CHAP, v.] TELEGRAPHIC LINES. 615 



the enveloping metal wires. Lastly, for the portion destined to be 

 submerged in the open sea, in very deep water, the smallest size is 

 adopted (Fig. 402), the cable having no longer to withstand the 

 agitations of the surface, and being much more easily laid when of 

 less weight. 



This weight is, indeed, something enormous for submaiine lines, 

 even if not of great length. The cable from Dover to Calais, laid in 

 1851, which is only 41 kilometres long, weighed nevertheless more 

 than 180,000 kilogrammes. The first of the transatlantic cables join- 

 ing Valeritia and Brest to America, weighed 865 kilogrammes per 

 kilometre, the second 836. This makes for the total weight 4,300 

 tons for the first, and nearly 4,000 tons for the second, comprising 

 only the section between Brest and the island of St. Peter. One ship 

 only, the Great Eastern, the colossus of the seas, was capable of 

 carrying such a burden. But the' disadvantage of such a weight, 

 which diminishes certainly by the part of the cable immersed, is 

 chiefly felt when it has to be laid in great depths, the portion hanging 

 down reaching a depth of 2,500 fathoms. But we are not about 

 to describe here the process of laying a submarine cable over 

 so long a distance. We must return to the physical side of the 

 question. 



Before it was accomplished, many persons doubted the possibility 

 of transmitting submarine signals to great distances, as from the 

 European continent to America, across the Atlantic. It was not so much 

 the distance itself, as what might happen to a cable plunged to enor- 

 mous depths in so eminently a conducting medium as sea-water, that 

 frightened them. How would the wire conduct itself when the electric 

 currents were thrown into it? Would its insulation be insufficient? 

 Would the force of the current be sufficient to pass through it without 

 disturbance from one end of the immense line to the other with no 

 relay. These fears, which were at first stated but vaguely, seemed 

 for a moment justified, when in August, 1857, after a few messages 

 had been exchanged between the United States and Ireland, the 

 apparatus were seen to give gradually more confused signals, and 

 finally to cease working altogether. The cause of the interruption 

 remained at first unrecognized. 



It was necessary then to learn, afresh, or rather to commence 

 seriously the experimental and theoretical study of the transmission 



