صور الصفحة
PDF
النشر الإلكتروني

His friend, the Hon. Samuel Molyneux, had commenced a series of observations, with the view of solving the great problem of the parallax of the fixed stars; or of determining whether these bodies changed their apparent place in the heavens when seen from the two extremities of the earth's annual orbit,-a distance of nearly 200 millions of miles. Dr Hooke had long before constructed an instrument for making these observations, and had fixed upon the star (viz., Draconis) which was suitable for the purpose. Mr Molyneux made choice of the same star, and had bis instrument constructed on the same principles, by that celebrated artist Mr George Graham.

As soon as the apparatus was completed, it was erected, in November 1725, in Mr Molyneux's house ; a large mansion at the western extremity of Kew Green, which belonged to his wife, Lady Elizabeth Capel, and which afterwards became the residence of George III. The first observation was made with it on y Draconis on the 3d December 1725; and the observations were continued on the 5th, 11th, and 12th of the month, without any change of place being noticed in the star. On the 17th, however, Bradley observed that the star passed a little more southerly than before. Ascribing this result to the uncertainty of observation, the star was again observed on the 21st, when it was found to have passed still more southerlya result which surprised them extremely, as it was in a direction opposite to what would have been produced by an annual parallax. Being now satisfied that this southerly motion of the star was not owing to inaccuracy of observation, it naturally occurred to them, that it might have been occasioned by some change in the materials, &c., of the instrument. This cause was soon excluded, not only by a determination of the great exactness of the instrument, but by the continued southerly motion of the star, which began to return northwards again about the middle of April, and returned to its original place in December 1726, after having described an apparent orbit about 39 seconds in diameter.

The cause of this singular motion now remained to be ascertained; and it is curious to observe the progress of Bradley's mind in this enquiry. He proceeds by the method of continued hypothesis, excluding, by new observations which the hypothesis indicates, each successive conjecture, till he arrives at the one which is consonant with the whole mass of his observation. This is not the Baconian method of investigation; but it is the method by which all original minds pursue truth through the mazes of error, till they at last surprise her in her strongholds.

Bradley himself informs us, that a nutation of the earth's axis I was one of the first things that offered itself on this occasion,' as the cause of the apparent motion of the star. This hypothesis was excluded by observing the motion in other stars; and particularly in a small one opposite, in R. Ascension, toy Draconis, and nearly at the same distance from the North Pole of the Equator, which changed its declination only half as much as y Draconis, whereas the changes should have been nearly equal, if they had been produced by nutation.

The next cause that suggested itself was an alteration in the direction of the plumb line, with which the instrument was constantly rectified; but Bradley himself observes, that this upon trial proved insufficient.'

Another, and certainly a most ingenious hypothesis, presented itself. He considered what refraction might do,'-not of course the ordinary refraction of the atmosphere--but a refraction arising from an alteration in the figure of the atmosphere, which it might be supposed to experience if the earth moved in a resisting medium. On this supposition, the upper surface of the atmosphere would assume the figure of an oblong spheroid, having its largest diameter in the direction of the earth's motion, and the earth being nearest to the advancing end of the spheroid. · Hence it was inferred that y Draconis would appear to describe nearly a circle round its true place; and it was considered, upon the whole, that the telescopic star in Auriga, and the star in the head of Perseus, had motions consonant with this hypothesis. Plausible, however, as it was, it rested on a hypothetical assumption by no means probable, and was speedily banished by new and more accurate observations.

Although no progress was yet made in discovering the true cause of the phenomenon, still, as Professor Rigaud observes, the observations to which they had had recourse, as tests of the hypotheses which had occurred to them, had given a clue to fur

ther investigation;' and they had found that the motion in declination was somehow connected with the latitude of the star. In this difficulty, Bradley resolved to erect a new zenith sector for himself at Wansted, in order that he might observe a greater number of stars than could be seen in Molyneux's instrument. It was completed in 1727, and he inferred, from some of his earliest observations, that the maximum apparent motion of different stars was proportional to the sine of their latitude, if not, as he had at first supposed, to the latitude itself. This new hypothesis was disproved by more numerous observations; and being thus baffled in all his attempts at generalization, he began to despair of success. An idea, however, occurred to him by the merest accident, which suggested that cause of which he bad been so long in search. When he was sailing with a pleasure party on the river Thames, he remarked that every time the boat put about, the vane at the top of the boal's mast shisted a little, as if the wind had slightly changed its direction. Upon expressing to the sailors his surprise at the regular change in the wind, they assured him that it was only apparent, and was owing to the change in the direction of the boat. From that moment he conjectured that all the phenomena which he observed, arose from the progressive motion of light, combined with the earth's motion in its orbit.

In order to convey some idea of this curious discovery, let us suppose a shower of hail to fall perpendicularly on a calm day, and from a small height in the atmosphere, so that the hailstones may descend with very little velocity. If a carriage with its front windows open is standing beneath such a shower, not a single hailstone will enter it. But if the carriage advances in any direction with great velocity, the hailstones will enter the front windows as if they fell obliquely. When a given hailstone has reached the top of the window, the carriage and the window will have advanced a certain distance before the hailstone reaches the bottom of the window; so that it must necessarily fall within it. Now, as this must take place whether the carriage moves north, south, east, or west, the person within would necessarily conclude that the hailstones did not fall perpendicularly, or from the zenith, but from a point on one side of it; and that this point would describe a small circle round the zenith of the carriage, moved in every possible direction.

Hence it is easy to understand how a star in the zenith must appear at a little distance from the zenith to a spectator who is carried along with the earth in its annual motion.

Roemer, the discoverer of the successive propagation of light, had computed that it moved from the sun to the earth in 11 minutes, while other astronomers had adopted 7 minutes as the most accurate result. Bradley found, from his observations, that is his theory was correct, light should move from the sun to the earth in 8 minutes 12 seconds, a result intermediate between those above mentioned.

It is scarcely necessary to inform the reader, that the happy idea which Bradley had been so fortunate as to seize, furnished him with a complete explanation of all the varied phenomena which he had observed. The discovery of the aberration of the stars, as it was called, established his reputation as the first astronomer of the age, and extended his fame throughout all Europe.

Among the conclusions which Bradley deduced from this discovery, we may mention two of considerable importance in physics. He inferred, that in the same medium, light was propagated with the same velocity after it had been reflected as before it; and that the light of small as well as of great stars moved with the same velocity.

* All those computations,' says Professor Rigaud, speaking of the velocity assigned to light, 'must of course be founded on the supposition of the light reaching us with the same uniform velocity from all the heavenly bodies. Bradley considered this to be established, and so did Clairaut; but if a difference should be clearly ascertained in the maximum of aberration, as derived from the motions of different stars, some modification of this supposition will become necessary. This, however, is a question which requires all the resources of modern astronomy to determine it.'

The important question to which Professor Rigaud here refers, may be considered as determined, within certain limits, by a series of accurate experiments made by M. Fraunhofer and M. Soldner, at Munich. With an instrument of great delicacy, which required to be used simultaneously by two observers looking through different telescopes, they determined that the refractions of the light of different fixed stars did not differ from one another, nor from that of the planets. A difference, amounting to the scoth part of the whole refractions, could be distinctly perceived with this instrument; and this difference would not amount to the fourth part of a second in the horizontal refraction of the atmosphere. Fraunhofer intended to continue these observations with still nicer instruments ; but a premature death has deprived science of his invaluable aid.

In the year 1729, Bradley undertook to deliver the Lectures on Experiinental Philosophy at Oxford, a duty which be continued to discharge till April 1760. In 1731 he was an unsuccessful candidate for the office of Keeper of the Ashmolean Museum ; and in 1732 he quitted Wansted, and took up bis residence at Oxford.

During the next ten years of our author's life, he does not seem to have been much occupied with his favourite science; and Professor Rigaud has not been able to collect much information respecting his labours in this period. He communicated to the Royal Society in 1734, a paper on the vibrations of the pendulum in different latitudes, and on the figure of the earth which might be deduced from them; and in the year 1738 he transmitted to the same learned body his observations on the comet of 1737.

Bradley was now destined to reach the highest object of his ambition. After the death of Halley in 1742, he was, through the influence of George, Earl of Macclesfield, appointed Astro

nomer-Royal of England. Upon his arrival at Greenwich, in June, he found the instruments in the Observatory in the most wretched condition, and his first cares were devoted to their repair and improvement. With an assistant, carefully trained by himself, he began his labours with a zeal and ardour which it is difficult to estimate. No fewer than 18,000 observations were made in the year 1743 by Bradley and his nephew. He made also a series of experiments on the length of the pendulum, between 1743 and 1749; and be observed with great care the comets of 1743, 1744, and 1748.

In his observations on the aberration of the stars, Bradley remarked that the stars near the equinoctial colure, changed their declination about two seconds more in a year, than they would do if the precession of the equinoxes was only fifty seconds; while those near the solstitial colure changed their declination less than they ought, if its precession were exactly that quantity: Satisfied of the accuracy of his instrument, he continued to observe these stars with the utmost care, with the view of discovering the cause of this apparent anomaly; and having completed his series of observations throughout a complete revolution of the moon's node, he succeeded, in 1747, in establishing his second great discovery of the nutation of the earth's axis. A motion in the earth's axis had been anticipated from theoretical considerations ; but Bradley had the sole merit, not only of determining it by accurate observations, but of ascertaining that it arises from the action of the moon upon the equatorial parts of the earth. The radius of the small circle which the earth's axis thus describes during a revolution of the lunar node, was proved by Bradley to be 9 seconds; and Dr Brinkley, Bishop of Cloyne, from a great number of the finest observations, has, in our own day, proved it to be 9 seconds.

This remarkable discovery was communicated to the Royal Society of London, in a letter to George, Earl of Macclesfield, dated December 31st, 1747; and we owe it to the memory of that distinguished Astronomer and patron of science, to insert the introductory paragraphs of Dr Bradley's letter, which we can assure our readers are not written in the language of adulation.

• The advantages arising from different persons attempting to settle. the same points of astronomy near the same time are so much the greater, as a concurrence in the result would remove all suspicion of incorrectness in the instruments made use of. For which reason I esteem the curious apparatus at Shirburn Castle (the seat of the Earl of Macclesfield), and the observations there taken, as a most valuable criterion, whereby I may judge of the accuracy of those that are made at the Royal Observatory; and, as a lover of science, I cannot but wish that

« السابقةمتابعة »