PRINCIPLES OF DESIGN. 79 



Breat-St. Pierre cable in 1888 and is still doing duty where it 

 lies in 1,800 fathoms. The core, therefore, remains intact, bub 

 it is impossible to say whether the sheath, after this long period 

 below water, retains anything approaching its original strength. 

 It was found impossible to raise a 38-naut length of this cable 

 inserted in the 1873 cable during repairs 217 miles offValentia 

 in 1891. This, however, was laid on a bottom at only 400 

 fathoms, and proved to be of a highly corrosive nature to iron 

 wires. This section parted on lifting so often that it had to be 

 abandoned and replaced by a new piece of steel-sheathed cable. 

 Although the inventors never meant or recommended their 

 hempen sheath for shallow waters, it is well known that the 

 very best hemp decays even when unmixed with iron to an 

 extent sufi&cient to render its recovery in deep water a matter 

 of impossibility. For this and other reasons, such as the 

 weakness of the sheath to stand chafe and the difficulty in 

 laying to properly sink it into ocean cavities, the experiment of 

 its use will probably never be repeated. 



Considering now a modern deep-sea cable with a core of say 

 3001b. of gutta-percha and 3701b. of copper per naut, 12cwt. 

 of jute, tape and compound, and 17cwt. of steel sheath, the 

 gross weight would be 35c wt. per naut, and the specific gravity 

 about 2 '4. The specific gravity of sea water being 1'028, the 

 weight of the cable in water would be less than that in air by 



— — X 100 = 43 per cent., 

 2*4 



that is by 15cwt., leaving 20c wt. as the weight of the cable in 

 water. 



The sectional area of the sheath would be 



-?^^ = 0-935sq. in. 



182 



(where 182 is the weight in cwts. per naut square inch of steel), 

 equivalent to twelve No. 13 B.W.G or eighteen No. 14 wires, 

 and using steel of 96 tons breaking strain the limit of tensile 

 strength of the sheath would be 9 tons. The weight in water 

 of 2,500 fathoms deep being 



