“Square des Arts et métiers”, Paris

"Vue prise du boulevard Sébastopol"
1870ca.
Albumin, 110x160 mm
Private collection

Paris was the destination of Father Secchi’s last two trips abroad: the first in August 1870, the second between September and October of 1872. Although the reason was not touristic, and thus almost certainly the astronomer would not have wandered among palaces and monuments (Figure 1), in his last visit, he will certainly have seen the terrible consequences of the siege of the city during the Franco-Prussian War.
The reason of the journeys was scientific: the participation, as representative of the papal State, in the work of the International Commission for the definition of the Meter.
Right in Paris, at the National Archives was preserved, from the end of the eighteenth century, the “standard meter”,a platinum bar, long the ten-millionth part of the terrestrial meridian that had been measured by astronomers Jean Baptiste Delambre and Pierre Méchain. The passing of the years and the diffusion of many “sample meters” not exactly of the same length had made necessary the realization of a new prototype of the unit of measure.
Secchi went to Paris in August 1870 as a member of the commission in charge of this revision, which met up at the Conservatoire des Arts et Métiers. The French Government had invited 26 countries, but due to the war only 16 delegations reached Paris. The works were interrupted and resumed only in 1872.
Between the two meetings, something had changed in Italy: on September 20, 1870, the conquer of Rome, with the breach of Porta Pia, ratified the annexation of the city to the Kingdom of Italy and the end of the Papal States as a political entity. Then the invitation of Secchi at the meeting of 1872 as delegate of the Holy See “was seen by the Vatican as an act in which the French Government acknowledged the right of the Pope to send a delegate to the Commission.” to the point that, compared to the request of Secchi of 600 Francs for travel expenses, the Holy See assigned to him 1000 Francs!
The Jesuit soon found himself in the midst of a diplomatic crisis. The papal state was granted the right to vote and so the clash with the Italian delegation, led by General Agostino Ricci, was unavoidable: the Italians withdrew from the works. In his diary, Secchi described with his usual humor the affair in which he was involved into. The diplomatic solution was finally reached: the Commission was to be considered only as a scientific institution, while the adherence to the metric system defined by the scientists was up to the States and not to the scientists. As a matter of fact, in 1875, only the political authorities signed the final convention. Among them, the Holy See was not there.

Agnese Mandrino, INAF-Osservatorio Astronomico di Milano

Bibliography
Chinnici I. (2018), Deconding the stars, in press;
Chinnici I. (2012), “Il profilo scientifico e umano di Angelo Secchi”, in: Altamore A. & Maffeo S., a cura di, Angelo Secchi. L’avventura scientifica del Collegio Romano, Foligno, Edizioni Quater, pp. 43-64.