Barometer built in 1793 by the famous Paris instrument-maker Nicolas Fortin. The mercury is missing. The instrument is fixed to a semi-spherical brass plate with graduations in French inches. The cistern is made of boxwood. Appreciated for its robustness, Fortin's barometer was used, together with the thermometer, in late eighteenth-century laboratories for accurately measuring the "atmospheric" conditions of experimental processes. Once the chemically active role of air had been recognized, chemists understood that changes in pressure and temperature could influence the results of chemical reactions.