ATLANTIC TELEGRAPHY. 
545 
Sardinia. As shown in the sections (fig. 10^ PI. XXI. the elec- 
trical cord is composed of seven small wires twisted together 
and insulated hj a thick layer of gutta percha. The deep-sea 
portion is protected by eighteen small iron wires; while the 
shore end is covered by an armour of ten stout iron wires_, and 
which is consequently much heavier than the deep-sea cable. 
The weight of the deep-sea section of this cable is 1^960 lbs. 
per mile^ and its total cost was £125^000. These may be 
regarded as typical examples of nearly all the submarine elec- 
trical cables which have been laid. It becomes necessary to 
give a little more in detail the description of the Atlantic cable 
of 1858 (fig. 9;, PI. XXI.). The electrical conductor was a 
copper strand^ consisting of seven wires — six laid round one^ 
forming in fact^ as far as electricity is concerned_, only one 
wire. This weighed 107 lbs. per nautical mile of 2^028 
yards. It was carefully insulated with gutta percha, laid on 
in three coatings, and weighing 261 lbs. per nautical mile. 
Eighteen strands of charcoal iron-wire — each strand composed 
of seven wires, six laid round one — laid spirally round the 
core. This was, however, previously padded with a serving 
of hemp, saturated with a mixture of Stockholm tar. The 
weight of this cable in air was 20 cwt. per nautical mile, while 
in water it was only 13*4 cwt. This made it equal to 4*85 
times its weight in water ; or capable of bearing its own weight 
in a little less than five miles deep of that fluid. Its breaking 
strain was estimated to be 3 tons 5 cwt. 
The unfortunate failure of this enterprise — in which both 
Europe and America were equally interested — led to consider- 
able discussion, and to a careful examination of all the conditions 
which were supposed to influence such electric cords. All the 
laws regulating the flow of an electrical wave were re-examined 
by some of our best electricians, and many previous conclusions 
received some correction. It had been supposed that an insu- 
lated submarine wire conducts according to a different law to 
that of a suspended circuit. This difference was thought to 
depend upon a s^^stem of induced earth currents influencing 
the submerged wire, which would not influence a wire when 
suspended in the air. A simple example to render this clear 
to the non-scientific reader may be necessary. If a wire 
insulated by means of gutta-percha, or any other non-conduct- 
ing substance, is connected, so as to complete the circuit, 
with the two ends of a voltaic battery, a current is said to 
flow through it. This means that an electrical wave of motion 
is propagated along it in a given direction. If another 
copper- wire is placed near it — though not in contact with 
either it or the battery — another current, or wave, is gene- 
rated, that is, induced, in it. This induced current interferes, 
