Astronomical Discovery Part 7

But we have a.s.sumed the star to lie in the plane of the earth's...o...b..t; 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.

[Sidenote: Delay in finding real explanation.]

[Sidenote: Bradley sets up another instrument at Wansted.]

[Sidenote: Finds the right clue.]

[Sidenote: A wind-vane on a boat.]

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 acc.u.mulate more facts. Molyneux's instrument only allowed of the observation of two stars, [gamma] Draconis and the small star above mentioned. 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 pa.s.s it through. The object-gla.s.s 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. We all know the story of the apple falling and setting Newton to think about the causes of gravitation. It was a similarly trivial circ.u.mstance 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. 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 a.s.sured 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 ill.u.s.tration which has become cla.s.sical.

[Ill.u.s.tration: FIG. 5.]

[Sidenote: a.n.a.logy 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 a.n.a.logy 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 recoit pas la moindre goutte de pluie; si la voiture avance avec rapidite, 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.

[Sidenote: Aberration.]

Applying it to the phenomenon which it is intended to ill.u.s.trate, 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.

[Sidenote: 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 quant.i.ty 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 const.i.tution 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 quant.i.ties in the air. But meantime this discovery was made in another way.

[Sidenote: Still something to be explained.]

[Sidenote: Probably nutation.]

[Sidenote: His nineteen years' campaign.]

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. 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 pa.s.sed into other hands. 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.

[Sidenote: 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 cla.s.ses 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 cla.s.ses 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 a.s.signed 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.

[Sidenote: Astronomer Royal at Greenwich.]

[Sidenote: Letter from Earl of Macclesfield.]

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 a.s.surance 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. "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 compet.i.tor 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.

[Sidenote: 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 a.s.sistant, 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!

[Sidenote: New instruments.]

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. 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 princ.i.p.al 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.

[Sidenote: 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 t.i.tle to fame. Bradley accomplished them all in addition to his great discoveries of aberration and nutation.

[Sidenote: Might have found variation of lat.i.tude.]

And with a little more opportunity he might have added another great discovery which has shed l.u.s.tre 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 Kustner 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 lat.i.tude." 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.

[Sidenote: Oxford's tardy recognition of Bradley.]

The att.i.tude of our leading universities towards science and scientific men is of sufficient importance to justify another glance at the relations between Bradley and Oxford. 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 a.s.sembly 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."

[Sidenote: 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.

CHAPTER IV