16 - Petronio Matteucci and Giovan Battista Guglielmini - The weather-station and the final XVIIIth-century contribution to astronomy.

Eustachio Zanotti died in 1782 after more than 40 years of hard work and an incredible number of observations of the Sun, the Moon, the planets, comets and light variations of the stars, as well as a more varied range of activity that included restoration of the meridian line in San Petronio and the study of perspective (146).

His successor in the post of director and the chair of Astronomy was the abbot Petronio Matteucci, assistant at the Specola since 1740 and esteemed aid, together with Giovanni Angelo Brunelli, of Manfredi and Zanotti.

Matteucci was director of the Specola for almost twenty years, faithfully pursuing the age-old programme of calculus and publication of Ephemerides. Although he succeeded in renewing a good part of the equipment, with the purchase in England of a regulator clock by Ellicot [file 9], a wide-field telescope by Gilbert [file 43], a parallactic telescope, a long telescope and a reflecting telescope, all by the Londoners Dollond [files 37, 38, 39] - at the time sole producers of achromatic lenses - his research did not stand out particularly until the introduction of meteorology.

Bologna became part of the European network of meteorological observations, carrying on, albeit with various interruptions, the collection of data begun privately by in 1714 by the Bolognese physicist and chemist Jacopo Bartolomeo Beccari (1682-1766). The daily recording of temperature, pressure, winds and meteors was logged from 1782 to 1792, being later resumed by Matteucci’s successors and continuing without interruption from 1813 to today (file 92). It is thus possible to document the Bolognese climate almost uninterruptedly from the XVIIIth century, i.e. for almost three hundred years (147).

Of note as regards Matteucci’s interest in meteorological events is his participation in an experiment carried out on July 27, 1752, by the physician Giuseppe Veratti (1707-1793) to demonstrate the electrical nature of lightening. This latter, following the advice of Benjamin Franklin (1706-1790) in a letter of 29 July 1750, and almost contemporaneously with what Georges-Louis Leclerc de Buffon (1707-1788) was doing in France, had a metal rod placed on the terrace of the Specola. Touching the rod with keys at the approach of a thunder storm he felt, as he accurately described in a paper, effects "that were exactly the same as those of common Electricity." (148)

The last important XVIIIth-century Bolognese contribution to astronomy came from the abbot Giovan Battista Guglielmini who should have been assistant astronomer but whose nomination Matteucci refused to recognize in that it dated back to the period of the Cisalpine Republic.

Professor of Mathematics at the University, Guglielmini would turn down the position of director of the Specola that was offered him in 1802.

In 1790 he had an opening made at the foot of the spiral staircase of the tower to prolong to 29 m the fall of a heavy body from the top of the tower and measure the deviation south-eastwards from the vertical that Newton, who in 1679 had proposed a similar experiment to verify the Earth’s rotation, had predicted.

The 4.5 mm displacement from the vertical that Guglielmini obtained was in close agreement with the deviation predicted for that height of 3.9 mm and the following year he repeated the experiment using the bigger drop (78 m) of the internal stair-well of the Asinelli Tower.

Although the Earth’s revolution around the Sun had already been proved, rotation around its own axis had not and Galileo himself had suggested using the experience of the fall of heavy bodies to provide the necessary proof.

Guglielmini’s experiment was accurate to the tiniest detail, including carrying it out at night so that vibrations from passing carts would not interfere with the lead balls as they were dropped (149). Despite this however, and the extremely positive results too, his conclusions had very little resonance even though they did anticipate the celebrated experiment of Léon Foucault at the Pantheon in Paris by 50 years.

  1. - E. Zanotti: 1779, La meridiana del Tempio di San Petronio ..., Bologna.
    E. Zanotti: 1766, Trattato teorico-pratico di prospettiva, della Volpe, Bologna.
  2. - E. Baiada: 1986, Da Beccari a Ranuzzi: la meteorologia nell'Accademia Bolognese nel XVIII secolo, in Le meteore e il frumento, R. Finzi (a cura di), Il Mulino, Bologna.
  3. - G. Veratti: 1752, Osservazione fatta in Bologna l'anno MDCCLII de i fenomeni elettrici, della Volpe, Bologna.
    G. Tabarroni: 1966, La torre dell'Università a Bologna e l'elettricità atmosferica, Coelum, XXXIV, p. 102.
  4. - G.B. Guglielmini: 1792, De diurno terrae motu, Bononiae.
    G. Tabarroni: 1983, Giovanni Battista Guglielmini e la prima verifica sperimentale della rotazione terrestre (1790), Angelicum, 60, p. 426.
    A. Braccesi: 1983, Un dimenticato experimentum crucis: la prova fisica della rotazione terrestre ottenuta nel 1791 da Giovan Battista Guglielmini osservando la deviazione a sud-est della verticale dei gravi in caduta libera, Giornale di Astronomia, 9, 319.
    S. De Luca: 1991, La prima verifica sperimentale della rotazione terrestre effettuata da G.B. Guglielmini a Bologna nel 1790-1791. Tesi di laurea in Astronomia, A.A. 1990/91, Università di Bologna (contiene la traduzione dell'opera citata di Guglielmini).