CHAPTER V.
THE TOWER BRIDGE.
“Whyshould they not have a drawbridge?”
“What! To draw up from each bank of the river?”
“No, I did not mean that exactly. Could they not get piers farther in towards the centre of the stream, and let the drawbridge rise and fall from them?”
“The river is too crowded for many piers.”
“It is. But I cannot help thinking a drawbridge—a bascule bridge as the engineers call it—is the best solution of the difficulty.”
“Well, a bridge is wanted sufficiently low to spring from the flat banks of the Thames for foot passengers and carriage traffic, and yet sufficiently high to permit tall ships to pass underneath.”
“And apparently these two requirements are incompatible.”
“Not altogether,” remarks a third speaker.
“You are partly right in your idea of a drawbridge. That is Sir Horace Jones’s idea. And, further, there is literally to be a high and also a low-level bridge; for there are to be two levels—that is, two roadways—one at a high, and one at a low, level across the middle span.”
“And is the low level to be a drawbridge—a roadway that can be drawn up to permit vessels to pass? Is that so?”
“Exactly. And this drawbridge will be in two parts, one on either side; they will be worked from two massive piers giving a clear span of 200 feet in the middle of the stream, through which span big vessels can pass. The usual traffic of the river will be able to pass even when the drawbridges are down.”
“And above the bascules or drawbridges will run the high-level bridge?”
“Yes, a girder bridge for footpaths, and people will reach it by lifts and staircases in the piers—which, by-the-by, will be more like huge towers. These towers will also contain the machinery for raising and lowering the drawbridges.”
“And what sort of bridge will be used for the other spans—that is, to cross the river between the piers and the shore?”
“Suspension bridges; so that the Tower Bridge as it will be called, for it will cross the Thames by the Tower of London, will embody the suspension, the bascule (or drawbridge), and the girder bridge principles, while in the centre will be two levels.”
“It promises to be a splendid piece of work.”
“It does. And it is very much needed, for the congestion of traffic on London Bridge is terrible.”
“And people have often to come round a long way to reach it.”
The promise of the Tower Bridge, as set forth by these speakers, has been amply fulfilled. It is indeed a fine piece of work; and although it does not embody any new idea, yet in its combination and development of old principles and in its size it is very remarkable. It was opened in June, 1894, and is, or was at the time of building, the biggest bascule bridge in the world.
Within its handsome Gothic towers are steel columns of immense strength, constituting the chief supports of the suspension bridges and of the high-level footways. The architect was the late Sir Horace Jones, and the engineer Mr. J. Wolfe Barry, while the cost was, including land, about £1,170,000.
The problem was to combine a low-level bridge providing for ordinary town traffic with a high level, under which ships could pass, and it was accomplished by a union of principles. In its oldest shape the drawbridge was probably a huge piece of timber, which was hauled up and let down by chains over the moats ofcastles. In the Tower Bridge there are two of such huge “flaps” or leaves, each about 100 feet long, one rising and falling from each pier and meeting in the centre. Large bascule bridges are usually constructed in this manner, and there is an excellent specimen over the Ouse, for the passage of the North-Eastern railway; one man at each half of the bridge can raise it in less than two minutes. Another fine bascule may be seen at Copenhagen.
The bascules are raised and lowered by chains, which, in the case of the Tower Bridge, are worked by superb hydraulic power from the massive pier towers. When drawn up, which is done in less than five minutes, the bascules are even with the sides of the towers, and full space is given for the vessels to pass.
The two side spans of the bridge, crossed by the suspension bridges, are wider than the centre, being 270 feet each, and the total length of the whole bridge is 800 feet between the abutments. There are also piers on the shoreward side for carrying the chains of the suspension bridges at each extremity.
The massive tower piers, sunk 27 feet below the river bed, are built of gray granite, and are also fitted with strong break-waters to resist the action of the tide. The high-level bridges across the central span are for foot passengers, and are 135 feet over high-water mark. The bascule bridges, when closed for vehicular traffic, are 29½ feet above high water, while the side suspension spans are 27 feet. The roadway is 50 feet wide, which is also the width of the approaches. The foot passenger traffic is never stopped, as persons can pass by the hydraulic lifts or the stairways in the tower piers to the high-level bridges above.
Sir Horace Jones died before the great work was completed, and was succeeded by Mr. G. D. Stevenson, who had been his assistant. Sir William Arrol & Co. supplied the iron and steel, and Sir William Armstrong the hydraulic machinery. Various contractors carried out different portions of the mighty work, which occupiedabout eight years in building. Near by stands the ancient Tower of London, looking not unkindly on the great constructive effort to which it has given its name.
Sometimes a bridge is made movable by swinging it round on a pivot instead of drawing it up on a hinge or axis; and sometimes, as in the case of a bridge over the Arun for the Brighton and South Coast Railway, it is made to slide on wheels backwards and forwards from the abutment. Floating or pontoon bridges are made by placing planks on pontoons, or boats anchored by cables. The longest in the world is probably at Calcutta, across the Hooghly. It is 1530 feet in length, there being twenty-eight pontoons in pairs. These are of iron, 160 feet long, and with ends shaped like wedges; they support a road-way of 3-inch timbers, forty-eight feet wide, and raised on tressel work. An opening can be made for ships by removing four pontoons and floating them clear of the passage way.
Great bridges present some of the most remarkable triumphs of the engineer. They rank beside the express locomotive and the ocean liner as among the great constructive achievements of mankind. Daring in design, and bold in execution and in sweep of span, they have been developed along several principles; and so solidly have they been built, so sound are the laws of their being, that it seems as though they will live as long as the everlasting hills.
a bridge