SUMMARY
An unusual instrument, named “Barothermometer”, belonging to the collection of Villa Vigoni, on the Como Lake, Italy, is analysed. The instrument is composed of a U-shaped thermoscope and a spirit thermometer. Both instruments have their tubes placed side-by-side on the same frame and use the same scale. It measures the air temperature and the tendency of the barometric pressure to predict weather changes. A study is made to clarify the operating principle and produce the instructions for use. The development of the ideas to which the instrument has been inspired follows a tree starting with the Galileo’s thermoscope, the Amontons air thermometer and finally the Adie sympiesometer. The measuring method is typical of the first half of the 19th century, but the construction details in aluminium, the fonts used in the printed instructions and the colour fading suggest that the instrument was built around 1930-40.