We are at once struck with its self-dependence as a machine, what is electric cable and even its resemblance in some respects to a living creature. In some of the newer cables wires covered with cotton soaked in paraffine are used instead of gutta-percha-covered wires. In round numbers we may estimate the total cost for one thousand wires at $150,000 per mile, or $150 per mile per circuit. The cost of the piping and chambers is in round numbers $50,000 per mile, and these pipes are intended to accommodate one thousand wires. The distances within this city are so short that neither induction nor retardation has to be considered in the telegraph wires. As the two lines constructed in Boston are short, only about one quarter of a mile each, it was deemed best to use single-line circuits, hoping that the induction and retardation on so short lines would not be serious. For any other apparatus than telephones, retardation and induction would not be felt on so short cables.

In such an arrangement, too, there is a minimum of retardation. We have seen that in Paris the retardation and induction are both obviated by the use of double and twisted wires in metallic circuit. The American Bell Telephone Company has recently constructed two short lines of under-ground wires in the business section of Boston, and these give us excellent data from which to judge of the extent of technical practicability and the expense of putting all wires under-ground. Babbage was among the first to suggest that a lighthouse might be made to signal a distinctive number by occultations of its light; but Sir William pointed out the merits of the Morse telegraphic code for the purpose, and urged that the signals should consist of short and long flashes of the light to represent the dots and dashes. From each of these points to the various subscribers run short stretches of ordinary house-top wire. If, then, the present method of running wires overhead is objectionable, and the expense of running them under-ground is so great as to put the cost of telephones, electric lights, and other electrical appliances out of the reach of would-be users, how are the wires to be run?

The telephone wires are in Paris protected from these evils by an extremely simple though expensive device. Such a device is called a "metallic circuit." Any outside disturbing circuit tends to induce, in the two wires of the metallic circuit, equal and opposite currents, which neutralize and disappear. In this way hundreds of single wires would be gathered into small and inoffensive cables, and the enormous wooden structures would be replaced by small cable supports of brick or iron. The Agamemnon was now almost becalmed on her way to the rendezvous; but the middle splice was finished by 12.30 p.m. In almost all of the large cities the question is now being asked, Why can not all of these wires be buried along with the gas and water pipes under the streets? It is thus demonstrated that it is technically possible to place all of the wires in a city under-ground. In no place would there be the offensive multiplicity of wires. The cost per conductor thus increases enormously as the number of conductors diminishes, so that it would be clearly impossible to follow out the wires of an exchange system in all of their bifurcations. The cables, of which several kinds are in use, run out from the basement of the central office through these pipes and up the side of buildings to roofs, from which they spread out to the subscribers by means of ordinary overhead lines.

For many purposes, as telephony or electric lighting, a considerable number of wires start out from a central office together, but continually bifurcate until single wires run to the houses of the subscribers. If these wires are run on poles, they not only disfigure the streets, but seriously interfere with the operations of firemen in case of fire, as we have repeatedly seen during the last few years. It is necessary that all of the wires be in metallic circuit, for, if a metallic circuit be connected to a single-line circuit, the disturbances are not removed. The cost of the cables is from $60 to $150 per mile for each circuit, according to the kind of cable used. Instead of a single wire for each circuit, two wires twisted together are used, the current going out over one and returning over the other. The cost of piping and chambers would be nearly as great for one hundred circuits as for one thousand, as the cost of chambers and the labor of excavating and filling would be the same; so that the cost for one hundred wires may be estimated at $50,000 per mile, or $500 per mile per conductor. It was known that the conductor should be of copper, possessing a high conductivity for the electric current, and that its insulating jacket of gutta-percha should offer a great resistance to the leakage of the current.

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