In Fig. 14 is shown a 12-duct vitrified clay conduit entering a manhole. Conduits of this type are made in blocks of two, three, four and six ducts. The four- and six-duct blocks are six feet long; the two- and three-duct block& are three feet in length. These blocks are laid in cement, end to end, and are held in position rela tive to one another by dowel pins. The walls of these blocks are five-eighths of an inch thick. A wrapping of wet muslin is laid around each joint and over the muslin cement mortar is placed. Earthenware conduits are also made in single ducts, 18 inches in length.
Wood pipe conduits consisting of wooden tubes which have been especially prepared to withstand decay are used quite largely. The single tubes are about eight feet in length and have socket joints. The tubes are laid in the trench in tiers, the lower tier resting on plank ing. In the various types of underground con duits mentioned and others the ducts are °broken* to add strength to the structure. Care is taken in laying these ducts to exclude cement, stones or any other obstacle that would ob struct or injure the cables in the °drawing in" process.
In the case of conduits for electric traction, the ducts are laid at the side of the tracks, underground, and wide manholes are provided at street intersections. For the °feeders" and other cables of the New York subways, conduits are laid in the wall of the structure and access is given by openings in the wall at suitable in tervals.
It is well known that frequent interruptions to overhead telegraph and telephone lines are occasioned by severe wind, snow and sleet storms in this country and Europe, and in con sequence the question of placing all such wires in underground conduits has frequently been raised. The great cost of such an undertaking, however, together with the fact that the speed of telephoning and telegraphing would be greatly diminished, owing to the increased electro-static capacity of the cables as compared with overhead lines, combined with other elec trical obstacles, has been prohibitory of the at tempt to carry out such a plan in this country on a large scale. In Great Britain an emer gency underground cable system has been laid between Birmingham, London and Edin burgh. For this purpose an iron pipe contain ing a 76-conductor telegraph cable, with open ings at certain intervals to give access to the cable, is employed. In the United States long stretches of underground telephone cables have been constructed, notably between New York, Philadelphia and Washington, D. C. The suc cessful operation of underground telephone (metallic) circuits at such distances has been made possible by the use of the Pupin induct ance coils placed at certain intervals along the circuit and by the use of the audion telephone repeater.