Reversing mercury thermometer for taking sea temperatures at depth, made by Negretti and Zambra, London, 1896. The frame design is credited to Admiral Magnaghi, Hydrographer to the Royal Italian Navy, and was marketed by Negretti and Zambra from 1885. Stamped with mark of the South Kensington Museum.

Surveyors and navigators struggled to take accurate readings of sea temperature at specific depths. Previous modifications of conventional maximum and minimum thermometers were used in the mid 1800s, but it was not always possible to know at what depth the minimum temperature had been reached. Reversing thermometers attempted to overcome this problem. They incorporated mechanisms that would abruptly capsize the thermometer once it reached the required depth.