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and ^oad for loaded mules to pass along them with safety.. Thina, these flying bridges are constructed of an almostincredible magnitude ; the Philosophical Transactions containe figure of one, consisting of a single arch, 400 cubits long,and 500 in height. °

-A-great change in modern bridge-building has been effected/ tlie introduction of iron, and the use of chain or suspen-mn-bridges. The invention of Iron Bridges is said to be^clusively English , but Duhalde gives the merit of it to thennese; be that as it may, there is no country where thereV. ^ een so extensive an application of the discovery, or inq J lc 1 ' las been erected so many fine bridges of iron as inSl* eat ^ r *tain. The first was set up at Colebrook Dale,iiopslnre, in the year 1797, and was speedily succeeded by( l 1116l 'ous others in all parts of the United Kingdom ; for ae ^ript; on of which we must refer to Iron Bridge.

T aw Bridges are of wood or iron, sometimes of both, withr - ne a hutments. They are placed over navigable canals and°ut ° r Used * n fortified places for the purpose of shuttingfast t le enern y> and are of varied construction. Some arer , ■ e 'j ed at one end by hinges, so that the other end may bedoi ° r - ^ owered at pleasure. The most common method ofd '' s is by a kind of balance, called pipers, in which caseacr f ° e '" dlen drawn up stands erect, to preclude a passageTack 53 a rnoat ’ Others are so constructed as to be drawnc „ j or thrust forward, as occasion may require. On small] a Sj &c., draw-bridges consist of one leaf only; but on^ 7y. r Navigations, wet-docks, &c., they are of two pieces,° r | o ' n S ln the middle, and forming an arch, which are raisedof vered hy means of balance frames, movable on the topss Uch^ ri ^* tS su 'ted in height to the magnitude of the bridge;ev er If *l la * at Bristol, over the Frome. Such bridges, how-Ca tch‘ avin S been found inconvenient from their tacklinga ]>i n * n S the yards and riggingof vessels passing through them,Ve nted° bridge, diverse from all the preceding, has been in-of Qne ’ c ahed a Swivel Bridge: these, on small rivers, are onlytollers or Taf and turn on a centre, or series of balls or

Parts ’ ^ Ut " dlen ma( le on a wider scale, they consist of twocentre ° ne ° n ea<dl s ' de °f tde channel, and meeting in thestru c f'j oaost complete of this kinij are those con-s Pans 4o , * le West-India and London Docks; the lattersi sts 0 p° f eet , and is 15 feet wide in the road-way. It con-ber 0 f cast "!ron ribs, about 11 inch thick, turning on a num-cast-i r COn P entr ‘ c rollers, which move between two circularlets (j° n rin S s > v ery nicely turned: each leaf has a flap, whicheither ^ a screw > and abuts upon the stone-work onCa pable f ^ omdn S the whole bridge, when shut, into an archThe wl i faring any weight that can possibly pass over it.Hutch e *° 6 a PP ara tus weighs 85 tons; but it moves with sotainute^ 86 ’ t ' lat ^ can f* e opened and shut in less than three

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this Cc ^ e ” Siow bridges have only lately been introduced intoPeriod .^ 10u l~h known to the Chinese from a very earlybee® er lron ‘ c hain bridge of Yunnan is supposed to have.Ming”; e « ed . about A.D. 6oI in the reign of the emperorhammer''• f 3 dcscr 'bed as very similar in principle to the® l ina l Suspension Bridge near London . In Kircher’slength ** ' s stated, that the chord-line is of the

gives a °0 cubits. In the Asiatic Researches, Turnererected , ei y mteresting account of the singular bridgesv aried con t 6 . rlatdves °f Bootan. These bridges are ofstances f 0J . S 1 . u . ct ' on > hut admirably adapted to the circum-Bo 0 t an Z ■ they are intended. Over the widest river

Ir °n chains ' S an ' ron bridge, consisting of a number of

ar ® stretch* su Pport a matted platform; and two chainsborder, p , , ove > parallel to the sides, to support a mattedUc h is absolutely necessary for the safety of the

passenger, who is certainly not quite at his ease till he haslanded from this swinging, unsteady footing. At anotherplace, a bridge for foot-passengers is formed of two parallelchains, round which creepers are loosely twisted, from whichplanks are suspended, the end of one plank resting upon theother without being confined.

In the rude suspension bridge of South America , withits ropes of twisted bark, and its platform of cross pieces ofwood interwoven in them, or the platform attached imme-diately to the sustaining ropes, the form assumed, the cate-narian curve, is the same as in the more perfect structures ofmodern times—and one traces easily the transition from thesimple but effective contrivance of the untutored Indian, tothe master-pieces of the genius of a Telford. See Suspen-sion Bridge .

Bridges of Boats are made of boats, either of copper, tin,or wood, fastened across the stream by means of anchors orstakes, and laid over with planks. The earliest instanceupon record of this kind of bridge, was that laid by DariusIlystaspes over the Ister, or Danube, in his Scythian expedi-tion, 508 years before the Christian era. The same monarchalso crossed the Thracian Bosphorus with 700,000 men, bymeans of a bridge of boats ; the strait being five stadia, or1,008 yards in width. Modern armies carry with them tinor copper boats, called pontoons, to be ready on any emer-gency: several of them, placed side by side, across the river,till they reach the opposite shore, with planks laid uponthem, form a plane for the soldiers to march on. At Beaucaire ,Rouen, and Seville , are very fine stationary bridges of boats,which rise and fall with the tide : that at Rouen is nearly300 yards long, and paved with stone, so that laden carriagesand horses, as well as foot-passengers, go over it in safety.In the absence of pontoons, military bridges have been madeof blown bladders, hollow casks, sheaves of rushes, &c.covered over with planks.

When bridges of this kind do not extend .over the wholebreadth of the river, but are contrived to float from one sideto the other, they are termed Flying or Floating Bridges.A bridge of this description is generally composed of severalboats connected with each other by a flooring of planks, andsurrounded by a railing. This stage or raft is furnishedwith one or more masts, according to its dimensions, towhich is fastened a strong cable, supported at proper dis-tances by boats, and extending to an anchor, in the middle ofthe water, where it is made secure. The bridge thus becomesmovable, like a pendulum, from one side of the river to theother, with the assistance only of a rudder. Such bridgeswere formerly sometimes constructed of two stories, for themore expeditious passage of a great number of men.

Another kind of flying bridge is formed of two platforms,laid one upon the other, and by means of cords and pulliesthe uppermost is made to run out beyond the lower platform,till its farther extremity rests against the place it wasdesigned to reach. In the Histoire de VAcadcmie Royale desSciences, for the year 1713, page 104, is a description of afloating bridge, which lays itself on the opposite side of a river.

Under this head we have now to describe one of the mostuseful and ingenious constructions of modern science andengineering skill—the steam Floating Bridge invented byMr. J. M. Rendel, the eminent civil engineer. The firstbridge on this principle was erected by Mr. Rendel acrossthe estuary of the Dart at Dartmouth , about the year 1832,and a similar one was established about two years after,across the I-Iamoaze, between Torpoint and Devonport.

A very full description of the latter, accompanied byelaborate drawings, has been furnished to the Institution ofCivil Engineers by Mr. Rendel himself, and from the first