Length of Bridge with its approaches 2680 feet.
Width between N. and S. abutments 830 feet.
Depth of River at high water under central span 33½ feet.
Depth of River at lowest tides under central span 12 feet.
Clear headway at high water when the leaves are down (varies 20 to 29½ from one part of the bridge to another) feet.
Clear headway in centre span at high water with the leaves 143 feet. raised
The side spans are really suspension bridges, but the chains have only two links, connected at the lowest point by a pin 2½ feet in diameter, while their higher ends are supported on the columns of the piers, and on similar but shorter columns on the abutments. The horizontal pulls of the chains on the piers are made to balance each other by connecting the chains to tie bars stretching across the central span, and the landward ends of the chains, after passing over the lower columns of the abutments, are securely anchored in enormous masses of concrete.
Each of the opening parts, or _bascules_, or leaves, as they may be called, consists of four girders 18½ feet apart, rigidly braced together, and connected at the pier end with a great shaft, 48 feet long and 1 foot 9 inches in diameter, which turns in massive bearings, resting upon four fixed girders. The leaf is counterbalanced on the shore side of the pivot shaft by 350 tons of lead and iron; the short leverage of the centre-weight and small space available for it required the greater part of this weight to be of lead, rather than of the less expensive metal. The pivot shaft passes through the centre of gravity of the whole, so that, although the total weight is nearly 1,200 tons, no very great power is required to set it in motion, as the pivot shaft rests on rollers to diminish the friction. The power for moving the leaf is applied to toothed quadrants of 42 feet radius, of which two are fixed to the outside girders of each leaf, and are geared into cogs moved by eight large hydraulic engines, with six accumulators, into which water is pumped by two engines, each of 360 horse-power.
The total length of the bridge, including the approaches, is just half a mile, and the height of the towers from the foundations is 293 feet, so that if one of them were placed beside St. Paul’s Cathedral, it would compare with it in height as shown in the sketch, Fig. 147_f_.