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will show the different positions which the boatsof a bridge, thus constructed, may individuallyattain, without straining any of- the materials, orbeing so braced as might break the bridge.

If the boats be of the larger class, such asbarges, lighters, or other vessels of at least 20or 30 tons, the current rapid, and the stuff forbeams short, gunwale pieces of wood, havingcompartments to receive the beams, may befastened on both gunwales, and the beams laidacross the intervals, from the outside gunwaleof one boat to the nearest gunwale of thatadjoining; short beams, as for decks, being laidacross each boat. Thus the timber being appliedto span the intervals only, will go further in this,than in any other way; and the stability of largeboats of burthen, as above, is such, as to doaway the objection to laying the weight on thesides of the vessels, particularly when the bridgeis covered, throughout, with troops passing.

When there is not sufficient depth of waterto admit of boats floating within the properdistance from the river bank, and thus to formthe two hutments, wharves, rafts, or trestles,should be constructed ; and it is sometimes ne-cessary to prolong these to a considerable extent,into the river, particularly on the sides oppositeto the concave parts, where the water is alwaysshallow, fig. 12, plate l. For tide rivers, or those