106 THE SCIENTIFIC PAPERS OF 



deterioration of the india rubber, or the appearance of sudden faults. 

 A circumstance greatly in favour of a wire covered in this manner 

 is, that the gutta percha shrinks upon the india rubber, and when 

 any mechanical injury to the covering occurs, the yielding india 

 rubber is forced into the gap, by the elastic pressure exercised by 

 the gutta percha, and thus prevents the formation of a fault. 



The outer covering of cables, as hitherto constructed, is certainly 

 the least perfect part. An iron sheathing is very necessary to pro- 

 tect the insulated core in shallow waters, where it is subject to tidal 

 currents, and to the dragging of ships' anchors. The error which 

 has been committed is, that wires of insufficient thickness have 

 frequently been adopted. But, for deep-sea cables, that is, for cables 

 in more than thirty fathoms, or, under special circumstances, forty 

 fathoms of water, the iron sheathing is, the author submits, an 

 element rather of weakness than of strength, by rendering the cable 

 ponderous, and its shipment expensive. The paying out is also 

 rendered hazardous, partly on account of the heavy brake-power 

 required, and partly through the risk which would be occasioned, by 

 the breaking of a wire between the ship's hold and the brake-wheel. 

 Eepairs also could not be conveniently made, and in some cases 

 they would be impracticable, owing to the difficulty of safely 

 raising a heavy cable from a great depth, under any circumstances, 

 and the impossibility of doing so after corrosion of the iron wire 

 has made any progress. When the Falmonth and Gibraltar cable 

 was first contemplated, the author, in conjunction with Mr. Fordo, 

 proposed that each iron wire should be covered with gutta percha, 

 with a view to prevent oxidation ; but the system was not acted 

 upon, except experimentally.* 



Mere protection of the iron wire is, however, not sufficient, in 

 the author's opinion, to constitute a good deep-sea cable. It is 

 capable of mathematical demonstration, that in paying out a wire- 

 sheathed cable, with a considerable strain upon the brake-wheel, it 

 will untwist, while in suspension in the water, to a considerable 

 extent, causing elongation of the core to the amount of say one per 

 cent., or even more. On reaching the bottom, the strain and con- 

 sequent twist will be released, and throw the cable into frequent 



* The results of the experiments referred to are given in the Report of the 

 Joint Committee on the Construction of Submarine Telegraph Cables, Appendix 

 No, 10. 



