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

noxes.

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 equiThe 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 devotes 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 nature, neither were his 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

* 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.

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 fixed 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.

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

tingen, and with great success. Mayer's lunar tables, for which his widow received a large reward from the Commissioners of Longitude of Great Britain, first gave the moon's place in the heavens with sufficient accuracy to determine the longitude. at sea, which has conduced so much to the safety and rapidity of modern navigation, and to the immense improvements, which have been made in geography within the last fifty years. Mayer possessed much more mathematical knowledge than Bradley, but had not his fine tact in observing, neither were the instruments he used so perfect.

About the same time that Mayer and Bradley were observing in the northern hemisphere, La Caille, at the Cape of Good Hope, was forming his catalogue of ten thousand stars of the southern hemisphere. His labors were immense, and it has been asserted, that he made more observations and calculations, than all the astronomers of his time taken together.

The taste for making improvements in the instruments and in the methods of observing, which began with Dr Bradley, has continued to the present time, and a regular series of observations has been obtained, from which new and complete tables of the motions of the planets and satellites have been formed, exceeding all expectation; so that an astronomer could now predict, for a thousand years to come, the precise moment of the passage of any one of those bodies over the meridian wire of the telescope of his transit instrument, with such a degree of accuracy, that the error would not be so great as to remove the object through an angular space corresponding to the semidiameter of the finest wire that could be made; and a body, which by the tables ought to appear in the transit instrument in the middle of that wire, would in no case be removed to its outer edge. In this work of improvement there were many cooperators, and the artist, by the perfection of his instruments, the astronomer by his observations, and the mathematician by his analysis, have mutually assisted each other. With the excellent instruments, made by Graham, Dr Bradley discovered that apparent motion of the fixed stars, which depends on the nutation of the earth's axis, and soon afterwards D'Alembert explained, upon correct principles, the physical causes of that motion, and gave formulas for computing it, shewing at the same time,

that the apparent motion of the pole of the earth was not in a circle, as Bradley and Machin had supposed, but in an ellipsis of considerable eccentricity. Many instances of a similar nature have occurred during the last century, and it has almost always happened that the English have furnished the best artists, the best instruments, the best practical astronomers, and the best observations, except in the case of the four small planets, lately discovered, while their continental neighbors, particularly the French and Germans, have made the improvements in analysis, and the deductions from the English observations, which were necessary for the computation of the present accurate tables of the motions of the heavenly bodies.

The decided superiority of the English artists in the construction of astronomical instruments, for the measure of angles, has been generally acknowledged by all the astronomers of Europe. So long ago as the year 1736, when the French Academicians were sent to the north to make observations for ascertaining the figure of the earth, the famous English artist, Mr Graham, was thought the fittest person in Europe to supply them with instruments. No greater proof of his superiority could have been given, than his being thus employed by the ministers of a rival nation in a work of such celebrity. Graham and Bird furnished the instruments for the observatory of Greenwich, when Bradley was appointed astronomer royal, and the same instruments were afterwards used by Dr Maskelyne in making his important observations. Bird's mural quadrants were famed over all Europe for their accuracy. He made them for the observatories of Greenwich, Paris, Petersburgh, Oxford, Manheim, Gottingen, Cadiz, &c. To him succeeded Ramsden, whose skill as an artist far surpassed that of any other man of his time. The mural quadrant be made for the observatory at Blenheim, was considered a most excellent instrument, and his me'ridian circles were still more. complete. He made one of these circles for Piazzi at Palermo, with which that celebrated astronomer made the observations for his great catalogue of the fixed stars. He also made that, which Dr Brinkley is now so successfully using at the observatory of Dublin. His great theodolite, used by General Roy in the survey of the English coast, is famed for its accuracy and completeness.