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The cables on the Monongahela bridge are four and a half inches in diameter,on the Cincinnati bridge they will be over eleven inches, or more than sixtimes as heavy. Cables of this huge size compactly wrapped, so as to be nearlyas rigid as a solid cylinder, will, of themselves, possess a stiffness which couldnot be overcome by the passage of the heaviest team in any perceptible degree.How much the stiffness of cables diminishes with their size was plainly observedby all those hands on the Monongahela bridge, who had been previously atwork on the cables of the aqueduct, which are seven inches in diameter. Thegreater degree of rigidity which a wrapped cable possesses over a chain, consti-tutes one of its superior features. It may also be observed here, that one singlecable possesses more stiffness than if divided into two, as is the case on theFreiburgh bridge in Switzerland . This work forms one single span of 890feet long, and is supported by four cables, arranged in two pairs, one on eachside. When the floor of a suspension bridge is set in motion by a heavy galeof wind or a hurricane, then the strength of the cables will be tested moreseverely than by any loads which could possibly be brought upon the floor. Inthis case lateral motion as well as vertical oscillations would take place, andthen the superiority of one undivided cable over a number of cables wouldbecome apparent. The undivided cable would always resist with an undividedstrength, while of those which are separated, one may be overstrained at amoment when the other is comparatively slack.

As regards the solidity and stiffness of the floor, I will further remark, thatnothing will be omitted which is calculated to add to this much desired featureof a suspension bridge, without increasing its weight materially. It will bedifficult to convince those who are skeptical on this point; they will howeverhave to acknowledge that the Monongahela bridge possesses all the stiffnessrequisite for such spans. In place of convincing we can only persuade them topostpone their judgment until they shall have an opportunity of seeing it refu-ted on the bridge itself when finished. The plan of railing, devised for theCincinnati bridge, will contribute much to its stiffness, and at the same timemuch improve its appearance.

Before dismissing the subject of the floor, the question of its durability shouldbe considered. The principal material to be applied in its construction, will bewhite pine timber, with the exception of a few pieces of the upper floor of theroadway, which will be composed of white oak. We are accustomed to seeall bridges covered with roofs. Even for aqueducts, to which roofs are a posi-tive injury, they have been considered indispensable. That system of archingand trussing, Avhich has been most generally approved for wooden bridges,renders framing of great depth absolutely necessary. On the rise of an arch onthe depth of a transframe, its strength mainly depends. But high trusses andarches will only preserve their strength so long as they are kept in line andprevented from yielding laterally. When out of line their strength is lost. Itbecomes therefore important to connect and brace the trusses or arches laterallyin the most effective manner. This is done at the bottom and at the top, theformer serving for floor, the latter offering an easy opportunity for roofing.Thus the roof is in a manner an important part of the work, and renderednecessary to insure its stability and strength. Other modes of constructingwooden bridges have been extensively and successfully practiced for centuriesin Europe , by which roofing is dispensed with altogether. It is true, that whengreen unseasoned timber is applied in the construction of such works, roofingwill add much to its preservation. But at the same time, it cannot be denied,that the weather-boarding, as well as the roof, has in many cases favored the dryrot. As regards the appearance of covered bridges, we must acknowledge that5