MAKING AND LAYING SUBMARINE CABLES. 155 
account of the plastic nature of the gutta-percha, the cordage 
must not press too strongly on the core. And again, the hemp 
covering must not only serve as a protection from the wires of 
the iron sheathing, but must add its own strength to that of the 
external covering. The hemp used at first to be covered with 
tar ; but tannin is now preferred as a preservative, because it 
allows faults to be more easily detected. 
Sometimes the cores of short cables connecting countries that 
have much inter-communication, consist of a number of wires 
formed into one strand; but more frequently the core is only 
a single wire, because it has been found more advantageous to 
Fig. 96.—Cable between Cagliari and Bone, 
increase the number of cables than to incur the risk of a com¬ 
plete interruption of all the means of communication. 
The process by which cables are covered by the iron wires 
that form their external protection is in every respect similar 
to that used in the manufacture of iron ropes, and the ma¬ 
chinery used for this last purpose can be used in making cables 
if a central guide be added to protect the core against unequal 
pressure of the iron wires. In a well-made cable, the central 
part should be subjected to no lateral pressure from the iron 
wires that are wound round it. All the machines used for this 
purpose leave the wdre in its place without any twist, just as in 
wire ropes (fig. 97). 
Iron cables are now covered with two reversed layers of thick 
tow, mixed with mineral pitch or asphalt, combined with sili¬ 
cate of lime to give it consistence. The machinery used in this 
process resembles that referred to above, and the vessel of 
mineral pitch is kept at a suitable temperature by steam. It 
is necessary not to let the cable remain too long in the trough 
of fused asphalt, as that would soon spoil the insulation ; this 
