138 THE SCIENTIFIC PAPERS OF 



pletely, and in the years 184G and 18 i7 the two gums, india-rubber 

 and gutta-percha, were introduced, the former by Professor Jacobi 

 of St. Petersburg, and the latter by Dr. Werner Siemens of 

 Berlin. This last gum soon became almost indispensable to the 

 cable manufacturer on account of its remarkable plasticity at low 

 temperatures and its insulating property. 



The first outer sheathing used was a tube of lead drawn tightly 

 over the insulated wire, and this again was covered with pieces of 

 wrought-iron tubing connected by ball and socket joints ; in this 

 way the Elbe and other rivers were crossed successfully in 1848 

 50. This method was superseded later on by the spiral-wire 

 sheathing, first proposed by Mr. Brett in 1851 for the Dover and 

 Calais cable ; since then, with few modifications and exceptions, 

 this form has been universally adopted. 



The speaker next enumerated the casualties to which submarine 

 cables are liable, and the precautions employed to obviate them. 

 He showed specimens destroyed by rust and the ravages of a 

 species of teredo. On the Indo-European cable line a curious, 

 case of fracture occurred ; a whale, becoming entangled in a 

 portion of cable overhanging a ledge of rock, broke it, and in 

 striving to get free had so wound one end round its flukes chat 

 escape became hopeless, and so had fallen an easy prey to sharks, 

 which had half -devoured it when the grappling iron brought his 

 remains to the surface. Other enemies to be dreaded are landslips, 

 ships' anchors, and abrading currents. 



The new Atlantic cable consists, for the deep-sea portion, of 

 copper conductors, gutta-percha insulators, and a sheathing of steel 

 wires covered with hemp ; the shallow water part consists of similar 

 conductors and insulators sheathed with hemp, which in turn is 

 covered with iron wire. 



In paying out, no catenary is formed, as might be supposed, but 

 the cable passes in a straight line from the ship to the sea bottom 

 a proposition which the speaker demonstrated experimentally 

 by means of a long trough with glass sides filled with water. The 

 retaining force applied by the brake-wheel should be equal to the 

 weight of a piece of cable hanging vertically downwards to the 

 bottom of the sea. In picking up, a catenary is formed, but 

 a vertical position is the best, because it produces the least 

 resistance. 



