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"Mulenskie balls" - a French submarine-mail
In the years 1870-1871 in a letter delivered to the besieged Paris not only with the balloon and pigeon, but also in special zinc boxes with small blades. These boxes fell into the Seine above Paris at Fontainebleau, and submerged under the water and carried away by currents reaches Paris. In the letters, which were sent to Paris, where they were caught in the Seine special iron grids, put a special mark: «a Paris par Moulins» («Paris via Moulin"). Therefore, these unusual means delivery of correspondence were called «boules de Moulins» («Mulenskie balls"). Some of them contained undelivered messages were found and removed from the Seine in 1970 and 1982.
via factroom.ru