The New York Rapid-transit Subway

122 DISCUSSION ON NEW YORK SUBWAY. [Minutes of The Author, in the portion of the working-cost chargeable to general expenses in the Subway and in the Great Northern was due to the fact that rates and taxes were not included in the former percentage. The Author referred Mr. Haigh to the answers to Mr. Fitzmaurice for an explana- tion of the estimates of cost: in the total cost was included much overhead line that was much less expensive than subway, as were also the rock tunnels which were lined with concrete instead of with iron, like tubes. This explained the difference between the stated average cost of subways and the total average cost of the whole line. The length of open coffer-dam in the approaches to the Harlem River tunnel was 976 feet, of which 250 feet was held by 12-inch sheet piling; and the lengths of the two air-caissons were 218 and 306 feet. The junction between the iron and concrete sections had been made by extending the conci'ete arching a distance of 15 feet over the cast iron, while the junction between the two iron sections had been effected by building the second caisson to enclose the open end of the completed half of the tunnel. Traffic over the three-track section was worked by returning the central-track trains over the opposite outer track. Thus, when trains were running south-bound on two tracks they were all returned on the other or north- bound line. This was possible because at such hours, the traffic being less in the other direction, the station-stops were shorter and more trains could be accommodated. Many of the speakers had referred to the reinforced concrete and to buried steel beams and their pre- servation. A dozen years or more ago the late Sir Benjamin Baker stated to the Author that he seriously questioned the necessity, or even the propriety, of painting steel members which were to be en- cased in cement mortar. The Author’s subsequent observations and experiments had convinced him of the soundness of Sir Benjamin’s observation. When the whole surface was protected by a strong covering which incipient rust could not break away or dislodge, rusting was practically impossible. The Author had seen pieces of metal removed from Chinese masonry several hundred years old and still in perfect condition. But there must be immediate contact, which could not be obtained by using concrete mixed dry, with un- filled voids, and rammed into place. The mortar must be in excess of the voids in the stone, and the cement in excess of the voids in the sand, and the whole must be so fluid as to flow into place without ramming, aided only, if aid was necessary, by cutting the fluid concrete with shovels. Water carrying the fine mortar must not be allowed to leak through the forms. Concrete so proportioned and mixed became a compact monolith, and could be depended on to encase thoroughly the embedded steel, which, to secure