Documents concerning Langlois’s life are rare and fragmentary, but from many accounts we know that between 1730 and 1756 he was the most highly regarded maker of scientific instruments in France and may be considered to have restored this art among the French. He and the other members of his trade provided scientists—at first astronomers—with the means of making highly precise observations during their many astronomical and geodesic missions. At the end of the seventeenth century the British instrument makers had become the undisputed leaders in the opinion of all European scientists, having surpassed Dutch workshops in their specialty—demonstration instruments for physics—and Italian workshops in the field of astronomical instruments (which the Italians had revitalized), and of instrumental optics (which they had created). French workshops were relatively numerous toward the beginning of the eighteenth century and furnished to a still small clientele all the types of instruments known. A few masters, like Nicolas Bion and Michael Butterfield, were better known than their colleagues, but none could be compared with the British competitors. Langlois, around the middle of the eighteenth century, was the first master to establish a firm tradition of precision workmanship in France. In 1730, the earliest date associated with his name, he received his first commission from the Paris observatory, for a wall quadrant of six-foot radius. He must therefore have been quite well known to French astronomers at this period to have merited the confidence represented by their commission. The signature on this instrument indicates that Langlois was established at the sign of the Niveau, probably on the Quai de l’Horloge, where most of the Parisian makers of scientific instruments were located. He was probably about thirty years old to have acquired not only the skill necessary for constructing large observatory wall instruments, but also the commercial foundation that could allow him to engage in such undertakings without serious financial risk. For no matter what price the purchaser paid, the instrument maker’s investment in the form of tools, special equipment, and time was considerable in view of the very limited number of instruments of this size that a French workshop could hope to sell. Langlois was in fact the only French instrument maker to have produced such instruments for nearly thirty years. From his workshop came all the kinds of apparatus known at the time, for use in physics laboratories and schools, and by surveyors and navigators. Langlois’s innovations in these areas included an improved pantograph that he submitted to the Académie des Sciences for approval.

The result of Langlois’s activity seems to have been convincing enough for him to be chosen the official instrument maker of the French astronomers. About 1740 he was named ingénieur en instruments de mathématiques by the Academy and along with this title received lodgings at the Louvre, as did all officially designated “artists.” This honor apparently reflected the maturing of Langlois’s talent and his high reputation among astronomers, notably Cassini II, Cassini de Thury, Camus, Le Monnier, Maupertuis, and the Abbé de Lacaille. During the last twenty-five years of Langlois’s life the Paris observatory ordered from him several instruments that were employed, along with English instruments, in the extensive triangulations of the first half of the century. In 1738 he constructed another wall quadrant of three-and-a half-foot radius and a six-foot sector; in 1742 a six-foot portable quadrant with certain features specified by Camus; a large sextant in 1750; and a three-foot portable quadrant in 1756, the year of his death.

In 1733 the Paris Academy decided to organize expeditions to Peru and Lapland to measure a meridian arc of one degree and the length of a seconds pendulum. Langlois was commissioned to construct five quadrants, of which three were taken to Peru and two to Lapland. In 1735 he furnished two standard toises established on the basis of the Chatelet toise, and each mission took along one standard. The Peru toise, also called the Academy toise, later replaced the Chatelet toise as the official standard. Langlois’s successor, his nephew Canivet, was commissioned in 1766 to construct eighty copies, which were distributed to all the provincial parlements. Moreover, the Peru toise was used in 1793 to determine the length of the four platinum rulers constructed by étienne Lenoir to measure the bases of Melun and Perpignan for the great meridian triangulation carried out by Delambre and Méchain. Several of Langlois’s instruments, the six-foot sector of 1738 and a two-foot quadrant built in 1739, were employed in 1739 and 1740 by Cassini de Thury, Giovanni Domenico Maraldi,m and the Abbé de Lacaille for the first series of triangulations undertaken to establish a new map of France. In 1744, Le Monnier entrusted Langlois with the restoration of the meridian gnomon of the Church of St. Sulpice in Paris, laid out twenty years earlier by the clockmaker Sully. The operculum, seventy-five feet above the south portal, was fitted with a lens having a focal length of eighty feet. Langlois placed the long copper plate representing the meridian on the ground; on the terminal obelisk he traced the curve and divisions of the solar pointer as a function of the calendar.