Engineering Wonders of the World
Volume I

 THE BRIDGES OF THE MENAI STRAITS. 143 feet wide at high tide. The distance between abutments is just short of one-third of a mile. To span this, Telford specified two short em- bankments, 7 arches of 52j feet span, and a main suspension span over the channel of 550 feet between the centres of the towers. The last factor taxed Mr. Telford’s inge- nuity severely. Such a span was at the time unprecedented, and the safe accom- plishment of the task demanded that a vast amount of pre- liminary experiment should be devoted to the huge chains forming the distin- guishing feature of the structure. An Act empower- ing the building of the bridge was passed in 1819, and Telford lost no time in get- ting to work. The foundations of the two main piers, each 153 feet high, were taken in hand first, and while the piers rose the arches of the two approaches rose with them, the chief difficulty being that of providing sufficient stone to keep the army of masons engaged. As the piers would be subjected to severe lateral strains, their individual stones were bound together by iron clamps, in much the same manner as the components of a lighthouse. Four large cast-iron saddles, running on rollers, to carry the suspension chains, capped each pier. Their easy movements over the rollers provided for the expansion or contrac- THOMAS TELFORD, Designer and Engineer of the Menai Suspension Bridge. (From the Rischgitz Collection.) The Building of the Piers. tion of the chains as the temperature of the air should vary. Since the efficiency of a chain depends ulti- mately on secure attachment, every care was taken to en- sure firm anchorages for the chains of this bridge. Anchoring the Suspen= sion Chains. The method adopted was to drive four paral- lel tunnels obliquely down into the native rock for a distance of 20 yards or more, and excavate a chamber across their lower ends. In this chamber were built up massive trans- verse anchorage frames, resting against the walls of rock separating the tunnels, and there- fore immovable un- less the rock itself were torn away — a contingency that was practically ne- gligible. The chains, six- teen in number, were composed of j-inch bars of iron. Thirty- to the strands of a six bars—corresponding wire cable—were grouped together to make a square chain four inches on the side, the com- ponents of the chain being wrapped with iron wire. The weight of the portion of the chain between the two suspension piers was over 23 tons ; its length, 570 feet. The masonry completed, preparations were made for hoisting the chains into position—a process to which Mr. Telford looked forward with the greatest anxiety. In order to obtain exact figures as to the power required to hoist