10 THE ADDRESSES, LECTURES, ETC., OF 



same diameter, if six smaller wires were laid in between the inter- 

 stices of the larger ones, the object being to get within a given 

 diameter a maximum amount of conductivity. 



We come next to the insulating coating, which may be con- 

 sidered as the vital part of a submarine cable, especially the 

 deep-sea cable. The insulating covering of the conductor is that 

 portion of a submarine cable which requires the greatest amount 

 of care. The insulating substances to be found in nature far 

 exceed in number and amount those which are remarkable for 

 their conductivity, and comprise all the earths, silica, glass, porce- 

 lain, sulphur, besides bituminous and resinous substances ; yet, 

 notwithstanding this vast field for selection, we have as yet found 

 only two substances fulfilling the collateral conditions of admitting 

 to be moulded upon the conductor into a homogeneous and pliable 

 covering, capable of excluding sea water entirely under great 

 hydrostatic pressure. These substances are india-rubber and 

 gutta-percha. 



The substance which was used in the first instance for insulating 

 wire, was india-rubber. It has great flexibility, and its insulat- 

 ing power is very remarkable, but it is a substance which is at no 

 time in a plastic state, therefore considerable difficulties are en- 

 countered in dealing with it. Gutta-percha was next tried, and 

 from the time when it was first laid in the harbour of Kiel, in 

 1848, until the present day, it has been the material used in 

 preference to all others 'for insulating telegraph wires. The cir- 

 cumstance that gutta-percha, at a temperature of 150 Fahr., 

 becomes semi-fluid, is a great point in favour of its application. 

 It can be put upon wire by a process analogous to the making of 

 maccaroni or of lead tubes, and by putting several coatings of 

 gutta-percha one upon the other, and combining them by inter- 

 vening thin layers of a fusible compound known as Chattel-ton's 

 mixture, the most perfect workmanship is produced, the chances 

 of any flaw or leakage in such a coating being exceedingly 

 small. 



As regards insulation, pure india-rubber has an insulating power 

 40 times greater than that of gutta-percha ; but notwithstanding 

 this great superiority, the difficulty of putting it upon wire, and 

 certain other drawbacks to which I shall presently allude, have 

 hitherto prevented its application upon a large scale. Gutta- 



