Meteorograph by Secchi

Fratelli Brassart et al., Roma, 1867
Brass, wood, still
220 x 150 x 150 cm
Museum of INAF-Rome Astronomical Observatory, Monte Porzio Catone (Rome)

The first attempts to automate the tedious and repeated readings of meteorological instruments date back to the mid-seventeenth century. It was mainly during the nineteenth century that due to the increasing importance of climatic forecasts, and for the consequent need of increasingly systematic and capillary observations , many types of instruments (the Meteorographs) able to record multiple meteorological parameters at the same time were designed and built. Certainly one of the most sophisticated and complex instruments of this was that conceived and built by Father Secchi between 1857 and 1867. Secchi, stimulated by the studies of the American meteorologist and oceanographer Matthew Fontaine Maury, thought that in order to understand “the weather machine” it was necessary not only to collect a large number of data, but also to order them in a systematic and easily interpretable way. The genesis of Secchi’s meteorograph lasted about a decade: between 1857 and 1866. But it was not a systematic project: it developed in 1857 by one realization of Secchi,a barometer with a scale. The instrument allowed to determine the atmospheric pressure determining the weight of a mercury column instead than its height. The movement of the arm that supported the barometric tube that was caused by the pressure variations was then written down on a mobile board whose vertical movement was adjusted by a clock. Encouraged by the success of his barometer with data recorder, Secchi gradually added to the equipment other instruments. The first one was an anemometrograph equipped with a wind vane and an anemometer. Thanks to an electromagnetic system coming from the telegraphic technology these instruments sent a series of electrical impulses to the counters and to writing arms which recorded both the direction and the speed of the wind on the mobile table. The thermometrograph (Figure 1) was made of a long copper wire whose variations in length as function of temperature were connected to a writing arm. Subsequently a complex electromagnetic system was added: thanks to a cart driven by a cam it could determine the levels of the thermometers (“dry” and “wet”) of a psychrometer through platinum tips, and therefore record the hydrometric condition of the air. Finally, the quantity and hours of rain were determined by a pluviographer. Electro-mechanical servomechanisms were activated by electrochemical batteries enclosed in the unit cabinet. The instrument, which was continuously modified and tuned over the course of a decade, reached its definitive form in 1866 circa (Figure 2). It was built most by the Roman manufacturer Ermanno Brassart but the counters and clock were built by the famous Parisian watchmaker Constantin Louis Detouche. The wooden parts and barometric barrel were built in Rome by other skilled craftsmen. In the final version of the instrument the clock (equipped with a bell set on the quarter of hour) operated two boards at different speeds, on which the papers for records were fixed. One of them collected the data (the “meteographic curves”) for 48 hours, the other one for 10 days. These sheets were then published in the Meteorological Bulletin of the Roman College Observatory.
The instrument, installed in the Torre Calandrelli of the Collegio Romano, gave excellent results and was thus decided to exhibit it in Paris at the Universal Exposition of 1867.

Paolo Brenni, CNR-IGG