Optical telegraphy is simply a system for sending messages by light signals over long distances. This technology will be replaced by the radio telegraphy, that the signals will pass, later, by the radio waves instead of an optical signal. This is the X-ray that saved the Eiffel Tower, the army seeing a certain interest, and so that it could take place, it was necessary that the optical telegraphy be tried, by the army also, a few years more early.
We are therefore in the late nineteenth century, radio technologies are still in its infancy, unimaginable from a practical point of view. The army is trying optical telegraphy experiments.
When you know the visibility at the top of the Eiffel Tower, you can well imagine the importance From the point of view of national defense, it can be used as an observatory of optical telegraphy, either daytime or nocturnal, using the Mangin projectors. Also, the competent service of the Ministry of War had, in anticipation of this job, to experimentally determine a certain number of points situated on the extreme periphery, with which it was possible to exchange signals. Of course, these perfectly specified points are known to this service alone. If these communications with distant points had existed during the investment of Paris in 1870, it is perfectly realized what incalculable services they would have rendered to the defense.
Without going into any detail on this subject, we can, however, give some indications from the point of view of the defense of the entrenched camp of Paris, according to a note handed to us by the lieutenant-colonel of retired artillery, Mr. d'Esclaibes, author of the visibility map reproduced on the page visibility at the top of the Eiffel Tower.
"From the point of view of the relations with the province in the event of investment, one can communicate either directly or by a relays judiciously chosen in advance, with Beauvais, Soissons, Provins, Fontainebleau, Chartres and even Rouen.
As communication of the new forts between them and with Paris, supposing cut the telegraphic or telegraphic lines which reunite them, the Tower can serve as common relay, except to establish in the center of some of them a light turret very small in height.
The Tower can therefore render, in the field of military optical telegraphy, invaluable services and contribute in a great measure to the defense of the entrenched camp of Paris."
It was in view of this eventuality that it was stipulated, at the original convention with the State, that in case of war, the Ministry of War immediately took possession of the Tower and all its organs. and all the electrical lighting devices that depend on them.