A French absolute barometer of c. 1880 — a type documented in print but, until now, apparently unphotographed online.
The collection has acquired a by , French artillery officers who patented the design in 1872 and exhibited it at the Paris Universal Exhibition of 1878, where it took a silver medal.
The instrument abandons both mercury and the aneroid capsule. Pressure is read by comparing an ordinary thermometer with an open air thermometer, using a thread stretched between two sliding cursors — a geometrical construction that replaces calculation entirely.
It was never a commercial success. The aneroid was cheaper and demanded nothing of its owner, and the absolute barometer survives today almost solely in printed sources: the Comptes rendus of the Académie des Sciences, the proceedings of the 1878 International Meteorological Congress, and Gaston Tissandier's article in La Nature, no. 318 (5 July 1879).
Searches of museum catalogues, auction archives and collectors' publications turned up no photographs of a surviving example, and no description of one. The only known contemporary image is the engraving accompanying Tissandier's article — and it shows a later variant, not this rectangular form.
A full catalogue description, together with translations of the patent file, the congress proceedings and the instructions pasted to the back cover, is published here for the first time.
The photographs of the actual instrument published here appear online for the first time.
