But it was not until the Franco-Prussian War that the one thing for which the Montgolfiers had predicted their usefulness in warfare--that of sending messages out from a closely besieged city--was put to practical test. During the siege of Paris by the Germans in 1870-71, when every other possible means of communication had been cut off, the Parisians still kept in communication with the outside world by means of balloons and carrier-pigeons. On September 23rd, the first ascent of the siege was made by the aeronaut Durouf, who carried a large number of despatches from the city, landing near Evreux, after being in the air about three hours. The success of this journey and several others that quickly followed led the French Government to establish a regular balloon-post, and to undertake the manufacture of balloons for this purpose. The mere matter of balloon construction offered no difficulty but a more serious one was met in the lack of experienced aeronauts. In this emergency, however, it occurred to the authorities that sailors, accustomed to climbing about at dizzy heights, might be taught to take the place of trained aeronauts. This experiment proved most successful, and in subsequent voyages these mariners maintained their reputation for daring undertakings. Between September and January sixty-four balloons were sent up, all but seven of which fulfilled their mission and delivered their despatches; and the total number of persons leaving Paris in balloons during the siege was one hundred and fifty-five. These carried with them a total of nine tons of despatches and something like three million letters, the speed with which these journeys were made ranging from a minimum of twenty miles an hour to a maximum velocity, in one instance, of eighty miles.