Page:Popular Science Monthly Volume 24.djvu/523

Rh the whim of the owner, and they have to be continually removed from building to building as the good-will of each owner is exhausted.

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? In answer, I propose to describe briefly what has been done in this direction in European cities, then to look at some experiments lately made in this country, and thus to show how far such a plan is and how far it is not practicable.

In Paris, all the wires are carried in the sewers under-ground.

In London, the telegraph wires are carried from the central office to many of the branch offices and to the railways leading out of the city under-ground.

In Vienna, Prague, Brünn, Munich, Augsburg, Nuremberg, and many other cities, the telegraph-wires are carried under-ground by armored cables to the outside of the city.

In the German cities we have seen that many of the telegraph-wires are carried under-ground from the center of the city to connect with cables running to other cities.

Telephone wires, electric-light wires, and a large majority of telegraph wires in European cities are, however, as in America, carried over house-tops or on poles.

The cable most generally made in Paris consists of seven gutta-percha-covered wires laid into a cable covered with tarred hemp and drawn into a lead pipe; this pipe is fastened by hooks to the side-wall of the sewer. The cables are thus easy of access, and any new cables may be added as required without disturbing those already in use. In some of the newer cables wires covered with cotton soaked in paraffine are used instead of gutta-percha-covered wires. The distances within this city are so short that neither induction nor retardation has to be considered in the telegraph wires. Electric-lighting wires, we have seen, are not affected. The telephone wires are in Paris protected from these evils by an extremely simple though expensive device. 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. 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 such an arrangement, too, there is a minimum of retardation.

There are several thousand miles of wire in the sewers of Paris, and the cost of the gutta-percha-covered cables is about $140 per mile of wire, or about five times the cost of a pole line to do the same work. As telephone cables require two wires for each circuit, this estimate would have to be doubled. The paraffined cables are, however, considerably cheaper, though their durability has not yet been proved. The cost for repairs is very small, and some cables have not been touched for twenty years. In any other city than Paris, the