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HISTORY OF ENGINEERING.

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Fig. 52*. HAMMEUSMiril SI.SPONSION IlKIDGE.

sl>ort links and inch-round screw-bolts. The roadway is formed of transverse joists 12inches by 4, on which longitudinal beams are laid, and then the planking.

Marlow Bridge is nearly similar in its construction ; its total length is 426 feet, and thecarriage-way is 20 feet broad, the height of the platform is 12 feet above the water ; thereare four main chains composed of flat bars ; the deflection is 18^ feet, and the section of theiron in the chains altogether is 64 square inches.

Norfobi Bridge , at New Shoreham in Sussex, is another beautiful suspension bridge bythe same engineer. The chord line between the piers is 284 feet, and the deflection of thechains 20 feet 2 inches ; the breadth of the platform within the parapets is 28 feet 6 inches,the width of the carriage-way being 20 feet. There are on each side three lines of chains,the sectional area of the whole being 84 square inches. The bars are 8 feet lOj inches long,6) broad, and 1£ inches thick, with eyes 8=j inches in breadth ; where the chains are sus-pended are rollers of cast-iron, all 10J inches in diameter, with 2^ wrought-iron pivots.

There is a central arched opening through the main piers, and the abutments, which aresolid, weigh 900 tons ; the backstays are not put at the same angle as the central chains;the platform of timber is in length 268 feet, and the total weight suspended in the centralopening is about 356 tons, or at the rate of 62 pounds per square foot.

Mivhlewood Bridge , designed by Mr. James Smith of Doune, differs in its constructionfrom most others; the platform is not suspended, but supported upon iron frames, whichrest upon the chains. The span is 103 feet, and there are two supporting chains on eachside about 2jj inches in diameter, and 1 foot apart, which are fastened to iron frames. Thejoists which support the platform rest upon iron uprights, set on the chains, the lowerends being made like a fork to embrace them. There are twenty cross joints, the scantlingof which is 12 inches by 5, and the whole is cross-braced and secured.

Broughton , near Manchester; this suspension bridge has 145 feet 6 inches span, with a de-flection of 12 feet 6 inches ; the platform is 18 feet 3 inches wide, and 143 feet 3 inches inlength ; it is supported by four chains, two on each side, formed of rods 2 inches in diameter,and 4 feet 6 inches long, united by coupling links of inch-square iron and pins 2 inchesin diameter. The vertical suspension rods, 1 inch in diameter, spread out at top like afork, where they are pinned to an iron plate.

The main chains are supported by four cast-iron suspension frames, each carrying a pairof chains, and the points of suspension are movable to allow for contraction.

Suspension Bridge over the Avon at Twerton, near Bath , is in length 230 feet, and thewidth of the roadway 14 feet; it was executed under the direction of Thomas Motley.There are two pair of pyramids, placed on a bed of concrete, 22 feet by 12, and formedof six courses of Bath stone 30 inches deep, having two blocks on each course; thedimensions of the pyramids at their base is 5 feet 6 inches by 4 feet 6 inches, and at top