Three people came up with the idea of placing the mail in watertight zinc cylinders, given a spherical shape by means of fins, which were immersed in the Seine well upstream of Paris, not far from Montereau-Fault-Yonne.

Rolling downstream on the river bed, these balls were to pass through the lines of the besiegers and be collected in a net set up on the outskirts of the capital.

Despite promising trials, the process proved a complete failure, with none of the 55 Moulins balls dropped between January 5 and 28, 1871 reaching Paris before the end of the siege, although more than half of them have since been found, from the Seine-et-Marne to the mouth of the Seine.

The air route was difficult to use: only carrier pigeons managed to enter the capital, but their numbers were insufficient for the volume to be transported, and the failure rate was high.

[5] As early as September 1870, Mr. Castillon de Saint-Victor was considering the idea of using the Seine's current to carry watertight balls containing mail, based on the "message in a bottle" principle.

[7] Three people, Pierre-Charles Delort, Émile (or Louis-Émile) Robert and Isca (or Jacques) Vonoven,[8] developed a system based on the manufacture of zinc cylinders, a metal unaffected by water, which were watertight once the lid had been welded on.

The device conceived by the three men, who did not seem to have been aware of Castillon de Saint-Victor's project, was perhaps inspired by the one used for tobacco smuggling on the French-Belgian border, in the waters of the Yser.

[11] Two successive experiments, carried out over short distances in the Bièvre at Arcueil, followed by another on December 1 and 2, 1870 over a longer distance at Port à l'Anglais in the Seine, were conclusive: balls thrown into the water were caught in the nets designed to stop them; in the case of the Bièvre, the clarity of the water even made it possible to follow them on their way.

Although the document was financially very interesting for the designers, it was vague on many practical details: for example, the collection point, Clermont-Ferrand, was only agreed verbally, and although the amount of the remuneration for the operators was fixed, the payment terms were not specified.

[7] Delort and Robert left the capital the following night aboard the Denis Papin balloon mail, which took off from the esplanade of the Gare d'Orléans.

[19] In order not to arouse the enemy's attention, the letters are not marked with any special stamp; only the recipient's address must bear the words "Paris, par Moulins (Allier)".

[17][note 2] Delort stayed in Moulins to pack the balls - those brought from Paris were too few in number, so others were made in Lyon by a craftsman -, fill in the slips detailing their contents, and supervise their transport to Cosne-sur-Loire, where they were stored.

[10] Robert, dressed as a farmer driving a straw cart or as a poultry and egg merchant, transported the "agents" - the term used to designate the balls from the very first discussions with Rampont - from Cosne to the banks of the Seine.

After the war, the designers, whose remuneration clauses had not been respected by the Post administration, took action against the State to obtain financial compensation, going so far as to write to Marshal Patrice de Mac Mahon, but without success; Delort and Robert were only entitled to a commemorative medal for their balloon trip out of Paris.

[17] During the 1968 salvage operation at Saint-Wandrille, the French Post Office decided to affix an official stamp to the back of the letters recovered from the balls, authenticating them.

[19] In three cases, proof was provided that a letter dispatched from Paris to the provinces by mounted balloon had received a direct reply by Moulins Balls.