OPTICAL TELEGRAPHY. 
5 
and of the Valley of Luchon, were occupied by watchmen who 
rapidly transmitted news from the adjoining countries. 
The telegraph represented on Trajan’s column is the only 
description of a Roman telegraphic station that has come down 
to our days. This station is surrounded by palisades ; its 
second story has a balcony, and the building is surmounted by 
a small tower. 
The Arabs and the Asiatics practised the art of conversing 
by means of visible signals, and the Chinese erected fire- 
machines on their great wall of 188 leagues in length, in order 
that the alarm might be given along the whole frontier that 
divided them from the Tartars, when some horde of that race 
were threatening them. They, like the Indians, used lights so 
brilliant that they penetrated fogs, and neither wind nor rain 
could extinguish them. The English, having learnt in India 
the composition of these fireworks, used them in the operations 
of the great trigonometrical survey by which the observatories 
of Paris and of Greenwich were connected in 1787. 
These operations were conducted on one side by Cassini, 
Mechain and Legendre, and on the other by Roy and Blagden, 
and by them not only was a perfect triangulation effected by 
the use of fireworks, and even of ordinary reflecting lamps, but 
there was besides an exchange of signals from the opposite 
shores of the Straits of Dover. The possibility of a telegraphic 
communication across the Channel was therefore demonstrated 
as early as 1787. 
Francis Kessler, an enthusiastic patron of the occult 
sciences, was the anticipator of the optical telegraph, now 
used in the army. He enclosed his telegraph in a barrel 
containing a lamp with its reflector. In front of the barrel 
was a moveable screen which could be raised or lowered* by 
means of a lever. The screen raised once indicated the first 
letter of the alphabet, raised twice the letter b, raised three 
times the letter c, and so on. We shall presently see how the 
Morse alphabet signals may be represented by similar methods. 
The alphabetic system was in vogue at the period of which we 
are speaking, and it has reached down to the present time. 
In 1684, the celebrated Robert Hook described before the 
Royal Society of London, his system of signals formed of 
