Early hygrometric observations in Padua, Italy, from 1794 to 1826: the Chiminello goose quill hygrometer versus the de Saussure hair hygrometer

The paper includes the reconstruction and analysis of rare historic records of relative humidity (RH). After having highlighted the story of the development of the hygrometer, the paper considers two instruments that in 1783 were submitted to the prize of the Theodoro-Palatina Academy of Sciences, Mannheim, for a new hygrometer with comparable readings. De Saussure proposed a hair wound on a cylinder connected to a pointer and Chiminello a goose pen fixed to a glass tube and filled with mercury. Chiminello won the prize for the corrections of the temperature dependence. In the Astronomic Observatory, Padua, two rare parallel series of RH observations were made in the same place, and at the same sampling time (tree readings a day) with a Chiminello and a de Saussure hygrometer, over the 1794–1826 period. A study was made to know these instruments and interpret the readings. A replica of the goose-quill hygrometer was built to verify in the lab instrumental performances and calibration problems. After having recovered the data, calibrated the instrument, transformed readings to modern units (%), corrected errors and homogenised the series, the paper compares the RH variability in Padua between early and recent instrumental measurements. It includes predictions of RH for two periods of the 21st century, concluding that no major modifications are expected. The paper highlights the importance of looking for metadata about early station sites, instruments and observers, in order to reconstruct early series as correctly as possible.