The 1792-1798 meridian mission

The Rapport sur le choix d'une unité de mesure [2] outlined a plan to realize the new system of measures. One of the items of the plan was to re-measure the Lacaille-Cassini arc of the Paris meridian from Dunkerque to Perpignan,³²also extending the measurements down to Barcelona (a total of almost ten degrees in longitude, for a length of 1075 km -- see the map in Ref.[39] to get an idea of ``cette belle entreprise''[14]).

The motivation of the Academy in support of the enterprise was to improve the knowledge of the meridian length. The confidence on a possible improvement relied on several factors:

the ability of the astronomers presently involved in this job, the perfection that the mathematical tools and instruments have acquired in the last times, the magnitude of the measured circle, that is extended by more than nine degrees and one half, the advantage of being this

[the meridian arc]

cut in the middle by the forty fifth parallel, all that guarantees us the exact and perfect execution of this beautiful enterprise, the greatest of this kind. (Ref. [14], pp. 3-4)

The campaign of measurements started immediately, apart from delays of practical nature, after the Academy's proposal was accepted. A quote is in order to report the driving spirit of the commission, that also shows the yearning for an `egalité of measures', one of the political and ideological requirements of the new régime that can't be set aside any longer.

There is no need, in our judgment, to await the concurrence of other nations either to choose a unit of measurement or to begin our operations. Indeed, we have eliminated all arbitrariness from this determination and rely only on information equally accessible to all nations. (Ref. [2], p. 11)

Two astronomers were nominated responsible of the mission: Jean Baptiste Joseph Delambre, in charge of the northern part of the arc, up to Dunkerque, a French town on the North Sea, close to the Belgian border; Pierre François André Méchain, in charge of the southern part.

In principle, the task was simpler than that accomplished by Lacaille and Cassini fifty years earlier, because it seemed initially possible to use much of their work (like the triangulation stations). However, things were much more difficult, complicated by Revolution and wars (see Refs. [40,10] for a dramatic account of the enterprise). It was almost a miracle that Delambre and Méchain could meet again in Paris in November 1798 alive and with the logbooks of their measurements.

The unexpected long duration of the mission was the reason the académiciens were urged to provide the provisional length of the meter in 1793. In fact, ``the interests of the Republic and of the commerce, the operations initiated on the money and on the cadastre of France, require that the adoption of the new system of weight and measures is not delayed any longer''[14]. The Decimal Metric System was later established by law on April 7, 1795, well before the meridian mission was accomplished.

After the end of the meridian mission an international commission was convened to review the Delambre-Méchain data and to establish the length of the meter. In March 1799 the meter was determined in 443.296 lignes, also taking into account Earth flattening.³³The new standard differed by 0.114 lignes (0.32 mm) from the provisional unit. Compared with our present value (see Table 3) one can see that the new result slightly worsened the knowledge of the meridian.³⁴

The manufacture of the definitive model, based on the results on those measurements, was completed in June 1799. On 22 June, the prototype of the meter was solemnly presented to the Council of Elders and of the Five Hundred.