Jean Baptiste Joseph, an eminent mathematician and astronomer, born at Amiens, Sept. 19, 1749. He studied in the gymnasium of that town under the celebrated poet Delille, with whom he maintained an intimate friendship till his death.

Fortunately for Delambre, his family had formerly founded a gratuitous place for a limited time in one of the great colleges of the University of Paris, and placed it at the disposal of the town of Amiens; and this enabled him to carry on his studies, thus auspiciously begun. The expiry of this privilege, however, left him to struggle with great privations. During the interval in which he was awaiting permanent employment he devoted himself to historical and literary studies. He undertook extensive translations from Latin, Greek, Italian, and English; and at the same time entered on the study of the mathematical sciences.

By the advice of his friends, he resolved to devote his attention to the instruction of youth, and with this view he went to Compiègne; but he soon returned to Paris, where he pursued the same career, with the additional advantages, however, of competent and independent subsistence.

He now yielded to the natural bias of his mind towards the study of physics and astronomy. When he presented Delambre himself at the College of France to attend the lectures of Lalande, he had already read the works of that astronomer, and had made a complete commentary on them. This was first remarked when, in the course of instruction, an occasion offered to him of citing from memory a passage of Aratus. Lalande immediately foresaw the advantages his pupil was likely to confer on science, and from that moment regarded Delambre as a fellow-labourer. He entrusted to him the most complicated astronomical calculations, and prevailed on M. Dassy (whose son had received lessons from Delambre) to establish an observatory at his house, where Delambre applied himself to astronomical observations. Delambre at the same time undertook the most extensive researches; he formed the design of completing the astronomical tables; and he dedicated his life to the study and the description of the heavens, a resolution he had previously entertained during his stay at Compiegne.

In 1781 the discovery of the planet Uranus by Herschel excited general attention among astronomers; and the Academy of Sciences proposed the determination of its orbit as the subject of one of its annual prizes. Delambre undertook the formation of tables of its motion, and the prize was awarded to him for his labour. His next effort was the construction of solar tables, and tables of the motions of Jupiter and Saturn; he likewise undertook the construction of elliptical tables of the satellites of Jupiter; and in a few years he completed that laborious and extensive work. He took a part in the sitting of the Academy of Sciences when Laplace communicated his important discoveries on the inequalities of Jupiter and Saturn; and he formed the design of applying the result of that profound analysis to the completion of tables of the two planets. Delambre turned his attention more especially to the satellites of Jupiter—an undertaking of great difficulty and extent. In this he was sustained by two powerful motives—public utility, and the dignity of the subject. He had been engaged for several years in the composition of his elliptical tables, when the Academy of Sciences proposed the same question as the subject of a prize. This was awarded to Delambre; and soon afterwards (viz. in 1792) he was elected a member of the academy.

The great diversity of weights and measures, and the consequent inconvenience to commerce, engaged the attention of the French nation about the time of the first revolution; and in the year 1790, Talleyrand, then minister of foreign affairs, brought the subject before the constituent assembly, where it was resolved that the king should be requested to apply to his Britannic Majesty, with the view of getting the English parliament to concur with the national assembly in fixing on some natural unit as a standard. It was proposed that commissioners from the Academy of Sciences should meet an equal number of members of the Royal Society of London, in order to ascertain the length of a pendulum in some determinate latitude, and thence to deduce an invariable model for all weights and measures; and a commission—composed of Borda, Lagrange, Laplace, Monge, and Condorcet—was appointed to consider this grand project. Besides the length of the pendulum, two other units were suggested, namely, a quadrant of the elliptic meridian, and a fourth part of the earth's circumference at the equator. Of the three, the commission adopted the quadrant of the meridian, and its execution was begun by appointing Delambre and Méchain to measure an arc from Dunkirk to Barcelona. This undertaking, in itself laborious, was, by circumstances which arose out of the revolution, rendered highly dangerous to the personal safety of those engaged in it. Méchain died whilst the work was proceeding; and its successful termination was at last accomplished by the zeal, the talents, the perseverance, and the prudence of Delambre. The details of this grand labour have been consigned to posterity in his work entitled Base du Système Métrique Décimal, the first volume of which bears the date of 1806, and the third and last that of 1810. The labours of Delambre and Méchain were followed up by those of Biot and Arago; who have added as a supplement to the Base du Système Métrique their Recueil d'Observations Géodésiques, Astronomiques, et Physiques, published in 1820, in which the operations are detailed by which the measurement of the meridian was prolonged from Barcelona to Formentera in the Mediterranean. Altogether this work does honour to the nation which produced such mathematicians, astronomers, and artists as were united in the execution of the labours detailed in it, but particularly to Delambre, the great directing mind that in the course of eight years brought them to a successful termination. The opinion of the National Institute of France was solemnly expressed when, having considered what had been the most important application of the mathematical and physical sciences during the preceding ten years, the universal suffrages of the members decreed the prize to the author of the Base du Système Métrique.

Delambre, who had been chosen as an associate of almost every scientific body in Europe, and a member of the French Board of Longitude, was appointed perpetual secretary for the mathematical sciences in the Institute. He had succeeded Lalande in the chair of astronomy of the College of France, and he was appointed one of the principal directors (titulaires) of the university. For twenty years he performed the duties of his office in one of the classes of the Institute, and, in doing so, he enjoyed the reputation of being impartial, faithful, and just. His annual reports, his historical eloges, which have been published, and his exposition of the progress of science, are eminently distinguished by profound erudition, a talent for writing formed on the best models, and, above all, by a disposition of mind which inclined him to place in the most favourable light the works of others, as far as was consistent with historical truth. His literary and scientific labours were very numerous, and, in respect of excellence, of the highest order. His History of Astronomy (6 vols. 4to) is a work of prodigious research. It puts the modern astronomer in possession of all that had been done, and of the methods employed by those who have gone before him.

His Méthodes Analytiques pour la Determination d'un Arc du Méridien, his numerous memoirs in the additions to the Connaissances des Temps, and his Astronomie Théorique et Pratique, exhibit the finest applications of modern analysis to astronomy and geography; in fact, they have given a new direction to these sciences. Astronomers have now laid aside the purely geometrical methods, which, however elegant, always required at last numerical calculations; and, instead of these, have expressed everything to be determined in astronomy by compact analytical formulae, of which a great number are due to Delambre.

It is a remarkable fact in the life of Delambre, that he did not apply himself to astronomical observations until he was about thirty-five years of age; whereas, in general, the particular subject in which a man is destined to excel attracts his attention in early youth. Delambre was appointed a member of the Royal Council of Public Instruction in 1814; a place, however, which he lost in 1815. He was in Paris when it was taken by the allied armies; and, in a letter written at that time to a friend and pupil, he says that on the day of the siege, in the hearing of the cannonade, he laboured with tranquillity in his study from eight in the morning till midnight. He had a happier fate than Archimedes in a like position, for he was not molested by the victors, and no one was billeted on him, probably from respect to his high reputation. At the creation of the Legion of Honour Delambre was made a member of that order. He was appointed chevalier of St Michael in 1817, an officer in the Legion of Honour in 1821; but a long time Delaware, previous, he had been created a hereditary chevalier, with an endowment, which was decreed as a national reward.

The life of continued and hard study which Delambre led, at last affected his health. The disease by which he was cut off became apparent in the month of July 1822. His total loss of strength, with frequent and long-continued fainting-fits, gave warning of a fatal result. He foresaw what was about to happen, and preserved to the last moment the unalterable mildness of his character and the serenity of his mind. At last, Aug. 19, 1822, at the age of seventy-two, he sunk under his disease, not without having suffered, but without having complained, or betrayed any symptoms of impatience.

The following is a list of his works which appeared separately:

- Tables de Jupiter et de Saturne, 1789; Tables du Soleil, de Jupiter, de Saturne, et des Satellites de Jupiter, pour servir à la 3me édition de l'Astronomie de Lalande, 1792; Méthodes Analytiques pour la Détermination d'un Arc du Méridien, 1799; Tables Trigonométriques, théoriques et pratiques, Berlin, revues, améliorées, et publiées par M. Delambre, 1801; Tables du Soleil (publiées par le Bureau des Longitudes), 1806; Base du Système Métrique Décimal, &c., 3 vols. in 4to, 1806-1810; Rapport Historique sur les Progrès des Sciences Mathématiques depuis 1789, &c., 1810; Abrégé d'Astronomie ou Leçons Élémentaires d'Astronomie Théorique et Pratique, in 2 vols.; Astronomie Théorique et Pratique, 3 vols. in 4to, 1814; Tables Élégantes des Satellites de Jupiter, 1817; Histoire de l'Astronomie Ancienne, 2 vols. in 4to; Histoire de l'Astronomie du Moyen Âge, 1819, 1 vol. in 4to; Histoire de l'Astronomie Moderne, 1821, 2 vols. in 4to; Histoire de l'Astronomie au Dix-huitième Siècle, 1 vol. in 4to, 1827.

In addition to these, he furnished a very considerable number of memoirs (about 28) on various points of astronomy, to the Commissions d'Astronomie beginning with the year 1788. He also contributed to the Memoirs of the Academies of Stockholm, Petersburg, Berlin, and Turin, and to those of the first-class of the French Institute; and he composed elegies and epitaphs for his contemporaries at their death, among which may be noticed one on our highly distinguished countryman Dr Maskelyne, the second royal-astronomer of England. Delambre has also rendered essential service by disarming the delusion propagated by his friend Bailly regarding the antiquity of the Indian astronomical tables, which he held to be only about 700 years old.