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regulated by the intended width of the bridge-If the boats are large, and strong in the sides,the ridge-beam may be fastened upon beamslaid across the boat; hut if, as is often the casewith keeled boats, the sides and timbers areslight, though their hulk may be sufficient todisplace the necessary volume of water withoutmuch immersion, the trestle should he raisedupon the kelson, as shewn in the figure. Thebalks should he laid from trestle to trestle. Thebeams of all floating bridges should be nearlysquare, because if the depth be much greaterthan the width, according to the well-knownscantling of strength, they would be apt to turnon their side when the bridge is much agitated,and would, moreover, be deficient in lateralstrength, to resist the horizontal strains to whichthey may be exposed.

The length of the balks is regulated by thedistance the boats are placed asunder, and thisagain depends upon their size, their number,and the length and strength of the timber forbeams. If the boats are large and strong, andgood long beams can be procured, the intervalsshould be very considerable, in a strong current,so that it may be let pass as freely as possible,by having its surface, where the velocity isalways greatest (page 13), as little covered, and