This is a marine sextant with a gyroscopic artificial horizon. Georges Ernest
Fleurais, a French naval officer, designed the form in the late 1880s, for use
when the natural horizon was not visible. The sextant was made in the early 1890s by Ponthus & Lepetit, shortly after they had taken over the shop begun by E. Lorieux in the 1830s. The gyroscope, marked "PONTHUS & THERRODE PARIS," was made a few years later. The U.S. Naval Observatory transferred this instrument to the Smithsonian in 1963.
Ref: K. H. Beij, "Astronomical Methods in Aerial Navigation," Report of the National Advisory Committee for Aeronautics 198 (1924), pp. 20-21.
G. E. Fleurais, "Gyroscope Collimateur, Substitution d’un repPre artificiel B l’horizon de la mer,"
Revue Maritime & Coloniale (1887).
G. E. Fleurais, Horizon Gyroscopique, ModPle Définitif (Paris, 1891).
Syndicat des Constructeurs en Instruments d’Optique & de Précision,
L’Industrie Française des Instruments de Précision (Paris, 1901-1902), pp. 211-212.