47. Tycho Brahe, a Danish nobleman, began his studies about the same time with the Landgrave of Hesse, and observed the great conjunction of Jupiter and Saturn; but, finding the usual instruments very inaccurate, he constructed many others much larger and more exact. In 1571 he discovered a new star in the chair of Cassiopeia; which induced him, like Hipparchus on a similar occasion, to make a new catalogue of the stars; which he composed to the number of 777, and adapted their places to the year 1600. In 1576, by the favor of the king of Denmark, he built his new observatory, called Uraniburg, on the small island Huenna, opposite to Copenhagen, which he very amply furnished with many large instruments, some of them so divided as to show single minutes, and in others the arch might be read off to ten seconds. One quadrant was divided according to the method invented by Nonius, that is by forty-seven concentric circles; but most of them were divided by diagonals; a method of division invented by Richard Chanceler, an Englishman. Tycho employed his time at Uraniburg to the best advantage, till the death of the king, when, falling into discredit, he was obliged to remove to Holstein: he afterwards introduced himself to the emperor Rodolph, with whom he continued at Prague till his death in 1601. Tycho was the inventor of a system of astronomy, a kind of semi-Ptolemaic, which he vainly endeavored to establish instead of the Copernican. His numerous works, however, show that he was a man of great abilities; and his discoveries, together with those of Purbach and Regiomontanus, were collected and published together in 1621, by Longomontanus, the favorite disciple of Tycho.

48. Tycho, while residing at Prague with the emperor, prevailed on Kepler to leave the university of Glatz, and to come to him; and Tycho dying in 1601, Kepler enjoyed all his life the title of mathematician to the emperor, who ordered him to finish the tables of Tycho Brahe, which he published in 1627, under the title of Rodolphine. He died about A. D. 1630, at Ratisbon, where he was soliciting the arrears of his pension. From his own observations and those of Tycho, Kepler discovered several of the true laws of nature, by which the motions of the celestial bodies are regulated. He discovered that all the planets revolve about the sun, not in circular, but in elliptical orbits, having the sun in one of the foci of the ellipse; that their motions are not equable, but varying, quicker or slower as they are near to the sun, or farther from him; that the areas described by the variable line drawn from the planet to the sun, are equal in equal times, and always proportional to the times of describing them; and that the cubes of the distances of the planets from the sun, were in the same proportion as the squares of their periodica! times of revolution. By observations also on comets, he concluded that they are freely

carried about among the orbits of the planets, in paths that are nearly rectilinear, but which he could not then determine.

49. At this time there were many other good proficients in astronomy; as Wright, Napier, Bayer, &c. Wright made several good meridional observations of the sun, with a quadrant of six feet radius, in the years 1594, 1595, and 1596; from which he greatly improved the theory of the sun's motion, and computed more accurately his declination, than any person had done before. In 1599 he published also, an excellent work, entitled, Certain Errors in Navigation discovered and detected,' containing a method which has commonly, though erroneously, been ascribed to Mercator. To Napier we owe some excellent theorems and improvements in spherics, besides the ever-memorable invention of logarithms. Bayer, a German, published his Uranometria, or the figures of all the constellations visible in Europe, with the stars marked on them, and accompanied by names, or the letters of the Greek alphabet; a contrivance by which they may easily be referred to with distinctness and precision.

50. About the same time, astronomy was cultivated abroad by Mercator, Maurolycus, Maginus, Homelius, Schultet, Stevin, Galileo, &c. and in England by Thomas and Leonard Digges, John Dee, Robert Flood, Harriot, &c. The beginning of the seventeenth century was particularly distinguished by the invention of telescopes, and the application of them to astronomical observations. The more distinguished early observations with the telescope, were made by Galileo, Harriot, Huygens, Hook, Cassini, &c. It is said that, from report only, Galileo made for himself telescopes, by which he discovered inequalities in the moon's surface, Jupiter's satellites, and the ring of Saturn; also spots on the sun, by which he found out the revolution of that luminary on its axis; and he discovered that the nebula and milky way were full of small stars.

51. Mr. Harriot, who had previously been known only as an algebraist, made much the same discoveries as Galileo, and as early, if not more so, as appears by his papers in the possession of the earl of Egremont. And Mr. Horrox, a young astronomer of great talents, found out in 1633, that the planet Venus would pass over the sun's disc on the twenty-fourth of November 1639; an event which he announced only to his friend Crabtree; and these two were the only persons in the world that observed this transit. Horrox made also many other useful observations, and had even formed a new theory of the moon, taken notice of by Newton; but his early death, in the beginning of 1640, put a stop to his valuable labors.

52. Hevelius, Burgomaster of Dantzick, flourished about the same time, and observed the spots and phases of the moon; from which observations he compiled his Selenographia. An account of his apparatus is contained in his work entitled Machina Cælestis, a book now very scarce, as most of the copies were accidentally burnt, with the whole house and apparatus, in 1679 Hevelius died in 1688, aged 76.

53. Doctor Hook, a contemporary of Hevelius invented instruments with telescopic sights, and ceusured the others. This occasioned a sharp dispute between them; to settle which, Halley was sent over to Hevelius to examine his instruments. The two astronomers made several observations together, very much to their satisfaction; and amongst them was one of an occultation of Jupiter by the moon, when they determined the diameter of the latter to be 30′ 33′′.

54. Huygens and Fontana, before the middle of the seventeenth century, greatly improved the construction of telescopes. The former constructed one of 123 feet, with which he observed the moon and planets, and discovered that Saturn was encompassed with a ring. With telescopes too, of 200 and 300 feet focus, Cassini saw five satellites of Saturn, with his zones or belts, and the shadows of Jupiter's satellites passing over his body. In 1666 Azout applied a micrometer to telescopes, to measure the diameters of the planets, and other small distances in the heavens: but an instrument of this kind had been invented before, by Gascoigne, though it was but little known abroad. To obviate the difficulties arising from the great lengths of refracting telescopes, and the aberration of the rays, Mersennus, in a letter to Descartes, first started the idea of making telescopes of reflectors, instead of lenses; and in 1663 James Gregory of Aberdeen showed how such a telescope might be constructed.

55. Sir Isaac Newton, after spending some time on the construction of both sorts of telescopes, found out the great inconvenience which arises to refractors from the different refrangibility of the rays of light; for which not finding a remedy, and pursuing the other kind, in 1672, he presented to the Royal Society two reflectors, constructed with spherical speculums. The inconvenience, however, arising from the different refrangibility of the rays of light, has since been fully obviated by Dollond.

56. Towards the end of the seventeenth, and beginning of the eighteenth century, practical astronomy rather languished; but the speculative part was carried to the highest perfection by Newton in his Principia, by David Gregory, Keil, and others. Soon after this, great improvements in astronomical instruments began to take place, particularly in Britain. Graham not only improved clocks and watch work, but also carried the accuracy of astronomical instruments to a surprising degree. He constructed the old eight feet mural arch at the Royal Observatory, Greenwich, and a small equatorial sector for making observations out of the meridian; but he is chiefly remarkable for contriving the zenith sector of twenty-four feet radius, and afterwards one of twelve feet and a half, with which Bradley discovered the aberration of the fixed stars. The reflecting telescope of Gregory and Newton was greatly improved by Hadley, who presented a very powerful instrument of that kind to the Royal Society in 1719. He invented also the reflecting quadrant or sector, now called by his name, presented to the society in 1731, and now universally used at sea. It appears, however, that an instrument similar to this in its princiVOL. III.

ples, had been invented by Newton; and a description, with a drawing of it, was given by him to Halley, when he was preparing for his voyage in 1701, to discover the variation of the needle: it has also been asserted, that Godfrey of Philadelphia, in America, made the same discovery, and the first instrument of this kind.

57. About the middle of this century, the constructing and dividing of large astronomical instruments were carried to great perfection by Bird, and reflecting telescopes were not less improved by Short, who first executed the divided object glass micrometer, which had been proposed and described by Louville and others. Dollond also improved refracting telescopes, by means of his achromatic glasses: and the discoveries of Herschel are owing to the amazing powers of reflectors of his own construction. Thus, the astronomical improvements in the present century have been chiefly owing to the inventions of, and improvements in, the instruments, and to the establishment of regular observatories in England, France, and other parts of Europe.

58. Roemer, a celebrated Danish astronomer, first made use of a meridional telescope; and, by observing the eclipses of Jupiter's satellites, first discovered the progressive metion of light, concerning which he read a dissertation before the Academy of Sciences at Paris, in 1675. Flamsteed, appointed the first astronomer royal at Greenwich, in 1675, observed for forty-four years, and gave a catalogue of 3000 stars with their places, to the year 1689; also new solar tables, and a theory of the moon according to Horrox; likewise, in Sir Jonas Moore's System of Mathematics, he gave a curious tract on the sphere, showing how to construct, geometrically, eclipses both of the sun and moon, as well as occultations of the fixed stars by the moon. On his observations were founded both Halley's tabies, and Newton's theory of the moon. Cassini, the first French astronomer royal, made many observations on the sun, moon, planets, and comets, greatly improved the elements of their motions, erected the gnomon, and drew the celebrated meridian line in the church of Petronia at Bologna.

59. Flamsteed was succeeded, in 1719, as astronomer royal at Greenwich, by Dr. Halley, who had been sent at the early age of twentyone, to the island of St. Helena, to observe the southern stars and make a catalogue of them, which was published in 1679. In 1705 he published his Synopsis Astronomia Cometica, in which he ventured to predict the return of a comet in 1758 or 1759. Ile first discovered the acceleration of the moon, and gave a very ingenious method for finding her parallax, by three observed phases of a solar eclipse; published in the Philosophical Transactions many learned papers, and amongst them, some concerning the use that might be made of the next transit of Venus, in determining the distance of the sun from the earth; composed tables of the sun, moon, and all the planets, which are still in great repute; and recommended the method of determining the longitude, by the moon's dis tances from the sun, and certain fixed stars; a method which was first proposed by Warner,

and which has since been carried into execution.

60. A dispute concerning the figure of the earth took place about this time. Newton had determined, from a consideration of the laws of gravity, and the diurnal motion of the earth, that the figure of it was an oblate spheroid; but Cassini, from the measures of Picart, supposed it to be an oblong spheroid. To settle this dispute it was resolved, under Louis XV. to measure two degrees of the meridian; one near the equator, and the other as near the pole as possible. For this purpose, the Royal Academy of Sciences sent to Lapland, Maupertuis, Clairault, Camus, and Lemonnier: who were accompanied by Outhier, and Celsus, professor of anatomy at Upsal. On the southern expedition were sent Godin, Condamine, and Bouguer, to whom the king of Spain joined George Juan and Antonio de Ulloa. These set out in 1735, and returned at different times 1744, 1745, and 1746; but the former party who set out only in 1736, returned the year following; having both fulfilled their commissions. Picart's measure was revised by Cassini and De la Caille, which, after his errors were corrected, was found to agree very well with the other two; and the result of the whole served to confirm the determination of the figure before laid down by Newton. On the southern expedition, the attraction of the great mountains of Peru was found to have a sensible effect on the plumb-line of one of their largest instruments, deflecting it seven or eight seconds from the true perpendicular.

61. In 1742 Dr. Bradley succeeded, on the death of Dr. Halley, as astronomer royal at Greenwich. The accuracy of his observations enabled him to detect the smaller inequalities in the motions of the planets and fixed stars. The consequence of his accuracy was, the discovery of the aberration of light, the nutation of the earth's axis, and a much greater degree of perfection in lunar tables. He observed the places, and computed the elements of the comets which appeared in the years 1723, 1736, 1743, and 1757; made new and more accurate tables of the motions of Jupiter's satellites, and, from a multitude of observations of the luminaries, constructed a table of refractions; which has ever since been in very general estimation for its accuracy, though it is now generally admitted that it gives the refractions too small. He also, with a very large transit instrument, and a new mural quadrant of eight feet radius, constructed by Bird in 1750, made an immense number of observations for settling the places of all the stars in the British catalogue, together with nearly 1500 places of the moon, the greater part of which he compared with Mayer's tables. Bradley died in 1762. 62. Astronomers elsewhere were equally assiduous in their endeavours to promote this science. The theory of the moon was particularly considered by Clairault, D'Alembert, Euler, Mayer, Simpson, and Walmsley, and especially Clairault, Euler, and Mayer, who computed complete sets of lunar tables: those of the last of these authors, for their superior accuracy, were rewarded with a premium of £3000, and brought into use in the computation

of the Nautical Ephemeris, published by the Board of Longitude. The most accurate tables of the satellites of Jupiter were composed from observations by Wargentin, an excellent Swedish astronomer. But these have again been superseded by the more recent ones of Delambre. There is much room for improvement, however, in our knowledge of the elements of Jupiter's satellites, even with respect to the first satellites, the predicted and actual times of immersion or emersion sometimes differ to the extent of two minutes.

63. Among the many French astronomers who contributed to the advancement of the science, it was particularly indebted to De la Caille for an excellent set of solar tables. He, in 1750, went to the Cape of Good Hope to make observations in concert with the most celebrated astronomers in Europe, for determining the parallax of Mars and the moon, and thence that of the sun, which it was concluded did not much exceed ten seconds. Here he re-examined and adjusted, with great accuracy, the places of stars about the southern pole; and also measured a degree of the meridian. In Italy the science was assiduously cultivated by Bianchini, Boscovich, Frisi, Manfredi, Zanotti, and many others; in Sweden, by Wargentin, already mentioned, Blingenstern, Mallet, and Planman; and in Germany by Euler, Mayer, Lambert, Grischow, and others.

64. In 1760 all the learned societies in Europe made preparations for observing the transit of Venus over the sun, which had been predicted by Halley more than eighty years before, with the use that might be made of it in determining the sun's parallax, and the distances of the pla nets from the sun. The same exertions were repeated, to observe the transit in 1769, by sending observers to different parts of the world; and from the whole, Short computed that the sun's parallax was nearly 83 seconds, and consequently the distance of the sun from the earth about 24,114 of the earth's diameters, or ninety-six millions of miles. Bradley was succeeded, in 1762, in his office of astronomer royal, by Bliss, Savilian professor of astronomy; who, being in a declining state of health, did not long enjoy it.

65. In 1765 Bliss was succeeded by Nevil Maskelyne, who, in January 1761, was sent by the Royal Society, at a very early age, to the island of St. Helena, to observe the transit of Venus over the sun, and the parallax of the star Sirius. The first of these objects partly failed, by clouds preventing the sight of the second internal contact; and the second also, owing to Short having suspended the plumb-line by a loop from the neck of the central pin. However, he indemnified himself by many other valuable observations: thus, he observed at St. Helena, the tides; the horary parallaxes of the moon; and the going of a clock, to find by comparison with its previous going, which had been observed in England, the difference of gravity at the two places; also in going out and returning, he prac tised the method of finding the longitude by the lunar distances taken by Hadley's quadrant, making out rules for the use of seamen, and teaching the method to the officers on board the ship. This method was explained in the Philo

sophical Transactions, for 1762, and more fully afterwards in the British Mariner's Guide, published in 1763. In September 1763, he sailed for the island of Barbadoes, to settle the longitude of the place, to examine Harrison's watch, and to try Irwin's marine chair. While at Barbadoes, he made many other observations, and amongst them, many relating to the moon's borary parallaxes, not yet published.

66. Maskelyne returning to England in the end of 1764, recommended to the board of Longitude the lunar method of finding the longitude; and proposed to it the project of a nautical almanack, to be calculated and published to facilitate that method. This the board agreed to, and the first volume was published for 1767, and has continued ever since to the great benefit of naviga

tion.

67. In consequence of a proposal, made by this astronomer to the Royal Society, the project was formed of measuring accurately the effect of some mountain on the plumb-line, in deflecting it from the perpendicular; and Schehallien, in Scotland, having been found the most convenient in this island for the purpose, he went into Scotland to conduct the business; by this experiment he showed that the sum of the deflections on the two opposite sides was about 11 of a degree; and proved to the satisfaction of the whole world, the universal attraction of matter. From the data resulting from these measures, Dr. Hutton computed the mean density of the whole matter in the earth, to be about 4 times that of common water.

68. The discoveries of Dr. Herschel form a new era in astronomy. In 1781, he began with observations on the periodical star in Collo Ceti, and a new method of measuring the lunar mountains, none of which he made more than half a mile in height; and having constructed telescopes far more powerful than any former ones, proceeded to other observations; such as, on the rotation of the planets round their axes; on the parallax of the fixed stars; catalogues of double, triple, &c. stars; on the proper motion of the sun and solar system; on the remarkable appearances of the polar regions of the planet Mars; &c. Above all his discoveries of a new primary planet, on the 13th of March, 1781, called by him the Georgian Planet, but named the Hersschel, and sometimes Uranus, by foreign astronomers, and of its six satellites, discovered since that time, has greatly enlarged the bounds of the solar system, this new planet being more than twice as far from the sun as the planet Saturn.

69. M. Piazzi, astronomer royal at Palermo, discovered on January 1st, 1801, another planet moving in an orbit between Mars and Jupiter. This planet has been named Ceres. Another was discovered on March 28th, 1802, by Dr. Olbers of Bremen, and named Pallas; a third was discovered and named Juno by Mr. Harding of Lilienthal; and a fourth by Dr. Oliers, and named Vesta, on March 29th, 1807. These planets are all very small, and all so nearly at the same distance from the sun, and movin in orbits differing so little either in eccentricity or declination, that they have by some Con

jectured to be fragments of a larger planet, which from some explosion had been burst, and its parts scattered abroad in space.

It is probable that as astronomical instruments become more improved, further discoveries of the same kind will be made, and that the boundaries of the solar system may be enlarged by the discerning of planets which circulate round the sun even beyond the orbit of the Georgian planet.

71. Dr. Maskelyne was succeeded at the Greenwich observatory in 1811, by J. Pond, esq. the present astronomer royal, under whose managment the business of this important institution has been kept in full activity. The number of instruments has been greatly increased. The use of the mural quadrant has been abandoned for that of the circle, two of which, one by Troughton, and one by T. Jones, are in constant use, and give results which accord with each other in a manner altogether surprising. The most important discoveries may be hoped for from the skill and activity with which the splendid instruments at Greenwich are managed. All indeed that appears wanting in that institution, is a telescope of the first class to follow up the discoveries in siderial astronomy, which conferred such splendor on the name of Herschel. But we are glad to perceive that this department of the science is likely to be carried to a degree of perfection which few would have hoped for, by Mr. Herschel, junior, the worthy and able son of the great astronomer, and his friend Mr. South, whose recent publication on the motion of double stars does them the highest credit.

He

72. On the continent of Europe, the greatest ardor is at present evinced in the cultivation of this science. The labors of Schumercher at Altona, are unintermitted and most valuable. may be considered at present as a common bond among astronomical men. Greass at Gottingen, Littrow, at Venice; Bressel, at Konigsberg; Struve, at Dorpat; Zach, at Genoa; and a host of other individuals distinguished for their labors and their zeal, have devoted themselves to astronomy.

73. In our own country, it would be injustice to pass over the names of Woodhouse and Brinkley, whose eminence in this science is of the most distinguished kind.

74. Another striking feature of the present day is the formation of The Astronomical Society of London,' an institution whose only object is the cultivation of astronomical science. This society includes among its members almost every individual known to the world as distinguished for astronomical knowledge. The memoirs of the society, of which the third part is just ready for publication, are very valuable and interesting.

75. The university of Cambridge has recently evinced its sense of the importance of a practical knowledge of this science, by the erection of an observatory on the most splendid scale; and the English government has also shown by the recent order for the establishment of an observatory at the Cape of Good Hope, that the importance which it has always attached to the cultivation of this science, has suffered no abatement.