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which are exposed to be much swollen, and, con-sequently, the surface-level suddenly raised, as inthe vicinity of a mountainous country, the trestlesused for this purpose should be formed of twostrong upright pieces at each end, fig. 4, plate 3,with a moveable ridge-piece, A, which may beraised or lowered, at pleasure, and readily ad-justed to any level, by the iron pins, b c, onwhich the ridge, A, rests. In rivers subject tofloods, the boats forming the hutment (andwhen the river bank is high, sometimes the twoor three nearest boats at each end of the bridge)should each be fitted with elevating trestles ofthis construction, by means of which theirridge-beams may likewise be lowered, or raised,in due proportion, to give an easy degree ofslope from the hanks to the bridge, when theseare so high, or the water so low, as to requirethe slopes to be thus prolonged. The mostconvenient method of providing for this verynecessary adjustment, is to have vertical screws,I) E, fig. 4, plate 3, placed between the uprightsof the trestles within which the ridge-beam, A,is laid. This is the mode practised in the bridgeof boats at Coblentz.

It has already been observed, page 65, thatit is indispensable to provide, in the construc-tion of all floating bridges, for making, readily,an opening in that part which lies in the most.