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Recent A dvcmces in T elegraphy. 
IV. RECENT ADVANCES IN TELEGRAPHY. 
By J. Munro, C.E. 
,F recent years the most important development of 
practical telegraphy has been the duplex system of 
sending messages. The idea of transmitting more 
than one message along a single wire at once appears first 
to have been conceived by M. Zantedeschi, as early as 1829 J 
but we may take it for certain that it was not until 1853, or 
about twenty-five years ago, that any serious attempts were 
made to carry it out in practice. From that time the atten- 
tion of electricians, both in Europe and America, has been 
directed to this interesting problem, but, until the last ten 
years, with so little success that in the 1867 edition of 
Sabine’s “ History of the EleCtric Telegraph ” we find the 
following sentence : — “ Both these systems of telegraphing 
in opposite directions, and of telegraphing in the same di- 
rection more than one message at a time, must be looked 
upon as little more than 4 feats of intellectual gymnastics ’ 
— 'Very beautiful in their way, but quite useless in a practical 
point of view.” 
Experience, however, shows that it is unwise to repudiate 
an eleCtric novelty ; and since these words were written the 
duplex system of telegraphing in opposite directions has 
become the ordinary means of communication over thousands 
of miles of land-lines in England and America, and of sub- 
marine cables in Europe and the East ; while the multiplex 
system of sending several messages in the same direction is 
rapidly being brought into practical service by means of the 
Meyer instrument in France and the telephones of Elisha 
Gray in America and La Cour in Denmark. Indeed, by a 
future combination of the duplex and multiplex systems, we 
may yet have a single wire transmitting as many as twenty 
or more distinct messages, ten either way. 
In order to explain the general principle of duplex or 
counter transmission, it is necessary to be perfectly familiar 
with the ordinary method of simplex or simple transmission. 
An ordinary telegraphic circuit invariably consists of 
the battery or source of the eleClric current ; the key or 
sending instrument, by which the circuit is opened or closed 
