Chap. XXIII.

HOPE BRIDGES.

1393

by blocks and pulleys to carry the timber platform : such an arrangement in all probabilitygave the idea of the several wire suspension bridges that have been constructed : for rivers

Fig 2314.

ROPE BRIDGE.

PLATFORM OF A ROPE BRIDGE.

of considerable breadth, it is often necessary to erect a framing of carpentry on each shore,to which the several ropes which are to carry the platform are suspended.

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Fig. 2316.

ROPE BRIDGE.

Bridges of Boats were of very general use during the middle ages, and are still foundconvenient over the Rhine, and in many parts of Germany : one at Strasbourg yet remains,upwards of 1300 feet in length, and another at Cologne: they are frequently describedby ancient authors, and we have already alluded to them. That over the Seine atRouen was constructed in 1700 by an Augustine monk, named Nicolas: various formsare given to the boats, which are flat-bottomed, and placed at a distance from centre tocentre, equal to their length, or a trifle more; they are loaded with stone or ballast, soas to enable them always to remain at the same height above the water, and are securedtogether by the longitudinal timbers that pass over them, or by chains that cross diagonallyfrom one to the other.

Anchors dropped in the stream, to which the boats are moored, enable them to maintaintheir position, and, when it is required to make an opening to let the vessels pass, two ofthe boats with the platform upon them are floated on one side, and held in their new'position by anchors laid down for the purpose. The bearers on which the platform is laidmay be formed of three or more longitudinal timbers, over which are short joists to receivethe planking, or by at once laying the planks transversely on a greater number of longtimbers; the nature of the stream and the strength requisite for the bridge must determinewhich method is preferable.

Floating Bridge across the Hamoaze , between Torpoint and Devonport , consists of a flat-bottomed vessel, 65 feet in length, and in width at midships 45 feet; it is further lengthenedby a drawbridge at each end, where its width is only 38 feet 6 inches : when freighted, itsdraught of water is 30 inches, and the clear depth of the hold is 4 feet 3 inches; it has itssides curved vertically, in order that the spray may not so readily mount, and to prevent

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