138 
THE TELEGRAPH 
tunnels of the South Eastern Railway. Siemens, about the 
same time, also covered copper wires with a layer consisting 
chiefly of gutta-percha. About the spring of 1848 the Prussian 
government adopted the system of subterranean wires covered 
with gutta-percha. By this time Siemens had laid the founda¬ 
tion of a rational system of testing, and even devised a method 
for ascertaining the exact position of a fault, whether in the 
continuity of the insulating covering, or in the wire itself. 
At the same time he pointed out the electrical phenomena 
that occur in a long line, in which the subterranean wire with 
its insulating covering acts like a huge Leyden jar, and 
the induction causes a return current, as in Volta’s well-known 
experiment. 
The plan originally adopted in England for laying under¬ 
ground lines has been continued. Cast-iron pipes are used in 
places where the ground is liable to much vibration, and 
earthenware pipes in places not so liable. The sizes of these 
tubes vary according to the number of wires that require to 
be placed in them. Within each pipe an iron wire is left 
which is joined on to another, so that the cables can be pulled 
through the tubes. 
Nearly the same method is adopted in France, and M. Ch. 
Bontemps gives a complete and interesting description of 
the laying down of subterranean lines. In the English and 
French system, however, the cables are introduced into tubes 
of narrow dimensions. So long as the tube passes in a straight 
line, there is no difficulty about dragging the wires through it; 
but it is a different matter when even a slight curve occurs ; 
the guiding wire often breaks, and the tubes have to be opened 
at a great loss of time and money. 
We are therefore surprised that the use of the Delperdange 
system has not extended. This system has been adopted in 
Belgium, and we shall describe it here in order to show its 
superiority. 
The tubes for containing the cables are of cast iron, and all 
have a length of ten feet. Their diameters, however, range 
according to the number of cables they have to contain; for 
three, five, and seven cables, the internal diameters are two 
inches, two and one-third inches, two and three-quarter inches 
respectively. 
