PRINCIPLES OF DESIGN. 75 



a constant wearing action going on due to the tides, or if tlie 

 <3able is near tlie mouth of a river it is subject to the scouring 

 action of all kinds of dihis, such as roots and trunks of trees, 

 carried down and discharged into the sea. Cables laid from 

 ooasts ice-bound during a period of the year are also subject to 

 a good deal of wear. Hence, the heaviest protection is given 

 to shore-ends, and these weigh, when complete, 15 to 20 tons 

 per naut. 



On the other hand the deep-sea portion is liable to no such 

 action, but rests undisturbed on the ocean bed. It is therefore 

 practicable to construct a comparatively light cable for great 

 depths, which will not require an excessive brake power to lay 

 and yet have sufficient tensile strength to stand the strain 

 when picked up for repair. The factor of safety usually 

 adopted is such that the cable will stand from three to four 

 times the estimated picking-up strain. 



The sheathing is either of steel or homogeneous iron wire 

 (English Bessemer) galvanised, the elongation being about the 

 same in each — namely about 5 per cent. — while steel has a 

 breaking stress of about 100 tons, and homogeneous Iron from 

 50 to 90 tons per square inch. Steel is now being largely used 

 for sheathing the entire length of cables. 



For deep-water cables the breaking strain of the sheathing 

 wires must not be less than 50 to 60 tons per square inch, and 

 ■for the deepest water is usually nearer 100 tons. 



The weight of iron or steel sheath for cables to be laid in 

 depths exceeding 2,000 fathoms is usually from 13cwt. to 

 17 cwt. per naut. Homogeneous Iron of from 70 to 80 tons 

 brealiing strain per square inch has been largely used for the 

 sheathing of deep-sea cables, chiefly on account of its greater 

 ductility than steel and its slightly lower specific gravity ; but 

 -steel of improved quality of about 4 or 5 per cent, elongation 

 and 90 to 100 tons breaking strain is now almost exclusively used. 



It is important as a preventative against corrosion that the 

 wires are well galvanised. 



The total weight in air of deep-sea cables is usually from 



\^ to 2 tons per knot. The weight in sea water, which is 



more important, varies with the specific gravity, and depends 



upon the respective weights of copper, gutta-percha, jute, com- 



jpound, and iron or steel employed in the construction. In the 



