G12 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



followed. But after this the successful laying and working of the 

 immense transatlantic cable in 1857, joining Europe and America, 

 between Ireland and Newfoundland, was the starting point of a 

 prodigious development of the universal telegraphic network. At the 

 present time, the globe is traversed not only across the continents, but 

 in the depths of the sea, by wires which carry everywhere with the 

 rapidity of lightning, the private and public messages of all civilized 

 nations, the length of all combined exceeding 380,000 miles. 



We may now give some details with respect to the structure of 

 the cables, and the mode of immersion adopted. 



FIG. 400. Submarine cables -outside view ami section. 



The conducting wire of a submarine cable is covered with several 

 envelopes, whose object it is either to insulate it, or to protect it 

 against the chances of destruction. It is either a copper wire of 1 or 

 2 mm. diameter, or a cord formed of seven very fine wires twisted in 

 a spiral. This last arrangement is now preferred as more pliant, 

 because in case of accident or rupture of these inner wires, if one or 

 two of them escape the communication will not be interrupted. 



The point of the highest importance is that the wires should be 

 surrounded by an insulating covering of gutta-percha; 3 or 4 layers of 



