328 THE ADDRESSES, LECTURES, ETC., OF 



suspended in the air require to be placed at some distance from 

 telegraphic line wires, and to be supported by separate posts, 

 Another way of neutralising interference consists in twisting two 

 separately insulated telephone wires together, so as to form a 

 strand, and in using the two conductors as a metallic circuit to 

 the exclusion of the earth ; the working current will, in that case, 

 receive equal and opposite inductive influences, and will therefore 

 remain unaffected by them. On the other hand two insulated 

 wires instead of one are required for working one set of instru- 

 ments ; and a serious increase in the cost of installation is thus 

 caused. To avoid this Mr. Jacob has lately suggested a plan of 

 combining pairs of such metallic circuits again into separate work- 

 ing pairs, and these again with other working pairs, whereby the 

 total number of telephones capable of being worked without 

 interference is made to equal the total number of single wires 

 employed. The working of telephones and telegraphs in metallic 

 circuit has the further advantage that mutual volta induction 

 between the outgoing and returning currents favours the transit, 

 and neutralises on the other hand the retarding influence caused 

 by charge in underground or submarine conductors. These con- 

 ditions are particularly favourable to underground line wires, 

 which possess other important advantages over the still prevailing 

 overground system, in that they are unaffected by atmospheric 

 electricity, or by snowstorms and heavy gales, which at not very 

 rare intervals of time put us back to pre-telegraphic days, when 

 the letter-carrier was our swiftest messenger. 



The underground system of telegraphs, first introduced into 

 Germany by Werner Siemens in the years 1847-48, had to yield 

 for a time to the overground system owing to technical difficulties, 

 but it has been again resorted to within the last four years, and 

 multiple land cables of solid construction now connect all the 

 important towns of that country. The first cost of such a system 

 is no doubt considerable (being about 38 per kilometre of con- 

 ductor as against 8 10s. the cost of land lines) ; but as the 

 underground wires are exempt from frequent repairs and renewals, 

 and as they insure continuity of service, they are decidedly the 

 cheaper and better in the end. The experience afforded by the 

 early introduction of the underground system in Germany was 

 not, however, without its beneficial results, as it brought to light 



