rable for the unexampled activity and great discoveries, which have been made. So important have been the labors of the practical astronomers, that a complete systein of the planetary motions might be deduced from the observations made during this time, and if all the previous observations, even to the most remote antiquity, were lost, the effect on the tables of the sun, planets, and satellites, would hardly be perceived, since the great accuracy of modern observations more than compensates for the shortness of the interval. It is proposed in this article to give a short account of some of the most noted discoveries during this period, to take a slight view of the latest and most correct tables of the motions of the planets and satellites, and to make such remarks on the labors of astronomers and mathematicians, as may be necessary in the notices of the works proposed to be reviewed.

The career of modern improvement was begun by Dr Bradley, one of the most indefatigable astronomers of the last century. He was remarkable for bis skill and accuracy, in tracing those minute changes in the places of the heavenly bodies, which had so much perplexed the astronomers who preceded him, and his labors were crowned with the most brilliant success, by the discovery of the Aberration of light and the Nutation of the earth's axis. His observations were so numerous, accurate, and important, that he may justly be placed in the same rank with Hipparchus and Tycho, the greatest and most accurate observers of ancient and modern times. He published an account of the aberration and nutation. His observations of the moon were also made public, and used by himself and others, in comparing and improving the lunar tables. A table of the places of 389 fixed stars was likewise deduced from his observations, and published by Dr Hornsby, but the great body of his observations, made at Greenwich while he was astronomer royal, were taken from the observatory by his executors, under the pretence that they were his private property, with the expectation of being paid for them by the government. A suit having been commenced for their recovery, the executors, in order to avoid it, presented them to Lord North, (so well known in the history of the American Revolution,) who gave them, in the year 1776, to the University of Oxford, of which he was Chancellor, upon the express condition, that they should immediately be

printed and published. But to the great disgrace of the University, and of Professor Hornsby, who had charge of the papers, they were withheld many years, notwithstanding the repeated solicitations and remonstrances of the Board of Longitude, who, in 1796, published several spirited resolutions, under the form of an appeal to the public, upon this very improper conduct.

These observations were made between the years 1750 and 1762, but it was not till the year 1798 that the first volume was published, and the whole was not completed till the year 1805, almost half a century after the observations had been made; and during the whole of this time, while unexampled progress was making in all branches of astronomy, these invaluable observations, which would have facilitated very much the calculations of astronomers, were lying almost useless.

But it may well be questioned whether this delay will, on the whole, be any disadvantage to the future progress of astronomy. For if these observations of the stars had been published soon after Bradley's death, they could not then have been reduced so accurately, as at the present moment, becarise the precise values of the small reductions to be made to the observations for precession, nutation, aberration, and refraction, were not so well known, and it was not then usual to take such pains in computing and combining together inany observations. Moreover, if the great labor of reducing the observations had been once gone through, even in a somewhat imperfect manner, it is probable that no one would have undertaken a new revision, as is the case with Flamsteed's observations. But, at the time of the publication of the observations, a considerable degree of interest had been excited, from the difficulties attending them, and this, with the well known accuracy of Bradley, was sufficient to procure an early and careful examination. Fortunately, at that time Bessel, the present astronomer royal at the observatory of Konigsberg, had just relinquished his mercantile pursuits, and with great success had devoted himself to astronomy. Having been furnished with a copy of Bradley's observations by Dr Olbers, he voluntarily undertook the task of reducing them, and no one was better qualified to do it, since he possessed, what is rarely united in the same individual, mathematical talents of the very first order, with great accuracy in


observations. The result of his labors is the important volume mentioned at the head of this article.

This work is divided into thirteen sections, in which Bessel successively treats of the various subjects connected with Bradley's observations, namely, the Instruments he used, and the corrections to be made to them. The Right Ascensions of his fundamental stars compared with the sun near the equi

The Latitude of Greenwich. The Refraction of the heavenly bodies, deduced solely from Bradley's observations, combining them together by an excellent theory, and with tables for the calculation, being more accurate than any tables of refraction, that had before been used by astronomers. The Obliquity of the Ecliptic from the observations of the solstices from 1753 to 1760. The Aberration of the fixed stars, with tables peculiarly adapted to the reduction of Bradley's observations, and an investigation of the quantity of the aberration, deduced from a great number of those observations, by which it would seem that the value of the aberration as found by Delambre, from the Eclipses of Jupiter's satellites, ought to be increased about a fortieth part. The Precession of the Equinoxes and Nutation. The Parallax of the fixed stars, which, by comparing a great number of Bradley's observations of the right ascensions of two stars on opposite meridians, (by which the effect is nearly doubled,) seems to be insensible.

But the most important part of the work is his excellent catalogue of 3222 fixed stars, in which the situation of each star is most commonly ascertained by several observations. In this catalogue he has given Flamsteed's numbers, their characters and magnitudes, also their right ascensions and declinations for the year 1755, with the annual precession for 1755 and 1800. The differences between the places of the stars and those in Piazzi's catalogue are likewise noted, with various references to other authors, who have observed the same stars. To this table is subjoined a smaller one of fortyeight stars, observed by Bradley, which cannot now be found in the places where he had marked them. Several were, without doubt, inserted by mistake, like that of writing down a wrong hour or minute of the time of observation, as is evident from the remarks on this table by Bessel, Burg, and Burckhardt, in Zach's Monatliche Correspondenz. One of these missing stars was, however, the planet Uranus, which was observed by Bradley, Dec. 3, 1753, and marked as a fixed star, without the least suspicion of its being a planet; he being less fortunate in this respect than Herschel, who, about thirty years afterwards, by repeating his observations on successive nights, detected its planetary nature by the change of place. Finally, Bessel devates one of the sections of his work to the consideration of the proper motions of the fixed stars, and pursuing the observation of Herschel, directs his attention, particularly, to the double stars, some of which indicate a mutual attraction between each other, and a revolution about their common centre of gravity. This is particularly the case with the star 61 Cygni, which is estimated by Bessel to perform its revolution in 350 years. The double star $ Ursæ Majoris, in 60 years, the double star p 70, Serpentarii, in about 50 years, and many others, noticed by Herschel and since by Struve, who has lately made many observations on such stars at the observatory of Dorpat.* J. W. Herschel has also given a valuable paper on these stars.

Bradley's chief excellence consisted in noting, with unexampled accuracy, the times of the transits of bodies over the meridian and their zenith distances, and he was not remarkable for noticing celestial phenomena of a different pature, neither were lis mathematical talents of the first order.

To form some idea of the accuracy of Dr Bradley's observations, and to shew at the same time what is now required of a first rate observer, it is only necessary to compare the results of the transits of fixed stars of the first and second magnitude, observed during one night, for the purpose of fixing the rate of the clock. From the mean of twelve observations of this kind, Bessel found the error of the clock to be about seventeen seconds and one fifth part of a second ; nine out of twelve observations did not differ one tenth of a second from the mean result, and the greatest difference did not exceed one third part of a second. The same degree of accuracy exists also in his right ascensions of the stars, since the results of successive years, when reduced to the same epoch, differ from each other but a small fraction of a second. His measures of zenith distances of the heavenly bodies were equally correct. An instance of which may be mentioned in the obliquity of the ecliptic for January 1, 1755, determined by the observations of fifteen solstices, from 1753 to 1760, to be 23° 28 15.” 44, and eleven out of the fifteen observations did not differ a single second, and the extreme difference was less than 3 seconds. Moreover, he found the observations of the summer solstices gave the same result as those of the winter, and in this respect, bis observations were free from the noted error, which existed for many years in those of his successor, Dr Maskelyne, who found, about the year 1795, the summer solstice gave for the obliquity 4 or 5 seconds more than the winter solstice, and a similar difference having been observed about that time by Piazzi, the question was started and much discussed, to account for this difference, and various hypotheses were proposed for that purpose. Among them the one that seemed most plausible was, that the refraction of the sun's rays was different from that of the fired stars, and, as the tables of refraction were founded on observations of the stars, a modification was proposed for solar observations. This discussion continued several years, and the true cause was not discovered, till Bradley's observations were published. It was then found by Bessel, that no such difference existed in the observations made by Bradley, when the instrument was new; that the error was not perceptible till the instrument had been used many years by Dr Maskelyne, and had become defective by constant use, so that at length there was an error of nearly 3” in the measure of these angles. Upon procuring a new circular instrument, this difference in the observations of the solstices ceased, and astronomers were enabled to determine the obliquity to a great degree of accuracy, which is a very important point, since this element enters in some way or other into almost every calculation of astronomy, and a change of a few seconds would, in some cases, affect the calculations considerably.

* It is rather strange that one of the best Observatories in Europe, as that at Dorpat undoubtedly is, should be situated in so high a latitude, being on the same parallel with the cold regions of Siberia. Notwithstanding this, the indefatigable Struve, overcoming the difficulties of the climate, has, in the course of a few years, published several volumes of excellent observations, which he has made at that place.

While Bradley was making his observations in Greenwich, his cotemporary, Tobias Mayer, was devoting his short, but extremely laborious and useful life, to the same pursuits in Got

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