INTRODUCTION. 
3 
After electricity, light is the natural agent with the greatest 
velocity of propagation. It is also the one that most readily 
impresses our senses, and it does so from the greatest distance. 
Its velocity is such that it can be expressed only in the smallest 
fraction of time, even in its passage over a distance equal to the 
longest line of telegraph. 
Then, whether we employ artificial light for transmitting 
signals, or utilise the direct or diffused light of the sun, no 
special conductors are required between the stations in order to 
make luminous beams or opaque bodies perceptible; the signals 
are conveyed with the speed of thought. After electricity, light 
therefore presents all the advantages of velocity, variety, cer¬ 
tainty, and economy, except only that atmospheric conditions 
may frequently interfere with the transmission of signals. In 
spite of this inconvenience, optical signals are still much used, 
and even at the present day are of great service to ships, and 
to armies in the field. 
Electricity is the true telegraphic agent, and to it especially 
does the telegraph owe its present development. And as it has 
now been allied with acoustical phenomena, it probably will 
soon be allied with optical phenomena also, and indeed be asso¬ 
ciated with other serviceable physical agents. We may then 
hope that an extension of the range of visual sense will soon 
follow that of the auditory sense, and that electricity is des¬ 
tined to greatly enlarge the present sphere of our material and 
intellectual life. 
Science certainly has much yet to reveal to us, and the four 
physical agents, the effects of which we have just now briefly 
mentioned, will no doubt at some time be so combined as to 
supply mankind with" the most perfect means of telegraphy. 
In the following pages we shall examine the different systems 
of telegraphy, founded on the four above-mentioned physical 
agents, but we shall of course devote the largest space to the 
electric telegraph. 
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