140 
THE TELEGRAPH 
easily made for the discovery of any faults that might occur. 
They are covered by the pavement, but their positions are 
marked on the plan of the line. 
Conductors insulated by gutta-percha, and collected together 
in the form of a cable, are also used for passing through water¬ 
courses, tunnels and drains. 
Figs. 81, 82. 
Fig. 83. 
In the first case, the cables are identical with those laid 
along the bottom of the sea to connect distant shores. The 
piotective iron covering must always be of large size if the 
river is navigable, on account of the small depth of water in 
which the cable is laid. 
In tunnels, the cables are formed singly of copper wires, 
insulated by a double layer of gutta-percha surrounded by 
hemp, and covered with marine glue. They are attached to 
