The reader can hardly have failed to observe that the chief element in the stability of the structure depends upon balancing a great mass of metal on the one side of a pier by an equal mass on the other side. But while each end of the central cantilever bears half the weight of a central girder, the two shoreward cantilevers have this load at their inner ends only. How is their balance maintained? In this way: the shoreward arms are made about 10 feet longer than those that stretch over the water and their extremities are also loaded with about 1,000 tons of iron, built up within the shore piers.
The lofty columns of the piers were erected without any external staging, from a temporary platform surrounding the piers and supporting the necessary machinery. The weight of this platform with the machinery on it was about 400 tons, and as the work proceeded it was raised as required by hydraulic machines placed within the vertical columns. As the height of these increased, the men and materials had to be conveyed to the platform by cages moving between guide ropes and worked by steam engines. From this platform were constructed not only the main columns, but the great diagonal tubes, the bracing girders, and the viaduct girder. The cantilevers were also put together without scaffolding. When the first few feet of the lower member had been built out from the base, a movable platform was hung round it, and on this platform were the cranes for putting the plates into position, the furnace for heating the rivets, and the hydraulic riveter of specially designed construction, without noise or hammering, the riveting being completed by the application of a pressure equal to 3 tons per square inch. The building up of the cantilever arms on either side of each pier always proceeded at the same rate, so that the balance was constantly maintained. This building out from each side of the pier, without the necessity of relying upon any temporary scaffolding from below, is one great advantage of the cantilever system, as it is both easier and safer than a system which relies upon the temporary scaffolding raised from below. The Forth is for the time the longest spanned bridge in the world; but it may not retain that honour long, for the legislature of the United States has already authorized the construction of a cantilever bridge, the spans of which are to be 2,480 feet. Still more gigantic is the project lately put forward by some competent French engineers of bridging the English Channel from Folkestone to Cape Grisnez in 70 spans on the cantilever system. The designs have been completed and the calculations made, and no one doubts of the engineering practicability of the scheme. But the cost is estimated at about 34 million pounds sterling, or nearly six times as much as that required for constructing the proposed Channel Tunnel; so that the scale could be turned in favour of the bridge only if the political reasons that were opposed to the tunnel were held not to be applicable to the bridge. But it is difficult to conceive that the existing traffic could ever be developed to such an extent as to make an undertaking of this magnitude a commercial success.