148 
POrULAK SCIENCE IlEVIEW. 
conducting substance into the course of the circuit, and con- 
necting the ends of the wires with it, so as to give it the 
appearance of one of the ordinary joints or splices of the line. 
At the same time a few poles can be pulled down in another 
place, and the wires cut, and the probability is that the 
engineers who repair the line will not discover the hidden 
interruption of the circuit until after they have restored the 
gap, and found that the wire is still cut somewhere else, and even 
then the place where the non-conducting substance is intro- 
duced will not be discovered until some time has been em- 
ployed in carefully testing the line with the galvanometer. 
But there are other dangers to telegraphic communication in 
the field besides the mere damage to the line. If the enemy’s 
cavalry get possession of a station, they can easily send mes- 
sages containing false information or delusive orders to well- 
known officers of the opposing force, while the place from 
which they are sent, and the assumed name in which they are 
despatched, will give the messages an appearance of authen- 
ticity which, if it does not completely deceive the recipient, 
will at least be the cause of considerable doubt and perplexity 
to him, and perhaps make him hesitate to accept the accurate 
information or authentic orders received from other sources. 
Again, even without occupying a station, it is possible to read 
the messages which are passing along a telegraph line, and thus* 
perhaps discover important secrets. All that is required for 
this purpose is a small portable receiving instrument, and a few 
yards of copper wire to connect it with the line. A single 
individual, thus equipped, can “ tap ” a telegraph line, in the 
day time, by receiving the message in the ordinary way ; and at 
night (when, of course, it would be easier to approach the line) 
by listening to the clicking of the armature against the electro- 
magnet of the instrument. But all these dangers are only of a 
partial or temporary character. By carefully patrolling and 
testing the line, it cannot be interrupted for any length of time 
without the damage being observed and repaired. By adopting a 
secret arrangement that there shall be a certain number of 
letters in the two or three words at the beginning or end of 
every message, a despatch sent by an enemy can in most cases 
be detected ; and again, by employing a cipher alphabet, it 
will be difficult for anyone who taps the line to obtain informa- 
tion from the messages which fall into his hands. 
From this brief sketch of the structure and uses of the field 
telegraph the reader will understand what an important part it 
plays in modern war. On the march it directs the movements 
of advancing columns, on the battle-field it flashes orders and 
information with the speed of thought to right, centre, and left 
of the immense lines extended over mile after mile of country ; 
