PBINCIPLES OF DESIGN. 77 



tively more or less iron in the sheath. Some deep-sea cables 

 have been laid with iron sheaths as much as 63 per cent, of the 

 entire weight and about 26 per cent, of jute and compound; 

 but the experience gained in repairing has shown that the bed 

 is a particularly good one and little or no corrosion has occurred. 

 The result of this is that the next cable laid over the same 

 route (as a duplicate line) is designed with less weight of iron, 

 making a lighter cable and of less specific gravity than before, 

 the tensile strength being maintained at about the same figure 

 by the use of wires of higher breaking strain. While con- 

 sidering durability the fact must not be lost sight of that pieces 

 of modern type are often put in during repairs on old cable?, 

 so that the latter become in course of time almost entirely 

 renewed. 



It will be seen from the foregoing that the conditions to be 

 met in the mechanical protection of a cable are somewhat 

 conflicting, and therefore can only be approximated to or 

 compromised. It is, of course, all-important that a cable 

 should be capable of being picked up from great depths for 

 repair without parting, and for this reason it should be as 

 light in weight as is consistent with bearing the strain. Light- 

 ness or low specific gravity also facilitates the laying of a cable 

 from the point of view of the brake arrangements. On the 

 other hand the cable should be heavy enough to sink into the 

 irregularities of the bottom to avoid chafe, and the advan- 

 tage of lightness in laying and recovering must not be carried 

 so far as to sacrifice durability, which is just as important a 

 consideration. In the early days of the enterprise, before the 

 art of laying and recovering was understood as well as it now 

 is, the armouring was lightened by the addition of hemp. The 

 iron sheathing wires were each enveloped in hemp, as first 

 suggested in 1864 by Messrs. John and Edwin Wright — the most 

 famous rope-makers of that time — for the second Atlantic cable, 

 to meet the necessities of recovery. But the alternate hemp 

 and iron cable proved itself wanting in durability and became 

 superseded by what is termed the " close-sheathed " cable, in 

 which the wires were laid in touch with each other all round 

 without intermediate packing. This abutment of the wires 

 gave the maximum strength of the complete arch, and at the 

 present time more or less close-sheathed cables are almost 



