.s'/A 1 WILLIAAf SIEMENS, F.R.S. 173 



hitherto to give to such compounds all the properties necessary in 

 the dielectric substance covering the conductor, viz., a low induc- 

 tivc capacity and high insulation, coupled with considerable 

 toughness and permanency at all ordinary temperatures and the 

 requisite plasticity at higher temperatures. 



The supply of gutta-percha has hitherto been sufficient for the 

 di iiKiud, but a large extension in the use of insulated conductors 

 both by sea and laud will, it may be apprehended, outrun the 

 supply, and it is well on this account that we should steadily fix 

 our attention upon such compounds as are likely to furnish a 

 suitable substitute. Regarding a continued supply of gutta-percha 

 and india-rubber, it is satisfactory to observe that the Indian 

 Government have turned their attention seriously to the question 

 of making plantations of trees bearing these gums, chiefly in the 

 Malay Peninsula, under the able direction of Sir Joseph Hooker, 

 and of Dr. Brandos, the Director of the Forest Department in 

 India. It is to be hoped that by these wise measures a continued 

 supply of these invaluable materials will be secured, while their 

 quality for insulating purposes will probably be improved by means 

 of cultivation. 



The outer covering now generally applied to shallow sea cables 

 consists of a sheathing of iron wire covered with a double layer of 

 hemp steeped in asphalte, and applied to the cable in a heated 

 condition, and this, if properly carried out, affords very efficient 

 protection for the iron sheathing against corrosion. 



In the construction of deep-sea cables, steel wires are generally 

 used, each wire being covered in the first instance with jute with 

 a view to reduce the weight of the cable. This construction affords 

 the advantage of lightness combined with strength, and thus 

 facilitates the operation of submerging the cable, but is objection- 

 able, inasmuch as it affords no complete metallic sheath against 

 the inroads of the teredo and xylophaga to the core, and, in the 

 case of a cable having to be raised from considerable depths, it is 

 apt to untwist, and run itself into kinks at the bottom. 



The use of a light cable for deep seas has been ably advocated 

 by some electricians, and its adoption has the one great argument 

 in its favour, that its first cost is much below that of a strong 

 cable ; on the other hand the risk incurred in successfully sub- 

 merging such a cable is much greater, and in the case of a fault 



