Book III. 



USEFUL DECORATIVE BUILDINGS. 



351 



arch will bend down, or yield in some places and rise in others ; in which case the whole 

 acts by its tenacity. 314 



^^^^«^^ 



1787. A very light and strong bridge may be formed by screwing together thin boards in 

 the form of a segment, or by screwing together a system of triangles of timber. This 

 principle may be carried to a great extent ; by using so many lamina the elasticity of 

 the materials is lessened without rupturing their parts, and though from the form of 

 such arches, they would appear to act by gravity, yet in truth, they act more by 

 tenacity, for the ends of the segment cannot be pressed out without rupturing the soffit, 

 or crushing the crown of the arch. For broad tame rivers in flat grounds, such arches 

 may be considered appropriate, as attaining the end without any appearance of great 

 effort. [Fulton on Bridges; Howard on Military Bridges.) 



1788. Bridges of common carpentry (Jigs. 315, 316.) admit of every variety of form, 

 and either of rustic workmanship or with unpolished materials, or of polished timber 

 alone, or of dressed timber and abutments of masonry. 



315 31G 



1789. Bridges of masonry (Jig. 317.) may either have raised or flat roads ; but in all 



317 



cases those are the most beautiful (be- 

 cause most consistent with utility) in 

 which the road on the arch rises as little 

 above the level of the road on the shores 

 as possible ; notwithstanding the pre- 

 judices of some eminent engineers 

 (Telford, in Ed. Encyc. art. Bridge) in 

 favor of the old practice of always 

 forming the extrados of a considerable curve 



It is only where masted vessels are to 

 pass under, that the raising the arches higher than what is necessary for the transit of the 

 stream can be considered in good taste. 



1790. Cast-iron bridges are necessarily curved; but that curvature, and the lines 

 which enter into the architecture of their rails, may be varied according to taste or 

 local indications. 



1791. The boat, as to construction, belongs to naval architecture. In gardening, 

 it is sometimes used as a substitute for the bridge, sometimes worked by a mechanical 

 power, as the wheel and pinion, and commonly with the deck arranged as part of the 

 gravel walk, which approaches the edge of the water. But where a river with a cur- 

 rent is to be crossed, the flying boat, with the deck arranged as part of the walk (Jig. 318.), 



