Chap. VIII.

II lUTA IN.

3 feet by 2 feet 6 inches. The distance or span of the middle compartment is 120 feet fromcentre to centre, and between them and the land 55 feet at each end. At the base of eachpyramid, level with the roadway, is a cast-iron bed, secured by iron bolts inserted into otheriron plates worked in the foundation; an iron bar, 3 inches by 1 inch, passes through thecentre of the pyramids into a cast-iron plate at the top.

The suspending bars average 2 inches by 1 inch, and are placed 2 feet 6 inches apart;these are fastened by gibes and keys into a cast-iron plate, which passes up the side of thepyramid from the base to the top. The main beam is formed of two wrought-iron bars,7 inches by | of an inch, arranged in lengths of 18 feet, breaking joint, and connected bybrace plates. At the edge of each suspending bar an upright piece of iron, about 12 incheslong, is welded, to which the upright supports are attached ; in these uprights are eyesthrough which the suspending bars, which are all arranged parallel, pass, and are madetight by wedges above and below the bar. These suspending bars are attached to around bolt 2 inches in diameter, which passes transversely, and connects the two ribs orbeams. At the abutments the ribs are secured by cast-iron bolts and plates. The weightof the suspending and upright bars is 7 tons, and the total quantity of wrought-iron is18 tons, and of cast-iron 5 tons. The floor is of Memel joists and oak platform, and thecost was 2500/. As there are twenty-four suspending bars, whose united sections equal48 inches, and one inch will support 20 tons, this bridge would carry 960 tons, but as theleverage is as 4 to 1, we must only allow one quarter, including the weight of thematerials.

Turning or Swing Bridges —There have been a great variety constructed over the variouscanals and dock entrances : among the best in design are those at the West India Docks.

'Jlie roadway is carried upon cast-iron plates, which are secured to the ribs by iron boltsand nuts, the ribs being cast with flanches to receive them.

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Fig. 529.

st. Catherine’s dock swing bridge.

At St. Catherine’s Docks the bridge is equally divided in its length, one half being madeto swing each way, the tails working round three parts of a circle; the clear opening be-tween the walls being 44 feet 9 inches, and the rise in the middle 3 feet 8 inches. Theturning is effected by a wheel working horizontally on a number of rollers that runupon a bed of cast-iron, and so nicely is the whole balanced, that by means of a rack andpinion it is easily worked. Iron bridges of this kind are generally adopted in pre-ference to those of timber, which were formerly used ; although more costly in the firstinstance, there is greater economy in the end, and from the superior quality of themachinery applied, and the nice adaptation of the parts, and perfect balance maintained,there is little labour required to open and shut them. On canals in particular where asingle lock-keeper or attendant is on duty, it becomes indispensable to provide the meansof turning a bridge with the least possible labour, and to so great perfection have ourengineers brought the construction of these swing bridges, that they may be manoeuvredalmost by a child. The ribs, when rightly proportioned and put together, are calculatedto bear any weight they may be subjected to, and, when properly balanced at their turningend, there is no danger of their getting out of order, or sagging.