SUBMARINE TELEGRAPHY 209 



Probably the imperfection of the old cable was due rather to 

 the joints between the separate miles of wire as manufactured, 

 than to any extreme inferiority in the gutta-percha employed. 

 These joints are even now the weak places in the protection of 

 a cable. When the gutta-percha has been selected and purified 

 with care, and applied by mechanical contrivances of proved 

 excellence, there is little risk of a fault occurring ; but this manu- 

 facture cannot be so conducted as to produce one unbroken 

 length of wire, and even if it could, convenience in the other 

 processes of manufacture would require the division of this wire 

 into lengths. One-mile lengths are, in practice, usually made 

 without joint, and are joined together by a skilled workman as 

 occasion arises. The copper strands are soldered together with 

 a scarf-joint, two pieces of fine wire are then wrapped over this 

 joint, so that even if it is pulled asunder, electrical continuity 

 will be preserved, and so far the operation is one of no great 

 difficulty. This cannot be said of the next process, the insula- 

 tion of the wire by hand, and the welding, as it were, of the 

 new sheets of gutta-percha, so applied, with the old sheathing 

 on either side. The gutta-percha is warmed by a spirit-lamp ; 

 too much or too little heat is fatal, and the jointer must judge 

 of the temperature by experience ; the least moisture will spoil 

 a joint hence one reason for providing that no moisture can 

 percolate along the metal strand. A very little dirt or impurity 

 will also do much injury hence the rule that a jointer must do 

 no other work, and that the copper wire must be soldered by one 

 man. the gutta-percha applied by another. A joint may also be 

 spoilt by the presence of air under one of the insulating coats, 

 and as the writer cannot pretend himself to make a joint, other 

 causes of failure probably exist of which he is ignorant, but 

 enough has been said to show the difficulty of the process. 



tion must be due to the low temperature and high pressure. Mr. C. W. 

 Siemens, in a paper published in the British Association Report for 1863, 

 arrives at the conclusion that at 24 C. pressure does not affect the change 

 produced by electrification. The resistance of the copper conductor of the 

 1865 cable is 7,604, that of the 1866 cable 7,209, corresponding to 4-009 and 

 3-893 per knot respectively. The mean insulation resistance per knot, as 

 measured in the factory at 24 C., was 379 millions, after one minute's electri- 

 fication. All the resistance measurements are given in British Association 



ite. 



VOL. II. p 



