504 THE POPULAR SCIENCE MONTHLY. 



and that in Paris, whicli is the only city in the world having a com- 

 plete under-ground system, there are unusual facilities for the running 

 of wires, as sewers large enough to walk about in extend even under 

 the less important streets of the city. Moreover, it has been found 

 that, for delicate and quick-working apparatus, such as automatic 

 telegraphs, polarized relays, and, above all, the telephone, long under- 

 ground lines are far less efficient than pole lines. There are two rea- 

 sons, apart from the difficulty of securing good insulation, why these 

 long under-ground lines are comparatively inefficient : 



1. If an electric conductor be brought near to a large mass of con- 

 ducting matter, as is a wire when it is taken down from a pole and 

 buried in the earth, there appears in the current the phenomenon of 

 retardation, by which each signal, instead of being sharp and distinct, 

 is partly kept back, so that it overlaps and mingles with the next ; the 

 result is to limit the speed of working of the apparatus ; or if, like the 

 telephone, it be an apparatus in which the currents are necessarily ex- 

 tremely frequent, to confuse and destroy the signals altogether. With 

 ordinary Morse telegraphic apparatus, this is not very troublesome on 

 under-ground lines a hundred miles long. With delicate relays, and 

 more especially with quick working printing telegraphs, or automatic 

 telegraphs, such lines are very troublesome ; and, with telephones, 

 the retardation is a very troublesome matter on under-ground lines ten 

 miles long. 



2. The second difficulty is called induction, and is noticed when two 

 or more wires are run side by side and near together, as they necessa- 

 rily are in an under-ground cable. 



If the signals on one wire of such a cable be sharp and quick, they 

 cause fac-simile signals on all of the neighboring wires, and this too, 

 though the insulation may be absolutely perfect ; indeed, above a cer- 

 tain point, the more perfect the insulation the greater the induction. 

 The result of this phenomenon is, that messages sent over one wire are 

 liable to be received on all of the other wires, and, in the case of the 

 telephone, this phenomenon is noticeable on cables one thousand feet 

 long, and on a cable one mile long the parties on one wire can easily 

 understand what those on the other wires are saying. For any other 

 instrument, however, the interference only becomes annoying on much 

 longer lines. Steady currents, like those used with electric lights, are, 

 of course, not affected either by retardation or induction. 



In our own country there is little doubt that the proper method of 

 constructing electrical wires between cities is, to string them on poles 

 in mid-air. A brief review of some of the European systems that have 

 been constructed will convince us of this. Between the years 1847 

 and 1850 a system of cables, containing 2,648 miles of wire, was 

 laid under-ground to connect Berlin with the other principal cities 

 of Prussia. Gutta-percha-covered wires were drawn into lead tubes, 

 which were then buried in trenches two feet deep. The cost of this 



