux’s instrument only allowed of the observation of two stars, ? Draconis and the small star above mentioned.Bradley sets up another instrument at Wansted. Bradley determined to have an instrument of his own which should command a wider range of stars; and by this time he was able to return to his uncle’s house at Wansted for this purpose. His uncle had been dead for two or three years, and the memory of the loss was becoming mellowed with time. His uncle’s widow was only too glad to welcome back her nephew, though no longer to the old rectory, and she allowed him to set up a long telescope, even though he cut holes in her floor to pass it through. The object-glass end was out on the roof and the eye end down in the coal cellar; and accordingly in this coal cellar Bradley made the observations which led to his immortal discovery. He had a list of seventy stars to observe, fifty of which he observed pretty regularly. It may seem odd that he did not set up this new instrument at Oxford, but we find from an old memorandum that his professorship was not bringing him in quite £140 a year, and probably he was glad to accept his aunt’s hospitality for reasons of economy. By watching these different stars he gradually got a clear conception of the laws of aberration. The real solution of the problem, according to a well-authenticated account, occurred to him almost accidentally.Finds the right clue. We all know the story of the apple falling and setting Newton to think about the causes of gravitation. It was a similarly trivial circumstance which suggested to Bradley the explanation which he had been seeking for two or three years in vain. In his own words, “at last, when he despaired of being able to account for the phenomena which he had observed, a satisfactory explanation of them occurred to him all at once when he was not in search of it.” He accompanied a pleasure party in a sail upon the river Thames. The boat in which they were was provided with a mast which had a vane at the top of it. It blew a moderate wind, and the party sailed up and down the river for a considerable time.A wind-vane on a boat. Dr. Bradley remarked that every time the boat put about the vane at the top of the boat’s mast shifted a little, as if there had been a slight change in the direction of the wind. He observed this three or four times without speaking; at last he mentioned it to the sailors, and expressed his surprise that the wind should shift so regularly every time they put about. The sailors told him that the wind had not shifted, but that the apparent change was owing to the change in the direction of the boat, and assured him that the same thing invariably happened in all cases. This accidental observation led him to conclude that the phenomenon which had puzzled him so much was owing to the combined motion of light and of the earth. To explain exactly what is meant we must again have recourse to a diagram; and we may also make use of an illustration which has become classical.
Fig. 5.
Analogy of rain.
If rain is falling vertically, as represented by the direction A B; and if a pedestrian is walking horizontally in the direction C D, the rain will appear to him to be coming in an inclined direction, E F, and he will find it better to tilt his umbrella forwards. The quicker his pace the more he will find it advisable to tilt the umbrella. This analogy was stated by Lalande before the days of umbrellas in the following words: “Je suppose que, dans un temps calme, la pluie tombe perpendiculairement, et qu’on soit dans une voiture ouverte sur le devant; si la voiture est en repos, on ne reÇoit pas la moindre goutte de pluie; si la voiture avance avec rapiditÉ, la pluie entre sensiblement, comme si elle avoit pris une direction oblique.” Lalande’s example, modified to suit modern conditions, has been generally adopted by teachers, and in examinations candidates produce graphic pictures of the stationary, the moderate-paced, and the flying, possessors of umbrellas.
Aberration.
Applying it to the phenomenon which it is intended to illustrate, if light is being received from a star by an earth, travelling across the direction of the ray, the telescope (which in this case represents the umbrella) must be tilted forward to catch the light. Now on reference to Fig. 4 it will be seen that the earth is travelling across the direction of rays from the star in March and September; and in opposite directions in the two cases. Hence the telescope must be tilted a little, in opposite directions, to catch the light; or, in other words, the star will appear to be farthest south in March, farthest north in September. And so at last the puzzle was solved, and the solution was found, as so often happens, to be of the simplest kind; so simple when once we know, and so terribly hard to imagine when we don’t! It may comfort us in our struggles with minor problems to reflect that Bradley manfully stuck to his problem for two or three years. It was probably never out of his thoughts, waking or sleeping; when at work it was the chief object of his labours, and when on a pleasure party he was ready to catch at the slightest clue, in the motion of a wind-vane on a boat, which might help him to the solution.
Results of discovery.
The discovery of aberration made Bradley famous at a bound. Oxford might well be proud of her two Savilian Professors at this time, for they had both made world-famous discoveries—Halley that of the periodicity of comets, and Bradley of the aberration of light. How different their tastes were and how difficult it would have been for either to do the work of the other! Bradley was no great mathematician, and though he was quite able to calculate the orbit of a comet, and carried on such work when Halley left it, it was probably not congenial to him. Halley, on the other hand, almost despised accurate observations as finicking. “Be sure you are correct to a minute,” he was wont to say, “and the fractions do not so much matter.” With such a precept Bradley would never have made his discoveries. No quantity was too small in his eyes, and no sooner was the explanation of aberration satisfactorily established than he perceived that though it would account for the main facts, it would not explain all. There was something left. This is often the case in the history of science. A few years ago it was thought that we knew the constitution of our air completely—oxygen, nitrogen, water vapour, and carbonic acid gas; but a great physicist, Lord Rayleigh, found that after extracting all the water and carbonic acid gas, all the oxygen and all the nitrogen, there was something left—a very minute residuum, which a careless experimenter would have overlooked or neglected, but which a true investigator like Lord Rayleigh saw the immense importance of. He kept his eye on that something left, and presently discovered a new gas which we now know as argon. Had he repeated the process, extracting all the argon after the nitrogen, he might have found by a scrutiny much more accurate still yet another gas, helium, which we now know to exist in extremely minute quantities in the air. But meantime this discovery was made in another way.
Still something to be explained.
When Bradley had extracted all the aberration from his observations he found that there was something left, another problem to be solved and some more thinking to be done to solve it. But he was now able to profit by his previous labours, and the second step was made more easily than the first. The residuum was not the parallax of which he had originally been in search, for it did not complete a cycle within the year; it was rather a progressive change from year to year. But there was an important clue of another kind. He saw that the apparent movements of all stars were in this case the same; and he knew that a movement of this kind can be referred, not to the stars themselves, but to the plumb-line from which their directions are measured.Probably nutation. He had thought out the possible causes of such a movement of the plumb-line or of the earth itself, and had realised that there might be a nutation which would go through a cycle in about nineteen years, the period in which the moon’s nodes revolve. He was not mathematician enough to work out the cause completely, but he saw clearly that to trace the whole effect he must continue the observations for nineteen years; and accordingly he entered on this long campaign without any hesitation. His instrument was still that in his aunt’s house at Wansted, where he continued to live and make the observations for a few years, but in 1732 he removed to Oxford, as we shall see, and he must have made many journeys between Wansted and Oxford in the course of the remaining fifteen years during which he continued to trace out the effects of nutation. His aunt too left Wansted to accompany Bradley to Oxford, and the house passed into other hands.His nineteen years’ campaign. It is to the lasting credit of the new occupant, Mrs. Elizabeth Williams, that the great astronomer was allowed to go on and complete the valuable series of observations which he had commenced. Bradley was not lodged in her house; he stayed with a friend close by on his visits to Wansted, but came freely in and out of his aunt’s old home to make his observations. How many of us are there who would cheerfully allow an astronomer to enter our house at any hour of the night to make observations in the coal-cellar! It says much, not only for Bradley’s fame, but for his personal attractiveness, that he should have secured this permission, and that there should be no record of any friction during these fifteen years. At the end of the whole series of nineteen years his conclusions were abundantly verified, and his second great discovery of nutation was established. Honours were showered upon him, and no doubt the gentle heart of Mrs. Elizabeth Williams was uplifted at the glorious outcome of her long forbearance.
Residence at Oxford.
But we may now turn for a few moments from Bradley’s scientific work to his daily life. We have said that in 1732, after holding his professorship for eleven years, he first went definitely to reside in Oxford. He actually had not been able to afford it previously. His income was only £140 a year, and the statutes prevented him from holding a living: so that he was fain to accept Mrs. Pound’s hospitable shelter. But in 1729 an opportunity of adding to his income presented itself, by giving lectures in “experimental philosophy.” The observations on nutation were not like those on aberration: he was not occupied day and night trying to find the solution: he had practically made up his mind about the solution, and the actual observations were to go on in a quiet methodical manner for nineteen years, so that he now had leisure to look about him for other employment. Dr. Keill, who had been Professor of Astronomy before Bradley, had attracted large classes to lectures, not on astronomy, but on experimental philosophy: but had sold his apparatus and goodwill to Mr. Whiteside, of Christ Church, one of the candidates who were disappointed by Bradley’s election. In 1729 Bradley purchased the apparatus from Whiteside, and began to give lectures in experimental philosophy. His discovery of aberration had made him famous, so that his classes were large from the first, and paid him considerable fees. Suddenly therefore he changed his poverty for a comfortable income, and he was able to live in Oxford in one of two red brick houses in New College Lane, which were in those days assigned to the Savilian Professors (now inhabited by New College undergraduates). His aunt, Mrs. Pound, to whom he was devotedly attached, came with him, and two of her nephews. In his time of prosperity Bradley was thus able to return the hospitality which had been so generously afforded him in times of stress.
Astronomer Royal at Greenwich.
Before he completed his observations for nutation another great change in his fortunes took place. In 1742 he was elected to succeed Halley as Astronomer Royal. It was Halley’s dying wish that Bradley should succeed him, and it is said that he was even willing to resign in his favour, for his right hand had been attacked by paralysis, and the disease was gradually spreading. But he died without any positive assurance that his wish would be fulfilled. The chief difficulty in securing the appointment of Bradley seems to have been that he was the obvious man for the post in universal opinion.Letter from Earl of Macclesfield. “It is not only my friendship for Mr. Bradley that makes me so ardently wish to see him possessed of the position,” wrote the Earl of Macclesfield to the Lord Chancellor; “it is my real concern for the honour of the nation with regard to science. For as our credit and reputation have hitherto not been inconsiderable amongst the astronomical part of the world, I should be extremely sorry we should forfeit it all at once by bestowing upon a man of inferior skill and abilities the most honourable, though not the most lucrative, post in the profession (a post so well filled by Dr. Halley and his predecessor), when at the same time we have amongst us a man known by all the foreign, as well as our own astronomers, not to be inferior to either of them, and one whom Sir Isaac Newton was pleased to call the best astronomer in Europe.” And again, “As Mr. Bradley’s abilities in astronomical learning are allowed and confessed by all, so his character in every respect is so well established, and so unblemished, that I may defy the worst of his enemies (if so good and worthy a man have any) to make even the lowest or most trifling objection to it.”“After all,” the letter goes on, “it may be said if Mr. Bradley’s skill is so universally acknowledged, and his character so established, there is little danger of opposition, since no competitor can entertain the least hope of success against him. But, my lord, we live in an age when most men how little soever their merit may be, seem to think themselves fit for whatever they can get, and often meet with some people, who by their recommendations of them appear to entertain the same opinion of them, and it is for this reason that I am so pressing with your lordship not to lose any time.”
Such recommendations had, however, their effect: the dreaded possibility of a miscarriage of justice was averted, and Bradley became the third Astronomer Royal, though he did not resign his professorship at Oxford. Halley, Bradley, and Bliss, who were Astronomers Royal in succession, all held the appointment along with one of the Savilian professorships at Oxford; but since the death of Bliss in 1761, the appointment has always gone to a Cambridge man.
Instruments very defective.
When Bradley went to Greenwich, in June 1742, he was at first unable to do much from the wretched state in which he found the instruments. Halley was not a good observer: his heart was not in the work, and he had not taken the trouble to set the instruments right when they went wrong. The counterpoises of that instrument which ought to have been the best in the world at the time rubbed against the roof so that the telescope could scarcely be moved in some positions: and some of the screws were broken. There was no proper means of illuminating the cross-wires, and so on. With care and patience Bradley set all this right, and began observations. He had the good fortune to secure the help of his nephew, John Bradley, as assistant, and the companionship seems to have been as happy as that previous one of James Bradley and his uncle Pound. John Bradley was able to carry on the observations when his uncle was absent in Oxford, and the work the two got through together in the first year is (in the words of Bradley’s biographer Rigaud) “scarcely to be credited.” The transit observations occupy 177 folio pages, and no less than 255 observations were taken on one night. And at the same time, it must be remembered, Bradley was still carrying on his nutation observations at Wansted, still lecturing at Oxford, and not content with all this, began a course of experiments on the length of the seconds’ pendulum. Truly a giant for hard work!
But, in spite of his care in setting them right, the instruments in the Observatory were found to be hopelessly defective. The history of the instruments at the Royal Observatory is a curious one. When Flamsteed was appointed the first Astronomer Royal he was given the magnificent salary of £100 a year, and no instruments to observe with. He purchased some instruments with his own money, and at his death they were claimed by his executors. Hence Halley, the second Astronomer Royal, found the Observatory totally unprovided in this respect. He managed to persuade the nation to furnish the funds for an equipment; but Halley, though a man of great ability in other ways, did not know a good instrument from a bad one; so that Bradley’s first few years at the Observatory were wasted owing to the imperfection of the equipment.New instruments. When this was fully realised he asked for funds to buy new instruments, and such was the confidence felt in him that he got what he asked for without much difficulty. More than £1000, a large sum for those days, was spent under his direction, the principal purchases being two quadrants for observation of the position of the stars, one to the north and the other to the south. With these quadrants, which represented the perfection of such apparatus at that time, Bradley made that long and wonderful series of observations which is the starting-point of our knowledge of the movements of the stars. The instruments are still in the Royal Observatory, the more important of the two in its original position as Bradley mounted it and left it.
Work at Greenwich.
It seems needless to mention his work as Astronomer Royal, but I will give quite briefly a summary of what he accomplished, and then recall a particular incident, which shows how far ahead of his generation his genius for observation placed him. The summary may be given as follows. We owe to Bradley—
1. A better knowledge of the movements of Jupiter’s satellites.
2. The orbits of several comets calculated directly from his own observations, when such work was new and difficult.
3. Experiments on the length of the pendulum.
4. The foundation of our knowledge of the refraction of our atmosphere.
5. Considerable improvements in the tables of the moon, and the promotion of the method for finding the longitude by lunar distances.
6. The proper equipment of our national Observatory with instruments, and the use of these to form the basis of our present knowledge of the positions and motions of the stars.
Many men would consider any one of these six achievements by itself a sufficient title to fame. Bradley accomplished them all in addition to his great discoveries of aberration and nutation.
And with a little more opportunity he might have added another great discovery which has shed lustre on the work of the last decade. We said earlier in this chapter that the axis of the earth may move in one or two ways. Either it may point to a different star, remaining fixed relatively to the earth, as in the nutation which Bradley discovered; or it may actually change its position in the earth. This second kind of movement was believed until twenty years ago not to exist appreciably; but the work of KÜstner and Chandler led to the discovery that it did exist, and its complexities have been unravelled, and will be considered in the sixth chapter. Now a century and a half ago Bradley was on the track of this “variation of latitude.” His careful observations actually showed the motion of the pole, as Mr. Chandler has recently demonstrated; and, moreover, Bradley himself noticed that there was something unexplained. Once again there was a residuum after (first) aberration and (next) nutation had been extracted from the observations; and with longer life he might have explained this residuum, and added a third great discovery to the previous two. Or another coming after him might have found it; but after the giant came men who could not tread in his footsteps, and the world waited 150 years before the discrepancy was explained.
The attitude of our leading universities towards science and scientific men is of sufficient importance to justify another glance at the relations between Bradley and Oxford.Oxford’s tardy recognition of Bradley. We have seen that Oxford’s treatment of Bradley was not altogether satisfactory. She left him to learn astronomy as he best could, and he owes no teaching to her. She made him Professor of Astronomy, but gave him no observatory and a beggarly income which he had to supplement by giving lectures on a different subject. But when he had disregarded these discouragements and made a name for himself, Oxford took her share in recognition. He was created D.D. by diploma in 1742; and when his discovery of nutation was announced in 1748, and produced distinctions and honours of all kinds from over the world, we are told that “amidst all these distinctions, wide as the range of modern science, and permanent as its history, there was one which probably came nearer his heart, and was still more gratifying to his feeling than all. Lowth (afterwards Bishop of London), a popular man, an elegant scholar, and possessed of considerable eloquence, had in 1751 to make his last speech in the Sheldonian Theatre at Oxford as Professor of Poetry. In recording the benefits for which the University was indebted to its benefactors, he mentioned the names of those whom Sir Henry Savile’s foundation had established there: ‘What men of learning! what mathematicians! we owe to Savile, Briggs, Wallis, Halley; to Savile we owe Greaves, Ward, Wren, Gregory, Keill, and one whom I will not name, for posterity will ever have his name on its lips.’ Bradley was himself present; there was no one in the crowded assembly on whom the allusion was lost, or who did not feel the truth and justice of it; all eyes were turned to him, while the walls rung with shouts of heartfelt affection and admiration; it was like the triumph of Themistocles at the Olympic games.”
The study of “residual phenomena.”
These words of Rigaud indicate the fame deservedly acquired by an earnest and simple-minded devotion to science: but can we learn anything from the study of Bradley’s work to guide us in further research? The chief lessons would seem to be that if we make a series of careful observations, we shall probably find some deviation from expectation: that we must follow up this clue until we have found some explanation which fits the facts, not being discouraged if we cannot hit upon the explanation at once, since Bradley himself was puzzled for several years: that after finding one vera causa, and allowing for the effect of it, the observations may show traces of another which must again be patiently hunted, even though we spend nineteen years in the chase: and that again we may have to leave the complete rectification of the observations to posterity. But though we may admit the general helpfulness of these directions, and that this patient dealing with residual phenomena seems to be a method capable of frequent application, we cannot deduce any universal principle of procedure from them: witness the difficulty of dealing with meteorological observations, for instance. It is not always possible to find any orderly arrangement of the residuals which will give us a clue to start with. When such an arrangement is manifested, we must certainly follow up the clue; it would almost seem that no expense should be prohibitive, since it is impossible to foresee the importance of the result.
