CHAP. V.] 



TELEGRAPHIC LINES. 



G13 



this substance are generally used, of a total thickness of 3 or 4 

 millimetres. The gutta-percha is not only a very good insulator, 

 but it is almost unalterable in sea-water. At first this was the 

 only covering, but it was soon found necessary to protect it from 

 damage. Bound the whole is now placed a thick layer of hemp, 

 saturated with Stockholm tar, and outside this again, the layer is 

 supported and protected by a series of galvanized iron wires twisted 

 in a spiral. The following figures, natural size of some of the 

 cables now working on different submarine telegraphic lines, will 



I ' 

 FIG. 401. Transatlantic cables of the line from Valentia to Newfoundland (natural size). 



show these arrangements. We see that though these specimens 

 differ in size, the construction is nearly the same, though in the 

 old cables were placed several distinct wires in order to multiply 

 the communications. This method has been generally abandoned 

 because of the disadvantages found to be connected with multiple 

 wires ; they require in fact a considerable volume and weight in the 

 cable, which makes the operation of laying it difficult, but more 

 particularly the nearness of the wires causes induced currents to be 



