610 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



wires in the open air was not trusted to, because it was thought it 

 would be subject to too frequent causes of loss of electricity, and 

 would, besides, be liable to wilful damage. In Prussia particularly 

 and in Russia, the wires were buried in the earth at a depth of 50 to 

 60 centimetres. But this system of telegraphic lines was found to 

 be very expensive. It is only used now, as we have just said, in those 

 portions of the lines which pass through the middle of towns or 

 through railway tunnels. In these cases the various conductors are 

 arranged as follows. 



The wires are of copper, each covered with a layer of gutta-percha, 



FIG. 898. English stretcher : Siemens' 

 and Halske's system. 



FIG. 399. Stretcher on German lines. 



and all bound together into a cable, which is itself surrounded with 

 tarred hemp. This cable is then placed in an iron tube, or one of 

 creosoted wood or lead, and is buried at a depth of about a yard, on a 

 bed of sand or sifted earth. Such is the nature of the subterranean 

 lines which join the central telegraph office at Paris with the 

 Observatory, the Luxembourg, and the stations of Montparnasse, and 

 the Lyons and Orleans railways, and connect the Central Telegraph 

 Station, London, with the numerous branch offices. Another system 

 consists in the employment of galvanized iron wires, like those of 

 the air lines joined in cords of 4, 6, or 10 wires, insulated from 

 each other by masses of pitch. The cable thus formed is In id in 



