MAKING AND LAYING SUBMARINE CABLES . 157 
The Atlantic and Indian Ocean cables, which reach depths 
of 5,400 yards and 4,100 yards respectively, are necessarily of a 
more elaborate construction. In these cases, each steel wire is 
provided with a hemp or Manilla jute covering, which adds its 
own strength to that of the steel, whilst the weight of this cover¬ 
ing adds nothing to the weight of the cable in the water. When 
the plaiting is carefully made, it is found that the strength of 
the covered wire is greater than the sum of the strengths of its 
parts. This is because the parts separately would give way 
at their weak points, and these points do not coincide in the 
Fia. 98 . 
complete cable. Cables of this kind are the strongest and 
lightest that have ever been made (fig. 100). 
Their breaking strain is equal to eleven times the weight in 
water of a nautical mile of their length; that is to say, if they 
had to reach a depth of eleven nautical miles, they could bear 
the strain without rupture. The greatest depth of the Atlantic 
does not exceed 2,700 fathoms, or a little more than three 
miles, and the modulus of rupture is therefore four times the 
actual strain. 
Siemens also constructed the cables unsuccessfully laid by 
the French government between Bone and Bizerte, and between 
Carthagene and Ain-el-Turck, near Oran. In these cables 
(fig. 101), instead of the iron protecting wires there is a cover- 
