.S7A 1 \\'1I.LIAM .S7/-;j//iA'.S', F.R.S, 99 



construction is perfect, and should therefore be adhered to in the 

 future. The difficulties encountered in the progress of this work 

 are suggestive, both of the perishable nature of the outer sheathing, 

 and of the tenderness of the insulating medium employed. Past ex- 

 perience has taught, at great cost, that the unprotected iron sheath- 

 ing will be destroyed by rust, sometimes in the course of a year ; 

 that the hemp serving will be eaten by a marine insect * wherever 

 it becomes exposed ; and that the gutta percha is apt to develop 

 faults, by slow degrees, wherever an irregularity in the insulating 

 covering, although perfectly harmless at first, has escaped observa- 

 tion in the process of manufacture. Impressed with these views, 

 the author has for some years directed his attention to the con- 

 struction of a cable of a more permanent character, and he is 

 sanguine that his endeavours have led to some useful results, which 

 he now proposes briefly to lay before the members. 



INSULATING MATERIALS. Respecting the insulating covering, 

 nature seems to have provided only two suitable substances com- 

 bining permanent pliability, at all ordinary temperatures, with 

 high insulating property, namely, India rubber and gutta percha. 

 India rubber wrapped round the conductor in the form of strips, 

 or bands, was first applied for insulating underground line-wires, 

 by Jacobi, of St. Petersburg, about the year 1 840. Gutta percha 

 was applied for the same purpose, in 1847, by Dr. Werner Siemens, 

 in Prussia, partly at the suggestion of the author, the material being 

 put upon the wire in a heated and plastic state, by means of a die. 

 This latter process has since been exclusively adopted for insulat- 

 ing subterranean and submarine wires, until very lately when 

 india rubber has been again put forward, in various forms, claiming 

 a preference over gutta percha on account of its higher insulating 

 power, its lower specific induction, and its power to resist higher 

 temperatures.t On the other hand, gutta percha possesses the im- 

 portant advantage, that it admits of being put upon the wire in a 

 plastic state, by means of a die, thereby producing a homogeneous 

 covering, which may be applied in several layers, and which gives 

 greater security against faults, than the lapped india rubber cover- 

 ing. It is, moreover, harder and stronger than india rubber at 



* The Xilophaga, according to Huxley. 



t Vide ' ' Report of the Joint Committee on the Construction of Submarine Tele- 

 graph Cables," pp. xrii. and xxiii., and also Appendices No. 1 and No. 2. 



H l> 



