For communicating optically from ship to ship during the day, the
marine uses flags of different forms and colors, and flames. Between
ships and the land there are used what are called semaphore signals,
which are made by means of a mast provided with three arms and a disk
placed at the upper part. The combinations of signs thus obtained,
which are analogous in principle to those of the Chappe telegraph,
permit of satisfactorily communicating to a distance.
On board ship, hand signals are used like those employed by the army
for communicating between bodies of troops. For night communications
the marine employs lights corresponding to the day flags, as well as
rockets, and luminous rays projected by means of reflectors and
intercepted by screens.
In conclusion, it may be said that optical telegraphy, which has only
within a few years emerged from the domain of theory to enter that of
practice, has taken a remarkable stride in the military art and in
science. It is due to its processes that Col. Perrier has in recent
years been enabled to carry out certain geodesic work that would have
formerly been regarded as impracticable, notably the prolongation of
the arc of the meridian between France and Spain. Very recently, an
optical communication established between Mauritius and Reunion
islands, to a distance of 129 miles, with 24 inch apparatus, proved
that, in certain cases, the costly laying of a submarine cable may be
replaced by the direct emission of a luminous ray.
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