TUBULAR BRIDGE. The advantages of the tube for carrying a level roadway across a large spau were brought into general notice by Robert Stephenson, engineer of the Chester and Holyhead railway, in the construction of the bridges to carry that railway across the Menai strait. It was required by the admiralty that these bridges, called the Britannia and Couway„should be constructed, so as not to interfere with the navigation, with clear spans of upward of 400 feet. The largest arched spans that had been previ ously constructed did not exceed 240 ft.; and suspension-bridges not being suitable for he ivy and rapid railway traffic, the engineer was obliged to devise some new form,which should conform to the stipulated conditions. Mr. Stephenson having decided upon the tubular form, proceeded, in conjuuctiou with `Jr. Fairbairn, to make an elaborate series of experiments on tubes, to determine the most suitable arrangement of the wrought iron of which they were to be composed. They found that a rectangular tube, of which the top and bottom were cellular, gave the greatest strength with the least material. The span of the Conway tube was 400 ft.; while the tubular part of the Britannia bridge consisted of two spans of 460 ft., and two of 230 ft. each in the clear. The foundation stones of these bridges were laid in 1846 and 1847 respectively. Since that time, many important bridges have been constructed on this principle. One of the largest and most important is the Victoria bridge, over the St. Lawrence, near Montreal, in Canada.. The total length of this bridge is 9,144 ft., or nearly lt miles. It is built in 24 spans, of from 242 to 247 ft. each, and one of 330 feet. The greatest depth of the river is 22 ft., and the average rate of the current 7 in. per hour. The bottom of the center tube is 60 ft., and at the abutments the bottom is 36 ft. above the water, so that there is a rise of 1 in 130 in the roadway toward the center of the bridge. An idea of the stu
pendous n Awe of this structure may be formed from the facts that 9,000 tons of iron were used in the tubes, and 14 millions of rivets; also that the total surface of iron was 32 acres; and as it received 4 coats of paint, the total painting was 128 acres. There were 2.713,095 cubic ft. of masonry, and 2,280,000 cubic ft. of timber in the temporary works, dams, etc.; and upward of 3,000 men were employed. The first stone was laid July 20, 1854, and the first train passed over on Dec. 17, 1859. The total cost was E1:400,000, or about £37 per lineal foot. Notwithstanding the success of these structures, the tubular form has been to a great extent superseded in recent structures by the lattice or trellis. 'Phis has arisen from the great saving in the material of which the sides are composed, effected by the open lattice-work, as compared with the solid plated side of the tube. By the lattice arrangement, the material is more capable of arrangement in the direct line of the strains; and the section of the lattice-bars can be accommodated to the strain, so that there shall be no material which is not carrying its due share of the load. The first large structure cf this nature was the Boyne viaduct, on the Dublin and Belfast Junction railway. Mr. Barton, the designer of this structure, in a notice of this bridge, gives the relative weight in the sides of the different forms of girders, neglect ing the weights of the top and bottom, which are the same in every case, as follows: ordinary tubular girder, 100; girder as adopted in great exhibition, 117; Warren's girder (angle of bars 60'), 73; lattice girder (angle of bars4.5'). 67. Besides this considerable saving in material, the facilities this form gives for repairs and painting, and the expo sure of a smaller surface to the wind, are additional reasons for its preference.