the truths which he discovers, and the prospect of a just and grate. ful posterity, console him for the ingratitude of his contempo

raries.

Kepler had obtained pensions which were always ill paid: going to the diet of Ratisbon to solicit his arrears, he died in that city the 15th of November, 1630. He had in his latter years the advantage of seeing the discovery of logarithms, and making use of them. This was due to Napier, a Scottish baron; it is an admirable contrivance, an improvement on the ingenious algorithm of the Indians, and which, by reducing to a few days the labor of many months, we may almost say doubles the life of astronomers, and spares them the errors and disgusts inseparable from long calculations ;-an invention so much the more gratifying to the human mind, as it is entirely due to its own powers: in the arts man makes use of the materials and forces of nature to increase his powers, but here the whole is his own work.

The labours of Huygens followed soon after those of Kepler and Galileo. Very few men have deserved so well of the sciences, by the importance and sublimity of their researches. The application of the pendulum to clocks is one of the most beautiful acquisitions which astronomy and geography have made, and to which fortunate invention, and to that of the telescope, the theory and practice of which Huygens considerably improved, they owe their rapid pro gress.

He discovered, by means of excellent object-glasses which he succeeded in constructing, that the singular appearances of Saturn were produced by a very thin ring, with which the planet is surrounded his assiduity in observing made him discover one of the satellites of Saturn.

He made numerous discoveries in geometry and mechanics and if this extraordinary genius had conceived the idea of combining his theorems on centrifugal forces with his beautiful investigation on involutes, and with the laws of Kepler, he would have preceded Newton in his theory of curvilinear motion, and in that of universal gravitation. But it is not in such approximations that discovery consists.

Towards the same time, Hevelins rendered himself useful to astronomy, by his immense labours. Few such indefatigable ob

servers have existed; it is to be regretted that he would not adopt the application of telescopes to quadrants, an invention which gave a precision previously unknown to astronomy.

At this epoch astronomy received a new impulse from the establishment of learned societies.

Nature is so various in her productions and phænomena, of which it is so difficult to ascertain the causes, that it is requisite for a great number of men to unite their intellect and exertions to comprehend and develop her laws. This union is particularly requisite when the sciences in extending approximate, and require mutual support from each other.

It is then, that the natural philosopher has recourse to geometry, to arrive at the general causes of the phænomena which he observes, and the geometrician in his turn interrogates the philosopher, in order to render his own investigation useful, by applying them to experience: and to open in these applications a new road in analysis. But the principal advantage of learned societies is the philosophical feeling on every subject which is introduced into them, and from thence diffuses itself over the whole nation. The insulated philosopher may resign himself without fear to the spirit of system; he only hears contradiction at a distance; but in a learned society the shock of systematic opinions at length destroys them, and the desire of mutually convincing each other establishes between the members an agreement only to admit the results of observation and calculation. Thus experience has proved that since the origin of these establishments true philosophy has been generally extended.

By setting the example of submitting every opinion to the test of severe scrutiny, they have destroyed prejudices which had so long reigned among the sciences, and in which the highest intellects of the preceding ages had participated. Their useful influence on opinion accumulated in our own time, with an enthusiasm which at other periods would have perpetuated them. Finally, it is among them, or by the encouragement they offer, that those grand theories have been formed which are placed above the reach of the vulgar by their comprehensiveness; and which, extending themselves by their numerous occasions in which they are applicable, to nature and to the arts, are inexhaustible sources of delight and intelligence.

Of all the learned societies, the two most celebrated for the number and importance of their discoveries in the sciences, and

particularly in astronomy, are the Academy of Sciences in Paris, and the Royal Society in London.

The first was created in 1666, by Louis XIV. who foresaw the lustre which the arts and sciences were to diffuse over his reign. This monarch, worthily seconded by Colbert, invited many learned strangers to fix themselves in his capital. Huygens availed himself of this flattering invitation; he published his admirable work De Horologio oscillatorio, in the bosom of the academy, of which he was one of the first members. He would have finished his days in France, had it not been for the disastrous edict which, towards the end of the last century, deprived it of so many valuable citizens. Huygens, departing from a country in which the religion of his ancestors was proscribed, retired to the Hague, where he was born the 14th of April, 1629, and died there the 15th of June, 1695.

Dominic Cassini was likewise induced to go to Paris by the offers of Louis XIV. During forty years of useful labours, he enriched astronomy with a crowd of discoveries: such are the theory of the satellites of Jupiter, the motions of which he determined from observations of their eclipses; the discovery of the four satel lites of Saturn; those of the rotation of Jupiter, of the belts parallel to his equator, of the rotation of Mars, of the zodiacal light, a very approximate knowledge of the Sun's parallax, a very exact table of refractions, and, above all, the complete theory of the libration of the Moon.

The great number of astronomical academicians of extraordinary merit, and the limits of this historical abridgment, do not permit me to give an account of their labors; I shall content myself with observing that the application of the telescope to the quadrant, the invention of the micrometer and heliometer, the successive propagation of light, the magnitude of the earth, its ellipticity, and the diminution of gravity at the equator, are all discoveries due to the Academy of Sciences.

Astronomy does not owe less to the Royal Society of London, the origin of which is a few years anterior to that of the Academy of Sciences. Among the astronomers which it has produced, I shall cite Flamstead, one of the greatest observers that have ever appeared. Halley, rendered illustrious by his travels, undertaken for the advantage of science, by his beautiful investigation concerning comets, which enabled him to discover the return of the

comet in 1759; and by the ingenious idea of employing the transit of Venus over the Sun, in the determination of its parallax. I shail mention, lastly, Bradley, the model for observers, and who will be for ever celebrated for two of the most beautiful discoveries ever made in astronomy, the aberration of the fixed stars, and the nutation of the axis of the earth.

When the application of the pendulum to clocks, and of telescopes to quadrants, had rendered the slightest changes in the position of the celestial bodies perceptible to observers, they endeavoured to determine the annual parallax of the fixed stars; for it was natural to suppose, that so great an extent as the diameter of the terrestrial orbit, would be sensible even at the distance of these stars. Ob. serving them carefully, at every season of the year, there appeared slight variations; sometimes favorable, but more frequently contrary to the effects of parallax. To determine the law of these variations, an instrument of great radius, and divided with extreme precision, was requisite. The artist who executed it, deserves to partake of the glory of the astronomer who owed his discovery to him. Graham, a famous English watch-maker, constructed a great sector, with which Bradley discovered the aberration of the fixed stars, in the year 1727. To explain it, this great astronomer conceived the fortunate idea of combining the motion of the earth with that of light, which Roemer had discovered at the end of the last century, by means of the eclipses of Jupiter's satellites. We are surprised that none of the distinguished philosophers who then existed, and who knew the motion of light, should have paid any attention to the very simple effects which result from it, in the apparent position of the fixed stars. But, the human mind, so active in the formation of systems, has almost always waited till observation and experience have acquainted it with important truths, which its powers of reasoning alone might have discovered.

It is thus that the invention of telescopes has followed by more than three centuries that of lenses, and even then was only owing to accident

In 1745, Bradley discovered by observation, the nutation of the terrestrial axis. In all the apparent variations of the fixed stars, observed with extraordinary care, he perceived nothing which indicated a perceptible parallax. The measures of the degrees of the

terrestrial meridian, and of the pendulum, multiplied in different parts of the globe, of which France gave the example, by measuring the whole arc of the meridian, which crosses it, and by sending the academician to the north and to the equator, to observe the magnitude of these degrees, and the intensity of the force of gravity. The arc of the meridian, comprised between Dunkirk and Barcelona, determined by very precise observations, and forming the base of the most natural and simple system of measures; the voyages undertaken to observe the two transits of Venus over the Sun's disk, in 1761 and 1769, and the exact knowledge of the dimensions of the solar system, which has been derived from these voyages; the invention of achromatic telescopes, of chronometers, of the sextant and repeating eircle, the discovery of the planet Uranus, by Herschel, in 1781; that of its satellites, and of two new satellites of Saturn, due to the same observer, all the astronomical theories being brought to perfection, and all the celestial phænomena, without exception, being referred to the principle of universal gravitation:—these, with the discoveries of Bradley, are the principal obligations which astronomy owes to England, which, with the preceding, will always be considered as constituting the most glorious epoch of the science. [La Place, Exposition du Systême du Monde.]

CHAPTER IV.

GENERAL NOTICES ON ASTRONOMY

To the preceding observations of La Place, the Editor of the present work has thought right to subjoin the following singular or learned opinions of several of the most esteemed writers on the subject.

Professor Playfair, in an article inserted in the Edinburgh Philosophical Transactions, supports the very high pretensions of the Bramins to Astronomy, and conceives that they are in possession of some observations not less than five thousand years old. Mr. Costard, in a paper on the Chinese Chronology and Astronomy, print