Measurement of the trigonometric base performed on the Appian Way

Angelo Secchi
“Veduta generale dell’apparato per la misura della base al principio avanti al monumento di Cecilia Metella”
Misura della Base Trigonometrica eseguita sulla Via Appia
per ordine del Governo Pontificio nel 1854-55
, Roma, Tipografia della Rev. Camera Apostolica, 1858
[Plate in the front of the frontispiece]
incisione, 300 x 200 mm
Library of INAF-Astrophysical Observatory of Turin

The measurement of the geodetic base on the Appian Way, coordinated and accomplished by Fr. Angelo Secchi, was a threefold interest: the purely geodetic one, because the base was the side measured on the ground which provided a scale to the whole triangulation, the historical-scientific one, because the base on the Appian Way, already been measured by Boscovich a century earlier, had been the subject of a scientific controversy, and the historical-archaeological, since in those years the Appian Way and many Roman monuments that meet nearby had returned to new light following a great excavation work by the architect and archaeologist Luigi Canina. In the introductory chapter of the memory dedicated to measurement operations, Father Secchi eviscerates all three of these aspects, obviously describing the importance of the measure for the geodetic and topographical operations in the Papal States and in the whole Southern Italy, not neglecting the historical-archaeological aspect: moreover, along the Appian Way were still recognizable the places of the original milestones and it was therefore possible to establish a good correspondence between the ancient and modern linear unit of measure.
The measurement operation of a geodetic base, carried on by the means of the time, is the most delicate one can imagine: it was necessary to measure a tract of open terrain, several kilometers long, overcoming the local asperities and fixing the extremes of the measured tract with signals visible from away and possibly at horizon sight for subsequent operations. In the placement of the extremes, and therefore in the measure of length, it was hoped to attain a relative precision of the order of the millionth: for a base of 12 kilometers, like the Roman one, this meant measuring the length with an error of the order of the centimeter. Despite the difference between the principle of the base (in front of the mausoleum of Cecilia Metella), and its end (at the Frattocchie) was quite hard (125.21 meters according to levelling for the purpose instituted, described in the chapters III and IV of the memory), the Appian Way, with its almost exactly rectilinear conformation, did not require demanding preparatory work; nevertheless, every precaution would be needed to ensure the result. In the various chapters of the Father Secchi’s Memory , after an historical introduction (head I), shows with high detail the criteria that led to the choice of instruments (head II) and the way in which they were used, as suggested by the direct experience of his staff and the assistants assisting him, for alignment (head V).
In particular, the instruments, being of new conception (Secchi adopted the method conceived by the Italian engineer Ignazio Porro, who had installed in Paris a workshop for the production of instruments of geodesy and topography, and which substituted the numerous “Tese” To be approached in succession with a series of microscopes bearing banquets, using only one “tense” to measure the distances between the microscopes themselves (Figure 1) are the subject of a detailed description, including the many modifications introduced to facilitate the Operations (head VI). Follow the methods of reduction and calculation (heads VI and VII), also very detailed, and of course the results of the measure (chapters IX-XI), referred to the amplitude allowed by keeping the volume within a reasonable size. The volume ends with an appendix, where the previous measure of Boscovich and Maire and the subsequent criticisms of the French geographers engineers, are rediscussed in order to resolve the dispute.

Giuseppe Massone, INAF-Osservatorio Astrofisico di Torino