Lacaille, who was at that time about to take his departure for the Cape of Good Hope, with a view to the more exact determination of the moon's parallax, had called upon the astronomers of Europe to forward the object of his voyage by making observations at their respective observatories, similar to those which he contemplated making himself at the Cape. The favourable position of Berlin, which has nearly the same longitude, while it differs in latitude by nearly the fourth part of the earth's entire circumference, suggested to Lemonnier the peculiar advantages which would accrue from obser vations made at the observatory of that city. But it so happened that there were uo instruments of any value at that observatory, and no person of ability had been appointed to its superintendence. Lemounier instantly offered the use of his own instruments, and at his recommendation the academy confided to Lalaude the respon sibility of making the necessary observations. When Maupertuis presented Lalaude to Frederick, the latter, as might he expected, expressed his surprise at receiving so young an astronomer—for Lalande bad not then completed his nineteenth year,—but after many flatteriog expressions he gave orders that everything should be done which could tend to the attainment of the object in view. Here, during the latter part of the year 1751, and the early part of 1752, Lalande passed most of his nights in the observatory; his mornings, in studying the mathematics under Euler ; and his evenings, in the society of Maupertuis, Voltaire, D'Argene, aud La Matrie. After completing his observations, the substance of which be communicated in a memoir to the Academy of Berlin, he returned to Paris, where the Royal Academy expressed their unqualified approbation of his conduct, and immediately elected him a member of their society. From his election till within a few years of his death, be contributed regularly to the Transactions of the Academy, and from this time his popularity as an astronomer may be dated.
The expected return of Halley's comet had led Clairaut to investi gate the amount of the perturbations to which it would be subject. Lalande, with the assistance of Madame Lepaute, supplied him with all the numerical computations of which he had need ; and when the appearance of the comet had realised their predictioua, he wrote its history, which appeared in 1759, appended to a translation of Valley's planetary tables. In 1760 he was appointed editor of the 'Con naissancea des Temps,' in which he introduced many important altera tions, and gave to it the form which it has since retained. In 1762 be succeeded Delille as professor of astronomy to the College of France, and continued to discharge the duties of his office with zeal and assiduity for more than forty years. From among his pupils he was in the habit of selecting those who manifested peculiar attach ment to aetronomical science, and these he wend invite to his house, where he perfected them in the calculations necessary for applying their theoretical knowledge to objects of utility. His residence was
in fact a school wherein many of his pupils nut only received a scientific education, but likewise board, lodging, and other necessaries, and from whence they afterwards removed either to conduct some observatory, to fill an astronomical lectureship, or- as professors of navigation and nautical astronomy ou hoard the vessels of tho government.
In 1764 ho published his large treatise on astronomy, which he afterwards extended to four volumed 4to. Before the appearance of this work there existed several able treatises on the theory of astronomy by Lacaille, Cassini, and Lemonnier ; but these contained little or no information as to the practice of astronomy. To supply this omission was the main object of Lalande. The work contains many biographical and historical notes, which will always be inter esting, and the results of numerous observations to which it will always be usefnl to recur.
In 1772 he published his 'Account of the Transit of Venus,' observed on the 3rd of Juuo 1769, which was drawn up with con siderable labour from the communications of those persons who, at his recommendation, had been sent by several of the European governments to different parts of the globe, in order to observe the phenomenon.
Lalonde died at Paris, 4th of April 1S07, in his seventy-fifth year As an observer. an author, and a tutor, he undoubtedly did much for the promotion of astronomy; but looking to the state of the mathematics at the time in which he lived, his knowledge of them appears to have been very limited. The candour and the warmth of his disposition gave full relief both to his virtues and his defects. lie regarded concealment of any kind and under any circumstances as disreputable to an honourable man; rind acting up to this opinion, he invariably express-a his sentiments without the slightest reserve, even when by so doing be prejudiced his own interests and those of his dearest friends. His love of truth, and the boldness with which he attempted to subvert all systems and opinions which did not accord with his own, and which sometimes partook rather of a spirit of fauaticion than of pure philosophy, excited against him a crowd of detractors and enemies. The extreme irritability of his temper led him on several occasions to acts of ingratitude towards Lemonnier, his early tutor and friend, who, to use Lalande's own expression, "refused to see him during an entire revolution of the moon's nodes." Ilia attachment to his native town was such that ho made a point of visiting it every alternate year during the college vacation ; and upon these occasions be gave public lectures, founded an Academical Society, and neglected nothing which might inspire a love of acieuee and of lettere. Hie filial affection induced him frequently to attend the devotions of his mother, although the creed which she had so zealously endeavoured to inculcate had been greatly modified, if not altogether eradicated, by his intercourse with Voltaire and others while at Berlin.