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As early as 1745 Pierre Bouguer, former professor of hydrography at Le Croisic, then at Le Havre, was solicited by the Academy of sciences of Paris and the successive ministers of the navy, Maurepas and Rouillé, to examine several treatises and projects on the discovery of the secret of longitudes submitted to the authorities. Having returned from Peru since 1744, after an expedition to Equator under the auspices of the Academy, which represented ten years taken away from personal research, Bouguer sought to replace Pierre-Louis Moreau de Maupertuis, who had left for Berlin, as “officer for the improvement of the navy.” In 1749 Rouillé granted him the pension of 3 000 pounds for this position, which had been created especially for Maupertuis in the autumn of 1739. Bouguer acquitted scrupulously this task until his death in 1758. An examination of the style, ideas and contradictions of Bouguer leads us to bring out a leitmotiv in his reports : faced with clocks whose functioning was not regular enough for determining longitudes at sea, such projects were held to be doomed to failure and were not to be encouraged. Thus Bouguer’s practical expertise appears to have been an obstacle to the development of marine clockwork in France during the 1750s. Was this obstinacy responsible for the lag of French clockmakers with respect to their British counterparts during this crucial period of the conquest of longitude at sea ? Such is the question this article seeks to answer.