LEMONNIER, PIERRE (CLAUDE) CHARLES, a diligent and accurate astronomer, born at Paris, on the 23d of November 1715, was the son of Peter Lemonnier, of St Sever, in the province of Normandy.
His father was a professor of philosophy in the College d'Harcourt, and member of the Academy of Sciences. He distinguished himself, as a teacher, by his activity in promoting the introduction of mathematical reasoning into the Cartesian philosophy. Under such auspices, the son might have enjoyed facilities for the cultivation of any of the sciences; but he soon showed a decided preference for astronomy, and began to be a practical observer before he was sixteen. In the month of November 1732, Mr Fouchy procured him the use of a mural quadrant of 3 feet radius, and he soon applied himself with diligence to the determination of the sun's greatest equation, an investigation which he continued for many years; and in 1741 he found the equation to amount to ; a quantity differing only by 7" from Delambre's latest determination.
He presented to the Academy of Sciences, in 1735, an elaborate map of the moon, accompanied by some remarks;
and, in the month of April of the following year, before he was twenty-one years old, he was made a member of the Academy, in the character of adjunct geometer. He continued to be a constant contributor to its Mémoires for more than fifty years. The winter of 1736 and 1737 he passed at Torneo, with Clairaut and Maupertuis, as a member of a committee appointed by the Academy for measuring a degree in Lapland; and he was not less zealous than any of his colleagues in the execution of that arduous undertaking.
He is considered as having effected, in conjunction with Lacaille, a complete reformation in the practical astronomy of France. He entered very early into correspondence with the English astronomers of the day; they had carried their instruments and modes of observation to a higher degree of perfection than their neighbours; and Lemonnier was of great use to the science in making known to his countrymen the practical methods of Flamsteed, and in introducing the instruments of Graham and Bird. In April 1739 he was made a foreign member of the Royal Society of London, and for the last twelve or fourteen years of his life he is said to have been the senior member of the society. He was the first that introduced the effects of nutation, then lately discovered by Bradley, into the solar tables; and he complained bitterly of Lacaille's injustice in not giving him due credit for the improvement.
The Duc de Noailles made him known to the King of France, who was fond of his company, and showed him many kindnesses. In 1742, he gave him apartments at the Capucins in the Rue St Honoré, where he continued to live till the Revolution. On another occasion, when he had taken great pains to fix an accurate meridian at St Sulpice, the king made him a present of 15,000 livres, which he expended in the purchase of instruments, as the greatest luxury that he was capable of enjoying.
The places of the stars, which he determined in 1740, served Lalande for the purpose of computing their proper motions, though they differed a little from the results of Bradley's observations. About 1746 he was much engaged in examining the inequalities of Saturn, produced by the attraction of Jupiter; and Euler employed his computations in the theory which obtained a prize in 1748; each confirming the accuracy of the other. He continued to observe the moon, without intermission, for fifty years, though a small part only of his observations was published.
In the year 1748 he went to England, partly for the purpose of obtaining further information from a personal acquaintance with the astronomers and opticians resident in London, and partly in order to observe the solar eclipse of that year, in a situation where it would be very nearly annular. He proceeded, accordingly, to Scotland with the Earl of Morton, accompanied by Short the optician, and they observed the eclipse together at Aberdeen, an ancient residence of Lord Morton's, in Fife-shire. They obtained their time from the College at Edinburgh, where there was a transit instrument, by means of the flash of a cannon fired from the castle at twelve, and another five minutes after. Lemonnier was particularly anxious to measure the moon's diameter, which "he found 29 47," agreeing precisely with the computed diameter, and not requiring any correction for the supposed effects of irradiation. A similar remark was made by a very accurate practical astronomer in the eclipse of 1836.
In order to verify the position of his mural quadrant, which was of eight feet radius, and made by Bird, Lemonnier felt the advantage of having a moveable one to compare with it, and he procured a block of marble 8 feet by 6, and 15 inches thick, turning on an axis, to which he fixed his smaller instrument of 5 feet radius, in order to be able to reverse its position. He devoted a considerable portion of his time to the investigation of the laws of mag-
netism, and especially to the variation of the compass; and he endeavoured to ascertain the effect of the moon's influence on the winds, and on the atmosphere in general.
Lemonnier had long disputed the accuracy of the Parisian base, measured by Cassini and Maraldi, but he was at last convinced that his objections were groundless. He was originally a most zealous friend and patron of Lalande; but afterwards, having taken offence at some slight cause, he refused to see him for many years. In fact, he appears to have been somewhat obstinate and irritable; but he is said to have had genius, zeal, activity, and intelligence, as well as credit in the world, and reputation among men of science. He was a voluminous writer; he had much learning and sagacity, but he often wanted precision in his language and his reasoning. In November 1791 he had a paralytic attack, which terminated his scientific career, though he survived it till the 2d of April 1799, when a second stroke carried him off, at Héril, near Baieux. He was made, in the meantime, one of the 144 original members of the National Institute, as a testimony to the merit of his past labours.
He had married, in 1763, Mlle. de Cussy, a lady of very respectable family in Normandy. He had three daughters; the first married M. de Parfourny, who was an early victim of the Revolution; the second the celebrated Lagrange; and the third her uncle, Lemonnier the physician, who was also a man of considerable science, and a member of the Royal Society of London.
From 1735 to 1790 there were very few volumes of the Mémoires of the Academy without one or more of Lemonnier's papers; but though not unimportant in the aggregate, they are somewhat uninteresting in the detail. They relate almost exclusively to astronomical observations; eclipses, occultations, appulses, oppositions, and conjunctions; solstices, longitudes and latitudes, with some accounts of astronomical instruments and apparatus. There are also some memoirs relating to the sun's equation and diameter, and on his place, as compared with Arcturus: on the motion of Saturn, and on his fifth satellite; on the expansion of wooden measures; on the transit of Venus, and on the diameter and the tables of that planet; on Euler's formula for parallax; on the variation of the needle; on lunar altitudes; on the tides; on horizontal refraction; on Saturn's ring; and on some currents of wind. He also published separately some extensive works, which acquired considerable celebrity.
bands, and including also some considerations on refraction and on magnetism.
13. Description et Usage des principaux Instruments d'Astronomie, 1774, folio, forming part of the collection of Arts et Métiers.
14. Atlas céleste de Flamsteed, 1776, in 4to, revised.
15. Lois du Magnétisme, 2 parts, 8vo, 1776-8, with an elaborate chart.
16. Traité de la Construction de Valseaux, par Chapman, 1779, folio, from the Swedish, and said to be less perfect than the translation of Vial du Clairbois.
17. Mémoires concernant diverses questions d'Astronomie et de Physique, 4 parts, 1781-84-85-88, in 4to.
18. De la Correction introduite pour accourcir la Ligne éclipse du Soleil de dix-huit points, 8vo, 1790.
19. Lettre au sujet d'une Éclipse, Paris, 1791; together with some remarks on navigation, and on the currents of the South Seas, all apparently from the Memoirs of the Academy. The eclipse, which was observed as annular in China, should have been total, according to the computed distance of the luminaries concerned. (T. V.)