Masterpieces of Science 



worthless from faulty insulation and the lack 

 of armour against dragging anchors and fretting 

 rocks. In 1851 the experiment was repeated 

 with success. The conductor now was not a 

 single wire of copper, but four wires, wound 



Fig. 58. Calais-Dover cable, 1851 



spirally, so as to combine strength with flexibility; 

 these were covered with gutta-percha and sur- 

 rounded with tarred hemp. As a means of im- 

 parting additional strength, ten iron wires were 

 wound round the hemp a feature which has 

 been copied in every subsequent cable (Fig. 58). 

 The engineers were fast learning the rigorous 

 conditions of submarine telegraphy; in its essen- 

 tials the Dover-Calais line continues to be the 

 type of deep-sea cables to-day. The success of 

 the wire laid across the British Channel incited 

 other ventures of the kind. Many of them, 

 through careless construction or unskilful laying, 

 were utter failures. At last, in 1855, a sub- 

 marine line 171 miles in length gave excellent 

 service, as it united Varna with Constantinople; 

 this was the greatest length of satisfactory cable 

 until the submergence of an Atlantic line. 

 40 



