PBINCIPLES OF DESIGN. TS 



better than hemp for the purpose of protecting the core and 

 forming a bed for the iron sheathing wires. If the inner 

 serving yarns are to be efficient a3 a close packing, they must 

 be applied in a short lay to render them useless for the purpose 

 of adding to the breaking strain of an ordinary iron-sheathed 

 cable. This serving will be seen in Fig. 37 and Fig. 38, which 

 show the successive protective coverings laid over a core such 

 as now adopted in an intermediate or main cable and a shore- 

 end cable. 



In the intermediate and deep-sea types there is only one 

 sheathing of iron or steel wires : in the shore-end type there 

 are two sheathings for giving greater weight and strength to 

 resist abrasion. 



The sheaths or iron wire armourings are covered overall by 

 two servings of jute laid in inverse lays, with intermediate 

 layers of compound. 



This construction, especially with larger cores, reduces the 

 proportion of lighter material, the jute tape and compound 

 comprising only about 35 per cent, of the whole weight. Con- 

 sequently the specific gravity is increased (from about 1'5 to 2'6),. 

 the cable sinks quicker and the retarding strain in laying is 

 increased. 



The outer serving is adopted mainly with the view of binding 

 the wires firmly together and holding them in their places as 

 well as helping to preserve them. Instead of yarns, canvas tape 

 is largely used for this purpose, being less costly, and a large 

 amount of old cable picked up on repairs is, after the removal 

 of bad places, treated in this way, the wires being bound 

 together by a serving of strong canvas tape, impregnated with 

 Stockholm tar. The tape is put on in two lays, with inter- 

 mediate coatings of compound, thus presenting a smooth 

 external surface. 



It may here be mentioned that coal tar has been found to 

 attack gutta-percha, causing softening, and therefore is never 

 used as an impregnation, either with the inner jute serving or 

 the outer lap of canvas tape. But a small proportion may be 

 used with other compounds for pickling the sheathing wires so 

 as to form a good coating over them. 



In shallow waters the cable is subject at times to considerable 

 strain due to being dragged by ships' anchors. There is also 



