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.