MONNIER (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 sixteen 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 Macleod, 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 thro' 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, six 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 sixteen 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 suis moi-même le principal resultat de son zele 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
Monnier esteem his instructor and master to the day of his death.
"I have not ceased to exclaim (writes Lalande), as Dio-
genes exclaimed to his master Antilhenes, You can-
not find a stick strong enough to drive me away from
you!"
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 mas-
ter; but Le Monnier would not read it. This is not
the place to give a circumstantial account of this intri-
cate quarrel; we shall only further remark, that La-
lande was the warm friend and admirer of the no less
eminent astronomer La Caille, whom Le Monnier mor-
tally 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 Henwart, the celebrated geometer and pro-
fessor 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 Pisse, where
I frequently made observations in company with Mes-
sier. Le Monnier was the friend of D'Alembert; and
consequently an opposer of Lalande."
This great man, who had, for some years, ceased to
exist either for the science of astronomy, or for the com-
fort of his friends, died at Lizeaux, 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 an-
nounced so early as the year 1741; among which was
twice to be found the new planet Uranus: (See La-
lande's Astronomic Tables, p. 188, (A). The more he
was requested to communicate his observations, the
more obstinate he became; he even threatened to de-
stroy them. At the breaking out of the revolution,
Lalande was greatly alarmed for the safety of these pa-
pers; 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 No-
vember 1791, Lalande apprehended, lest, if no one ex-
cept himself should know where he had hidden his pa-
pers, the infirm old man might perhaps have himself
forgotten it. He hopes, however, that La Grange, who
married his second daughter, may have some informa-
tion concerning them. Le Monnier left behind him no Mon-
nier.