(Peter Charles Le), was born at Paris on the 20th of November 1715. The profession of his father, or the rank which he held in society, we have not learned; and we are equally ignorant of the mode in which he educated his son. All that we know is, that young Monnier, from his earliest years, devoted himself to the study of astronomy; and that, when only fifteen years of age, he made his first observation, viz., of the opposition of Saturn. At the age of twenty he was nominated a member of the Royal Academy of Sciences at Paris. In the year 1735 he accompanied Maupertuis in the celebrated expedition to Lapland, to measure a degree of latitude. In 1748 he went to Scotland with Lord Macsfield, to observe the annular eclipse of the sun, which was most visible in that country; and he was the first astronomer who had the pleasure to measure the diameter of the moon on the disk of the sun.

Louis XV. it is well known, was extremely fond of astronomy, and greatly honoured its professors; he loved and esteemed Le Monnier. I have seen the king himself (says Lalande) come out of his cabinet, and look around for Le Monnier; and when his younger brother was presented to him on his appointment to the office of first physician, his Majesty was pleased to wish him the merit and reputation of his brother the astronomer. All the remarkable celestial phenomena were always observed by the king, in company with Le Monnier. Thus he observed with him, at his chateau of St Hubert, the two celebrated transits of Venus through the disk of the sun in the years 1761 and 1769; as appears from the Memoirs of the Royal Parisian Academy of Sciences. It well deserves to be here recorded in what manner the king behaved during these important observations, and how little he disturbed his astronomers (the celebrated La Condamine being likewise permitted to observe the transit in his presence) in this occupation; the proper time for which, if permitted to pass by, could not be recalled. Le Monnier relates in his Dissertation, that "his Majesty perceiving that we judged the last contacts to be of the greatest importance, a profound silence at that moment reigned around us." At the transit of Venus in 1769, the king allowed the Marquis de Chabert, an intelligent and expert naval officer, who was just returned from a literary voyage to the Levant, to assist at the observation. In a court like that of Louis XV. so scrupulously observant of etiquette, these will be allowed to have been most distinguished marks of honour, and of royal favour and consideration.

In the year 1750, Le Monnier was ordered to draw a meridian at the royal Chateau of Bellevue, where the king frequently made observations. The monarch on this occasion rewarded him with a present of 15,000 livres; but Le Monnier applied this sum of money likewise in a manner that redounded to the honour of his munificent sovereign and of his country, by procuring new and accurate instruments, with which he afterwards made his best and most remarkable observations. In 1742, the king gave him in Paris Reu de la Poste, a beautiful free dwelling, where, till the breaking out of the revolution, he resided, and pursued his astronomical labours, and where his instruments in part yet remain. Some of them the present French government has, at the instance of Lalande, purchased for the National Observatory. In 1751, the king presented him with a block of marble, eight feet in height, five feet in breadth, and fifteen inches in thickness, to be used for fixing his mural quadrant of five feet. This marble wall, together with the instruments appended to it, turns on a large brass ball and socket, by which the quadrant may be directed from south to north; thus serving to rectify the large mural quadrant of eight feet, which is immovably made fast to a wall towards the south.

With these quadrants Le Monnier observed, for the long period of forty years, the moon with unwearied perseverance at all hours of the night. It is requisite, to be a diligent astronomer, to be able to conceive to what numberless inconveniences the philosopher is exposed during an uninterrupted series of lunar observations. As the moon during a revolution may pass through the meridian at all hours of the day or night; the astronomer who, day after day, prosecutes such observations, must be prepared at all, even the most inconvenient, hours, and sacrifice to them his sleep and all his enjoyments. How secluded from all the pleasures of social intercourse, and how fatiguing such a mode of life is, those astronomers, indeed, know not who then only set their pendulum clocks in motion, when some of the eclipses of the sun, moon, or of the satellites of Jupiter, are to be viewed. At this time, and in the present state of the science, these are just the most insignificant observations; and an able astronomer, well supplied with accurate instruments, may every day, if he take into his view the whole of his profession, make more important and more necessary observations.

Le Monnier was Lalande's preceptor, and worthy of such a scholar; and he promoted his studies by his advice, and by every other means in his power. Le Monnier's penetrating mind, indeed, prefaced in young Lalande, then only fifteen years old, what in the sequel has been so splendidly confirmed. In his twentieth year, he became, on the recommendation of his preceptor, a member of the Royal Academy; and in 1752 he was proposed by him as the fittest person to be sent to Berlin, to make with La Caille's, who had been sent to the Cape of Good Hope, correspondent observations, for the purpose of determining the parallaxes of the moon, then but imperfectly known. Le Monnier lent his pupil for this expedition his mural quadrant of five feet. His zeal for astronomy knew no bounds. For this reason Lalande, in his Notice des Travaux du C. Le Monnier, says of himself: "Je fis moi-même le principal résultat de mon zèle pour l'astronomie."

Le Monnier was naturally of a very irritable temper; as ardently as he loved his friends, as easily could he be offended; and his hatred was then implacable. Lalande, as he himself expresses it, had the misfortune to incur the displeasure of his beloved preceptor; and he never after could regain his favour. But Lalande's gratitude and respect for him always continued undiminished, and were on every occasion with unremitting constancy publicly declared: patiently he endured from him undeserved ill treatment; so much did he love and esteem. What a noble trait in the character of Lalande, who in 1797 wrote likewise an eulogium on Le Monnier in the style of a grateful pupil, penetrated with sentiments of profound veneration and esteem for his beloved master; but Le Monnier would not read it. This is not the place to give a circumstantial account of this intricate quarrel; we shall only further remark, that Lalande was the warm friend and admirer of the no less eminent astronomer La Caille, whom Le Monnier mortally hated. An intimate friendship likewise subsisted between Le Monnier and D'Alembert; but Lalande had no friendly intercourse with the latter.

Among the scholars of Le Monnier may likewise be reckoned Heunert, the celebrated geometrician and professor of mathematics at Utrecht; who, in a letter to Von Zach, astronomer to the Duke of Saxe Gotha, dated the 26th of May 1797, says, "Le Monnier is a penetrating and philosophical astronomer: I learned much from him in Paris; though I lodged with the late De l'Isle, where I frequently made observations in company with Mfssier. Le Monnier was the friend of D'Alembert; and consequently an opponent of Lalande."

This great man, who had, for some years, ceased to exist either for the science of astronomy, or for the comfort of his friends, died at Lizieux, in the province of Normandy, in 1799, aged 84 years. He left behind him some valuable manuscripts, and a number of good observations; with respect to which he had always been very whimsical, and of which in his latter years he never would publish any thing. He had by him a series of lunar observations, and a multitude of observations of the stars, for a catalogue of the stars, which he had announced so early as the year 1741; among which was twice to be found the new planet Uranus: (See Lalande's Astronomie, Tables, p. 188, (a).) The more he was requested to communicate his observations, the more obstinate he became; he even threatened to destroy them. At the breaking out of the revolution, Lalande was greatly alarmed for the safety of these papers; he wished to preserve them from destruction, and made an attempt to get them into his possession; but all his endeavours were in vain. He was only able to learn, that Le Monnier had hidden them under the roof of his house. Le Monnier having been first seized with a fit of the apoplexy so early as the 10th of November 1791, Lalande apprehended, lest if no one except himself should know where he had hidden his papers, the infirm old man might perhaps have himself forget it. He hopes, however, that La Grange, who married his second daughter, may have some information concerning them. Le Monnier left behind him no son.