the suspenders, and in some instances parted the chains. No accident of thiskind is known to have taken place on a wire cable bridge. In a former part ofthis report I have explained the necessity of a stiff floor, which of itself willprevent short undulations. Lateral motions can never take place to a greatextent on account of the inclined position of the cables, which, with their greatweight and stiffness, will defy an ordinary storm. Let us, for instance, supposea hurricane or a tornado, like those which sometimes have occurred on thelower Mississippi , the force of which has been estimated to be equal to about30 pounds pressure upon one superficial foot. The outside vertical surface ofthe floor and railing, as far as exposed to the wind, measures 7 superficial feetfor every foot of floor. The pressure of the air would, therefore, be equal to 210pounds against one foot of bridge, or 159,600 pounds, or 80 tons, against onespan. This is certainly a great force; but the great stiffness of the floor alonewould be sufficient to resist it. As the ends of the floor will be firmly connectedwith the masonry, the strength of the timbers arranged in the line of the bridge,will be competent to resist a far greater force. But for the express purpose ofguarding against hurricanes, a system of under-floor stays will be applied,calculated to check these injurious movements. It is true that a hurricane,such as was experienced at Natchez, might force the centre of a span a few feetout of line, but the yielding nature of the work would prevent any furtherinjury. With a view of checking lateral vibrations of the cables, they will beconnected by diagonal stays, made of wire rope.
The action of vertical currents of air upon the underside of the floor, will beeffectually resisted by its great weight and stiffness. Admitting the up-liftingpower of an under-current to act with a force of 30 pounds upon every super-ficial foot of floor, it would be resisted by a weight of 50 pounds, which includesthe cables at the centre of a span, where they are firmly connected with thefloor by the suspenders. It is not probable that a vertical current would actagainst the whole extent of the floor at once, and with equal power. It is morelikely that its force will be directed against one portion of the floor, while theother remains at rest. An undulatory movement of the structure would be theresult, but soon counteracted by its own stiffness.
The maximum tension of the cables was stated at 1427 tons. To resist suchan enormous tension, equal to the weight of a frigate, may seem impossibleto those who are not used to the application of great forces. But the mostsceptical will not deny the practicability of resisting a large force as well as asmall one, provided the means applied are duly proportioned to the magnitudeof the object to be attained. The estimate of resistance afforded by theanchorage, has been made with a full appreciation of the forces to be opposed.Its certainty is based upon a simple and unerring calculation. It was stated inthe 4th section that the end of each cable is to be divided into 7 parts, each toconnect with an anchor chain. As these chains are curved about 65 degrees,their tension diminishes as they descend. In our examination of the strengthof the cables we have allowed as the greatest tension to which the wireshould ever be subjected, 20,500 pounds for every square inch of solidwire. In determining the requisite section of the anchor chains, which are tobe constructed of solid bars, I propose to allow one superficial inch of section forevery 14,000 pounds, or 7 tons of strain. The aggregate section of the chainswill, therefore, be 1427 divided by 7, or 204 superficial inches. Each of the 14chains will have a section of 14.57 inches; they will be composed of links, the