IO 



the following might, however, be taken as the average cost per 

 mile of a single wire under-ground telegraph : 



s. d. 



The gutta-percha and lead-coated wire . . . 21 14 

 The trench 2 feet deep, including filling up again .600 

 The instruments and sundries . .860 



36 



or about 30 when the lead coating was dispensed with, and the 

 gutta-percha was increased to about 100 pounds per mile. 



In Germany the trench work was generally taken by contract, 

 at 2 or 3 silver groschen per 12 feet, which was somewhat less 

 than G per mile. Eecourse had, in some cases, been had to a 

 species of plough, which was propelled at a walking pace along the 

 line by a locomotive engine, and whereby the cost of ground 

 work was much reduced. 



It appeared at first to be a serious difficulty to discover the 

 places of rupture, or of bad insulation, in the under-ground line 

 wire ; this had, however, been successfully removed by a simple 

 system. To discover a place of rupture between two stations, a 

 battery was inserted at one station, between the line wire and the 

 earth ; an officer then proceeded about midway between the 

 stations and connected a galvanometer with the earth on the one 

 hand and the line wire (which was at intervals accessible by being- 

 brought up into testing posts) on the other. If he observed a de- 

 flection of the galvanometer needle, the rupture could not be 

 between him and the station with the battery. He therefore pro- 

 ceeded to the next post in the direction of the other station, and 

 so on, until he found upon repeatedly attaching the galvanometer 

 that no deflection took place. The rupture must therefore be 

 situated within the distance between the two testing-posts, which 

 distance was therefore halved and quartered by digging holes, and 

 thus obtaining access to the wire. In the course of an hour the 

 position of the rupture w r as generally ascertained to within a few 

 yards, which were taken up and a fresh piece of wire soldered to 

 the main wire in two places. 



The position of a place of bad insulation was discovered by an 

 application of Ohm's law, without proceeding along the line wire. 



