Engineering Wonders of the World
Volume I

148 ENGINEERING WONDERS OF THE WORLD. in which Mr. William Fairbairn took an im- portant part. Model tubes of one-sixth full size were constructed and tested, and from the results so obtained was established the superiority of rectangular tubes, specially strengthened at the top, over tubes of circular or elliptical section. The following was the general scheme of the bridge, for the erection of which parliamentary powers were sought. On the Britannia Rock, and on the shores of the straits, at about high-water mark, would be built three huge piers of masonry, having openings 100 feet above high-tide level (for the tubes). The four tubes of the two main spans were to be 460 feet long, 15 feet wide, and 30 feet high at the Bri- tannia Tower, whence they tapered slightly vertically towards the shore. For the ASSEMBLING A 460-FOOT TUBE ON TEMPORARY PLATFORM. (From an Old Print.) two land spans, four tubes, 230 feet long each, would be needed. All eight tubes were to be built up of riveted boiler plates, ranging from | inch to Chief - . thickness, supported Features. 8 1 r internally by strong ribs of angle iron, and. strengthened at the corners, to prevent distortion, by triangular gussets. The roof of a tube was composed of eight flues, 21 inches deep and 20 inches wide, as experiment had shown that a group of flues gave, for a fixed weight of metal, greater strength to the top member than could be obtained from plates assembled in the same way as those of Tubes the sides. Stephenson de- cided to build the short land tubes in their final positions on “ falsework ” of stout timbers, and, as the Admiralty conditions prohibited this system for the main spans, to take a leaf out of Tel- ford’s book and con- struct the tubes for the latter on plat- forms on the Car- narvon shore, float them between the piers, and raise them to their final eleva- tion by means of hydraulic presses. As soon as Steph- enson’s plans had received official sanc- tion, preparations were begun for push- ing the work ahead vigorously. Fifteen hundred workmen were collected, and soon the shores of the straits echoed with the blows of mallets and hammers. When the wharves, workshops, and other tempor- i i -i Busy ary structures nad been built, Scene large gangs of masons attacked the Britannia Rock and the sites of the two land towers and abutments, while labourers, aided by many horses and carts, heaped up the approach embankments of the bridge. Ship after ship came to anchor in the straits