Observations and measurements in the Real Gabinetto Fisico in Naples

Starting in 1840, an extensive observational work, primarily related to meteorology and geodesy, was conducted within the Real Gabinetto Fisico located on the second floor of the Royal Palace in Naples. Its establishment dates back several decades, but a large number of apparatus and instruments were commissioned during this period to the most renowned instrument makers of the time. The significant increase in the Royal Physics Cabinet's equipment allowed for the launch of a thorough observation campaign, not limited to meteorological observations. Many thermometers and barometers were available there for measuring temperature and pressure, including a Fortin barometer and an aneroid barometer. Fortin's instrument was a technically improved version of Torricelli's mercury barometer, with the mercury container consisting of a deformable leather bag placed inside a cylindrical tank. The lower level of the mercury could be adjusted via a jackscrew until it coincided with a reference mark. Humidity measurements were then taken with hygrometers, particularly using de Saussure's hair hygrometer (Fig. 1). Its operation is based on the property of hairs to absorb water vapor and change its length in response to changes in air humidity. For geodetic operations, several instruments were available in the Royal Physics Cabinet of Naples. Measurements of the Earth's magnetic inclination (i.e., the angle between the direction of the Earth's magnetic field and the plane of the horizon) were taken with a Gambey inclinometer (Fig. 2), consisting of a twenty-four-centimeter-long magnetic needle free to rotate around a horizontal axis within a graduated vertical circle. This circle, in turn, was fixed to a brass support, equipped with a spirit level and capable of rotating around a vertical axis. The steady growth of the Royal Cabinet's instrument collection, the ongoing maintenance of its instruments and apparatus, and the extensive scientific activity conducted there testify to the leading role this institution played in the Neapolitan scientific landscape from 1840 to 1860, when Garibaldi's entry into Naples put an end to the Kingdom of the Two Sicilies.

___Salvatore Esposito & Adele Naddeo